Who thinks Faster Than Light travel is possible?

[Swank]

If FTL turns out to involve attaching an onion to your belt (which was the style at the time), it will be important to peel the onion in a manner that allows ships to travel on the surface of the different layers of onion in the shortest distance to connect those layers, allowing multi-layer travel at superluminal speeds via this onion layer hopping.

Eventually, a method to "slice the onion" might be devised, whereby the layers would be bypassed entirely via the imposition of a two dimensional field effect. There would be major complications to this method, the least of which are the hazards of "space allicin", and that space marines have no time for tears.

 

[SilentRoamer]

I realise that weird ways have been suggested for making this happen in a way that would make some sort of significant difference (dragging, at FTL velocities, the ends of wormholes about being one, I believe)...

...but if one took two days to reach Proxima Centauri and then immediately took another two days to get back to Earth, one would arrive on Earth four days after one had left.

If one had, while at Proxima Centauri, sent a light-speed message to Earth saying that you'd arrived there, one would, of course, have reached Earth about 4 years before that message did... but how is this significantly different from posting a letter to somewhere then immediately driving to that somewhere (however far away it was)?

The problem with the above is that it assumes a Universal time constant. Remember as well - that for the person travelling FTL, less time has actually passed. Ignoring that any mass requires infinite energy to accelerate to C and assuming it is possible through some future unbeknown tech then the reason FTL causes a problem is because it inviolates the laws of causality. Which as far as we have ever been able to observer, hold true. It's entirely possible that our understanding of light and information exchange and the extrapolation of this into World Lines is not complete - I mean black holes and the singularities themselves are an indication that either 1. We don't understand the maths or 2. that finite beings cannot understand infinites.

Whatever the observer has seen, the ship must have left the Earth before it can arrive at its destination, so just because the observer sees the ship's departure after they see its arrival (both in their own time frame), it doesn't mean that they can get to Earth before that departure. How could they?

Again - this assumes a Universal Time constant.

If we take for an example a super advanced space ship, as it crossed the Event Horizon an external observer sees the ship become red shifted to infinity - from the PoV of the ship you cross the horizon in normal time, from the external observer you never cross. Both of these are both simultaneously true and counter intuitive.

 

[Dave]

Sorry, but I haven't read any of the, currently, 13 pages of this thread (so this may have been covered already) but I keep seeing the question on the forum every day, and it annoys me every time. Whatever answer is being elicited, the question shouldn't be "who" thinks FTL travel is "possible" or not.

Firstly, that is posing it as a question about "belief" and it makes no difference if I believe it or not. If I try to believe it harder, that still isn't going to make it happen. If more people believe in it, that won't make it more likely either.

Secondly, it is equivalent to going back to the 11th Century and asking "Do you think it's possible that a man can run faster than a horse?" Of course a man can't run faster than a horse, but he can take a car, a train, an aeroplane or a rocket ship; only to explain those concepts to the 11th Century man is going to be quite difficult.

 

[Ursa major]

The problem with the above is that it assumes a Universal time constant.

I wasn't assuming that; I was suggesting that if an observer sees a traveller arriving at that traveller's destination, the observer (however fast either of them might be travelling) cannot then get to the traveller's departure point before that traveller left it...

...and throwing in a lot of stuff that is irrelevant to explaining how that might happen (infinite energy, etc.) won't explain what you are not explaining but merely asserting.

 

[SilentRoamer]

I wasn't assuming that; I was suggesting that if an observer sees a traveller arriving at that traveller's destination, the observer (however fast either of them might be travelling) cannot then get to the traveller's departure point before that traveller left it...

...and throwing in a lot of stuff that is irrelevant to explaining how that might happen (infinite energy, etc.) won't explain what you are not explaining but merely asserting.

If we assume an external observer watching Bob and Alice. Bob sends Alice a message at C, Alice replies at C. The external observer sees the reply arrive before the original message is sent. There are hard maths behind this that I can't claim to understand, although I think I understand the concepts.

C isn't just a certain speed, its the speed where any mass needs infinite energy to accelerate to, as you approach C then mass curves to infinite, which is forced causality.

Apologies for my poor explanation.

 

[Venusian Broon]

If we assume an external observer watching Bob and Alice. Bob sends Alice a message at C, Alice replies at C. The external observer sees the reply arrive before the original message is sent. There are hard maths behind this that I can't claim to understand, although I think I understand the concepts.

Heh SR!

Essentially your summary is correct.

The original situation you responded to did not involve any obvious breakage of causality, therefore no obvious conclusion could be made that this was (non-standard!) 'time-travel'.

However if we had the ability to travel/signal faster than the speed of light as was posited, you can relatively easily construct a scenario that gives an observer the ability to see the effect of a FTL signal or operation before its cause. And because we're going to give this observer the same faster than FTL abilities (why not?), that also gives them the ability to contact/visit the bit of space before the 'cause' has occurred and presumably, in theory, break our understanding of causality by being in the position to essentially time-travel/pre-warn and change what they had observed happened. Grandfather paradox rears its head, in effect, etc...

I could go through it, but it would involve drawing loads of Space-time diagrams and trying to explain special relativity simultaneity, and I don't have time right now.

Now it might be the case that causality might be able to be 'attacked' in this manner and we just haven't got the tech to really see it yet, or that there are other constraints that will allow it - while maintaining our naïve view of causality. But we haven't really observed anything weird happening that suggests our universe operates like that.

Hence, for now until something fundamental in our understanding changes or we consistently observe space-time weirdness happening, FTL is not on the menu!

 

[Swank]

If we assume an external observer watching Bob and Alice. Bob sends Alice a message at C, Alice replies at C. The external observer sees the reply arrive before the original message is sent. There are hard maths behind this that I can't claim to understand, although I think I understand the concepts.

C isn't just a certain speed, its the speed where any mass needs infinite energy to accelerate to, as you approach C then mass curves to infinite, which is forced causality.

Apologies for my poor explanation.

Part of the problem is referring to "external observers" and "universal time". Those don't exist in this universe.

 

[Ursa major]

If we assume an external observer watching Bob and Alice. Bob sends Alice a message at C, Alice replies at C. The external observer sees the reply arrive before the original message is sent. There are hard maths behind this that I can't claim to understand, although I think I understand the concepts.

In your second sentence, the first half refers to the timing of something having being seen to have occurred and the second half refers to the timing of it occurring, i.e. two different things. I can see how one can compare the timing of observations, and can compare the timing of actions, but surely comparing an instance of one type with an instance of the other provides a poor basis for arguing anything.

its the speed where any mass needs infinite energy to accelerate to, as you approach C then mass curves to infinite

I thought that, where infinity is involved, all bets are off... but we're discussing FTL, not masses travelling at c, so what does the maths say about that?

in theory

These two words are doing a lot of work, aren't they (particularly where the means of achieving FTL, if there is/are such means, is purely a matter of conjecture)?

 

[SilentRoamer]

In your second sentence, the first half refers to the timing of something having being seen to have occurred and the second half refers to the timing of it occurring, i.e. two different things. I can see how one can compare the timing of observations, and can compare the timing of actions, but surely comparing an instance of one type with an instance of the other provides a poor basis for arguing anything.

I suppose so, but observation and action are interchangeable when we are talking FTL because an external observer also capable of FTL, from a non preferential reference frame can observe FTL occurrences and then FTL before those instances have taken place. Mathematically, any WorldLines that have an FTL function are capable of intersecting themselves. I don't understand the mathematical proofs for this but I am sure @Venusian Broon does.

I thought that, where infinity is involved, all bets are off... but we're discussing FTL, not masses travelling at c, so what does the maths say about that?

I mean sure - assuming some Alcubierre space compression or some exotic form of wormhole then perhaps FTL would be achievable without accelerating mass to C. The maths suggest that anything that can travel FTL (whether via acceleration of mass or some hitherto unknown method) that any travel exceeding C allows for sending of signals as replies before messages were sent.

These two words are doing a lot of work, aren't they (particularly where the means of achieving FTL, if there is/are such means, is purely a matter of conjecture)?

Oh absolutely - and all of this conjecture is taken from the constraints of a finite species that haven't even properly hopped off of their own planet. My gut feeling is that finite beings are unable to comprehend infinities, that the mathematics behind the singularity will always be broken.

It is a really fun discussion though if ultimately fruitless.

 

[hitmouse]

Sorry, but I haven't read any of the, currently, 13 pages of this thread (so this may have been covered already) but I keep seeing the question on the forum every day, and it annoys me every time. Whatever answer is being elicited, the question shouldn't be "who" thinks FTL travel is "possible" or not.

Firstly, that is posing it as a question about "belief" and it makes no difference if I believe it or not. If I try to believe it harder, that still isn't going to make it happen. If more people believe in it, that won't make it more likely either.

Secondly, it is equivalent to going back to the 11th Century and asking "Do you think it's possible that a man can run faster than a horse?" Of course a man can't run faster than a horse, but he can take a car, a train, an aeroplane or a rocket ship; only to explain those concepts to the 11th Century man is going to be quite difficult.

Basically:
Wishful Thinking 1-0 Scientific Rationalism

 

[Dave]

I think what I should have said there was how do you explain to the 11th Century man about telephones, radio, television, holographic projection - that would allow you to get to the horse's destination and observe it before it arrived.

What I do think, is that for our species to survive we must leave Earth at some point and go elsewhere. The speed of light is a constant but there is much we don't know about the multidimensional physics of the universe which may allow us to travel without breaking any scientific rationalism. We just don't know how yet, but that may also be wishful thinking on my part.

As for Relativity, I discovered the Theory of General and Theory of Special Relativity when I was about eleven. I actually read them then and read lots of books on the subject. That was exciting Physics to me, while at school, we were playing with cars on ramps doing F=ma or else exploding capacitors in electrical circuits. I found all the infinite mass, infinite length, infinite energy stuff that came with near c velocities, and the complications that came with different observers at different near c velocities all mind-blowing, including that such an observer could see something before it had happened. While travelling at near c velocities may not be possible, it does make for some great theoretical thought exercises that you just can never quite get your head around.

 

[Daysman]

Then you should, because it's at the very least counterintuitive.

Whatever the observer has seen, the ship must have left the Earth before it can arrive at its destination, so just because the observer sees the ship's departure after they see its arrival (both in their own time frame), it doesn't mean that they can get to Earth before that departure. How could they?

Just re-read my previous reply. Not very useful. Thought I'd try again. I may fail. You may have moved on. That's fine.

I originally responded because your post presented a concrete scenario based on real world expectations. The question is, I think, why would an FTL journey be different to any other?

Well, it's all about the end points. Unless one event is in the future of another, they don't have a fixed order.

Your argument assigns order by appealing to a universal ‘now’, but this landscape tilts whenever you change your speed and direction.

Why? Well, everything's about the speed of light.

The universe conspires to make the relative speed of light the same for anyone that measures it, and employs time dilation, length contraction, etc. to achieve it.

For the same reason, time aboard a fast moving spaceship doesn't just move slower compared to that experienced by a ‘stationary’ observer, it also slips into the past as we move from the front of the ship to the back.

Note: ask Google about lightning strikes on Einstein's train…

Clearly, in reciprocation, for everyone on board, the rest of the universe is similarly skewed.

Your FTL ship travels 4.3 light years in two days. This is such a high speed that, if our interloper at the destination is moving at around 200 km per second, their ‘now’, when stretched all the way back to Earth, will be two days before your departure. That's 4 days ago. So, using the same two day jump, they'll arrive just after your departure.

If we tilt ‘now’ just a little more, they'll arrive before it.

Whether this causes a paradox really depends on how the universe that allows FTL actually works… it may be that everything's fine, and that you have to deal with it if you want interstellar travel.

I've no idea.

Well, no, actually I've got a few, but they're all science fiction…

Have a great weekend.

 

[Ursa major]

Whether this causes a paradox really depends on how the universe that allows FTL actually works

Quite.

(I ought to add that the very existence of the universe -- or the complete cast of multiple universes, in parallel and/or in series -- would seem to be paradoxical.)