# If the speed of light was not a barrier



## Brian G Turner (Jun 23, 2015)

In another thread, some of the curious effects of relativity were mentioned - and it got me thinking.

Simply as a thought experiment, if the speed of light was not the barrier to travel we currently understand it to be, then what sort of quirky effects might we see as a consequence of this?

Thinking aloud, I recall one reason why time travel is not possible - for example, merely observing the beginning of the universe - is that the observer would have to be able to observe outside of time cones defined by the speed of light. So would one possible consequence be the ability to observe the past directly, let alone potentially interact with it?


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## Venusian Broon (Jun 23, 2015)

I'm not sure I get your premise Brian, but aren't we _always_ observing the past directly. And the further away the object that we can see is, the further back in time we are observing it? The only thing in the the 'now' is that voice inside your head that 'speaks' your thoughts? (And that might even be a little lagging in the now, according to some experiments...)

The past time cone that we all share does include the beginning of the universe in some manner - we can't see it, but we can measure the cosmic background radiation and anyway all this time cone stuff is really about the transmission of useful information in some manner, rather than sensing it directly!

If you mean that the speed of light was infinite, then we would see everything as it is NOW. (Whatever 'now' means) and the past would forever be lost to us. Of course an infinite speed of light destroys E = mc2, to take the venacular expression - E would be infinite too and I guess the universe would be a wildly energetic place. That would also take out all relativity, although I'd have to think about that...


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## Ray McCarthy (Jun 23, 2015)

Venusian Broon said:


> but aren't we _always_ observing the past directly. And the further away the object that we can see is, the further back in time we are observing it?


Yes.
But we can't get closer to those far off events by travel, unless we travel faster than light, which would break causality.


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## Venusian Broon (Jun 23, 2015)

Ray McCarthy said:


> Yes.
> But we can't get closer to those far off events by travel, unless we travel faster than light, which would break causality.



Correct but the first part of Brian's question was about observing the past.


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## Brian G Turner (Jun 23, 2015)

Indeed, my apologies if I messed up a little - the dangers of posting too quickly, without enough forethought. 

But considering the curious relativistic effects of travelling near to the speed of light, has anyone considered what may have if we could travel faster than light? Aside from speculation, is there anything in physics that suggests what might happen?

And, to clarify my point about time cones - would this make travel backwards in time possible?


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## farntfar (Jun 23, 2015)

It's must be at least than 30 years since I read it, but I remember a delightful little book called Mr Thompkins in Paperback, by George Gamow who was a pioneer in Big Bang theory, about a land where the speed of light was 10 mph (or something) and so relativistic effect were observed when travelling by train.
Not quite on subject, but has anyone else read this?


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## Venusian Broon (Jun 23, 2015)

Brian Turner said:


> Indeed, my apologies if I messed up a little - the dangers of posting too quickly, without enough forethought.
> 
> But considering the curious relativistic effects of travelling near to the speed of light, has anyone considered what may have if we could travel faster than light? Aside from speculation, is there anything in physics that suggests what might happen?



The problem is, I suspect, that anything travelling faster than light will eventually lead to paradoxes of logical impossibilities. Hence as physics is about observable entities we should note that we have not really come across a logical impossibility in nature or lab. Thus you might argue that if you want real FTL transmission/travel then you are suggesting that the logic that seems to 'run' our universe is far stranger than we know. This is perhaps a possibility, but what grounds have we for questioning some of the very basic logic that our universe seems to run on? That grandfather paradoxes occur constantly but we just don't observe the consequences??? (My head is starting to hurt thinking about it )

Ignore Tachyons - I see that as a bit of playful mathematics, taking a theory to breaking point and saying, 'Well what if particles went faster than light...' All a lot of fun, but zero experimental evidence for them. 



Brian Turner said:


> And, to clarify my point about time cones - would this make travel backwards in time possible?



I'm not sure, depend on your new theory that describes everything. Using General relativity as the framework I think it could, but it would generate our friendly paradoxes ten a penny.


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## Ursa major (Jun 23, 2015)

Can someone explain to me, in words of few syllables, how FTL travel might produce the same sort of grandfather paradoxes as time travel (with and without the use of one end of a wormhole being carried on an FTL ship, if that's the actual cause of the problem).


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## Venusian Broon (Jun 23, 2015)

Ursa major said:


> Can someone explain to me, in words of few syllables, how FTL travel might produce the same sort of grandfather paradoxes as time travel (with and without the use of one end of a wormhole being carried on an FTL ship, if that's the actual cause of the problem).



We're writing about it in this thread https://www.sffchronicles.com/threads/553151/

There is an explanation of how *true* FTL transmission leads to paradoxes - its an explanation from someone on reddit. It's on a post I gave on page 3 somewhere. To be frank my relativity is a bit rusty so I'm still scratching my head over it, but I think it is kosher.

I think though if you have wormholes that distort spacetime - so that you get *apparent* FTL (these are solutions to General Relativity, and GR no matter how hard you squeeze it, says no FTL transmissions) then you don't necessarily derive any paradoxes, but I may be wrong. 

There are also solutions to GR that conceivably can time travel - although the earliest you can travel is to the point that the time machine was turned on. These might be used to achieve a grandfather paradox, but there is debate that a more complete Quantum Gravity theory will be able to rule out paradoxes being produced ('cause we ain't seen any so far...) Also the fact is that a lot of these solutions require vast amounts of energy/mass etc... and therefore look pretty much theoretical. But who knows.


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## Ursa major (Jun 23, 2015)

The reason I'm asking is that my WiPs use neither any form of FTL (i.e. vehicles travelling faster than _c_) nor wormholes (which might allow literally instantaneous travel between two points), but still have normal reaction craft (with a typical maximum velocity of 10km/s, set at this for various practical reasons too complicated to mention here) reaching other, sometimes very distant, star systems through conduits whose internal dimensions are such that the distance in the direction of travel is very much reduced. (There is conservation of the _volume_ of space -- these conduits are not TARDISs -- but that's neither here nor there. ) The conduits, when being created, are generated at no faster than _c_ (if that).

I'm not sure that there would be anything akin to time paradoxes: I can't see any way of even information (communications travel, naturally, at _c_ through the conduits) arriving back at its source before it can be transmitted.


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## chrispenycate (Jun 23, 2015)

And if the speed of light were (subjunctive mood) fixed, non-infinite, and the universe had a regular frame of co-ordinates so a beam of  light from A to B would interpenetrate a beam of light from B to A at twice the speed of light, rather than at once, as it does now? A rigidly Newtonian universe where light has a speed (or you couldn't generate a rainbow with a prism) but nothing except engineering prevents you from overtaking it. Where there are currents in the luminiferous ether, and light going downstream goes faster than that going across, or down? Actually, it wouldn't change much for us, or our grandchildren, at all. The Newtonian universe wasn't a bad approximation at all. We might lose Doppler shift, as the different energies can be taken up by different relative velocities - or not, should photons turn out to be waves rather than particles. Black holes would get much easier to understand, which wouldn't change my life. Gravity would still be incomprehensible, and ubiquitous, as would most quantum effects.

As regards our space travel attempts the light speed barrier is like putting 120mph speed panel on a canal towpath - it's going to take you some considerable time to breed a horse who can read the panel, and a lot longer to get one who cares . The only place matter is getting fast enough that we care about Relatavistic effects is in particle accelerators like the LHC - and then it's very little bits. I don't believe the matters going to come up during the exploration and colonisation of the solar system, either; too much energy needed to gain too little time.

Which. I suspect, gives us a century or more to find some solution for the universal speed limit, be it finding a loophole or a way of living with it. 

And we can do a lot in a century.


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## Ray McCarthy (Jun 23, 2015)

chrispenycate said:


> We might lose Doppler shift


No, we'd still have doppler shift.


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## steelyglint (Jun 26, 2015)

I quite liked the way the universe worked in Vernor Vinge's 'A Fire Upon the Deep'. The further you got from large masses, the faster light could travel and the more high-tech your automation could be. The outer reaches of the galaxy are the 'Slow Zones' and the galactic disc itself is in the 'Unthinking Depths'. Out in inter-galactic space things could really get a serious shift on.

That seems to fit with the ideas of 'Dark Matter' and 'Dark Energy', as there would be much less of it out in the spaces between galaxies and clusters.

.


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