# Why Is radiation the highest form of entropy?

1. Dec 23, 2015

### memento

Hello everyone, I have heard in a sminar that radiation is the highest form of entropy. Why?
I was wondering too if all matter would transform at the end in radiation somehow

2. Dec 23, 2015

### Demystifier

Because photons have zero mass.
Larger entropy means larger number of different states with given energy. The smallest possible energy of a given particle is equal to its mass (times $c^2$), so when particles do not need to "spend" a part of their energy for having mass, more energy remains for contributions to the entropy.

3. Jan 16, 2016

### memento

Thank you. Do you know how is the entropy involved in combustion (the heat emited in form of radiation)? More exactly, why are the photons entangled in this procces?

4. Jan 18, 2016

### Demystifier

Entanglement is related to the conservation of energy, momentum, and angular momentum. For example, if a radiated photon has certain energy $E$, then the source must have energy reduced by the value $-E$. This means that their energies are correlated. For quantum systems, the correlation is described by entanglement.

Is your question motivated by a desire to understand radiation by black holes?

5. Jan 18, 2016

### DrDu

You can't say this in general. For example, laser radiation has very little entropy.

6. Jan 18, 2016

### Demystifier

7. Jan 18, 2016

### sciencejournalist00

Because it is dispersed in a very large volume. Unlike it, laser is highly directional.

8. Feb 16, 2016

### memento

Not exactly, but the physical meaning of the conservation of information in quantum mechanics. If something related with black holes or anything cast light to the subject, please let me know,

Thanks

PD: Following the reasoning of high entropy of ligh I suppose that conversion of mass into light is spontaneus, why don´t we see it?

9. Feb 17, 2016

### Demystifier

We do see it, but it is perhaps not so obvious because the process of mass reduction is usually quite slow.

10. Feb 17, 2016

### DrDu

There are also conservation laws for charge, baryonic number etc. which don't allow for the conversion of mass into light.

11. Feb 17, 2016

### A. Neumaier

This is a misunderstanding. Entropy is not conserved in Nature.
is something completely classical, unrelated to entanglement.

12. Feb 17, 2016

### A. Neumaier

This is impossible since baryon number is conserved.

13. Feb 17, 2016

### Demystifier

This is both true and wrong, depending on the precise definition of entropy.
For example in classical statistical mechanics, fine grained Gibbs entropy is conserved but coarse grained Gibbs entropy is not conserved.
Similarly in quantum statistical mechanics, von Neumann entropy of a closed system is conserved, but von Neumann entropy of an open system is not conserved.

14. Feb 17, 2016

### A. Neumaier

But post #1 makes sense only in the context of macroscopic (hence coarse-grained), increasing entropy. (Even then only superficially, since it ignores conservation laws that are actually valid.)

15. Feb 17, 2016

### memento

16. Feb 17, 2016

### Demystifier

I agree. But to understand how can it be compatible with conservation of information in QM, one has to have in mind the existence of different kinds of entropy.

17. Feb 17, 2016

### memento

Really? Could you give me more information about the velocity of this process, a formula would be nice, thank you

18. Feb 17, 2016

### memento

I supposse this conservation law don´t allow convert light into matter, but this is FALSE, since in 2014 was announced that this year we will see it!

19. Feb 17, 2016

### Demystifier

I'll not give you a formula, but a physical idea. The Sun needs a few billion years to convert a significant fraction of its initial mass into light.

20. Feb 17, 2016

### DrDu

Before judging, I would propose at least to wait whether this prophecy becomes reality. Btw. from whom is this announcement?