# Red shift

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Two assumptions followed by a question

1) Red shift of distant galaxies is determined by measuring the shift in absorption lines in a spectrum of light, however, it is not just the lines that shift but the entire spectrum.

2) Higher frequency light has more energy than lower frequency light.

Q. If light is emitted with more energy than when it is received where does that energy go?

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Nowhere. Assume someone shines a laser off a set frequency of x mhz, as measured by him and he shines it for 1 second by his clock. By his measurement,  he is sending at 100 watts, so in the one second he transmits 100 joules of energy.

He is moving away from you at some fraction of c so that you see the laser red-shift by a factor of n, so that you measure the frequency of the laser as being 1/n what he measures it to be  this means you also measure the beam to be 1/n of 100 watts.

However,  the red-shift doesn't just effect the frequency, but also how long you measure the laser beam from start to end.  From the moment you first detect the beam to when the end reaches you, you will measure n seconds. SO While the source says transmitted at 100 watts for 1 second, you receive the beam for n seconds at 1/n sec.  You both say that the beam contained a total of 100 watts of energy.

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57 minutes ago, Janus said:

Nowhere. Assume someone shines a laser off a set frequency of x mhz, as measured by him and he shines it for 1 second by his clock. By his measurement,  he is sending at 100 watts, so in the one second he transmits 100 joules of energy.

He is moving away from you at some fraction of c so that you see the laser red-shift by a factor of n, so that you measure the frequency of the laser as being 1/n what he measures it to be  this means you also measure the beam to be 1/n of 100 watts.

However,  the red-shift doesn't just effect the frequency, but also how long you measure the laser beam from start to end.  From the moment you first detect the beam to when the end reaches you, you will measure n seconds. SO While the source says transmitted at 100 watts for 1 second, you receive the beam for n seconds at 1/n sec.  You both say that the beam contained a total of 100 watts of energy.

Joules? I think I need to digest this a little. I suspect I might get some incite from this as it doesn't seem quite right to me. (not that that holds any water)

1. Given that they're not in the same frame, and energy is frame dependant...why do they both measure 100 joules?

2. It's the same number of photons...at different energies.

3. Does the expansion during transit not matter?

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3 hours ago, J.C.MacSwell said:

Joules? I think I need to digest this a little. I suspect I might get some incite from this as it doesn't seem quite right to me. (not that that holds any water)

1. Given that they're not in the same frame, and energy is frame dependant...why do they both measure 100 joules?

2. It's the same number of photons...at different energies.

3. Does the expansion during transit not matter?

Digest it? You mean the answer given was wrong and you're pretty sure of that? Is there some unwritten rule on this site that it's poor etiquette to disagree with a resident expert? If people ask questions and see a wrong answer from a resident expert, and no one authoritative corrects it, they tend to trust the wrong answer.

1. They don't, which you know. 2. That's right. 3. I don't know, hopefully someone else does.

Photons have kinetic energy, which is different in different frames. There's no conservation of an object's energy between different frames. Where conservation of energy comes in is... in either frame, the galaxy loses the energy of each photon that leaves (ie. different in the 2 frames), and I guess is pushed in the opposite direction of each photon a tiny amount (different in each frame).

4 hours ago, Janus said:

However,  the red-shift doesn't just effect the frequency, but also how long you measure the laser beam from start to end.  From the moment you first detect the beam to when the end reaches you, you will measure n seconds. SO While the source says transmitted at 100 watts for 1 second, you receive the beam for n seconds at 1/n sec.  You both say that the beam contained a total of 100 watts of energy.

The red-shift also lowers the intensity of the light (or the number of photons received per your second), so the energy you receive is less than 100 joules (I think you receive 100/n^2 watts for n seconds, 100/n joules total?).

Edited by md65536
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31 minutes ago, md65536 said:

Is there some unwritten rule on this site that it's poor etiquette to disagree with a resident expert?

There is a common sense rule that disagreeing with someone who has proven himself time and again should be done cautiously.

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It should be noted here also that in curved space-times, conservation of energy-momentum is a purely local conservation law; it does not necessarily apply globally across regions of non-Minkowski geometry, unless the spacetime in question has the specific symmetries that give rise to such conservation laws.

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2 hours ago, md65536 said:

Digest it? You mean the answer given was wrong and you're pretty sure of that? Is there some unwritten rule on this site that it's poor etiquette to disagree with a resident expert? If people ask questions and see a wrong answer from a resident expert, and no one authoritative corrects it, they tend to trust the wrong answer.

2 hours ago, zapatos said:

There is a common sense rule that disagreeing with someone who has proven himself time and again should be done cautiously.

This. Especially as it is Janus. But I also have not had the time to work through it, as I was also watching both the Democrat debates and a hockey game...

3 hours ago, md65536 said:

The red-shift also lowers the intensity of the light (or the number of photons received per your second), so the energy you receive is less than 100 joules (I think you receive 100/n^2 watts for n seconds, 100/n joules total?).

I think this is correct.

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1 hour ago, Markus Hanke said:

It should be noted here also that in curved space-times, conservation of energy-momentum is a purely local conservation law; it does not necessarily apply globally across regions of non-Minkowski geometry, unless the spacetime in question has the specific symmetries that give rise to such conservation laws.

That means if you're talking about the galaxy as an object moving away from you through flat spacetime, then the light energy it emits and the light energy you receive will be the same in a given frame of reference? But if you're talking about spacetime expansion, you can't treat the source and receiver as having a common frame of reference (or can you speak of a global frame of reference for other laws etc.)?

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3 hours ago, J.C.MacSwell said:

This. Especially as it is Janus. But I also have not had the time to work through it, as I was also watching both the Democrat debates and a hockey game...

Everybody makes mistakes. But as a general rule the people that are resident experts and mods are the sort that will acknowledge them when they are pointed out (and there are others, to be sure, who behave this way).

So it's not that it's forbidden to disagree — it's not a problem to point out mistakes, if you're sure it's a mistake. I think the "I need time to understand this because it doesn't make sense" is a good response, as would be phrasing an inquiry as a question to get more information. If there was an error, then the poster has a chance to re-assess their earlier response.

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So if a photon is emitted as blue light and received as red light is it the case that it still has the same total energy but it's energy is delivered over a longer period of time?

or is a photon all or nothing in which case it has less total energy?

Edit: I am thinking about expansion of space and not movement through it. I am trying to change my thinking not yours

Edited by between3and26characterslon
clarification
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1 hour ago, between3and26characterslon said:

So if a photon is emitted as blue light and received as red light is it the case that it still has the same total energy but it's energy is delivered over a longer period of time?

or is a photon all or nothing in which case it has less total energy?

Edit: I am thinking about expansion of space and not movement through it. I am trying to change my thinking not yours

You can't make the comparison because the two measurements are in different frames of reference. Energy being conserved means it stays the same within a frame of reference.

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7 hours ago, swansont said:

Everybody makes mistakes. But as a general rule the people that are resident experts and mods are the sort that will acknowledge them when they are pointed out (and there are others, to be sure, who behave this way).

So it's not that it's forbidden to disagree — it's not a problem to point out mistakes, if you're sure it's a mistake. I think the "I need time to understand this because it doesn't make sense" is a good response, as would be phrasing an inquiry as a question to get more information. If there was an error, then the poster has a chance to re-assess their earlier response.

My experience has been that yes, experts tend to correct their own errors when they're realized, and that it requires an expert to correct an expert (even if it's the same person) because expert replies are far more trusted and accepted than others, and that experts tend not to acknowledge errors in other experts' posts.

I agree that a good way to disagree is to ask a question (like, aren't the number of photons sent and received the same?) because even when you're wrong, you're not wrong in asking! But too much stuff like "I need time to understand" says you think the problem is with yourself and that there's no reason for the other person to reconsider what they wrote. When other people read that, and compare a reply labelled "Expert" to one that says "I don't understand", it's easy to brush off the latter.

Edited by md65536
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13 hours ago, md65536 said:

That means if you're talking about the galaxy as an object moving away from you through flat spacetime, then the light energy it emits and the light energy you receive will be the same in a given frame of reference? But if you're talking about spacetime expansion, you can't treat the source and receiver as having a common frame of reference (or can you speak of a global frame of reference for other laws etc.)?

You can. But it's not an inertial frame. It is a set or continuum of inertial frames.

2 hours ago, md65536 said:

My experience has been that yes, experts tend to correct their own errors when they're realized, and that it requires an expert to correct an expert (even if it's the same person) because expert replies are far more trusted and accepted than others, and that experts tend not to acknowledge errors in other experts' posts.

I agree that a good way to disagree is to ask a question (like, aren't the number of photons sent and received the same?) because even when you're wrong, you're not wrong in asking! But too much stuff like "I need time to understand" says you think the problem is with yourself and that there's no reason for the other person to reconsider what they wrote. When other people read that, and compare a reply labelled "Expert" to one that says "I don't understand", it's easy to brush off the latter.

There's some truth to this. I may have replied differently if it wasn't Janus, but I've gained a lot of respect for his posts over the years.

But it isn't that uncommon for me to reply in the form of a question, even if it is just an average poster.

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21 hours ago, md65536 said:

That means if you're talking about the galaxy as an object moving away from you through flat spacetime, then the light energy it emits and the light energy you receive will be the same in a given frame of reference?

The conserved quantity is energy-momentum, not just energy (which is always a frame-dependent quantity); specifically, it is the energy-momentum tensor.
If receiver, emitter, and the electromagnetic field itself are all located in a patch of Minkowski spacetime, then energy-momentum is always conserved:

$\int _{V} \partial _{\mu } T^{\mu \nu } dV=0$

But the same is not necessarily true if the spacetime in question is non-Minkowski (but it can still hold in specific cases).

21 hours ago, md65536 said:

But if you're talking about spacetime expansion, you can't treat the source and receiver as having a common frame of reference (or can you speak of a global frame of reference for other laws etc.)?

I understand your concerns, and you are absolutely correct in that they are not within the same frame of reference. However, the universe as a whole is described by a known solution of the field equations (FLRW metric), so we know how the two frames are mathematically related. So even if the usual conservation laws do not technically hold, we can still calculate what happens to light that travels to us from a distant galaxy. The result is the known redshift relation.

This is again one of those cases when I would simply sit down and employ the mathematical machinery, rather than trying to figure out the situation by intuition based on local laws.

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On 1/16/2020 at 5:35 AM, Markus Hanke said:

...This is again one of those cases when I would simply sit down and employ the mathematical machinery, rather than trying to figure out the situation by intuition based on local laws.

I don't disagree but not everyone has the ability to do that.

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14 hours ago, between3and26characterslon said:

I don't disagree but not everyone has the ability to do that.

Yes, you are right of course. This is why one can go and ask someone knowledgeable in the subject matter - or learn the skills needed to do it themselves.

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