first

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

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.

@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.

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.

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

Thanks

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

Causality has left the room

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

What’s the difference between the two?

Entropy

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

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

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...

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

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

@@malto_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.

Missed that too

Yup simply observation will not do anything

when fast observer sends FTL signal, to stop the bomb , they read his message and dont release bomb. So,. we have the effect (explosion) , but no cause ( bomb release) . hence causality is broken

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.

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

Haha, thanks!

​​@@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.

@@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.

I would say it is the speed of PERCEIVED causality.

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.

@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

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.

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

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

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

@@abdulqader1829 You can imagine it going arbitrarily quickly, or even instantly, after the "boom" detection as the animation plays.

Closed timelike loops

​@@abdulqader1829On 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.)

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.

Someone was a missileer

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.

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

@@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.

@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.

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).

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.

Took me a moment.

@@astrokevin92 glad someone did 😉

snap, crackle and pop .....xD

I was really proud to have understood that too 😅

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.

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.

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

@@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.

@@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”

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.

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.

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.

@@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.

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.

She is a failed physicist but she is funny.

Par yeh dil maange more…Ahaaa!

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

If you send this message FTL, from another observer travelling near FTL this could ring true, on your recommendation!

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

Time dilation in this video????????

There is a video by ScienceClic, "Visualising General Relativity", the best video I've seen. Everything "falls" down at 9.8m/s² because they don't, they are stationary but the ground us accelerating outwards which seems to the people who are accelerating with the ground(us) as if the objects were falling down. Space is being contracted continuously because mass bends Spacetime, so the accelerating ground seems to be in the same place because where it moves, space itself moves back.

There are some good videos recently posted here on KZfaq which explain how light propagates through a transparent material, and how it "slows down". The full explanation is really long and complicated. I'll try to summarize, but this is the incomplete explanation. The atoms (or molecules) of the transparent medium are displaced by the electric field oscillation of the light, and then returned to their original equilibrium location by the other atoms/molecules of the medium. Because the atoms/etc. are accelerating, they now radiate their own light at the same frequency (usually) as the incoming light wave, but with a phase delay. So now the original light that wasn't absorbed, and the new radiated light from the material itself are constructively and destructively interfering with each other. Anyway, if you do the math, when the light leaves the material it will be delayed... there's more stuff going on I didn't mention here.

Yes. There's some videos on KZfaq here of someone who wrote a raytracer for rendering 3D models moving at relativistic and superluminal speeds.

Velocities don’t add up like that!

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!

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. )

I'd love to hear the explanation

@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.

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

Any interaction in which things located 299,792,458 meters apart in space, that occurs in less than 1.000000000 seconds (as measured by an inertial observer) will always have the "effect" occur _before_ the "cause". This video doesn't really give a good explanation of the complete logic of FTL signals, and I'm not going to write an explanation here right now either, but yeah, it's the speed of light and anything else that is massless, in the vacuum, for all inertial reference frames, to make a long story short.

The expansion of the universe isn't the boundary of the universe flowing outward, it's that every single spot in the universe is slowly getting bigger simultaneously. Like technically everywhere is the center of the universe due to inflation which is a little odd but it's accurate. So the space itself isn't really moving which is why it can seem to exceed the speed of light.

That space is expanding faster than light can cross that distance, which means that the light will never reach the far side... at all, ever. There's an "event horizon" where very distant locations will never have any cause and effect relationship with each other. (I guess you could flip the coordinate transformation around and say that light is slowing down and stopping.)

That raises a really interesting point... in the obsolete idea that the universe might expand and then contract again back to a singuarity, I wonder if (for the sake of argument, assume the theory was true) there would be a point at which spacetime collapses inwards faster than the speed of light, and the utterly bizarre way the universe would behave from that point on.

@h14hc124 You wouldn't be able to see the collapse coming until it was 'too late'... The exact details depend on... well, the exact details of this situation, but everything in the shrinking volume of space will hit you all at the same instant from your point of view. It may not be meaningful to talk about time or space (or you) existing after that happens... but like I said, it depends on the exact details of what is collaping, where and how much, and for how long.

It doesn't actually... so... I recently got around to reading up on all the old luminiferous aether theories and... to make a long story short, the Lorentz-Larmor-(Poincaré) aether theory has the exact same 'effect before cause' thing happen as Special Relativity does when anything propagates faster than light. This involves a really _long_ explanation that I don't really want to write in this KZfaq comment right now unless someone replies to this and asks me to.

Why? Are saying it’s not fact?

This "effect before cause" situation will always happen with faster than light signals, under _all_ circumstances. Ask me and I'll explain.

@@juliavixen176 ok, why? Wouldn’t we see ripples of time running forward and backwards between some multiple of the speed of light?