#Science - Do astronauts age faster in space? (Question inspired by a documentary on Albert Einstein...)

Question

Do astronauts age faster in space?

Answer

It depends... sometimes they age slower and sometimes faster. See below for more... 

Analysis

I think the best way to approach this question is to gently lead us on a trip down Science Lane...

In 1687, Sir Isaac Newton, one of the preeminent thinkers ever, wrote his Principia (also known as Mathematical Principals of Natural Philosophy). Within that great book was Newton's Three Laws of Motion (remember Action/Reaction from school? That was Newton. Remember that whole thing with observing the apple falling from the tree and then seeing the Moon not fall and realizing that the laws governing the apple should also govern the Moon? Newton.)

And so the question of "Do astronauts age faster in space?" in the days of Newton, the answer would be: No. Astronauts age the same as everyone on Earth. And by the way, what's an astronaut?

Fast forward a couple of hundred years and we have James Clerk Maxwell who wrote A Dynamical Theory of the Electromagnetic Field in 1865. In it, Maxwell found that light operates the same as electrical and magnetic waves and that they all have a maximum speed (that of light).

And then, another 50 or so years later, comes a patent clerk, 3rd class from Austria by the name of Albert Einstein, who read both books and realized there was a problem.

With Newton, all events for all observers happen at the same time. Information (such as visual information - like the light bouncing off that building that lets you see it, or the light coming from your computer screen right now) would be received by everyone at the same time. For example, for someone sitting directly in front of the computer screen and for someone on the other side of the room and looking over your shoulder, the light from the computer screen arrives at the two of you at the same exact time. For Maxwell, that isn't true - the light arrives in your eyes first and then, very shortly after, arrives in the eyes of the person at the other side of the room. Einstein said "no no - this can't be! Someone is wrong". He probably said this in German, but you get the idea.

Einstein went on to work out that Newton, while approximately correct (to the degree that the two of you reading that computer screen would never notice), was ultimately wrong - that his theories needed a rework. And Einstein did just that. And the world of physics experienced a collective "mind blown" moment.

Einstein worked out that space and time are interconnected, that light indeed sets the absolute speed record for potential velocities, and that the curvature of "space time" affects everything. You, me, light... everything. These are his Special and General Theories of Relativity.

So what does this have to do with astronauts and aging? Plenty. The thing we'll be focusing on is how Time is affected by the expansion and compression of "space time".

Note the important word in the theories - Relative. We're getting into an explanation that deals with, not so much What I Experience, but What I Observe Someone Else Experience. Which can get complicated, but let's see how this works.

Before getting into that, let's first set the stage for our Astronaut (labeled A) and our Earth-bound observer (labeled E).

E is on the Earth, experiences a certain level of gravity (for argument's sake, let's have E be at sea level) from the Earth and a tiny tiny amount from the Moon and the Sun (the two celestial bodies that produce ocean tides). E also experiences a certain velocity from the movements of the Earth (revolving around it's axis to produce day and night, the revolutions about the Sun producing the seasons, the movement of the Solar System revolving around the centre of the galaxy, the movement of the galaxy in the universe, etc). That is the gravity and velocity that E experiences. For all that, E experiences 24 hours in a day and lives his life, all without being aware of most of this.

So now let's shoot A up into space. She's living a lovely astronaut life in her astronaut spaceship. She experiences a lot less gravity (being much farther away from sea level than E), but she also, being in orbit around the Earth, experiences a much higher velocity than E (she experiences all the movement that E does, except for the rotation of the Earth. That rotation speed is replaced with an orbital speed, which is much higher than the Earth's rotation). Like E, A experiences 24 hours in a day and lives her life, all without being aware of most of this.

Now remember that Newton, asked the question about A and E, would say that they age at the same rate. A and E will experience their 24 hour days in the same way - they both work, sleep, and do all the things they do in a day, all without ever feeling like their day is shorter or longer than it should be. Clocks record all 24 hours of every day - no seconds or minutes mysteriously are added or subtracted. For A and E, it all feels very "Newton" - there's no difference between the two of them.

But now let's watch as they observe each other and we'll see Einstein's theories pop up:

For A, who is experiencing less gravity than E, time will flow faster relative to E. Why? Because as gravity decreases, time speeds up.

However, also for A, since she is experiencing a faster velocity than E, time will flow slower relative to E. Why? Because as velocity increases, time slows down.

How these two effects net out is the difference A and E will observe in each other. For arguments sake in this example, let's have the effect of the decreased gravity outweigh the effect of the increased speed for A, and so she will experience time slightly faster (and age slightly faster) than E.

Let's say that the two of them decide that E will call A at 6pm exactly each day. As the days progress, A will notice that E's call comes later and later.

This is exactly the situation with every single GPS satellite in orbit around Earth. Because of relativistic effects, this difference in the passage of time experienced by the satellites as compared to the Earth-bound receiving towers needs to be programmed into system so that the satellites and the towers operate on the same time for each communication back and forth.

There is a "sweet spot", an orbit where the time flow differences from decreased gravity and increased speeds balance out and time will be experienced for A and E without a difference. That sits at roughly 0.5 Earth radii above sea level, or about 3,000 km (roughly 1860 miles) up. Orbits below that experience slower time (the International Space Station) while orbits above that experience faster time than E (GPS satellites). For GPS satellites, the difference is roughy 2 minutes per year.

So do astronauts age faster than those of us on Earth? It depends on the local gravity and velocity that the astronaut experiences.

Major help in answering this question came from wikipedia discussions of Newton, Maxwell, Einstein, and Socratic.org's discussion of this very question.

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As always, questions and comments are welcome!