Time Dilation & Aging: Why Do Astronauts Age More Slowly in Space?

[InquiringMind]

TL;DR

Time-dilation and Aging

I can't seem to get my head around this. It's my understanding that time moves slower in the heavy gravitation than it does in space. So why to I read that astronauts age move slowly in space, where time moves faster, than the would on earth? I would think if time is moving faster, they would age more quickly. Would it have to do with time actually moving slower because they are moving so fast?

 

[malawi_glenn]

https://en.wikipedia.org/wiki/Time_dilation

Gravitational time dilation is experienced by an observer that, at a certain altitude within a gravitational potential well, finds that their local clocks measure less elapsed time than identical clocks situated at higher altitude (and which are therefore at higher gravitational potential).

Gravitational time dilation is at play e.g. for ISS astronauts. While the astronauts' relative velocity slows down their time, the reduced gravitational influence at their location speeds it up, although to a lesser degree.

 

[phinds]

TL;DR Summary: Time-dilation and Aging

I can't seem to get my head around this. It's my understanding that time moves slower in the heavy gravitation than it does in space. So why to I read that astronauts age move slowly in space, where time moves faster, than the would on earth? I would think if time is moving faster, they would age more quickly. Would it have to do with time actually moving slower because they are moving so fast?

You are confusing time dilation, which is frame dependent, with differential aging which is not frame dependent. Time NEVER moves locally at anything but one second per second. This is called "proper time". "Coordinate time" is the time "seen" by an observer, not the subject and varies according to the observer. You are not moving relative to the chair you're sitting in but you are moving very fast relative to a bullet train in France and VERY fast relative to a particle in the CERN accelerator. Each of those three things sees your coordinate time differently, but you see it as proper time.

There are two reasons for differing coordinate times. One is speed (Special Relativity) and one is depth in a gravity well (General Relativity). I suggest more reading (there are many hundreds of threads here on PF about both)

 

[PeterDonis]

It's my understanding that time moves slower in the heavy gravitation than it does in space.

That's one effect, but not the only one.

So why to I read that astronauts age move slowly in space, where time moves faster, than the would on earth?

Because there are two effects involved, not one: the gravitational effect due to altitude, and the time dilation effect due to speed. In low Earth orbit, the time dilation due to speed outweighs the faster time flow due to higher altitude (compared to the surface), so the astronauts age more slowly.

Conversely, at the altitude of the GPS satellites (semimajor axis of 4.2 Earth radii), the gravitational effect dominates, so the clocks on the GPS satellites naturally run faster than ground clocks, and their rates have to be adjusted accordingly.

In between there is a point where the two effects cancel out and a clock in free fall orbit runs at the same rate as a clock on the Earth's surface. IIRC this point is at 1.5 Earth radii (1.5 times the Earth's polar radius, to be exact).

 

[PeterDonis]

You are confusing time dilation, which is frame dependent, with differential aging which is not frame dependent.

No, he's not. We are not talking about SR inertial observers that only meet once. We are talking about observers in orbit about the Earth, for which "meeting" can be defined (at least to a good enough approximation for this discussion) as each time the astronaut in orbit passes over the same fixed point on the ground. (We are assuming a perfectly circular orbit in the equatorial plane.) For those observers, both the gravitational altitude effect and the effect due to orbital speed are invariant; both observers will agree on them. In other words, both effects are differential aging effects in this scenario.

If you are still doubtful, you are welcome to apply the most general test and compute the actual proper times using spacetime geometry. (For this case, it's actually not that hard since you can find closed form expressions for the proper times.)

 

[phinds]

We are talking about observers in orbit about the Earth, for which "meeting" can be defined (at least to a good enough approximation for this discussion) as each time the astronaut in orbit passes over the same fixed point on the ground.

Good point. Thanks.

 

[vanhees71]

No, he's not. We are not talking about SR inertial observers that only meet once. We are talking about observers in orbit about the Earth, for which "meeting" can be defined (at least to a good enough approximation for this discussion) as each time the astronaut in orbit passes over the same fixed point on the ground. (We are assuming a perfectly circular orbit in the equatorial plane.) For those observers, both the gravitational altitude effect and the effect due to orbital speed are invariant; both observers will agree on them. In other words, both effects are differential aging effects in this scenario.

If you are still doubtful, you are welcome to apply the most general test and compute the actual proper times using spacetime geometry. (For this case, it's actually not that hard since you can find closed form expressions for the proper times.)

Here's an example for such a calculation:

https://www.physicsforums.com/insights/the-twin-paradox-for-freely-falling-observers/