How does light travel create time travel violating causality?

Question

Saw a question about faster than light travel... I still have the same question though none of the answers offered any resolution for me. It is so summarily assumed by all physicists and commentaries that exceeding the speed of light would turn back the clock. I can't see the relation.

Doubling the amount of any speed halves the time taken to travel a given distance. Keep doubling the speed and that time is halved (or otherwise divided). Divide any quantity (time in this case) and you always end up with a fraction of it but never zero and certainly never a negative amount as would be the case for the causality conflict. So it seems to me that whatever speed one attains, there is always a positive time element in the travel no matter how tiny!!

The speed of light is only unique to me in that it is the fastest observed speed but is otherwise just another speed quantity set by nature (just like the speed of sound etc) could it be that other elements in nature are travelling faster than light but we lack the means to detect or measure them (like the rebellious neutrino)?

I also don't understand time as an independent element that can be slowed sped up etc. It seems to me that time is simply a relative measure of the ever-changing state of matter relative to other states of matter.

If every thing in the universe stopped- that is all state of matter everywhere frozen, all electrons frozen in place etc wouldn't we observe that time had stopped? Isn't it therefore our observation of the changing state of matter around us that gives the perception (perhaps illusion) of time?

I can therefore only understand time as a subjective sense of changing states relative to an observer! It should be the rate of change of these states that slow down or speed up (in relation to the observer or instrument) and not the universal rate of change or universal time that changes! It would also debunk any notion of time travel, as it would involve the manipulation of every particle in the universe to a previous of future state...

Disclaimer.. I hate calculations, stink at them and have no idea what mathematical formulas are used to arrive at the accepted conclusions so I'm not trying to dispute any findings etc by the experts, just trying to align my lay understanding to their conclusions.

Answer 1

Disclaimer.. I hate calculations, stink at them and have no idea what mathematical formulas are used to arrive at the accepted conclusions so I'm not trying to dispute any findings etc by the experts, just trying to align my lay understanding to their conclusions.

Main thing is, all these strange theories come up only after scientists try to match experimental data with "mathematical formulas". Which is why plain intuition won't get you too far.

I also don't understand time as an independent element that can be slowed sped up etc. It seems to me that time is simply a relative measure of the ever-changing state of matter relative to other states of matter.

There are some phenomena which take fixed amounts of time in a given frame. You can always use these as benchmark "clocks". Time is a relative measure, but in more ways than one. Other than what you say, time is also relative to the observer, and there is no "absolute" time. Time isn't being "slowed down", really -- just that some events that take a certain amount of time appear to take a different amount of time when viewed from a different reference frame.

If every thing in the universe stopped- that is all state of matter everywhere frozen, all electrons frozen in place etc wouldn't we observe that time had stopped? Isn't it therefore our observation of the changing state of matter around us that gives the perception (perhaps illusion) of time?

Yes, completely true. That changes nothing..

I can therefore only understand time as a subjective sense of changing states relative to an observer! It should be the rate of change of these states that slow down or speed up (in relation to the observer or instrument) and not the universal rate of change or universal time that changes! It would also debunk any notion of time travel, as it would involve the manipulation of every particle in the universe to a previous of future state...

Except that there is no "universal timeline". The minute we talk about relativity, we discard any notions of a "special" reference frame. Everything is derived assuming that all reference frames moving at constant velocities are equivalent--thus there is no frame which you can point out and say "this is the Universe's record of events, it is more correct than the others". It's the intuitive way of looking at it, but here intuition fails us. We just can't look at it this way because physics does not allow such a privileged viewpoint.

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With that in mind, we can address your final issue:

Doubling the amount of any speed halves the time taken to travel a given distance. Keep doubling the speed and that time is halved (or otherwise divided). Divide any quantity (time in this case) and you always end up with a fraction of it but never zero and certainly never a negative amount as would be the case for the causality conflict. So it seems to me that whatever speed one attains, there is always a positive time element in the travel no matter how tiny!!

You've got to remember that we have multiple reference frames here. The time we're talking about isn't the time in "speed=dist/time". It's a different time interval we're measuring. Let's look at a spaceship travelling at some speed. In the spaceship, we have a clock. Now, for a viewer outside the spaceship, he will perceive that the clock is going slow. How? He can send light pulses to the clock to measure the motion of the hands of the clock. Light travels at a finite speed, so he must recompensate for that. After doing that, he stil gets the result that the clock is slower from his point of view. If the velocity was increased, it gets even slower -- with the limiting condition that if velocity reaches $c$, then the clock does not tick at all from the outside viewer's point of view. By this logic, if speed is increased further, the clock ought to start ticking backwards. Note that speed is still (distance travelled by spaceship)/(time taken by ship). The time we're dealing with is the time taken between events generated by a "clock".

Answer 2

If you have a clock at earth and a lot of clocks on a spaceship setting off at faster than the speed of light. When you send each clock back to earth you are saying each of the clocks will show a different time to the one on earth when it arrives back. (And I'm talking about a clock unaffected by the lack of gravity). Why as there is nothing to physically act on each clock?

An object moving up to and faster than the speed of light would send an image back once at t0 and multiple times at t+n. Imagining each image to be a timestamped postcard accurate to the trillisecond saying here I am. Up to c the images would appear to slow. But beyond c it would all appear to be the same image sent at different times/places but the trillisecond clock would still be showing differences. To the external viewer this collision of light images would probably appear as a bright flash or a streak. There would no ticking backwards - the postcards would not lie you cannot go back in time. As a spaceship arrives at its destination the observers would see a reverse of this. As the clock image postcards of arriving and arrived all appear at once. Its the same as a sonic boom just a photonic flash.