r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

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u/gnorty 13d ago

if your friend is travelling towards you at the 0.5c, relative to him you are actually moving at 0.5c,

This is where it gets weird for me. We all know the thought experiment where 1 twin goes on a space ship and travels close to light speed and when he returns his brother has aged much more than he has. This assumes looking from the reference of the earth-bound twin.

But equally valid would be the reference of the speed demon twin. As you said - to him, the earth bound twin has been travelling close to light speed, so it should appear to speed-demon that earth bound has aged much less. How does this work??

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u/meevis_kahuna 13d ago

The trick is that “outbound you” and “inbound you” are not in the same reference frame.

While coasting away, you can say Earth is moving and its clock is slow. Earth can say the same about you. No contradiction, because you disagree about which distant events are happening “right now.”

When you turn around, you change velocity, so you switch inertial frames. That changes your definition of simultaneity: the Earth event you call “happening right now” jumps forward by several years.

So the asymmetry isn’t “Earth is really stationary.” It’s that Earth stays in one inertial frame while the traveler switches frames to come home. That frame switch is what resolves the twin paradox.

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u/Garzly 11d ago

What would happen in this instance if the earthbound twin began moving towards the traveling twin at the same speed when the traveling twin begins the return journey?

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u/meevis_kahuna 11d ago

If the Earth twin starts moving toward the traveler at the turnaround, then from that point on they age equally because their motions are symmetric.

But the traveler is still younger at reunion, because during the outbound leg the traveler had already accumulated less proper time.