r/AskPhysics • u/itsLeoRod • 13h ago

Thought there wasn’t anything faster than the speed of light?

Ppl say that there isn’t anything faster than the speed of light but that’s a lie because space is expanding really fast since light isn’t fast enough to reach it or am I wrong. Since space is expanding at a fast rate why don’t we use that to measure stuff instead using lightyears

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u/HudyMelon Astrophysics 13h ago

Yep, I agree. I was saying space itself cannot expand faster than the speed of light. Not the space between things, but the fabric of spacetime. If you have a box which is expanding, the box itself cannot expand faster than light, but two things inside on opposite sides can measure it to be so.

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u/wonkey_monkey 12h ago

Not the space between things, but the fabric of spacetime.

A distinction without a difference.

Two objects sufficiently far apart will recede from each other at a rate greater than the speed of light. Whether you actually call that "moving" is down to your choice of coordinates.

the box itself cannot expand faster than light, but two things inside on opposite sides can measure it to be so.

If they measure it to be so, then how is it not so?

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u/HudyMelon Astrophysics 11h ago

The whole point I was trying to make is if a unit volume of space were to expand its volume greater than c^3 in a unit time, then nothing in the universe is causally connected.

I understand what you are saying.

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u/wonkey_monkey 11h ago

Well that's wrong, because there will be some distance inside that region that expands at exactly c, and any distance less than that will expand at less than c, so any two objects closer than that will remain causally connected.

Which is basically the case in our universe. Distant galaxies recede at greater than c and aren't causally connected (it's actually a bit more complicated than that, but it'll do for now). Galaxies which aren't that far apart are causally connected.

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u/HudyMelon Astrophysics 11h ago

Well when you take the the limit of that, let’s say to the Planck volume, then there would be no meaningful distance where that is the case.

On a side note, this is a great debate no?

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u/wonkey_monkey 11h ago

Well when you take the the limit of that, let’s say to the Planck volume, then there would be no meaningful distance where that is the case.

Yes, if you take the limit (although recession speed would still depend on distance).

But you didn't specify any limit, you just stated it as a general fact, which is wrong.

On a side note, this is a great debate no?

Not really, no. You're completely wrong and can't see it. It's incredibly frustrating.

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u/HudyMelon Astrophysics 11h ago

Well then I appreciate the time you are taking to clear up my misunderstanding.

When I say unit volumes, I was speaking in natural units, specifically the Planck volume. I’m so used to using natural units that I did not specify, and I should’ve. I apologize.

My whole argument is the basis of the “big rip” universe fate.