Scientists discover how to turn light into matter after 80-year quest (2014) | Hacker News Reader

Scientists discover how to turn light into matter after 80-year quest (2014)

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107 pointsfelipemnoa30 comments

Since 2014 I can find 34 citations, none of which are an actual experimental realization of their scheme [1]. Anyhow,

TL;DR: They theoretically propose the following experiment:

First, accelerate electrons to just below the speed of light and fire them into a slab of gold. This creates the beam of nessesary high-energy photons.

Second, fire a high-energy laser at the inner surface of a tiny hohlraum (German for ‘empty room’) gold can, to create a thermal radiation field which generates light similar to that emitted by stars.

Finally, direct the photon beam from (1) through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can.

===

[1] https://www.researchgate.net/publication/270858006_A_photon-...

I know it's from the article, but the expression "Accelerate Electrons just below the speed of light" tells you pretty much nothing. An electron travelling at .9 the speed of light has 1.1 MeV of energy.

Nature seems to be down, so I can't access the abstract, but the google tagline has the following.

'Photons emitted above 100 MeV as a function of target width, for 1 × 109 incident electrons of energy 500 MeV (blue), 1 GeV (black) and 2 GeV (red).'

A table for comparison

        .9    1.1 MeV 
       .99    3.6 MeV
      .999   11 MeV
     .9999   36 MeV
    .99999  114 MeV
   .999999  361 MeV
  .9999999  1.14 GeV

So it's somewhere in the .999999c-.999999999c. Assuming that snippet was accurate. The LHC gets particles up to the TeV range, but, as the name suggests, they are much larger particles. An actual particle physicist can probably tell you more about whether it is significantly harder to get the much lighter Electron (and associated radiation losses) up to those energies.

SLAC (https://en.wikipedia.org/wiki/SLAC_National_Accelerator_Labo...) apparently can get electrons up to 50 GeV, so if those energies from the snippet are the target ones, that part should be achievable.

I'm not a particle physicist but a particle accelerator engineer.

GeV energies for electrons are achievable with a reasonably sized synchrotron like Diamond Light Source[0]. The magnets required to keep electrons in line are quite a bit smaller than those required for protons due to the mass difference. Small refrigerator sized verses small car sized.

[0] https://en.wikipedia.org/wiki/Diamond_Light_Source

That struck me as odd as well. 57 citations none of them where they actually ran the experiment and created electron / positron pairs. This was fun though (from 2013) "A table-top laser-based source of femtosecond, collimated, ultra-relativistic positron beams" -- https://arxiv.org/pdf/1304.5379.pdf

Comments such as this one make me always check comments before reading the submission. Thanks!

To be fair very few labs in the world have the ability to do that. Possibly only the NIF has powerful enough lasers

For a split second I thought, "well, can we even do the reverse?" Then I realized yeah, pretty much all light comes from matter. And a less useless question came to mind: Can light emerge from any source other than matter?

I don't know. I'm no physicist, and it's not an area where I could even be mistaken for a knowledgeable amateur, and it seems like I'm asking "can a photon give rise to another photon". But it did lead me to a very interesting treatment of a similar question concerning the de novo creation of photons [0]

[0] http://curious.astro.cornell.edu/about-us/137-physics/genera...

Depending on what you mean precisely, I'd actually say that most of our light sources other than the sun don't come from matter, in the sense that they don't turn matter into light but rather light is created when the energy level of electrons changes (whether in electric or chemical light sources).

Of course indirectly, this energy comes from matter being turned into energy, whether by the sun or by nuclear fission.

What I find more interesting is that today, all our energy ultimately can be traced back to stars, even if sometimes extremely indirectly (e.g. in the case of fossil fuels or nuclear fission). But this could actually change -- if nuclear fusion becomes used in practice.

> they don't turn matter into light but rather light is created when the energy level of electrons changes (whether in electric or chemical light sources). > Of course indirectly, this energy comes from matter being turned into energy, whether by the sun or by nuclear fission.

This is incorrect. It directly comes from matter as the rest mass of the low energy state is a minuscule amount less than high energy state. Specifically it is E/c^2 less, where E is the energy of the photon.

Parent is talking about different processes that produce photons: classical acceleration of charged particles vs particle-antiparticle annihilation.

You brought in a fact from general relativity which has no bearing on which processes are more common in the universe.

Of course you can always be pedantic and analyze problems at the wrong level. For example, you can say it's all the same QED vertex in the end.

They are fundamentally the same mechanism by our current understanding of physics. It is not a pedantic point.

> What I find more interesting is that today, all our energy ultimately can be traced back to stars, even if sometimes extremely indirectly (e.g. in the case of fossil fuels or nuclear fission). But this could actually change -- if nuclear fusion becomes used in practice.

Wow, that's a cool thought. In an attempt to continue, even human-made fusion energy is an extremely indirect output of the sun. Photosynthesis, animals, evolution, human history, and even fossil fuels enable the right conditions for more fusion energy. The system understands itself well enough to recreate its most massive and awesome force in miniature.

matter/antimatter collisions seen in particle accelerators is the most dramatic form of matter into light; the matter is completely consumed. Nuclear reactions are somewhat less dramatic, but still result in a substantial loss of mass.

Arguably we are not turning matter into light with classic black-body radiation; that is rather thermal energy turning into light.

I’m not sure if it’s what you’re asking, but one photon can give rise to two, lower frequency photons under certain circumstances:

https://en.m.wikipedia.org/wiki/Spontaneous_parametric_down-...

They already discovered how 80 years ago. This is them theorizing that they can do it with modern technology.

TL;DR: They still cannot do it.

This article started with such a bafflingly breathless tone about 80-year-old physics that I couldn't even finish it. Pair production is not exciting or controversial.

Pair production usually can't happen unless an atomic nucleus is nearby. This is necessary for the conservation of momentum. I don't think photon-photon pair production has been achieved yet.

I believe you, but can you explain in more detail?

The classic diagram in undergrad physics is of a single particle going in one direction converting to two particle/antiparticle pairs going off at angles with a total momentum equal to that of the first particle. Why can that situation not happen?

That can't happen if the incoming particle is a single photon. One way to convince yourself is to realize that a closed system consisting of a single photon is massless, therefore it cannot later contain a massive particle without violating conservation of mass (in the sense of E^2 - p^2).

Another way to think about it is that a photon contains the maximum amount of momentum possible for a given amount of energy. (E = p). If you transform any of its kinetic energy into rest energy, there's nothing that can carry the excess momentum.

However if the photon is in the presence of a massive particle, that particle can soak up the excess momentum, and pair production is possible. Alternatively if you have two photons, their momenta can cancel and pair production is again possible.

A fun physics problem is a photon striking an electron and producing an electron-positron pair; what was the minimum energy of the photon, in terms of electron rest energies? It's not two, for the reasons given above!

Thanks, that makes sense.

*By humans. It's almost certainly happening in particularly large stars, and around black holes, not to mention just... in the vacuum, although in that latter case it's annihilation as well.

Read that entire article to find out what type of matter turned out...only to realize that they have not actually created anything, or run the experiment.

So, any idea what type of matter would it be created?

>>electrons and positrons

dumb question: what happens to them? Suppose you get lots and lots of energy converted into electrons and positrons...do they go and forth with each other until they create something bigger? If so, what?

To be fair it's more like "turn matter into light and then into matter", since they still need to accelerate electrons first to produce the gamma photons.

They're not turning those electrons into photons; they're using the electrons to transfer energy from the accelerator into gamma photons. The electrons aren't annihilated in the process.

Thinking about it, that would mean they emit some kind of Bremsstrahlung, or not? If that is the case, I am wondering about the angular distribution of the emitted gamma photons... I would consider it rather random. This would beg the question, how do you focus and direct the gamma beam into the gold can?

SLAC, at least, works by deflecting the electrons, rather than by slowing them down via Bremsstrahlung.

https://lcls.slac.stanford.edu/overview

The result is a nice, tight, collimated beam of hard X-rays. Pretty awesome! But I'm not sure that SLAC has the power for this experiment - they probably would have done it already if they did...

I have a theory that it is possible to travel inside the light, most likely at close to the light speeds and travel thru universe literally on the beam of light scattering thru the space.

We know its possible for laser to have different spectrum and resonate with different speeds (imagine putting very close to each other million lasers and creating one large "tunnel", where outlayers vibrate at higher speeds thus creating "walls/ceiling/floor". That's probably the "easy" part. It might be super expensive in terms of used electricity to power such travel channel but tiny bits comparing to manned trip to Mars. Most likely the biggest problem here would be putting an object, or a person inside it and make it not accelerate to speed of light in split second (as light travels it creates a wave, similarly to a river creating stream and you getting in it are being pushed by it). The object put inside such laser tunnel (lets call it "lunnel") would have to be incredibly heavy so that it starts accelerating slowly and won't disintegrate hitting max speeds before reaching destination.

As a result of building such vehicle, imagine seeing a blimp or red light on the sky that least less than half a second. What happened? Oh, just another cargo delivery to Mars, no biggie.

One day I will write a paper on it, so it can bare my name.

The laws of physics and special relativity would like to have a... word... with you in the other room.

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