Viewing a single comment thread. View all comments

RecognitionUnfair500 t1_jco38jl wrote

Reply to comment by BrotherBrutha in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Red shift and blue shift or astronomical terms for the tops of shift, which has to do with relative velocity. Not just velocity.

There is so much misunderstanding here that I feel obliged as a physics professor to jump in.

Doppler shift is a relative effect between two observers, it is in effect based on the velocity of either the source or the observer. It is not an intrinsic unitary property of an electromagnetic wave or a photon.

2

BrotherBrutha t1_jco3s0w wrote

To be fair, if it’s a mistake, it’s a pretty common one - for example, from here:

https://astronomy.swin.edu.au/cosmos/c/cosmological+redshift

​

>In cosmological redshift, the wavelength at which the radiation is originally emitted is lengthened as it travels through (expanding) space. Cosmological redshift results from the expansion of space itself and not from the motion of an individual body.

2

RecognitionUnfair500 t1_jco5xbf wrote

Yes, the point is that the redshift is what we see when we look at distant galaxies. Nothing is intrinsically happening to the energy of the photon. That’s what seems to be missing in a lot of these discussions.

1

BrotherBrutha t1_jcoa5jh wrote

>Nothing is intrinsically happening to the energy of the photon.

I think that's my point: the energy of the photon really is reducing (in the case of a cosmological redshift, not a doppler one).

From here :

>Question:.... If light is redshifted in an expanding universe, and this results in photons losing energy, where does that energy go to?

​

>Answer:
..... The short answer, though, is that light loses energy as the Universe expands, and that energy goes into the expansion of the Universe itself, in the form of work.

1

RecognitionUnfair500 t1_jcpvyqg wrote

That is false, and is a violation of the conservation of energy. And you seem to be contradicting yourself as well about the change of the energy of the photon

1

BrotherBrutha t1_jcpyji9 wrote

It’s not just random blogs that say this though; I’m doing the online ANU EDX astrophysics course at the minute, and it was exactly the explanation they gave (one of the presenters is a Nobel prize winner, so I feel like it’s reasonably trustworthy!). And there are many places that give the same description.

Of course, I appreciate it may not necessarily be the full story, but it at least seems to be more than a daft idea!

1

RecognitionUnfair500 t1_jcq3mmq wrote

Is there a chance you may have misunderstood what the presenter said?

1

BrotherBrutha t1_jcq4loj wrote

I don’t think so, it was pretty specific. And it matches the answer given in the NRAO link I gave above.

Of course, I could be wrong!

Edit: is it possible that the physics can be interpreted in a bunch of different ways, and some will describe as I have, and some as you’ve done? Perhaps it’s just different conventions in Cosmology vs straight physics?

1