Comments
jmarkmorris t1_jcnbw8p wrote
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
​
>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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ZealousidealClub4119 t1_jcngki0 wrote
Cutecumber_Roll t1_jcnfrdo wrote
From the frame of reference of the photon, time moves infinitely quickly, so it arrives at its destination immediately after departing its origin. If it were to travel forever without ever being absorbed, it would still experience that infinite journey in a single moment of time.
ZealousidealClub4119 t1_jcnhbmz wrote
This is one of the things that periodically blows my mind: look up, and that photon from that galaxy has traveled for millions of years and for a jiffy, and just here its wavefunction has collapsed on one cell in my retina.
Varsect t1_jco1nwn wrote
Its not really accurate to say that infinite time has passed infinitely quickly. The best way to put is that distance for a photon doesn't exist.
Zealousideal_Loss898 t1_jcocnz8 wrote
This 👆 The photon has left and arrives at the same time from its frame of reference and time is irrelevant. So your question has no meaningful answer.
[deleted] t1_jcnrdzm wrote
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DudeWithAnAxeToGrind t1_jcnjzmm wrote
Let assume that photon is emitted from Earth, just so we can have convenient frame of reference. The same would be true if it was emitted from anywhere else.
Assuming nothing ever absorbs it, the photon would just keep going forever. However, even if the Universe itself is not infinite, and has an edge, it will never get to that edge. Not even close. I.e. as far as your question goes, the difference between infinite and finite Universe is irrelevant. The final fate of that photon and how far it can get is the same.
As it travels, its frequency will get lower and lower because the space it is traveling through is expanding; it will redshift more and more until its energy becomes so low to be undetectable.
It will only be able to reach a region of space that is finite distance from where it started its journey. This furthest point in space that it can reach is within our currently observable Universe. However, by the time that photon reaches it (after it was traveling for infinite amount of time), that point in space will be far outside of our future observable Universe. Again because space is expanding, and the space beyond observable Universe is moving away from us much faster than the speed of light, it can never get to those regions of space (it actually can't even reach the edge of currently observable Universe, because that region of space is also already moving away from us faster than the speed of light).
This also means that there is a sphere around the Earth from which the photons emitted right now towards us, Earth's current location in space is the furthest they will ever be able to get to. By the time they get here after travelling for trillions of years, they'll be so much red shifted as to be undetectable.
House13Games t1_jco64y1 wrote
But where do they go?
DudeWithAnAxeToGrind t1_jcqwb8m wrote
Are you trolling me here? Assuming you are not, the above answers exactly where they'll go. It'll simply take them forever to get there, because space they have to travel through is itself expanding.
To make an analogy, imagine an ant on the surface of a balloon trying to get from the bottom to the top of the balloon. Imagine you can just keep inflating this balloon indefinitely, making it bigger and bigger. If you are inflating balloon fast enough so that it increases in size faster than ant can move over its surface, the ant will make a progress in its journey, but it will never be able to reach the top of the balloon. If ant can keep going infinitely, there's a spot on the balloon it will eventually reach after infinite amount of time has passed.
This is what happens to a photon traveling through Universe. The universe is expanding, and the far regions of space are receding from this photon faster than the photon can travel through space.
ReadditMan t1_jcnft7u wrote
🎶 Where do photons go when they die? They don't go to heaven where the angels fly. 🎶
^(I'm sorry)
Snushine t1_jcnhmct wrote
They go on forever way up in the sky, and you'll see even more on the fourth of July.
Anonymous-USA t1_jcnhzpq wrote
First off, yes to the part about photons (as waves) traveling indefinitely until absorbed. However, no to the second part — a photon will never reach the “edge of space” for two reasons. First, space itself expands faster than light speed. And second, since space expanded everywhere at once and is isotropic, any random photon has (today) 46B light years of its own observable event horizon in all directions, and there’s plenty of matter filling that space just as within our own observable universe. Whether our own galaxy is within that photon’s event horizon or not.
triffid_hunter t1_jcnj4f4 wrote
> Where do photons go if they've been emitted but are destined to never be absorbed
They just keep going forever.
We're still receiving photons from the first moment that the universe was transparent, they're called the CMB
> would these photons traveling ad infinitum define the edge of the universe
There's no edge.
Varsect t1_jco20n0 wrote
Wrong (kinda) Well, yes, there is no edge but there is an edge.
p-d-ball t1_jcnu84f wrote
I believe it'll just continue to lose energy as the universe continues to expand. In the same way that the CMB has lost energy.
Pegajace t1_jcnwgbu wrote
>would these photons traveling ad infinitum define the edge of the universe
They would if the Big Bang had been an explosion at a specific point in space, but it wasn’t. There isn’t an expanding sphere of photons defining the outer edge of the universe because the universe did not start at a central point. The Big Bang was a rapid growth of spacetime that happened to space, not in space, and it happened everywhere simultaneously.
FullOfStarships t1_jcoc9ps wrote
This answer is simultaneously completely correct, and completely wrong.
TL;DR the question you are looking for is "edge of the Visible Universe", and the answer is "yes, that is the dictionary definition of Visible Universe".
The Cosmic Microwave Background was created 370,000 years after the big bang. The photons that reach us have travelled for the lifetime of the universe, minus 370,000 years. These have a redshift of 1,100.
The Cosmic Neutrino Background was created about 10 minutes after the big bang. The neutrinos that reach us have travelled for the lifetime of the universe, minus 10 minutes. These have a redshift of 10^10.
You can only go another ten minutes further back in distance / history. That's it. No more.
More to the point, if you could get back to the exact "zero" point, the radiation would be infinitely redshifted.
In fact, that point is recognised as an Event Horizon. Apparently it actually emits Unruh radiation. It perfectly describes the edge of the "Visible Universe". This is "our universe".
There are good theoretical reasons to believe that the big bang created space billions, trillions, quadrillions, etc... times bigger than the Visible Universe, which has the same physical laws as us. But, for all we know, the universe one micron "further away" than the event horizon could be dragons packed nose to tail. We would have no way to know.
Ironically, if there is an intelligence which exists 99% of the distance to the edge of our visible universe, they would see a sphere the same size as our visible universe, but centred on them.
The visible universe is centred on the observer. To an utterly irrelevant extent, people on the opposite sides of the Earth perceive slightly different edges to their visible universes.
Varsect t1_jco1ty6 wrote
Well, I mean, they spend their very peaceful life zipping around at 299,752,458 m/s till it hits any random atomic nucleus. The thing with ''destined to never be absorbed'' is just the fact that the photon doesn't experience distance. You not experiencing distance is you not really experiencing time.
space-ModTeam t1_jcoc79m wrote
Hello u/mysteryofthefieryeye, your submission "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)?" has been removed from r/space because:
- Such questions should be asked in the "All space questions" thread stickied at the top of the sub.
Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.
Civil_Willingness298 t1_jcnoq2r wrote
Photons do not experience space or time. It reaches its destination 13 billion light years away or two inches away at the same exact moment it departs. Let that cook your noodle for a minute or two.
mysteryofthefieryeye OP t1_jcnpe2o wrote
I'm sorry, that's incorrect, and no noodle has been cooked. From the point of view of the photon, sure, you are right. But the physical packet of information still travels at the speed of light, and even then that speed can be altered by an intervening atmosphere or interstellar medium. By your logic, the starlight I see outside is both here and just leaving the star simultaneously, which is 100% inaccurate.
Civil_Willingness298 t1_jcnpz88 wrote
No, it is scientific fact. https://phys.org/news/2014-05-does-light-experience-time.amp
mysteryofthefieryeye OP t1_jcnucsf wrote
I know light doesn't experience time. From the photon's vantage, you are right. But in physical space, it still takes time for the photon to reach a destination. Downvote me all you want but your original comment is plainly wrong.
Civil_Willingness298 t1_jcnq7p7 wrote
It is the extreme end of time dilation. Well accepted physics for a century.
decrementsf t1_jcncnlk wrote
Space is round. It eventually travels around and comes back from the other side.
Anonymous-USA t1_jcni8or wrote
Assuming that’s true, a photon could never circumnavigate the universe because the event horizon for that random photon is expanding faster than light speed. It can’t ever catch up. Any random photon anywhere in the universe at this moment has a 46B light year event horizon in all directions. And the universe itself (regardless of its geometry) is larger than the event horizon anyway.
[deleted] t1_jcnd8zi wrote
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extra_specticles t1_jcnas87 wrote
A photon only has particle like behaviour when interacting with matter, otherwise it behaves like waves and spreads out and is essentially excitations in the electromagnetic field which permeates all spacetime. If there is no interaction, then it's just travelling energy in the field. In effect all travelling photons are in superposition in the electromagnetic field.