• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Strong Equivalence Principle Question

My notes say that a photon travelling from a high altitude to an observer at low altitude appears to have a higher frequency i.e. gravitational blue shift.

now im having trouble getting this:

the strong equivalence principle says we can substitute this "laboratory" with a laboratory that is undergoing uniform acceleration -g in deep space.

i then drew a box with the photon being emitted from the roof and travelling towards the floor. the box is moving downward with an acceleration g. so as we are moving away from the source we should use the following formula for gravitational doppler effect:

[itex]\nu^{obs}=\nu^{source} \sqrt{\frac{1-\beta}{1+\beta}}[/itex] where [itex]\beta=\frac{v}{c}[/itex] which predicts a gravitational redshift as [itex]\beta[/itex] is positive as we are moving away from the source with velocity v. i'm messing up somewhere but i can't see it.

help!
 

turin

Homework Helper
2,323
3
... a photon travelling from a high altitude to an observer at low altitude ...
... substitute ... with a laboratory that is undergoing uniform acceleration -g in deep space.
... photon ... emitted from the roof and travelling towards the floor. the box is moving downward with an acceleration g.
... we are moving away from the source ...
In order to emulate earth's gravity, so that things fall to the floor, the floor should accelerate upwards, not downwards. We are "moving" toward the source (if we are on the floor).

[itex]\nu^{obs}=\nu^{source} \sqrt{\frac{1-\beta}{1+\beta}}[/itex] where [itex]\beta=\frac{v}{c}[/itex]
How are you going to determine β?
 
ok. i think what was confusing me was that whilst gravitation on earth is [itex]-\vec{g}[/itex] downward. the strong equivalence principle equivalent of this will be a laboratory in deep space which is accelerating "up" with magnitude g, correct?

this would certainly explain why we observe a gravitational blue shift...

its just an introguction course to GR so we were just examining whether [itex]
\beta[/itex] was positive or negative and noting if we see blue shift or red shift as a result...
 

Related Threads for: Strong Equivalence Principle Question

  • Posted
Replies
2
Views
814
  • Posted
Replies
5
Views
31K
Replies
7
Views
938
  • Posted
Replies
1
Views
2K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top