Can light meaningfully travel an infinite distance?

In summary, the conversation discusses the possibility of light signals "petering out" or ceasing to exist due to various factors such as expansion of the universe, quantum thresholds, and detection technology limitations. The concept of unlimited vs infinite distances is also brought up, along with the Cosmic Microwave Background Radiation and its potential for providing information about the early structure of the universe. The conversation ultimately concludes that there is no universal lower limit for energy and that the sensitivity of detection equipment is a practical engineering matter.
  • #1

SimplePrimate

TL;DR Summary
Since any light signal attenuates over distance, can it become so weak that it becomes undetectable?
In particle terms I'm thinking this could be when the signal's photon density drops low enough to be indistinguishable from the vacuum energy. In wave terms, I'm thinking maybe either the amplitude or frequency might drop below any quantum threshold needed to budge an electron shell in any atom of a detector.

(Arts Graduate here, please be gentle)
 
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  • #2
The frequency drops due to expansion of the Universe, so it can drop below a quantum threshold of a detector. The amplitude drops independently of the Universe expansion, but there is no quantum threshold associated with this.
 
  • #3
First, it's important to distinguish between "unlimited" and "infinite". Take, for example, the Voyager space probe. Theoretically there is no limit to the distance it may travel from Earth, but that distance will never be infinite.

For a light source, the question depends on how you intend eventually to measure the light. If you are far enough from the Sun, then eventually the detection of light becomes photons per square metre per unit time. Does one photon per square metre per million years still count as detecting the Sun? How would you even know that photon came from the Sun in the first place?
 
  • #4
That's what I'm getting at. It's probably extreme empiricism, but I'm wondering if when a light signal can no budge an electron shell then, functionally, it ceases to exist.

If so then this raises some flow on questions for me:
a) Size of the universe vs. the observable universe (Olber's Dark Night Sky Paradox)
b) How optimistic can we be to define a 'Law of Conservation of Information', if signals can just 'peter out'
c) Galactic formation where light pressure is finite.
 
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  • #5
SimplePrimate said:
That's what I'm getting at. It's probably extreme empiricism, but I'm wondering if when a light signal can no budge an electron shell then, functionally, it ceases to exist.
The energy needed to excite a hydrogen atom from the ground state is ##10.2 eV##. There is no sense in which photons below this energy do not exist. A radio wavelength photon has energy of less than a billionth of this. Yet, your radio still works!
 
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  • #6
OK my lack if physics is hampering me here. I understood that within a radio aerial an electron shell had to be excited before any current could flow.
 
  • #7
SimplePrimate said:
OK my lack if physics is hampering me here. I understood that within a radio aerial an electron shell had to be excited before any current could flow.
The point is that there is no lower limit on energy generally. A particular system may have defined quanta of energy, but there is no universal lower limit for energy.

The sensitivity of detection equipment is a practical engineering matter.
 
  • #8
Yeah, thinking on it some more. Even the energy of a signal was undetectably low, merely accelerating a detector towards the signal source would raise the signal's wave frequency again to detectable levels. So yeah right, light shouldn't be expected to peter out conclusively, in any universal fashion.

Another pet theory heads to an early grave.
 
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  • #9
SimplePrimate said:
Yeah, thinking on it some more. Even the energy of a signal was undetectably low, merely accelerating a detector towards the signal source would raise the signal's wave frequency again to detectable levels. So yeah right, light shouldn't be expected to peter out conclusively.
That's a good answer to your own question.

SimplePrimate said:
Another pet theory heads to an early grave.
If you want to move physics forward in 2021, the you are going to have to dedicate your life to it.
 
  • #10
Not expecting to do that. I suspected my notion was in error, otherwise it would rate as an issue in the popular science press at least. Still needed to scratch that itch though (it's a primate thing). Thanks for the help.
 
  • #11
SimplePrimate said:
That's what I'm getting at. It's probably extreme empiricism, but I'm wondering if when a light signal can no budge an electron shell then, functionally, it ceases to exist.
Are you familiar with the Cosmic Microwave Background Radiation? That is light that has traveled for almost the entirety of the age of the universe, approximately 13.7 billion years. There is actually a detail map of that light, which shows it is still within range of our current detection technology. In fact, it is even detectable on an old fashioned TV/antenna. I don't know how low we can go, but it is a lot lower.
 
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  • #12
DrChinese said:
Are you familiar with the Cosmic Microwave Background Radiation? That is light that has traveled for almost the entirety of the age of the universe, approximately 13.7 billion years. There is actually a detail map of that light, which shows it is still within range of our current detection technology. In fact, it is even detectable on an old fashioned TV/antenna. I don't know how low we can go, but it is a lot lower.

My Reply:
Ah, I thought with ultra low-frequency radio telescopes we're close to detecting an 'opaque era' before light could travel freely (a natural problem with my earlier proposal that at great distance light might get stretched beyond and detectability, due to expanding universe's accompanying redshift).

Can we hope to see much more detail of the early structure of the universe?

(you can see I'm having trouble getting the quotes to work)
 
  • #13
SimplePrimate said:
Ah, I thought with ultra low-frequency radio telescopes we are close to detecting an 'opaque era' before light could travel freely (a natural problem with my earlier proposal that at great distance light might get stretched beyond and detectability, due to universe expansion and accompanying redshift).

Can we hope to see much more detail of the early structure of the universe?

(you can see I'm having trouble getting the quotes to work)
The CMBR we will observe in the future is from the same period of the universe's development but from further away. We will never see the previous era because the light from that era was lost in the various interactions that took place when the universe was "opaque".

Here's a recent thread on the subject:

https://www.physicsforums.com/threads/is-the-supply-of-the-observable-cmb-radiation-limited.1005787/
 
  • #14
Hopefully DrChinese is right and we do get to see some more detail, after the opaque era. As a TV station CMBR is fine, it's just that there's not many program changes.
 
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  • #15
DrChinese said:
Are you familiar with the Cosmic Microwave Background Radiation? That is light that has traveled for almost the entirety of the age of the universe, approximately 13.7 billion years. There is actually a detail map of that light, which shows it is still within range of our current detection technology. In fact, it is even detectable on an old fashioned TV/antenna. I don't know how low we can go, but it is a lot lower.
I'd say the CMBR hasn'd traveled at all. Rather it's radiation filling the entire universe. Until about 380000 years after the big bang it's radiation in thermal equilibrium with the charged particles. Then the corresponding photon distribution decoupled from the matter, because the latter formed neutral atoms at this point in the cosmological evolution. Now free photons have no scale. So all that happens then is that the thermal radiation undergoes the cosmological Hubble-Lemaitre red shift as all unbound systems and that's why today it still is an utmost perfect Planck distribution of thermal filling the entire universe utmost homogeneously and isotropic (when observed in the local rest frame of this radiation, i.e., by "comoving fundamental observers").
 
  • #16
vanhees71 said:
I'd say the CMBR hasn't traveled at all.

I would say that is a "unique" description about CMBR photons. Perhaps a reference on why light detected from that era has not been traveling at c through space for the last 13.77 billion years?
 
  • #17
vanhees71 said:
I'd say the CMBR hasn'd traveled at all.
"The CMBR", if it refers to the entirety of the radiation from the surface of last scattering that fills the universe, could be said to not be "traveling", I suppose, since it's a radiation gas filling the entire universe. But the particular CMBR radiation that we are observing on Earth now certainly has traveled through the universe. Light does not stand still. The CMBR radiation we observed yesterday traveled a slightly shorter distance than the CMBR radiation we observe now, and the CMBR radiation we will observe tomorrow will have traveled a slightly longer distance than the CMBR radiation we observe now.
 
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  • #18
PeterDonis said:
"The CMBR", if it refers to the entirety of the radiation from the surface of last scattering that fills the universe, could be said to not be "traveling", I suppose, since it's a radiation gas filling the entire universe.
We were discussing the very long time range movement of light, so referencing the CMBR in toto is not within that context.

But even if we were... That's a stretch - and I'm sure you can see why. Virtually all light we detect from the CMBR has never been in causal contact - it never existed within the same light cone (and only recently came to us). So trying to define a proper reference frame for that is going to be hard (or meaningless). How could you ever even call that stationary? And besides, we currently move with respect to the WMAP field as a whole at perhaps .001c in a direction I'll call north. And that's not stationary at all. And who can say if that is stationary, or if we are stationary? That's not so simple a question to answer, even considering the usual relativistic guidelines?

Further: No one has the slightest idea which of us, if either, are moving relative some larger, absolute reference frame, nor at what velocity (if in fact such a frame even exists). What we do know is that every CMBR photon we detect today has traveled a very long distance. The volume of space where the photon originated has long since become unreachable to us by any means.

-----------------------------------------

Now here's a question to consider: The Hanbury Brown Twiss effect calls for bunching* of photons from far away places/stars. Would that quantum effect manifest itself even across such fantastic distances, with CMBR radiation?

*Edited to correct my labeling as "anti-bunching".
 
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  • #19
DrChinese said:
Now here's a question to consider: The Hanbury Brown Twiss effect calls for anti-bunching of photons from far away places/stars. Would that quantum effect manifest itself even across such fantastic distances, with CMBR radiation?

Not exactly. In fact, light from stars is not expected to show antibunching, but it should show bunching instead. See, e.g. here: https://arxiv.org/abs/1403.7432 or here: https://academic.oup.com/mnras/article/472/4/4126/4344853

If I remember correctly, at least some current CMB detectors are indeed already operating at the limit where photon number noise is the dominant noise source and one indeed finds that it is not sufficient to consider only Poissonian shot noise in determining the noise equivalent power. One indeed finds additional contributions to the magnitude of the photon number noise that arise due to photon bunching, especially at low frequencies. However, CMB physics is not really a topic I know much about and my information might be outdated.
 
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  • #20
Cthugha said:
Not exactly. In fact, light from stars is not expected to show antibunching, but it should show bunching instead. See, e.g. here: https://arxiv.org/abs/1403.7432 or here: https://academic.oup.com/mnras/article/472/4/4126/4344853

If I remember correctly, at least some current CMB detectors are indeed already operating at the limit where photon number noise is the dominant noise source and one indeed finds that it is not sufficient to consider only Poissonian shot noise in determining the noise equivalent power. One indeed finds additional contributions to the magnitude of the photon number noise that arise due to photon bunching, especially at low frequencies. However, CMB physics is not really a topic I know much about and my information might be outdated.

Great stuff, thanks for the references! I think HBT is a very cool quantum effect...
 
  • #22
DrChinese said:
I think HBT is a very cool quantum effect...
Indeed, for a long time I was quite happy with a convenient instrumentalistic interpretation of QM. But after reading about HBT in Feynman's QED, I had to realize that it (=my version) was not good enough. But this doesn't prevent me from continuing to rely on that convenient instrumentalistic interpretation. I just have added a note in my head that it is not the whole truth.
 
  • #23
In which sense does the HBT effect contradict a intrumentalistic interpretation?
 
  • #24
vanhees71 said:
In which sense does the HBT effect contradict a intrumentalistic interpretation?
It only contradicted my version. I am not a big fan of the wavefunction of everything. My convenient instrumentalistic interpretation prefers the density matrix for the stuff near my current position in space-time that I care about, and then uses some more or less optimal coherent decompostion of that density matrix to get wavefunctions as input for the actual computations. But HBT shows that just caring about my immediate neighborhood can sometimes be not enough.
 
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  • #25
I found Alain Aspect's presentation on the HBT effect very interesting and easy to understand. He provides many good visuals to help explain this effect:
 
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  • #26
vanhees71 said:
For a very readable introduction to HBT (and it's indeed "bunching" for bosons)...
I flipped that around badly... :smile: Thanks.
 
  • #27
kurt101 said:
I found Alain Aspect's presentation on the HBT effect very interesting and easy to understand. He provides many good visuals to help explain this effect:
Thanks for posting! It looks very interesting, so I will watch the entire video now.
 
  • #28
kurt101 said:
I found Alain Aspect's presentation on the HBT effect very interesting and easy to understand. He provides many good visuals to help explain this effect:
I watched the entire video yesterday and I really liked it. There was a wealth of food for thought in it for me.

I was particularly fascinated by the Hong–Ou–Mandel effect, which I did not know about.
(http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.59.2044)

Alain Aspect describes it here at 1:02:47:
It was also fascinating that the Hong–Ou–Mandel effect has been demonstrated with two atoms emitting photons, described here at 1:10:13.

Thanks again! :smile:
 
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  • #29
How about a triple negative? Just because something is not detected doesn't mean it isn't there...
 

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