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We have all heard of the famous twin paradox, in which the twin traveling at relativistic speeds ages slower, and finds his twin brother is now years older than him.

What about the other way around?

Is it possible to move so slowly compared to the rest of the galaxy, that time moves faster for the twin in space than the one on Earth, so that it is the astronaut twin that comes back older? If all speed is relative, can you get infinitely slower, so that a thousand years pass in the spacecraft for every second on Earth?

The first issue I notice is, relatively speaking, what's the difference between decelerating, or accelerating in the opposite direction of what you're slowing down from? Would that mean you couldn't go slower than the rest of the galaxy, without your time still slowing from your perspective? Would that mean there's also a minimum speed limit of the universe?

Ben Warner
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    As time is relative, the question has to be "can time dilation make time go FASTER compared to X". But note that, if I have a clock strapped to me, it will show my proper time, and it will have zero velocity in my frame. Any other clock moving with constant velocity with respect to me will tick slower since, well, it has a non-zero velocity in my frame. So in that sense, zero is the minimum speed limit. The other source of time dilation is my location in a gravitational field, so a clock "higher up" will certainly tick faster than my clock. – Marius Ladegård Meyer May 31 '22 at 21:39
  • "If all speed is relative, can you get infinitely slower, so that a thousand years pass in the spacecraft for every second on Earth?" But we're talking about relative to earth here. You can't go infinitely slow relative to earth, you can only be at rest with Earth or moving relative to it. – Señor O May 31 '22 at 22:01
  • No, the processes can only run slower, due to time dilation. It is all about the interruption of said processes. Surely this makes sense to you. – shawn_halayka May 31 '22 at 23:54
  • No. It can't make time go slower either. – WillO Jun 01 '22 at 00:17
  • What do you mean by infinitely slower? The slowest speed relative to anything else is zero. – Marco Ocram Jun 01 '22 at 10:15

2 Answers2

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What you are asking about does happen, but the time differences are incredibly small. For example, for gravitational time dilation the difference between your man in space (say at an altitude of 10,000 kilometers) versus the man on earth is only 1 second per century. And at the very slow speed of the earth moving through space, the kinetic time dilation would also be very very small. The curve of kinetic time dilation only starts to get steep once you get very close (say 95%) to the speed of light. Even the Bullet Cluster of galaxies is only moving through space at 1% of c.

foolishmuse
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We have all heard of the famous twin paradox, in which the twin traveling at relativistic speeds ages slower, and finds his twin brother is now years older than him.

He doesn't really age slower. In special relativity, there is no absolute time for comparison of the aging process. You just have your own clocks and you get older by one second each second of your own time. The time is actually defined to work that way.

What actually happens in twin paradox is that the twins get separated at certain event A (i.e. at some place and some time) and then meet again at event B. They then compare how long their journeys from A to B through spacetime lasted, simply by checking watches on their wrists (assuming they work properly and show proper time). It so happens, that the twin in a spaceship found a "shortcut" through the spacetime, so that for him, the journey took less time and thus he will be younger upon the meeting.

If all speed is relative, can you get infinitely slower, so that a thousand years pass in the spacecraft for every second on Earth

First of all, speed is relative. You did not specify relative to what you are trying to travel slower than Earth. In the twin paradox, the twin in spaceship moves with zero speed relative to itself (obviously no one is moving relative to oneself), while Earth is moving pretty quickly, possibly very close to the speed of light. Note, that if you move with any velocity relative to Earth, no matter how small, it will be infinitely slower in your frame, since zero is infinitely smaller number than any nonzero number.

But to answer the question if we can get older than citizens of Earth after undertaking some journey through the universe after which we meet again on Earth, we can. The longest time travel between two events A and B is for the inertial observer, i.e. observer that moves at constant velocity. Earth is of course noninertial, meaning, we can become older by simply moving inertially to the meeting event. How big the difference will be will be determined by how much Earth is noninertial and how much in the future (in, say, Earth's frame) is the meeting event.

Umaxo
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