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

[Skhandelwal]

I read in physics that it is impossible to get at the speed of light b/c it requires infinite force and I HEARD that if you do you can go to the future and if you cross it, you go to the past. Now I am having problem understanding the HEARD part. I mean what makes you reverse or accelerate entropy?

btw, they were talking about some objects that nevertheless do travel faster than light. I think one of them was photons and then there were some that havn't been proven to exist. Do any of you guys know what were they?

 

[selfAdjoint]

I read in physics that it is impossible to get at the speed of light b/c it requires infinite force and I HEARD that if you do you can go to the future and if you cross it, you go to the past. Now I am having problem understanding the HEARD part. I mean what makes you reverse or accelerate entropy?

First of all speed in relativity is relative, duh. So all statements about going some speed or other need to be qualified with stating the observer ("inertial frame") which the speed is relative to. Observers seeing another body at a speed relative to them will experience that body's length, time, and energy as altered by a formula that depends on the relative speed. Lengths are shorter, times are dilated, and kinetic energy increases, relative to the observer's own values. Note that many differently moving observers are possible, each one with its own relative speed experience of the body and its own formula for adjustment.

As the relative speed approaches the speed of light, lengths and time durations shrink toward zero, relative to any observer, while kinetic energy increases without limit. So no, a body which has mass cannot get to the speed of light if it has mass. On the other hand, a body which is massless, like the photon must travel at exactly the speed of light.[/quote]

btw, they were talking about some objects that nevertheless do travel faster than light. I think one of them was photons and then there were some that havn't been proven to exist. Do any of you guys know what were they?

You have mixed up a discussion of massless particles, including the photon, which must travel at the speed of light relative to every observer; that is AT the speed of light, not over it!

The math of relativity allows for particles that always travel faster than light. They have the same length, time and energy problems slowing down to c that our particles do in accelerating up to it. So they can't cross the c-limit from above and we can't cross it from below. These particles are entirely hypothetical; no-one has ever detected one, and many people have tried. These FTL particles (there could be many types of them) are generically called tachyons, and our kind of particles are similarly called bradyons. These names are from the Greek words for "fast" and "slow" respectively. Some physical theories, notably string field theory, predict tachyons, and there is theoretical work on what properties they would have. One thing that relativity requires is that tachyons have mass a multiple of \sqrt{-1}.

So there are generically three kinds of particles in pure theory:
1. Bradyons, our kind, have masses > 0 and must travel slower than light relative to all observers.

2. Massless particles, have mass = 0, and travel exactly at the speed of light.

3. Tachyons have masses that are multiplied by \sqrt{-1} and must travel at speeds greater than light.

 

[Skhandelwal]

Does inertial frame really exists? Since the whole galaxy is traveling and who knows, may be the universe is too!

 

[selfAdjoint]

Does inertial frame really exists? Since the whole galaxy is traveling and who knows, may be the universe is too!

If you can't detect any acceleration or forces, you can assume your frame is inertial fo all practical purposes. In practice, even on the rotating earth, laboratories and their instruments are considered inertial with respect to fast moving subatomic particles.

 

[Skhandelwal]

So you are saying that even if it isn't, it doesn't really make a difference. Right? Btw, how did they came up w/ tachyons? I mean I am sure it wasn't a random guess.

 

[michael879]

So you are saying that even if it isn't, it doesn't really make a difference. Right? Btw, how did they came up w/ tachyons? I mean I am sure it wasn't a random guess.

the equation you can see evidence of them from is m_r = \gamma*m_0 and \gamma=1/\sqrt{1-v^2/c^2} if v < c then m_r increases with speed. if v = c then m_r is infinite and if v > c then m_r is imaginary. Also, \delta t = 1/\gamma for a reference frame moving at v relative to you. If something goes at c, its time difference compared to yours goes to 0 which means they are basically stopped in time relative to you. If it goes over c it has imaginary time relative to you but I believe there is a way to manipulate a situation in which something goes faster than c in order to send information to the past.

o yea also, E = m_r*c^2 shows the energy of something with a velocity (not sure if its the right equation it may be E = 1/2*m_r*v^2) so as v->c, E -> infinity (or -infinity if its from the >c direction).

 

[Skhandelwal]

What about that time traveling thing?

 

[DaveC426913]

When I turn on the engines of my rocket, am I now in an ertial frame?

 

[selfAdjoint]

When I turn on the engines of my rocket, am I now in an ertial frame?

As long as your engines were the only (measurable) source of acceleration, yes. Usually in these thought experiments that is assumed to be the case. When you are coasting through space at a constant speed relative to other observers you are inertial, and your physical experiments should work just as if you were at rest.

 

[DaveC426913]

As long as your engines were the only (measurable) source of acceleration, yes. Usually in these thought experiments that is assumed to be the case. When you are coasting through space at a constant speed relative to other observers you are inertial, and your physical experiments should work just as if you were at rest.

Your humouradar is busted.

Is an object's tendency to be pushed around easily known as its ertia?


"Now most of the these landmines are inert, however some of them are ... ert."
- Sgt Hulka, Stripes (1981)

 

[michael879]

What about that time traveling thing?

if you have something going faster than light there is a way to send information to the past. It has to do with the time difference being imaginary, not really sure how to do it tho.

 

[Skhandelwal]

Is it like that time keeps getting slower the faster we travel and at the speed of light, it stops so after, we imagine that it will will traveling in the past?

But then how do we go into the future?

 

[michael879]

Is it like that time keeps getting slower the faster we travel and at the speed of light, it stops so after, we imagine that it will will traveling in the past?

But then how do we go into the future?

not exactly. If you have something going at faster than c relative to you, it's clocks will go some imaginary time for every second the clocks go in your frame. Its hard to say exactly what that means. However if you multiply two imagary numbers you get a negative number. A negative time represents the frame going backwards in time relative to you. Like I said, I am not even positive if its possible but I've read multiple times that if you can find something going FTL you can send information to the past. I gave you the equation that shows that time slows down relative to you as something speeds up and then becomes imaginary.

to go into the future you just do nothing lol. You can't really prevent yourself from going into the future without going >= c. To go into the future faster than everyone else, you just move relative to them.

 

[Skhandelwal]

What do you mean by saying, moving relative to them inorder to go to future faster?
Btw, what is making us move to the future? I heard something about entropy but didn't exactly get it.

 

[JesseM]

What about that time traveling thing?

It has to do with the fact that different reference frames in relativity disagree about "simultaneity", which means that if you have two events that happen at the same time in one frame, they happen at different times in another frame. But it works out so that if you send a signal at the speed of light or slower, then if the event the signal being sent happens before the event of it being received in one frame, then every frame will agree the event of being sent happens before being recieved. However, if you could send a signal faster than light, it would always work out so that some inertial frames would see the signal being received before it was sent! Not only that, but since relativity says the laws of physics must work the same in every frame, that means if it is possible in some frames to send a signal such that it is received before it is sent, then that means it must be true in every frame. Because of this, if FTL signals were possible and the laws governing these signals worked the same way in every frame, then if I was moving away from you at slower-than-light speeds, you could send me a signal FTL in your frame and backwards in time in my frame, then I could send a reply FTL in my frame and backwards in time in your frame, and you would actually receive my reply before you sent the original signal.

 

[michael879]

What do you mean by saying, moving relative to them inorder to go to future faster?
Btw, what is making us move to the future? I heard something about entropy but didn't exactly get it.

well if you move at some high velocity relative to Earth and then come back at that same velocity, Earth will have aged more than you. (due to general relativity, not special). One theory explaining time (that I like to use to visualize it) is that the universe is a hypersphere with our 3d universe on its surface. The radius of this hypersphere is time so that the farther away from the center you are, the farther in time youve traveled. The big bang is what makes the hypersphere expand.

 

[JesseM]

well if you move at some high velocity relative to Earth and then come back at that same velocity, Earth will have aged more than you. (due to general relativity, not special).

This can actually be understood using either special relativity or general relativity--see this FAQ on the twin paradox for more. SR does only deal with inertial reference frames, but you can calculate how much time will elapse on an accelerating clock by integrating the rate of its ticking throughout the journey as seen in a given inertial frame.

 

[Skhandelwal]

Ok, so if I invent a time machine, and I go to the past, would I meet myself, or would time reverse?

 

[michael879]

Ok, so if I invent a time machine, and I go to the past, would I meet myself, or would time reverse?

thats a good question. Does anyone really know what would happen if you traveled into the past? Its theoretically possible..
I know in movies youd actually see everything like it was back in the past and youd be able to change the future but time isn't like that in reality is it? Like if the universe were to contract, would everything replay itself except in reverse? or would it still seem like time were going forward?

 

[Skhandelwal]

the thing is, if you mess up w/ time, the universe corrupts(grandfather paradox) so I don't think that would happen but even if we do back in time, just reversing the time, it would create this thing where you come to future, you go back to time, you come to future again, then go back to time, till eternity. One thing I am unsure about is that would you put the whole world in cycle or just yourself? Another thing that can happen is just seeing the past but how you described it to me, I don't think that would happen by this method, what do you think?

 

[DaveC426913]

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

Time would march forward. Entropy would increase. Glasses would not miraculously jump off the floor and reassemble themselves on the counter.

 

[Skhandelwal]

What did you mean by contract michael?

 

[michael879]

Time would march forward. Entropy would increase. Glasses would not miraculously jump off the floor and reassemble themselves on the counter.

yea that's what I figured, so going into the past doesn't mean you can actually see past events.

 

[michael879]

What did you mean by contract michael?

If there were enough mass in the universe, the gravity would eventually slow the universe's expansion down to 0, and then contract it all into a point. However it has been shown that there isn't enough mass and that the universe won't contract. I am pretty sure I've read that the universe's expansion is actually accelerating (in a hyperspherical universe with a constant time expansion two points on the surface would accelerate away from each other so I am not sure what they meant by it).

 

[JesseM]

yea that's what I figured, so going into the past doesn't mean you can actually see past events.

He wasn't talking about going into the past, he was talking about whether entropy would reverse if the universe began to contract (and in fact no one is sure about the answer to this, since no one really knows the explanation for the low entropy of the big bang--a cosmological model actually was once proposed by physicist Thomas Gold with low-entropy boundary conditions at both the big bang and the big crunch, and this would imply that entropy would reverse when the universe began to contract). When physicists talk about the possibility of actually visiting the past (the technical name for this is a 'closed timelike curve'), they mean it in the literal sense of meeting your younger self, although of course most physicists would guess the laws of physics prevent you from doing this in the first place (even though general relativity seems to allow it in some circumstances, and no one has definitively ruled out the possibility of faster-than-light particles called 'tachyons' in special relativity, which would allow you to send signals into your own past, so in theory this would let you travel to the past as well if you had a star-trek style transporter).

 

[Skhandelwal]

WOW I would like to know more about this stuff, it seems cool, do you have any google terms you could share for me to seach for it?(may be that guy's name or his theory's name, etc.)

 

[JesseM]

WOW I would like to know more about this stuff, it seems cool, do you have any google terms you could share for me to seach for it?(may be that guy's name or his theory's name, etc.)

Going backwards in time, you mean? You google for stuff like "closed timelike curves" and "wormholes", maybe with "time travel" as an added search term, to find more. I also highly recommend the book https://www.amazon.com/dp/0393312763/?tag=pfamazon01-20 by Kip Thorne, the physicist who first realized that general relativity seems to allow for "traversable wormholes" which could make time travel possible (although most physicists would probably bet that when quantum theory is integrated with general relativity, it will eliminate these time travel solutions).

 

[michael879]

He wasn't talking about going into the past, he was talking about whether entropy would reverse if the universe began to contract (and in fact no one is sure about the answer to this, since no one really knows the explanation for the low entropy of the big bang--a cosmological model actually was once proposed by physicist Thomas Gold with low-entropy boundary conditions at both the big bang and the big crunch, and this would imply that entropy would reverse when the universe began to contract). When physicists talk about the possibility of actually visiting the past (the technical name for this is a 'closed timelike curve'), they mean it in the literal sense of meeting your younger self, although of course most physicists would guess the laws of physics prevent you from doing this in the first place (even though general relativity seems to allow it in some circumstances, and no one has definitively ruled out the possibility of faster-than-light particles called 'tachyons' in special relativity, which would allow you to send signals into your own past, so in theory this would let you travel to the past as well if you had a star-trek style transporter).

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

 

[Skhandelwal]

thx. a lot! just one more thing, from photon's pov, is the world not moving or is it moving at an infiniteous rate?(I'd pick not moving)

 

[Skhandelwal]

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 definate answer, guess there isn't one. But what is the current theory on what happens when you travel faster than light?(be specific)

 

[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 definate 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?