Is Faster Than Light Travel Really Traveling Backwards In Time?

[byron178]

On wikipedia it says that nothing travels faster than light but yet their things that do travel faster than light,even though these processes do not carry information do they still travel backwards in time?

In the context of this article, FTL is transmitting information or matter faster than c, a constant equal to the speed of light in a vacuum, 299,792,458 meters per second, or about 186,282.4 miles per second. This is not quite the same as traveling faster than light, since:

Some processes propagate faster than c, but cannot carry information (See Examples section immediately following)).
Light travels at speed c/n when not in a vacuum but traveling through a medium with refractive index = n (causing refraction), and in some materials other particles can travel faster than c/n (but still slower than c), leading to Cherenkov radiation (see phase velocity below)

Neither of these phenomena violates special relativity or creates problems with causality, and thus neither qualifies as FTL as described here.

In the following examples, certain influences may appear to travel faster than light, but they do not convey energy or information faster than light, so they do not violate special relativity.

 

[russ_watters]

O...but yet their things that do travel faster than light...

What things?

 

[Dale]

E.g a laser pointer dot can move faster than c, so in some frames I think it will sweep left to right and in others right to left.

 

[byron178]

E.g a laser pointer dot can move faster than c, so in some frames I think it will sweep left to right and in others right to left.

so that would be time travel backwards?

 

[byron178]

What things?

The example with the laser pointer swept across,where it can be faster than light,would it be swept so fast that it travels backwards in time?

 

[Dale]

No information is transmitted so what would "travelling backwards in time" even mean?

In one frame it goes left to right and in another it goes right to left. Those two situations are the time reverse of each other, but would you really call one going "back in time" and the other going "forward in time"?

If so, which one is forwards and which one is backwards? Is left forwards and right backwards or is right forwards and left backwards? I don't think it is reasonable to say it that way.

 

[JesseM]

The example with the laser pointer swept across,where it can be faster than light,would it be swept so fast that it travels backwards in time?

No "thing" goes FTL or backwards in time, it's just a bunch of separate photons hitting the surface at different points in space and time, each one creating the appearance of a "dot" at the position and time the photon hits the surface. If you rotate the laser pointer from left to right, in some frames the photons going to the right have a shorter distance to travel than the ones going to the left, and thus the photons hit the right side of the surface before other photons hit the left side, so the dot appears to go from right to left. In other frames the photons going to the left have a shorter distance and thus the photons hit the left side before other photons hit the right side, so the dot appears to go from left to right. So if you have a marker L on the left side, and another marker R on the right, in some frames the dot seems to travel FTL starting at L and ending up at R, in others it seems to travel FTL starting at R and ending up at L. We wouldn't normally think of either one as going "backwards in time" since the dot carries no information FTL, but if you had an actual FTL signal emitted from L and received at R, then if there was a frame where it was received at R before it was emitted at L, in that case we could say the signal went backwards in time.

 

[berkeman]

Thread is re-opened.

 

[byron178]

No "thing" goes FTL or backwards in time, it's just a bunch of separate photons hitting the surface at different points in space and time, each one creating the appearance of a "dot" at the position and time the photon hits the surface. If you rotate the laser pointer from left to right, in some frames the photons going to the right have a shorter distance to travel than the ones going to the left, and thus the photons hit the right side of the surface before other photons hit the left side, so the dot appears to go from right to left. In other frames the photons going to the left have a shorter distance and thus the photons hit the left side before other photons hit the right side, so the dot appears to go from left to right. So if you have a marker L on the left side, and another marker R on the right, in some frames the dot seems to travel FTL starting at L and ending up at R, in others it seems to travel FTL starting at R and ending up at L. We wouldn't normally think of either one as going "backwards in time" since the dot carries no information FTL, but if you had an actual FTL signal emitted from L and received at R, then if there was a frame where it was received at R before it was emitted at L, in that case we could say the signal went backwards in time.

What about the example with a shadow,it can travel faster than light,my question is even though it does not carry information will somebody see the effect then the cause i.e causality violation?will it travel backwards in time without any information?

 

[Drakkith]

What about the example with a shadow,it can travel faster than light,my question is even though it does not carry information will somebody see the effect then the cause i.e causality violation?will it travel backwards in time without any information?

You're shadow isn't an object. It is the result of you blocking the path of light. If you blocked a laser that was shining on the moon, it would take a second or so for someone on the moon to see that you had blocked the laser. In the case of a shadow moving across a surface, your shadow isn't anything physical. Nothing is moving.

 

[Dale]

will somebody see the effect then the cause i.e causality violation?

No, this will not happen.

 

[byron178]

No, this will not happen.

i read that it does because the shadow travels faster than light,and from what i understand a different observer will see it go backwards in time.if you scrol down to shadows and light spots: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

 

[Drakkith]

i read that it does because the shadow travels faster than light,and from what i understand a different observer will see it go backwards in time.if you scrol down to shadows and light spots: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

No. Again, a shadow is not an object.

 

[JesseM]

i read that it does because the shadow travels faster than light,and from what i understand a different observer will see it go backwards in time.

No, it's just like with the laser dot, some see it go from L to R and others see it go from R to L, but there's no reason to call either motion "backwards in time". It's only when some signal or causal effect is being transmitted from one to the other that some observer has reason to say it went "backwards in time" (because the effect happened before the cause, or because the event of the signal being received happened before the event of it being sent)

 

[byron178]

No, it's just like with the laser dot, some see it go from L to R and others see it go from R to L, but there's no reason to call either motion "backwards in time". It's only when some signal or causal effect is being transmitted from one to the other that some observer has reason to say it went "backwards in time" (because the effect happened before the cause, or because the event of the signal being received happened before the event of it being sent)

but isn't the laser dot a physical thing?Correct me if I am wrong,Relativity says that if you or soemthing such as shadows travel faster than light one observer will see it going backwards in time am i right,to the past?

 

[Dale]

i read that it does because the shadow travels faster than light

The shadow can travel faster than light, but that was not what you asked. You asked if the effect could preceed the cause in any frame, and that is not possible. Suppose we have a shadow which travels from A to B at 2c. Then, it is true that in some frame "shadow at B" preceeds "shadow at A". However, if "shadow at A" is the cause then "shadow at B" cannot be the effect since no information is transferred from A to B. Therefore, the fact that the shadow goes from B to A in some frame does not imply that effect preceeds cause in that frame.

 

[JesseM]

but isn't the laser dot a physical thing?

No, it's just a series of photon hits from different photons. Imagine I have a machine gun which I point at a wall, and as I continuously fire I rotate the machine gun from left to right. In this case you'll see a row of bullet holes moving across the wall from left to right, and potentially if the speed of the bullets was great enough and the distance to the wall was large enough it might be possible for this row to move faster than light, even though each individual bullet had moved slower than light from the gun to the position on the wall it struck.

Correct me if I am wrong,Relativity says that if you or soemthing such as shadows travel faster than light one observer will see it going backwards in time am i right,to the past?

You are wrong. If one observer sees it starting at one point L and later moving to another point R, then a different observer will see the order reversed so it starts at R and later moves to L, but with no causal influence or signal being transmitted from L to R, there's no reason to see the R to L version as "backwards in time", it's just a disagreement about the order.

 

[byron178]

E.g a laser pointer dot can move faster than c, so in some frames I think it will sweep left to right and in others right to left.

how could this happen if a shadow is not a physical thing?

 

[Drakkith]

how could this happen if a shadow is not a physical thing?

Think back to the laser dot. The path each photon takes is a straight path (ignoring gravity and such) and travels at c. Let's say when you sweep the pointer across, say the moon, the point sweeps across at greater than c. But remember, the laser is made up of photons. Each photon travels at c. The dot is simply the reflection of these photons.

Now, imagine that you were standing on the moon and my laser point came screaming across the surface towards you. Since the point is traveling greater than c, the photons reflected when the point is closer to you will reach you before the photons from that are reflected when the point is further away from you. It will look like the laser dot is traveling away from you instead of towards you because of this, but it is merely an "optical illusion" effectively. There is no traveling through time.

 

[byron178]

Think back to the laser dot. The path each photon takes is a straight path (ignoring gravity and such) and travels at c. Let's say when you sweep the pointer across, say the moon, the point sweeps across at greater than c. But remember, the laser is made up of photons. Each photon travels at c. The dot is simply the reflection of these photons.

Now, imagine that you were standing on the moon and my laser point came screaming across the surface towards you. Since the point is traveling greater than c, the photons reflected when the point is closer to you will reach you before the photons from that are reflected when the point is further away from you. It will look like the laser dot is traveling away from you instead of towards you because of this, but it is merely an "optical illusion" effectively. There is no traveling through time.

so the dot and shadow propagation is an illusion?

 

[Drakkith]

so the dot and shadow propagation is an illusion?

Perhaps illusion was the incorrect word. The dot and shadow will move greater than c, but it doesn't violate any laws because a shadow or laser dot is NOT a physical object. Both are the result of light being obscured or shone on a surface. And as I said before, the dot or shadow might LOOK like it is traveling backwards to you, and in fact that is completely valid from your frame, but the light that had to be transmitted to cause the laser dot did indeed sweep towards you, not away from you. Measuring the "velocity" of the dot from your frame will tell you that the laser dot traveled towards you, not away from you, even though it looks like it did. That's probably kind of confusing, but that's about the best way I can describe it.

The light from the laser STILL takes a second or so to reach the moon, so even if you wanted to you could not transmit any information faster than light.

 

[byron178]

Perhaps illusion was the incorrect word. The dot and shadow will move greater than c, but it doesn't violate any laws because a shadow or laser dot is NOT a physical object. Both are the result of light being obscured or shone on a surface. And as I said before, the dot or shadow might LOOK like it is traveling backwards to you, and in fact that is completely valid from your frame, but the light that had to be transmitted to cause the laser dot did indeed sweep towards you, not away from you. Measuring the "velocity" of the dot from your frame will tell you that the laser dot traveled towards you, not away from you, even though it looks like it did. That's probably kind of confusing, but that's about the best way I can describe it.

The light from the laser STILL takes a second or so to reach the moon, so even if you wanted to you could not transmit any information faster than light.

On the faqs page in sent you it also talks about virtual particles traveling faster than light? how is this possible if they are not real?

 

[DaveC426913]

Again: a laser dot and a shadow are not "things". A photon hits here, then another photon hits there, very far away. The two photons have nothing to do with each other. The only way they are a "thing" at all is in the mind of an observer with persistence of vision who decided to correlate two otherewise unrelated events. At no time has anything moved from here to there.

Shadow likewise.

 

[DaveC426913]

Perhaps illusion was the incorrect word. The dot and shadow will move greater than c

No, illusion is the correct word. The "moving dot" itself is an illusory thing. It does not exist at all.

If a star exploded in Regulus, then 1 second later another star exploded in Canis Major, would we call that a "moving dot"? No.

No need to go into whether "it" is carrying any signal; there's no "it" in the first place.

 

[byron178]

No, illusion is the correct word. The "moving dot" itself is an illusory thing. It does not exist at all.

If a star exploded in Regulus, then 1 second later another star exploded in Canis Major, would we call that a "moving dot"? No.

No need to go into whether "it" is carrying any signal; there's no "it" in the first place.

Even though the shadow carries no information does it still travel backwards in time?

 

[Vanadium 50]

There is no "it" to travel anywhere. Or anywhen.

 

[Quinzio]

Between the points of the shadow there is no causality.
SR states that object cannot influence each other if they are not reachable by light.
Eg. A is out of the light cone of B, so be cannot B send signals to A.

Once the photons that will form the shadow has been thrown away (and they hit the object of the shadow), the form of the shadow is already decided, and there's nothing we can do to change it.
Hence there's nothing carrying any signal, all is decided.

Yes, the shadow may give the impression to carry a signal, but the signal is not modifiable anymore, it's been already shaped.

Eg. this message maybe will be read by Byron and Dave in the same instant, but it was me to send some signals to Dave and Byron, it's NOT Byron sending signals to Dave faster than light (although they read in the same moment).

 

[Drakkith]

Even though the shadow carries no information does it still travel backwards in time?

Nope. Remember that a shadow or a laser dot isn't a "thing".

 

[Drakkith]

Actually, think of it like this. Each individual photon in the laser beam is it's own "dot". So each split second or whatever that you see the dot moving towards you (or away from you), it is a new dot, because new photons have been reflected.

 

[byron178]

Actually, think of it like this. Each individual photon in the laser beam is it's own "dot". So each split second or whatever that you see the dot moving towards you (or away from you), it is a new dot, because new photons have been reflected.

I asked around and got this:For anything moving faster than light you cannot assign a temporal order to points on its worldline, so the question does not really make sense.

If you pick two points where the shadow falls on some objects (call them A and B), some observers will see the shadow fall on A first and then B while others will see it fall on B first and then A. This is true even if the observers take into account the time light takes to get to them from A and B, because simultaneity is relative.



The observed order of the shadow passing over A and B may be opposite to the order in which the object casting the shadow passed in front of the light, however this is not so amazing because it is also true in ordinary Newtonian physics given a finite value for the speed of light. Just imagine that B is much closer to the light than A to see why. If you want to call this “travelling backwards in time” you can do so, but there is no useful meaning to it.