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# Faster Than Light and Time Travel

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I'll second that

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Just a slight somewhat off topic remark if I may. I learn from these boards, but they really exist to exchange views, not teach. If one doesn't understand a topic one can find many things to help them get the basics, so that when one comes here one doesn't seem quite so stupid.

There is of course no such thing as a dumb question, so long as it is a sincere question, but so often questions are asked not for information but as challenges because the person doesn't understand and is not willing to admit that something they don't understand might nevertheless be true. Arguing against well-established scientific theory all across the board is the most egregious example of this sort of, what can I call it (?) -- arrogant ignorance? -- where an inability to understand leads to the conclusion it must be false.

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Now I will ask my dumb question. Why is there a maximum response time of the universe and why is it what it is? Also might I put in that it might be better to say "maximum response time of space-time?"

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That one still bugs me, just like Simultaneity of moving train stroke by lighting on both ends

Both are correct in their own frame of reference, but does that means both events did happen or only a single event?

Both events happened.

Both lightning strikes occurred at specific coordinates in space-time.

Exactly what those coordinates are depend on your frame of reference - and therefore they are arbitrary.

However the space-time interval between these two events is a scalar invariant - and therefore is ``real''. Every observer, regardless of reference frame, will agree on the space-time interval between these two events.

However measurements like ``past'' and ``future'' are characteristics of a set of coordinates.

what if we installed sensors on both ends and just measure which one is correct once and for all?

We can't. There is no correct time-difference between two events. Only a correct space-time interval.

for this the maximum response speed of the Universe, t is still greater than 0 right

It might not be - as I mention above, time depends entirely on your coordinate set. The space-time interval between the two events will be non-zero unless they are connected by a light-speed trajectory.

What people perceive as ``time'' is actually the progressive increase in the space-time interval of your own personal reference frame. This is what sets your ``age'', and everyone will agree on how old you are regardless of what reference frame you are in.

But as the so-called ``twin paradox'' shows, people can accumulate a different age over the same period of time (as measured by only one observer).

Now I will ask my dumb question. Why is there a maximum response time of the universe and why is it what it is? Also might I put in that it might be better to say "maximum response time of space-time?"

I am not sure there is a good answer to that, other than ``because''.

Well, to be more specific, the ``maximum response speed of the Universe'' is 1 in ``natural units''. So there is no real mystery why it has a specific value; it is the natural length/time scale by which we measure everything. Our definition of 1 metre and 1 second, in contrast, are somewhat ad-hoc and totally arbitrary - they just happen to be convenient scales to measure everyday experiences.

But to your deeper question, I don't think there is a good explanation why there is a ``maximum response speed of the Universe''.

If there was no maximum response speed, then many things that are connected in our Universe would be separated (for example electric and magnetic fields would be totally separate and unconnected, there would be no mass-energy equivalence). But this doesn't mean that things wouldn't work - just that they would work differently.

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Both events happened.

Both lightning strikes occurred at specific coordinates in space-time.

Exactly what those coordinates are depend on your frame of reference - and therefore they are arbitrary.

However the space-time interval between these two events is a scalar invariant - and therefore is ``real''. Every observer, regardless of reference frame, will agree on the space-time interval between these two events.

However measurements like ``past'' and ``future'' are characteristics of a set of coordinates.

We can't. There is no correct time-difference between two events. Only a correct space-time interval.

It might not be - as I mention above, time depends entirely on your coordinate set. The space-time interval between the two events will be non-zero unless they are connected by a light-speed trajectory.

What people perceive as ``time'' is actually the progressive increase in the space-time interval of your own personal reference frame. This is what sets your ``age'', and everyone will agree on how old you are regardless of what reference frame you are in.

But as the so-called ``twin paradox'' shows, people can accumulate a different age over the same period of time (as measured by only one observer).

I am not sure there is a good answer to that, other than ``because''.

Well, to be more specific, the ``maximum response speed of the Universe'' is 1 in ``natural units''. So there is no real mystery why it has a specific value; it is the natural length/time scale by which we measure everything. Our definition of 1 metre and 1 second, in contrast, are somewhat ad-hoc and totally arbitrary - they just happen to be convenient scales to measure everyday experiences.

But to your deeper question, I don't think there is a good explanation why there is a ``maximum response speed of the Universe''.

If there was no maximum response speed, then many things that are connected in our Universe would be separated (for example electric and magnetic fields would be totally separate and unconnected, there would be no mass-energy equivalence). But this doesn't mean that things wouldn't work - just that they would work differently.

Just the highlighted bit (a well structured post as always Sepulchrave )

..."as measured by only one person" is crucial in this because an external observer (in a separate RTF) will not be able to distinguish which twin is actually accelerating away from the other, and BOTH will "age" at the same rate.

This is the Head - Hurting bit : at every possible moment in time alternate Universes MAY be created, therefore I suspect that going back in time and killing your father would simply (whoa that is a bit of an understatement!!) create yet another alternate Universe and when you return to your own Universe your Father was not killed....that ocurred in a different Universe.

What this leads me to wonder is whether time travel could even possibly lead to a violation of Causality? (left it as a question)

In a somewhat different vein, but possibly connected, the Fermi Paradox. (I paraphrase) "If the Galaxy is full of intelligent alien beings then why are they not here?". For "alien beings" replace with time travellers - if every time a paradox is created by time travel then a new universe MAY be created in which they do not travel to our time and , by extension, WE would never know that it happened....

OK, I am going to lay down for a bit and take a couple of paracetamol

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I have always kinda thought without good reason that the reason space time has this maximum response time has to do with that space time is not a "true" void but is a something that has geometry and other properties and is able to hold fields. It is however also able to "expand" and thereby carry objects in it with it, something a "true" void would not be able to do. You can tell I'm not beyond the undergraduate level with this stuff, but I do like having my brain knocked about.

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That sounds plausible to me.

I don't know much about string theory/brane theory (etc.) either, but at the root of all scientific theory - no matter how advanced - is a core set of axioms that are simply accepted as true.

I suspect (but I don't know for sure) that the invariable speed of light (or something related, like the linkage between electric and magnetic fields, or energy/mass equivalence) is often an axiom of these theories - not a derived property.

The best argument I can think of for requiring a ``finite response speed of the Universe'' is to accept the axiom of ``finite information density''. If there were an infinite response speed, then (for example) every astronomical object in the Universe would instantly react to the motion of every other object in the Universe - no matter how slight that reaction might be.

By having a finite response speed, the motion of one object will only affect objects in a limited range - greatly reducing the possible number of interactions.

I think an axiom of ``finite information density'' is slightly better than the axiom of ``finite response speed of the Universe'' because I believe the former axiom can also be used to explain the Uncertainty Principle of Quantum Mechanics.

However this is still a rather vague idea - it isn't detailed enough to be considered a theory or even a hypothesis. (I also forget where this idea came from, I think I heard it first from Seth Lloyd but I am not sure... Anyway it is quite possible that someone smarter than me has formalized this concept already.)

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SO far as to the extent of the 'evidence available' goes ... much is hindered by the capabilities of the machines that is relied on to gather them ... or our capacity to come up with better machines to gather the evidence ...

~

Edited by third_eye

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If we accept that "finite information density" is axiomatic then, by definition, we cannot prove it mathematically as it is a "starting point". In a "Brane Universe" then the propogation of the effects of gravity is instantaneous, and if that is true then Gravity waves cannot exist, and further cannot be detected because every atom in the Universe is affected simultaneously therefore all measurement moments are equally affected.

Even further "out there" is the possibility that Space - Time itself is an incorrect construct of Einsteinian Math - that the real "fabric" (for want of a better word) of the Universe is Gravity. If this were true then dimensions become another awkward QM construct as there would be an infinite number of physical and Spacial dimensions rather than the current 11 posited to allow suffificient DOF´s in String Theory.

This whole field of endeavour is extremely challenging, counter - intuitive, and - dare I say it - possibly beyond the current evolved capabilities of mankind to resolve...

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Would it then follow that were gravity waves to be detected the brane-universe idea your talking about would fall apart?

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Until then I'm gonna stick with religion subforum

That's what they said right, "Science too difficult? Try Religion"

Edited by wuhugm
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Would it then follow that were gravity waves to be detected the brane-universe idea your talking about would fall apart?

Only from my inadequate understanding of the subject

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According to Newton f=mA which is force equals mass times acceleration. This can be rewritten A=f/m. From this it would seem that for particles 0 rest mass like photons any force would make acceleration infinite so it would seem to me that if it was possible to go ftl light would already go faster than it does