Stimbo Wrote:We don't need to look that far out. Our Galaxy 'only' has a span of around 100,000 ly, which is still a hell of a walk. Millions of lightyears gets us out to our nearest large galactic neighbour, M31 at 2.2 million. There are about two thousand stars within 50 ly of Earth, about 15% of which could have planets with the proper conditions to qualify as life-sustaining., and about a dozen such potentials have been discovered. There could be tens of 'billions' (hate that term) in our Galaxy alone. No rush to go so far from home just yet.

I suspect that there's a big disconnect between the odds of 'can have life' and 'can have civilization'.

Alex K Wrote:

Quote:According to what we currently know about the laws of physics, there doesn't seem to be a feasible way to break the speed-of-light barrier.

Quantum Entanglement - if you look at how causality plays out in Quantum Field Theory (that's currently the best experimentally secured description of quantum physics which also takes into account relativity), yes, there's this spooky action at a distance thing. But if you write down the mathematical formula for a communication at superluminal speeds, you find that the spooky particles and the spooky antiparticles exactly cancel each other out and you get zero; It's a famous textbook example, it's in my book as well - nature makes sure that no information can be transported until enough time has passed - as soon as enough time has elapsed, the quantum formula returns a number different from zero and communication is possible. It's the strangest thing, as if the universe had built-in machinery set up to hide superluminal messages from us.

On the plus side, that sounds like it could still be a basis for very long-distance reliable communication, just not FTL communication.

(August 10, 2016 at 9:01 am)RozKek Wrote:

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(August 10, 2016 at 4:06 am)Alex K Wrote: According to what we currently know about the laws of physics, there doesn't seem to be a feasible way to break the speed-of-light barrier.

Quantum Entanglement - if you look at how causality plays out in Quantum Field Theory (that's currently the best experimentally secured description of quantum physics which also takes into account relativity), yes, there's this spooky action at a distance thing. But if you write down the mathematical formula for a communication at superluminal speeds, you find that the spooky particles and the spooky antiparticles exactly cancel each other out and you get zero; It's a famous textbook example, it's in my book as well - nature makes sure that no information can be transported until enough time has passed - as soon as enough time has elapsed, the quantum formula returns a number different from zero and communication is possible. It's the strangest thing, as if the universe had built-in machinery set up to hide superluminal messages from us.

Our only hope would be messing with space-time itself in order to shorten the path to the remote point by changing the geometry or topology of space. If you'd have a (as far as we know inexistent) substance with negative Energy, that would upset the maths of the above calculation, and information could travel faster than light (the recently often-discussed example is the Alcubierre Drive). But my personal opinion from what I've seen is that as soon as this kind of device would actually work, one would unavoidably run into time travel paradoxes. If wormholes in spacetime, connecting distant reaches via a shortcut, would exist, that would work, but it's not clear whether they can be stable and traversible without erasing everything that tries to pass them. So far, we haven't seen one. To make wormholes, one would probably again need this weird negative energy substance to keep them stable.

So my conclusion so far is that it's not completely out of the question that it might one day be feasible if some surprising physics discoveries happen, but so far, not much speaks in favour of the possibility.

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Which paradoxes are we talking about here, and what would some possible solutions?

Generally, if you have a means to send information or travel faster than light, it is possible to send
information into your own past. This will then form a time-loop which would feed into itself like an infinite
feedback loop. We are literally
talking about the "killing your own grandmother" thing, after which you wouldn't exist, obviously preventing
you from travelling back and killing her, which means that
your grandmother is still alive, which means that you can do the going back and killing her after all, etc., you
get the idea. Though much more subtle influences will already change the past by so much that it would
cause everything to become inconsistent.
Ways out? Maybe whenever you do that, the universe splits up and you create a new timeline (i.e. you don't really send
the info back to your own past, but instead cause a forking where one parallel version of you receives the message.
This is somehow imaginable if the many worlds picture of quantum mechanics were true. Another thing that could happen
is that this timelike causality loop will oscillate wildly through all kinds of realities until it settles in a self-consistent state
where past and future including the time travel are self-consistent, and that's the reality you'll think you were in the entire time
(this means that upon further investigation you will find out that you were indeed messing with your own past, but that's
precisely why you exist now as you do, and you have no free will to not go and do the time travel.

I think there is the possibility to have faster-than-light without time travel paradoxes, namely if faster-than-light travel is only
possible with respect to a certain frame of reference. I haven't done the maths, I'm not 100% sure and need to check.
It would certainly require a drastic modification of relativity theory.

To elaborate on the latter idea - imagine that there is a way to send info with exactly ten times the speed of light (to pick a number) with respect to for example (to pick an obvious choice) the reference frame in which the cosmic microwave background rests, for example because there is some kind of medium. In this case, information will always take positive time to travel in this frame, which is the frame in which the age of the universe is measured, and hence I *think* that will never create a paradox, because if you have the info returning to the same point, the event of returning will be at the same place at a later time, and this ordering of time is always the same for any observer.

(August 10, 2016 at 9:18 am)Alex K Wrote:

(August 10, 2016 at 9:01 am)RozKek Wrote:

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Which paradoxes are we talking about here, and what would some possible solutions?

Generally, if you have a means to send information or travel faster than light, it is possible to send
information into your own past. This will then form a time-loop which would feed into itself like an infinite
feedback loop. We are literally
talking about the "killing your own grandmother" thing, after which you wouldn't exist, obviously preventing
you from travelling back and killing her, which means that
your grandmother is still alive, which means that you can do the going back and killing her after all, etc., you
get the idea. Though much more subtle influences will already change the past by so much that it would
cause everything to become inconsistent.
Ways out? Maybe whenever you do that, the universe splits up and you create a new timeline (i.e. you don't really send
the info back to your own past, but instead cause a forking where one parallel version of you receives the message.
This is somehow imaginable if the many worlds picture of quantum mechanics were true. Another thing that could happen
is that this timelike causality loop will oscillate wildly through all kinds of realities until it settles in a self-consistent state
where past and future including the time travel are self-consistent, and that's the reality you'll think you were in the entire time
(this means that upon further investigation you will find out that you were indeed messing with your own past, but that's
precisely why you exist now as you do, and you have no free will to not go and do the time travel.

I think there is the possibility to have faster-than-light without time travel paradoxes, namely if faster-than-light travel is only
possible with respect to a certain frame of reference. I haven't done the maths, I'm not 100% sure and need to check.
It would certainly require a drastic modification of relativity theory.

I thought wormholes only "bent" space and made the distance between two spots closer, but didn't actually make you travel faster than the speed of light. If that is the case (that it bends space) then why would/would any paradoxes arise?

Wormholes could remain without paradox. yes

(August 10, 2016 at 9:05 am)Mister Agenda Wrote:

Stimbo Wrote:We don't need to look that far out. Our Galaxy 'only' has a span of around 100,000 ly, which is still a hell of a walk. Millions of lightyears gets us out to our nearest large galactic neighbour, M31 at 2.2 million. There are about two thousand stars within 50 ly of Earth, about 15% of which could have planets with the proper conditions to qualify as life-sustaining., and about a dozen such potentials have been discovered. There could be tens of 'billions' (hate that term) in our Galaxy alone. No rush to go so far from home just yet.

I suspect that there's a big disconnect between the odds of 'can have life' and 'can have civilization'.

Oh, obviously. From our point of view, at best we're only tooled up to discover the first of the two. That reduces the possible number of targets on which we need to expend time looking for the second.

Quote:I'm always surprised by people wondering why we haven't found any alien civilizations as of yet.

They took one look at us and decided to hide.

. . . Fermi paradox . . .