Does gravity affect how the body ages?

15 November 2016


Elderly man holding a watch



I've listened your latest podcast where you had a question regarding a travel to Jupiter for 50 years and what would be the effect on a human compared to other human beings.

Well that - and many, many other theoretical ideas about Einstein’s theory of general relativity has been bugging me for quite a while. In essence the question is; if you travel very fast or close to a massive gravity your time does slow down but does the ageing slow as well? And if yes, how?

Let me rephrase my question;

Say we send a hypothetical Mr. Smith close to a black hole to make a hypothetical podcast from there for one hour. In theory, as far as I've understood, once MR. Smith get's back to his home planet everything around him has aged several years (or something like that) but not MR. Smith. (Let's disregard the travelling time to and from this black hole)

But how is it possible that MR. Smith's body has slowed it's "natural" ageing process while on this trip? Do we have some internal clock in our bodies that slows everything down or does the increased gravity slow the cells and genes down?

Obviously it's just a theoretical funny game or idea so the other question is - is there even point thinking this? Well obviously not, but still it bugs me. Can our bodies ageing process slow down in increased gravity or speed according to Einstein’s general relativity?

Just a question for your fantastic show! Hope it get's attention :)

Yours Sincerely

Tuomo Seppala
A dedicated listener and science fan from Helsinki, Finland


Chris Smith put this to physicist Andrew Norton from the Open University...

Andrew - Well, Tuomo is absolutely right. There are two effects here, and what we're talking about here is relativity. We're talking about special relativity and general relativity. And, as Einstein first figured out over a hundred years ago now, if things move very fast then time will slow down for the rapidly moving body - that's special relativity. And if you come close to a very massive object, such as a black hole, then time will slow down in a very strong gravitational field - that's general relativity. And these thing have been measured; they're absolutely measured to high precision that we can in many experiments.

If we take the example say of a GPS satellite (Global Positioning Satellite), they're in orbit above the Earth at about 20 thousand kilometers up. Now they're orbiting the Earth, so they're going quite fast and as a result of the speed that they're going, a clock on a GPS satellite will run slow by about 7 microseconds in the course of 24 hours.

Chris - 7 millionths of a second?

Andrew - 7 millionths of a second, yeah, in the course of 24 hours. Now that's one effect, that's the special relativity effect. But, of course, the GPS satellite is also further away from the Earth than we are, so it's further away from the strong gravity of the Earth, if you like. And, as a result of that effect, the clock on the GPS satellite would run fast by about 46 microseconds over the course of 24 hours. So these two effects are going in opposite directions - 46 microseconds fast due the general relativity and 7 microseconds slow due to the special relativity, so there's a net effect of one minus the other there.

As to what is causing it? If it's a person on that satellite would their body clock feel those same effects? They would indeed. It's nothing to do with the body, or the clock, or the satellite, it's that time itself changes the rate that it flows. It doesn't necessarily flow forwards at the rate of one second per second as a result of high speeds in special relativity or strong gravitational fields in general relativity.

Chris - But it does for you. You're the person experiencing the time effects so for you your watch continues to tick one, two three. You think that a year has gone or however long but the people who are not in the same frame as you, for them time is running faster?

Andrew - That's exactly right Chris, yeah. So it's a real effect, but the time as you experience it is affected by the speed that you're moving, by the location that you're in relative to other people of other anything. It doesn't need another person there. But time doesn't always flow at the same rate.

Chris - Peter.

Peter - Yes. So if you take an airline pilot who spends the whole time - I mean how many miles do they do in a lifetime? A hundred million miles, that's seven hundred miles an hour. Any idea how many microseconds younger is it, than they would be from the person staying on the ground?

Andrew - Well again, there's both effects. They're moving fast so they're getting the slowing down due to special relativity but they're also at high altitude, so they're getting the speeding up due to being further away from the gravity of the Earth. I haven't worked it out but I would guess that for people in aeroplanes it's probably a similar effect to being on a satellite, so one of the effects will outweigh the others. There will be a certain distance above the Earth at which the two effects would cancel out, actually.

Chris - There's another factor which is that being that much higher up in the atmosphere you exposed to more incident radiation coming in from space. So people who pilot aeroplanes, and cabin crew have higher rates of cancers and lymphomas (cancers of white blood cells). So actually there is an advanced ageing effect owing to radiation exposure which outpaces the effect of relativity. Sorry to rain on your parade there boys.


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