B Red shifted tired light doubts

[MikeG]

How did you find PF?: Searching for answers about redshift

I have a friend who is a skeptic of the "expanding Universe". Neither of us are physicists, but he is an engineer. He claims that red shift is due to "tired light" not the "doppler"-like effect of light coming from stars moving away. He believes that light tires either due to collision with particles or by the effect of gravity. II have looked on this forum enough to debunk the collision explanation, so enough said about that.

I argue that since photons have no mass, they are not affected by gravity, other than the fact that they travel through space, which of course is curved by gravity. His question remains, however, what happens to the energy in the photon when it downshifts toward red?

The answer presented here to that question is that conservation of energy is a local law, not a universal law. I found that unsatisfying, not necessarily wrong. I tried explaining it to him this way:

"With the emitter and receiver moving away from each other the photon would, in classical physics, 'hit' the receiver at less than the speed of light. But since light only travels at "e" relative to any observer, it can NOT arrive at the receiver at a speed less than e from the receiver's point of view. So, something has to give and it's the amount of energy 'seen' in the 'collision', i.e., the wavelength. My amateur answer is close to the two presented on the forum, which state that conservation of energy is a "local" rule and doesn't apply in the relativistic, expanding geometry of space. I just find my formulation easier to grasp. But is it right?"

 

[phinds]

He claims that red shift is due to "tired light".

which has been totally debunked here over and over. Arguing this is pointless.

I argue that since photons have no mass, they are not affected by gravity

Incorrect. Photons have energy, which means they ARE affected by gravity. Look up Einstein Rings.

The answer presented here to that question is that conservation of energy is a local law, not a universal law. I found that unsatisfying,

The universe really doesn't care what we find satisfying. It does what it does. The energy of a photon is not conserved as it travels through an expanding universe.

"With the emitter and receiver moving away from each other the photon would, in classical physics, 'hit' the receiver at less than the speed of light. But since light only travels at "e" relative to any observer, it can NOT arrive at the receiver at a speed less than e from the receiver's point of view. So, something has to give and it's the amount of energy 'seen' in the 'collision', i.e., the wavelength.

Yep.

 

[OmCheeto]

which has been totally debunked here over and over. Arguing this is pointless.

I disagree.

Incorrect. Photons have energy, which means they ARE affected by gravity. Look up Einstein Rings.

They also have momentum. I believe I learned that from YOU!

Which I inferred meant that photons affected the universe gravitationally, and the universe reciprocated. Hence, tired light might be a valid solution.

The universe really doesn't care what we find satisfying. It does what it does.

I can go along with that.

The energy of a photon is not conserved as it travels through an expanding universe.

So what happens to the energy of a photon in a non-expanding universe? That appears to be the OP's question.

Yep.

I've forgotten the question.....

ps. This question was actually the motivation for my 'Interstellar asteroid' post.
God knows we are not allowed to question the status quo, and have to finagle our questions in via nefarious means.

 

[Janus]

How did you find PF?: Searching for answers about redshift

I have a friend who is a skeptic of the "expanding Universe". Neither of us are physicists, but he is an engineer. He claims that red shift is due to "tired light" not the "doppler"-like effect of light coming from stars moving away. He believes that light tires either due to collision with particles or by the effect of gravity. II have looked on this forum enough to debunk the collision explanation, so enough said about that.

I argue that since photons have no mass, they are not affected by gravity, other than the fact that they travel through space, which of course is curved by gravity. His question remains, however, what happens to the energy in the photon when it downshifts toward red?

The answer presented here to that question is that conservation of energy is a local law, not a universal law. I found that unsatisfying, not necessarily wrong. I tried explaining it to him this way:

"With the emitter and receiver moving away from each other the photon would, in classical physics, 'hit' the receiver at less than the speed of light. But since light only travels at "e" relative to any observer, it can NOT arrive at the receiver at a speed less than e from the receiver's point of view. So, something has to give and it's the amount of energy 'seen' in the 'collision', i.e., the wavelength. My amateur answer is close to the two presented on the forum, which state that conservation of energy is a "local" rule and doesn't apply in the relativistic, expanding geometry of space. I just find my formulation easier to grasp. But is it right?"

Let's take the effect of gravity in consideration for a moment. Yes, gravity will cause a red-shift, but this requires moving from a lower gravitational potential to a higher one, such as climbing out of a gravity well. However, falling into a gravity well has the opposite effect, a blue-shift. Thus if you have two equally massed galaxies, light going from one to the other will have no net shift on arrival, because the red-shift from climbing out of one galaxy's well, is undone by it falling into the well of the other galaxy upon arrival. Any measured red-shift would need to be due to something else, such as the galaxies receding from each other. Now, is you want to attribute cosmological red-shift to gravity, that means every distant galaxy, in all directions, is deeper in a gravity well than we are. This would be extremely hard( if not impossible) to account for in a static universe. However, if we consider that looking further away is also looking back into time, and if the universe was smaller and more compact back then, then it would appear as if we were looking deeper into a gravity well. But, this requires that the universe has expanded since then.

 

[PeroK]

How did you find PF?: Searching for answers about redshift

I have a friend who is a skeptic of the "expanding Universe". Neither of us are physicists, but he is an engineer. He claims that red shift is due to "tired light" not the "doppler"-like effect of light coming from stars moving away. He believes that light tires either due to collision with particles or by the effect of gravity. II have looked on this forum enough to debunk the collision explanation, so enough said about that.

It's not enough to think something, you have to show that what you think matches the data. When Einstein developed the General Theory of Relativity (GR), he found that a static spacetime was not a possible solution for the entire universe. Space must either be expanding or contracting, but cannot be static.

The perceived wisdom at the time, however, was that the universe was static, so Einstein introduced a fudge factor, in the form of a cosmological constant, to make the universe static. Later, when evidence of an expanding universe was found, Einstein was able to take out the cosmological constant. He should have had the courage of his convictions and predicted the expanding universe before there was any evidence for it.

I argue that since photons have no mass, they are not affected by gravity, other than the fact that they travel through space, which of course is curved by gravity. His question remains, however, what happens to the energy in the photon when it downshifts toward red?

Energy depends on the reference frame. This is true in all physics. A photon does not have an intrinsic energy, although the energy in the rest frame of its source is quite important. Its energy depends on the reference frame in which it is measured. The energy measured at the detector depends on the relationship between the source and detector. From that point of view, the energy doesn't go anywhere. The redshift is a function of the relationship between the source and the detector.

The answer presented here to that question is that conservation of energy is a local law, not a universal law. I found that unsatisfying, not necessarily wrong.

That's largely irrelevant to the redshift of photons.

I tried explaining it to him this way:

"With the emitter and receiver moving away from each other the photon would, in classical physics, 'hit' the receiver at less than the speed of light. But since light only travels at "e" relative to any observer, it can NOT arrive at the receiver at a speed less than e from the receiver's point of view. So, something has to give and it's the amount of energy 'seen' in the 'collision', i.e., the wavelength. My amateur answer is close to the two presented on the forum, which state that conservation of energy is a "local" rule and doesn't apply in the relativistic, expanding geometry of space. I just find my formulation easier to grasp. But is it right?"

The speed of light is invariant, but the energy-momentum of light is not. A photon's energy and momentum are frame dependent. This is covered in any introductory book on special relativity.

Again, the concept of global energy conservation is not relevant. You can measure the redshift of light in the lab.

The key argument against the tired light theory is that the redshift predicted is different quantitatively from that measured. It would depend only on the time between emission and detection. Whereas, this is not the redshift that is measured by experiment.

It's not a theoretical debate, as such. Tired light is simply a theory that fails to match experiment.

 

[Bandersnatch]

Wright's tutorial pages list issues with tired light. https://www.astro.ucla.edu/~wright/tiredlit.htm
It's good to keep those in mind as one talks to, uh, a proponent.

 

[PeroK]

Neither of us are physicists, but he is an engineer.

It should be said that many engineers are antagonistic to physics. I was talking to someone recently who was reading about Quantm Mechanics from a popular science book. He said that his friend, who is an engineer, believed that QM was all nonsense and that the microscopic world was simply classical and that physicists had got it all wrong.

Many engineers, when they retire, set about trying to develop their own "theory of everything", free from all the "obvious" problems, paradoxes and contradictions that physicists choose to ignore. A suprising number of the "crackpot" theories we see on here are from retired engineers (and, increasingly, retired software engineers).

Now, of course, not all engineers are crackpot physicists! But, it's worth bearing in mind, that many engineers cannot accept modern physcis and will often prefer a theory like tired light for reasons that seem to be wholly unscientific.

 

[PeterDonis]

I have a friend who is a skeptic of the "expanding Universe".

We don't do discussions by proxy here. If your friend wants to ask about this here, he can open his own PF account.

That said, your own understanding appears to contain some errors:

I argue that since photons have no mass, they are not affected by gravity

This is not correct. Photons have energy. Everything with energy is affected by gravity.

what happens to the energy in the photon when it downshifts toward red?

There is no such thing as "the energy in the photon" independent of some observer. Photons have energy, as above, but that doesn't mean you can quantify the energy they have in the abstract. You can't. (This is true for any object, not just photons, by the way.)

Also, the photon doesn't "downshift towards red". You have a curved spacetime in which some distant object emits a photon. Relative to the emitter, the photon has a certain energy. (You mention wavelength, but wavelength, frequency, and energy are all inter-related, if you know one, you know all three. They don't change independently.) Then, a billion years later (or whatever), someone on Earth detects the photon with a certain energy, that's lower than the energy relative to the emitter. But some other observer, moving relative to Earth, might detect that photon with a different energy, which could be the same as, or even higher than, the energy it had relative to the emitter. What's the "real" energy of the photon? There is no such thing. So asking what happens to that nonexistent thing is a meaningless question.

The answer presented here to that question is that conservation of energy is a local law, not a universal law.

More precisely, it's a local law, not a global law. There is no "global energy of the universe" that's conserved. But locally, stress-energy can't be created or destroyed, it's conserved.

However, what's conserved, "stress-energy", is not the same thing as what you intuitively think of as "energy". That's illustrated by the photon example. The photon's stress-energy is conserved in that example! But that conservation does not mean that every observer measures the energy of that photon to be the same. The actual conservation law is more subtle than that, and requires looking at the actual math to understand.

 

[PeterDonis]

I disagree.

Disagreeing that tired light has been debunked is not a viable position here.

Which I inferred meant that photons affected the universe gravitationally, and the universe reciprocated.

That's correct. And our best current models all take that into account.

Hence, tired light might be a valid solution.

Wrong. The reasons why tired light was debunked have nothing whatever to do with photons not affecting or being affected by gravity.

 

[PeterDonis]

God knows we are not allowed to question the status quo, and have to finagle our questions in via nefarious means.

Posting further along these lines will get you a warning. Please be advised.

 

[Jaime Rudas]

Many engineers, when they retire, set about trying to develop their own "theory of everything", free from all the "obvious" problems, paradoxes and contradictions that physicists choose to ignore. A suprising number of the "crackpot" theories we see on here are from retired engineers (and, increasingly, retired software engineers).

Can you provide any references to support this claim?