Thread: It's a small world, after all.

Originally Posted by Sebastiani

Again, the law of relativity states just that. If there was no space/time dialation that would be true, but there is, so as measured from either ship C is never exceeded.

Hmm... let's get something straight here.

The special relativity law establishes that no two objects can be moving at a speed relative to each other larger than the speed of light. Or in better words, the speed of light is the same for all observers regardless of their motion. So far, we agree.

But the expansion of the universe doesn't have that constraint. I need to remind you that here we are not talking of moving objects respecting the laws of inertia, but of a paradoxical moving inert object respecting the Metric Expansion of the Universe. There's no e=mc2 in the expansion of the universe.

EDIT: Link is even better than I first thought. Check the second and third paragraphs.

It would not make the round trip, because the transmission of information cannot exceed the speed of light BUT the relative velocity of two objects CAN.

Ok, so following that logic, if you sent a signal to the opposing ship, it would never arrive, correct? But in fact the signal would propagate through space at C, and the observer would measure the opposing ship moving somewhat slower than C, and thus the signal would eventually overtake the opposing ship. Once reflected, the converse would be true, and so the signal does in fact make the round trip. How long would it take? Well, from the point of view of the observer, much longer than 2C. But that isn't important, the real question is how long would it take as measured from the two ships? The answer is less than 2C.

Again, consider the time dilation factor and how you measure speed; for the ship travelling at near light velocity, time dilation is such that for the people on the ship, the ship is travelling thru the galaxy at much more than c.

Or perhaps it's not that you've exceeded C, but that time and distance have been forshortened.

Also, I believe that some particles are considered to exceed C for durations but are not detectable or effectively "present" during that time (again qv the wikipedia article), and the phemomenon can be described in other terms to do with the energy level of the particle -- it becomes something else for us, not a ftl particle.

The article does pose some interesting (if not speculative) ideas, but I don't think any of them have been proven - yet. Who knows, though? The universe is a very strange place, and it would be foolish to insist that relativity will never be amended by new, as yet undiscovered, laws. It happended to Newton, after all.

Originally Posted by Sebastiani

Or perhaps it's not that you've exceeded C, but that time and distance have been forshortened.

That's why I said it is a semantic problem and not a scientific one -- eg, you can construct circumstances were the "contention" is about something that only seems meaningful. If you are on a ship travelling "objectively" slightly less than C, but time is so foreshortened that on the ship it "seems" as if only 56 years have passed while you travelled 100000 light years, it is still true that you travelled 100000 light years in 56 years of your own time, but if you want you could still say "perhaps it's not that you've exceeded C, but that time and distance have been forshortened" -- which is true, but only in relation to an abstract and objectified reality. Such an "abstraction" is totally valid on a Newtonian scale, but as Einstein says in the opening pages of The Theory of General Relativity, it does not work on an interstellar scale. The only constant is "the speed of light"; the flow of time relative to you, or something else, your relationship to space which is always a *particular* one, etc, none of that is a constant that must be respected and included in some decontextualized explanation of how things which do not have relationships beyond certain effects (via light and gravity) "must relate in a proper and constant way". In other words, Sebastiani, the light beam does not catch the ship, honestly

You cannot observe time travelling at different rates directly, even tho, as I said, it happens faster in the sun. Doesn't that mean the sun must be older than the earth by now -- or rather that it exists in a future we have not arrived at yet? You can do it by inference, as with a caesium clock, or I imagine on the ship by triangulating some information from the incredibly distended wavelengths of light reaching the ship from other objects, which will still be travelling at C.

So its like using BB's to accelerate a tin can, and then declaring that because it takes more and more BB impacts to accelerate the can and the velocity falls off as you approach B, that nothing can travel faster than a BB.

I like that.

The universe is a very strange place, and it would be foolish to insist that relativity will never be amended by new, as yet undiscovered, laws. It happended to Newton, after all.

Even more! I think it will be very similar in fact; Einstein does not really "disprove" Newton; he contextualizes him.

Originally Posted by abachler

Well, one of the major weaknesses of the theory is that all observed 'proofs' rely on electromagnetic acceleration of particles, and electromagnetism is propagated by none other than photons. So its like using BB's to accelerate a tin can, and then declaring that because it takes more and more BB impacts to accelerate the can and the velocity falls off as you approach B, that nothing can travel faster than a BB.

I see your point, but then again we can only speculate about what we can observe, right? I mean, for all we know the entire universe may be a mere sub-atomic particle of some other world, but what's the point of debating that if it can neither be proven nor disproven?

Originally Posted by Sebastiani

I see your point, but then again we can only speculate about what we can observe, right?

I'd say you can only speculate about what you cannot observe.

Originally Posted by Sebastiani

I mean, for all we know the entire universe may be a mere sub-atomic particle of some other world, but what's the point of debating that if it can neither be proven nor disproven?

It might be harder than you think to reach a point where this could "neither be proven nor disproven", since you have decided to give up without trying. AFAIAC possibilities such as that do shadow any debate about the physical nature of reality.

Originally Posted by Mario F.

Hmm... let's get something straight here.

The special relativity law establishes that no two objects can be moving at a speed relative to each other larger than the speed of light. Or in better words, the speed of light is the same for all observers regardless of their motion. So far, we agree.

But the expansion of the universe doesn't have that constraint. I need to remind you that here we are not talking of moving objects respecting the laws of inertia, but of a paradoxical moving inert object respecting the Metric Expansion of the Universe. There's no e=mc2 in the expansion of the universe.

EDIT: Link is even better than I first thought. Check the second and third paragraphs.

Well, I'm not sure I can accept that. If the metric of the universe is expanding then things should "grow" along with this expansion and hence it would be unobservable anyway. It just doesn't make sense to me.

It's not an easy subject indeed. As stated on the 3rd paragraph and linked to some interesting articles.

EDIT: I cannot pretend to follow that type of discussion. I can only go by the conclusions.

Originally Posted by Mario F.

I cannot pretend to follow that type of discussion. I can only go by the conclusions.

Sorry Mario, I'll try to simplify next time

Originally Posted by Sebastiani

Well, I'm not sure I can accept that. If the metric of the universe is expanding then things should "grow" along with this expansion and hence it would be unobservable anyway. It just doesn't make sense to me.

Oh come on you didn't even finish the first paragraph:

This model is valid in the present era only at relatively large scales (roughly the scale of galactic superclusters and above). At smaller scales matter has clumped together under the influence of gravitational attraction and these clumps do not individually expand, though they continue to recede from one another.

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be moving away from each other at a speed greater than the speed of light (meaning that one cannot be observed from the other). The size of the observable universe could thus be smaller than the entire universe.[...]Spacetime is highly curved at cosmological scales, and as a result the expansion of the universe is inherently general relativistic; it cannot be understood with special relativity alone.

IMO large objects travelling at near light relative speeds are operating on a cosmological scale.

Yes, I can relate to that. Though in the movie, they had no reason to do it.

You mean Zeno's Paradoxes of Motion? They have been resolved already.