# I What turns energy into matter and vice versa?

1. Jul 22, 2016

### Cozma Alex

How does the universe decide if transforming energy into matter or vice Versa?
Is there a particle or a field that decides so?
Is a probability process?

2. Jul 22, 2016

### Orodruin

Staff Emeritus
Energy is not a substance that transforms into matter. Energy is a property of matter, just like momentum or mass.

3. Jul 22, 2016

### Cozma Alex

OK, maybe I said it wrong, with matter I was meaning mass

4. Jul 22, 2016

### Orodruin

Staff Emeritus
Yes, the answer is still the same. There is nothing being converted. Just energy changing form from some other form of energy to energy in the form of mass. This is no stranger than energy changing from potential energy to kinetic energy.

5. Jul 22, 2016

### Cozma Alex

Yes, but what decide whenever to convert or not?
For example why doesn't all the mass around us convert in another form?

6. Jul 22, 2016

### Orodruin

Staff Emeritus
You might as well ask why kinetic energy is converted into potential energy. It happens in nature and we have physical theories that describe how this occurs in a very good manner.

The answer is that nature simply foes not behave in that way. If you want a "deeper" answer you are no longer asking s physics question.

7. Jul 22, 2016

### Cozma Alex

Ok, thanks, so there's not a field or something in quantum mechanics theory that mediate this conversion process and "decides" how much is converted or not

8. Jul 25, 2016

### Khashishi

I think what you meant to ask is what turns photons into matter and vice versa. Both photons and matter have energy.
If that is the case, photons can turn into matter and antimatter in a process called pair production. And matter and antimatter can become photons in a process called annihilation. There is a certain probability of the former happening if a high enough energy photon hits matter. And a certain probability of the latter happening whenever a matter and antimatter particle are close to each other. The probabilities are given as cross sections--the larger the cross section, the more likely they are to react (or interact). The probabilities depend on which particles are interacting, and can involve lengthy calculations to compute.

It's almost certainly possible to collide two photons to produce an electron and positron, but rather difficult to demonstrate in a lab due to the low probability and high energy gamma rays needed.

Last edited: Jul 25, 2016
9. Jul 25, 2016

### Cozma Alex

You got the point, thanks, so it's a matter of probability that something turn into energy? For example in nuclear decay is there a probability that when the nucleus decays some mass turns into photons? It is possible to know before how much of its mass turn into photons?

10. Jul 25, 2016

### Zafa Pi

There is. The vase is sitting on the shelf Schrodinger's cat pushes it off thus converting potential energy to kinetic energy. Quantum mechanics does it again.

11. Jul 25, 2016

### Cozma Alex

So... there is a field, does the field act based on probability or is something that can be easily predicted, for example in a nuclear decay, can we predict the products of the decay and the mass that has transformed in photons?

12. Jul 25, 2016

### BvU

Decay goes from higher mass to lower - the difference goes into kinetic energy and rest energy of the other decay products. Nuclear decay tables are full of half-life and decay product energies info.

13. Jul 25, 2016

### Cozma Alex

But how can we predict these products? (Theoretically I mean)

14. Jul 26, 2016

### Khashishi

For a reaction to occur, it has to obey certain conservation laws, such as angular momentum conservation, momentum conservation, energy conservation. It usually also obeys approximate conservation laws like lepton number, baryon number, etc. To make a prediction, you have to list all the possible reactions, and calculate their probabilities. I can't help you with the details there.

15. Jul 26, 2016

### Tanelorn

How might a photon create, perhaps with other constituents, a proton?

Can other bosons create fermions?

16. Jul 26, 2016

### Khashishi

Yes, other bosons can create fermions.

As far as how, two photons come together and, bang, a proton and an antiproton come out. You have various Feynman diagrams depicting possible pathways for the reaction, but what actually happens is some superposition of possible pathways. You can't say exactly what happens between what goes in and what comes out.

17. Jul 27, 2016

### Tanelorn

ok thanks I didn't know that Protons were also possible, I thought it was only electron positron pairs.
So it is theoretically possible to collide photons as a way of making a proton source?

18. Jul 27, 2016

### Khashishi

All particles are theoretically possible, as long as various conservation laws are obeyed. Theoretically possible, but not practical.

19. Jul 28, 2016

### Tanelorn

Thanks, it really makes one realise that every form of matter and energy is made of the same thing just in a different form.
And it all started with a maximum density, homogeneous form of matter/energy right at the start of the BB.

20. Jul 28, 2016

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