Is Photon Energy Lost or Conserved in Cosmological Redshift?

[(Q)]

I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

 

[drag]

Originally posted by (Q)
I would like to hear some interpretations
of this phenomenon, especially in regards
to what happens to the energy of photons
in cosmological redshift – where do YOU
think it goes? Is it lost or is it conserved?

Space-time expands and the EM waves do too.
I'm not certain about how the energy is
treated, I suppose you could say nothing
dissappears and it's just like normal
space-time curves or there are those theories
dealing with quentessense or something like
that and say that it "fuels" the expansion.

Live long and prosper.

 

[marcus]

Originally posted by (Q)
I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

this question is one reason why physicforums is the greatest thing since sliced bread. this is a great question

i want to hear what some gurus say about this (tom, damgo, etc)


note that your questions assumes that it goes somewhere
which means you are assuming a conservation law
but there is no energy conservation law in GR, I think.
or any momentum conservation law either.

you are assuming that if energy is lost (from the light being stretched out and getting a longer wavelength) that it must
GO somewhere----because in local SR physics energy is conserved.

but expansion of space is a GR thing----so you must ask if
there is a conservation law in that context. Maybe there is!
I am no expert. But I do not know of one, for whatever that is worth.

I heard a neat thing about GR. An amoeba can move in curved space merely by changing shape.

this seems to contradict conservation of momentum

he does not need any rockets to push him. he just changes is shape a couple of times and behold! he is in a different place.

this was at an MIT archive of recent online physics papers or something, they thought it was cool. It is good peer-reviewed physics even tho it sounds wacky.

so if anybody thinks he knows of a conservation law for momentum in GR he will have to explain this traveling amoeba

(Q) you are great for asking this question. maybe it will get a good answer from the likes of damgo

love it!

 

[Labguy]

Originally posted by (Q)
I would like to hear some interpretations of this phenomenon, especially in regards to what happens to the energy of photons in cosmological redshift – where do YOU think it goes? Is it lost or is it conserved?

I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

Remember, the Alien looking from the "other side" sees the same light source blue-shifted, same light source = same photon energy. The photon energy and the perceived wavelength are two different animals.

 

[drag]

Originally posted by marcus
I heard a neat thing about GR. An amoeba
can move in curved space merely by changing shape.

WHAT ?! LINK, PLEASE !

 

[FZ+]

Oh that... well... it's not entirely accurate. It's a study of how amoebas move in water that gave a theoretical way of cheating Newton's 3rd law - if space is curved, you can exploit the gravitational gradient by generating motion through moving the craft's centre of gravity. It's sort of akin to tacking a yacht in a headwind...

 

[LURCH]

Originally posted by Labguy
Remember, the Alien looking from the "other side" sees the same light source blue-shifted, same light source = same photon energy.

Not so. An alien on the other side would see the light red-shifted, just as we do. If he is the same distance away, he'll see the red-shift about the same amount, too.

 

[Labguy]

Originally posted by LURCH
Not so. An alien on the other side would see the light red-shifted, just as we do. If he is the same distance away, he'll see the red-shift about the same amount, too.

Whoa? Let's say we are here and the alien is way over there, and moving about equally relative to us. Place a fast-moving light source smack in the middle between us and the alien. The light source moves quickly away from us and toward the alien. We see a "redshift" (wavelength change) because its moving away from us. The alien sees a blueshift because it is moving toward him.

Why is this "not so"?

I am talking redshift from motion, not gravitational redshift.

 

[marcus]

Originally posted by Labguy


I am talking redshift from motion, not gravitational redshift.

Then you are talking SR and there is a conservation law.

Say a receding spaceship is flashing lights at us. In the spaceship frame the light is not redshifted and retains its energy.
In the earthbased frame looking at the spaceship flashing lights at us, the light was always of a lower energy lower frequency longer wavelength, so it retains its energy.

Everything is normal.

But this guy (Q) was talking about the COSMOLOGICAL redshift which happens from space stretching out while the light is under way towards us. this is a GR thing. It is not relative motion of source and receiver, but an effect of expansion of space itself.

He is saying 'what happened to all that energy?' I think IMHO that it just went away. What a huge amount of energy to have just ceased to exist! All the light from all the galaxies, for the whole history of the universe, most of which is still flying on its way somewhere, has been deprived of a large percentage of its energy!

Hey drag! I do not have the URL. There is an MIT archive of online physics goodies and I found it there. Maybe I will come across it again. Of course it was not a real amoeba, I just called it that. The journal article was about an object that can change its shape and, by using locally curved spacetime, can move around by changing shape thru a cycle that leaves it the same original shape but in a different place. If I see it again I'll post it.

 

[drag]

Originally posted by marcus
Hey drag! I do not have the URL. There is an
MIT archive of online physics goodies and I
found it there. Maybe I will come across it
again. Of course it was not a real amoeba,
I just called it that. The journal article
was about an object that can change its
shape and, by using locally curved spacetime,
can move around by changing shape thru a
cycle that leaves it the same original
shape but in a different place. If I see
it again I'll post it.

Well it seems like an EXTREMELY "slow and
painfull" process, unless you can do
some near c movements of course but that's
a bit too much for an amoeba I guess.

 

[FZ+]

Oh it is extremely slow... but there is speculation on using nanotechnology to do this on a large scale...

 

[Labguy]

Marcus said:

But this guy (Q) was talking about the COSMOLOGICAL redshift which happens from space stretching out while the light is under way towards us. this is a GR thing. It is not relative motion of source and receiver, but an effect of expansion of space itself.

And also some things about all of the "missing energy".

Ok, I'll buy that in the context of "cosmological" redshift. But, even GR, Hawking, Wheeler, et al won't throw out thermodynamics; they require it. So, the energy takes an equivalent transformation to "something". Your (Marcus') description of the huge amount of energy available from this effect since expansion began might be "the" source of the "repulsive force" ("dark energy"). Just thinking out loud. This might seem to fit in with what Linde recently proposed, which was posted a day or two ago.(Expansion then contraction)

 

[LURCH]

Originally posted by Labguy
Whoa? Let's say we are here and the alien is way over there, and moving about equally relative to us.

But that's just it; he won't be (moving equally, that is). If things at great distances from one another were stationary relative to one another, there would be no cosmological redshift.* If we look at a light source one billion lightyears away, and an alien is also one billion lightyears away from that same light source (but on the "far side), he will see about the same redshift as we. If the alien were to look at us, and we at him, we would each measure the other's redshift to be about twice what it is for the intermediate object (the light source).


*At least, not in theory.

 

[russ_watters]

Originally posted by Zefram
But doesn't the relation between wavelength and energy for a photon imply that when a photon is Doppler shifted (from our point of view) that the amount of energy we measure for it has changed? So the photons reaching the alien actually have more momentum (if absorbed) than photons absorbed over here.

Is an "apparent" change in the energy of a photon different than an actual one (what is an "actual" one, anyway)?

Therein lies the problem - the photon doesn't change its wavelength. It is created at that wavelength because the source is moving. There is no missing energy.

Doppler shift works the same way for sound as well. With a moving source, the sound is created with a doppler shift.

 

[selfAdjoint]

Take a galaxy that we see receding, and its light red-shifted. People living in that galaxy do not see their light redshifte, it seems normal to them and it doesn't occur to them that any energy has been lost. What they do see red-shifted is our light. And maybe they ask where the energy went in our case.

Neither we nor the aliens are speeding, the distance beteen us is growing and it produces the effect of speed including red-shift. The stars in galaxy X are glowing with their usual spectra.

 

[drag]

selfAdjoint and russ, if you maintain a certain
permanent rest frame in the Universe you will
see the expansion of space-time decrease the
energy of at least photons and possibly
also the kinetic energy of all rest-mass particles
(though the rest masses appear to stay the same).

Live long and prosper.

 

[marcus]

the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

this energy can be calculated---most simply in c=G=hbar=k=1 units.

Almost all the redshift energy loss since year 300,000 after BB has been from CMBR photons which were released around year 300,000 and have experienced redshift 1100.

This loss is overwhelmingly greater than that in light emitted by galaxies.

The temp of CMB is 2E-32
The energy density (energy per unit vol) of CMB is equal
to the fourth power of the temp, multiplied by π²/15.
that factor is about 2/3
So the density comes out close to E-127
and multiplying by 1100 one gets about E-124.
It is a small energy density compared with dark energy
have to go will proofread later

 

[russ_watters]

Originally posted by drag
selfAdjoint and russ, if you maintain a certain
permanent rest frame in the Universe you will
see the expansion of space-time decrease the
energy of at least photons and possibly
also the kinetic energy of all rest-mass particles
(though the rest masses appear to stay the same).

Live long and prosper.

Aether? Unnecessary. Just like sound - but now both the observer and the source are moving. And it doesn't matter what frame of reference you use (Relativity) you still get the exact same doppler shift. The frequency of the light does not change while it is traveling unless acted upon by a gravitational field.

 

[(Q)]

Labguy

I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

A change in the wavelength IS a change in the energy.

 

[(Q)]

Marcus

the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

Has the energy really been lost? Redshifted photons would indicate it has been lost.

But is because it is lost and not conserved or is it because our frame of reference has changed and conservation does not apply?

 

[marcus]

Originally posted by (Q)
Marcus

the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

Has the energy really been lost? Redshifted photons would indicate it has been lost.

But is because it is lost and not conserved or is it because our frame of reference has changed and conservation does not apply?

there is no lorentz transformation to a different frame
that would recover that energy, Q

it is LOST

GR and cosmology are different from SR
and in cosmology there is a rest frame
being at rest with respect to the Hubble flow (as they say)
or at rest with respect to the CMB
so if you want, do all your calculating in that rest frame
and just think about the energy of the CMB
as it started out 13 billion years ago and as it looks now
(much cooler now, by factor of 1100)

there is no way by speeding to the right or left you can make
the whole CMB have more energy

the whole CMB has cooled, not from doppler (which you could hope to compensate for by changing framed) but by the actual
stretching out of the waves by the stretching out of space.

the only way to recover the energy would be to start re-collapsing space again preparatory to a big crunch
and then you would see the CMB get hotter and hotter as
the wavelengths were compressed again

 

[schwarzchildradius]

OF course the photons are redshifted everywhere, so long as the universe is isotropic. Their energy is less, as we observe them at this Time because there is more Space now than when they were generated (from decoupling event). So the energy is indeed conserved, what is not conserved is space.

 

[marcus]

Originally posted by schwarzchildradius
OF course the photons are redshifted everywhere, so long as the universe is isotropic. Their energy is less, as we observe them at this Time because there is more Space now than when they were generated (from decoupling event). So the energy is indeed conserved, what is not conserved is space.

Schwarzschildradius's Law-----energy plus space is conserved


to paraphrase: when energy goes away at the same time that space is created, then it just balances out so that the quantity E+V is conserved where E stands for energy and V for volume


I do not like this as well as an alternative law which is Marcus's
Law-----energy is conserved, just plain energy, if you include the vacuum energy represented by the cosmological constant.

It is highly speculative though plausible in this sense:

I calculated the energy that the CMB photons now found in a cubic kilometer have lost by stretching since they were emitted, and it was within about a factor of ten of the dark energy now believed to occupy the same cubic kilometer

 

[russ_watters]

Originally posted by (Q)
Labguy

I think that Redshift is a change in the wavelength of the light reaching us, not a change in the energy of any particular photon(s) at all.

A change in the wavelength IS a change in the energy.

Here's one for you then: since light has no medium that it travels one, can it even HAVE a wavelength except at the instants of emission and absorption?

But is because it is lost and not conserved or is it because our frame of reference has changed and conservation does not apply?

Ding, ding - this is what I am getting at. The energy of a photon depends on your frame of reference. This is the same as the energy of ANYTHING. In order to know what the energy is, you have to define the frame of reference and different frames of reference will result in different (and equally valid) calculations of the energy.

Schwarzschildradius's Law-----energy plus space is conserved

No. Schwarz's law is a restatement of the density equation: d= e/v

As volume decreases and energy remains constant, density decreases. From that you derive the equation for light intensity vs distance from source.

 

[marcus]

Originally posted by (Q)
Marcus

the original post by (Q) called attention to the energy that has been lost from the universe by wavelength stretching caused by expansion of space.

Has the energy really been lost? Redshifted photons would indicate it has been lost...

conservation of energy has not been proven in GR
and so we can't assume that energy is conserved

at baez' website at the Usenet Physics FAQ this is
discussed and one of the possibilities is that the
energy is simply lost

since there is no conservation law that is one possibility
but also, it could be that someone will invent some new
mathematics, and a new theory will be tested and supplant
GR and in this new theory there will BE energy conservation
and people will find some place they hadnt thought of, like
dark energy, where the energy went and they will say
"Ahah!, there was conservation all along, it just went in to
the xyz blahblah."

but for us now, by occam's razor we kind of have to say simply
it is lost---because that is the simplest thing to suppose
and it does not violate any law to suppose it.

I assume you see the flaw in what RW says about d=E/volume.
It is Astro 1 fact that the universe was once light-dominated and now is matter-dominated because the radiation density falls off
with the fourthpower of stretch while the matter falls off with the
third. The number of photons per volume goes down (third power)
and the energy of each one goes down (linear). So light energy thins out faster than matter energy. Is this clear or do I have to be more articulate (this is coffee time)

 

[drag]

Greetings !

Originally posted by russ_watters
The frequency of the light does not change
while it is traveling unless acted upon by
a gravitational field.

So, the Universe does not expand
for you ?

Originally posted by schwarzchildradius
OF course the photons are redshifted everywhere,
so long as the universe is isotropic. Their
energy is less, as we observe them at this
Time because there is more Space now than
when they were generated (from decoupling
event). So the energy is indeed conserved,
what is not conserved is space.

Originally posted by marcus
Schwarzschildradius's Law-----energy plus space
is conserved

I think that if you could also prove this
energy-space conversion and explain it
Schwarzschildradius, the next Nobel prize
is yours.

Live long and prosper.