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

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Discussion Overview

The discussion revolves around a smartphone app that calculates time dilation effects based on relativity, inspired by the twin paradox. Participants explore the app's functionality, its implications for aging, and the factors influencing gravitational time dilation, including motion and altitude.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants discuss the app's use of accelerometer and GPS data to calculate time dilation based on velocity and altitude.
  • Others suggest that the gravitational effect is significant unless one is traveling at high speeds, such as in an airplane.
  • There is mention of the impact of changing height on gravitational time dilation, with specific numerical effects noted.
  • Some participants express skepticism about the health implications of seeking lower gravitational potential, such as in Death Valley or the Dead Sea.
  • A participant raises a hypothetical scenario about being weightless at the center of the Earth and its relation to time dilation.
  • One participant notes a correction made during the program's production regarding the equipotential nature of the Earth's surface and mentions the inclusion of the Sagnac effect in the app's calculations.

Areas of Agreement / Disagreement

Participants express a range of views on the factors affecting time dilation, with no consensus on the best approach to utilize the app or the implications of altitude and motion on aging. The discussion remains unresolved regarding the optimal conditions for experiencing time dilation.

Contextual Notes

Some assumptions about the app's functionality and the effects of gravitational time dilation are not fully explored, and there are unresolved questions about the accuracy of the app's calculations and the health implications of suggested activities.

m4r35n357
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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.
 
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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.
 
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.
 
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The gravitational effect dominates unless you use an airplane or drive around all day.
 
mfb said:
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.
 
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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.
 
mfb said:
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!
 
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.
 
mfb said:
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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mfb said:
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?
 
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Chris Miller said:
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.
 
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PeterDonis said:
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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