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Intense laser experiments provide evidence that light can stop electrons

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Re: Intense laser experiments provide evidence that light can stop electrons

#31

Post by jpmattia »

chopin wrote: post #16

How is this different from Compton scattering? Unfortunately the article does not state this.

As near as I can tell: Compton scattering treats the photon and electron as two colliding balls, assigning momentum to the photon based on its wavelength. However, because the electron changed momentum during the collision, it must have experienced acceleration. Maxwell's eqns tells you that an accelerating charge radiates, but this radiation is not accounted for in the elementary Compton calculation. Compton got away with the elementary calculation because the effect is apparent at high energies only.

[This is the first I've heard of the effect, and I'm thinking it out in real time, so apply grain of salt.]

Re: Intense laser experiments provide evidence that light can stop electrons

#32

Post by hossbeast »

pif wrote: post #30
greendesk wrote: post #13

Can this phenomenon be used as a shield for spacecraft? If a laser travels inside a fiber cable, then the laser beam can wrap around the spacecraft. If the beam can stop electrons, it can potentially stop the larger particles traveling in space.

> the laser beam can wrap around the spacecraft. If you want the laser beam carry enough impulse to stop a rock flying towards you at a few thousands km/h, good luck keeping the fiber in place! Every time the fibre tries to make the light change direction, the light tries to straighten the fibre, just like water running through a pipe.

I think GP was talking about shielding against radiation of various forms, not rocks.

Re: Intense laser experiments provide evidence that light can stop electrons

#33

Post by sofasurfer »

hossbeast wrote: post #32
pif wrote: post #30

Earlier quoted context omitted.

> the laser beam can wrap around the spacecraft. If you want the laser beam carry enough impulse to stop a rock flying towards you at a few thousands km/h, good luck keeping the fiber in place! Every time the fibre tries to make the light change direction, the light tries to straighten the fibre, just like water running through a pipe.

I think GP was talking about shielding against radiation of various forms, not rocks.

I think a better way would be to make a mini magnetosphere around the space craft - most of the radiation that’s dangerous in space is high speed protons in the solar wind. By surrounding the space craft with a magnetised plasma you can make these deflect around it, just like Earth’s magnetic field protects us from the solar wind.

Re: Intense laser experiments provide evidence that light can stop electrons

#34

Post by sofasurfer »

jpmattia wrote: post #31
chopin wrote: post #16

How is this different from Compton scattering? Unfortunately the article does not state this.

As near as I can tell: Compton scattering treats the photon and electron as two colliding balls, assigning momentum to the photon based on its wavelength. However, because the electron changed momentum during the collision, it must have experienced acceleration. Maxwell's eqns tells you that an accelerating charge radiates, but this radiation is not accounted for in the elementary Compton calculation. Compton got awa…

Radiation reaction is related to Compton scattering. In Compton scattering, the interaction is between one electron and one photon producing one photon at a new energy, with the electron changing energy. Non-linear Compton is one electron interacting with many photons to produce an electron and a high energy photon. Radiation reaction is essentially one electron scattering off many photons many times, producing many high energy photons. The paper itself is about the first observation of radiation reaction, there is work showing electrons losing energy in non-linear Compton scattering from the 1990s.

Re: Intense laser experiments provide evidence that light can stop electrons

#35

Post by ISL »

shdwban wrote: post #20
ISL wrote: post #10

The HN title is incorrect -- perhaps the first direct evidence is Compton's original work, showing that electrons scatter photons. If electrons scatter photons, then photons scatter electrons. Every time a photon scatters off an electron, there exists a reference frame in which the electron is brought to rest. I'm certain that one could find an earlier argument than Compton scattering, too. Maxwell surely would have…

Genuine question out of curiosity, what is your line of work? I don't see a lot of physicists around here

Physicist.

Re: Intense laser experiments provide evidence that light can stop electrons

#36

Post by ISL »

pif wrote: post #28
jerry40 wrote: post #18

Earlier quoted context omitted.

> If electrons scatter photons, then photons scatter electrons. Does it mean that photons are able to scatter photons?

Yes, they are. They cannot interact directly, but they can via "virtual" electron/positron pairs (or actually any other pair of charged elementary particle + antiparticle).

To expand a little -- the process is known as light-by-light scattering.

Searching will turn up plenty of good references, like this one: https://home.cern/about/updates/2017/08/atlas-observes-direc...

Re: Intense laser experiments provide evidence that light can stop electrons

#37

Post by noobermin »

sofasurfer wrote: post #25
noobermin wrote: post #12

This is a potentially impactful experimental result in my niche field, the physics of relativistic laser phenomena. People have been running simulations and imaging what RR would look like for years, and this looks like the first experimental result that shows it The money shot, fig. 9 is a bit underwhelming to me though, because it is supposed to show where the "classical model" diverges from the quantum model, and…

I’d say that the main conclusion is the observation of a statistically significant (> 3 sigma) radiation reaction effect which is pretty consistent with both the classical model and the quantum model. The data “hints” that the quantum one fits the data better, but not at a very significant level (1 sigma). Moving to higher electron energies and/or laser intensities will help make that difference clearer. The paper is…

Exactly. 10^21 W/cm^2 is actually a little below the threshold of where we expected RR (which is what the second Gemini beam shot at), and hitting 0.5 GeV electrons head-on helped alleviate that restriction. With a host of planned laser systems exceeding 10^22 W/cm^2, we can imagine more statistically significant divergences between QRR and Landau-Lifshitz (the "classical" RR model).

Re: Intense laser experiments provide evidence that light can stop electrons

#38

Post by sofasurfer »

fastelephant wrote: post #17
chopin wrote: post #16

How is this different from Compton scattering? Unfortunately the article does not state this.

It's not. It's an inverse Compton scattering experiment. Abstract: "The generated gamma-rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme..."

I do think there is a difference, but they are aspects of the same thing. One significant difference is that in the classical limit (ie taking the limit hbar -> 0) non-linear Compton scattering doesn’t produce energy loss for the electrons. The classical limit of radiation reaction (ie n photons scattering of an electrons m times) does produce energy loss (because the loss per scatter scales like hbar but the number of scatters scales like 1/hbar). The sentence you quote is about the energy of the photons produced (which determined by Compton scattering) but the energy loss of the electrons is what we call radiation reaction.

Re: Intense laser experiments provide evidence that light can stop electrons

#39

Post by sofasurfer »

rambojazz wrote: post #14

Was this predicted by theory (when?) or is this a unexpected result discovered from experimentation?

Classical radiation reaction was predicted in the early 20th century. I think that quantum theories for radiation reaction in with non-relativistic particles in weak fields has been around since the 1960s. The “hard” bit, that is currently an active research area, is the interaction of relativistic particles with very strong fields.
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