# Why can't energy or information go faster than light?

jbriggs444
Homework Helper
actually it is what I am trying to express
All right. So you have decided that we have this hypothetical capability to send objects or signals between two arbitrary events in space-time. And just so we are not cheating too outrageously, we will require that the points be "space-like separated". That means that you cannot directly link one event with another that is unambiguously in its past. In other words, you can't send a signal directly to an event from which you could then hand-carry a response back the other way.

The point you seem to want to make is that this capability does not count as "Faster Than Light travel" because you can divide distance covered (on the hypothetical short-cut non-folded path) by elapsed time (on a wrist watch carried on the short-cut non-folded path) and arrive at a figure less than the speed of light (c).

That's true. You do not need to call this faster than light if you do not want to. But that's just word choice. It does not affect the physics. If you can get a signal from point a to point b and the signal can get there faster than light could (following the normal long-way path) and if you can do this for any pair of space-like separated events then that is all that is needed to construct a tachyonic anti-telephone and start generating paradoxes.

You could also Google for "closed timelike curve". A path that goes from point a, FTL to point b, FTL to point c and then snail mail back to point a counts as a closed timelike curve.

danielhaish
Dale
Mentor
2020 Award
actually it is what I am trying to express
This does not negate the problem identified above at all. Even with this type of spacetime with a shortcut you still wind up with the possibility of closed timelike curves and hence a breakdown of causality.

danielhaish and jbriggs444
All right. So you have decided that we have this hypothetical capability to send objects or signals between two arbitrary events in space-time. And just so we are not cheating too outrageously, we will require that the points be "space-like separated". That means that you cannot directly link one event with another that is unambiguously in its past. In other words, you can't send a signal directly to an event from which you could then hand-carry a response back the other way.

The point you seem to want to make is that this capability does not count as "Faster Than Light travel" because you can divide distance covered (on the short-cut non-folded path) by elapsed time (on a wrist watch carried on the short-cut non-folded path) and arrive at a figure less than the speed of light (c).

That's true. You do not need to call this faster than light if you do not want to. But that's just word choice. It does not affect the physics. If you can get a signal from point a to point b and the signal can get there faster than light could (following the normal long-way path) and if you can do this for any pair of space-like separated events then that is all that is needed to construct a tachyonic anti-telephone and start generating paradoxes.
can you explain the paradox in the fallowing example , I am a little confused . let assume that there is two location with difference time frames and in each location there is one quant , and the two quant are entanglement to each other . lets take a look at the properties of the location of each quant . each quant have a possibility to be in every location but when you put measure device on one quant it like sending a signal to the past that change the wave function of the quant . but no one can tell that the measured has taking place in the other location according to the No-communication theorem . and also it not like the location of the quant has changed because before the measure of the location of the quant it still could be where the measurement took place (the result of the measure). so the past didn't practically changed till you reach to the future . I meant that the observer can see the signal only after some time and adter data about weather the other quant has been measured so it not effect him and also the location of the quant is not necessarily changed . let assume there is two people in each location and one should jump to the pool when signal arrive according to the paradox it not possible that the signal is being send faster then the light because then it would change the past and make one man jump to the pool in the past but the other person cant tell that the signal have change in the past

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Grasshopper and PeroK
jbriggs444
Homework Helper
can you explain the paradox in the fallowing example , I am a little confused . let assume that there is two location with difference time frames and in each location there is one quant , and the two quant are entanglement to each other . lets take a look at the properties of the location of each quant . each quant have a possibility to be in every location but when you put measure device in one quant it like sending signal to the past that change the wave function of the quant . but no one can tell that the measured has taking place in the other location according to the No-communication theorem . and also it not like the location of the quant has changed because before the measure the location of the quant still could be where the measurement took place. so the past didn't practically changed till you reach to the future . I meant that the observer can see the signal only after some time and adter data about weather the other quant has been measured so it not effect him and also the location of the quant is not necessarily changed
I thought I had a prayer of understanding what you were talking about this time.

The prayer was not answered.

Probably it has something to do with quantum entanglement not transmitting actionable information due to the no-communication theorem. [I was careful to use the phrase "actionable information" in my posts up-thread in order to avoid discussions like this about quantum entanglement]

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Grasshopper, Vanadium 50 and phinds
I thought I had a prayer of understanding what you were talking about this time.

The prayer was not answered.
I add something in the end . I am referring to the pardox that changing the past cant change the future

Staff Emeritus
I am a little confused

That is clear.

t assume that there is two location with difference time frames

That is not.

and in each location there is one quant

Nor is that. What is a "quant"?

but when you put measure device on one quant it like sending a signal to the past

I understood that! But it is not correct.

PeterDonis
Mentor
2020 Award
when you put measure device on one quant it like sending a signal to the past that change the wave function of the quant

No, it isn't. The measuring device only affects the thing being measured at the spacetime location of the measurement. It doesn't affect anything to the past.

Dale
Mentor
2020 Award
@danielhaish I am sorry but your posts simply do not meet our quality standards. I am closing this thread. I don’t know that we will be able to help you at all here. You may want to find a forum in your native language where the others may have a chance to understand what you are asking.

berkeman