Why do faster than light signals break spacetime?

[@Mahesh_Shenoy]

To try everything Brilliant has to offer-free-for a full 30 days, visit brilliant.org/FloatHeadPhysics . You’ll also get 20% off an annual premium subscription.

[@a_lgaming3368]

first

[@SurajgupthaMuppa-vn9sp]

Sir,unable to get 30 days free trail even after clicking the link.

[@classicalmechanic8914]

Space time diagrames use 45° angle for light only as CONVENTION. If you decide angle for light is 0°, light particles would not travel back in time but travel in present and wouldn't experience time. Misconception of this video is it suggests faster than light particles would travel back in time. Retarded action is the reason why physics cannot make progress, since relativity suggests every particle travelling faster than light travel into the past. But the truth is faster than light particles travel into the present from the perspective of the source, they don't actually travel back in time. Relativity of simultaneity opens a possibility for faster than light propagation, since synchronization convention prevents you from measuring one way speed of light.

[@classicalmechanic8914]

@TriTr-qd2bd If there was no speed of light, universe would look the same but eveything would happen all at once. Speed of light is actually speed of causality, which suggest c is round trip distance divided by time.

[@cesarblsjr]

nah. this episode was way too forced. in real life faster than light travel dont cause time travel.. it just shortens time and the people you used in the example dont know about FTL and cant do proper calculations.

[@abebuckingham8198]

If you don't understand the shirt in calculus if we consider x to be position and t to be time then the rate of change of the position over time is called dx/dt. The rate of change of velocity is acceleration and so it's d^2x/dt^2. The rate of change of acceleration is d^3x/dt^3 and the name for that is jerk. So the shirt says "Don't be a jerk".

[@anonymes2884]

And bonus fun fact for cereal fans: the 4th, 5th and 6th derivatives are called snap, crackle and pop.

[@Bildgesmythe]

Thanks

[@gamingversatile6617]

bro i legit came here to explain the same thing but u beat me to it. kudos!

[@rioowosama1545]

Thanku I'm always curious about his shirts what they mean sometimes when I didn't get it and someone explain it in the comment section it's always heaven to my heart 💝 😊😌😌😌

[@rajanvenkatesh]

Many videos ago, you said 'speed of light is actually speed of causality'. With every fresh video, that is becoming clearer and clearer. Thanks!

[@112313]

I would say it is the speed of PERCEIVED causality.

[@timothynorman962]

c as in causality

[@AlekThunder47]

"What will it be?" bartender asks. Tachyon walks into a bar.

[@Mahesh_Shenoy]

Causality has left the room

[@kaleijuka8532]

​@Mahesh_Shenoy bomb moves backwards from bomb or the event occurs chronologically in reverse?

[@Mahesh_Shenoy]

What’s the difference between the two?

[@sileightynz5274]

Entropy

[@skasev]

Well not really, just seems like it to the near speed of light observer

[@Life-my9tl]

How is the causality broken in any of the cases discussed? As you said, what you see is not what is happening. So, even if the explosion is observed to happen before the missile is being launched. In reality, the effect is still following the cause. For example, we see lightning before the thunder. But anyhow the thunder occurred before the lightning. So, even if we are seeing the causality to break just because of seeing light signals in wrong order, that does not mean that the events have also occurred in wrong order. So the causality should not be broken even if the missile is travelling faster than light. Consequently, the argument that causality will break if an object travels faster than light should not stand. As an analogy, a supersonic aircraft travels faster than the speed of sound resulting in different effects without breaking the causality. You explain well, in a very simple and entertaining way. Thank you, for sharing. Keep educating us.

[@sonofcronos7831]

The lighting example dont work because one event is not causing the other. The sound and the light comes from the same event, but is not one that is leading to the other. And examples using sound waves also dont work because sound uses air as a medium. Most experients bases itself in a vacuum. But sound not travel in a vacuum.

[@galaive]

@life-my9tl I was wondering the same thing! I wrote a comment wondering if you redid the thought experiment, but with a supersonic missile and observers LISTENING for these events, would the causality also be broken? And @sonofcronos7831 I think it’s ok to just add air to the thought experiment so that sound can propagate, or assume sound is also an EM wave for the sake of thought experiment

[@terra_creeper]

It's not a real explanation, but I think a good way to think about it is like this: You are always constantly travelling at the speed of light. But that speed is distributed between time and space. The faster you move through space, the slower you move through time and vice versa. But no matter what the distribution is, both speeds must sum to lightspeed. If you then want to travel through space faster than light, while the sum stays fixed, you have to have a negative speed through time.

[@Mahesh_Shenoy]

In the spaceship’s frame, the explosion did happen before launching the FTL missile. Check that section of the video again :)

[@musthaf9]

Supersonic travel doesn’t contract space, so it’s not an appropriate analogy. This weird concept is happening because traveling at the speed of light is doing a weird effect on spacetime. Any other speed can’t be used as an analogy

[@cdamus]

Hands down the best relativity physics content on KZbin. Your approach of leading the audience to discover the meaning of each concept for themselves with the help of animations and Socratic dialogue is wonderful. A superb teacher.

[@EscanorAbd]

I wish you SHOWED us at 22:14 how causality is broken when the fast observer send FTL message to stop the bomb. That was the most important part of the entire visualization of events

[@ricfwolff]

Missed that too

[@morticias5043]

Yup simply observation will not do anything

[@bluzfiddler1]

Even if he had some instantaneous transmission device, his trigger would be the reception of the light signal. This, by nature, would have taken a year (relative to the ship) so the signal would arrive at the exact moment the launch signal arrived at the second ship. Still not breaking causality.

[@nickwalden6425]

The blue ships trigger is the explosion. The blue ship is right next to explosion when it happens, so the time it takes for the light signal of the explosion to reach the blue ship is negligible. If they send an instant/ftl/faster than missile signal to the destroyer, that’s the paradox. Rewatch starting at 20:00, with key points at 22:00

[@stylis666]

@@nickwalden6425 It would seem so, but it isn't, is it. No matter when you send the signal and how fast you send it, the missile was already fired from the firing ship's perspective and the moon has already been blown up by the missile before any signal reaches the blue ship. Here's my idea of what actually happens: Ship a fires an FTL missile, the missile blows up the moon, ship b sees the moon blown up, ship b sends an FTL signal to ship a, ship a can receive this signal anywhen between the moment that the explosion was seen by ship b and infinite time from now in the future, depending on which direction and speed ship a has in relation to ship b's message signal. From ship b's perspective their message will reach ship a in the future, after they see the moon explode and the ship firing the missile, and not in the past. The light from ship a at the time when it receives the missile will reach ship b waaaaaaaay after it has all happened. There is no breach of causality. It's only that some observers will not be able to calculate the correct order of events without additional maths that corrects for the incorrectly observed time that passes for each event from their perspective. Some events, like the path the missile traverses, will seem to go backwards, but in reality they don't, they just go really really fast. To actually break causality you'd have to find a way to send the signal to a time before the missile was fired and that didn't happen, and wouldn't, no matter the speed. Even at infinite speed you'd still be stuck with the present. We have to remember that we can see the past because light takes time to get here. Things in the past have already happened, regardless of the time or speed with which our information is updated with the events.

[@binbots]

General relativity and quantum mechanics will never be combined until we realize that each individual observer is observing them both at different moments in time. Because causality has a speed limit (c) every point in space where one observes it from will be the closest to the present moment. When one looks out into the universe they see the past which is made of particles (GR). When one tries to look at smaller and smaller sizes and distances, they are actually looking closer and closer to the present moment (QM). The wave property of particles appears when we start trying to predict the future of that particle. It is a probability wave because the future is probabilistic. Wave function collapse is what we perceive as the present moment and is what divides the past from the future. GR is making measurements in the observed past and therefore, predictable. It can predict the future but only from information collected from the past. QM is attempting to make measurements of the unobserved future and therefore, unpredictable. Only once a particle interacts with the present moment does it become predictable. This is an observational interpretation of the mathematics we currently use based on the limited perspective we have with the experiments we choose to observe the universe with.

[@parthhooda3713]

Ain't reading all that

[@kenten]

Read all of it. Makes perfect sense. Thank you for writing it!

[@aster2790]

That would mean that we can predict the future of a particle if we look at it from far away, but that's not true as far as I know

[@ckpioo]

your first point what it basically says is that whenever you look into your past lightcone, you see particles and when you try to derive a outcome of the future lightcone by observing the past lightcone, then particles behave like waves?

[@binbots]

@@aster2790 celestial objects are far away and made of particles and we can predict their motion.

[@sharmanraval7041]

i have to admit your are really smooth with the promos

[@Mahesh_Shenoy]

Haha, thanks!

[@pwinsider007]

​​@@Mahesh_Shenoy breaking of causality is not a paradox but an usual phenomenon.rocket's light will travel slower than rocket therefore we will see that rocket hasn't hit anything but in reality rocket would already have smashed into the object and the light of the moment when rocket hit the object will take time to reach us therefore we will see destroyed object first then we will see rocket smashing into object.

[@h14hc124]

@@pwinsider007 What you've described was the first scenario, with the astronaut right next to the planet - there's an illusion that makes it look like it happened in reverse, but it actually didn't. The second scenario, with the near-light-speed space ship passing the planet at the time of impact shows that for some observers, the events *actually* happen in reverse, it's not just an illusion that makes it look that way.

[@pujamathssolution9906]

Relativity is seriously a amazing topic to talk to the people's who likes it

[@malemsana_only]

Yeah, its great if you have friends that share same intrest

[@asahmosskmf4639]

it is interesting their finding that, technically its possible to look back in time. but the idea is nothing like back to the future movie or anything... i mean deterioration of the universe, rotting, aging, ( whatever you call it.. ) - could go slightly backwards just walking around. but in our eyes this would be like 1 in 1000th of a second, i mean you wouldnt even notice it. you couldnt even do the dejavu cat from the matrix. and its a 1 in billion possibility in every day life...

[@chrisoakey9841]

Our observation of reality, and reality aren't the same thing. Models need to remember that perception and reality are not the same thing.

[@malemsana_only]

@@chrisoakey9841 objective reality don't exist tough, atleast we can't see.

[@chrisoakey9841]

@@malemsana_only we cant see, but in general dont need to as the stuff that affects us enough to make a difference are seeable. we dont worry about the pull of gravity from proxima because it is insignificant. but models like general relativity are fine until we extrapolate concepts like space compression etc because of taking the model of our observation and suggesting that we therefore know... which results in idiotic things like the expanding universe, dark matter and dark energy and twin paradoxes etc.

[@igorbondarev5226]

Before this video I didn't understand what the problem with seeing things backwards is, now thanks to the faster than light signal "don't shoot!" I understand. Bravo, as usual! Event circles is also a good depiction

[@EscanorAbd]

I wanted to see how the faster than light "don't shoot" signal traveling, he said it will arrive before the light of the moment they "shot" the bomb, but how though? I wish he showed us instead of just saying it does

[@igorbondarev5226]

@@EscanorAbd You can imagine it going arbitrarily quickly, or even instantly, after the "boom" detection as the animation plays.

[@DrDeuteron]

Closed timelike loops

[@juliavixen176]

​@@EscanorAbdOn a regular 1D+1D Minkowski spacetime diagram, two inertial observers at physically distant locations in space, usually get drawn as parallel vertical lines... But... The "same time" for each of those observers are connected with 45° diagonal lines. (It's not a horizontal displacement on the graph.) To shift from one observer's coordinate system to the other, you slide the parallel lines up and down (in time) so that points intersecting on the same 45° diagonal line, will be moved to match on the diagonal line perpendicular to the first one (i.e. -45° or 135°) Faster than light signals will intersect with the "past" of each observer's vertical line after transforming to the other observer's coordinate system. (This happens _both ways_ symmetrically.)

[@112313]

If the boom is the triggering event to send a signal to stop the boom, then it is irrelevant because by virtue of the boom happening, the firing had already happened. Does sending a signal back to the destroyer to tell them to stop firing erases the boom from happening? Of course not. Therefore, causality is maintained.

[@93thelema777]

A simple way to rework this is to imagine the default refresh rate of the universe is C (Light speed) so if something could move faster than the speed of light it wouldn't be drawn properly . It might look something like a laterally, directionally stretched object that flashes in time/space cycles as it moves through large areas of space and if you were to cut out all the gaps when it wasn't visible it would seem to be moving at C , but when you add the dark gaps in it's illumination you can deduce how much faster than C it's going . If time stands still at the speed of light then moving closer and closer to C would be like reducing the frame-rate until it's approaching zero frames a second which would be invisible . A simple way to think of it is how cameras make wheels going a certain speed start to appear to turn backwards . If you had an infinately powerful camera and you wanted to reduces the movement of light to a completely still image when reduced back to 24 frames a second , the best you could ever acheive is smaller and smaller fractions of a frame , which is why it would take infinite power to acheive 1c . But if you could go from 0c to 1c without accelerating , then you should be able to go over C . But it's just possible that going C+ looks like a ghostly still image beaming in and out of space in such a flash you might not see it if was right infront of your computer screen . Anything visible would be reduced to the same laws as seeing something move at lightspeed because it would be visual abberations of C speed photons being disturbed by a partially drawn mass . Maybe it would look more like a streched out collection of flickering entangled point particles . Maybe faster than light travel has an embedded quantum probability mechanic . Not really something I've given a lot of thought . Fun to imagine though . Anyway , just because you see an effect before a cause doesn't mean it actually happened that way . Could be little difference between that and using different speed communication devices to hear an answer before a question - it doesn't mean you have the ability from your perspective to get an answer before asking a question .

[@linuxp00]

That's How I think about, and that also, maybe things move at discrete steps (yet really small ones) like a Planck's length. Because of light speed is limited and a field can't transmit information to all particles simultaniously (even though, entanglement effects could happen between bunch of particles, that wouldn't change the overall perspective for a macroscopic observation, so we could ignore it, if things go at speeds lesser than C).

[@x.s100x]

I love you bro before watching your videos i hated physics but now i love it more than anything

[@devinfaux6987]

There's a couple things about this sort of thing I've found fascinating for a while now. First is that if there was a stationary observer sitting somewhere between the destroyer and the moon, when the FTL missile passed them they would get the optical equivalent of a sonic boom. They would see the image of the missile appear out of nowhere at the point of its closest approach, then *split in two.* One image would race forwards towards the moon, the other backwards towards the destroyer. Like the astronaut they could do the math later and work out the order of events, but I still find it neat. Second is that there's a relationship between the speeds of the spaceship and the FTL missile in order for causality to break. If the ship isn't traveling close enough to lightspeed, it won't see causality break. Similarly, if the missile isn't travel as far above lightspeed -- let's say, only two or three times lightspeed instead of four -- the spaceship won't see causality break. As demonstrated, at exactly the right combination of speeds the spaceship sees it all happen simultaneously. I don't know the math well enough to figure this out exactly, but I have a hunch it's something close to an inverse relationship between the speed of the missile and the time dilation/length contraction observed by the spaceship. It's not the raw speed of the spaceship because the relativistic effects don't scale linearly; you don't get 50% time dilation/length contraction at 50% of lightspeed, you get it at about 86.6% of lightspeed. So for a spaceship observing 10% time dilation/length contraction (41.7% lightspeed), you would only start to see causality break from things traveling more than ten times faster than light. At 20% TD/LC (55% lightspeed) you'd see it break for things above five times faster. At 50% TD/LC it would break for anything above two times lightspeed.

[@juliavixen176]

You can break causality with any signal velocity greater than light and a much clearer demonstration of this is to do a round-trip journey from "Location A" to "Location B" and then back to "Location A" again. If the trip is done faster than light [FTL] it will arrive at its destination "Location A" *_BEFORE_* it departed from "Location A". I was hoping that this video would demonstrate this case, but it didn't.

[@DrDeuteron]

The math is a line, y = Mx + b, so you can do it. For a launch at t,X = 0,0 in years, light years, and an impact at (1/v, 1) where v is the missile speed. For a rocket ship going u and launch it 0,0. The hit occurs at t’ = gamma(1/v - u), so Lorentz contraction and time dilation are irrelevant, but the break point is indeed inverse u > c^2 / v

[@vichav3167]

@@juliavixen176 it’s from pov of B. From pov A sequence is normal. You can’t see spacecraft coming at point b from pov b, but once it arrives, images of it’s travel will appear like moving backward, and then you’ll see launch from point A. And if before that spacecraft launches from B to A, from perspective of B, that didn’t see launching yet, it will seems like spacecraft will return before it was launched. But when light reaches B all sequences will be in order. From Pov B the’ll see two spacecrafts flight towards A. One of them moving backwards, and another moving forward. But they reach A with same delay as between arrival at B and departure. And as far as I understand, we don’t really understand what means (-dt)^(1/2) (result of v > c). Maybe it’s just limit of theory, or maybe time travel in some way. If it’s later, than causality can be broken, but it’s likely former. It can be considered as time travel in a way. Imagine B observing caveman on A in far system, and suddenly those ”caveman” arrive to B on FTL spacecraft.

[@juliavixen176]

@vichav3167 Location A and Location B _are both in each other's past_ symmetrically. The FTL object/signal arrives in the past of the other location _each way_ A round trip puts the FTL thing in *everyone's past* including the original location where it started. In Special Relativity, time *is* space. Every location in space is a location in time, and every location in space is in the past of every other location in space. (The use of " _i_ " on the time coordinate is a mathematical way to deal with this.) When you look with your eyes, in a straight line from the tip of your nose out into distant space, what you are currently seeing _right now_ is the past. The straight line distance away from you in space is the 45° line on a Minkowski spacetime diagram. Everything you see and interact with *_right now_* is on this 4D light cone. Anything not on this light cone is not happening to you _right now_ That's time; time is the radial distance in a "straight line" away from you. Velocity is just the conversion factor between two observers of how much of spacetime to label "space" and how much to label "time" for each other... because all inertial observers are at rest with respect to themselves and their clock always ticks at one second per second.

[@MichaelPiz]

The more I learn about light speed, relativity, FTL, etc, the more intuitive my understanding becomes. I followed this video easily! I'm also reading _Faster than Light_ by Robert Nemiroff, which is also helping a lot.

[@astrokevin92]

Well done. I really liked the nested cause and effect circles. Great way of looking at this.

[@paraax]

The circles are fine, but if you've invented ftl then you have sped up the cause circle. Pretending that the speed of light circle is the cause circle doesn't get to the core paradox, the claim that you could get a signal back to the cause before it happened given you have observed the effect.

[@josephbenjamin6426]

Yo!! You could have LEAD with the circles diagram FIRST! That was the CLEAREST I’ve ever seen this explained! 👏🏽👏🏽👏🏽

[@ImposterMalone]

First Video I see and I'm mainly disappointed because you're not just a floating head explaining physics.

[@JacobAbraham-twozerosix]

Insane.... Your explanations are traveling FTSL... I feel the effects even before you start explaining...

[@catastrophe3049]

Ek hi to dil.hai mahesh bhai Kitni baar jeetoge

[@Mahesh_Shenoy]

Par yeh dil maange more…Ahaaa!

[@ParM-s5v]

MAHESH SIR YOU HAVE TO SEE THIS! Hello Mahesh Sir, I’ve been exploring the concepts you discussed in your video about FTL and causality, and I came up with a thought experiment that I believe could allow for FTL travel without violating relativity. In your video, you assume that the missile firing is the cause, and the moon's destruction is the effect. However, in my thought experiment, I propose that the missile hitting the moon is the cause, and the moon's destruction is the effect. By redefining the cause and effect this way, causality can still be preserved. Additionally, I consider the impact of length contraction. As objects near the speed of light undergo significant length contraction, the light would still reach them at the speed of light in their reference frame. This keeps relativity intact while allowing FTL travel in a way that avoids causality violations, as light's speed would remain constant from all perspectives. I really admire your work and wanted to share this idea with you. I’m only 14, but I hope this thought experiment could add something new to the discussion! THANK YOU FOR YOUR TIME!

[@jpe1]

I have a suggestion for your next t-shirt: a graphic with three cartoon characters eating a puffed rice breakfast cereal, each character labeled d^4x/dt^4, d^5x/dt^5, and of course d^6x/dt^6

[@astrokevin92]

Took me a moment.

[@jpe1]

@@astrokevin92 glad someone did 😉

[@akshatpratapsingh5476]

snap, crackle and pop .....xD

[@CastorQuinn]

I love this idea that the cause signal remains contained within the effect signal even under transformation for signals less than or at the speed of light. That is a fantastic way to arrive at the relationship between reference frames without calling on any maths. I'm going to watch this a few times to really bed down this representation.

[@DrDeuteron]

also watch some animations of Minkowski diagrams (where the expanding circle here is repented by the light cones' X....it never moves, while the (t, x) axises flip flop around, that is: all references frame agree the effect envelope is a sphere expanding at the speed of light.

[@madlep]

The missile knows when it is in all reference frames. It knows this because it knows when it isn’t.

[@bluzfiddler1]

Someone was a missileer

[@djayjp]

Sabine Hossenfelder stated that FTL doesn't actually result in backwards causation because the paradox is due to only solving for SR, not GR, and the paradox goes away in the case of the latter.

[@varunshah4971]

The transformations on the cause and effect loop, the length contractions are being made according to special relativity, which assumes the speed of light to be the limit. So using special relativity to say that faster than light travel doesn't exist while using it on a case where faster than speed of light travel occurs doesn't make sense to me.

[@sonofcronos7831]

No. Is exactly because faster than light breaks casuality that we know that nothing can travel faster than light, because one of the laws of physics is the law of casuality.

[@juliavixen176]

There's a much better demonstration, that wasn't covered in this video, of taking a round-trip voyage faster than light and arriving at where you started *BEFORE* you left. I was hoping that this would be in the video.

[@YeOldeBelmont]

I love the energy you have while explaining things!

[@mgostIH]

There's a Sabine Hossenfelder video about ftl not necessarily breaking causality, what do you think of it?

[@PerryNguyen]

I think that's somewhat clickbait, but at the same time, because there is not a combined theorem of GR and QM. We can't be *absolutely* certain.

[@Rudyard_Stripling]

She is a failed physicist but she is funny.

[@vishnu_m]

Thank you! I've been waiting soo long to find video explaining this in simple way and since I found your channel I was hoping that one day you will touch this subject. Big thanks!

[@stylis666]

4:08 Hang on a second. If she sees both the explosion and the missile being launched at the same time, wouldn't she also see the entire path of the missile covered with the missile? In other words, wouldn't she also see a lightyear long missile from the ship to the moon?

[@StickManShortsofficial007]

Bro, sleep. 😢

[@stylis666]

@@StickManShortsofficial007 **sleeps** Hey, you're really good at that hypnosis thing! Or is it just that you're really boring and you have no depth aside from a very thin layer of toxicity

[@domvasta]

Yes

[@SecondAccount-eo4oj]

Even if I didn't understand, just by your reaction and joyfulness makes the video fun❤

[@Mahesh_Shenoy]

I don’t think I explained this well!!

[@TheSmokingLizardSWE]

I don´t understand why it matters that causality is broken to observers as long as its not broken from the cause and effects "reality" as observed by them. Even if something as in the example is launched 4xFTL in a 1LY distance, the message from an FTL observer to the effect would still reach the cause from the cause point of view after it has acted no matter how fast the message was transmitted.

[@bfsobnfs]

I've been binging physics content for 2 days because I have the same issue. If an observer is subjected to space/time dilation and causality is broken for them, why does it matter for everyone else not subject to that space/time dilation and for whom causality is not broken...

[@blesskurunai9213]

@@bfsobnfs well there are more complicated reasons like entropy or thermodynamics that could explain why casualty being broken at any point of universe will affect everything everywhere and everywhen. But for a simplified example, let's use the analogy used in this video. Say that after the spaceship saw the moon being destroyed, it decided to move in the trajectory of that missile that hasn't been (from their perspective) launched yet. And after some time they get hit by that very same missile, diverting or blocking its path. So from the perspective of the people who launched that missile, it never hit the moon. So it's not blown up. Yet from the perspective of the spaceship the moon did get blown up. So which reality is true?

[@dennisposadas882]

Intuitively understood so easily by the end; marvelous, thank you!

[@ScottAtwood]

Your shirt! “Don’t be a jerk!” 😂

[@akaHarvesteR]

I was really proud to have understood that too 😅

[@imagiro1]

22:10 Intuitively I'd say, the problem is _transitioning_ a signal between slower-than-light and faster-than-light speeds. As long as the stl-"world" and ftl-world stay cleanly separated, causality doesn't break. Goes with what I heard, that the problem is _crossing_ the speed of light, not if you are below or above. I have to think about that. However: Reeeealy great video! Me like 😁

[@anrwlias]

It took me a minute to get the joke on your shirt. Very clever.

[@jpe1]

At first I thought the shirt was saying “don’t be an accelerationist” (a sentiment I agree with!) but acceleration is second order, not third, so I was confused and stopped thinking about it. When I saw your comment I thought about it again, and remembered that third order force (jolt) is sometimes called “jerk” thus “don’t be (a) jerk”. Very good indeed! Next he needs a shirt with three characters eating breakfast cereal, each character labeled d^4x/dt^4, d^5x/dt^5, and of course d^6x/dt^6

[@abebuckingham8198]

@@jpe1 Jerk is better than jolt because when you jerk something around you're changing the acceleration but when you jolt something you're probably throwing lightning around.

[@jpe1]

@@abebuckingham8198 I don’t disagree. When I learned physics in high school (_many_ years ago) it was jolt, but it seems jerk is now the more common term. Like, back then, my dad would have said (describing my mom’s driving) “don’t jolt the transmission” but now I think the more common phrase would be “don’t jerk the car around”

[@hebruixe9125]

You're an phenomenal teacher! Einstein would be proud of you.

[@RiiDii]

You missed a critical step. We MUST change the frame of reference back to the destroyer's frame of reference when the message not to shoot is received. When we do that, there is no paradox. In your example, the moving ship observes the order of events as its message arrived before the missile was fired, but the destroyer still doesn't get the message until after from their frame of reference.

[@RiiDii]

Here's an analogy sonic experiment to demonstrate why. We have a gun pointed at a target down range. The gun has a light sensor that will lock the gun when the sensor activates. The target has a laser aimed at the gun's light sensor and will activate when a bullet hits the target. With the precision of our setup, we'll have an observer safely positioned near the target, ready to witness the sequence of events. 1) The gun fires. 2) The observer hears the bullet hit the target (I know this because I've been in this scenario). 2a) The laser fires. 3) The laser hits the gun's light sensor and locks the gun. 4) The observer hears the gunshot *BANG! Even though the observer sees the gun lock before hearing it fire, we know the gun fired before the bullet hit the target. I know folks may say, "But this is sonic, not the speed of light. They're different." Yes, but apply the same logic to the ships. If we keep the destroyer's frame of reference, they will never receive the message not to fire before they fire. You can even use instantaneous communication, like hypothetical portals, and there is no way to create a paradox.

[@MarshallForLifeOfficial]

i love how you make everything so understandable keep doing what others don't i love it!!

[@earlhaiger]

I'm thinking though, for the FTL missile: even if we see the missile's explosion's first and then the missile going backwards to the ship, if we knew the missile was FTL... can we incorporate that knowledge into our thinking and deduce that we saw the events in reverse?

[@akaHarvesteR]

I was thinking the same thing. Shouldn't the spaceship people have accounted for the fact they were themselves travelling in the same direction at relativistic speeds when they back-calculated where the missile came from? Wouldn't that account for the disparity in their view of cause and effect events? I was left with the impression that there was a missing coordinate frame transformation there.

[@juliavixen176]

There's a much better demonstration, that wasn't covered in this video, of taking a round-trip voyage faster than light and arriving at where you started *BEFORE* you left. I was hoping that this would be in the video.

[@Gedof]

@@akaHarvesteR They did. What they "see" is different than what they calculated, and they still reached the conclusion that it happened backwards. The only way for them to conclude that the cause happened first would be to assume they are not valid observers, or that the astronaut POV is more valid (remember, from their perspective she is the one traveling backwards at relativistic speeds). That also goes against relativity, because all inertial observers are valid regardless of velocity (and they all observe the speed of light to be the same). See the astronaut, she also saw the explosion first, but she could calculate it backwards and realize that the missile is FTL and was launched before the explosion without assuming she isn't a valid observer. The ship did the same and reached a different conclusion. You could do the same experiment with the destroyer and the target both traveling close to the speed of light and the ship being "stationary". They would still find that the explosion happened first (the target would still see it happening first but conclude it happened afterwards). EDIT: Just to be clear, the ship will be able to conclude that the missile was shot first from the reference frame of the destroyer or the astronaut. But since they are also a valid reference frame, you can't just do that and call the other reference frame "more correct". There is nothing that makes the ship a less valid reference frame.

[@vvc7943]

Amazing video sir ! each and every video is getting even better! waiting for the next one !

[@goloth]

It's a good video explaining how FTL can create the illusion of the violation of causality, but nothing in it shows that causality was actually violated.

[@zenastronomy]

the only problem i have with this is that to me it means therhetically we can still travel ftl. All we need to do is create a dilation contraction bubble. if an object or signal travels faster than light whilst interacting with the universe, it breaks causality sequence of events. but isn't it theoretically then possible, that if we can create a bubble that surrounds the missile, that pulls it out from time dilation and length contraction affects of the universe. we could fire a missile. once it would hit ftl speed. it would disappear like travelling into a wormhole and then only reappear once it drops below light speed and hits the moon. basically ftl travel is akin to teleporation, or similar to travelling through wormholes, or similar to how 5th dimensional objects come into out dimension and dissappear from our dimension. from the ships perspective, all they would see would be a missile suddenly pop into existence and hit the moon. they won't know what happened until months later when they'd see a missile being fired and disappearing from existence. so they wouldn't be able to send a message to the cruiser to not fire the missile. so causality would not be broken. and we could still have ftl travel. all it would mean is that ftl travel involves a type of teleporation / 5th dimensional movement of going out of our 4 dimensions and then coming back in. So that causality can never be broken through the time dilation length contraction of other observers.

[@DJ_Force]

These are some of the best physics explanations there are. Period. Also, what does the shirt mean?

[@112313]

How every observer observes some event does not change how an event happened. Using the faster than light weapons you mentioned, objectively, the missile would be launched, before the impact...regardless of how other observers perceived it. What you just shown does not mean causality is broken, and thus it meaning faster than light is possible.... If, hypothetically the spaceship were to detect such an event, we can conclusively proof that faster than light travel is 100% possible. The final example about a signal being sent back to the destroyer to tell them not to fire, from the destroyer's perspective....the signal should be received after they fire. Reality is reality. Something causes, something happens. Just because one sees it differently doesn't makes impossible. Even if the hypothetical weapons is an instantaneous weapons with zero travel time (infinity speed), the moment the weapon is launched, it already hit. Even if the observer spaceship is travelling at the speed of light at the target, and saw the boom, from their perspective, the boom happened, then the light of the launch arrives...so, whatever fancy reconstruction of the event from their perspective is irrelevant. The spaceship's signal to the destroyer would've been red shifted to heck.

[@TheCruisinCrew]

Exactly... I've been arguing this here ad nauseum... good to see that there are at least a few people left here that can think logically!

[@siddharthrana9216]

When an object travels faster than the speed of light (or the speed of causality) it surpasses photons, thus after reaching at the destination, the effect would get hit by those photons (which were lagged behind, due to faster travel), thus revealing the effect first than the cause. An observer would see a "delayed" future of an object travelling at the speed of light (or causality). That's my take on the faster than light travel. PS: I haven't yet watched the video, this is my initial understanding over this topic. However, I will be watching the video, for my future.

[@nickwalden6425]

When we are in the original scenario, we looked at the length contraction from the missile POV. However, when looking at the FTL missile, you completely skip that step. I understand that the length contraction would make the distance imaginary, but it still seems like an important part of why things break at FTL.

[@MARKZ137]

I have seen variations of this thought experiment several times, yet it makes no sense to me. Imagine if FTL message is sent along with the missile informing about sending the missile. Even if you send FTL message to them, that they should not fire the missile afterwards, they will just answer that they have already fired the missile (and informed us about that). As we can see, the problem in these simulations is in confusing Speed of Light and Speed of Causalty. Both are exactly c, so it does not matter normally. But once you start sending FTL objects, then either you make Speed of Light < Speed of Causality, or Causality is broken directly by this very act. The trick at the end of the video does not really work - it only proves what we already know, that we can send FTL signal (in this example). Interestingly, this example does not have to break relativity; Once you decouple Light from Causality (replace Speed of Light with Speed of Causality in 2nd Einstein's postulate), for example making Light 4x slower, you can move (little less than) 4x faster than Light. Just like Sound is something like 10^6-times slower then Light and does not serve as barrier for Causality and medium for Relativity, Light will no longer be able to serve in the same function and be instead replaced with whatever medium you use to send these FTL signals (missiles, maybe? :D). When you then compare these points of view using this FTL medium, cause and effect is very much preserved.

[@geneticjen9312]

Some people will turn off when it's about FTL that won't happen but the relativity of simultaneity means similar things can really happen, where A comes before B for one observer and B before A for another

[@paraax]

The real paradox occurs only if you can send a signal back in time.

[@juliavixen176]

This video is correct about FTL signals reversing cause and effect, although the presentation kinda burries the actual reason in a bunch of descriptive stories. It's a bit easier to understand the problem with "FTL anything" by analyzing a FTL round-trip journey between two distant locations in space. To make a long story short, observers at each location will see the FTL thingy come from the distant location's own past... _Both_ ways! If something goes back and forth FTL several times, it will time travel further and further backwards in time each trip. I'll write out a long explanation of this if anyone here wants to read it.

[@thenippletwister3457]

I'd love to hear the explanation

[@juliavixen176]

@mistersadfaceman4257 Woo-hoo! I should have prepared some text beforehand, but I'll try to summarize the important things. (I'm in a hurry at the moment and need to be doing other stuff than writing right now. ) Things to know about Special Relativity: • Every location in space is also a location in time. • When you look in a straight line directly in front of your nose to the distant stars, everything you see and feel and can effect you in any way *_right now_* is the 45° surface of hyperspace "light cone" on a 4D Minkowski spacetime diagram. (Like slicing a 3D cone into circles, you are looking at concentric spheres centered on your eyes, and spheres are slices of a 4D hypercone. It's a 45° line on the regular 2D diagram everyone draws in books and videos. ) • What you see *_right now_* is the past of everything everywhere else in the universe. • Everything everywhere at every location in space is also located in the past of every other location in space. Your feet are six nanoseconds in the past from your head and your head is six nanoseconds in the past from your feet. The Moon is 1.2s in the past of Earth, and Earth is 1.2s in the past of the Moon. The Earth is ~600s in the past from the Sun, and the Sun is ~600s in the past from Earth. The Earth is 2,537,000 years in the past from the Andromeda Galaxy, and the Andromeda Galaxy is 2,537,000 years in the past of Earth... ... _right now_ • If you flip this around, every location in space is also located in the future from every other location in space. This is if you count t=0 as everything you can see *_right now_* which is everyone else's past. • This is symmetric, both ways. • The _only_ way for two things to actually happen "at the same time" is by being located "at the same place" • Syncronizing remote clocks is a bitch. • There is a gap of time between any two distant locations in space, equal to the amount of time it takes for light to travel between those two locations, during which events do not have a strict cause-and-effect ordering. Event "A" at one location and Event "B" at a distant location can occur "before", "simultaneously", or "after", each other if they both occur during this time period. (It's the diamond shape between two light cones on a Minkowski diagram. ) All arrangements are valid, because who gets to be called "right now" is an arbitrary choice. • Oh, I should mention: Everyone and everything's own "proper time" clock always ticks at exactly one second per second, _always_ no matter what they do. Time dilation is everyone else's problem. I think that's most of the basics. So, everyone's current moment of *_right now_* is synchronized with light (or any kind of light speed signals, but light is the most practical.) Everyone arbitrarily chooses whether or not they will align their own current "right now" time to be named the "past" or the "future" on someone else's clock. Alice and Bob are located on planets or space stations or whatever, four light years apart. Let's pretend that they are standing still with zero relative velocity with each other to keep this simple. Alice broadcasts a radio message: "At the tone, it will be 00:00:00 January 1, 2000... *BEEP* " From Alice's reference frame, Bob will receive this signal on New Years Day 2004 _on Alice's own clock_ But Bob sets his clock to match Alice's clock. So the instant when he receives Alice's radio signal, _it _*_IS_*_ Jan 1, 2000 for him_ (Back to Alice for a bit) When Alice was broadcasting that message on New Years 2000... from Alice's reference frame, it was "currently" 1996 at Bob's location. Ok, got all that? Here's the thing: This is symmetric. Swap the names "Alice" and "Bob" in the text above, and it's exactly the same. It's valid for either one or even both to decide when to set the "zero" time to start counting seconds from. They could even use a third location halfway between them, that doesn't change their timekeeping situation. So, Bob declares that it's "now" the year 2000-Bob-Time, and so Alice is in 1996-Bob-Time. Alice declares that it's "now" 2000-Alice-Time and so Bob is currently "right now" in 1996-Alice-Time. You can slide these scales back and forth however you want as long as the offset _is less than four years_ As soon as light can get from Alice to Bob (and the other way) the order of cause and effect becomes frozen into a single reality... because they have both been "at the same time" for each other's "current time right now". If a Baby is born on Alice's planet in 2001-Alice-Time, that's 1997-BobTime. If a baby is born on Bob's planet in 2002-Bob-Time, that's 1998-Alice-Time. Which Baby was born first? The answer is that it is valid to say that both babies were born before, simultaneously, or after each other. If a Baby is born on Alice's planet in 2005-Alice-Time, that's 2001-Bob-Time. So, a baby born on Bob's planet in 2000-Bob-Time *IS* born _before_ that baby on Alice's planet. (1996-Alice-Time) Cool, got all that? Faster than light signals travel from the future to the past. They outrun the t=0 "right now" present moment synchronization that keeps cause and effect and "the present instant" in order. When Bob receives Alice's radio signal, he's hearing it live, exactly as it is broadcast "right now". It's not a recording, it's really happening. If Alice transmits a Faster Than Light [FTL] signal to Bob in 2000-Alice-Time, and Bob receives it in 1998-Alice-Time.... and then Bob immediately replies with his own FTL signal back to Alice. Bob is broadcasting his FTL signal in 1998-Alice-Time... which should be 1994-Bob-Time... which means that Alice will receive Bob's FTL signal in 1996-Alice-Time.... *_Four years before Alice broadcasts the original message in 2000_* (I did this math in my head, and so if it's off by 2 or 4 years: oops! But the round-trip time is always negative. ) Slower than light round-trip: positive time length Light-speed Round-trip: zero time length Faster than light round-trip: negative time length. I have to go do other stuff. Ask if you have any further questions.

[@thenippletwister3457]

@@juliavixen176 Okay, so I think I understand now. Thanks

[@eeman1335]

So, what you're saying is that the special effect showing Star Trek's Enterprise arriving with a light stream behind it is backwards. Actually, we should see the Enterprise pop on screen and then the light effect go backwards behind it to where it came from after.

[@MrTriple3D]

for people who don't understand: you have to remember that the beginning of the light wave IS reality itself, the wave itself is a map of where an action has been seen or is being seen at the current moment. beginning of the light wave is the exact moment for every single person where an action happens. so in specific frames of reference (going near the speed of light) FTL quite literally changes reality.

[@backfire10z29]

This I’m still having trouble understanding. Why does an action have to be (able to be) seen in order for it have occurred?

[@MrTriple3D]

@@backfire10z29 it doesn't need to be seen in order for it to have occured. think of the place where the light wave collapses (where it begins) is reality, it happens for everybody at the same moment, that's where an action starts. and the wave is a guideline or a map to show where an action has been seen. where it has been observed. under FTL the observer would never be able to influence the action but in FTL he would. say like in the video there is a spaceship shooting a planet and another spaceship is above it. lets call them spaceship A (the one shooting the planet) and spaceship B (shooting spaceship A) say they both have 2x FTL rockets and they are a lightyear apart now spaceship B shoots a rocket the moment it sees spaceship A has blown the planet up. so from spaceship B's perspective the planet would blow up and shoots a rocket towards spaceship A, the moment that rocket hits spaceship A, the light coming from spaceship A would hit spaceship B from the moment spaceship A was loading the rocket, so from spaceship B's perspective he shot spaceship A the instant it shot spaceship A's rocket, (again you have to remember that the beginning of the light wave is reality itself) now the problem begins when you start tweaking the numbers a bit, like the distances or the speed of the rocket. if you make the distances smaller or the speed bigger, it wouldn't just make spaceship B hit spaceship A the instant it shoots his rocket but BEFORE he shoots his rocket, breaking casuality

[@h14hc124]

Light has nothing to do with causality (aside from it travelling at the same speed as causality in a vaccum) - events will still occur regardless of whether they emit any light - in the examples given in this video, the emitted light is used to allow observers to react to the events, but observation is not necessary for the events to have occurred.

[@MrTriple3D]

@@h14hc124 of course events will still occur if no light has been emitted, there is no point talking about it because if you can't see the emitted light you can't use any of the information because there is no information. reason i say the beginning of the lightwave is reality itself is because all calculations will arrive at the same conclusion regardless of frame. once you go past that all of that breaks. for 1 person event A would happened before event B and for another person event B would happened before A

[@vichav3167]

I don’t know about shattering spacetime, but photons do exactly that every time. So if create a mind experiment, in which photon released from Point A which lead to destruction of Point B, then spacecraft must see it in reverse too. It’s impossible to see “rocket” at speed of light approaching. But light reflected of rocket still should exist. I think, that light reflected from rocket while it’s travelling must be taken in consideration, and shown as separated circle expanding at speed of light. Or maybe it’s effect of sqrt(-dt).

[@jamesopio4898]

I only get so attracted to any discussion regarding relativity.

[@dfcastro]

Assumption 1: nothing can travel through space time that has positive rest mass at speed of light. Assumption 2: nothing forbids space time itself to expand in faster than light speeds. (They do indeed for very far distances from us) So if you pack the missile into a warp bubble and that warp bubble is responsible for contracting space in front of missile and expanding behind the missile could be still in its patch of space but the space could be displacing at speeds higher than c. No relativity violation! Than pretty close to the target the bubble breaks apart and the missile becomes subluminal and fires it’s engines and hits the target. So, from the point of view of anyone the missile was launched just after the bubble explodes. There was a super luminal travel without any violation. No issues about light cone for observers.

[@juliavixen176]

No, this still has _the exact same problem_ It doesn't matter *_how_* you technologically accomplish the FTL motion. And.... Spacetime in General Relativity... ok, the full explanation is going to take a long time to explain, but what your warp bubble is actually doing _is not moving a bubble of spacetime itself to a different location in spacetime_ What it is doing is "shrinking" the space in front of it, so the actual distance needed to travel is less, and "stretching" the space behind it. Yes, this destroys anything along its path. Also doing this requires several times the mass of the Sun in both positive *AND* "negative energy", which doesn't exist as far as we know. (It's like having inertial mass be a complex number. )

[@pranaypaul6361]

Just brilliant explanation

[@ajayjaiswal2232]

Hello sir please answer my question that " Why does current not decrease when the potential difference across the resistance is decreased in series connection " . Please sir reply me as soon as possible 🥺🥺

[@allbopable]

You got a new subscriber! More! More! I want more mind bending videos like this!

[@bojanmerela5892]

I really enjoy your videos... I am learning a lot from them :)

[@kinshuksinghania4289]

This is the best physics channel

[@PrashantGijare]

13:36 sounds similar to supersonic jet that we experience on earth. Sound comes after the plane has passed. Another example is lightning where you see it first followed by the sound of it!!

[@XREADTHISTODIEX]

I love this video and your channel. Thank you so much for sharing knowledge in such an entertaining way. I wanted to ask: why is light the determining factor in causality? Is it because it’s constant speed? Given that there are indeed particles and phenomena that travel faster than light (like the expansion of space) isn’t it a matter of choosing an entity whose speed is faster than speed of light as a determinant of causality? Just throwing random questions from my shallow understanding of the matter. Again thank you so much for the videos, I enjoyed a lot.

[@Boahz]

13:00 guys save you some time. This is absolutely correct!

[@sam1812seal]

It makes it easier to understand once you realise that the speed of light = the speed of causality.

[@h14hc124]

I really wish that it had been called the speed of causality (the word would even match the letter c used to represent it in equations), because calling it the speed of light has led to people thinking that light has some sort of special status as a conveyor of causality, but it's actually the other way around. Lights moves at the speed it does because that's the speed at which causality occurs. Of course we didn't know about causality and relativity when the speed of light was calculated, so that's why it was named that way. A pity really.

[@AmritaSingh-g5f]

Sir, when things travel faster than the speed of light, it's only the effect that is visible and cause is not visible or visibly delayed. But cause effect relationship is not broken . It's maintained. It's just that we see only the effect and not the cause. That's it.

[@andriykovach2736]

You turned my world upside down !!!

[@Marinealver]

More like time appears to be moving backwards, much like being inside a moving train. Now here's a thought experiment, have the train grow or shrink and see how object appear to move both on and off the train.

[@juliavixen176]

So, one of the big things not mentioned in this video is that the spaceship who launched (will launch) the missile, watched it flying backwards towards them for several months after the moon went "boom". When the backwards missile finally reached them, _they launch it_ Seriously, faster than light stuff travels backwards in time.

[@wargod1722]

No, it still takes 1 year to travel boom effect to the one who launched and he will still see normal, not backwards because the closer images of missile reach him first.

[@juliavixen176]

@@wargod1722 Think about it: when do they see the light from the missile, and where is the missile. The missile is further away than its emitted light. When do they see the "boom" and when do they see the missile just before it reaches the "boom"? Do they see the missile _before_ or _after_ the "boom"? Do they see the missile _before_ or _after_ they launch it? Which events are seen in which order? When the missile is _halfway_ in between, is that seen before or after launching it and before or after the "boom"? Seriously, try to write these events down in order, the missile must be traveling backwards in time to make it all fit together.

[@martf1061]

As soon as an object moves faster than light, we ( our eyes ) will stop seeing it. Not because its too far away, only because of the reason that we SEE things. We see things because some light was able to reach the object, and some parts of that light bounced off of it and reached our eyes. Faster than light = invisible to human eyes.

[@juliavixen176]

@martf1061 It emits light... that light propagates away from the thing _at the speed of light_ It's regular old *_light_* So we can see it just like any other light.

[@wargod1722]

I mean light from boom still take 1 year to travel because two locations had distance of 1 light year.

[@stavi82]

I'm still just impressed the astronaut can see a tiny ship and missile a light year away.

[@juliavixen176]

Hey everyone, in Relativity, if you see light from something, it's really actually physically there interacting with you. It's not an illusion or a recording or something. When you see the FTL missile flying backwards from the target to where it was launched, _it is actually really there_ You can shoot it with a powerful laser beam after it has detonated at the planet, but before it has been launched, and destroy the missile in flight before the missile has destroyed the planet after it destroyed the planet.

[@AlGreenLightThroughGlass]

The speed of light in a vacuum is the speed of causality which is why it can’t be exceeded or causality broken

[@Deoxys_da2]

I mean we are talking about an impossible event so if impossible event occurs things happens where human brain can't comprehend Like we get infinite voltage if the rate of change in current in just a small capacitor increase abruptly like in time period of 0 according calculation hence whole universe will be destroyed because of this which is an impossible event

[@willmcgo8288]

Reminds me of videos where the distance is far enough, and a bullet is fired and travels faster than the speed of sound towards you, and you hear the bullet hit a target close by, then the sound of the gun firing is heard. Cody's Lab did a video on that. Not sure how you could be traveling at the speed of sound past the target and still hear it get hit; that might be non-trivial.

[@maanavallampallam4864]

YOUR SHIRT IS AWESOME!!!!

[@edwinov]

The fact that I liked this video before I even watched it is proof that FTL exists!

[@StuMas]

It seems to me that, all the emphasis is placed on what different observers will see. Regardless of differing perceptions due to proximity and speed, the cause, in and of itself, always occurs before the effect. A person cannot be shot and wounded by a bullet before the trigger is pulled. Scenarios to the contrary, defy the logical linearity of observed reality which underpins our understanding. Could it even be possible to explain how an effect came into existence before its cause?

[@quentinfool]

I am so happy to figure out your shirt (a friend coincidently discussed it too) Hint: it's a third derivative of distance over time

[@urbanarchery26]

Have experienced this with sound standing next to a rifle target being shot at a huge distance. You hear the bullet hit the target before you hear the gun fire.

[@vitovittucci9801]

In the same way the lightning (effect) arrives before the sound of the thunder (cause)

[@shohanshamim6478]

Best experimental video❤

[@rezaastaraky8376]

This video delves into why Einstein struggled with quantum mechanics. It suggests that when one electron's spin changes, its entangled partner responds instantly, potentially disrupting causality and challenging the speed limit of light, which Einstein proposed in his theory of relativity.

[@DrDeuteron]

since Schrödinger's equation doesn't have a "c" in it....its not surprising there's no "c" limit.

[@lyrion0815]

As always, great video! But with this in mind, wouldn't an alcubierre drive also break causality? Mount the rocket on one to get it to the moon in 3 months (but still slower than light, because thats how alcubierre work) and use another one to get the "dont shoot" message to the firing ship... !?

[@nareshjain4626]

To explain causalty you need to explain lorentz length contraction in detail. Without it the understanding wouldnt be complete. It is not like the causalty is "perceived" to be broken from the spaceship point of view but actually broken. Which can have an effect on other observers in other frame of references

[@oldmandice2731]

I have a theory, based on several other theories I've read, that seems to remove the time paradox. Imagine you are moving through time and your position is (A). (A) is constantly moving through time in a loop at a set "time speed", lets also imagine a prior time (B) moving at the same "time speed". If you were to travel back to (B) any change you made would always stay with (B) as (B) moves through the time loop but would never reach (A) as (A) is also constantly moving at the same "time speed". If, after making changes to (B), you returned to (A) you would not see any effect. Note that however much time you spent in (B) would also pass in (A). So, if 1 unit of time passes you would then be returning to (A+1) from (B+1). However, if you were to remain in (A+1) for a period of time, say 10 units of time or (A+11), then return to (B), which would now be 10 units of time to the future of when you 1st visited, or (B+11), you would see the effect of any changes you made.

[@juliavixen176]

I didn't really follow this, because everyone travels through time at exactly one second per second (in inertial motion, i.e. no acceleration (long story)) That's just it... It's always one second per second. Where Relativity kicks in is that every location in space is also a _location in time_ and every location in space is in _the past_ of every other location in space. Yes, both ways symmetrically. Everything is in the past of everything else, and the further distance away something is in space is exactly how far away it is in time (in the past). The only way two things will ever be "at the same time" is when they are "at the same place". So, if you have two clocks located at the same place at the same time, you can synchronize them to start counting seconds together, 1, 2, 3, etc. If you take one (or both) of the clocks and move them around through a different path in space, you also move them a different path through time. The clocks still tick one second per second, but when you bring them back together, they will have counted different quantities of seconds _because they traveled different distances in time_ and space from the first time their paths crossed to this second time their paths crossed. (That's all those "Twin Paradox" setups.) If the clocks are not at the same location in space, then what is considered "right now" (the same numbered clock tick) is the 45° lines of their "light cones" on a Minkoeski diagram. Also, the -45° or 135° diagonal lines. Both are equally valid and you can't say that either clock is "before" or "after" the other clock as long as they are separated in space. (Seriously, events within the time window duration equal to their distance in space in light-speed units, can *not* be definitively ordered into "cause" and "effect". Any order is valid within that duration of time. (The window is of course zero when the clocks are zero distance apart, hence why you *can* synchronize them then.))

[@oldmandice2731]

@@juliavixen176 I understand your confusion. I's a hard concept to explain in a format like this. Imagine a film reel every frame on that reel a snapshot in time. Each frame a separate reality, one that has its own past and its own future but following the same path. So, there are an infinite amount of you, each a slice of time existing in their own frame and moving through time at the same speed, so 1 second ago you, you and 1 second in the future you never meet, present you is always present you, moving through time. So, if you were to travel back 1 second in time you could meet your past self without creating a paradox as that self would be its own present moving through time and would never reach "your" present. Any changes made by you would remain in that present and if you were to return to "your" present you would not see any changes that you made nor would you suddenly have a memory of being visited by your future self as it would have never happened for the "present" you. Your "present" however would now have moved into the future by the same amount of time you had spent in the past and were you to return to the same exact time you left that would actually be a different reality, in the past relative to your actual present.

[@nedmerrill5705]

And the reverse is true, too. If a phenomena requires a reversal in time, it breaks the special theory of relativity. This happens in certain double-slit experiments.

[@lazyliar9744]

I always had this question ,thanks 🙏

[@loganhodgsn]

So, it seems the most impressive thing we will ever see in our universe is the death star laser! We will witness the explosion and firing sequence simultaneously.

[@sharthakghosh970]

How do you explain quantum effects like quantum tunneling where there seems to be some communication that happens faster than light. Is that a right way to think?

[@aroundandround]

In a mathematical model, I can imagine a thought experiment where the spaceship sends an FTL signal to undo the missile launch after seeing the explosion (and the missile itself), but it seems like they could also “correct” their perception using physics to conclude that entropy cannot decrease and therefore they are the entity moving and must recompute their perspective from a hypothetical observer that is either stationary or moving at any speed at which entropy is not seen as decreasing; can they not in principle?

[@matthieumallavan1827]

Hi, I love your explanation ! But I have a problem to understand why we care about someone perceive something? Light is a wave, so the sound is, so when a lightning strike it appends before we hear it, doesn't make a mater. And the animation would be the same with someone shoot a missile faster than sound (mach4 for example), at a distance from the strike, and a plane moving near the speed of sound... Because an observer perceive something earlier doesn't make a mater from traveling speeder than sound or light ?

[@joshuascholar3220]

I would think that to show causality is broken you would show that be RESPONDING with a faster than light anti-missile missile, you could, you could hit the source before it launched.

[@andruss2001]

Thanks! But I still believe that FTL communication doesn't break anything. What gets broken is a subjective illusion of causality in certain observers' frame of reference. But Einstain's bending of time-space reminds me of an equation: if you adjust the left side, then something gets automatically adjusted on the right side, so the balance is saved. I also believe that this space-time bending has nothing to do with time travel.

[@NavyaMenon25]

yay new video! i look forward to them all the time!

[@The_Man42]

I love the fact that the example he uses is a dark forest strike

[@sebastiantornberg5179]

I love your video, you expalin physics so well

[@martf1061]

Every effect is caused by something previously. To every reaction, there is a previous action. But every actions are in fact, reactions to previous actions, that are reactions.... So on and so on... Therefor, CAUSALITY CAN'T "REVERSE".

[@John-zz6fz]

A better way to state this is "FTL breaks objective causal determination." If we alternatively accept that we can't always determine causality in some reference frames then FTL is consistent. The idea that the laws of physics "MUST" be consistent with objective reasoning might be a nice idea but it's not fundamental, I see no reason why nature shouldn't provide circumstances where objective event knowledge is limited or even impossible. For example another interpretation of the spaceship sending a FTL signal to stop the missile launch is that it's ignored and the missile is launched anyway. A more complicated scenario is that the missile is remotely controlled by the spaceship. In that scenario the spaceship send the launch command, time passes and the moon blows up then they see the launch. They can then infer the correct order. The astronaut sees the moon blow up, the launch signal is sent and the launch occurs but is able to back out the order. A second spaceship however could lose objective causal determination which is only a problem if we say it is.

[@juliavixen176]

No, it's worse than all that, and I was hoping that this video would mention that if anything takes a round-trip journey FTL, it will arrive at where it started *BEFORE* it originally left.

[@John-zz6fz]

@juliavixen176 No, it really wont, trace it out in a light cone.

[@paraax]

​@@juliavixen176 That is the fundamental claim and this video did not prove that you could get a signal back to the ship prior to it launching. Spacetime (from PBS) tries it hand at this proof, causality comes (light cones) based on different frames. I remain unconvinced but admit I haven't fully worked through that case. The main claim here says that cause and effect light circle don't intersect, which of course would be the whole point of FTL. If it is possible watching it's effects would be wildly unintuitive.

[@juliavixen176]

@John-zz6fz Assuming that the FTL travel is two straight (inertial) lines in Minkowski spacetime, transform from the starting location's coordinate system, into the coordinate system at the turn-around location, and then transform from _that_ spacetime coordinate back into the coordinate system of the original starting point. (It's going to be a line greater than 45° from the (vertical) time axis on a standard diagram.) You can swap the space locations of the "start-end" point and the spacally separated "turn-around" location and get the same result, in fact, I recommend that you do draw the FTL path both ways on your Minkowski diagram, because you will clearly see that the paths arrive before the other departs. It's the coordinate system shifting... the _zero_ time for two inertial observers at rest with respect to each other _separated by a distance in space_ is on _each other's 45° light cone_ ! It is *NOT* on a horizontal line drawn along the _x_ axis!

[@WWLinkMasterX]

None of this is an issue if you consider that physical interactions are symmetric over time, so any "effect" is mathematically consistent with being a cause. What the FTL ship sees is debris in space coming together in just the perfect way to form a moon and a missile, which travels backwards, collecting exhaust, until it slides perfectly into the launching tube of a destroyer. Entropy has been reversed, but no other laws of physics have been broken. Consider if the FTL ship tries to "prevent" the missile launch by firing something like a brick wall into the missile's path. Then from the astronaut's perspective (who's moving through time "normally"), debris from a wall will appear to spontaneously come together from all directions in space to form a missile. Issues only arise from the assumption that agents have the "free will" to change future events and a desire to say entropy can only increase/decrease one way. Put even more abstractly, the common sense notion of causality creates an asymmetry in time, where past events can't be changed, but future ones can. But the mathematics of physics have no such asymmetry, only a requirement of continuous change between states. You can tack-on the second law of thermodynamics, but that relies on a sense of probability that's not causal. I want to rewatch TENET now...

[@BracaPhoto]

Lets now consider how the light from the explosion will interfere with the incoming light from the missile -- Will the interactions between the light give you a "hologram" of the cause and effect inside 1 piece of film ??

[@cslcchy]

22:21 But the problem is that this reverse phenomenon is only seen in the spaceship's perspective, no? Even if he sends a signal back, in the perspective of the shooter, the rocket has already launched. It is because of the limit of light's speed that the rocket is only seen after the boom occured.

[@timelapseofdecay9028]

I think Mahesh's videos on relativity are some of the best on the internet, but this is not one of them. A much better explanation on the paradoxes can be found here: kzbin.info/www/bejne/l5-TfmCtmK2qa6M