Intense laser experiments provide evidence that light can stop electrons | Hacker News Reader

Intense laser experiments provide evidence that light can stop electrons

www3.imperial.ac.uk

89 pointsjoeyespo37 comments

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 feel queesy making a trend out of three points (one point in the blue even looks like it could touch the classical model). I'd have to read into it more (will do, just not tonight). This is, however, an extremely difficult experiment to perform, hence why they only have four data points. They definitely have demonstrated that RR does occur (no duh, which is more of why it's a good paper) but I'd hold off on the claim they've really distinguished the two.

Cheers to ICL on this first.

The article is a little hard to understand. First I thought the electrons were in frontal collisions with light and was like "duh of course it slows down". But then they talked about the electrons hit a "sheet" of light. So is the correct interpretation that the light and electons hit at perpendicular angles (in the lab reference frame)?

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 not called “first observation of quantum radiation reaction” for a reason.

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).

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 agreed that light could exert force upon charges.

The experiment to which the HN title links, however, is awesome, and is perhaps the first time one has demonstrated stopping 0.5 GeV electrons in a wall of light.

OK we've taken "first" out above.

> If electrons scatter photons, then photons scatter electrons.

Does it mean that photons are able to scatter photons?

Photon photon scattering is a thing [1] but I am not sure whether that is implied by the interaction of light and electrical charged particles acting both ways. I slightly tend to think that is not implied but I am not a physicist.

[1] https://en.wikipedia.org/wiki/Two-photon_physics

Interesting! Thanks

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...

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

Physicist.

What’s the difference between this and radiation pressure on charges (which is classical physics)?

Good question! In radiation pressure the fact that the charges radiate energy isn’t included, it’s just the change in momentum due to to radiation being absorbed or reflected that produces a force. In radiation reaction the charges are accelerated and decelerated by the electric field of the light. Classical electrodynamics says the electrons must give off em radiation, and so must lose energy in the process, so must be an additional force associated with this emission. Physicists such as Dirac looked at classical radiation reaction in the early 20th century, but I don’t think it’s effect has been observed in the interaction of electrons with light before. At very high electron energies and/or laser intensities the classical description of radiation reaction should break down.

Study: Experimental Evidence of Radiation Reaction in the Collision of a High-Intensity Laser Pulse with a Laser-Wakefield Accelerated Electron Beam

Citation: J. M. Cole, K. T. Behm, E. Gerstmayr, T. G. Blackburn, J. C. Wood, C. D. Baird, M. J. Duff, C. Harvey, A. Ilderton, A. S. Joglekar, K. Krushelnick, S. Kuschel, M. Marklund, P. McKenna, C. D. Murphy, K. Poder, C. P. Ridgers, G. M. Samarin, G. Sarri, D. R. Symes, A. G. R. Thomas, J. Warwick, M. Zepf, Z. Najmudin, and S. P. D. Mangles. Phys. Rev. X 8, 011020 – 2018-02-07

Link: https://doi.org/10.1103/PhysRevX.8.011020

DOI: 10.1103/PhysRevX.8.011020

Abstract: The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today’s lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (ε > 500 MeV) with an intense laser pulse (a0 > 10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (γ rays), consistent with a quantum description of radiation reaction. The generated γ rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy εcrit > 30 MeV.

Here's a great Royal Institution Lecture by Kate Lancaster talking about Ultra Intense Lasers from 2017 :) https://www.youtube.com/watch?v=hcGgaa2mFc4

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.

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.]

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.

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.

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.

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.

Where would this occur naturally?

Reading the article I saw this quote "This radiation reaction is thought to occur around objects such as black holes and quasars "

If you combine this with interference do you get another door into quantum computing?

Ah, yes. The "Deer-in-headlights" effect.

What on earth, light by definition is the quantifiable (quanta) of energy levels between electron orbital states. This is such a nonsensical title, as the electron would be changing electron states based off the energy level of the laser - of course! Any suggestions on what the title should be changed to?

Is that what this article is about? I read this article as being about a quantum phenomenon where the photon beam absorbed the momentum from an electron beam in a manner quite different from how they would interact at lower intensities.

Ok, not even even about electrons in orbit. First couple paragraphs mention an electron beam.

Reading the article might help.

> light by definition... of energy levels between electron orbital states

No, that's not the definition of light, photons or quanta.

The fact that electrons on an atom emit or "absorb" energy in a quantized way is a consequence of several basic elements of QM

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