eli5: What does it mean when people say that time works differently in space?

Question

I’ve heard people talk about how people living on a space station their whole lives would age at different rates to people on Earth but…how?

In: Physics

Answer 1

This doesn’t really have anything to do with being in space per se, but instead has to do with relative movement and gravity.

It all comes back to the concept that there is a maximum speed limit for things to move. This is the speed of light; light moves at the maximum possible speed for anything in our universe. But movement is relative so there is no underlying frame of reference upon which this is based. You cannot for example say that an object is moving at half the speed of light in one direction and therefore cannot accelerate beyond another half of light speed.

As a consequence of this, everyone sees light as moving at the same speed regardless of their frame of reference. Shoot a beam of light out in front of you and it recedes at “c” (short for light speed), and if you accelerate to 90% of c then shoot a beam of light… it still seems to recede at c!

This seems pretty weird and it results in all sorts of strange consequences. Observers in different frames of reference will always agree on the maximum speed limit of c, but they won’t agree on either the distances between locations or the time which passes between different reference frames. If someone is moving very quickly they will start to see the universe compress in the direction of their travel, distances along their path seeming to be shorter than a “stationary” observer. The traveler will also experience less time passing than the observer that doesn’t move.

It is important to understand that this impact on aging has nothing to do with exposure to radiation, or some subtle influence on how our bodies grow and change. It has everything to do with the fundamental passage of time; a very fast traveler might experience only 24 hours while others experience 1000 years! Both of them are “correct” about how much time passed for them, our expectation that time and distance are the same for everyone is just untrue.

Getting back to people in space, our astronauts are orbiting Earth at about 7,700 meter per second. This means that over the course of a year they experience about 0.01 seconds less than people on Earth. Obviously this isn’t a huge difference, but it is there and significant enough that GPS satellites need to take it into account in order to function.