
Scientists discover how to turn light into matter after 80-year quest (2014) - felipemnoa
http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_16-5-2014-15-32-44
======
splittingTimes
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-...](https://www.researchgate.net/publication/270858006_A_photon-
photon_collider_in_a_vacuum_hohlraum)

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

~~~
PoachedSausage
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](https://en.wikipedia.org/wiki/Diamond_Light_Source)

------
ineedasername
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...](http://curious.astro.cornell.edu/about-
us/137-physics/general-physics/particles-and-quantum-physics/805-how-are-
photons-created-and-destroyed-advanced)

~~~
nhaehnle
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.

~~~
kobeya
> 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.

~~~
eternauta3k
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.

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

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graphememes
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.

------
nategri
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.

~~~
whatshisface
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.

~~~
aidenn0
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?

~~~
ridiculous_fish
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!

~~~
aidenn0
Thanks, that makes sense.

------
tryingagainbro
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?

~~~
felipemnoa
>>electrons and positrons

~~~
tryingagainbro
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?

------
grondilu
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.

~~~
samcheng
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.

~~~
splittingTimes
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?

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

[https://lcls.slac.stanford.edu/overview](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...

------
joering2
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.

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

