# The Naked Scientists Forum

### Author Topic: Does time dilation take place during free fall?  (Read 4938 times)

#### flr

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##### Does time dilation take place during free fall?
« on: 05/11/2010 00:30:43 »
If I am at the surface of a planet under a gravitational field of (say) g=9.8N/m^2, I experience a time dilation effect relative to a far-away frame.

My question is: If I am free-falling under the same gravitational field (of g=9.8N/m^2), do I still experience a time dilation effect as if I would be sitting of the planet surface?

« Last Edit: 05/11/2010 08:37:46 by chris »

#### maffsolo

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##### Re: Does time dilation take place during free fall?
« Reply #1 on: 05/11/2010 04:11:39 »
Would you be able to see the event of your body hitting the ground while you free fall or look up to the point where you previously existed to see yourself before falling??
« Last Edit: 05/11/2010 04:14:49 by maffsolo »

#### flr

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##### Re: Does time dilation take place during free fall?
« Reply #2 on: 05/11/2010 07:32:57 »
My question was: Does the free falling objects experience time dilation?

maffsolo, it is possible to be in free fall motion around a planet, without ever colliding with that planet. Ex: Moon motion around Earth. not sure what's your point (if any).
« Last Edit: 05/11/2010 07:39:12 by flr »

#### imatfaal

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##### Does time dilation take place during free fall?
« Reply #3 on: 05/11/2010 09:49:25 »
FLR - yes and no.   So if the gravitational potential is the same you will get no time dilation - but it is very hard to imagine a real example of free fall where the gravitational potential will remain the same as that on the earth's surface. The gravitational will not be equal at different heights - and this difference will be manifest, obviously falling from a chair isn't going to be recognizable. As your example of the moon made clear an object can be in longterm free fall and under the effects of gravity (and nothing else thence the name free fall) - and under the effects of this situation's differential gravitational potential one will have time dilation.

One of the best and neatest examples of time dilation are the GPS satellites - the atomic clocks on board each satellite must be accurate for the system to work and thus are adjusted for time dilation due to relative velocity and gravitational potential.

I dragged these figures off the net - the presentation looked pretty good but I haven't checked them
Gravitational
Orbital altitude 20,183 km
Clock runs fast by 45.7 μs per day

Time dilation
Satellite velocity 3.874 km/s
Clock runs slow by 7.1 μs per day

« Last Edit: 05/11/2010 09:58:32 by imatfaal »

#### maffsolo

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##### Does time dilation take place during free fall?
« Reply #4 on: 05/11/2010 11:45:51 »
My question was: Does the free falling objects experience time dilation?

maffsolo, it is possible to be in free fall motion around a planet, without ever colliding with that planet. Ex: Moon motion around Earth. not sure what's your point (if any).
Sorry Flr. I was a bit short on an explanation, just an odd abstract attempt to refine an explanation.
« Last Edit: 05/11/2010 11:51:07 by maffsolo »

#### Bill S

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##### Does time dilation take place during free fall?
« Reply #5 on: 05/11/2010 15:05:42 »
Quote from: imatfaal
The gravitational will not be equal at different heights - and this difference will be manifest

Does this mean that as one falls, gravitational potential varies constantly, and perceived time dilation in that F of R varies with it?

#### imatfaal

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##### Does time dilation take place during free fall?
« Reply #6 on: 05/11/2010 15:44:17 »
As FLR mentioned you need to be careful when you say freefall.  If someone turfs you out of a u2 spyplane you will freefall to earth at first (very thin atmosphere);  the moon is also in freefall - ie they follow courses in which the only force is gravity.

If you change height (ie distance from centre of mass) then the gravitational potential changes - and there will be a dilation effect between the two points.

#### flr

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##### Does time dilation take place during free fall?
« Reply #7 on: 05/11/2010 19:55:53 »
As I understand, an object will 'experience' the same time-dilation when (i) it is free-falling under a constant gravitational field or (ii) it is sitting on the surface of a planet (again under constant gravitational field) , if the gravitational field is constant and the same in both cases.

#### maffsolo

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##### Does time dilation take place during free fall?
« Reply #8 on: 05/11/2010 21:07:20 »
As FLR mentioned you need to be careful when you say freefall.  If someone turfs you out of a u2 spyplane you will freefall to earth at first (very thin atmosphere);  the moon is also in freefall - ie they follow courses in which the only force is gravity.

If you change height (ie distance from centre of mass) then the gravitational potential changes - and there will be a dilation effect between the two points.

Doesn't the gravitaional potential also change below the surface of the mass, decending to the center?
In other words, isn't the surface of the mass the turning point for gravity's potential and its direction is pointed to the center of the mass?
« Last Edit: 05/11/2010 21:12:00 by maffsolo »

#### flr

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##### Does time dilation take place during free fall?
« Reply #9 on: 05/11/2010 21:46:12 »
Yes, it changes.
Circular orbits around a planet would correspond to free-fall under constant potential, right?

#### jartza

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##### Does time dilation take place during free fall?
« Reply #10 on: 05/11/2010 22:06:20 »

Gravitational potential energy is m*g*h
Gravitational potential is g*h

time's speed in place a / time's speed in place b = potential in a / potential in b

clock's speed in place a / same clock's speed in place b = potential energy in a / potential energy in b

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##### Does time dilation take place during free fall?
« Reply #10 on: 05/11/2010 22:06:20 »