I thought light does not experience time...I was wrong!

[@Mahesh_Shenoy]

Here's a cool physics T-shirt: floatheadphysics.com/products/dont-be-a-jerk-dark

[@krislogy]

Minor correction. Fact. Tu. Ality. Not Faculty. :)

[@BaseSixBasics]

But, what if an object travels so close to the speed of light that it travels across the entire observable universe in femtoseconds of proper time? Would the object continue on its trajectory until it hits into another object? Or would it wrap around the universe infinitely or stay in place in empty space as the universe expands so that the object HAS TO experience time?

[@tokajileo5928]

where to get that t-shirt? XL size

[@petevenuti7355]

So when I'm playing chess with my computer and our kings wind up next to each other, and im pissed because the computer wont let me take its king saying its an illegal move but it's king takes mine‽ I shouldn't be learning chess from a computer.

[@harrisbinkhurram]

I think your explanation is better, and it proves Tyson's experiment wrong: kzbin.info/www/bejne/eGPHdKmupbCKfM0

[@mduftube]

This has nothing to do with scientific analysis, this is a video/channel analysis. He always sets himself up as the ignorant foil asking the questions. Einstein or whatever author he’s read is always correcting him, even though he’s the guy who understands what he’s teaching us. This is what makes him charming and charismatic and approachable from our point of view as learners: let’s learn together. That’s the literal definition of a good teacher.

[@user-cd6vy2jg6f]

Veritasium did a video on this teaching technique as well. It’s more important to explain what something is not / incorrect than it is to simply list truths

[@kornklown420]

Yes, this channel is amazing for all the reasons you just said, not to mention how excited he is about all the information, it's impossible to be bored watching him 😂

[@p3ter9000]

I also find it so charming. I'm always cheering with him, "Come on Mahesh!"

[@davidbordwell8346]

Wish i had this man as my teacher at some point. Hes an excellent communicator

[@davidbordwell8346]

But what if you were riding a beam of light? Einstein famous thought experiment.

[@michaelbell5984]

I love the hypothetical conversations with Einstein and Feinman. A great vehicle for explanations.

[@TheGreekSneak]

This addition to his explanations are what made me subscribe immediately. 💯

[@fernandogajo8800]

Absolutely! "I'ts approaching the speed of light... NOT QUITE, NOT QUITE! DON'T GET ANGRY WITH ME EINSTEIN!" That made me chuckle.

[@kyo_.]

man your topics are always so interesting i just have to watch

[@Mahesh_Shenoy]

Thank you :)

[@arkdark5554]

No. At the speed of light…the time stops. According to Gödel's interpretation of Relativity.

[@DABmonger]

I'm with that line of thinking, but then if equations are allowed to be blown up do we assume that singularities exist?! Perhaps time not moving and singularities are connected in some way, at least in telling us that we're missing something major in our knowledge!

[@kellyrobinson9564]

You don't understand inertial reference frames

[@DABmonger]

​@kellyrobinson9564 Who are you replying to? Why do you say what you say? Anything moving at the speed of light obviously blows up our current equations, such that some physicists prefer to say that special relativity just doesn't apply, and there's no valid frame of reference (having contracted down to zero length). In the limit of travelling at the exact speed of light, as well as tending to zero length above, we also have time elapsed tending to zero. So many physicists posit that light/photons do not experience time at all. And that a photon is at all places on its path at the same time. You could argue that at the speed of light nothing exists - there's no reality. BUT photons do exist. In respect of this content creator, I think it would be better to give both sides to the question of whether light experiences time. Light exists. And singularities (maybe) exist, as we have black holes. We are clearly missing something rather big in our knowledge of nature ...

[@luciddreamworks]

I have a masters in Math, and your videos have allowed me to appreciate axioms, postulates, and modeling so much more.

[@vyvianalcott1681]

I'm two minutes in and you are already blowing my mind about concepts I thought I understood. I absolutely love your videos, you are by FAR the best physics presenter and explainer since Feynman. Keep up the great work, I'm hanging on your every word now!

[@Mahesh_Shenoy]

Wow, that’s super encouraging to hear. Thank you :)

[@everythingisalllies2141]

I love it when people actually believe that they can really KNOW this stuff. They really have convinced themselves that they have actually understood what we can never possibly understand. Understanding what does light experience, is a classic case of over confidence. google the free ebooks, "Dave vs Hal 9001" for more information. You may change your mind about what you think is real and true.

[@auriuman78]

@@Mahesh_Shenoy it's true my man, I look forward to seeing your presentations when they drop in my notifications. They're actually fun dude, you make science what it's supposed to be, awesome 👍

[@Littleprinceleon]

​@@Mahesh_Shenoy, you help us to have some glimpses from the shoulder of science giants. That's the POV worth to achieve. Many thanks 🙏👍

[@Robienko]

Because it's not correct

[@effectingcause5484]

"There comes a point in every man, woman, and child's life that they wonder - Does light experience time?" See, that's the mindset of a great theoretical physicist..

[@Pit.Gutzmann]

who does not know normal people 🙃

[@laurencerilling5873]

You know that was attempted humor, ?

[@paranormalbirdman]

@@laurencerilling5873that question is meaningless! 🧐

[@Poolboys690]

It was a joke of a theoretical physicist.

[@mmicoski]

If an object travels almost at c, from its reference frame the universe is traveling at almost c and is extremely contracted in the direction of the movement. So, from its perspective, it would traverse the entire universe almost instantaneously. I think looking this way we could say this object experiences almost no time, meaning it does not see the universe evolve in the very short time (from its perpective) it took to traverse the universe. If you think regular objects do not traverse the entire universe, but a smaller distance between object creation and destruction, for this object it existed during almost no time and was almost instantly destroyed

[@QuantenMagier]

That is also how I see light, it's as if the photon was using a wormhole through space-but-not-time, and that wormhole is called the EM-field, but it's an imperfect wormhole due to redshift..

[@DABmonger]

I view light in the sense of limits and infinities, that blow up equations. Like singularities. We're clearly missing something significant in our understanding.

[@petejohnston5880]

You're absolutely right. As every point in the universe sees light traveling at the speed of light relative to it, then light in turn sees every point in the universe moving at the speed of light relative to it self and hence the whole universe is length contracted to be infinitely thin in the direction of travel. The universe becomes an infinitely thin but very wide pancake. This means that the point in space where it starts its journey is the same point in space where it ends its journey and it take zero time from its perspective to travel from start to finish. So does light experience time, well its life is over before it has a chance to experience anything. We see light moving but for it, it all happened and finished it's journey before time even moved.

[@DABmonger]

​@@petejohnston5880It can be argued that light does not experience time, and a photon is in all places on its path at the same time.

[@QuantenMagier]

@@DABmonger Nope, photons are just the exchange particles, there exist no photons on the path of light, just electromagnetic waves, the photons are only created from those electromagnetic fields by interactions with matter.

[@Sadlander2]

I'll admit, I almost always fast forward the ad(s) but I love it when there's a smart transition to the ad, which was the case here! Very smart, very well done! I wish my teachers had your passion. Except one teacher, with all of my other teachers, instead of being taken for a "ride", I had to find the motivation to make me curious about whatever the teachers were talking about and try to remain curious and motivated when it was obvious that the teachers didn't even want to be there! It is so easy to listen to you and you make it easy to understand. I struggled to understand how C was always constant, no matter how fast _YOU_ were travelling... until I watched your video and it clicked right away!

[@AS-zc8mr]

I followed your recommendation in a party, and the dude telling the story punched me in the face!!!

[@Mahesh_Shenoy]

What recommendation? :D

[@rodschmidt8952]

@@Mahesh_Shenoy probably saying "you're breaking the rules, you cannot use SR to conclude anything once you've broken SR"

[@fuseteam]

💀

[@kriiistofel]

I read somewhere how we could imagine photon 'perspective'. From it's 'point of view' there is only act of creation and then instantaneous act of annihilation (when it interacts with some object). There is nothing in between for photon, it does not 'experience' time.

[@Vexas345]

But why assume it's instantaneous?

[@G0ldbl4e]

@@Vexas345 They experience infinite time and length contraction as per special relativity

[@auriuman78]

Well as far as it (the photon) is concerned, it might as well be instantaneous, even if it took 13.8 billion years in reference to me? Zero time experience is pretty well instant right? Kinda baffles the logic of normal intuition when you realize that time is truly based around the fastest thing in the universe, that's how it ought to be reasoned at least, if you want to truly understand what time is. Base your reference at the fastest thing known, kind of like temperatures and zero. There's a limit and we reference from there in science, hence Kelvin. Why not do the same with light\photons\emr? I'm not trying to turn our time reference understanding on it's head, Just like we don't use Kelvin in everyday life. But for scientific matters, yeah 👍 I think it would simplify the problem of medium speeds not matching to do that way, cause it's all light speed, just different frames of reference right? I'm not gonna even attempt to address the things that would happen sub-planck though, haha, not qualified. It'd be weird for sure, in case this isn't weird enough for you. I am in no way certain there's anything meaningful below Planck pixel, but I'm certainly not opposed to it being more than not meaningful, so what if the Schwartzchild radius = black hole. Another thing we don't really know anything about other than they're there and they're doing something deeply interesting.

[@Vexas345]

@@G0ldbl4e They do not. The math for time dilation/length contraction doesn't work for things moving at c. They don't have reference frames, so special relativity doesn't apply.

[@auriuman78]

The very idea that the photon does not experience time, as far as it's concerned it's just sitting there, good Lord man what a concept. I mean really if you think about it 🤯 trillions of trillions of trillions of photons, all different amplitudes and wavelengths, interacting with different interferences to form stuff, all infinitely small and large at the same time, this is weird stuff seriously. It makes me pull back to the holographic model, you know what I mean, if it's just there then not, everything else around it being what's in motion... Which is also a bunch of other photons at rest, blipping on and off here and there. Kind of leads to the idea that maybe it is in some way intricately holographic. I mean it's pretty much all electrical\em. I learned that some years ago during a heroic experience that I did not plan on taking but accidentally did anyway. Idk I think a lot about stuff but this one I think is above human thinking. I'm definitely not smart enough to grapple with that one, but I'd be willing to bet my life that it's so ridiculously elegant and simple that we're just looking right over it. It's probably right in front of us 😅

[@CyberSystemOverload]

I learn more from you in a 14 minute video than all my years of high school. Thank you sir!!

[@daniellindforsbernholm3682]

I think the more intriguing point with the "experience" of something moving "almost" at the speed of light is that it would experience ending up at any destination "almost" instantaneously. If it keep going without any destination it would experience ending up infinitely far into the future (whatever that means) "almost" instantaneously. If it could do this indefinitely it would for sure experience the clock ticking as usually. But that it would even have time to experience one tick of the clock before either smashing into something or getting to the end of times is not likely.

[@siddhant5631]

I was literally looking for a person who could explain things that comprehensively. The way you crack things, like the scientists who are much more eager to break apart the subatomic particles in the Large Hadron Collider to understand the behavior or laws of this Universe. The power of visualization and the interpretation you have mastered is just Astonishing. I don't have words for you, you are just limitless. Keep continuing the series I am learning a lot by opening different ways to visualize things for Better interpretation. My regards to you.

[@kornklown420]

Instead of simply saying that light has no reference frame "because Relativity says so", I think we can explain and conceptualize it a little better. Light has no reference frame because light does not experience time. Period. It is moving through 3 dimensional space at maximum speed, and therefore it is moving through the fourth dimension of time at a speed of zero. Because it does not experience time it has no reference frame, from the "perspective" of time it is essentially teleporting from point a to point b, the same moment it is emitted it is absorbed at its final destination, and therefore light does not actually have a "perspective", or reference frame.

[@fuseteam]

There's nothing to experience, if there's no perspective for it But there's another way to frame it in the thought experiment at the beginning of the video, we were not looking at it from "light's perspective", if we did we would get the same answer as the reference frame of "almost at the speed of light" that is: light moving away at the speed of light aka "proper time"

[@longhoacaophuc8293]

light does have a reference frame. An "observer" in such frame experiences time as any other "observers" do, just at different rate. Beside, he does not see the photon that his frame sticks to. The problem appears when you use Lorentz transformation.

[@bernardoj54]

Light isn't instantaneous. It takes time for light to travel to distances bigger than its speed limit, so if from point X to point Y light takes, let's say, 10 minutes to reach, light would 'experience' that 10 minutes trip. It doesn't experience anything though, because it's not a sentient being such as a living being.

[@Cythil]

I regard to relativity it just a question that do not make sense. The video I think showed this very well. One could take the perspective of a photon, but then you are outside what relativity can handle, and so you can not draw conclusions from it. Like the video points out, you need to start to work out your own system where you even do basic things like define what you mean by taking a perspective. Relativity simply does not allow for the perspective of a photon. And you can also ask yourself what you are trying to achieve by taking this perspective. What insight are you looking for? Since no thinking being can actually have the perspective of a particle moving at C. There may be a valid reason to do so, but you need to be careful. Relativity is simply such a good model that it can handle most situations. Except when we move in to the quantum realm. So I guess if you could use this perspective change to combined predictive power of both quantum physics and relativity, then you are all set. But you are working out a new theory. Theories have a scope and beyond that scope thing get ill-defined. It is like a Married Bachelor. It makes no sense. Or what is north of the North Pole. Also do not make sense. You have to redefine what you mean at that point. The reason why going north from the North Pole makes no sense is not that you can physically unable to move a certain direction when you have reach the North Pole. It is that when you use that sort of coordinate system you have to reach a singularity in your model and all other directions are to the south. It is just how things are defined. And all models have those limitations where the question you ask become nonsensical.

[@nbrown6648]

I would like to present a different view: light does not exist. It is purely a human construct that conceptually connects two point (typically electrons) separated in space that interact with each other. One loses a quantum of energy, the second one gains a quantum a certain time later proportional to the distance between them. The probability wave radiates out in all directions, collapsing on the electron whose sum of all possible routes is greatest (as famously described by Feinman). Nothing is physically travelling, it is purely an interaction between two remote quantum states.

[@Pavan_Gaonkar]

What a video! I think this series is one of the greatest assistant to understand special theory of relativity

[@Mahesh_Shenoy]

Wonderful to hear that :)

[@gerbenhoutman9348]

9:22 in chess the result would be determined by who's move it is. Of course, that can't happen, because it's against the rules of chess

[@jensphiliphohmann1876]

05:40 f _Looking from s.o.'s perspective basically means looking from s.o.'s reference frame._ I'd put it differently: By "looking from s.o.'s perspective" physicists mean *describing the scenario from a rest frame of her/ his.* ▪︎A frame in this context is a coordinate system which basically maps all of spacetime, so you can't really looking _out of_ it. Rather you express physical quantities in those coordinates. ▪︎Everything kind of has infinitely many rest frames with different orientations or origins. ▪︎Your reference frame is the frame you use to describe a scenario. This _might_ be a rest frame of yours but it _doesn't have to._ And in everyday life, it seldom is. Mostly we implicitly use a frame bound to Earth; otherwise, if I went to Cologne by train, I'd have to say "the train has come to rest (a very active rest like that of someone running on a treadmill) and lets Cologne approach it".

[@everythingisalllies2141]

google the free ebooks "dave vs Hal 9001" you will be surprised what you will discover.

[@that80sLoverboy]

I watch a lot of these types of videos, and you just make things so much easier to understand than pretty much any other channel or there.

[@justinhageman1379]

As always these videos are amazing! By far the best most easily digestible explanations of physics concepts I’ve ever seen.

[@johnmagnotta8401]

Hear hear

[@johnmagnotta8401]

Or is it "Here, here?"

[@dogcarman]

Mind duly blown. I had never considered that consequence of the second postulate. Wonderful. Thank you.

[@danielcopeland3544]

OK, fine. I can't help noticing the animations all say "length contraction ignored". So let's talk length contraction. Suppose we have three objects: P, Q, and R. P and Q are at rest with respect to each other, while R is moving with respect to both. P and Q can be road signs and R can be a car, or something. In P and Q's reference frame, they are some distance d apart. Due to length contraction, in R's reference frame they are some smaller distance d*k apart, where 0 < k < 1. As the speed of R with respect to P and Q approaches c, the factor k approaches what number? In R's reference frame, after P has gone past, it takes some time t for Q to reach it (since in R's reference frame it is P and Q that are moving). What happens to the time t as R's speed with respect to P and Q approaches c?

[@danielcopeland3544]

@silverrahul Then isn't that a meaningful way of framing the question?

[@danielcopeland3544]

@silverrahul ...the question in the title of the video? "Does light experience time?"

[@danielcopeland3544]

@silverrahul The thought experiment I started this comment thread with. Did you watch the video? Mohesh says that "does light experience time?" isn't a meaningful question, because it can't be framed in a way that has an answer within the laws of special relativity. I'm querying that.

[@danielcopeland3544]

@silverrahul Calculating what happens as one _approaches_ an unreachable limit, as a proxy for what happens _at_ that limit, is a routinely used and uncontroversial method in calculus. Mahesh himself discusses it in this very video (9:39--11:48).

[@danielcopeland3544]

@silverrahul I know that an object with mass cannot reach c. If you were concerned that I didn't understand that, you can rest easy. Thought experiments, in physics just as in mathematics and philosophy, reach out into the impossible, if only to understand _why_ it is impossible. We wouldn't say to Einstein "But you can't accelerate a train to half the speed of light, you'll kill all the passengers." We can turn my question around, if you like. Let's suppose again that we have objects P and Q, stationary with respect to each other, and object R, moving at speed v with respect to P and Q. P and Q are a distance d apart in their own reference frame, and a shorter distance d*k apart in R's reference frame due to length contraction. Let's ask now: what would v have to be to make k = 0? Now, is there any _other_ value of k that would yield the answer v = c? (There are certainly other impossible values of k. k can't be negative, or >1, or an imaginary or complex number. If you plugged _those_ values into the equation and solved for v, what answers would you get? Not c. v = c is associated with k = 0 and only 0.)

[@Krokodil986]

I also asked myself this question, and, hopefully, i have a similar answer to what Einstein would have given. I phrased the question like this: If I approach the speed of light relative to the centre of mass of the Milky Way, I see the Milky Way's length along my direction of travel as approaching 0. So I should cross the Milky Way in a length of my proper time that approaches 0. But an external observer (who is at rest with the Milky Way's centre of mass) will see my velocity as approaching c, and hence my length will approach 0, while the galaxy's stays "normal". So the external observer would see me cross the galaxy in a long amount of time, nowhere near 0.000...01 seconds. So whats the solution to this paradox? After 0.00..01 seconds have i, or have i not, crossed the galaxy? Who's right - me or the external observer? The answer i could come up with is this- There is no shared definition of "right now" between me, and the external observer "B". So while my journey takes 0.00...1s in my time, it takes some years in B's time. B and I, however, are using different standards to measure the distance between two events - me setting off, and me crossing the galaxy. That's why we don't agree on the time between those events. As you can see, until now, the situation is perfectly symmetrical and both of us are equally "right". So we just don't agree on the time between two events. But thats ok, it happens all the time in special relativity. Also B would agree on the length of *my* time it would take me to cross the galaxy because he sees my clock as very slow, approaching not ticking at all. So he would agree that it takes me less than a second of *my* time for me to cross the galaxy. I think it's beautiful how this fits so nicely - i measure so little time due to velocity which manifests as length contraction, while B measures the same length of *my* time, again, due to velocity, but this time it manifests as time dilation. And the result is exactly the same in both measurements. Maybe this shows the deep similarities between space and time, and why one can't be without the other, why we speak about spacetime together, not separately. But what if i turned around and came back to B, and we compared our ages? This is called the twin paradox. To come back, I'd have to accelerate. Time, space and simultaneity are relative but acceleration is absolute. So the moment i accelerate towards B, i break the symmetry. Now both of us have traveled thru spacetime, between two events - me leaving and me coming back. However my path was more thru my space and less thru my time, B's was more thru B's time and not at all thru B's space. So B has aged more than me when we meet again. And this comparison is only valid if and when we do meet up, otherwise you can't ask both of us how old we are right "now" because there is no meaningful way to define a shared right "now" for both of us.

[@Krokodil986]

​​@@bobpower9189 you are correct - the middle clock, C, will see both A (who is at rest with the Milky Way) and B (who is crossing the Milky Way) as aging both slower than C and at the same rate as each other, as they are moving away from C at equal but opposite velocities. As you pointed out near the bottom of your comment, this situation is 100% symmetric. *If A and B are in relative motion, A sees B slow and B sees A slow.* That is, until acceleration happens, or more specifically, until one of these observer assumes a distinct inertial frame, and abandons his initial inertial frame. Everyone would agree on who changed their inertial frame and he would be found to have aged less when all three observers meet up again. This is consistent with the time animation you provided a timestamp to. Try drawing out a spacetime diagram for this accelerating observer. Initially, in the first inertial state, the light bounces off between two lines parallel to the proper time. But when the proper time changes direction, the light suddenly starts bouncing off much less frequently than before. So when your three observers meet up again, we can assert that B accelerated the least (didn't accelerate at all), C accelerated a little bit (less than A) and A, who travelled at nearly c, accelerated the most. Hence A ages the least, C a bit more and B the most.

[@Krokodil986]

@@bobpower9189 amazing question So rate of acceleration is how much you are accelerating per second, "jerk". The lower the jerk, the slower the change to the new inertial frame, but also the more time spent in acceleration. So I think it cancels out (to be honest I'm not too sure about this) Deceleration is the same as acceleration, but viewed from a different perspective. Imagine me walking along the street. I can start running. I think I'll be accelerating, right? Well someone who is already running will think I'm decelerating, because I'll be decreasing the relative velocity between me and that person. But for me I'm still accelerating - so the effects of acceleration and deceleration must be the same, time dilates. Put in other words, the direction of acceleration does not matter because the universe is fair - it treats all directions equally. In a spacetime diagram, deceleration may be shown as acceleration to the left instead of to the right. Directions are equal, the universe is fair so it doesn't matter. The only thing that may change is simultaneity lines may bunch up or spread out depending if you're accelerating toward or away from an observer onto his world line But the only meaning of this spreading or bunching is it changes from what point in time causal action propagating from this observer can reach you in your own "right now". I didn't explain this last point very well, if you don't get it I'll try to explain it better, let me know

[@Krokodil986]

@@bobpower9189 I've done some digging around... also let's define two velocities u and f let's say u -> c And f -> -c If you go from v = u to v = f, even if your change in velocity is instantaneous, ie delta t = 0, the age disparity will still approach infinity. In fact the quicker you get the acceleration part over, the quicker your two two inertial frames will reach their maximum difference. The quicker this happens, the longer they will be maximally different from each other. And the age disparity comes from this difference. So the longer the difference exists for, the larger the age disparity. So rate of acceleration does play a small part, but what's more important is the difference between the two inertial frames - ie the difference in initial and final velocity, rather than the time spend in transition.

[@henno3889]

"After 0.00..01 seconds have i, or have i not, crossed the galaxy?" You are mixing up two different time scales here, so you are comparing apples and oranges. You cannot do maths (subtract, add) between two entities that use different units, it's like adding inches to centimeters, that doesn't make sense. So, you would travel through the galaxy within one second of YOUR time scale, but it would take MANY seconds on the (different) time scale of the external observer. Ever heard of time dilution?

[@Krokodil986]

@henno3889 I can't tell if you read all of my comment and are staying it's completely wrong, or you read the top part only

[@photelegy]

I always just thought about it like: If you're nearly at the speed of light (c) others would see your time go very, very slow. But for you in this reference frame your time goes "normal" but everything around you seems to be very, very fast. So if you would be at the speed of light for the others your time stands still (∞ slow) but for you everything around you would be ∞ fast. So for you the whole age of the universe would be over in 0 seconds. So you can't experience anything outside of your reference frame. And that's the problem that arises like you described the problem in another way.

[@wailer27]

That's not how it works as you're skipping relativity. You see the universe moving slowly too as it is also moving relative to you, therefore your clocks would agree. Only when you accelerate or spend time in a gravitational field will your clock slow down relative to everything else.

[@grayaj23]

I think some of the confusion comes in trying to understand length contraction in this context. Traveling very close to the speed of light, you experience proper time, yes. But the distance between the start and end points would be much shorter and take less time than it would for an observer watching you or for a traveler going at non-relativistic speeds. That makes it sound like you'd experience "no time".

[@Pranav_pundir]

Hypothetically, if I make a phone call to my friend who is near the sun, than he would recieve it after more than 8 mins 16 secs and we will have conversation at each interval of more than 8 mins 16 secs?

[@PocketManaster]

Duh

[@Farming-Technology]

I'd check with your mobile provider first to see if interplanetary calling is included in your package. Could be expensive 🤠

[@ashutoshsethi6150]

He is toasted.

[@Fluxikator]

The Interval would be double that. You Record your message and send it away. After 8min 16 sec your friend will recive it. He Records an answer and sends its back. After another 8min and 16 sec the answer has reaced you. So for you the time it takes from sening your message and getting an answer is 16min and 32 seconds. + the time he has used to reply to your message. Thus the interval is at 16mins and 32 seconds at a minimum.

[@Pranav_pundir]

@@Fluxikator It means if I say "Hello", I would hear his reply "Hi" after around 17 mins?

[@stevehorne5536]

But if the question is "Does light appear to experience time?", that refers to appearances from an observers POV. The speed of light in a vacuum is then c as expected, but from the observers perspective (that's any observer in any inertial frame, precisely because of that second postulate), the photons clock has stopped. It's just the observers POVs that has the photons clock stopped, not the photons POV. Any well-defined frame is a valid frame, it's just that many frames are not inertial, so presumably even though the frame of the photon is at constant velocity relative to observers frames and has no acceleration, it's not inertial - ie we can claim that the photon appears not to experience time according to an observer (if we can actually decide what appearing not to experience time means for something that's moving at the speed of light regardless) but we cannot say that the photon itself doesn't experience time, as that's a claim about what's happening in the photons own rest frame, which isn't an inertial frame. We cannot convert from any inertial frame to the photons rest frame because the Lorentz transformation equations blow up, generating infinities (for mass dilation at least), again suggesting that the equations aren't valid for that conversion, therefore again suggesting that the photons rest frame is not inertial (or else the conversion should be valid). The behaviour of a photon can only be understood relative to frames that are inertial, which excludes the rest frames of any massless objects, thus we can only understand the behaviour of a photon as seen by an observer that has a non-zero rest mass - not as seen by a massless observer traveling at light speed such as another photon, and in particular not as seen by the photon itself. Though what is the relevant behaviour of a photon anyway? OK, it never decays from observers POV, but is there any reason to believe it should if it were experiencing time? And any interactions AFAIK are periods where the photon ceases to exist (energy absorbed then re-emitted by something else) so the whole logic about constant velocity c no longer applies anyway. And we're certainly not expecting one photon (A) to overtake another photon (B) from behind, so it's good for causality that we can't argue that photon A is traveling at velocity c in photon Bs rest frame. At least I think that makes some kind of sense.

[@IterativeTheoryRocks]

I studied this subject some decades ago. My interpretation of this question was somewhat different. You are focusing on time for the photon and the photons rest frame. You don’t have to do that. Instead think of travelling ‘almost at the speed of light’ and what you will see is that your own time passes normally, but the entire rest of the universe is Lorentz contracted in distance. In other words, you will (due to length contraction) arrive almost immediately after you leave - as the distance to travel is minuscule - even light years would (if you go fast enough) shrink to millimetres. The reason light does not experience time is nothing to do with rest frames, but rather that at the speed of light it arrives at the same instance it leaves / because the ‘length’ of its travel has shrunk to zero. This is in a perfect Vacuum. If the vacuum is not perfect, then the light will experience some minuscule time.

[@damc7456]

Excellent... Also, I think the video would better serve viewers by comparing clocks. If a photon were to look at its wristwatch, it would observe the hands moving in proper time. Sure, but an observer watching the photon go by would observe that the photon's wristwatch has hands that aren't moving at all. As you point out, this is explained by the photon observing that all the points through which it travels at the speed of light have contracted into one infinitesimal. I'm super curious how this topic relates to entanglement. Could it be that entangled pairs, despite potentially being separated by light years, "think" that they continue to occupy the same point because per their observation, zero time/space has passed between before they were split and when they become disentangled?

[@Jim-uq1mc]

In a reference frame the observer is at rest; not the photon is at rest. The photon needs to travel at the speed c with respect to any observer - even if the observer would himself travel at c with respect to some other observer . . .

[@Littleprinceleon]

​@@Jim-uq1mc two photons moving at the same direction: do they travel at c from each others' perspective?

[@damc7456]

@@Jim-uq1mc In my comment I make the photon the observer. "If a photon were to look at its wristwatch..."

[@kylelochlann5053]

Time cannot be applied to a null curve, that is, for a photon. Light in a medium will travel along a world-line (time-like curve) so there's no difference between us and light as far as time goes.

[@bakshiavijit]

I really liked how you presented your understanding step by step emerging from your confessed confusion. Also loved how you made it a conversation between Mahesh, Einstein and Feynman chipped in, that made it both interesting and funny. A honest presentation full of excitement. Liked and subscribed!

[@stephanevernede8107]

Light is a null vector of space time which mean that is proper time is always 0 . This is can be seen from the equation of proper time `dtau^2 = dt^2 - 1/c^2 dx^2' which is always 0 for a photon whatever is the reference frame. So saying that light as zero proper time is perfectly legit, and this in all reference frames.

[@kylelochlann5053]

Well... not exactly. The equation you should have is general flat-space metric: ds^2=-dt^2+dx^2. It is only in the special case of a time-like curve that ds^2=-dτ^2. The proper time along a null curve is not 0, it is undefined.

[@stephanevernede8107]

@@kylelochlann5053 Thanks for your comment. If I understand well your point, you agree that light is a null vector of space time and has 0 norm, but you point that this norm can not be called `proper time` but should only stick to `norm`. How is this more that a play on word ?

[@kylelochlann5053]

​@@stephanevernede8107 Given a spacetime curve, S, in arbitrary spacetime coordinates with tangent vector, U, the "norm" is then the inner product on the tangent space, g(U,U). This would have nothing to do per se with how the curve is parameterized. For a time-like curve, the norm is a constant, g(U,U)=1, we can use a clock to measure off the distance along any time-like curve (if measurement shows that all identical clocks tick at the same rate, everywhere in the universe, and under all circumstance of motion and orientation). A null curve has no spacetime length, so in what sense can a clock be used to define time as a parameter to measure length along a length-less curve?

[@zemm9003]

​@@stephanevernede8107 you are correct in your original argument. He is just writing word salads.

[@zemm9003]

​@@kylelochlann5053proper time is by definition the length of the spacetime interval measured in the rest frame. However due to Lorentz invariance this number is always the same regardless of the reference frame. So we can just choose a different one to perform the calculations. We can choose any so it doesn't matter. The photon will always travel with spacial velocity c which means that Δs = 0 for all reference frames and hence Δτ = 0.

[@AnnaLangdon-d5u]

So glad I saw this. In a previous video you explained time dilation and space contraction, where as an object approaches the speed of light the numerator of that equation (time dilation) approaches infinite while the denominator (space contraction) approaches 0. The answer you gave in this video says that it would appear from “lights perspective” that it would travel at infinite/0. But that would also break math because you can’t divide by 0. So that equation doesn’t make sense. Light doesn’t have a reference frame fits perfectly and is understandable!

[@Jupiter1423]

And you might say, mahesh - i always wondered if we could have a beer together. But einstein says yes, of course you can have a beer with mahesh.

[@Andre_Alvim]

Something is not 100% clear for me in the example of the moving cart. When the photon is "fired" upwards and the cart starts moving, shouldn't the photon collide with the side of the rectangle in which it is successively reflecting, that is, the wall perpendicular to the horizontal mirrors (above and below)? We can say this because the horizontal speed of photon does not depend on the speed of the cart itself (since it's a wave and its speed depends only on the medium), so at the moment the photon is fired, if the firing angle is 90º in relation to the horizontal, it will only have vertical speed (horizontal speed equal to zero). In this case, can we "fix" this problem just by "tilting" the device that will fire the photon, at an angle such that the horizontal speed of the photon will be equal to that of the cart (and the photon "stays" reflecting inside the clock)? In this way, the total speed of the photon will be "c" and the vertical component will be smaller than in the case where the cart is stationary, since part of this speed was "used" to maintain the same horizontal speed of the cart? Or am I wrong in thinking that when firing the photon at 90º it will NOT have horizontal speed (and, in fact, it WILL have horizontal speed)? But if I'm wrong, then how does the photon know that at the moment it was fired it had horizontal speed (of the cart) and, therefore, it should go partly up and partly forward? Its like the photon always know what to do based on who is observing it... If the observer is outside the cart, then the photon "knows" that it should go a little forward and a little upward, but if the observer is inside the cart, the photon knows that it only has to go upwards. One way to better understand my reasoning is to think about how the equipment that fires the photon should be positioned: "at 90º" or "tilted forward"? If the answer is "at 90º", then the observer outside the cart will be confused, since the photon's trajectory will not be parallel to the "photon exit pipe". And if the answer is "tilted forward", then the observer inside the cart will be the one confused, since the photon's trajectory will not be parallel to the "photon exit pipe". Whenever I think I understand relativity, I later realize that I don't understand absolutely nothing.

[@Nuovoswiss]

That asterisk "ignoring length contraction" is doing a lot of work here, since in "light's reference frame" the universe is 2-dimensional, and its velocity is zero in what we would call its direction of travel (because for a photon, that direction doesn't exist). But real photons don't travel in only one direction, they propagate with a probability cone. Might be some insightful math to be done there...

[@lewis7515]

How have you concluded with such certainty that Light's universe is 2-Dimensional?..... Light's velocity is zero in its, "direction of travel" - purely from our, practically-useful, perspective, of Light's perspective. However, surely, Light only has a, "direction of travel", at all, from our perspective, of Light's perspective? That is: isn't it the case that Light's, "travel", is only perceived? Light doesn't travel, it cannot - we call how it presents in our Dimensions, "travelling", because it's intuitive and helpful. Meanwhile, isn't it actually the case that Light only simply _propagates_ - from the point that it manifests? If that is the case, then Light must propagate, at the speed of Light. If that is so, then Light simply propagates, from wherever it manifests, in every Dimension, in every direction, equally - which is: at the speed of Light. Wouldn't that then mean that every direction and every Dimension: is zero? Isn't the conclusion, then, that while there is no, "direction of travel", from Lights perspective - the exact same terms apply to all other, "directions": that it could only move to, at the speed of Light....So, wouldn't there, equally, be, no, "Direction" - in any non-direction it could move to?.. Must it not then be the case that there are no, "Directions", whatsoever - and, by that token, no, "Dimensions", in any non-direction, for Light to not, "travel", to? Isn't it the ultimate conclusion that, for Light, there is no, "Direction", at all, that it could travel to at anything less than the speed of Light - meaning there is no, "Dimension", whatsoever, to not, "traverse": with the result that there is no, "Space"; no, "Time"; no, "Thing"; no, "Where" - and there was not, is not, and never could be... Would it not be that, for Light, _all_ is simply null - and moot?

[@GIRGHGH]

I thought this was gonna talk about how light oscillates and if something changes throughout it's existence it must have time or something.

[@jonathanhinkel8813]

To be honest, I hadn't wondered until clicking on this video.

[@aeomaster32]

I love that you embrace the questions I puzzle about and answer them in conceptual ways that one can visualize. The definition of time is - the measurement of motion. My understanding now, is that we measure time by comparing the speed of an event to the speed of light, in the frame of reference of the event.

[@amaze2708]

Dumb question perhaps.. if an object is traveling close to C does it appear colder to a stationary observer? My thinking is if time slows for that object including all particle momentum, average kinetic would appear lower. And I suppose all stationary objects would appear colder to the moving object too?

[@Mahesh_Shenoy]

I think “colder” is a very lose word here. If you are specifically thinking about the temperature, then you will have to measure it in its rest frame, no? (I mean you need to stick a thermometer in there somewhere, and now the thermometer is in the rest frame of that object). But, the average thermal energy should slow down. So, that’s an interesting question. Let me add it to my list of topics to research more. Thanks for the question. It’s anything BUT DUMB.

[@a64738]

That is actually a very interesting question...

[@amaze2708]

@@Mahesh_Shenoy thank you for the reply. To expand on this.. remote observation of temperature is possible (we measure temperature of everything in space remotely). Also colder == more red shifted.. what if part of the red shift we observe from distant objects is because they are at relativistic speeds from our reference frame due to expansion? Obviously Doppler shift is still relevant. If we see their “clock” moving slower, we should also detect the temperature as lower; all time based events are affected, and temperature is time based. I just solved dark energy (Kidding).

[@FocusingOnStudy]

​@@amaze2708 Well , Temperature is time based as it measures total kinetic energy of molecule and kinetic energy is dependent upon velocity which depends upon time and as time is relative , temperature is also relative . I am not a physicist but I think a way to measure temperature can be thermal radiation, as thermal radiation is electomagnetic radiation it's speed is not relative but it's total power should as no of photon emitted in some time is different for both of them

[@mpmpm]

@13:45: "...in vacuum": The plot thickens: there exists no vacuum. Therefore, light always travels with a speed lower than c.

[@kitmoore9969]

Light always travels at the speed of light. This speed depends on the medium.

[@armaan7381]

Oh god I recognize your voice from khan academy

[@rumpaghosh7929]

Yeah you're right

[@ManoFury7]

Sorry I am posting this question 10 months later. Isn't the concept same for all the particles/objects in a specific frame relative to space? why limit this to only light? i think every object moves at the same speed within the frame regardless of how fast the frame itself travels in space. For. eg. if i can throw a ball at 100kmph, then the ball would travel at 100kmph until slowed down by surrounding resistance like gravity/air etc. This would not change based on whether earth is travelling closer to speed of light (c) or a speed that is a fraction of 'c'. so nothing within a frame never experiences time dilation.. but why limit this to only light here in this video? Sorry if i am totally way off the point explained here

[@AdritoMitra]

Sir another question why the speed of light is less in water if the speed of light is always same for all observer? Love your videos

[@AdritoMitra]

And also if the speed of light is less than the speed of light then Photons will have mass. As you said in that video.

[@Mahesh_Shenoy]

I think I should cover this in a separate video. Adding it to the list. Short answer is, it doens’t make sense to think of “speed of photons” inside a medium

[@AdritoMitra]

​@@Mahesh_ShenoySir it will be very helpful for me because I can't find the answer. Love you and your videos from West Bengal ❤❤❤

[@Farming-Technology]

​@@Mahesh_ShenoyI would look forward to that video. Some points of interest for me would be, is causality slowed in a medium? When you say vacuum do you just mean free from baryonic matter? Also how does the light accelerate¿ when entering vacuum from a medium? I don't know enough to ask the questions correctly but it is very interesting to me and the way you explain mathematics to us laypeople is some of the best I've seen.

[@wynq]

@@Mahesh_Shenoy I also look forward to this video. Can you also include in that video an explanation of what's going on in the experiment where Hau and Harris "stopped" light in a cloud of ultra-cold sodium atoms? I'm having trouble understanding how the photon wouldn't see its own velocity as 0 instead of c.

[@Yaaalala]

I needed this channel for so long and didn’t even know 😱 Finally instead of just knowing the right answer I can understand what is going on. TY so much, great work! 👍🏻👍🏻👍🏻

[@Simon-fg8iz]

There is another counter-attack: how much time does an observer experience between photon emission and photon absorption, if he is tracing the same path as the light, travelling with a speed limiting to the speed of light? In that sense, you do get that the events (emission and absorption) happen basically immediately one after the other, leading to a loose statement that a photon doesn't experience any proper time. In "photon's frame", it is born and dies at the same time - by definition, because the space-time interval is 0 on a light-cone.

[@Mahesh_Shenoy]

That’s because, from that frame, the emission and absorption locations were infinitesimally close to each other. If my neighbors house is infinitesimally close to my own, the moment I step out of my house, I would have stepped into the neighbor’s almost instantly. That doesn’t mean I don’t experience time, right?

[@Simon-fg8iz]

@@Mahesh_Shenoy If you only exist while traveling from your house to your neighbour's house, and the threshold is infinitesimally narrow, you could say that you only exist for a single moment, not any finite interval of proper time during which you could experience anything. I'm thinking of neutrinos here, who do have "time" to oscillate during their flight. Again, of course, Einstein's argument holds that the limit c→∞ isn't strictly reasonable. p.s. Just keeping the back-and-forth discussion with Einstein here, the conclusion in the video is of course correct :)

[@ramankhatri]

Photon experienced time but unfortunately nothing happens in that time as the space outside is not changing. It's as if it went out of the universe and popped right back in just before annihilation.

[@lewis7515]

Isn't that just sophistry? You've just used different words to suggest the same thing as an imagined frame of reference that is travelling in equality to light.. The logical conclusion of SR is that there is no, "basically immediately, one after the other", by Light's terms - that is is fudge the conclusion for the convenience of human interpretation. The natural consequence must be that Light does _not_ perceived Time - because by Light's terms, there is no such thing as, "Time": and there is no such thing as, "Space". Those attributes only mean anything, to us and other entities of those dimensions... For Light, there is absolutely nothing. That is: Time and Space are emergent properties - dimensions in a Universe that Light, by nature, has no access to: because they and that Universe are, by nature, coiled up to zero and contracted out of existence. Every single thing, is beyond Lights event horizon - where Light, is it's own event horizon. That's why the question makes no sense - but to say that the question makes no sense isn't necessarily a mature or complete answer to be shared among adults. The simple answer is that no, Light doesn't experience Time. The expanded answer is that Light cannot experience Time l, even if it wanted to - because neither Time nor anything else actually exist on Light's side of Light's own event horizon. That is to say: if Light could communicate, it couldn't process a question on whether it experiences Time? It wouldn't - couldn't - even have any single idea, whatsoever, of what you were even talking about: "Time", and, "Space"?..... Utterly meaningless.

[@leonardopizzini1443]

@@lewis7515 i don't realy get this. Why shouldnt light experience time if we see time as the change happening to matter in space . it should in my opinion. Giving light a perspectiv as a human doesnt make sense to me but looking at it from the outside its clear that change is happenig while it moves so how could it be different from the inside perspektiv of light? is it just a hypothetikal question that doesent realy apply to the real world?

[@AbhiramGSrivathsa]

I have a doubt (not on this topic) : WHY DOES RESISTANCE DECREASE WITH INCREASE IN AREA? it may be a silly question, but how? when we increase area, the atoms are also more right? then resistance must remain same

[@otaku-chan4888]

my intuitive (disclaimer: I'm not a physicist or electrician) answer to this is: most of matter _is free space._ With an increase in area (and hence increase in the region where matter exists to provide resistance) there are more atoms but a LOT more free space as well. You'll understand it easier this way: imagine there's a tube with the free area of an atom inside. If there's atoms inside the tube's hole, the hole is blocked- except for the bit of space between the nucleus and the electron orbits. (assume) 40% of the 'stuff' inside can provide resistance. Now imagine the space increases to have a free area of two atoms inside the tube. Sure, there's one more atom- but twice the amount of free space. 80% out of 200% provides resistance, but a whopping 120% of 200% doesn't. Percentage wise nothing much's changed, but there's a lot more room for the same probability of resistance to _not_ play out.

[@thedeemon]

When light goes from A to B, we can compute the spacetime interval and corresponding "proper time" - for a photon it will be zero and it will be a valid computation. This is enough to say that time doesn't tick for a photon, and thus it "doesn't experience time". And it's an idea with practical measurable consequences: particles moving at light speed must not change along the way, they must be "frozen in time". That's why we now think neutrinos must move a bit slower than light, as they do change during their travel.

[@kriiistofel]

Photons also change, their wavelength gets longer as they travel through spacetime

[@thedeemon]

@@kriiistofel Not in special relativity ;) In GR yes, but there the topic gets more nuanced.

[@kylelochlann5053]

No, the proper time for a photon is not zero - it's undefined.

[@kylelochlann5053]

@@kriiistofel No, it is impossible for a photon to change, or have any intrinsic frequency/wavelength.

[@kylelochlann5053]

@@thedeemon No, there is no distinction between SR and GR (SR is simply describes the ground state gravitational field).

[@flatisland]

I think a famous physicist - was it Brian Greene? - said a photon experiences the whole life of the universe in an instant. which pretty much sounds like light does *not* experience time. If you travel near the speed of light you can reach any star within your life-time. Thus, would it be wrong to extrapolate that to travelling at the speed of light means going anywhere in an instant?

[@flatisland]

I get the point though. literally speaking :D that's why SR (and GR) are *probably* MATHEMATICALLY wrong when it comes to singularities both time-like and space-time-like ones. Because it uses conventional maths, i.e. the real numbers - which in reality probably don't exist. Growing number of people are realizing this - beginning with Professor Wildberger from Australia.

[@donnyfanizzi5360]

Thanks again great videos as usual. Love the energy!

[@Mahesh_Shenoy]

Thanks, Donny :)

[@skoobastories]

hi, should not the light appears to bend from the moving perspective just like what you said in case of the gravity?

[@Rationalific]

Could you say that the faster you are moving, that the objects approaching you from the front seem to have their time speeding up (like a Doppler effect with a sound pitch getting higher)? And could you say that as you get arbitrarily closer to the speed of light, the passing of time of objects in front of you gets arbitrarily closer to infinitely fast? Or am I off base here?

[@renedekker9806]

_"Could you say that the faster you are moving, that the objects approaching you from the front seem to have their time speeding up"_ - if you are asking about the light that you see from the objects that approach you, then that is fully correct, yes. That is mostly due to the Doppler shift of the light. If you asking about what you would conclude about the clocks of the others, then your conclusion would be that they tick slower than yours. That is due to the relativistic time dilation.

[@Rationalific]

@@renedekker9806 I see... Thanks for the reply!

[@FocusingOnStudy]

​@@renedekker9806I think you are saying opposite , if we are moving near speed of light our clock tick slower than theirs and other object appear travelling faster as they are moving fast in time relative to us

[@renedekker9806]

@@FocusingOnStudy _"if we are moving near speed of light our clock tick slower"_ - our clock never ticks slower or faster. It always ticks at the same rate. It's that rate that is called proper time. It is always our perception of OTHER clocks that appear to tick slower (or faster in some circumstances). _"other object appear travelling faster"_ - other objects always appear to travel at the speed that they travel relative to us.

[@FocusingOnStudy]

@@renedekker9806 If time is relative how can we say that clock ticks at same rate ,proper time should not exist even if proper time exist , then it is different for everybody If we are moving with speed near speed of light then other thing at rest only does not appear moving faster in time they are actually faster, and in this situation clocks at rest will always tick faster than our clock (if both clock have same mechanism)

[@sgiri2012]

Mahesh sir please cover quantum mechanics stuff also......

[@Mahesh_Shenoy]

Yes, yes! On my ever growing backlog of videos.

[@tomasvanderlaan2375]

I think you are confused by what a frame of reference is in physics. A frame of reference is a set of coordinates to describe a velocities of objects in that frame. A key feature of an INERTIAL frame of reference (the one we use in physics, which you are describing) is the frame is not accelerating, which does not mean that the speed is zero (which is also a “loose” term as describing velocity in respect to something, like the ground or earth)

[@ToyyinnAuslander]

Exactly. At rest also extends to straight line, non-accelerated motion which is exactly how photons travel through the vacuum of Space. I think this explanation is inconsistent with the very foundational basis on which it claims to sit. In another video, he explains why everything at rest in the universe is moving through Time at the speed of light. Using that thought experiment, he goes on to agree that the faster one travels, the greater their displacement through Space and the less their displacement through Time, till, eventually, the particle in the thought experiment reaches c and employs all its speed in the spatial dimension with nothing left over for Time hence, objects travelling at c experience no time. This video appears to contradict that. 🤷🏾‍♂️

[@undercoveragent9889]

@@ToyyinnAuslander Yes, I agree. To be honest, I always had a problem with the proverbial 'light-clock' being used in these thought experiments. I never understood how it was possible for the path-length between the reflector and detector to be altered by the velocity of the clock as it travels through space. These experiments seem to assume that the motion of the clock provides some forward momentum to the photon being emitted and detected in the clock but doesn't that contradict the postulate that all observers experience the same 'c'? I mean, if the clock is traveling along the x-axis and the photon is emitted along the y-axis then the photon would travel along a path that is perpendicular to the direction along which the clock is traveling, right? The photon would have traveled in a straight line but the location of the detector would have changed during the time of the photon's journey. Also, one would assume that each photon can be detected only once but in the model used here, the photon is depicted as a wave travelling from the emitter to the reflector. The fact is, according to the postulate, when the clock is at rest, the experiment is set up so that the photon travels the path between the emitter, the reflector and the detector. When the clock is in motion however, we can see that 'at rest' is a 'special case' since from the photon's point of view, the positions of the clock-parts are arbitrary. As soon as we put the clock in motion, the photon is no longer traveling the path between emitter/reflector/detector; it travels from point 'a' on the y-axis to point 'b' on the y-axis and arrives back at point 'a' on the y-axis. Right? The photon _never_ deviates from the y-axis but the emitter of the photon does. It's not the photon traveling along a path that is 45° to the direction of travel, it's the emitter that has moved to a new position on the x-axis that is at a 45° angle with respect to the photon's position along the y-axis; the photon is 45° _behind_ the emitter. The length of the photon's journey however remains constant. In other words, if you had a photon emitter firing photons between two parallel plates, the top one is uniformly reflective and will reflect any photon back the way it came; the bottom one will detect and register any photon that interacts with any point on its surface and we can have the plates spaced at some arbitrarily small distance so that even at light-speed, no photons will be lost to the system. There is a tiny hole at the centre of the detector where the emitter injects photons between the plates. With a clock like this, no matter how fast you travel, the clock will tick at a rate that is proportional to the distance between the two plates and the speed of light, both of which remain constant in all frames. And even assuming that 'length contraction' is an actual thing, that would not change the rate at which the clock ticks either. One might argue that the length of the clock becomes so contracted that its displacement along the x-axis takes the reflector out of the space occupied by the photon which is never detected. In that case, the clock simply stops but until then, the rate at which it ticked remained constant. So yeah, these thought experiments make it appear that the velocity of the clock alters the path of the photon somehow but how, no-one ever explain that to me.

[@jonathanlister5644]

Excellent argument, your logic is very sound - it brings to mind constructor theory. Also love the chess analogy reminds me of Feynman's use of a chess analogy. Thanks very thought provoking.

[@Mahesh_Shenoy]

Thanks, Jonathan. :)

[@magnuszakrisson]

But you can have two kings next to each other in chess. That would have required an illegal move though but it can still happen. There has even been a real chess game where the judge ruled that the one who played a move after an illegal move lost, because it was illegal to to continue after this illegal position and create a new illegal position. When the player who made the first illegal move pointed that out the judge concluded that the last player who made an illegal move lost! ;) Not sure what conclusion we can draw about lights perspective from this though lol

[@CyrilleParis]

Your videos are always so clear and your explanations so well put! Before watching your videos, I knew (not always) ; after watching them I understand. I'm amazed by and love your channel.

[@taggartaa]

Can you constantly accelerate to closer to the speed of light such that, from an outside perspective, the light never reaches the top of the clock and ticks? I suspect from inside the vehicle, you would just see space continually contract such that you would arrive at any destination point before light had a chance to tick the clock.

[@rodschmidt8952]

I like this question. I think you are right

[@_abdul]

Now that the "experience" part is ingeniously taken care of, Shall we move to the next innocent but tricky term i.e, "Time"?. What is it? is there any intuitive sense of this term? Is it universal or it's just an emergent phenomenon as a statistical byproduct of entropy? I thought I almost got it from the Arvin Ash's video on the topic but would really love to have it reimagined with your enthusiasm and care. Great work brother, I absolutely admire your explanations. ❤

[@PHIplaytesting]

This is a semantic issue. Someone could choose to describe the idea that _light has no reference frame_ as "light does not experience time." In that case, they would not be incorrect. In my opinion that's a perfectly valid way to describe what you've outlined in this video. Your argument seems to consider the idea of _time standing still_ as the meaning of "light does not experience time." If that's the meaning we take then your argument is valid. But I would argue (semantically) that the fact that light neither sees time passing or time stopped means that light doesn't "see" time at all, which could also be stated as "light does not experience time."

[@fuseteam]

This has the same energy as quantum superposition: not a, not b, not a and b, not a nor b Not time passing, not time not passing. Just no time 🤔

[@georgedabre5711]

I think you've misunderstood the second postulate. Speed of light in vacuum is a constant in all inertial frames, which is to say that observers from all inertial frames will always experience light as travelling at a constant speed. You cannot apply this when light is the observer, because an object cannot observe itself. The second postulate is about observing light, from frames of reference other than light's own inertial frame of reference, which may or may not exist.

[@ThomasGutierrez]

Love the channel. Thought provoking as always, great content and delivery. I would note that events along light cones in SR are defined as having zero spacetime separation, implying zero proper time elapsed for particles moving along those worldlines. There is nothing pathological about that. This does suggest no time elapses for observers moving along those worldlines. It also suggests that space is infinitely contracted along the direction of motion for those observers and time is infinitely dilated. Your observation that the light cone frame isn't inertial is a good one, because it seems to be excluded since there apparently is no rest frame -- but I'm not sure it is really correct. All reference frames are the same in that frame, including the rest frame. Weirdly, light still moves at the speed of light in the light's reference frame. Light cone coordinates are a common tool used in certain branches of physics and are just as valid as ordinary spacetime ones for certain calculations.

[@kriiistofel]

The more I learn about special relativity, the more I think that our movement is just an illusion, rather we all stay in place and only light is truly moving.

[@nHans]

Lemme guess-still working from home? 😁😂🤣

[@BenGreen1980]

Does "time" just mean "perpendicular to displacement vector in 4-D space"? From time's perspective, since it's displacement vector is fully in what we call spatial dimensions, from its perspective, time is one of those spatial dimensions? Is light light in its own reference frame? What if what we perceive as light is just everything that's moving perpendicular to the direction we call time? As something approaches the speed of light from our reference frame, it will compress along its direction of movement. If it was moving fully perpendicular to our direction of time, it would be fully compressed and maybe appear to us as a wave?

[@jadioj]

The beauty of this format is that we leave with two amazing takeaways: a better understanding of physics and for me is a new way of thinking about hard subjects like physics.

[@kirjuschaks]

How's the music called in the animation?

[@richardoldfield6714]

My understanding is that everything is moving at the velocity of light in vacuum, with this velocity being split between motion through time and motion through space. For photons of light, time does *not* pass because - being without mass - all of their velocity is through space, leaving them with zero velocity through through time. Thus for a photon, the journey from its emission to its absorption is instantaneous from its own perspective, regardless of the distance travelled in space. However, the notion of "time experienced by a photon"is not meaningful because time is tied to the reference frame of an observer .... and photons do not have a rest frame, as it is impossible for them to be stationary.

[@Peoplearedumb13]

The only thing incorrect is everthing isn’t moving at C. Or relationship to the limit isn’t a speed all things go.

[@richardoldfield6714]

@@Peoplearedumb13 I said that everything is moving at the *velocity* of light - speed and velocity are not the same thing in physics. Velocity has magnitude and direction, whilst speed has only magnitude. Thus I stand by my statement, because there is velocity through space *and* velocity through time.

[@Peoplearedumb13]

@ you are confused. We are not moving in a direction and magnitude of C.

[@richardoldfield6714]

@@Peoplearedumb13 Yes, we are. In the case of objects with mass, like the human body, most of that velocity is through time and a only a little through space, but the total still adds up to c (light velocity in a vacuum).

[@365dongle3]

Can you do a video based around time? What is Time? What is Space Time? We on earth use clocks in order to make some kind of routine for ourselves, which in turn are connected to the movements of earth around the sun, but is there another way of looking at or explaining that indescribable distance between one moment and another?

[@mrstevecox7]

Good video! The whole answer to this is that light neither experiences time nor distance. The concept of a 'physical wave' between emission and absorption of light is not real. The 'wave' is only a mathematical conception which informs us where and when the light energy packet is "transmitted" to. There is Nothing in between the emission and absorption points, either in time or in space..

[@a64738]

Good point and so many seems to not understand that part of the physical properties (or lack of that) in light. It explains a lot of what we think of as strange behavior like "spooky action at a distance" and the many strange things with quantum physics.

[@Vexas345]

Light takes up physical space (wavelength) based on the number of oscillations in its E-field per unit of time (frequency). It's energy (E=hv) is based on these physical properties. Photons have to physically change to be measured, so I don't know how those concepts would make sense if light didn't interact with time or space.

[@kylelochlann5053]

@@Vexas345 No, light itself does not energy/frequency as per E=m. Given some world-line, we can assign a photon an energy/frequency, which are frame dependent values.

[@Vexas345]

@@kylelochlann5053 how do you define a photon then? Our interactions with photons are based entirely by the transfer of energy. A photon with zero energy essentially doesn't exist, since it can't interact. Sure, its energy may change depending on the observer's frame, but that also applies to real objects. Real particles have relativistic momentum and so their measured energy is frame dependent.

[@kylelochlann5053]

​@@Vexas345 We define a photon as an excitation of the free electromagnetic field that couples to electric charge and associated with the quantum state of some system. Let's say we have a hydrogen atom in state A that transitions to A', that then is coupled to a detector (which also undergoes a state transition) at light-like separation. The "photon" is the exchange between the systems. Keep in mind that energy is a description of matter, specifically being the Noether charge of time-translation symmetry, and not something that exists. Or to put it more leisurely, energy is an arbitrary number assigned to a system that represents a constraint on the dynamics. The equation E=m is a statement about the internal dynamics of a system. We say an electron has an intrinsic energy because there's an internal interaction between the electron matter field and the Higgs field. Given an arbitrary time-like curve we can assign the electron a coordinate energy/momentum: E=γm and p=γmu, constructed in terms of the internal dynamics represented by m. We can do no such thing with a photon - there are no internal dynamics. The energy of a photon is zero (intrinsically), E=m=0. However, as the photon couples to the electric charge, and the electromagnetic interaction is constant over spacetime, we can associate the photon with conserved quantities with regards to the space and time components of some observer, E=p=ω.

[@bartomiejbadura499]

You are absolutely right - that kind of question uses lots of words with imprecise meaning. What I found talking about light not experiencing time is a person I talk to has in mind is what other reference frames look like from a reference frame approaching speed of light.

[@michaeljorgensen790]

You left out the the thought experiment most people want an answer to. A spaceship already accellerated close to C and moving past earth and then past the nearest star 4.3 light year away at close to C. Would the astronauts experience less time than 4.3 years? If so , then did the distance get length contracted for them? Isn't the entire universe smaller for something traveling at close to C.?

[@sylarkane8883]

Ok. Your videos are absolutely best at explaining basic physics. Do you have video about why speed of light is same in all reference frames?

[@78dentedhead]

Isn't an inertial reference frame simply one that is not accelerating/decelerating (which is equivalent to being "at rest")? So a photon traveling at a constant velocity (C) is an inertial reference frame.

[@sebastiankaifrost2939]

So, my question is, from the object traveling very close to the speed of lights reference frame. (Say a spaceship) If you look out the window you experience yourself at rest right? But the universe appears to be traveling past you very quickly. So all object you pass seem to have clocks running much faster than yours. So there would, theoretically come a speed at which the entire universe outside seems to age and die in a few ticks of your perfectly normal ticking light clock yes? So while you can could yourself at rest. An observer seeing you go past would see your clock going VERY slowly. But conversely you would see theirs going fast? Or would theirs also appear slow to you as from your reference frame you are at rest and THEY are traveling close to the speed of light. Unless your clock seems slower as you travel away from them but quicker if you travel towards them? Could you do a video on this scenario? It's the one part of the whole thing that doesn't clock for me. We talk about what one observer would see all the time. But what about the inverse view. Or the view of the universe from the ship traveling close to light speed. What would they experience?

[@chrisbragg7909]

I asked myself this when I was young. Like late teens early 20's. I would mention it to people and they would have no idea what I was talking about. My friends weren't physics geeks. lol. Thanks for sharing. Love this videos

[@HeriJoensen]

Great video, man. I’ve been wondering about this specific issue for a long time.

[@SteveLawrance]

This is an awesome video, very well explained, thank you…

[@noelwalterso2]

What confuses me (not difficult) is that whichever trolley you choose as the reference frame, the clock of the other one appears to run slow. They can't each run slower than the other, can they? To make things more confusing, you need some way for people on each trolley to observe the clock on the other. Perhaps use light pulses or something to signal the time at regular intervals. The further the two trolleys move apart the longer the signal will take to travel between them, but, assuming neither trolley accelerates, both trolleys should each receive the same number of signals from the other one. Imagine both trolleys now slow to a halt, and then head back towards each other, and then slow down again to meet up at zero relative velocity. Assuming people on both trolleys were keeping count of all the signals received from the other, then they must both have the same tally. So surely their clocks must agree? Yet from each frame of reference the other, moving frame of reference should have experienced less time. Is this what is meant by the twins paradox?

[@noelwalterso2]

Oh! I just figured it out. It must be typing all that out that did it. Obviously in order to compare clocks they have to be in the same frame of reference. So one or the other or both will need to change velocity in order to do that. The discrepancy only occurs while they are moving relative to each other.

[@sophiasalleythedawnofsadie3313]

Does Time experience Light? is the question i get asked the most. Right behind Do you know what time it is? And How are you doing? Thank the good Lord i have the answer now.

[@Orchestration1983]

Things that confuse me: 1. Normally you couldn't catch up with light but what happens if it slingshots around a high gravity object and comes back towards you. Have you not caught up with it? 2. Does light experience existence? It is emitted from a source, so from our point of view it does. And then it either continues into infinity or gets absorbed by something, ceasing to exist? Did it ever exist at all? I know the question is probably meaningless but it almost feels like light it self is a singularity that exists simultaneously everywhere at once and nowhere at all. It almost feels like it asks the question does time even exist? Are all events in the universe actually happening at the same time and how does that influence cause and effect?

[@fra_trk]

Nice content! The moving clock ticking slower points directly to the twins paradox which has one of the hardest wikipedia pages I've ever seen lol

[@billt3922]

Love watching floatHead Physics! Your enthusiasm is infectious and I will be showing my teen daughter your videos to help her intuition of physics. Thanks, Mahesh!

[@fishingpervert]

Forgive me, as this is from a non-college education. Obviously, "light" does not experience time. Requires a mind that thinks to experience anything. Thought experiment assumption: we can "click a switch" and be travelling at C. (takes the acceleration/deacceleration out of the equation) The true question is, does a person travelling at the speed of light experience time? A space craft travelling AT the speed of light would reach our nearest neighbor in 4 1/4 years, Earth time. On Earth, our clock would tick one second for each 186,000 miles travelled. I do not expect time to stop for those onboard the craft, but what would they experience? Maybe a few minutes? Perhaps an instant? Click on/click off and we're there? I also don't believe time can go backwards. You can't unburn the match. So, carrying on the thought experiment, the craft can go 4 times C. On Earth, a little more than one year would pass. The people on the craft would again be there in an instant. They could go to that star system, explore for a month or so, and return. For them, the only time spent would be that time at the system. For us, it would be 2 years and a month or so before they returned.

[@Gabriel-sn6yg]

I was under the impression that light did not experience time because it didn't had a reference frame...

[@ANunes06]

10:50 - That *asterix* caveat is pretty important though. If you *don't* ignore length contraction, by approaching the speed of light, an object doesn't experience time dilation from its reference frame, but it *does* experience nearly infinite contraction of space in the direction of travel. So from its perspective, its clock ticks normally, but it arrives at its destination before the clock moves (more accurately the clock would *barely* move). In this sense, that object does experience no time. You can also visualize this as the object itself "expanding" from its perspective until it is both at its source location and end location at the same time. Stretching the spacceship or whatever into a a line describing its entire geodesic life-line.

[@Sol-En]

It is possible that the photon has an incredibly small mass and is moving incredibly close to the maximum speed in the universe

[@kylelochlann5053]

Sure, though this would no bearing on relativity.

[@joelrivardguitar]

Interesting question. Do the Lorentz transformations show light remains outside of any reference frames in SR? If a photon was at rest, would the energy be 0 and can you have that with the uncertainty principle? Light is the propagation of the waves where a photon is a detected particle so each would have different reasons for not having a rest frame I would think?

[@erictaylor5462]

You know that scene in the 1980 Cosmos. Sagan is talking about the effects of Relativity and the teenager takes off on his little scooter, leaving his little brother sitting on a bench. When he returns the little boy is gone, and an old man sits in his place, then you find out the old man is the little boy. I was 10 when I fist saw this and it really freaked me out. I had nightmares from that. I still find that scene extremely disturbing. Imagine, nightmares caused by Cosmos!

[@samgordon9756]

But the speed of light is not the same in all reference frames. c is the same. c is not l. (l for light. c is for... causation.) So what happens to an object moving at the speed of causation? That is to say when we jump into the proper time of a photon does the photon's clock have time to tick before its journey is over for any arbitrary distance? I'm not suggesting this video must be wrong, but it seems to be leaving something out. Light is still constrained by relativity. "The question is meaningless" is ultimately as much of a dodge/misconception as having it the other way around so long as one doesn't explain the difference between the maximum speed of a photon in a vacuum and causality. Those are not the same thing, and only one is actually fundamental under relativity. In any inertial frame, observers will always measure the speed of CAUSATION to be the same. Light moves at that speed, but that's still not the same as saying that is light's speed. I don't have the math to say what the truth is/means. I do have enough knowledge to know that this didn't actually answer the question. It just swapped one arbitrary metaphor for another. Maybe it's not possible to actually give an answer in layman's terms. But it would be nice to see someone try. After all, mathematicians and engineers will tell us it's perfectly valid to take the limit as speed approaches c. Here, the answer is that is not valid. Why? Because it's not. That's not actually an answer or an advisor explanation. I would suggest that you can do better, but can you? Is that even possible?

[@kylelochlann5053]

There is no speed of causation as causal curves can be time-like.

[@samgordon9756]

I'm sure physicists the world over will be shocked to hear this. For the education and information of those who might think your answer makes and sense in this context,@@kylelochlann5053, I'm replying to say it doesn't. That is to say, physics discusses the speed of causation (c) all the time. It is in fact (as I already said) why the symbol is "c." The "c" stands for causation. As such, saying it doesn't exist by referring to a relativistic effect that we literally can't know if it is even real is not, in any way relevant to the question at hand. I'm not further bothering with you so feel free to scream into the void if you want. Anyone who saw your response is now permanently dumber for having read it. I award you no points and may god have mercy on your soul. For those who have been infected by this nonsense and may wonder why I claim it IS nonsense: A photon moving at c on a closed time-like curve (CTLC) would, by this commenter's logic, have no speed and thus LIGHT IN ALL forms on all curves would have no speed because light can travel CTLCs and therefore the universe could not exist as we experience it. We can discount the logic because we are here to discuss it. To be clear, this means that the explanation given is wrong even if it is somehow true to say causality has no speed. CTLCs, if they can even exist, are not the reason.

[@catastrophe3049]

Bhai re addict ho jata hu teri video ka Mat banaya kar aisi itni badhiya video Subah 5 baje teri video ko play kar diya dekhna nhi tha bas yuhi save karne ka irada tha Ab nahane nhi ja paa rha. Teri wajah se school jane me late ho jaunga😢 BTW I am physics teacher Feynman lecture ki book kharid ke rakhi thi padhne ka time nikalana bhul gya tha Ab tu aisi aisi chije lata hai to padhna padga😅

[@Mahesh_Shenoy]

Ab bas bi Karo bhai. Rulayega kya?

[@nishantsaha710]

so interesting man, my mind is blown after every video you release

[@mweave]

Amazing video Mahesh. The production values are going up too. Nice!

[@captain-hooked]

I'd love to see a video where you focus on the reference frame more, with more than one observer. Observer 1 at rest, observer 2 moving at almost the speed of light relative to observer 1. Observer 3 accelerating at 1000 m/s/s relative to observer 2. Do this and show each observers perspective and how they experience light. I think it would make a fascinating video.

[@alexanderdede6354]

First, why are you so good at incorporating your sponsors in your videos? You're simply too good. Secondly, I totally agree with correcting people's understanding about certain subjects and misconceptions. Though, I would totally commend them for thinking about these things and being curious. I also can understand others telling others "click bait" concepts. I feel it is to get then intrigued and interested to learn more. Promoting education, critical thinking and scientific literacy is definitely a great thing to do.😊

[@rykehuss3435]

It doesnt. Time dilation, length contraction, all that jazz. Work out the equations and light (and anything moving at the speed of light) does not experience any time. From its frame of reference, it is emitted and absorbed instantly no matter what distance it travels before absorption. This causes all kinds of fun and interesting phenomena inside a black hole's event horizon, where spacetime (and everything its 'carrying') flows faster than c. A topic you didnt explore

[@kylelochlann5053]

No, it's just not meaningful to use proper time as an affine parameter for any null curve. To say "light doesn't experience time" really doesn't make sense. Also, it's the "spatial coordinates" that flows faster than light inside a black hole (not spacetime) when considering the Gullstrand-Painleve coordinates.

[@rykehuss3435]

@@kylelochlann5053 "No, it's just not meaningful to use proper time as an affine parameter for any null curve. To say "light doesn't experience time" really doesn't make sense." Okay so special relativity disagrees with you. Due to length contraction and time dilation, as an object approaches c, all distances seem to contract and that object finds itself traveling distances faster than its velocity would indicate. At exactly c, that object can travel to any destination on a geodesic instantly. Ergo it experiences no time. You can use either time dilation or length contraction to get this result.

[@kylelochlann5053]

​@@rykehuss3435 Given a spacetime, S=[M,g], where g_{jk} is the flat-space metric, g_{jk}=η_{jk}, and given a world-line, x(τ), of arbitrary spacetime coordinates, the world-line then defines a space-like foliation, Σ. We then considers some other time-like curve, T(τ), called the traveler world-line. The "length contraction" is the projection of the traveler world-line onto the foliation defined by the observer world-line, and the "time dilation" is the projection of the traveler world-line onto the observer world-line. Given that the speed c is not an element of the Poincare group there are no spacetime events reachable by either the observer or the traveler existing at light-like separation, and therefore the notions of "time dilation" and "length contraction" have absolutely no meaning at the speed of light.

[@RohanSlazar]

I think this is a problem with how the question is phrased. As we know, an object moving close to lightspeed would see the universe outside evolve as if it was in fastforward thanks to time dilation. Because a photon it's moving at C, this fastforward evolution would basically be instantatious. From its frame of reference the photon would see the universe around it evolve instantatiously from the moment it was radiated to when it was absorved, while it itself still experiences time passing normally. But I'm only a hobbyist and not a scientist. I'll be glad if you could tell me if I'm wrong. Otherwise good vid 👍

[@saikatroy6225]

The way You explaining things is just mind blowing. Keep up the good work.❤❤❤😊

[@hdthor]

Doesn’t light experiencing time invalidate Penrose’s CCC since that theory relies on massless particles not experiencing time? Edit: thinking about it more, I think CCC only relies on a totally massless universe being unable to experience time. And Penrose is correct in that. A single massless particle in a universe with mass can still experience time, but mass is the originator of time, even for massless particles in its vicinity.

[@orangeanarchy235]

How about the question: How much time would an object going 0.999C experience if it was traveling a certain distance? It would experience time normally, but the amount of time to go a specific distance would, from its perspective, be severely shortened. Like, how fast would you have to go for your experience of a trip from the Andromeda Galaxy to the Milky Way to only be one second long from your perspective? I feel like this phrasing better gets to the heart of the question without breaking relativity.

[@juliavixen176]

If Andromeda (M31) is 2.537 million light years away, then _from Earth_ an object going 0.999999999999999999999999999922 C should arrive at Andromeda after experiencing 1 second of proper time.

[@juliavixen176]

That's 28 nines, so from Earth's perspective (or I guess the Milky Way) you're only going 7.8×10⁻²⁷ percent slower than light. You will arrive at the Andromeda galaxy one second after light/radio from Earth arrives. I'm not taking into account that the Andromeda galaxy is actually "falling" towards the Milky Way galaxy and will collide will us in about four or five billion years. This calculation I did assumes that these two galaxies are not moving relative to each other.

[@godfreypigott]

_"It would experience time normally, but the amount of time to go a specific distance would, from its perspective, be severely shortened."_ You appear to be referring to time in one reference frame and distance in another.

[@lewis7515]

​@@godfreypigott No, he isn't - he describes effects perceived by the subject in motion, and he is correct.

[@godfreypigott]

​@@lewis7515 Yes he is. When he says "a specific distance", he is referring to distance measured in a "stationary" frame. Ignoring the relative motion of the earth and Sirius as it is insignificant compared to the speed of light : Sirius is 8.6 light years away as measured in the reference frame in which the earth and Sirius are stationary. So an object travelling at 0.5c will take 17.2 years to travel to Sirius as measured in our reference frame. In the reference frame of the moving object, it will still take 17.2 years to move *that specific distance,* but it will take it WAY past Sirius. Instead of saying "to go a specific distance" he should have said "to travel between two specific objects".

[@jaumeparra6891]

Why it has to keep up with the car 1:40? Why doesn't it just go straight up and miss the mirror at the top? I thought the speed of light was independent of the speed of the emitting object. If it has to keep up, then is clearly not independent.

[@dmitriy9053]

The light that goes up from the perspective of "stationary" observer just hit the wall, so let's assume that the moving observer send the signal straight up from their perspective, then the "stationary" observer will see it like light was sent sideways like is shown in the video. As all movement is relative there is no "true" movement or staying still. So if you send a light signal to the target you should see it going straight up, and somebody moving from your perspective would see it moving sideways.

[@dmitriy9053]

So, in another words the speed of light is independent, the direction of the light can depend on the movement of the emitter. So, if the emitter is moving straight from you and sending a laser signal at 90 degrees to your left it's movement would bend the laser away from you, the speed would be the same. It is the nature of waves, sound is the same. Moving emitter bends the front of the wave towards it's movement. Just that the speed of sound is not the same to all observers.

[@jaumeparra6891]

@@dmitriy9053 Hi, thanks for replying. I understood that point. But have we proven the speed of light is same for any observer? My understanding is we have measured speed of light only on our "stationary" reference. But I am not aware of any measurement of it from another reference moving form the "stationary" one. So, we don't really know if speed would be same. We are assuming it should be. I know I must be wrong. But my way to learn is to challenge that so I get the right answer. I can see all the logic of time dilation, etc. lays on light speed being same for any observer. But my question is: are we sure it is same? Thanks for replying and have a nice day!

[@dmitriy9053]

@@jaumeparra6891 I don't get your understanding. Any reference frame is stationary from it's own perspective, so what are you talking about? We measured the speed of light in different circumstances like moving towards it, from it etc. It is still the same.

[@jaumeparra6891]

@@dmitriy9053 every ref is stationary from their perspective. But you can have a moving one from yours. Then you can compare what they see to what you see, exactly same as in example, where rocket sees straight line, and stationary sees oblique line. Due to technical impossibility, we haven't experienced that. We have only done measurements where both emissor and exterior observer are at rest from each other. Never an emissor have been at a significant high speed movement from the observer to see what the example shows (or at least, I'm not aware of it). Also, all our measurements don't get away from it or towards it. Speed of light is so huge that all we can do is to make it bounce in mirrors back and forth several times, and thus we cannot say we are moving away or moving towards it, as we do both in a single experiment. Don't get me wrong, just trying to get some further understanding. Thanks again for engaging in discussion.

[@WonderUniverse_]

Hi! Congratulations for another amazing video 💪 I have a question: if everything moves at the speed of light in space-time and if space and time are basically "made of the same thing", are 4 equal dimensions, why it seems that light is completely different? What is the difference between me and light? I mean, if I am at rest then I move at "c" in a dimension we call time, while light moves at "c" in a dimension we call space (from our point of view). So why light seems to be completely different (for example there is no reference frame in which it is at rest)? Ok, Einstein postulated it, but why the movement and the perception of space-time is highly dependent on the mass of the observers? What do you think about it? Thank you in advance and have a nice day! 👋

[@ChuckTBA]

there’re also the redshift/blueshift depending on where the light comes to you from is located. This is the actual effect when moving close to the speed to light

[@zachariemelanson485]

I don't think this is the best way of getting to the result "the question doesn't make sense". The way I view it is as you approach speed c, length contraction becomes extreme. At c, the distance you would perceive between yourself and anything becomes precisely 0. Light doesn't experience time, but it also doesn't experience space, which is why the question doesn't really make a lot of sense.

[@Vexas345]

But it doesn't become 0, that's the problem. The Lorentz equation works out to 1/0 at c, which is undefined.

[@zachariemelanson485]

@@Vexas345 L' = L*sqrt(1-(v^2/c^2)) L is the proper length of the object (or a distance) and L' is the lengh the observer would see the object (or distance). When v=c it gives 0

[@Vexas345]

@@zachariemelanson485 That's a simplified form. It's L*1/(Lorentz factor), which is undefined at c. But light doesn't have a reference frame to calculate from anyway.

[@kylelochlann5053]

@@Vexas345 You can't apply the Lorentz transform to light as the speed of light is not an element of the Lorentz/Poincare group.

[@Vexas345]

@@kylelochlann5053 True, that's what I was trying to get at.