And as a photon experiences no time, at what point does this energy transfer(?) take place?

__________________

But from the Photon's point of view, there is no redshift, since there is no time, and therefore no gaps between the "wavelengths". So the question needs a little more work IMO.

__________________
Numbers are not case sensitive. (me)

You can answer the question yourself if, as NEOWatcher
suggests, you think about the question some.

What is red shift? What causes red shift? Or blue shift?
Simply answering those basic questions will probably tell
you where the energy and momentum go.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Excellent question!

If you fire a photon at a receding receiver, the receiver will tell you it arrived with less energy & momentum than you sent it with.

However, from your perspective, everything in receiver's frame-of-reference has more energy/momentum than it would were it at rest with respect to you. So, from your perspective, the red-shifted photon imparted just as much energy/momentum to the moving frame as it left with.

Does that make sense?

__________________
PW -- Plant Whisperer

I hear this question fairly often, but Peter's answer does not address directly the intent of the askers. They are asking about where the energy and momentum go WRT Cosmological Redshift. This is not the mere measurement of the sending vs. receiving frame of reference.

__________________
Forming opinions as we speak

Hum,
the laws of thermodynamics dictate that energy is conserved. If in a universe where space expands, the light has to `spread` it energy over a bigger distance (relative to the observer). In a universe where space does not expand, there is no redshift.

__________________
`Irony` actually does mean `metal like`...

Actually, the photon energy in an expanding universe, like any adiabatic expansion, is not conserved. This is why gas cools when you expand it. For a gas, you make up energy conservation by doing work on the walls of the enclosure, or you convert it into bulk motion like in an explosion. For the universe, there are no walls, and there is no bulk motion, and there is also no need to conserve energy globally. That's right, there is simply no principle of global conservation of energy in general relativity, so if that is what the OP is really about, then that is the answer. Why should that bother us? We're the ones who cooked up that principle anyway, and invented "potential energy" to make it work. As soon as gravity stopped being a force, it stopped having "potential energy", and bye bye conservation of energy. No biggie, we can still use it in local contexts or in Newtonian gravity. But if you really want to have cosmological conservation of energy, you can kind of get away with imagining that the energy lost in the redshifting photons went into the gravitational potential energy of the universe. Now bring in "dark energy", and you have a real headache-- best to just let the principle of energy conservation go. If it doesn't return, it was never really ours.

If you tell her you love her and that you do not want her to go but if she leaves anyway then she was never yours ...

but if you just let her go without expressing your desires then she will think you do not care and she will feel justified in leaving you and therefore never come back - even though she was yours.

The moral of the story is: To let her know how you feel and then let her make the decision so that you know her true desires.

The first law of thermodynamics was called into question when radioactivity was first being studied. Many prominent scientists of the day wondered if at a small scale, micron for instance (Georges Gouy), or “that energy conservation might not hold strictly” the Bohr-Kramers-Slater proposal (ref: A. Pais; Inward Bound;105-108), because an energy source for the radioactivity could not be determined at that time (early 1900’s). I see a parallel with the redshift energy question and our current level of understanding. Stay tuned for future developments……

Quote:
Originally Posted by Ken G
... If it doesn't return, it was never really ours.

Okay this is not that hard to understand. The energy does not go away or dissapear, it just gets spread out over a larger area of space. Redshift is the lengthening of wavelengths. As the wavelengths of light get longer, the space between the waves becomes greater. Because the space becomes greater between the wavelengths, that means the beam of light becomes longer. The entire beam of light still contains the same amount of energy, the energy is just spread out over a larger area of space.

Is there really a problem? A light beam has less power when red shifted, but because of time dilation at the source it will last longer to compensate?

All I'm saying is, cosmological red-shift is a can of worms.

__________________
PW -- Plant Whisperer

Sorry, that's just wrong. Look up "adiabatic expansion", and see if that just spreads energy over a larger volume of space. It does spread energy over a larger volume, but that only accounts for part of the reduction in energy density-- there's also a loss in total energy in the photons (or the gas). This is an important point about adiabatic expansion that quite a lot of people don't understand.

Why, because it doesn't conserve total energy? Who said it had to? Do we dictate our laws onto the universe, or does the information flow in the opposite direction? That is the fundamental difference between good and bad science, in a nutshell.

That isn't right either, but it's a good stab at it. The wave mechanics doesn't work that way, because the same physics that spreads out the time also weakens the energy emission rate. Those two compensate, but the redshift is never compensated, it's just gone. Energy is generally not conserved when you compare two reference frames, only when you stay within one frame, and that's exactly what you do not do in general relativity.

Hum,
isn't the point that ultimately energy isn't lost?
I would agree with triclon, and add that what we really see is the collapse of the photons wavestate.
We could say that the photon travels at a constant speed so if the geometry of spacetime changes relative to the observer then the photon is recorded with a certain energy; this model would be likening a photon to a small bullet.
So if an observer were travelling towards the Virgo cluster at the same speed as it is receding there would be no redshift. If the traveller stops then redshift suddenly appears.
So rather than saying that somehow the photon knows what the traveller is doing, or is going to do, and loose or gain energy accordingly , it is better to assume that to the photon no energy is lost, and that as soon as it is recorded that the wave state collapses to whatever the observer is doing. Relative to the photon there is no change. Relative to the moving observer there is change.

__________________
`Irony` actually does mean `metal like`...

I would say the point is that energy conservation is a local concept, it's not meant to be applied for propagation of light from one end of the universe to the other, and it doesn't work for that.
Then you would be wrong with triclon, and for the same reason. Adiabatic expansion does lose energy from the gas, it shows up as work on the boundaries (like a piston) or in the bulk expansion (like an explosion). But the universe doesn't work that way on the grandest scales, it has no walls and it's not an explosion. There's simply not energy conservation unless you cook up some kind of pseudo-gravitational potential energy that really doesn't exist in general relativity.

"To the photon" there's no such thing as energy. It's a photon. Energy is a concept of an intelligent observer, and must be applied in the reference frame of said observer. And the observer concludes that cosmological redshift does not conserve energy, nor does it need to, but energy is conserved in all experiments done in the observer's local reference frame.

To Ken G., I agree with your statements in respect to GR completely. However, the act of observation thermalizes the radiation. Intercepting any mass thermalizes the radiation. How could it be determined if photons from a distant source are not a mix of thermalized and non thermalized photons, and to what degree? The position of the emission lines would not be changed.

I'm afraid I don't understand what you are asking. What do you mean by a mix of thermalized and unthermalized photons? The CMB is all thermalized, and has no emission lines. Are you talking about quasar spectra? That is never formally thermalized, as it is all going in one direction when we see it. But it may have come from a thermalized source, and that's always one of the questions one asks.

@Ken G
Hum,
ok i accept what you are saying - but are we not saying the same thing?

“Relative to the photon there is no change. Relative to the moving observer there is change.”

i suggest we don't even have to use the 1st law of thermodynamics...

__________________
`Irony` actually does mean `metal like`...

Yes, I was referring to the quasar spectra.

[unless you cook up some kind of pseudo-gravitational potential energy]

Which would be 'gravitational radiation' IMHO.

This is kinda like your 'star' thread for me...everyone keeps trying to explain gravity with 'energy'! The 'base' Planck size/mass element that makes of the non baryonic DM, has a small mass but NO energy (it is inert) and is all of gravity.

So, the gravity well that any body (of baryonic matter) creates in space/time, is just that...a gravity well that baryonic matter must follow the curve of, but there is no gravitational energy.

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

Surely energy has to be conserved. If you are in a ship and are flying away from our galaxy, you see our galaxy's light redshifted. This is because you are flying with the wavelengths of light and see less wavelengths per second then if someone was in a ship sitting still looking down on our galaxy. If the guy sitting still sees 500 waves in 10 seconds, you in the ship might see the same 500 waves in 30 seconds. You both see the same amount of energy but over different time periods. The same 500 waves of light coming from the milky way do not have more or less energy because they are seen over different periods of time, it is still the same 500 waves thus energy is conserved.

No.

That is the subtle difference between an adiabatic expansion and a "free expansion". With the former, the temperature drops, with the latter it does not (for ideal non interacting particles -- if internal energy is a function of more than just temperature, then a free expansion will lower the temperature, but not as much as an adiabatic one). Now, there is something I can't remember about calculating entropy changes -- you can model the free expansion as an isothermal expansion (but it's different because no heat is transferred). You get the correct entropy increase *for the gas itself*, but there is no corresponding decrease of entropy for "the outside", and so the net entropy of the universe increases. There's a lot of subtle stuff like this I've forgotten. Adiabatic is "isoentropic", but free expansion is not!

You can sort of imagine a free expansion as one where the expanding particles don't get a chance to do work. For example, pull a piston out so fast that the gas particles don't have a change to collide with it. I think that translates to moving faster than the speed of sound for the gas inside.

Thermodynamics is one of things I've never really liked. I enjoy (love, really) EM and (now even more) gravity, but I never enjoyed thermodynamics for some reason. It was something I have to learn in order to understand things, but not something I enjoy, so I tend to forget a lot of the subtle things.

-Richard

How can energy be lost? Take the cosmological redshift. The expansion of space streatching out the wavelengths of light right? As space expands, so do the light waves, thus you get the same amount of waves but over a larger area of space right? thus there is no loss of energy, just energy being spread out over a larger area right? I'm lost so tell me where I'm wrong here.

I'm not sure what you mean there, I would think of a free expansion as the conversion of random kinetic energy (i.e., temperature) into ordered kinetic energy (i.e., fast flows), which certainly does lower the temperature. Those were the two types of expansion I was talking about (but it's good that you are clarifying this distinction), neither of which applies to cosmology when one is using general relativity and the comoving reference frame.

That much is true. It's because for an adiabatic expansion, you trade off lower temperature (which lowers entropy) for larger volume (which increases entropy). For free expansion, you have the lower temperature and the larger volume, but you also have the bulk flow kinetic energy. It is the entropy of the expanding flow that is where you get your entropy increase, not because the temperature isn't dropping (it is). A nice way to think about entropy is whether or not what is happening is reversible, i.e., can you tell if something weird is going on if you watch the movie backwards. Expansion against a piston is reversible (entropy conserving), an explosion is not. What is happening in cosmology is reversible-- the cosmology movie running backward would not look strange, so the entropy of the universe is not changing (due to cosmological expansion, that is-- it certainly is increasing due to what stars are doing), and that's one reason it is sometimes modeled as adiabatic expansion of the CMB even though there are no walls.

All true. The particles in a free expansion don't do work on anything else, however you can certainly imagine that they are doing work on themselves. Pressure acts just like a force, so PdV work applies whether there are walls to do work against or not. If there are no walls, the PdV work shows up in the kinetic energy of the flow, i.e., the "explosion" energy.

Ken,

But there is no bulk flow, save in the brief transient period. The temperature of an ideal gas does not drop in a free expansion, although that of real gas will slightly because of the self-interaction behavior.

Consider a test jar, say a quart jar of gas inside a 10 gallon jar which is completely evacuated. We open the quart jar so fast (or imagine the walls just dissappearing) no work is done against it. The gas has free expanded into 40x the volume. There is no bulk flow because it is now contained.

If internal energy is a function of temperature alone as it is for an ideal gas, there will be no change in temperature.

Some of the thermodynamics greats of the 19th century and maybe before did lots of experiments trying to prove this, basically trying to realize the above idea of the small jar inside the bigger jar. They were hampered by the relatively low heat capacity of air.

-Richard

That's not right. There is bulk flow-- the expanding gas! (Bulk flow does not mean the center of mass as a whole is moving, it means that the atoms have locally correlated motion, i.e., there is an outward flow.) And the temperature does drop, or you would not conserve energy. This is all ideal behavior.

Even ideal gases have flow kinetic energy, which is not counted as "internal" energy because the latter is always determined in the local comoving frame of the fluid (not the total center of mass). Nevertheless, energy is globally conserved here.

Energy is not lost. Energy is relative. The wavelengths,
frequencies, and energies of photons you see coming from a
given source vary when you change your motion relative to the
source. If you accelerate away from a light source, you cause
the light to become redshifted. If you accelerate toward the
light source, you cause the light to become blueshifted.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves