If the fabric of spacetime isn't bound by the limit of the speed of light (the universe is expanding faster than the speed of light), could humans somehow wrap a spaceship in a bubble of the fabric of space time and go faster than the speed of light?
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4$\begingroup$ Spacetime isn't made of fabric, it is a popsci phrase. $\endgroup$– Marius Ladegård MeyerCommented Mar 5 at 17:21
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$\begingroup$ I know, but that is what it is called, how it is represented. $\endgroup$– Kellan HeerdegenCommented Mar 5 at 17:32
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$\begingroup$ Check alcubierre-drive $\endgroup$– Agnius VasiliauskasCommented Mar 5 at 17:38
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$\begingroup$ "the universe is expanding faster than the speed of light" -- this is just a coordinate artifact arising from time dilation. A spaceship could travel "faster than light" if you used the same coordinate conventions. $\endgroup$– StenCommented Mar 5 at 19:06
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2 Answers
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If the fabric of spacetime isn't bound by the limit of the speed of light (the universe is expanding faster than the speed of light),
If you consider cosmic expansion to be faster than light, then there is certainly no theoretical obstacle to a spaceship traveling faster than light in the same way.
Simply send the spaceship away from Earth at, say, 96% the speed of light. Now transform into "synchronous" coordinates, similar to what we use to describe the universe. The idea is that we want clocks on the Earth and the spaceship to advance at the same rate. To do this, we boost into an intermediate reference frame, in which the Earth and the spaceship are receding in opposite directions at the same speed. Due to relativistic velocity addition, this mutual speed turns out to be 75% the speed of light, since $2\times 0.75/(1+0.75^2)=0.96$. But now the trick is that we measure time with respect to the synchronized clocks on the Earth and spaceship, which are slowed by a factor of about $(1-0.75^2)^{1/2}\simeq 0.66$ due to time dilation. By comparing distance traveled to the time elapsed on these clocks, we would conclude that the Earth and the spaceship are traveling in opposite directions each at $0.75/0.66 = 1.13$ times the speed of light.
If you don't think that this is faster-than-light travel, then you shouldn't think the universe is expanding faster than light either!
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current understanding of physics allows for the expansion of the universe exceeding the speed of light, achieving faster-than-light travel for individual objects like spaceships remains incredibly challenging, and wrapping them in a "bubble" of spacetime wouldn't work due to: infinite Energy Requirements and many other reasons. but it's crucial to understand the significant theoretical and practical hurdles that currently prevent us from achieving it. Currently many scientific groups are trying to explore theoretical possibilities and technological advancements, but achieving true interstellar travel within a reasonable timeframe might necessitate breakthroughs beyond our current understanding of physics.
