
Intense laser experiments provide evidence that light can stop electrons - joeyespo
http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_5-2-2018-12-35-5
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noobermin
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.

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sofasurfer
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.

~~~
noobermin
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).

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ISL
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.

~~~
jerry40
> If electrons scatter photons, then photons scatter electrons.

Does it mean that photons are able to scatter photons?

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danbruc
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](https://en.wikipedia.org/wiki/Two-photon_physics)

~~~
jerry40
Interesting! Thanks

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randomdrake
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](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.

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

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

~~~
jpmattia
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.]

~~~
sofasurfer
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.

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greendesk
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.

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pif
> 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.

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

~~~
sofasurfer
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.

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

~~~
sofasurfer
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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matte_black
Where would this occur naturally?

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

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

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djole103
Ah, yes. The "Deer-in-headlights" effect.

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marknadal
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?

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

