Unraveling the Mysteries of Faster-Than-Light Travel in Physics

[DaveC426913]

We know of no way to go faster than light. Idle speculation is all that's left.

 

[Skhandelwal]

OMG I know that, I was just wondering what is the current theory of the predictable outcome.(the most recognized one, the one w/ most data and analysis, w/ most funding, whatever)

 

[JesseM]

Actually he specifically said that "Glasses would not miraculously jump off the floor and reassemble themselves on the counter." which implies that going to the past doesn't mean seeing past events.

But he was responding to your question:

Like if the universe were to contract, would everything replay itself except in reverse? or would it still seem like time were going forward?

Your question was only about what would happen during the contracting phase of the universe, not about what would happen if you could send signals faster than light or if you could travel back in time through a wormhole.

But I guess he's wrong lol, thanks for the answer. general relativity really doesn't say anything about what going backwards in time means?

Again, there are only a few specific situations where general relativity allows backwards time travel, like the "traversable wormhole" solution found by Kip Thorne, and there are some plausible arguments that quantum effects would prevent them from actually working. But in terms of general relativity alone, with no quantum effects, the theory says you could actually take a trip that would result in your ending up in the past light cone of some earlier point on your worldline, which means "travelling into the past" just like in any time travel movie.

 

[JesseM]

So what is the answer? When I travel faster than light, does the entropy reverse? Or for entropy to reverse, universe has to comeback to one point?
Does going faster than speed of light means just YOU are going back to past, meeting yourself...I am confused. unlike normally, this time, you guys even got me more confused. But i am thankful for that, however, I was hoping for a definite answer, guess there isn't one. But what is the current theory on what happens when you travel faster than light?(be specific)

There's no meaning in relativity to what things would look like from the point of view of a tachyon, just like there's no meaning to what things would look like from the point of view of a photon...there'd be no known method of building clocks or rulers out of either photons or tachyons, so you can't get meaningful answers to how they would perceive time or distance. But if tachyons existed, they could be used by slower-than-light observers to send signals into their own past, according to relativity. In other words, you at age 45 could send out a message in morse code using tachyons, and your younger self at age 35 could receive it and learn about his own future.

 

[michael879]

But he was responding to your question: Your question was only about what would happen during the contracting phase of the universe, not about what would happen if you could send signals faster than light or if you could travel back in time through a wormhole. Again, there are only a few specific situations where general relativity allows backwards time travel, like the "traversable wormhole" solution found by Kip Thorne, and there are some plausible arguments that quantum effects would prevent them from actually working. But in terms of general relativity alone, with no quantum effects, the theory says you could actually take a trip that would result in your ending up in the past light cone of some earlier point on your worldline, which means "travelling into the past" just like in any time travel movie.

if the universe were to contract wouldn't everything start to travel into the past rather than the future?

 

[JesseM]

if the universe were to contract wouldn't everything start to travel into the past rather than the future?

No, why would you think that?

 

[Skhandelwal]

So you are saying that we don't even have any theories w/ any mathematical analysis for time travel?(except the fact that time gets square root negative after the speed of light so we imagine that we would go back in time) So it is possible that the formula we came up w/ only complies for objects traveling less than or equal to the speed of light and that whole time traveling thing is nothing more than a philosophy.

 

[michael879]

No, why would you think that?

well, if you take the hyperspherical model of the universe where time is the radius of the hypersphere, while the universe is expanding time is increasing but if the universe were to contract time would decrease back to 0. Are you saying the hyperspherical model is wrong? I like it so much...

 

[michael879]

So you are saying that we don't even have any theories w/ any mathematical analysis for time travel?(except the fact that time gets square root negative after the speed of light so we imagine that we would go back in time) So it is possible that the formula we came up w/ only complies for objects traveling less than or equal to the speed of light and that whole time traveling thing is nothing more than a philosophy.

its possible, we've never observed anything going >c so there is no experimental evidence to suggest they apply. However it would be strange for different rules to apply once you go faster than light. Also, simply going FTL doesn't make you go back in time. If you were going >c compared to earth, Earth clocks would appear to move x*i for every second yours do. This doesn't really make sense and doesn't seem to send you into the past. The only way I've heard of "traveling" to the past is by using a tachyon to send information to the past. Although I don't even see how that is possible.

 

[JesseM]

well, if you take the hyperspherical model of the universe where time is the radius of the hypersphere, while the universe is expanding time is increasing but if the universe were to contract time would decrease back to 0. Are you saying the hyperspherical model is wrong? I like it so much...

Ah, I gotcha. Well, normally when we picture a curved 2D surface, we have to imagine it sitting in a higher-dimensional 3D "embedding space", and you seem to be thinking in these terms, with the curved surface of the sphere as space and the higher embedding space as time; but the mathematics of differential geometry in which general relativity is formulated actually described curved surfaces in totally intrinsic terms, without the need for any such embedding space. As an alternate visualization, you could think of a closed universe as a curved one-dimensional space, ie a circle, then you could picture spacetime as a whole as the surface of an american football, with the bottom tip as the big bang and the top tip as the big crunch, and each successive cross-section would be a larger and larger circle during the expansion phase, and each successive cross-section would be a smaller and smaller circle during the contraction phase. But it's important to understand that any picture of curved space or curved spacetime in terms of a curved surface in 3D space is just a kind of mental crutch, and that features of our visual picture may not be reflected in the actual mathematical description of curved spacetime in general relativity.

 

[JesseM]

So you are saying that we don't even have any theories w/ any mathematical analysis for time travel?(except the fact that time gets square root negative after the speed of light so we imagine that we would go back in time)

No, the fact that tachyons could be used for backwards-in-time signalling has nothing whatsoever to do the imaginary answers you get if you plug v>c into the Lorentz contraction formulas, or into the formula for time dilation. It just has to do with analyzing the path of an FTL particle from the perspective of various slower-than-light reference frames, in which case it becomes clear that any signal moving faster than light in one frame would be moving backwards in time in some other frames, due to the way different frames have different definitions of simultaneity.

 

[michael879]

Ah, I gotcha. Well, normally when we picture a curved 2D surface, we have to imagine it sitting in a higher-dimensional 3D "embedding space", and you seem to be thinking in these terms, with the curved surface of the sphere as space and the higher embedding space as time; but the mathematics of differential geometry in which general relativity is formulated actually described curved surfaces in totally intrinsic terms, without the need for any such embedding space. As an alternate visualization, you could think of a closed universe as a curved one-dimensional space, ie a circle, then you could picture spacetime as a whole as the surface of an american football, with the bottom tip as the big bang and the top tip as the big crunch, and each successive cross-section would be a larger and larger circle during the expansion phase, and each successive cross-section would be a smaller and smaller circle during the contraction phase. But it's important to understand that any picture of curved space or curved spacetime in terms of a curved surface in 3D space is just a kind of mental crutch, and that features of our visual picture may not be reflected in the actual mathematical description of curved spacetime in general relativity.

thanks a lot for clarifying that (and for giving me a better model). I know that we can't picture anything 4d, but 3d physics equations all have a 1d form. So wouldn't there be some model that is the "correct" model of a 1d universe? i.e. it "explains everything". Either that or general relativity doesn't specify it, I don't know much about GR.

 

[Skhandelwal]

1. Wait, if that imaginary # isn't the evidence that something moves back in time as it exceeds the speed of light then what it?
2. I used to know, for some reason, it is really basic so please don't laugh but I can't understand it. velocity doesn't depends on the object's velocity emmiting it, why is that?

 

[michael879]

1. Wait, if that imaginary # isn't the evidence that something moves back in time as it exceeds the speed of light then what it?
2. I used to know, for some reason, it is really basic so please don't laugh but I can't understand it. velocity doesn't depends on the object's velocity emmiting it, why is that?

a negative time indicates going backwards in time. velocity depends on the object that "emits" it. The velocity addition formula is (v1+v2)/(1+v1v2/c^2). Light goes at c so that no matter what the emitter's velocity is, the total is c.

 

[JesseM]

a negative time indicates going backwards in time..

No, it just means that the Lorentz transformations give nonsense-answers for v>c (and they don't say that the tick of an FTL clock would be negative, they say the tick of an FTL clock would have an imaginary time-interval). Again, the reason for the "FTL=backwards in time" idea has to do with simultaneity issues, namely the fact that for an FTL signal, there will always be some inertial frames where the signal was actually received before it was emitted (this wouldn't be true for slower-than-light or speed-of-light signals--if they are emitted before they were received in one frame, this will be true in every frame).

 

[michael879]

No, it just means that the Lorentz transformations give nonsense-answers for v>c (and they don't say that the tick of an FTL clock would be negative, they say the tick of an FTL clock would have an imaginary time-interval). Again, the reason for the "FTL=backwards in time" idea has to do with simultaneity issues, namely the fact that for an FTL signal, there will always be some inertial frames where the signal was actually received before it was emitted (this wouldn't be true for slower-than-light or speed-of-light signals--if they are emitted before they were received in one frame, this will be true in every frame).

yea I was saying if the lorentz equations gave a negative time it would indicate going backward in time. The imaginary time doesn't mean anything. I forgot the causality thing tho, FTL travel makes it variant with respect to your speed.

 

[JesseM]

yea I was saying if the lorentz equations gave a negative time it would indicate going backward in time.

Right, but there isn't any v you can plug into the time dilation equation for the amount a moving clock slows down, \sqrt{1 - v^2/c^2}, that will specifically give a negative answer. The square root function doesn't even specifically give you a positive or negative answer--the square root of 4 can be either 2 or -2, for example.

 

[Skhandelwal]

I know it happens b/c of equation but physically, WHY does it happen? Why does the light don't care about the object ommiter's velocity?

If my car is moving at 60 miles per hour, and I turn on the light and it travels on the inside of the car, well, isn't it traveling faster than light then?

 

[DaveC426913]

I know it happens b/c of equation but physically, WHY does it happen? Why does the light don't care about the object ommiter's velocity?

If my car is moving at 60 miles per hour, and I turn on the light and it travels on the inside of the car, well, isn't it traveling faster than light then?

Nope. You see the light traveling at c.

Let me anticipate your next question: "What if my car goes 99% of the speed of light?" Well, you still see light in your car going at the speed of light.

 

[Skhandelwal]

Look, I know what happens, I am well aware of that, I just don't know why it happens. Why does light doesn't care about the velocity of the ommiter's or the velocity of the space she is traveling in?

 

[lightarrow]

Look, I know what happens, I am well aware of that, I just don't know why it happens. Why does light doesn't care about the velocity of the ommiter's or the velocity of the space she is traveling in?

If you want a kind of analogy, you can think about sound: if you are in a car moving at 60 miles per hour and you ring the horn, the sound's speed doesn't change, it's the same as at rest (but here the explanation is different from that of light). What changes is the sound's frequency: someone in front of you measures an higher frequency, while someone behind you measures a lower one.

So, even this should be impossible, but it happens!

In this case the reason is that sound moves relative to the air.

In the case of light the reason is that, actually, light's speed is infinite. So, infinite + 60 miles per hour = infinite and the result doesn't change.

 

[MeJennifer]

Look, I know what happens, I am well aware of that, I just don't know why it happens. Why does light doesn't care about the velocity of the ommiter's or the velocity of the space she is traveling in?

In the SR interpretation it is simply because there is no absolute space (and time). As a consequence you cannot say that something is traveling at a certain speed, in the local frame it is always standing still, hence a light beam leaving the local frame leaves it with the speed of light.

Alternatively there are theories that assume an absolute space and time, theories such as the Lorentz ether theory and the reason here that we observe a beam of light separating us at light speed even if we were to travel at a certain speed relative to absolute space has to do with the contraction of space and time as described by the Lorentz contractions.

While in special relativity the constancy of the speed of light is a principle it is an effect in the Lorentz ether theory.

 

[Skhandelwal]

I heard that ether theory was proven wrong for some reason.(would appreciate if someone could remind me what coz I forgot) I got an analogy in my mind and that made all the sense...light is just like transfinite numbers. So 1+infinity=infinity but infinity+1doesn't. Wow, I love when everything pieces together and makes sense.

By my complete(from my pov) understanding of light, I got some ideas/challanging questions.

1. Is it possible to accelerate energy to the speed of light?(b/c if it is, and if we have a method to convert energy into mass then we can accelerate energy to the speed of light, then convert it into mass. What would happen then?) My guess is that it will immediately(in zero time, even from photon's pov, convert into a point where it will have infinite density but no volume, thus, turn into a black hole)

2. IF we travel at the speed of light, the world will be still, right? B/c time has stopped for me, but if I can manage to do it, would I become immortal from this world's pov b/c from mine, I will age normally?

3. IF we can create a vacuum, and in that vacuum, we will have a spaceship which will travel at the speed of light but will be bounded by an mirroric type of area(something that reflects instead of damaging us) so we will be at the same place, just keep reflecting. We, inside the spaceship, won't have to worry about galaxy crashing b/c time for us will be stopped, just as long as we keep recycling stuff in that region, we will live forever,(not individuals, but our race)

4. I used to know this, just wanted to check my basics, as our velocity increases, does our mass increases, and lenght, width, and height decreases ? So if we do get the speed of light(as if we we were in event horizon, we end up w/ 0 volume and infinite density, right?)

5. Does light ever accelerate?(I know it bends and I also know that it has some negligable negative acceleration b/c of friction, etc. but that's about it, right?)

6. Does light have a size?(be specific, include lenght, width and height)

7. If something has no size, but infinite densite, does its gravity becomes the speed of light? (same as black hole?)

Its ok, I understand my questions are long, therefore, you don't have to answer all at once, just answer one at a time, or something.

 

[Aether]

In the case of light the reason is that, actually, light's speed is infinite. So, infinite + 60 miles per hour = infinite and the result doesn't change.

No, light's speed is not infinite. For example, we can measure the distance between a RADAR antenna and a target by measuring the round-trip travel time of a reflected radio pulse. We couldn't do that if light's speed were infinite.

I heard that ether theory was proven wrong for some reason.(would appreciate if someone could remind me what coz I forgot)

There are some ether theories that have been proven wrong because their predictions do not agree with experiments. The Lorentz ether theory agrees with all experiments to the same degree as special relativity (it is actually the same physical theory as special relativity but cast in a different set of coordinate systems), and it has not been proven wrong.

I got an analogy in my mind and that made all the sense...light is just like transfinite numbers. So 1+infinity=infinity but infinity+1doesn't. Wow, I love when everything pieces together and makes sense.

By my complete(from my pov) understanding of light...

The speed of light is not infinite, so how does that change your pov?

 

[lightarrow]

No, light's speed is not infinite. For example, we can measure the distance between a RADAR antenna and a target by measuring the round-trip travel time of a reflected radio pulse. We couldn't do that if light's speed were infinite.

What I mean is this I wrote in another thread: https://www.physicsforums.com/showthread.php?t=132528&page=2

Many people don't understand the fact such a body cannot reach or exceed the speed of light, because, maybe, they don't know that, in practice, light's speed is infinite:

The fact is that we have chosen a "bad" definition of speed, when we have chosen v = S/t, since space and time are not actually independent from each other; but we didn't know it! (Anyway, that one is the definition which we can deal better).

If we defined the speed of a body in a more appropriate way, that is, exactly in the way we define lenght, mass, time, that is, using a sample of it and adding n equal samples to make a sample n-times bigger, it's possible to show, mathematically, that the speed of light would become infinite.

This means: 299,792,448 m/s is the maximum number anything speed can have, mathematically.

 

[ZapperZ]

What I mean is this I wrote in another thread: https://www.physicsforums.com/showthread.php?t=132528&page=2

Please be very careful here:

1. You have ignored experimentally-verified measurement of its value using the standard definitions. Furthermore, one can show that using "covariant systems", that is the speed one measures. There is a tremendous difference between c and "infinite", and one should certainly detect that.

2. You have made a speculation that isn't supported by any empirical evidence. Speed of light would become infinite, and so, it must be c?

3. Re-read the PF guidelines on speculative posts.

Zz.

 

[lightarrow]

Please be very careful here:

1. You have ignored experimentally-verified measurement of its value using the standard definitions. Furthermore, one can show that using "covariant systems", that is the speed one measures. There is a tremendous difference between c and "infinite", and one should certainly detect that.

2. You have made a speculation that isn't supported by any empirical evidence.

3. Re-read the PF guidelines on speculative posts.

Zz.

Once you have seen that 1/SQRT[mu(0)*epsilon(0)] is the speed of light, and you have calculated this value from
mu(0) and epsilon(0) values (for example), formally, you don't need much more, assuming Maxwell equations are correct (and so, that c doesn't depend on ref. frame).

Then, you can derive Lorentz rules of transformations; then, you define a body's speed in the way I wrote up, then you see mathematically that, using the usual definition of speed, the infinite value (with the other definition) becomes exactly c.

I saw this demonstration in a book I don't have, and then I have repeated some years ago this computation. If you don't believe me, give me some days and I'll write you the proof of it (it's not difficult at all, but it's a long time I don't make these mathematics).

This is the concept: you take a train, which holds railtracks attached on top of it. You can impose a certain speed v to this train (measured in the usual way, for example); then you put another train on the first, and impose this second train's speed is still v respect the first one. You can define a new concept of speed, saying that this second train has speed 2*v relative to the ground. But, using Lorentz rule of speed addition, you have, according to the usual definition of speed, that the second train moves at a speed:
v2 = (v1+v2)/(1+v1*v2/c^2) = 2v/(1+beta^2). Repeating the same for a third train on the second and so on, you have v3,...vn. At the limit for n-->infinite, the speed is:
1.infinite, according to the new definition
2. c, according to the usual definition.

 

[ZapperZ]

Once you have seen that 1/SQRT[mu(0)*epsilon(0)] is the speed of light, and you have calculated this value from
mu(0) and epsilon(0) values (for example), formally, you don't need much more, assuming Maxwell equations are correct (and so, that c doesn't depend on ref. frame).

Er.. back up. You are basing this on EXPERIMENTAL MEASUREMENTS! Now think, what exactly are you measuring here and how exactly are such measurements defined so that you are able to measured those values? I challenged you to find ANY technique of measuring the permittivity and permeability WITHOUT using the value of the speed of light in it.

And yet, you then turn around and tried to "derive" the speed of light from this? This is like assuming the value of g to solve some mechanics problems, and then using its results to recalculate g. You do not see the absurdity in this?

You cannot "derive" the speed of light from First Principles.

Zz.

 

[lightarrow]

Probably I didn't explain myself well.

Of course the value of c is obtained sperimentally, I don't want to find it Theorically.

What I mean is that, since it comes from properties (mu(0) and epsilon(0)) of space itself, once you have this value, given that the space properties doesn't change, it becomes an absolute, fixed value.

When I said this is the maximum speed "mathematically", it's because of what I said about the other kind of speed definition. That is, it comes out "mathematically" that light's speed, that is, 299,792,458 m/s, according to the usual definition of speed, is equivalent to infinite m/s, according to the other (more "appropriate") definition of speed.

I hope I expressed myself correctly, this time.

 

[ZapperZ]

What I mean is that, since it comes from properties (mu(0) and epsilon(0)) of space itself, once you have this value, given that the space properties doesn't change, it becomes an absolute, fixed value.

But how do you think those two values were obtained in the first place? They are not obtained from any First Principles calculations. If you open the CODATA tables, you'll discover that those values totally depends on knowing what c is, and that causes you to really go in circles if you use them to get c.

Zz.