Achernar
Correct me if I’m wrong but, I think the description and picture under the title, “The Cosmological Redshift” in the following link contradicts your explanation.
Well, the photon isn’t really well described as a “point” particle because there is no such thing in physics due to Heisenberg’s Uncertainty principle. The longest wavelength photons don’t behave like point particles at all. Gamma rays, on the other hand, are a different story.
What’s important to realize is that the rulers that we are using to measure a photons’ wavelength are changing if space is expanding. These rulers are cosmological in nature and have nothing to do with the actual “size” of things expanding but rather have to do with the manner in which energy is transformed from one location to another. When you look at how the energy passes between one comoving reference frame and another, the difference in energy is simply determined by one over the wavelength difference multiplied by Planck’s constant times the speed of light. This is basically where redshift comes from in the expanding space model. The effect is basically to have all frames of reference at recessional velocities from all other frames of reference, and more importantly, the basic solution must be a linear relationship between distance and this velocity. In short, the answer is that the photon’s change even a single photon’s change is a result of redshifting of the wavelength. It’s simply related to the velocity difference between the frames via the “Doppler” redshift.
Since a spaceship is not traveling at the speed of light, it itself defines a specific reference frame against which you can measure an expansion. Since the expansion is on cosmological scales, interaction with a cosmologically “redshifted” spaceship will not happen. This is simply because said spaceship will be moving through space between comoving frames. Photons are unique because they trace space in an interesting way due to their lack of a rest frame. Upon annihilation (detection) of a photon you can get information about it’s trip. That’s why we see redshifts. What we could do is ask the question as to what a spaceship that moved at a constant velocity starting at galaxy A and moving at nearly the speed of light until it reached us (but didn’t stop to join us in our reference frame) would look like. That’s actually a special relativity problem where you add the recessional velocity of the incident galaxy to the velocity of the spaceship using the SR-velocity addition formula. Not as glamorous as redshift, but an answer nonetheless.
JSPrinceton
I was under the impression that cosmological redshift had nothing to do with the Doppler redshift or SR.
Does not GR explain cosmological redshift on its own?
Cosmological redshift has everything to do with perspective and is spoken to by classical physics and the Doppler Effect. It has to do with a source moving away from you. In that way, no relativity is really required, though you can use relativity to talk about the differences in energies and the “expanding” of space-time.
Yes, if you solve it properly. However, that doesn’t negate the Doppler Effect treatment of the problem.
This stuff is great.
So look:
A “photon” is a “particle.” But it has no rest mass, no rest frame, and no size. It isn’t made of anything. You can’t “split” it, or slow it down, or hold it in your hand, or pile 'em up. It has no charge, no polarity, no grav field.
In other words, of the various properties we expect particles to have, photons have almost none. It seems to be a sort of moving location in space with which is associated a measurable capacity to do work (in the physics sense).
Why, then, do we call it a “particle”? Tradition?
What is the difference between the photons that convey work in a positive electrical field (eg, around protons) and those in a negative electrical field (around electrons)?
If photons can’t be slowed down, how is a prism able to bend their paths? (For it seems that departing from the straight-and-narrow must, geometrically, alter the ratio of unit-space per unit-time during the swerve–and isn’t that what we mean by speed?)
I recall that electrons were once treated as point-masses, but that it proved useful to calculate how the field of one part of the electron might interact with another part (self-interaction). How was it determined that photons have absolutely zero diameter? (I don’t see how any number of experiments could arrive at such a conclusion.)
And, say, now that I think of it, what happens when the path of this moving-location-in-space arrives at a neutron? Why doesn’t it just pass right on through? Just what IS this “absorption” we hear about?
And what is that “very rare photon-photon interaction”?
(Yes, I’ve read QED.)
Photons are polarized if that’s what you mean by polarity? A very energetic photon will ‘split’ (the I use the term cautiously as it may imply that he photon isn’t a fundamental particle, which it is) into other particles. The property that makes it a particle is that it is that it mostly obeys the behaviour of what we would think of classically as a particle. For example it can be subject to inelastic collisions with electrons.
Well, it has a polarization. It also has a gravitational field because it has an associated stress-energy tensor. You can’t slow individual photons down, but you can slow light down… in fact, you can even stop light in some substances. You can split photons and pile them up to your heart’s content. Just make sure you don’t violate local conservation laws.
In a word, yes.
You mean the virtual photons? It’s a delicate question, because the answer depends on how the interactions proceed. The fact is that the sources are different creating different potentials in the “quantum foam”, so to speak. You get preferential directions based on these potentials and the way virtual photons wink in and out of existence. It’s all very tidy and neat.
By scattering. The shortest distance between two points is a straight line. If you get something to travel a whole bunch of straight lines at different angles you do get a sort of “acceleration”… but it’s not so much an acceleration as it is a type of interaction (or reaction) that creates a new photon going in a different direction.
This is all in the QED sense. Classically, of course, we just say that light only travels the speed of light in a vacuum, not in any given medium.
Velocity, really, but see if the above answer gets at what you’re trying to probe.
Who said they have zero diameter?
In truth, neither do I!
It all depends on the details of the Feynman diagram. A naked neutron has a very low cross-section for absorbing a photon true, however, it is not non-zero, and especially not for highly energetic photons.
Do you mean photon-photon scattering?
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JS Prinseton, though I hesitate to argue with a Doper of such evident knowledge, I believe that the doppler-effect explanation of cosmological redshift is problematic. Since there are no global inertial reference frames, a distant object does not have a velocity relative to us in the conventional SR/Newtonian sense (because we can’t measure its velocity using a rigid grid of clocks and rulers in our inertial reference frame, even in principle). In order to speak of the velocity of a distant object relative to us we must first define what that velocity means, and that is conventionally done by plugging its redshift into the special-relativistic redshift formula. This is an entirely reasonable thing to do, but in my opinion it means that explaining cosmological redshift as a consequence of velocity is circular. I suspect this explanation continues to be used largely because better explanations are far more complicated, as seen in this thread.
Scott Dickerson, I’ll try to answer your questions to the best of my ability. Why do we call photons particles? The answer is partly tradition and partially, as MC said, that a photon does behaves in certain ways like a classical particle. In addition, a photon obeys exactly the same dynamical laws of motion and interaction as a more traditional fundamental particle like an electron does, save that it has a different charge, mass, spin, etc.
The photons that are said to convey work through electrical forces are different in a fundamental way from both the photons that make up ordinary electromagnetic radiation and the particles that make up ordinary matter. They are “virtual” particles rather than “real” particles. A good description of virtual particles can be found http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html]here.
Photons can’t be slowed down in free space. At a quantum-mechanical level, the transmission of light through a transparent medium is quite complicated, and the boundary between two media more so. I believe it can be validly described in terms of photons that travel at c at all times but are continually being absorbed and reemitted. Conservation of momentum prevents them from being reemitted in any direction other than that in which they were originally traveling. The vector sum of all these photons is a wave that travels at a speed less than c.
I believe neutrons can potentially interact with photons despite being neutral because they are made up of quarks, which themselves are charged. Truly fundamental neutral particles like neutrinos do not interact with photons at all except through gravity.
I’m afraid I don’t know anything about the photon-photon interaction. Based on my understanding of physics photons should be unable to interact with other photons in free space, but the presence of additional particles might allow such interactions to occur without violating momentum or energy conservation.
I don’t know how much QED you’ve read, so hopefully this isn’t too basic for you but at the same time is at least partially comprehensible to those without such a background.
Here’s a lovely little article on the blueshiftng on the cosmic microwave background by “dark energy”:
Repulsive Astronomy: Strengthening the case for dark energy
I’ve already started writing up the patent on the obviously implied perpetual motion machine. It’s a good thing the USPTO doesn’t still ask for working models, the real thing will be ~100 million light years across 
You say this in jest, but it’s actually a rather interesting point. This is not an illusory gain in energy, but a real one. The energy given to the photons traversing concentrations of matter by the dark energy-driven acceleration of universal expansion is only an infinitesimal fraction of the energy given to the matter itself.
The ultimate source of this energy, in current theory, is the continuing collapse of whatever field or fields drive inflation. Most of this energy was released very early in the history of the universe; it is in fact what powered the Big Bang. Modern observations of dark energy could be explained if some utterly miniscule fraction of this energy remains unspent, and is slowly leaking out into the universe of matter through expansion driven by its internal pressure. Small as it is in comparison to the primeval fireball, this energy would be quite vast on modern cosmological scales. Moreover, I see no reason it could not in principle be tapped through mechanical means.
Engineering difficulties present themselves; to produce useful quantities of power a device might need to be millions of light-years across, as you suggest. Nonetheless, this is a potential power source that would remain long after all the stars have burnt themselves to cinders. It might be of interest to our distant descendants.
Thanks JS Princeton et al.
Here I thought the photon was the easier-to-understand twin of the wave-particle duality. Maybe it is, but whoaaa, we’re a long way from itty-bitty billiard balls, eh?
So photons DO have a diameter, and are not just points? OK then, what’s the diameter?
I knew LIGHT could be polarized. I own a pair of sunglasses. So you’re saying that one individual photon can be polarized? (Hmmm, I now recall that those “Bell” experiments had to do with polarized photon pairs…but that’s not a single photon, in a sense.)
The words “very rare photon-photon interaction” come either from the pre-senile Arthur C. Clark or the pre-dead Isaac Asimov.
I gather, then, that bending light in a prism has to do with “biasing” the directions of successive re-emissions.
And now there’s this thing about photons gaining and losing energy. But energy equates to mass. Photons, however, have no rest mass, so it seems like gaining energy would by like multiplying a zero by something…
Well, obviously I’ve got to study photons with a bit more seriousness of purpose. At least I know now that these things are a little tricky.
The polarization of a photon is equivalent to the direction of it’s spin anglar momentum.
photons do have energy which they can gain or lose they just don’t have any rest mass.