# Is it possible that photons lose a small amount of energy over time to become redshifted instead of being redshifted because of an expanding universe? [duplicate]

Can photons lose a small amount of energy over time when traveling large distances due to either weak interactions with magnetic fields (Faraday Effect) or due to the way all objects cool and give off thermal radiation (Second law of Thermodynamics) or gravitational interaction with nearby dust particles? If so, would this make the universe seem to be expanding when really, photons are being red shifted because of a small energy loss over time? To me, it seems like the universe is neither expanding nor contracting. Objects such as planets in the solar system or stars in a galaxy rotate and reach an equalibrium that lasts for billions of years.

## marked as duplicate by user10851, John Rennie, ACuriousMind♦, HDE 226868, fibonaticSep 6 '15 at 12:58

We live in the age of measurements and observations and specific mathematical theories that fit measurements and observations beyond any doubt.

Photons are elementary particles. . They have zero mass, and other characteristics which separate them from other elementary particles .

Can photons lose a small amount of energy over time when traveling large distances due to either weak interactions with magnetic fields (Faraday Effect)

This is an electromagnetic vertex for a photon, the faraday effect is for the emergent light beam from innumerable photons.

or due to the way all objects cool and give off thermal radiation (Second law of Thermodynamics)

No elementary particles behave thermodynamically. Thermodynamics is an emergent theory over many particles (statistical mechanics). Energy is gained or lost through interactions.

or gravitational interaction with nearby dust particles?

gravitational interactions with a strong gravity well, yes. Dust particles do not have enough gravitational potential for a measurable difference.

In all cases the photon would lose energy by interacting randomly. Random interactions would destroy the spectra coming from stars and galaxies that show a redshift, (loss of energy). After all it is from the displacement of spectral lines of known atoms that the study of observational astronomy took off.

That the universe is expanding we know from measurements of the velocities of clusters of galaxies. None are moving towards us. They are all moving away, and the raisin bread analogy might give an intuition how this can happen.

So the time when one could think up alternate models for the universe by handwaving is long gone. One needs hard measurements and concise mathematical models.

• Anna, I placed a reply as an answer. – HolgerFiedler Sep 5 '15 at 8:00
• "We live in the age of measurements and observations and specific mathematical theories that fit measurements and observations beyond any doubt." And if they don't physicists can always resort to this "hocus-pocus" or "shell game" or "dippy process" as Feynman called it, which officially goes by the name of "renormalization". – bright magus Sep 5 '15 at 9:49
• @brightmagus: renormalization is not an arbitrary procedure but actually an intrinsic property of quantum field theories. What nature tries to tell you by making it necessary is that your naive thinking about "scales" is flat out wrong. There are two intellectual responses to that: 1) you can leave snipe comments on SE or 2) you could actually think harder about what nature is trying to tell you. – CuriousOne Sep 5 '15 at 11:00
• @CuriousOne: You are flat wrong. What Nature is telling you is that the teories do not fit measurements and observations. And actually, the snipe comment is not mine, but Feynmam's. Still, if you persist in your "naive" believe in these theories, than perhaps you should consider what Jon Butterworth (physics professor at University College London and a member of the UCL High Energy Physics group who works on the Atlas experiment at Cern's Large Hadron Collider) has to say on the subject: theguardian.com/science/life-and-physics/2011/oct/11/… – bright magus Sep 5 '15 at 15:43
• @brightmagus: Not listening to nature is not a winning recipe in science, especially when the theory fits observation to 14 digits... as in case of Feynman's QED. Try to open your ears... she is basically screaming at you in this case. :-) – CuriousOne Sep 5 '15 at 15:46

Yes, photons do lose energy because of the expansion of space. Their wavelengths are increased by a factor of $(1+z)$ between when they were emitted at redshift $z$ and when we detect them now. Their energies are therefore decreased by a similar amount.

The "tired light" interpretation of this effect has been discussed, debated and disproved. In my opinion, the most convincing evidence is that we see other phenomena (not photons) at high redshift that have also had their frequencies decreased by exactly the same $(1+z)$ factor. I am thinking specifically of the time decay of type Ia supernovae - these events are thought to be very close to a standard, redshift-invariant explosion. Yet high redshift examples have their light curves "stretched" by just the same cosmological space expansion factor.

• Wait, if photons do lose energy over time, is this a conservation law violation (allowed because the universe as a whole isn't time invariant), or is the energy extracted by the fabric of spacetime? – John Dvorak Sep 5 '15 at 10:36
• @JanDvorak: It's only a conservation law violation if you draw an arbitrary system boundary based on the assumption that the expansion of space is not coupled to some other physical effect (e.g. a geometric field). In reality we can't tell if it's a violation or not because we don't know what the total system really looks like. – CuriousOne Sep 5 '15 at 10:54

Anna's answer is perfect in the sense, that photons are indivisible particles, which during their life don't loose energy. Nor Faraday effect, nor magnetic fields or the dispersion with intergalactic dust made the redshift. At the end of the answer Anna pointed out

that the universe is expanding we know from measurements of the velocities of clusters of galaxies. None are moving towards us. They are all moving away,...

But this measurement is based on the fact of the redshift of the electromagnetic spectra. So primary is the fact of the redshift and secondary is the interpretation of the big bang.

Are there possible other interpretations of where the redshift comes from? Indeed there is an alternative, it is the gravitational redshift. First at all, how this redshift fits with the conclusion that photons are indivisible particles which are not losing energy during their life? If the emission and the receiving of photons takes place in different gravitational potential, in the higher gravitational potential the emission of photons say from hydrogen is redshifted compared to a point of less gravitational potential. And second, do we have the right imagination of how big the mass has to be, from which we able to receive photons over 14 billion light years? So perhaps the theory about gravitational redshift and a not expanding space has a chance in the future again.

• Still no takers for your personal "hypotheses"? – CuriousOne Sep 5 '15 at 10:51