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

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

www3.imperial.ac.uk

117 pointsLibertatea45 comments

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."

I prefer the phrasing, The only difference between theory and practice is that in theory, there is no difference between theory and practice.

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

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

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.

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?

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.

Solar panels add momentum to existing electrons.

This is actually quite exciting.

I see what you did there.

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.

It is an interesting idea, but there are other ways, probably easier, to get the positrons. For example, some radioisotopes emit positrons as part of their natural radioactive decay process. See:

http://arstechnica.com/science/2011/02/positrons-at-center-o...

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

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.

The idea is to store the positrons, and positrons aren't light.

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

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.

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.

Yes, it's infinite velocity. When you compute an amplitude from a Feynman diagram, you sum (integrate) over intermediate particle paths that are superluminal. The math is structured in a way that these contributions cannot actually be used to send signals, but the contributions are crucial and cannot be discarded. Essentially, they represent the relativistic generalization of the effect where one particle in a Bell pair "influences" another over a spacelike separation, but in a subtle and spooky way that cannot produce superluminal signaling.

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)

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.

In cases like this energy is not really what you want to talk about as that's always constant it's USEFUL ENERGY which is interesting.

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

Mass by itself is only conserved in nonrelativistic classical physics. When you are in a relativistic regime the conserved quantity is a combination of momentum and energy. (You've probably seen E = mc^2, which shows that mass and energy can be transformed into each other.)

Conservation of mass is not applicable in the conditions under discussion, see Wikipedia's fun list of exceptions to conservation of mass: http://en.wikipedia.org/wiki/Conservation_of_mass#Exceptions...

Nope. As energy equals mass times the speed of light squared you're just converting energy into matter. It's just the opposite of what the atomic bomb does.

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.

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.

Wiki:

The law of conservation of mass, or principle of mass conservation, states that for any system closed to all transfers of matter and energy (both of which have mass), the mass of the system must remain constant over time, as system mass cannot change quantity if it is not added or removed.

Wait, what? Photons have no mass, but they have energy. Ergo, that "both of which have mass" is flat-out wrong. Methinks wikipedia has problems.

Photons have no rest mass, but one of the key points in GR is that mass and energy are interchangeable by E=mc^2. Mass-energy equivalence is how Fission bombs work.

"Relativistic mass" is one of those concepts that was considered, bandied about in the 70s for a while, then ultimately discarded as not helpful.

Photons have no mass, period. An ensemble of photons may have a non-zero invariant mass, but there's just no way in which it's correct to say a photon has a mass.

This has nothing to do with saying that you can't convert mass to energy. Electron-positron pair creation from ultra high energy gamma rays has been a known thing for a long time. Pulsars are thought to run partly off of pair creation off the magnetic field, etc.

Stupid question maybe, but if you can create mass from photons, wouldn't that start to affect the shape of the universe if it happened enough? Could you use this, in theory, to turn an ever expanding universe into one that contracts? Just keep turning energy into mass?

Nope. In GR, energy has as much "gravity" as the equivalent mass, so it won't affect anything. (Energy is more tied to the expanding universe, though, as the wavelength increases too, so it's an important distinction in early-universe cosmology.)

Photons have zero rest mass.

Rest mass is mass. Photons have zero mass. "Relativistic mass" is more confusing than helpful and is generally not used in modern GR.

Cite: ask any actual physicist. I hate to appeal to authority, but this is the view I was taught in grad school (PhD, UCB, 2001), and I have never spoken to anyone in physics or astrophysics who thought "relativistic mass" was helpful.

I'm entirely on your side on this (PhD, UChicago, 2006), but there are still a bunch of physics teachers out there (mostly not doing research in related fields, is my rough impression) who will vigorously defend teaching the "relativistic mass" concept. They raise some decent points, but in my opinion none of it comes close to outweighing the arguments in favor of treating mass as an invariant property.

If this is a recent terminology change, then people who use the old terminology are not factually "wrong", they just use an outdated definition.

I do have a question though, if you or anyone else wants to clear this up for me. Which kinds of energy are no longer considered part of a system's "mass"? I gather that kinetic energy is not. What about various forms of binding energy?

Only kinetic energy does not contribute to mass. In fact, it's even narrower: only the kinetic energy of the center of mass does not contribute. For example, heat energy is really just kinetic energy of the constituent particles, and heat definitely contributes to mass.

The essential idea is that mass is Lorentz invariant, which means it does not change under changes of coordinate systems. So mass must be independent of velocity and orientation, which are just artifacts of how you measure anyways.

A dupe of https://news.ycombinator.com/item?id=7763711.

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

Black holes probably do this?

there is no way this is not sensationalized.

Imperial mages learn conjuration.

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

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

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