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FMLAdad t1_jcnga7l wrote

Reply to comment by jmarkmorris 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

My understanding is that redshift is caused by expansion, and one possible outcome is that photons will indeed redshift into nothing. At that point we would not see other galaxies and they may as well not exist to the observer.

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OffusMax t1_jcnle8d wrote

Redshift is caused by the motion of the object emitting the light and the fax that light behaves like a wave.

Consider the following example. Imagine a train sitting still on the tracks. The sound waves emitted by the engine propagate away from it as expanding, concentric spheres. There is no motion so an observer hears the sound at their natural frequency.

Then the train starts to move. At each interval, it emits a new spherical wave that has moved from the position where the last sphere was emitted. That means that the distance between the sphere just emitted and the previously emitted sphere in front of the train is closer than the distance between them in the back of the train. When the observer hears the sound, they hear a higher pitched sound as the train approaches them (blue shifted) and a lower pitched sound as it recedes from them (red shifted).

The same thing happens with the light emitted from stars in a galaxy. The color of the light changes because it’s light and not sound. But it’s all caused by the way we perceive light (or sound for the train) and not anything intrinsic about the universe.

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FullOfStarships t1_jco9rvz wrote

This is wrong (except for any difference proper motion that exists at the time the photon is emitted).

Cosmic redshift is an expansion of space.

Your analogy requires that the train is stretched by the expansion.

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jthtiger t1_jcnq4bb wrote

Redshift is (from my understanding) a single moment, not continuous. Light travels at a constant rate, so the wavelength is not CONTINUALLY expanding. If it did, then one of the wave fronts would have to be travelling at a different speed. The redshift is only caused by the difference in position of the object that emitted them from when two waves were emitted.

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ZylonBane t1_jcnt1c7 wrote

Redshift has nothing to do with position. Redshift is the photon equivalent of the Doppler effect. Just as sound sources that are rapidly receding sound lower-pitched due to their waveforms being stretched out, light from sources that are rapidly receding appear shifted toward red in the electromagnetic spectrum. So velocity is what matters.

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jthtiger t1_jcnttul wrote

Position isn't the right word probably. Velocity is more accurate yes, but it's the velocity of the object that emits that cause the wavelength to be stretched.

My point was that the wavelength does not continue to stretch over time. So a photon won't redshift into nothing-ness.

The velocity of the photons does not change over time and therefore will not drift apart.

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ZylonBane t1_jcnv4vx wrote

>So a photon won't redshift into nothing-ness.

It will if the rate of universal expansion is accelerating.

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jthtiger t1_jcnvjfn wrote

Hmmm. As always, physics answers brings more questions. Thanks for imparting some knowledge.

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BrotherBrutha t1_jco0ybo wrote

There is a redshift due to the expansion of space as the photon is travelling; this will keep happening and is continuous.

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RecognitionUnfair500 t1_jco38jl wrote

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.

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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

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>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.

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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.

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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.

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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

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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!

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RecognitionUnfair500 t1_jcq3mmq wrote

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

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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?

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