# Equivalence principle?

## Main Question or Discussion Point

The Equivalence principle says or at least this is what i learned , Is that being in free-fall is the same as being out in space , But in free fall if you shined a laser up it would get Doppler shifted and gravitationally red shifted but out in space it would not . Or do i have something wrong.

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The equivalence principle says that in a local region around you (the "laboratory") you cannot perform an experiment that tells you whether you are in outer space or free fall in a gravitational potential. Far away from your laboratory others can certainly determine a difference. Those others would be the ones measuring your Doppler shift and gravitational shifts or lack thereof.

You with your laser would not be able to tell which was the case.

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how local are we talking , because lets say I had some gas of neon or whatever and then I was shining my laser at , but I had the frequency specific enough to where if it was red shifted it would not absorb the photon. But now that i think about it when the photon is red-shifted from my falling elevator , the neon gas is moving towards the light so the blue-shift would cancel the redshift . But then we still have the gravitational red-shift but from the time the light was emitted the elevator has picked up speed so the increased speed might cancel the gravitational red-shift.

If the free fall is in a uniform gravitational field, then, there will be no red shift according to the freely falling observer.

so it has to be in a uniform field. Like falling towards earth would no work , what about the Doppler shift .

jambaugh
Gold Member
You need to be more specific about your thought experiment. If you are in free-fall within a tank of the gas which is free-falling with you then you won't see any effect.

If you are free falling through the gas then the gas is not in free fall (it is kept from it via a pressure gradient like we have in our own atmosphere) and the EP implies you'd see the same light interaction as if you were in space within a wind tunnel which was accelerating the gas.

Remember to apply the EP you must speak of nearby gas interactions. There are still tidal effects which distinguish falling a central gravitational potential vs floating in free space.

Finally also remember that the equivalence principle is used in GR when deriving the gravitational red & blue shifts so they will be consistent with it (if GR is correct).

Ok if i was in free-fall , The gravitational field would be slightly stronger at my feet than at my head . So if i have to atomic clocks one at my feet and one at my head and i am in free-fall the clocks would get a little bit out of sync . Or is this not ok to use special consequences of GR to disprove the EP.

Ok if i was in free-fall , The gravitational field would be slightly stronger at my feet than at my head . So if i have to atomic clocks one at my feet and one at my head and i am in free-fall the clocks would get a little bit out of sync . Or is this not ok to use special consequences of GR to disprove the EP.
This is what is referred to as "tidal effects". The gravitational field you are describing is non-uniform.

ok so it has to be uniform to apply the EP.
thanks for all of your guys responses by the way.