exchemist

I wonder if this may be about virtual photons as "force carriers" in magnetism. They do that, don't they, in QED? (I never learnt this stuff).

The figure derived from 2.5cm/secs is only (if my arithmetic is good) the increase in k.e. from a single absorption event, so yes, obviously to explore what happens in laser cooling is a lot more involved. But I was simply intrigued to see how significant the translational energy input is, compared to the energy of the photon, and what the effect on the temperature of a spectroscopic sample might be. I suppose since k.e. goes with the square of the speed, the kinetic energy transfer from the same amount of momentum to a lighter molecule, say hydrogen, would be somewhat greater.

This all has an air of Reiku/Graviphoton/Gareth Meredith...... and all stations to Cardiff......about it.

OK I've had a go at this, taking gas phase sodium atoms as an example, since I see that has been used in laser cooling experiments. My arithmetic may need checking but if it's right it goes like this: Take gas phase sodium atoms, absorbing at the D line. Wavelength of D line, λ = 590nm Momentum, p = h/λ (de Broglie) => p ~ 6.6 x 10⁻³⁴/5.9 x 10⁻7 ~ 10⁻²⁷ kg-m/sec = Δmv for a sodium atom absorbing a D line photon. ------------------------------ Mass of a sodium atom is 23g/mol, i.e. 0.023/6 x 10⁻²³ ~ 4 x 10⁻²⁶ kg. So for the absorbing atom, Δv = 10⁻²⁷/4 x 10⁻²⁶ = 2.5 x 10⁻² m/sec. Hence the increase in translational k.e. will be: 1/2 . 4 x 10⁻²⁶ . ( 2.5 x 10⁻²)² = 0.5 . 4 x 10⁻²⁶ . 6.25 x 10⁻⁴ = 1.25 x 10⁻²⁹ J ---------------------------- Translational k.e. = 3/2 . kT. So ΔT due to extra momentum = 2/3 . ΔE/k = 2/3 . 1.25 x 10⁻²⁹/1.4 x 10⁻²³ ~ 6 x 10⁻⁷ Deg. This is a “temperature” increase, for each atom that absorbs a photon, of under a millionth of a degree. Expressing that in terms of the energy of the photon, that energy will be hν = pc = 10⁻²⁷ . 3 x 10⁸ = 3 x 10⁻¹⁹ J. So the proportion of its energy that contributes to thermal k.e. is 2.4 x 10⁻¹⁰ , so almost infinitesimal. Now I understand why this is never mentioned in spectroscopy.

Of course. That's what I meant when I referred to collisional relaxation. What I have been trying to get at is whether radiation can directly stimulate the translational degrees of freedom. I think the answer is no, save for this issue of momentum transfer when a photon is absorbed by another form of excitation, which @swansontput me onto. (By the way, this whole area of relaxation from excited states, esp. the mechanisms for the redistribution of thermal energy among the available degrees of freedom, is something that seems to me to be quite complex and often rather glossed over, in particular how electronic excitation comes to be redistributed into vibration, rotation and translation.)

Sure, but that's just the redirected blue wavelength photons. I don't think the effect can be much because this is never discussed in spectroscopy. In principle what you are saying is that the absorbed photon has to do two things: excite a transition to an excited state AND contribute to an increase in translational k.e. But we ought to be able to calculate the k.e. increase from the momentum, shouldn't we? For the photon E=pc = hν.

Do you think Rayleigh scattering can produce a significant heating effect, then?

I have a memory of a sci-fi short story I read as teenager in the 1960s, in which, after a spacecraft crashes on an alien world, 4 members of the crew regain consciousness to find their bodies, apart from the brain and spinal cord, have been consumed by a sort of protoplasmic organism. The organism however supports and hosts the brain and spinal cord, as it is useful to its existence to have a directing mind. All four are in one blob and are able to communicate telepathically within the blob. The blob is shape-shifting and can take on forms instructed by the brains it hosts. So they can "make" arms, for example, by a process of mind control. The 4 crew members argue, fight and two eventually are killed, while the remaining two separate and try to reconstruct their human forms. I don't remember the author but I think the title was "Four In One". I've searched the internet for this without success. Does anybody recognise this story from my description and, if so, can you provide any more details about it? Update: I've found it: Damon Knight, 1953. There is even a pdf of the story: https://epdf.tips/four-in-one.html So now the question is different: does anyone else but me know this story? It's one that made quite an impression on me at the time, so that I still remember it, more than half a century later.

OK. The issue I am trying to bottom out is by what means, if any, radiation can transfer energy to the translational motion of molecules, as part of a heating process. To do that, energy has to be lost by the radiation and gained by the matter it is interacting with. That is the energy loss I am talking about. I started from the viewpoint that radiation can't directly stimulate translational motion, as there was no dipole for it to couple to. What you have pointed out is that when a photon is absorbed, it not only excites the molecule to a new internal energy state but also imparts its momentum to it, adding to its kinetic energy of translation. That's fine and seems to confirm my original thought that translational excitation directly, on it own, is not a possible process: it has to be a byproduct of an absorption due to excitation of some other degree of freedom rotation, vibration or electronic). Rayleigh scattering won't add kinetic energy to the scattering particle, presumably, as the wavelength of the scattered photon is the same as that of the incident photon.

Surely elastic means with no energy loss, does it not? In which case, if there is a net energy transfer from the radiation to the matter, the process is not elastic - and there has to be a net frequency shift. But, the issue of whether we call it elastic or inelastic aside, what I take out of this is that if photons are absorbed by matter, there will be a translational energy gain, in addition to the excitation responsible for the absorption. However radiation can't alter the translational kinetic energy of molecules alone - it has to be a side effect of absorption (or emission). Is that fair?

OK fair enough. So any absorption process will also, as a side effect, alter the translational kinetic energy of the entity that absorbs the photon. This is not elastic scattering, however.

In the visible, photons are absorbed by electronic transitions, creating excited states that relax by non-radiative means (i.e. intermolecular collisions) to populate vibrational and rotational excited states - and thence eventually translational motion. In the IR, they are absorbed by vibrational states, which then similarly cascade down by collisional relaxation. None of these radiative processes, so far as I know, can directly stimulate excitation of translational motion. The exceptions would be ionisation or bond breaking, where the absorption causes an electron to jump right out of an atom, or two part-molecules to fly apart because the bond strength has been exceeded. It seems to me you need a transition dipole moment in order to absorb a photon and I can't see how you get that in translational motion. So I think, subject to correction by one of the physicists here, that translational excitation is always the consequence of collisions, rather than radiation directly.

Surely elastic scattering, by definition, does not transfer any energy to the medium doing the scattering? If it idid, it would not be elastic, would it? If there were energy transfer there would have to be either absorption of photons or a change in frequency. This is a question of energy rather than momentum. Inelastic (Raman) scattering involves excitation of vibrational and rotational modes in the molecules and a concomitant change in frequency of the radiation. My issue is whether there is a process by which EM radiation can directly excite translational motion in molecules. I cannot think of such a process - but then I am a chemist, of course.

Oh you mean transfer of momentum by inelastic (i.e. Raman) scattering?

Can it in all cases, though? Rotational and vibrational excitation can change by absorption or emission of radiation, but translational motion? Doesn't that have to change by heat flow?

No, enthalpy would include various forms of internal potential energy associated with chemical bonding etc. One can only measure changes in that, rather than absolute values. I am thinking here of kinetic theory. What I mean is thermal energy, i.e. the famous 3/2RT at constant volume for monoatomic gases or 5/2RT for diatomic gases, due to thermal kinetic energy of the molecules. This does have an absolute value which goes to zero at absolute zero. (Though there is still some residual zero point motion at absolute zero, this is by definition unextractable.)

OK but surely one can define "heat energy" or "thermal energy" as a subset of total internal energy, meaning that portion of the internal energy due to kinetic energy of molecules, which is the same as that portion of it that can be made to flow out by means of a temperature gradient. Heat energy, so defined, is reduced to zero at absolute zero, is it not?

If it's gallium, it will turn into a puddle on a hot day in summer.

But isn't that just because we now speak of internal energy, and use "heat" to describe the flow of internal energy rather than the energy itself? I should have thought one could say that a body at absolute zero has no heat energy left in it. All that's left is is zero point energy, and various kinds of potential energy, including rest energy, none of which is extractable as heat.

Looks like ballocks to me: I don't see how a real robot, i.e. with a microprocessor capable of making decisions and mechanically executing them, survives liquefaction. But here's the abstract of the published paper (have to pay to get the whole thing): Magnetically actuated miniature machines can perform multimodal locomotion and programmable deformations. However, they are either solid magnetic elastomers with limited morphological adaptability or liquid material systems with low mechanical strength. Here, we report magnetoactive phase transitional matter (MPTM) composed of magnetic neodymium-iron-boron microparticles embedded in liquid metal. MPTMs can reversibly switch between solid and liquid phase by heating with alternating magnetic field or through ambient cooling. In this way, they uniquely combine high mechanical strength (strength, 21.2 MPa; stiffness, 1.98 GPa), high load capacity (able to bear 30 kg), and fast locomotion speed (>1.5 m/s) in the solid phase with excellent morphological adaptability (elongation, splitting, and merging) in the liquid phase. We demonstrate the unique capabilities of MPTMs by showing their dynamic shape reconfigurability by realizing smart soldering machines and universal screws for smart assembly and machines for foreign body removal and drug delivery in a model stomach. Perhaps someone else here can explain what this actually does. The reference to "locomotion speed" is intriguing.

Well, Montmartre is where the ladies of the night used to hang out.........

You mean like Evelyn, Beverly, Vivian, or Leslie/Lesley? I know Sidonie is a French girl's name, but that has 3 syllables. Perhaps Sidney for girls is a variant of that. But it sounds weird to my ears, I must admit. I suppose the geeks might have deliberately picked an androgynous name.

I suppose it's a trivial observation, compared with the scandal of not backing off from disruptive intrusion into someone's human relationships, but it also seems tone deaf to a person's likely sexual orientation, given that he is married to a woman and Sidney is a man's name. "Nul points" to the guys with spiky hair on this one.

I just wish these geeks would put half the effort they waste on this stuff into controlling the dissemination of falsehoods. Haven't they damaged society enough, without looking for new ways to do even more damage?

Yes, it doesn't bother me too much either. I lived in The Hague for a few years and came to appreciate Dutch directness. I also rowed for many years and am used to being coached, and then there is choral singing, in which coaching can also be fairly direct! I find your posts clear, knowledgeable and informative. And indeed, I think we are all here to learn, not just to pontificate.