# If anyone here doesn't yet get the twin "paradox" . . .

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## Main Question or Discussion Point

Jim al Khalili did a (basic level) TV programme in the UK last night where he featured a smartphone app which works as a space-time "odometer".

Now you can experiment on your own version of the legendary brain-teaser with your friends, no need to go to Alpha Centauri and back! Go for a walk, take a bus/tube ride, or just sit in an armchair - the choice is yours.

## Answers and Replies

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PeterDonis
Mentor
Interesting. The app's page doesn't say how it works, but I'm assuming it uses some combination of accelerometer/GPS to sense your velocity and altitude and then calculates your time dilation factor relative to a standard (the usual standard is being at rest on the Earth's geoid), and integrates that to get your elapsed aging.

PAllen
If this encourages people to move around quickly to get reduced aging from relativity, it will give a much larger benefit from biological 'age reduction'; of course, if a car is used, not so much.

mfb
Mentor
The gravitational effect dominates unless you use an airplane or drive around all day.

PAllen
The gravitational effect dominates unless you use an airplane or drive around all day.
Ah, but the gravity effect is the common baseline. You are not going to change anything by jumping (unless you can jump like a flea), most people fly only a couple times per year. Thus, motion is the controllable factor.

mfb
Mentor
Unless you live in an extremely flat region, just changing the position will change your height a bit, changing the gravitational time dilation.
Even going one floor up or down has an effect of 0.3 fs/second or 26 ps per day.

PAllen
Unless you live in an extremely flat region, just changing the position will change your height a bit, changing the gravitational time dilation.
Even going one floor up or down has an effect of 0.3 fs/second or 26 ps per day.
ah, good point. For a longer life (relative to others) get thee to Death Valley!

mfb
Mentor
I don't think that is good for your health, and it is not particularly deep (down to 86 m below sea level). Go to the Dead Sea (430m below sea level).
You can also go into mines, but that is probably even worse in terms of health and you don't have GPS there - the app won't work.

PAllen
I don't think that is good for your health, and it is not particularly deep (down to 86 m below sea level). Go to the Dead Sea (430m below sea level).
You can also go into mines, but that is probably even worse in terms of health and you don't have GPS there - the app won't work.
I LOVE desert hiking. Two of my best hiking memories are hiking all day solo in Death Valley in October, and hiking with buddy during cactus blooming season in the Sonoran desert.

Edit: but I'm sure I'd like the Dead Sea!

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You can also go into mines, but that is probably even worse in terms of health and you don't have GPS there - the app won't work.
But if you were in a (well air conditioned) sphere in the exact center of the Earth, wouldn't be weightless?

PAllen
But if you were in a (well air conditioned) sphere in the exact center of the Earth, wouldn't be weightless?
Yes, but time dilation relative to sea level would be maximal. To make a simplistic analogy, the more stair climbing energy you need to reach distant empty space, the slower you age relative to distant empty space. Acceleration of gravity at one point plays no role at all in gravitational time dilation.

Interesting. The app's page doesn't say how it works, but I'm assuming it uses some combination of accelerometer/GPS to sense your velocity and altitude and then calculates your time dilation factor relative to a standard (the usual standard is being at rest on the Earth's geoid), and integrates that to get your elapsed aging.
From what he said in the programme I think he worked out the SR and GR effects separately. Interestingly, he got it wrong the first time (he did not realize the surface of the earth is equipotential) during the making of the programme, and had to get it re-done!
I think the current version is more sophisticated as a result, somewhere I read that it now also includes the Sagnac effect.

It seems a good time to bring up this article again for those interested in the gory details.

[EDIT] Here is a link to a review of the programme.

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