# Time: Travelling twins Vs Black hole

## Main Question or Discussion Point

Forgive me if I have posted this in the wrong location. I'm trying to reconcile my understanding of spacetime, but am running into a paradox that I'm sure is arising from my own misunderstandings. As you get closer to a black hole, time, with respect to outside observers, begins to slow to a stop. My understanding is that it is the extreme gravity that is warping space and time around the black hole, to the point that, as an example, a clock just outside the black hole would move slower than one sitting outside of it's influence. Now the second issue I am having is with the travelling twins idea. Unless I'm misunderstanding, the twin that remains on the planet, will experience time at a faster rate with respect to the travelling twin. So what causes the clock just outside the black hole to move slower, the clock on earth to move faster, and a clock in space to move at what I will consider "normal" pace?

Edit: Updated references to focus on just outside the black hole.

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## Answers and Replies

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phinds
Science Advisor
Gold Member
2019 Award
All clocks move at the same local rate of one second per second. The confusing part is that while that is true, it is ALSO true that different paths through space-time can take differing AMOUNTS of time even though along both paths, the clocks are ticking at the same rate. This is a bit hard to get your head around at first but it falls out rather trivially from the simple math of Special Relativity and has been show experimentally to be true.

By the way, "inside" a black hole, things get exceeding weird. What you really mean in your question is "just outside the black hole", not "within it"

EDIT: Google "muons and special relativity" for a concrete example (and be careful ... you'll see a lot of lunatic sites saying relativity is wrong)

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Thank you for the quick response, I updated the post to reflect just outside the blackhole rather than within. I'm just trying to wrap my head around this concept which has me rather confused at the moment. Thanks!

phinds
Science Advisor
Gold Member
2019 Award
Thank you for the quick response, I updated the post to reflect just outside the blackhole rather than within. I'm just trying to wrap my head around this concept which has me rather confused at the moment. Thanks!
Hey, you're not alone. When I first started studying quantum mechanics (the very small) and cosmology (the very large), there were several times when I stomped around the room tearing at my hair and shouting "THAT CAN'T BE RIGHT !!!"

Dale
Mentor
So what causes the clock just outside the black hole to move slower, the clock on earth to move faster, and a clock in space to move at what I will consider "normal" pace?
Although the twins scenario is often posed as one twin staying on earth, the scenario is usually supposed to ignore gravity. So the only difference between the two twins is speed, not gravity.

PeterDonis
Mentor
2019 Award
Unless I'm misunderstanding, the twin that remains on the planet, will experience time at a faster rate with respect to the travelling twin.
As DaleSpam pointed out, this isn't because the traveling twin is "out in space". It's because he's traveling; more precisely, because he travels out and then turns around and comes back again. To make it clear that gravity is being ignored in the scenario, a truly precise presentation would have the "stay at home" twin floating out in space somewhere, while the traveling twin fires his rockets to start up, then to turn around, and to stop again when he returns.