B Was the Big Bang a quantum mechanical vacuum fluctuation?

  • Thread starter mark!
  • Start date

DaveC426913

Gold Member
18,312
1,910
Photon's energy is decreasing. Where does it go?
It gets attenuated.

Think of the universe as analogous to an ideal gas in a closed vessel. The temperature will change in inverse proportion to the volume change.
i.e. increase the volume, and the temperature drops, though no energy has left the system.
 
I’ve read the article. To summarise it, “Energy isn’t conserved; it changes because spacetime does”.

But the energy itself is still there. No boson or fermion has disappeared. So therefore, from that point of view energy ("stuff" on the Standard Model) “has been conserved”.
Wrong - energy and number of particles are completely different concepts.

Also, number of particles is not a conserved quantity at all. Say, one gamma-ray photon can be converted to millions of visible and infrared photons. Stars do this all the time.
 
106
8
one gamma-ray photon can be converted to millions of visible and infrared photons. Stars do this all the time.
Are you referring to this:
When a photon is scattering with matter or antimatter, part of energy from the photon is given to matter/antimatter, and a new photon with smaller energy is created. A particle which received energy from photon is accelerated. It can be repeated over and over again, and from an initial 511 KeV photon after millions of such interactions, you will have millions of photons with very low energies (e.g. visible spectrum, then infrared).
Energy has changed, yes, but it wasn't created or destroyed. You can divide '1' into 1/2 and 1/2, or into 1/4, 1/4, 1/4 and 1/4, but that doesn't mean that energy has been added. It still has been conserved.
 
Last edited:
784
361
phinds said:
The recent discussion has been about conservation of energy. Nikkkom's comment was pointing out that energy density is decreased because photons lose energy and it doesn't go anywhere, it just evaporates.

The best way to "get" the argument against your line of thinking is to learn the actual physics. Then your misunderstanding will evaporate like the energy of EM radiation as it travels over cosmological distances.
Saying that it "evaporates" is not the same thing as saying it "is not conserved". Vaporization is readily recognizable as a disaggregation and not as a non-conservation of the vaporized matter.

Bare remarks to the effect that on a cosmic scale energy isn't conserved are jarring to the sensibilities of many. The expression "is not conserved" is insufficient to account for what happens to the 'not conserved as such' energy. It dis-integrates and dissipates, and although not 'conserved as such', it is nonetheless 'conserved not as such', as a difference in a quantity of a different suchness that is proportional to the difference in the 'not entirely conserved as such' energy.

It goes not from somethingness to nothingness; it goes from a thisness to a thatness, just as everything else in flux does.
 
Saying that it "evaporates" is not the same thing as saying it "is not conserved".
In this context, phinds did mean that energy disappears. All CMB photons redshift everywhere.

Bare remarks to the effect that on a cosmic scale energy isn't conserved are jarring to the sensibilities of many.
Facts don't care about your sensibilities.

The expression "is not conserved" is insufficient to account for what happens to the 'not conserved as such' energy. It dis-integrates and dissipates, and although not 'conserved as such', it is nonetheless 'conserved not as such'
If it is "conserved not as such", whatever that means, where, in your opinion, energy of redshifting photons goes?
 
784
361
In this context, phinds did mean that energy disappears. All CMB photons redshift everywhere.
He said evaporates. I replied to what he said. Just as water seems to disappear from a dish left in open air, but is in fact somewhere else rather than simply gone, the energy diminution we encounter in our observations of photons should not be presumed to be un-accounted-for in the grand ledger of the universe.
Facts don't care about your sensibilities.
I'm fairly confident that for the most part you're right about that. :wink:
If it is "conserved not as such", whatever that means, where, in your opinion, energy of redshifting photons goes?
I suspect that it goes somewhere other than nowhere. We can observe that it doesn't appear to us to continue to inhere in or exhere from the photons we measure. We cannot thereby infer that it is leaked out of the entire universe.
 
38
11
Perhaps its useful to think about this photon energy question in a simpler setting. Consider photons emitted from someone speeding away from you. They arrive at a different frequency, which can be calculated based on relativistic doppler formulas.

However, the intensity of the light is also influenced. I think since E gets multiplied by gamma for a lorentz boost, the intensity should be reduced by gamma squared?

Can anyone fill in the dots? I'm interested to see whether photon number is actually conserved- if someone speeding away emits 10^10 photons at a certain wavelength over a finite period of time, does the stationary observer really see that same number? I don't think so. Photon numbers depend on choice of basis for hilbert spaces which can be done differently. This is part of the story behind spontaneous particle creation in Hawking radiation for example.

Related would be to do this for gravitational redshift.
 
784
361
Perhaps its useful to think about this photon energy question in a simpler setting. Consider photons emitted from someone speeding away from you. They arrive at a different frequency, which can be calculated based on relativistic doppler formulas.

However, the intensity of the light is also influenced. I think since E gets multiplied by gamma for a lorentz boost, the intensity should be reduced by gamma squared?

Can anyone fill in the dots? I'm interested to see whether photon number is actually conserved- if someone speeding away emits 10^10 photons at a certain wavelength over a finite period of time, does the stationary observer really see that same number? I don't think so. Photon numbers depend on choice of basis for hilbert spaces which can be done differently. This is part of the story behind spontaneous particle creation in Hawking radiation for example.

Related would be to do this for gravitational redshift.
That meander does not serve to clarify. It's clear that there's an energy loss that is not fully accounted for merely by the accepted paradigms. Although I've stated that I think that the energy must go somewhere rather than nowhere, I confess that I think that I'm as much at a loss to explain exactly where, as are the proponents of nowhere to explain exactly how.
 

phinds

Science Advisor
Insights Author
Gold Member
15,491
5,119
That meander does not serve to clarify. It's clear that there's an energy loss that is not fully accounted for merely by the accepted paradigms. Although I've stated that I think that the energy must go somewhere rather than nowhere, I confess that I think that I'm as much at a loss to explain exactly where, as are the proponents of nowhere to explain exactly how.
But there is no NEED to explain "how" since that question only arises if you insist on applying a local principle (conservation of energy) in a domain (large scale cosmological distances) where it does not apply.
 
784
361
But there is no NEED to explain "how" since that question only arises if you insist on applying a local principle (conservation of energy) in a domain (large scale cosmological distances) where it does not apply.
If you insist that energy exits the universe, when all you can show is that it became no longer to you findable, while I insist that it must be somewhere, when all I can show is that historically, energy once reported as missing usually turns up somewhere else, I think it's at least as incumbent upon you to account for how energy leaves the universe, as it is on me to account for where it went.
 
If you insist that energy exits the universe, when all you can show is that it became no longer to you findable, while I insist that it must be somewhere, when all I can show is that historically, energy once reported as missing usually turns up somewhere else
It's not a "he said, she said" competition. Energy conservation is a theorem. For time-translation invariant spacetime, energy must be conserved.

Since our local conditions, to a very high precision, are time-translation invariant, we should, and we are, observing that energy is conserved in all processes.

At the same time, on a cosmological scale, we observe that Universe is not time-translation invariant. Therefore, energy conservation on that scale is not required by any theorem.
 

fresh_42

Mentor
Insights Author
2018 Award
11,743
8,274
I cut off the discussion about energy conservation at this point, because this post summarizes it all:
It's not a "he said, she said" competition. Energy conservation is a theorem. For time-translation invariant spacetime, energy must be conserved.

Since our local conditions, to a very high precision, are time-translation invariant, we should, and we are, observing that energy is conserved in all processes.

At the same time, on a cosmological scale, we observe that Universe is not time-translation invariant. Therefore, energy conservation on that scale is not required by any theorem.
Everything else I've read so far is an argumentation about phrasing it. It is furthermore off topic, resp. will become off topic if we go on with it. So please return to the question, whether the big bang can be considered a quantum fluctuation, on which scale ever, i.e. possibly a real giant one. Otherwise we will be forced to close the thread, as personal debates about wordings won't help anybody to understand what has been going on before inflation.

Thank you.
 
63
8
To expand on that a bit, conservation of energy is universally true LOCALLY. That is, everywhere in the universe energy is conserved on small scales (~ within glactic clusters) but over cosmological distances it is not.
That is true in General Relativity which is not the full picture .. maybe energy will be conserved in a quantum theory of gravity.
 

phinds

Science Advisor
Insights Author
Gold Member
15,491
5,119
That is true in General Relativity which is not the full picture .. maybe energy will be conserved in a quantum theory of gravity.
Nothing about a quantum theory of gravity would change the fact that photons lose energy as they travel in an expanding universe, so no, it would not.
 
63
8
Nothing about a quantum theory of gravity would change the fact that photons lose energy as they travel in an expanding universe, so no, it would not.
It might explain where that energy goes.
 

phinds

Science Advisor
Insights Author
Gold Member
15,491
5,119
63
8
That was just one example. Another is that you can't even DEFINE kinetic energy between two objects separated by cosmological distances.
Yes I know that.

My point is that our current theories are not complete.. what appears to us as violation to the law of conservation of energy, and what makes it appear to only work "localy", could be our lack of knowledge on the complete picture.

Thats why we need a more complete theory such as a quatum gravity theory.
 

haushofer

Science Advisor
Insights Author
2,234
565
Yes I know that.

My point is that our current theories are not complete.. what appears to us as violation to the law of conservation of energy, and what makes it appear to only work "localy", could be our lack of knowledge on the complete picture.

Thats why we need a more complete theory such as a quatum gravity theory.
I think that these "pseudoproblems" are because we try to impose Newtonian concepts (e.g. energy conservation in fixed backgrounds) into general relativistic frameworks. It has nothing to do with a "lack of the complete picture". It's just a matter of oversymplifying, based on Newtonian intuition.
 
63
8
I think that these "pseudoproblems" are because we try to impose Newtonian concepts (e.g. energy conservation in fixed backgrounds) into general relativistic frameworks. It has nothing to do with a "lack of the complete picture". It's just a matter of oversymplifying, based on Newtonian intuition.
Conservation of energy is not Newtonian concept!! It is a law of physics.
In my view it is as fundamental as 2nd law of thermodynamics.
 

PeroK

Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
10,372
3,945
Conservation of energy is not Newtonian concept!! It is a law of physics.
In my view it is as fundamental as 2nd law of thermodynamics.
Why do you think it's a law of physics?
 

fresh_42

Mentor
Insights Author
2018 Award
11,743
8,274
Conservation of energy is not Newtonian concept!! It is a law of physics.
In my view it is as fundamental as 2nd law of thermodynamics.
I always thought it is Newtonian and the consequence of Noether's local theorem about invariant Lagrangians. I cannot see a system of differential equations for some particles be extended on the entire universe. That would result in serious problems with the atlas.
 
Conservation of energy is not Newtonian concept!! It is a law of physics. In my view it is as fundamental as 2nd law of thermodynamics.
"Laws of physics" are proposed by people, then tested against experiments. If experiments match, then the corresponding law might be true; if experiments contradict them, they are out. No law is secure against refuting by future developments.

In this case, conservation of energy fell victim to the discovery of expansion of the Universe, especially accelerating one.

Mathematically speaking, conservation of energy is consequence of time translation invariance ("the same experiment tomorrow has the same result as today"). Our local part of Universe on the scale of, say, our Galaxy, is very nearly time translation invariant. That causes conservation of energy to be not violated in our experiments.

As it turns out, most global solutions of GR are not globally time translation invariant. Therefore, globally, no mathematical logic exists for conservation of energy.
 

Related Threads for: Was the Big Bang a quantum mechanical vacuum fluctuation?

Replies
4
Views
2K
Replies
6
Views
3K
  • Last Post
Replies
12
Views
2K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
23
Views
6K
  • Last Post
Replies
12
Views
3K

Hot Threads

Top