jmarkmorris t1_jcnbw8p wrote
Reply to comment by extra_specticles 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
I've also wondered about Op's question. What happens as photons continue to redshift? Is there some ultimate redshift where the photon just fades away? Or is this a case of we don't know because we can't observe photons below frequency f. By the way, what is the lowest frequency longest wavelength photon that is observable by state of the art equipment?
triffid_hunter t1_jcnj657 wrote
> what is the lowest frequency longest wavelength photon that is observable by state of the art equipment?
We're still receiving photons from the first moment that the universe was transparent, they're called the CMB
FullOfStarships t1_jco9jx0 wrote
The big bang was what scientists call "very hot". So hot that the atoms were in a plasma (the "fourth state of matter") for the first 370,000 years. Towards the end of that time, the whole universe had cooled to the same temperature as the surface of the sun. That's your mental picture - surface of the sun but everywhere. White hot and glowy.
As it cooled a bit more the plasma "condensed" into gas as we're familiar with (the "third state of matter"). This is much like water vapour (third state) condensing to become water (second state).
Gas is transparent (you can see through the atmosphere) instead of glowy, so the photons that had been trapped (outrageous simplification) in the plasma were released. Fly, my pretties.
That's from about 370,000 years after the big bang. Expansion of the universe has redshifted (cooled) those photons by a factor of 1,100 - from ~5,000K (visible light) to 2.7K (microwaves).
But, frankly, that's peanuts.
Between the 2nd and 20th minutes of the universe, hydrogen was fused to helium. This phase of the evolution of the universe is under appreciated.
Start with ~10^80 protons.
Over a period of about 20 mins, ~10^79 helium atoms were formed by fusion. Strewth.
Don't forget that those fusions produce neutrinos, and they don't have a transparency problem. Can't cage those beasts.
The "Cosmic Neutrino Background" has been redshifted by 10,000,000,000 times since then.
Neutrinos have an absolutely tiny mass. So small that the neutrinos that came from SN1987A arrived at the same time as the photons after racing each other for 100,000 years.
The CNB may be the only neutrinos in the universe which have slowed down so much that they are no longer relativistic. There is no currently conceivable way to detect them, but we know that they are still there.
So, there's your answer - photons could be redshifted by 10^10 (ten million times more than the CMB) and they'd still "exist" as a moving probability wave. If the wave happens to interact with matter, then there will be a collapse of the waveform, and an incredibly low energy photon would be detected.
Much more boring answer...
Photons emitted near a Black Hole's event horizon are redshifted as they ascend. In theory, they could be redshifted by any amount, only depending on how close they were to the event horizon when they started out.
The issue is not whether they still exist, but whether there is any practical way to detect them.
bullett2434 t1_jcoabxl wrote
Except microwaves are way higher frequency than radio waves which we detect all the time. So that’s not the answer
triffid_hunter t1_jcobkbc wrote
> Except microwaves are way higher frequency than radio waves which we detect all the time.
Uhh microwave ovens, WiFi, and bluetooth all use the same radio band around 2.4GHz, and the definition of "microwave" is frequencies between 300MHz and 300GHz…
Orthoggy t1_jcodg5r wrote
I’m confused with what you’re trying to prove. All those examples (2.4GHz) is within the range 300MHz - 300GHz. So they are microwave frequencies, right?
triffid_hunter t1_jcodlzz wrote
> they are microwave frequencies, right?
Yep - I'm confused about /u/bullett2434 saying microwave frequencies are higher than radio frequencies, when the dramatic majority of our radios are using microwaves these days…
Orthoggy t1_jcody0u wrote
Ohhh, I see what you mean. Yeah, it looks like microwave frequencies are a subset of radio frequencies. So that point is confusing me too 😅
FMLAdad t1_jcnga7l wrote
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.
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.
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.
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.
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.
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.
[deleted] t1_jco5xhu wrote
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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.
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.
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.
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.
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.
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
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!
RecognitionUnfair500 t1_jcq3mmq wrote
Is there a chance you may have misunderstood what the presenter said?
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?
[deleted] t1_jcntqy6 wrote
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