

Scientists discover how to turn light into matter after 80-year quest - Libertatea
http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_16-5-2014-15-32-44

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JacobAldridge
To be clearer, Theoretical Physicists have a theory about an experiment that
would allow Experimental Physicists to prove a theory made by earlier
Theoretical Physicists. They haven't actually conducted the Experiment.

 _" In Theory, there is no difference between Theory and Reality. But in
Reality there is."_

~~~
scott_s
I prefer the phrasing, _The only difference between theory and practice is
that in theory, there is no difference between theory and practice._

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frozenport
I'll count my chickens when they hatch, lets wait until they do it.

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Noxchi
So it's turning light into electrons? Isn't that already done with solar
panels?

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pjc50
No, solar panels turn light into the _change of energy state_ of an existing
electron. This is why they operate at the characteristic band gap voltage of
the semiconductor from which they are made.

This experiment is about creating an entirely new electron/positron pair.

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tannerc
Solar panels wouldn't be the ideal use of this ability if it were made real,
would they? I haven't looked through the full article yet, but would this new
approach help overcome the problem of Carnot's theorem in any way?

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pjc50
Carnot's theorem is irrelevant to solar cells, as they aren't expanding-fluid-
driven heat engines.

Multijunction cells are the way to go for increased W/m^2, but are very bad
W/$ at the moment.

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NickM
Probably a long way off, but I can't help but imagine the kind of amazing
applications something like this might have. If we could reliably capture and
store the resulting positrons, then recombine them with electrons later to
release the energy and harness it somehow, I'd imagine this could make for an
energy storage system with some really interesting properties.

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fixedd
Like the fact that light doesn't take up any space, so you could theoretically
store an infinite amount of energy?

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gliese1337
Sorry, doesn't work. Light still exerts pressure, so even if you could
losslessly trap light in a storage container indefinitely, the container would
explode long before the energy density became 'infinite'. Furthermore, the
energy and momentum in light contributes to the gravitational field, so even
if you could make an infinitely strong mirror-box, pumping too much energy
into it would eventually result in collapsing into a black hole.

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ximeng
Can anyone explain the vertical lines in the space time diagrams? Presumably
not infinite velocity?

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yk
They are not space time diagrams, they are Feynman graphs. Technically they
are a clever way to write down the terms of a series expansion. In the
convention used, on the left hand side are the incoming particles and the
outgoing particles are on the right. The internal lines and vertices are just
representations of formulas, and the important thing is only the topology of
the graph.

However one of the really great things of Feynman graphs is, that they admit a
easy interpretation about what is happening. Then the interior lines are
interpreted as virtual particles that are created during the reaction, but
since they are virtual they do not really have a defined speed, so the lines
are just drawn like that.

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jessriedel
In fact, Feynman diagrams _are_ spacetime diagrams. (They are commonly
computed in momentum space, i.e. Fourier space, but you can also do them in
normal position space.) When you draw a Feynman diagram, the vertices
correspond to actual points in spacetime, which are then integrated over a la
the path integral.

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ToastyMallows
How much energy is this photon/photon collier going to take? If a future
experiment works, could we use this created matter to turn around and use it
as energy? Or would that not be possible (i.e. - The energy used is far
greater than the energy gained)

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timnic
No, the energy gained will be equal or (most likely) less than the energy
used. This is because we have conservation of energy, so a process like

energy (the original photons) -> matter (electron/positron) -> energy

can not produce more energy than was put in.

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nkozyra
By no means is this my forte, but would this violate the conservation of mass?

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timnic
Mass is not a conserved quantity. Energy for example is a conserved quantity.
So like in a nuclear fission reaction we loose mass but this lost mass is then
converted into energy. I have not read the article in detail, but I suppose
they want to turn part of the photons energy into masses for the electron and
the positron. Mass and energy are linked via E = mc^2.

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millstone
In fact, both energy and mass are conserved. If you accept e=mc^2 as "true,"
it's easy to show.

If energy is conserved and mass is not, then what can E=mc^2 possibly mean? Is
it a relation, like F=ma? Then if E is constant and of course c is constant,
then m must be constant too.

Or is it an exchange rate: you can convert this much mass into that much
energy? But then energy is no longer conserved, if you can make it from mass.

Last try. Maybe e=mc^2 means, within a given quantity of mass, there is
necessarily a minimum energy? And this is true, but notice that the equation
is phrased as an equality. Equalities go both ways: we can just as easily
derive m=e/c^2, and now the "minimum energy" idea runs aground.

In modern interpretations, energy and mass are both conserved, and e=mc^2 is
understood to be the energy in the rest frame of the system. Individually
photons are massless, but multiple photon systems can and usually do have
mass. The price we pay for conceiving of mass as conserved and invariant is
that it no longer adds nicely.

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dang
A dupe of
[https://news.ycombinator.com/item?id=7763711](https://news.ycombinator.com/item?id=7763711).

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menubar
Sounds like this theory could lead to creating one heck of a weapon, Dr.
Venture.

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th0ma5
Black holes probably do this?

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dmead
there is no way this is not sensationalized.

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cLeEOGPw
Imperial mages learn conjuration.

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gregorkas
1\. grab a flashlight 2\. make your friend grab a flash light 3\. turn them on
and point them at each other 4\. make it rain

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tlholaday
My friend is so obstinate I cannot complete step 2, and now there is severe
drought.

