Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 15/04/2017 08:13:20

Title: How is wavelength perceived in freefall?
Post by: jeffreyH on 15/04/2017 08:13:20

If an observer is in freefall in vacuum how would they perceive the shift in wavelength of light sources from different gravitational potentials to their own? What equations would be needed to calculate this?

Title: Re: How is wavelength perceived in freefall?
Post by: chris on 15/04/2017 13:33:38

Now that's a good question! Actually our latest question of the week looked at something a bit similar. It discussed the sound made by a falling bomb, and why Hollywood have been hoodwinking us for decades:

https://www.thenakedscientists.com/articles/questions/why-do-bombs-make-whistle-sound-they-fall

Title: Re: How is wavelength perceived in freefall?
Post by: jeffreyH on 16/04/2017 21:11:35

Can Rindler coordinates be used in this situation?

Title: Re: How is wavelength perceived in freefall?
Post by: jeffreyH on 17/04/2017 00:52:31

For more information see:
https://en.m.wikipedia.org/wiki/Rindler_coordinates

Title: Re: How is wavelength perceived in freefall?
Post by: evan_au on 17/04/2017 12:31:57

You could start here:
Special Relativity (velocity effects): https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
General Relativity (gravitation effects): https://en.wikipedia.org/wiki/Gravitational_time_dilation

Title: Re: How is wavelength perceived in freefall?
Post by: jeffreyH on 17/04/2017 13:24:04

While those are useful, I am currently looking at rain coordinates.
https://en.m.wikipedia.org/wiki/Gullstrand%E2%80%93Painlev%C3%A9_coordinates

Title: Re: How is wavelength perceived in freefall?
Post by: yor_on on 17/04/2017 15:54:13

Don't really know how you set that into Gullstrand–Painlevé coordinates, but, maybe, you could start with looking at Lorentz-invariant theory of gravitation (https://en.wikiversity.org/wiki/Lorentz-invariant_theory_of_gravitation)   All of it is observer dependent but you have those LorentzFitzgerald equations joining different frames of reference. Should be a possibility here.
=

Don't read this as I agree with all that is written there though, but it discuss what you're interested in. Another possibility is using GR for it, which is a rather advanced concept and as I would expect, rather difficult. General Relativity PHY-5-GenRel U01429 16 lectures  (http://star-www.st-and.ac.uk/~hz4/gr/HeavensGR.pdf)   

 

Title: Re: How is wavelength perceived in freefall?
Post by: yor_on on 17/04/2017 17:18:18

Weird, my answer disappeared almost as I logged out, got cut after the mess telling people that you can't follow the links unless you register? Also Jeffrey, I was thinking of it in terms of trying to get to a 'universal expression' aka Lorentz transformations. Are you thinking about how to calculate a frequency at one particular time, being in one gravitational potential, observing another?

Title: Re: How is wavelength perceived in freefall?
Post by: jeffreyH on 18/04/2017 12:24:02

Keep it up yor_on. It is good to be reminded of the alternatives to keep you on a steady course.