Recent comments in /f/space

RecognitionUnfair500 t1_jcpvyqg 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

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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triffid_hunter t1_jcodlzz wrote

Reply to comment by Orthoggy 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

> 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…

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Zealousideal_Loss898 t1_jcocnz8 wrote

Reply to comment by Cutecumber_Roll 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

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.

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FullOfStarships t1_jcoc9ps wrote

Reply to comment by Pegajace 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

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.

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space-ModTeam t1_jcoc79m wrote

Reply to 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

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.

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triffid_hunter t1_jcobkbc wrote

Reply to comment by bullett2434 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

> 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…

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BrotherBrutha t1_jcoa5jh wrote

Reply to comment by RecognitionUnfair500 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

>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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FullOfStarships t1_jco9rvz wrote

Reply to comment by OffusMax 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

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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FullOfStarships t1_jco9jx0 wrote

Reply to comment by triffid_hunter 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

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.

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RecognitionUnfair500 t1_jco5xbf 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

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_jco3s0w wrote

Reply to comment by RecognitionUnfair500 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

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.

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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.

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Varsect t1_jco1ty6 wrote

Reply to 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

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.

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