Let there be light, and then matter

[Kramodlog]

Scientist wanna make matter out of light. With lasers!

But researchers at Imperial College London say they have found a way to convert light into matter by using new equipment in the form of an extremely powerful high-intensity laser, capable of speeding up electrons to just below the speed of light.

 

[tomBitonti]

This seems to be a bit oversold.

The phenomenon was observed some time after 1933 by Patrick Blacket (or perhaps using his equipment), leading to his receiving the Nobel Prize in Physics in 1948:

http://en.wikipedia.org/wiki/Patrick_Maynard_Stuart_Blackett

My understanding is that electron and positron creation was observed in a cloud chamber using cosmic rays as the high energy photons.

According to this, pair production occurred with light from a man made laser in 2008:

http://en.wikipedia.org/wiki/Pair_production

With the Titan laser at Lawrence Livermore:

http://en.wikipedia.org/wiki/Titan_laser#Unique_facilities

A positron and electron, interacting, convert into two high energy photons. Two high energy photons, in the presence of a nucleus (which is necessary to absorb momentum to balance the reaction) can convert into an electron and a positron. The process has been known for quite a while. The problem is generating high enough energy photons.

Note that the "matter" in this case are a number of electrons and positrons, which are probably emitted in quite smallish numbers. Nothing special about the electrons. I would imagine the positrons are quite scarce, but are otherwise unremarkable.

Thx!

TomB

 

[Umbran]

Yeah, this is being a bit oversold in the media. It isn't like the Higgs, where we weren't already sure we could see it. We know this happens.

Now, if they intend to do ir *regularly*, to explore some of the details, that's awesome. But it making a positron-electron pair from photons isn't itself ground-breaking.

 

[kinem]

The cool 'new' part is they're doing it without the presence of a nucleus or electron; it's just x-rays colliding with each other. It's nothing surprising, though.

 

[Nellisir]

Two high energy photons, in the presence of a nucleus (which is necessary to absorb momentum to balance the reaction) can convert into an electron and a positron.

So you needed matter to make matter? And now they've figured out how to actually make matter without matter? That seems like a rather big difference to me.

 

[Umbran]

So you needed matter to make matter?

You can think of it kind of like a catalyst. The basic problem is that two photons colliding, and producing an electron-positron pair can't generally conserve momentum and energy at the same time. If you have an atom involved, it can take up the extra momentum.

And now they've figured out how to actually make matter without matter? That seems like a rather big difference to me.

Well, yes and no. You see, they still involve matter. Check out the article:

"The experiment would involve blasting electrons, sped up by lasers to just below the speed of light, into a slab of gold to create a beam of photons a billion times more energetic than visible light.

The next stage would need a tiny gold can called a hohlraum, German for 'empty room'.

Scientists would fire a high-energy laser at the inner surface of this to create a thermal radiation field, generating light similar to that emitted by stars.

They would then direct the photon beam from the first stage of the experiment through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. "

I would have to see the math, but I'm guessing that the thermal field as a collective whole can absorb the extra momentum.

 

[kinem]

The basic problem is that two photons colliding, and producing an electron-positron pair can't generally conserve momentum and energy at the same time. If you have an atom involved, it can take up the extra momentum.

Not quite. That's why a single photon can't produce pairs on its own. With two photons colliding, it's just the reverse process of an electron annihilating with a positron. The center of momentum moves slower than the speed of light, so momentum can be conserved.

The problem is the very tiny cross-section for two photons to interact. In classical electromagnetism, the waves wouldn't interact at all. In quantum field theory, they can interact by sparking the vacuum (aka Schwinger pair production), forming an electron-positron pair if enough energy is available.

 

[Mustrum_Ridcully]

So, when will we have our first matter replicators á la Star Trek?

 

[Nellisir]

Scientists would fire a high-energy laser at the inner surface of this to create a thermal radiation field, generating light similar to that emitted by stars.

It seems like the gold is necessary to set up the conditions for the reaction, not the reaction itself. That's rather like we need laser generators and electrical outlets and thumbs to set up the conditions.

In any case, y'all can be bored and say this is old hat, new ribbon; I'm going to continue to be impressed. It's more fun.

 

[Dannyalcatraz]

So, when will we have our first matter replicators á la Star Trek?

The 3D printers would seem to be that, in their infancy.