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Comment by sdfghswe | Hacker News Reader

Why is it surprising? Light can transfer momentum into the water, which is was matters for the process of evaporation...?

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Isn't 'transfer momentum' just a round about way of saying "heat up."

I think the interesting aspect here is that the evaporation is greater than what can be explained by heat alone.

Yeh I read it as though it’s a form of catalyst. Ultimately the light is changing the threshold energy at which water can evaporate.

Similar to the photoelectric effect. Similar say to an enzyme.

All these environmental changes to the reaction lower the ‘action’ energy making the reaction vastly more efficient or possible in an environment that it wasn’t possible in previously.

Enzymes catalyze the breaking of molecular bonds.

The embodied energy of a phase transition does not pay attention to what path you took to cross it. There’s no “threshold” between phases of water.

For H20 to move from liquid water to vapor, energy must be added. There’s no catalyst.

So either we’ve discovered some new physics since I last studied thermodynamics, or this isn’t an accurate analogy.

It could be that the energy required to break the hydrogen bond from random collisions is higher than the theoretical minimum because the angle of a typical collision "wastes" energy in imparting vibration or rotation of the water molecule rather than just imparting "escape velocity".

Just a wild guess though. Haven't yet read the article.

> For H20 to move from liquid water to vapor, energy must be added.

I think that is a wrong assumption. Liquids will naturally evaporate even with 0 external energy, assuming there is not too much pressure in the surrounding atmosphere.

The equilibrium partial pressure of water vapor with a source of water in another phase is not zero for most temperature and pressure regimes on earth, that is true.

But thermodynamics still hold; the water vapor is still in a more energetic state than liquid water or ice.

Energy levels are one thing, but liquids are just a fundamentally unstable state of matter. Absent external pressures, they will either evaporate or freeze depending on temperature. Most likely, with very low atmospheric pressures such as in the vacuum of space, the outer parts will quickly vaporize, consuming energy from the inner parts which will then freeze.

Basically, some amount of a liquid will move to the higher energy state, and other parts will move to a lower energy state. The energy to vaporize some of the liquid doesn't need to come from something external to the liquid.

> Light can transfer momentum

Momentum is mass x velocity; what’s the mass of a photon?

You can't use classical physics to calculate the momentum of a photon.

https://en.wikipedia.org/wiki/Photon#Relativistic_energy_and...

Classical electromagnetic waves from Maxwell's equations (i.e. non-quantized) also carry momentum, right?

https://www.youtube.com/watch?v=bvzr2HbbPC8

(Maxwell's equations are consistent with relativity)

...or another way of looking at it (that I presume Boltzmann would agree with). If your had a single black body mass at some temperature greater than absolute zero in an otherwise empty universe, it would radiate away heat and thus cool off. The cooler body means the individual atoms in the mass have less energy and less momentum. If momentum is conserved, then that momentum must have been carried away from the mass in the mass-less radiation. Another neat thing is that light can also have angular momentum.

> > Light can transfer momentum

> Momentum is mass x velocity; what’s the mass of a photon?

Photons have zero mass. What's your point?

E = ħf = mc^2.

For photon,

p = hλ

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