Why the Speed of Light is the Ultimate Speed Limit | The Physics of the Universe

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How can the speed of light be the same for everyone, regardless of their state of motion? First, investigate how the speed of light is determined. Next, consider the hypothesized medium for light propagation-the aether-which was dealt a fatal blow by the Michelson-Morley experiment in the 1880s. Finally, examine laboratory proof that the speed of light is constant for all observers.
This video is lecture six from the series "The Evidence for Modern Physics: How We Know What We Know"
Stream the full series now on Wondrium. www.Wondrium.com/KZbin
00:00 How Do We Measure the Speed of Light?
06:27 Proving the Speed of Light is Uniform for All Observers
11:11 Maxwell's Equations and Electromagnetism
12:52 The Michelson-Morley Experiment
19:15 Particle Accelerator Offers Further Proof for Speed of Light
23:41 Is the Speed of Light the Same for All Wavelengths?
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[@aethrya]

Man, I just love Don Lincoln. Fermilab has been a great source of knowledge on physics for me and I have always enjoyed his intellect and dry sense of humor that occasionally comes out.

[@emilaviles6703]

UFO= not exactly name,g because they can out gravity, right call space ship travel speed of light, the pilot is human, allein or et,,,,,100% handsome and pretty ,100% kindly

[@robertgoss4842]

I have grown to greatly value your programs. You presentation style and timing are very accessible and in fact, make your lessons a pleasure to watch. Thanks for your hard work, and for making some often knotty subject matter so much more comprehensible.

[@willsingourd2523]

Thank you for this quite excellent vid on the "C" constant along with its bonus, strong History of Science flavor! My subscription was instant, if not as fast...

[@danield.kidane4811]

Great presentation! as usual.

[@WooliteMammoth]

Great video...but doesn't answer the question in the title of why the speed of light is the limit. Perhaps change the title?

[@gasperstarina9837]

Fundamental constant...we don't know for many...but faster you move thru space slower time is going for you (its proven, with observations everywhere) and at 100% speed of light time would stop for you, literally. Yes ik what ou have in mind even if you know relativity but it seems its physical constant like many others and we simoly don't kbow why, how it just works-like quantum mechanics, we don't know why and how we just know it works and all technology since 50s is at one point based in QM

[@stefan24georgiev]

@@gasperstarina9837 why does time slow down ?

[@hassegreiner9675]

The major difference between the baseball thrower and lightemission is, that while the speeds of the thrower adds to the speed of the ball, light is just set free to travel at the speed it chooses given the medium - it cannot be affected (bothered) by the speed of its source.

[@markiv2942]

That's great explanation, for a six year old.

[@chunfuxu]

Good explanation. It doesn't seem to prove the speed of light is constant for all observers.

[@DrDeuteron]

What medium? There is no medium.

[@cjheaford]

So WHY IS SPEED OF LIGHT the speed limit then?? I hate it when videos don’t answer the very question they pose in their own title.

[@joebloggs396]

I don't think we know. But I think his videos on Fermilab will give more advanced ideas at least than this video. edit:I added my speculation in another comment

[@crewrangergaming9582]

Let's don't see it as speed of light, or even as a limit, it isn't a limit, it is just a speed that we humans have calculated it to be relative to our understanding. But what it really is simple - The more you move in space, the less you move in time, the more mass you have the less you move in space with the same energy, and light don't have any mass, so it doesn't move through time but is moving through space infinitely, you see, relative to a particle of light, it hasn't aged a day since the big bang. Think of it like, you can move through space and time, but only when the sum of it is 1. So maybe you can move through space 0.4 and through time 0.6 but never more than that, the Universe will limit you from doing so, if you reach the speed of light time slows down so you only move so much, so that casaulty is maintained.

[@cjheaford]

@@crewrangergaming9582 I understand all that. My complaint is the clickbait nature of the title. It didn’t even address it’s own question.

[@kylelochlann5053]

I don't think he knows why.

[@kylelochlann5053]

@@joebloggs396 Of course we know, though I'm confident Don Lincoln doesn't know.

[@stevem83]

Great work. Very interesting.

[@joemug4079]

This was that totally captivating and interesting. Even though I couldn’t grasp all of it. But I do understand it.

[@titleloanman]

Though I don’t implicitly doubt the conclusion of the final experiment, I’m having trouble understanding why it proves that the photon is moving at the speed of light with respect to the electron. It only seems to prove that it moves at the speed of light respective to the stationary detector, irrespective of the speed of the emitter. I don’t see how that experiment is demonstrating the perspective of the emitting electron at all.

[@NalitaQubit]

Thank you for a your simple way of explaining complex phenomena!

[@koantao8321]

I wish I had you as my physics teacher. I had a D in physics, yet I was fascinated by it and still am.

[@barryzeeberg3672]

"Why the Speed of Light is the Speed Limit" - but the video showed what the speed of light is, and how it is measured, but not why it is what it is, and not why it is the speed limit.

[@katieell4084]

Exactly. I suppose nobody can answer the question posited by the title. Unclear why it is titled wrong given how smart the content seems to be. Clickbait, I suppose?

[@moped975]

Das ist genau die Hilflosigkeit, die versucht, eine Behauptung so lange mit immer neuen Thesen zu untermauern, bis sie gesichert scheint.

[@popkorn256]

Technically, he didn't really answer the question of WHY the speed of light is the limit. I was hoping to see the reasonning of Einstein when he came to that conclusion. Great video though!

[@colleendougherty4475]

fancy words never convinced me .

[@DrDeuteron]

@@colleendougherty4475do the predictions of: Time dilation Length contraction Antimatter Black holes Non static universe Frame dragging Shapiro delay Convince you?

[@paulcarfantan6688]

@@DrDeuteron No.

[@maurjoy4104]

You've convinced me. Though there are caverns in my understanding. But you are a superb narrator.

[@Wondrium]

Thank you, Maur! We'll share your comments with the professor!

[@ballsrgrossnugly]

I like the video, it touches on some bigger concepts for anyone coming to physics for the first time. One thing I noticed, you mentioned that the dark patches on the double slit experiment are the peaks and troughs lining up, but wouldn't they be the double troughs? The grey sections in between would be any combination of peak/trough strength that isn't both 1 or both 0, including a peak/trough line up right on the 50% brightness line.

[@-_Nuke_-]

For anyone having trouble understanding why the speed of light even exists read the following: No matter which theory of relativity we use (The two most famous ones are Galilean and Einstein’s relativity). We always need a way to preserve causality That means -> that we need a way to be able to put EVENTS in causal order (we need to know which event caused which other event and at what order.) To do that, we always need to have some quantity that is absolute. For Galilean and Einstein’s relativity, those quantities are a certain specific speeds. For Galilean relativity, our speed through time is that quantity. Its absolute. Which means that is 1) invariant and 2) constant. 1) Invariant means, that the value of the speed doesn’t depend on any observer or their own speeds. 2) Constant means that its value can’t change. It’s a set value and there is nothing we can do to change that. For Galilean relativity, that quantity - our speed through time - is therefore constant and invariant and equal to 1s/s. Which means, that every object is aging at 1 second per second no matter who is observing or what their speeds are. That also means that there is nothing you can do, to travel in time faster than 1 second every second! (That by the way, shows us that Galilean spacetime is Euclidean). NOW! For Einstein’s relativity that has changed. An objects speed through time is not absolute anymore, and its not constant and its not invariant. We know this - because we have seen that our spacetime is not Euclidean as we once thought. Its hyperbolic and its depicted by the Minkowski metric. So the nature of spacetime - being hyperbolic - makes our speed through time to not be constant and or invariant as we once thought! So now we need a new relativity, Einstien’s relativity, which does 2 things. 1) Preserves causality. And 2) has speeds through time that are not constant or invariant anymore. So how can we do that? Well, Einstein comes up with a new quantity that’s absolute that does those 2 things. That new quantity is our speed through spacetime. And for Einstein - its absolute. Which means that its 1) constant 2) invariant and equal to 1ls/s (one light-second per second). That means, that every object moves through spacetime at the speed of 1ls/s which is the speed of light So… An object can move in spacetime with ONLY one speed. The speed of light. Me, you and everything is right now, moving in spacetime AT the speed of light relative to nothing. We don’t need relativity to say that, because the speed of light ISNT relative - its absolute, and its equal to 1ls/s. So now, just like you couldn't age faster than 1second per second in Galilean relativity. Here you can't travel in spacetime faster than 1 lightsecond per second. Just like it didn't make sense for an object to age faster in Galilean relativity, here again its meaningless to travel faster than 1ls/s in spacetime. Why? Well, lets take a look what means to travel at a certain speed in space. Since our speed in spacetime is the speed of light... That speed in spacetime is always the combination of our speed through space and our speed through time. If our speed through space is C then our speed through time is 0. And if our speed through time is C then our speed through space is 0. The 1st where our speed through space is C, is the case for light. All of its speed in spacetime is allocated in just space, and time doesn't pass for it. The 2nd case, where all our speed is in time, is freefall! Because that's what freefall is! To stay stationary in space and age at the speed of light. Anything in between is the speeds that everyday objects have. Some of it will be in space some of it will be in time but when combined they will give us C as a net result. That means! That the speed of light IS effectively infinity. At least for a hyperbolic geometry that is. Becuase when you travel close to the speed of light, time for you passes very slowly (your speed in time is very small). Which means that you can reach very far away distances withought time passing for you... So you can reach a 1light year away star in just 1 week because you didn't experince time. If you go closer to the speed of light in space still - then you can reach that 1 lightyear way star in 1 day! Closer? 1 hour... Closer? 1 second... Etc... So when you go AT exactly the speed of light in space, your speed in time is zero. So you can reach any position in space infinitely fast (in zero time). (For you at least...). So to answer you initial question: You can't go faster than C, because you can't go faster than infinity.

[@icerainstorm2592]

If you listen you can hear the answer to that "the speed of light is the same " if it wasn't you and I would see light differently

[@icerainstorm2592]

Oop wrong spot sorry about that

[@IterativeTheoryRocks]

What matters is the ‘size’ of the peak or trough. The brightness is dependent on the size squared. So A trough is the same as a peak in size - and brightness - just out of phase. The dark patch is where a peak is cancelled out by a trough, giving a size of zero.

[@Robinson8491]

@@-_Nuke_- Do you need hyperbolic geometry for the speed of light being constant as in special relativity?

[@bloodyorphan]

Loved this Thankyou. A couple of things, all the standard model particles that "Occlude" need to be the same size (12 Planque ish). The superposed nature of photon weight versus frequency... The "Particle" and the QM field wave are always "there" it's the spacial collapse and expansion for the raw weight that is frequency. So does the entire "magnetic" aperture of the photon disappear and reappear (Polarity question) ? Lensing is likely another double aperture addition, but if symmetrical flow energy is the vector driver, the "longest" distortion and therefore the lowest pressure is on the wrong side, but the planet side compression is 12 Planque higher than the outside so back to a kissing hawking reaction over time spinning the lowest flow density aperture length to the outside, resetting after frequency cycle completion ? 😁 Love your work DrDon. Thank you again Bernhard

[@jamesh9174]

What if there is simply a speed limit in our universe for light or anything and that is what we see in the experiment and not that light is the same speed for all observers?

[@bloodyorphan]

@@jamesh9174 Boundary not limit and yes space has a time dilation ratio compared to us of 9*10^70 seconds for our one second observed.

[@davidcolombier5673]

I had a great time listening to you. I unfortunately didn't learn physics,nor astronomy, but am very interested by it. So I rad and watch on Internet everything that is related to it.

[@DevinDTV]

the michelson-morley experiment is pretty impressive

[@darkososyt]

Thanks for the video, some good explanations, and there is a lot of them out there. I'm seeing this line of reasoning over and over again, and every time I'm left puzzled. It seems to me we have two different things here. First, since we don't really have a medium for light, i.e.the very space itself is the medium, and if we suppose it's fixed (although GTR says it's not, but lets leave that aside here), then it's no surprise that it can't be pushed by anything. Because it's not really pushed (or thrown or whatever), it is more generated. That very fact is enough for us to say that the photon could not travel faster than the speed at which the space itself could spread the electromagnetic disturbance (that is very similar to Maxwell's derivation from two different permeabilities). So that part is pretty obvious. The second part is what puzzles me, and that is hidden inside the following sentence: "..the electron sees photon traveling at the speed of light...". First of all, how we know this? I'd say that is not something that could be deduced from any of these experiments. I know it's one of the conjectures, but could you please say what exactly proves that part? And I think it is crucial. As I see this, that part is what makes the (Einsteinian) relativity so "weird" - it gives rise to time and length dilatation, effectively the usage of Lorentz's transformations.

[@kylelochlann5053]

The geometry of the gravitational field requires that the electron measures the speed of light to be c. There is no way to render a different result consistent with the results we obtain in the lab.

[@darkososyt]

@@kylelochlann5053 ok, but I don't see that in the line of reasoning in this video. I'm just saying that from presented reasoning and experiments we cannot say that the source of light, traveling in the same direction as that light, perceive it's velocity as c. I would really like to see explanation on that. The fact that emittion of light is not the same as throwing a ball from the train is far more obvious.

[@jamesh9174]

What if there is simply a speed limit in our universe for light or anything and that is what we see in the experiment and not that light is the same speed for all observers?

[@kylelochlann5053]

@@darkososyt Correct, the video presumes a basic understanding of relativity, which isn't obvious to anyone who has not studied it sufficiently. The constancy of the speed of light is owed to the local structure of the gravitational field and originally taken as a postulate by Einstein on the assumption that the electromagnetic equations of Maxwell are correct. Nowadays the local constancy of the speed of light is taken as a consequence of the spacetime interval being null for light, i.e. ds=0.

[@darkososyt]

@@kylelochlann5053 that's very interesting... For some reason I'm trying to understand this, and the more I watch videos on the subject the more questions I have :) Anyway, as much as appreciate people that make these kind of videos, seems to me that there's always something missing in line of reasoning. And often there is a mix of things like "light between mirrors on a train" and some math equation which already have many other thing hidden in it. I'm a mathematician by education, so to me, if we assume something and then make equations that are the consequence of those assumptions, even if the equations proves right, that doesn't mean that our assumptions are true. Logically, from false assumptions you can deduce right conclusions. So I'm trying to learn what exactly are facts and what are assumptions, or consequences of assumptions. I mean, if we knew everything, there would be no need to question anything...

[@antifajesus]

You're a really good presenter and the camera work is getting better thanks for posting

[@Wondrium]

Thank you for your nice feedback.

[@AmyWinehouse.914]

Another very well explained upload - thank you.

[@Rasta426]

I knew you were alive!

[@AmyWinehouse.914]

@@Rasta426 How did you know?

[@Rasta426]

@@AmyWinehouse.914 when is youre next single coming out?

[@AmyWinehouse.914]

@@Rasta426 The day after you become funny.

[@ttamdude]

Was this done in a single take? My goodness… this is great.

[@drbonesshow1]

We found the speed of light - how incredibly fast it was - but then we found out how incredibly slow it was - considering the size of the Universe,

[@George.Coleman]

There is no limit, time just slows down

[@Spugler2]

Yes, there is a limit.

[@MATT-xv4bh]

Interesting lecture. It causes me to ponder the subject.

[@soiledpants1499]

I would watch a vid of Dr Don explaining a cake rising. Thanx so much for all the fun physics.

[@ERPRocks]

The title is misleading. This video does not say why the speed of light is the speed limit, only that we observe the speed of light to be the same everywhere.

[@spiralsun1]

It’s because mass grows and becomes infinite at the speed of light. So if you shot a BB gun at the Earth at the speed of light it would destroy Earth according to Einstein and that kid from “A Christmas Story” who stuck his tongue to the flag pole. I think they both would agree. Hopefully without swearing. Thanks 🙏🏻 I hope this helped you out, but I thought it did say why, so I don’t know 🤷‍♀️ If you have any more questions I am ready. Thanks 🙏🏻

[@utee72]

The speed of light is completely a function of the medium it traversed. Specifically as Maxwell showed in 1860 = 1/(sqrtf(u0)**2 * sqrtf (e0)**2). With u0 magnetic permeability and e0 electrical permittivity. These last two may vary depending upon medium eg free space supports faster speed than water.

[@BlackPDigitalMedia]

@@utee72 so say the medium is the quantum realm then light could have a different speed limit there🤔🤷🏽‍♂️

[@utee72]

@@BlackPDigitalMedia No the "realm" is our universe's free space and thus the speed of light is constant there. Quantum objects in our universe cannot defeat this restriction. Note again that different "media" eg water will transmit light slower as they have different u0 and e0.

[@surendranmk5306]

@@spiralsun1 In EM radiation what is moving through space? If it is the photon, it have the energy h when it is emitted by an electron. Then the mass of a photon is h/c^2. If the photon is moving at the speed c then too it's mass have to increase towaards infinity. But it is not happening. It remains constant. Why?

[@moff321]

Surely from the perspective of the electron the speed of the photon is zero (based on the way you explained it)

[@kylelochlann5053]

No, in the frame of the electron the photon still moves at c. Of course, the explanation of Don Lincoln is wrong, as usual.

[@polyrhythmia]

@@kylelochlann5053 Also in the frame of the electron, the electron and photon do not arrive simultaneously. Relativity of simultaneity.

[@DrJohnNO]

From my probably messed up understanding, it must be due to time dilation at the speed of light from the electron perspective vs the outsider perspective with no time dilation.v

[@CharlesDeMar]

This guy is a good teacher. Altho beyond the scope, my understanding is that measuring the speed of light in one direction is not achievable. It’s measured by round trip b/c of a problem with keeping two separate clocks perfectly synchronized.

[@ruperterskin2117]

Right on. Thanks for sharing.

[@nkhmd]

Love your videos....reminds me of the joy of calculating particles in a box at U of Chicago undergrad, but that gamma ray burst example you gave was simply mind-altering / bending/expanding! Thank you, Doc, for your understated-profundity!! ;)

[@VeteranVandal]

Also, the rotating mirror type experiments to measure the speed of light are even easier (require less than 500ft, tho bigger distances is indeed better) to do than the ones mentioned in the video and get between 10% and 1% error depending on how careful you are in setup.

[@andymachala999]

We did both in Physics lab. as well as the Cavendish experiment.

[@naimyadally193]

Nice explanation. My only question: This experiment to measure the speed of light with 2 electronic timers, is it not too simplistic? (1) How far need they be placed apart knowing that the c = 300 000 km/s? (2) How do we synchronize the 2 clocks at the start?

[@MahmudNMashuk]

We can never know the one way speed of light, no matter what the method is. Let along with the method mentioned here. I feel you, bro.

[@georgevprochazka5316]

Thank you sir! The speed of light made sense to me for the first time ever.

[@KpxUrz5745]

The average person would benefit immensely from this channel. They just don't teach enough physics in schools, so most people walk around quite ignorant of basic concepts.

[@jrzygurl]

I'm an average person and I try to understand what my pea brain allows me to. I find all things about the universe quite interesting... I am not smart enough to understand even a small percentage but I do try

[@spiralsun1]

This video was AWESOME. ❤ One of the few to actually talk about the huge leap in thought that light has a finite speed. We so take that for granted. Ole Romer actually constructed a telescope with calibration to do the measurements then squinted through a telescope carefully night after night and figured that out. Just mind-blowingly amazing! I loved how clear and carefully the topics were covered thanks!!! ❤

[@hg6996]

Dr. Don Lincoln has his own yt channel, in case you are interested in more of what he has to say.

[@baarni]

If we detect gamma rays from 7.5 billion light years away they must have started out with vastly more energy than we detected…

[@AORD72]

Why? They aren't using any energy to travel. Surely they are set in motion when they are released from matter, then maintain that motion until they collide with something.

[@matthewphilip1977]

@@AORD72 Wouldn't they have collided with lots of stuff if they've travelled 7.5 billion light years? 7.5 billion light years is 300,000,000 x 60seconds x 60 minutes x 365 days x 7,500,000,000. That's a lot of metres (something with approx 25 zeros after it; more than a billion quadrillion metres!) Wouldn't there be friction involved?

[@michaelniederer2831]

Lucid and succinct. Strange but true. Thanks.

[@terryrogers8965]

The speed of light being the same for all observers is telling us something deep and fundamental about the universe. Have often thought the same thing but this is the first time I've ever heard it said in such a direct manner. Thank you!

[@matthewphilip1977]

Why do you believe that? And why might the nice man in the video believe it?

[@terryrogers8965]

@@matthewphilip1977 Well the short answer is I don't know, yet. Another thing that makes me wonder is photos traveling at the speed of light apparently do not experience time. If I figure it out I'll let you know or you'll hear about it when I pick up my Nobel prize. 🤣

[@graemej2599]

The Speed of Light (C) is constant because the Doppler Effect is altering Frequency (F) and Wave-length (WL) C = F x WL

[@DrDeuteron]

@@terryrogers8965drop that photons don’t experience time. There are no reference frames for light, so it is meaningless, and adds no understanding.

[@Bjowolf2]

The Lorentz factor really represents projections back and forth between the two frames of reference, if you interpret it as a cosine (!) instead. The apparent "length contraction" and "time dilation" arise from the 2D length and time "vectors" ( both complex variables ) each pointing in different directions in their respective complex planes between the two frames of reference , the more the faster an object moves relatively to the observer"s frame of reference. Gamma^(-1) = sqrt( 1 - v^2/c^2 ) So they don't shrink (contract) or expand (dilate) at all - it's their projections (!!!) - or their "presence" if you like ! - in the frame of reference of the observer that shrink or expands. ( and the other way around as well of course ). As v goes towards c, gamma will decrease to 0 - in which case the length dimension of the object along the axis of motion is entirely "out of" ( not present in ) the length dimension of the observer. Think of a "unit" circle in the complex plane with radius c and v along the imaginary axis (!)

[@starfishsystems]

Well said.

[@jamesh9174]

What if there is simply a speed limit in our universe for light or anything and that is what we see in the experiment and not that light is the same speed for all observers?

[@Bjowolf2]

@@jamesh9174 Well, there is - but then we would measure different values for the speed of light from different frames of reference - and we don't (!!!), which is very counter intuitive and weird. So something else "has to give" so to speak for that to be true - namely the concept of a universal time and fixed spatial dimensions. They (now) depend on the frames of reference of the observers. From the frame of reference of the photon it reaches our eyes INSTANTANEOUSLY from the light source, since no time is passing for it, as the spatial dimension along its direction of "travel" is contracted to zero length ( v = c, in the Lorentz factors ) - or its "presence" ( the projection of its length dimension ) is reduced to zero, as I prefer to see it 😉

[@daviddeutsch3392]

@@Bjowolf2 I like this "something has to give"! This is exactly how I have thought about these matters. Nothing shrinks or expands but the measurement of these parameters. It is not necessary to think of a an object actually shrinking in the direction of motion, only that the measurement of such changes with relative velocity. Of course, my view here raises the question of whatever "actually" means in these contexts.

[@Bjowolf2]

@@daviddeutsch3392 Thank you 🤗

[@jamesraymond1158]

Excellent. I am looking forward to your video on the speed of gravitational waves. Since they are not an electromagnetic phenomenon, there is no obvious reason why they should also travel at c.

[@jamesh9174]

What if there is simply a speed limit in our universe for light or anything and that is what we see in the experiment and not that light is the same speed for all observers?

[@goestovbhudi8716]

@@jamesh9174 That is in fact true. Light travels at the maximum speed that the universe will allow, and so do gravitational waves.

[@JSSTyger]

Thank you for your explanations.

[@Wondrium]

It is our pleasure!

[@courtneysmith6082]

Is there a theoretical consequence to traveling FTL? What would happen to matter if it were to exceed the speed of light (e.g., particle breakdown, going plaid, etc.)?

[@kylelochlann5053]

Nothing would happen and time (and all else) would be normal for you. You would be moving traveling a negative length and this doesn't make physical sense, and you'd be able to send messages to yourself about events that haven't happened yet. So, it's not so much a problem as that it is meaningless.

[@DrDeuteron]

Once you go faster than light, there are frames where you are teleporting instantly (to outside observers), and more in which you are going backwards in time. Just say no.

[@MrGyulaBacsi]

Very good video, though I think it did not answer to the question in the title. I saw another video about this topic on PBS SpaceTime where they tried to explain this question. If I got that correctly (sorry, I'm not a scientist just a poor electrical engineer) the takeaway of that video was that (and they were referring to some articles there) if we make some basic assumptions about the universe (and I'm talking about real basic stuff here, like the cause must always comes first and the effect is only later). So based on these assumptions it can be derived that there must be a cosmic speed limit with which NOTHING can travel faster. This speed limit is the speed of causality. And it happens to be that the light has this speed (and as a matter of fact because its stationary mass is zero it cannot go slower - I mean, in vacuum.). Perhaps some video about this topic on this channel would be useful.

[@jameskirk7762]

Always nice to see dr. Lincoln.

[@hypercomms2001]

It is no the speed of light that is the limit, but the speed of causality that is the limit. The speed of light can vary depending on the material properties of the medium it is passing through.

[@richdobbs6595]

I'd be really interested in a quantitative analysis of the risk associated with mercury in the Michelson-Morley experiment. I suspect that it would accord more with Michelson and Morley, rather than the modern practices. I suspect that Mercury is dangerous at elevated temperatures or when complexed in bio-chemically active compounds, but that room temperature mercury is far safer than smoking and driving cars, let alone rock climbing or hang gliders. Probably safer than living in Colorado.

[@invertedpolarity6890]

“It’s not true for light, and we know this to be true.” So true.

[@TheNuclearGeek]

The best thought experiment for understanding the speed of light I got when I was younger was the speed of light train traveling in space. What would the speed of a baseball thrown out the window parallel to the speed of light train be (first ignoring mass for simplicity and then discussing what would happen if the baseball weighed 1 kg). Then what would the speed of light be traveling when you turn on the train's headlight traveling at the speed of light?

[@goestovbhudi8716]

@@robertvangeel3599 In any relativity calculation, you begin by assuming that "you" are stationary; you cannot travel at 99.99%, or 0.000000000000000000001%, of the speed of light. See 0:49 in this very video.

[@skizmo1905]

Please tell your video editor that those side camera recordings are really horrible... they make these videos very awkward. Just use the front camera.

[@mhoover]

I advocate jail time for directors who do this. They would occupy cells near the people who do countdowns for everything.

[@caseyford3368]

Once we connect with all energies and anti energies in this and other universes and dimensions, we will be able to travel at the speed of thought. Then, we can be everywhere at the same time. I'd have 100 of myself in every existence. Always learning and helping those who want it.

[@caseyford3368]

@@baberoot1998 work on people? I was just describing my point of view.

[@CharlieLOL]

Fantastic! You are amazing!!!

[@mikeburda3038]

There is always something faster. Always!

[@fractalnomics]

Well done.

[@pravinpanchal9883]

Speed of light has to do with a nature of space. We know that electric permittivity (epsilon naught) of vacuum or free space and magnetic permeability (mu naught) of free space are constant everywhere, that's why speed of light in vacuum is the same everywhere and independent of frame of reference. It is simply observed that way. It is one of the fundamental constants. The constancy of speed of light is one of two conjecture of the special theory of relativity.

[@jamesh9174]

What if there is simply a speed limit in our universe for light or anything and that is what we see in the experiment and not that light is the same speed for all observers?

[@peolindstrom8570]

Great video👍

[@Wondrium]

Thank you for your feedback, Peo.

[@jackmagdiel1750]

Thank you for what you do.

[@jamesgerard4505]

"The speed of light, being the same for all frequencies and wavelengths, and the same for all observers says something very deep and fundamental about the universe. Mind you we don't know exactly what it is, but it is clearly telling us something." That is a profound and essential observation. I do know exactly what it is telling us. It is that the speed of light is a boundary phenomenon between two different levels or regions or planes of the universe. On our side of the speed of light, everything travels at or below that speed. On the other side of that speed, everything travels faster, up to the light of that region, which is another boundary. There are seven such boundaries, all held in the overall gravitation, which may be one of the things that can leak through these boundaries, and my guess is that may account for dark energy and dark matter.

[@DrDeuteron]

What it tells us is that frequency and wavelength are not properties of light. They are consequences of the frame in which the light is observed. It really is that simple.

[@atec6006]

The most elaborate effort I have come across so far to explain to the '"average Joe" why the Speed of Light is Constant for all reference frames

[@JamesJoyce12]

SR does not claim a speed limit for C - it simply says things can not accelerate through and beyond C - not that things are not already moving [unaccelerated] at beyond C - how come so many folks do not know this?

[@DrDeuteron]

Because there are no tachyons, and you don’t need to know that to describe experiment .

[@paulcarfantan6688]

@@DrDeuteron I guess there was no Higgs Boson either...until there was. Be patient.

[@geeks4greyson425]

Thanks for the detailed explanation of the Michelson - Morley experiment. And for mentioning that it was conducted in my home time of Cleveland. Ohio at The Case School of Applied Sciences now Case Western Reserve University. I contend, whenever and wherever the opportunity arises, that, in a very real sense, my hometown is the birthplace of Einstein's theory of relativity.

[@giulianoardis370]

As far we know at the moment , at quantistic level there are many speed unesplained

[@markyerges316]

This was a very helpful and informative video. One thing was confusing to me and it may be that my understanding, prior to watching this video, was just wrong. What I have understood the statement "the speed of light is the same for all observers" to mean is that speed is the same for each respective frame of reference. Meaning, if I'm traveling on a shuttle in the direction of a light beam at 1/2 the speed of light, that beam of light is traveling away from me at the speed of light. But, to a stationary person, that beam of light is still only traveling at the speed of light. I thought that was how time dilation occurred. After watching the final experiment here, though, I have more questions. Having a beam of light emit a photon and measuring that speed from a single point of reference does not feel the same as measuring one (or even two) beams of light from different reference points.

[@matthewphilip1977]

Hello. If you and the beam of light were in a race over a very long distance would you be covering half of the distance that the beam was covering?

[@josephbigler]

We are told the speed of light in a vacuum is 299,792.458 kilometers per second by definition. We are also told that everyone will always see light at that speed no matter their reference frame. An example is someone with a flashlight standing on an open flat train car traveling at 60 kilometers per second. Assume the person holding the flashlight is pointing the light in the direction the train is moving. We are told that if the person on the train car were to measure the speed of the light, they would measure it at 299,792.458 kilometers per hour. Now assume we have someone standing on the ground near the train. If the person on the ground measures the speed of the light from the flashlight as it passes by, they also measure the speed of the light from the flashlight as 299,792.458 kilometers per second, the same speed. But the light is not traveling in a vacuum, it is traveling through the air. Light traveling through the air at STP (0 degrees centigrade, 1 atmosphere) has a refractive index of 1.000293(1). The speed of light in the air would be 299,792.458/1.000293 which is 299,704.645 kilometers per second. The person on the train should read 299,704.645 kilometers per second for the speed of light coming from the flashlight. Does the person on the ground measure 299,704.645 kilometers per second, the same speed as the person on the train, or do they read 299,704.645 + 60 = 299,764.645 kilometers per second? This is still below the maximum speed of 299,792.458 kilometers per second of light in a vacuum. Am I missing something? Has something like this ever been tested experimentally? I know it would be difficult given the train would be traveling faster than any current train, but could some experiment be done to verify that light is either traveling the same speed as it would in the air or that speed plus the speed of the train? (1)en.wikipedia.org/wiki/Refractive_index

[@irabot]

Very interesting Great cause Keep it up mate your doing a good thing

[@tobyihli9470]

In the experiment with the electron and the photon, both traveling the same, it has nothing to do with the “observer.” It has to do with the fact that the photon has no mass, therefore there’s no inertia, or centripetal force as pertaining to gravity. You can’t sling, or throw something that has no mass. Nothing to do with “the observers.”

[@braaitongs]

That is for Transverse electromagnetic waves. And we will have to go back to the aether. I think it was Oliver Heaviside who simplified JC Maxwells 20 equations in the 4 know today as maxwells equations.

[@DrDeuteron]

All propagating modes are transverse, but it works for near field effects too, it’s just that studying those fields only adds difficulties, not elucidation

[@kindlin]

I just found this channel, I mainly know you from your other Fermi channel.

[@vesuvandoppelganger]

THE PARADOX THAT DISPROVES SPECIAL RELATIVITY: There is a triangle of lights which we will call A, B, and C. They flash simultaneously in the frame of reference that is at rest relative to these lights. There is someone moving at a high rate of speed from B to A. There is someone else moving at a high rate of speed from C to B. There is someone else moving at a high rate of speed from A to C. So A flashes first and then B flashes and then C flashes and then A flashes again. How can A flash twice? When A flashes has B already flashed or not yet flashed? or B flashes first and then C flashes and then A flashes and then B flashes again. How can B flash twice? When B flashes has C already flashed or not yet flashed? or C flashes first and then A flashes and then B flashes and then C flashes again. How can C flash twice? When C flashes has A already flashed or not yet flashed?

[@leonhardtkristensen4093]

You start up saying that they all flash simultaneously. Then each traveler will only see one flash from each point but at different times. The travelers speed only matter to make it possible to see any difference in when the light flashes arrive and possibly also the color of the flashes.

[@kylelochlann5053]

Is this a joke?

[@barryon8706]

You seem to be describing what different observers see and then asking how they can see different things. Relativity accepts that events that look simultaneous to one observer need not look simultaneous to another. Also, you could do this same experiment with Newtonian physics, or so it with sound. Or BBs.

[@leonhardtkristensen4093]

@@vesuvandoppelganger You started up by saying that they flash simultaneously and by that I understand that if seen from a point with equal distance to all three points they would be seen as flashing simultaneously. Any person at anywhere else around will see the flashes when they come to them. Time = c/distance. None of them will see more than one flash from each point. It is my belief that if an electromagnetic impulse (in this case photons) is emitted from a point in the universe this point is fixed meaning it doesn't move even though it is emitted from a moving object. Any movement can be noticed in the frequency of the signal but not the speed. An example it speed radar. I believe that you can't tell any difference if you check the speed of a house if tested from a moving car or if you are standing at the house and testing the speed of the car. Another example is the redshift from far away stars that supposedly move away from us. Some people say that electromagnetic waves (including light) are moving in some quantum field. My question is: Isn't that just another name for the eather that has been disproved to exist? I believe electromagnetic pulses (photons) are energy pulses or quanta's that can exist on their own just like a small space ship. They are pulsating giving them the appearances of waves. This in my opinion would explain the double slid performance. That free electrons supposedly also show up as both particles and waves in a double slid setup would indicate that they are also wavy particles. They are much bigger and have gained mass and as such can be accelerated but also move at much lower speed.

[@andrewmcevans465]

I am old enough to remember when the sound barrier was the ultimate speed barrier. Before that it was locomotives making 45 MPH..

[@dexw7094]

Nice job...

[@hanumantd]

Just one question from layman: While explaining the particle accelerator experiment at t23 you showed that the electron and the photon both reached the second detector at the same time. This showed that the speed of the photon was still c and not 2c. But how did you know that the electron observed the speed of that photon as also c instead of 0? To prove that the speed of light is same for all observers, you have to show both, right? That the stationary observer as well as the moving observer both observe the speed of the photon to be c?

[@kylelochlann5053]

The constancy of the local speed of light for all inertial observers is a consequence of the causal structure of the gravitational field and not something that can be covered in this video.

[@hanumantd]

@@kylelochlann5053 OK, so shall I infer from your comment that this video does not show that the speed of light is same for all observers?

[@kylelochlann5053]

@@hanumantd That's right, it doesn't. and I disliked it for the clickbait title. Based on his other videos, I'm confident Don Lincoln never took a course in relativity and doesn't know the answer. EDIT: Lincoln would have used relativity in his course work and professionally. In grad school the high energy group (the particle physics kids) could often calculate things in relativity quicker than we could, but they had no idea why anything was the way it was (and often had hilarious explanations of what they thought the physics was about).

[@roger7341]

The speed of light in a vacuum is the same as the speed of any electromagnetic wave in a vacuum. The speed of any electromagnetic wave in a vacuum is determined by the characteristic impedance of free space, which is equivalent to the square root of the proportion of penetrability of free space (henrys per meter) to the penetrability of free space (farads per meter). It is about 377 Ω, which is the characteristic impedance of the universe. In the presence of virtually anything, space is not entirely free and the speed of electromagnetic waves decreases.

[@JungleJargon]

*Speaking of thought experiments,* The speed of light is merely a mathematical construct. In reality gravity drops off exponentially outside of a galaxy allowing for time to speed up and the other thing that happens, which people seem to forget, is that less gravity also allows for distance to be expanded, which results in less distance compared to our contracted distance inside of a galaxy. So less gravity allows for our observation of the light to travel 186,000 miles at a faster rate of time over an inflated measure of distance relative to where we are inside of the galaxy causing the speed of light to be greatly increased relative to where we are in a more contracted measure of distance and a slower rate of time if GR is true and GR is now more of an observation than a theory.

[@rookiebird9382]

Love this channel

[@spjunkies]

I'm too dumb for this video, but thanks for trying to educate me.

[@dalemain8342]

Yes the tittle is misleading. No reason is given as to why ‘c’ is ‘c’ which is possibly one of the greatest unknown questions in physics. If we knew this it could possibly provide a great deal of insights into the physical world.

[@maarifawalcott5145]

Love this stuff

[@paulwright3252]

Nice to have a real person explaining and not an AI voicebot.

[@Benson_aka_devils_advocate_88]

It drives me crazy when people say never. Or in this case, scientists that fall into the trap of believing an idea is set in stone. I mean, think about _How Many Times_ in history we've said never. Speed of sound, traveling to the moon, medical advancements; so many times someone came along and thought, maybe? Dark matter and dark energy are the other two things in physics that should prove this point.

[@-_Nuke_-]

"It drives me crazy when people say never" reffering to us never being able to reach the speed of light or go beyond it! I too had the same reaction when I first learned special and general relativity. In this text bellow I will explain why this is indeed impossible! Lets start: ------------------------------------------------------------------------------------------------------------------------------------------------------------------ No matter which theory of relativity we use (The two most famous ones are Galilean and Einstein’s relativity). We always need a way to *preserve causality* That means -> that we need a way to be able to put EVENTS in causal order (we need to know which event caused which other event and at what order.) To do that, we always need to have some quantity that is absolute. For Galilean and Einstein’s relativity, those quantities are a certain specific speeds. For Galilean relativity, our speed through time is that quantity. Its absolute. Which means that is 1) invariant and 2) constant. 1) Invariant means, that the value of the speed doesn’t depend on any observer or their own speeds. 2) Constant means that its value can’t change. It’s a set value and there is nothing we can do to change that. For Galilean relativity, that quantity - our speed through time - is therefore constant and invariant and equal to 1s/s. Which means, that every object is aging at 1 second per second no matter who is observing or what their speeds are. That also means that there is nothing you can do, to travel in time faster than 1 second every second! (That by the way, shows us that Galilean spacetime is Euclidean). NOW! For Einstein’s relativity that has changed. An objects speed through time is not absolute anymore, and its not constant and its not invariant. We know this - because we have seen that our spacetime is not Euclidean as we once thought. Its hyperbolic and its depicted by the Minkowski metric. So the nature of spacetime - being hyperbolic - makes our speed through time to not be constant and or invariant as we once thought! So now we need a new relativity, Einstien’s relativity, which does 2 things. 1) Preserves causality. And 2) has speeds through time that are not constant or invariant anymore. So how can we do that? Well, Einstein comes up with a new quantity that’s absolute that does those 2 things. That new quantity is our speed through spacetime. And for Einstein - its absolute. Which means that its 1) constant 2) invariant and equal to 1ls/s (one light-second per second). *That means, that every object moves through spacetime at the speed of 1ls/s which is the speed of light* So… An object can move in spacetime with ONLY one speed. The speed of light. Me, you and everything is right now, moving in spacetime AT the speed of light relative to nothing. We don’t need relativity to say that, because the speed of light ISNT relative - its absolute, and its equal to 1ls/s. So now, just like you couldn't age faster than 1second per second in Galilean relativity. Here you can't travel in spacetime faster than 1 lightsecond per second. Just like it didn't make sense for an object to age faster in Galilean relativity, here again its meaningless to travel faster than 1ls/s in spacetime. Why? Well, lets take a look what means to travel at a certain speed in space. Since our speed in spacetime is the speed of light... That speed in spacetime is always the combination of our speed through space and our speed through time. If our speed through space is C then our speed through time is 0. And if our speed through time is C then our speed through space is 0. The 1st where our speed through space is C, is the case for light. All of its speed in spacetime is allocated in just space, and time doesn't pass for it. The 2nd case, where all our speed is in time, is freefall! Because that's what freefall is! To stay stationary in space and age at the speed of light. Anything in between is the speeds that everyday objects have. Some of it will be in space some of it will be in time but when combined they will give us C as a net result. That means! That the speed of light IS effectively infinity. At least for a hyperbolic geometry that is. Becuase when you travel close to the speed of light, time for you passes very slowly (your speed in time is very small). Which means that you can reach very far away distances withought time passing for you... So you can reach a 1light year away star in just 1 week because you didn't experince time. If you go closer to the speed of light in space still - then you can reach that 1 lightyear way star in 1 day! Closer? 1 hour... Closer? 1 second... Etc... So when you go AT exactly the speed of light in space, your speed in time is zero. So you can reach any position in space infinitely fast (in zero time). (For you at least...). So to answer you initial question: You can't go faster than C, because you can't go faster than infinity.

[@Benson_aka_devils_advocate_88]

@@-_Nuke_- Trust me, I understand the theory behind the why. As Neil deGrasse Tyson put it, you can't undo what's already been proven. However, there is a lot we still don't understand about the universe. A vast chunk of the predicted mass and energy in the universe doesn't line up with what we see. What that means is for laureates to discuss and ponder as students earn their PhDs. So again, to stick to this notion that what we know is absolute is absurd. Far too many times a person has come along and upended our understanding of our world around us. FTL as we know it isn't possible. But there is sooo much we do not know...

[@barryon8706]

@@Benson_aka_devils_advocate_88 If special relativity is supplanted by something in the same way that special relativity supplanted Newtonian mechanics, then sure. But you should also realize that a lot of the things people said we'd never do -- get to the moon, fly heavier than air craft, etc. -- were thought intractable engineering problems, not violations of established natural laws.

[@JonasWeezer]

Can you please tell me how the arrival times are measured how do you filter the light precisely? Also if you don't know the source of the gamma ray bursts how do you know how far away they are?

[@DrDeuteron]

I think the distance to GRBs are taken from the galaxy in which they occur.

[@hogg4229]

It’s nuts is what it is. Tells you that there is so much we just don’t understand. It’s wild.

[@billyjunior1917]

I seem to remember you pointing out that we lose a small number of stars every day. Not because they have moved outside some mystical area but that they are now traveling faster than the light they emit. But perhaps I misunderstood? Maybe that was before the JWST, but I hadn't heard anyone say "Hey, ya know what? We just found them stars we lost!

[@DrDeuteron]

The most distant thing we can see in EM is the CMB, and that will always be the case. The stars we a losing now are made from that stuff (plus or minus some distance, idk what that is), so there’s kind of a time lag at play

[@yeeil]

From the point of view of an electron moving at light speed the speed of the photon is zero and the detector is moving at light speed. So it's more accurate to say the speed of light is independent of the speed of the matter which is emitting it because it doesn't matter if the electron is traveling at twenty percent the speed of light or at the speed of light, the photon will travel at the speed of light.

[@chunfuxu]

In the electron/photon experiment, what if you move the detector towards the photon? Will you detect the photon sooner than a stationary detector does?

[@gomezmezcalero]

you are so cool thanks to make easy to understand

[@spook_dad]

at 6:11 you show a measurement of the one way speed of light, I was led to believe this had never been done as we can not maintain 2 synchronised clocks

[@jameskinyuru8076]

The speed of light is relative rather than absolute, we assume that is the speed of light because that's what our equipments can measure.

[@JeroHitsukami]

Putting vibration sensitive equipment underground would actually make it worse if anything made a sound. Sound travels faster through denser materials you even stated that.

[@mattmadden2501]

So I've always wondered. If you were traveling just 1 mph below the speed of light , and you take a baseball and bat that are traveling with you thru space at 1 mph below the speed of light. What would happen if you hit that baseball with the bat in the direction you are moving? Because from your vantage point you and the ball would be at rest but physically you and the ball are both moving thru space at just below the speed of light. So would the ball exceed the speed of light or would it just not move and shatter the baseball bat?

[@AMC2283]

To the stationary observer it might travel at .1 mph less than c. To you, 100mph because of length contraction and time dilation.

[@kitmoore9969]

@@AMC2283 To you, 100mph; to the stationary observer it might travel at .1 mph less than c because of length contraction and time dilation.

[@kokomokid4006]

Didn't understand anything you said...but you seem to be a nice photonic reflection of a mass of quantum field fluctuations.

[@waywardtycoon]

The velocity of light is not relative, space and time are. The velocity of light isn't just how fast light goes, it's a number somehow woven into reality itself - that is, space and time organize themselves around "c". Einstein commented that "light was an illusion".

[@rgc121044]

On the front of an electromagnetic wave an energy front travels also and it has its equivalent relativistic mass; hence it curves space. That is, it generates gravitational waves that propagate at the speed of light, the same speed at which the EM wave that curves space travels. But the fabric of the universe has inertia that opposes its curvature, it is an opposition to its displacement that faces light and that determines the speed of light (or gravitational wave).

[@rayjasmantas9609]

The speed in light is a energy clearance to empty space. An energy that allows the properties to being absorbed, thus having a natural strength, which could be said a force. It would thus being influencing added energy levels to empty space itself.

[@leesweets4110]

26:45 Okay, but why was there any difference at all?

[@The1stDukeDroklar]

There is a problem with the example of a particle accelerator and the speed of light to observers. The particle is not conscious and does not consciously experience time. What I mean is that the speed of light is a certain distance within a certain amount of time. That means that to the conscious observer traveling at the speed of light, the light emitted from a flashlight he holds pointed in the direction of travel, would appear to stand still. That light cannot move faster than the speed of light and since he is already moving at the speed of light, to him, it would appear stationary. To stationary observers outside the ship and looking in (not possible but), the light and the person within the ship would both appear to be moving at the speed of light.

[@kylelochlann5053]

A conscious observer cannot move at the speed of light. And even so, would still measure the speed of light to be c.

[@sandrajayne5446]

Video exploded my brain but I😊 loved it.

[@RandyFinch]

Great video. I love his presentation style. Just one question related to the electron emitting a photon. How can the speed of light in the electron's frame of reference be 300,000 km/s when the photon is traveling alongside the electron at the same speed as the electron?

[@utee72]

When the photon is emitted it starts at the speed of light since it IS light. The electron loses energy proportional to the emitted light frequency and thus it loses speed as well. The energy in must always equal energy out if ALL the components are properly accounted for.

[@RandyFinch]

@@utee72 Thanks. So what about this instead. Two photons are traveling alongside each other. Do they each see each other moving at the speed of light?

[@utee72]

They can interfere with each other but that interference will eventually decline to noise level as they diverge I'm the vastness of space. Multiple photons beyond just two can also interfere with each other and that can get really complicated but they will eventually diverge in free space. Notice that in fibre optics communication channels there can be a lot of interference in such a restricted channel and thus different frequencies are used to reduce the impact as long as they don't lose too much amplitude and thus become too difficult to decipher. Optical repeaters are used to address these kinds of problems in fibre optics communications. Interestingly the math always involved various methods in trigonometry since sine waves are always involved eg A*sin(wt + P) provides three different signalling mechanisms eg A, w=frequency and P = phase all of which can be modulated to encode data.

[@RandyFinch]

@@utee72 I don't understand what that has to do with each photon seeing the other going the speed of light in their own frame of reference.

[@utee72]

@@RandyFinch They don't "see" each other they just use the same free space medium to oscillate at their frequency thru the same medium eg electric and magnetic. Think of two rocks thrown into a pond such that their wave action interferes with each other until they due out. That's what happens in free space until a photon of light encounters something eg dust? And decoheres thus transferring it's energy to said particle.

[@YouKnow-re6rw]

In reference to GRB 090510: Seems intuitive to me (though I don't know what I'm talking about). ~Same Oregon, ~Same emission vector, same speed, different paths= different lensing from the Photonic aspect of the lens effect. If this is a thing please point me in the right direction.