Who's Older?

Classical Mechanics Level 1

Alfred and Jo are the exact same age before Alfred goes on a vacation to space. Alfred spends most of the trip at very high velocities or relaxing in the presence of very strong gravitational fields (compared to Earth).

When Alfred returns, how will their ages compare?

Alfred will be younger than Jo. Alfred will be older than Jo. Alfred will remain the same age as Jo.

2 solutions

Brahmaprasad R
Sep 15, 2015

That's due to time dilation. Watch Interstellar..!!!

if joe counts Alfred's age they have the same age, but only differs in appearance. meow

Jonathan Stephen Gaston - 5 years, 9 months ago

It is true that there will be time dilation but it will only happen if the spaceship moves towards or away from earth. The person on the spaceship will not get younger on returning to the earth since while returning he has to go from constant velocity frame of reference to accelerated frame of reference. The time dilation principle that you are talking about has been done for constant velocity frame of reference. in short there will be no paradox, if it were so then Alfred would find Jo younger because for Alfred, Jo's frame of reference is moving with constant velocity.

Abhishek Kumar - 5 years, 9 months ago

I agree Alfred Age will depend on what the frame of reference is. If Alfred returns back to Earth and the frame of reference is Earth Surface then his age wouldn't have changed from the last time his age was calculated on Earth.

If however he moves to another planet with greater gravitational pull and travelling at a faster velocity(assuming around sun), his age will be based on what his current age was when he landed to this new planet/spaceship(based on Earth as frame of reference) and the number of years on the new spaceship....a calendar year here will be based on revolution around a frame of reference similar to that of Earth's frame of reference and corresponding age change would be applied. If the new spaceship(outside the realm of earth) is taken as a frame of reference, his new age matters.

To me there isn't sufficient info to the options provided. Not Enough Info would be the answer.

Arunasree Swapna - 5 years, 9 months ago

No. you are going in different direction it has nothing to do with calendar year or revolution of planet around the sun. It's question of special theory of relativity given by Einstein according to which the a moving clock appears slower to a stationary observer.

Abhishek Kumar - 5 years, 9 months ago

Josh Silverman Staff
Sep 16, 2015

According to the theory of relativity, time progresses differently for observers traveling at different speeds. To start, this abolishes the idea of a universal clock, i.e. that there is one proper, agreed upon progression of time for all observers in the universe. If two people move with a relative speed of $v$ , the clock of the faster person will slow down relative to that of the slowly moving person's clock. As the faster person approaches the speed of light, time will all but cease to change.

This may seem like a far out concept, but in fact there is an abundance of experimental evidence for it. For instance, when cosmic rays enter the upper atmosphere and collide with molecules in the air, they produce muons, which have a half life of only 1.5 microseconds when measured in the lab. It takes much longer than 1.5 microseconds for these particles to reach the Earth where they're measured, about 20 times as long. After 20 half lives, we don't expect many of the muons to still be in existence, but in fact we measure quite a few of them making it to the ground. The explanation is that the muons are traveling very quickly, close to the speed of light, so that 1 second on Earth corresponds to only about 0.2 seconds in the frame of the Muon. When the decay is recalculated in the Muon's frame, the prediction agrees perfectly with what's measured. Thus it's really the case that time for the muon "slows down" relative to a clock on Earth.

Similar effects occur when in the presence of a very strong gravitational field.

For our space traveller, the high speeds we moves on and the strong gravity he relaxes near conspire make his clock progress more slowly than his friends on Earth. As such he can be significantly younger than them upon return.

So, if you're on vacation and see this coming, you should reset your watch when you get back:

Who's Older?

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