Undergrad The CMB drag on an object moving through space

[JimJCW]

An object moving through a fluid, such as the air, experiences a pressure drag caused by the difference in fluid pressure between the front and back surfaces of the object. Similarly, an object moving through a thermalized photon gas, such as the CMB, also experiences a drag. The cause in this case is that the object sees a blue shifted photon gas along the direction of motion and a redshifted one in the opposite direction (see Smith 1995, The drag on a sphere moving through a blackbody radiation bath). An example of this effect is the observed CMB dipole: The Earth travels at about 370 km/s relative to the CMB (see Planck 2018). In the all-sky map from the COBE satellite, radiation in the Earth's direction of motion appears 0.35 mK hotter than the average temperature, 2.725 K, while radiation on the opposite side of the sky is 0.35 mK cooler. The pressure of a thermalized photon gas is related to its temperature T by

https://www.physicsforums.com/attachments/288905

The observed CMB dipole, therefore, implies a radiation pressure difference between the two sides of the earth, causing a CMB drag on it. By plugging in suitable estimates of quantities involved, such as the cross-section of the Earth and the temperatures on the two sides, 2.725K + 0.35mK and 2.725K - 0.35mK, this drag can be estimated to be tiny,

https://www.physicsforums.com/attachments/288906

For comparison, please note that,

https://www.physicsforums.com/attachments/288908

For a photon traveling at 300,000 km/s through space, could there be a CMB drag too? How can we get some ideas about this?

 

[PeterDonis]

For a photon traveling at 300,000 km/s through space, could there be a CMB drag too?

No. Photon-photon interaction is negligible at CMB energies. A photon is not a little billiard ball.

 

[kimbyd]

No. Photon-photon interaction is negligible at CMB energies. A photon is not a little billiard ball.

Even if CMB photons did interact with one another, they'd just redistribute energy between one another, retaining the same thermal distribution. Drag doesn't make sense here, because drag implies a loss of energy. If CMB photons did interact, any "lost" energy in such an interaction would end up being picked up by another CMB photon.

So even if they did behave like billiard balls, drag still wouldn't make sense.

Drag from the CMB does make sense, however, when considering very high-energy particles, especially charged particles.

 

[JimJCW]

Even if CMB photons did interact with one another, they'd just redistribute energy between one another, retaining the same thermal distribution. Drag doesn't make sense here, because drag implies a loss of energy. If CMB photons did interact, any "lost" energy in such an interaction would end up being picked up by another CMB photon.

Drag from the CMB does make sense, however, when considering very high-energy particles, especially charged particles.

We are considering an optical photon traveling through the CMB photon gas, similar to what you said, “Drag from the CMB does make sense, however, when considering very high-energy particles, …” In the present case, by drag, I mean that on the optical photon.

Photon-photon scattering is allowed by quantum electrodynamics through virtual electron-positron pair. The interaction between the traveling photon and the photon gas is, therefore, unavoidable when it travels through the CMB; the question is how much. PeterDonis thinks it is negligible.

 

[PeroK]

We are considering an optical photon traveling through the CMB photon gas, similar to what you said, “Drag from the CMB does make sense, however, when considering very high-energy particles, …” In the present case, by drag, I mean that on the optical photon.

Photon-photon scattering is allowed by quantum electrodynamics through virtual electron-positron pair. The interaction between the traveling photon and the photon gas is, therefore, unavoidable when it travels through the CMB; the question is how much. PeterDonis thinks it is negligible.

I found this:

https://inspirehep.net/files/4aa16cf428ef41062d0fafa026485c01

 

[Vanadium 50]

Photon-photon scattering

We've been through this before in your previous, now locked, thread. There's more scattering in the lens of your eye than in photon-photon scattering in the entire universe.

Since you clearly don't believe the answers you are getting, I can only conclude you are not seeking information - you are instead pushing your own personal theory.

 

[PeterDonis]

PeterDonis thinks it is negligible.

It's not a matter of what I personally think. As @Vanadium 50 has pointed out, we already discussed this in a previous thread of yours, which has been closed. So this thread is closed as well.