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June 25

How does "force carrier" work for magnetism?

A photon is the force carrier for the electromagnetic force… how does that work for a magnet?

I can (or I think I can) imagine a photon carrying a radio wave, hitting an aerial, and then generating a current or field.

I can't imagine a photon carrying a magnetic field. If a (solid bar) magnet is static, do the photons travel around the magnet… get re-absorbed and re-emitted?

How is a bit of metal attracted (or repelled) by photons? What happens to the photons? Why isn't the strength of the magnet diminished? (I know that's like the action of gravity… but if it's a particle… or something… how is it interacting, and how is it perpetual?) MBG02 (talk) 13:29, 25 June 2023 (UTC)[reply]

A photon does not "carry" a radio wave. Depending on its wavelength, it is a radio wave, that is, an excitation of the electromagnetic field. There is no classical macroscopic explanation for the forces arising from the exchange of virtual particles, in this case photons. For a derivation of the macroscopic forces from the quantum picture, see Static forces and virtual-particle exchange. --Lambiam 14:07, 25 June 2023 (UTC)[reply]

Unless you're dealing with a truly microscopic bar magnet, a quantum description is needlessly complex. Maxwell's laws will do just fine. But it may be interesting to see how a quantum approach can lead to the same result. PiusImpavidus (talk) 08:47, 26 June 2023 (UTC)[reply]

The question posed is not, "What laws do describe magnetism?". It is, specifically, "How can I understand the role of the force-carrying photons in magnetism?". (Also, to explain how a magnet attracts a nail from first principles using Maxwell's laws as the point of departure is not a trivial exercise.) --Lambiam 09:11, 26 June 2023 (UTC)[reply]

The OP's question is a non-starter if one tries to perceive the Photon as a material particle. That view misguides one into expecting ballistic behaviour of the photon such as bouncing collisions, remaining inert or rolling around sated after an impact or being subject to Newton's laws of mechanical motion. The term "particle" is only loosely applied to the photon which has (at rest) no mass or material. Following Maxwell's formulation of electric and magnetic fields as components of one electromagnetic phenomenon and the demonstrations by Hertz of the reality of electromagnetic waves, experiments have isolated the fundamental wave energy packet or quantum, named the photon. This exhibits a few but not all properties of a particle, see the elaboration in the article Wave–particle duality. I suggest it as useful to the OP to regard the photon as nothing more than an insubstantial tiny scenario in which energies obey Maxwell's equations, possibly eternally in space. That view does leave unanswered questions such as Q1: Where are the photons responsible for the Earth's magnetic field (and what are their energy or frequency)? and Q2: How to explain the correspondence between energy and frequency in photoelectrically emitted photons? Pursuit of these questions led to the full development of Quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, Paul Dirac and others. Philvoids (talk) 22:19, 26 June 2023 (UTC)[reply]

Solid matter and heat death

Melting a solid would increase its entropy by the second law of thermodynamics.

So, would all the solid matter in the universe eventually melt away by the heat death of the universe? GeoffreyT2000 (talk) 14:28, 25 June 2023 (UTC)[reply]

No. In physics, the term "heat" is loosely used for thermal energy – also a loose term – as distinct from "available" energy – energy that can be used to do work. Heat engines do not work on heat but on heat differences. If all energy has become thermal, and the universe is in thermal equilibrium, no more work can be done. There are several theories for the ultimate fate of the universe. The one referred to as the heat death of the universe is also known as the Big Freeze. That should give you a clue it won't be very hot. --Lambiam 17:39, 25 June 2023 (UTC)[reply]

Indeed. If you take "heat" to mean "heat differences", it's "heat death" because there won't be any. Isaac Asimov wrote a story about this. {The poster formerly known as 87.81.230.195} 90.197.177.243 (talk) 19:18, 25 June 2023 (UTC)[reply]

Also, although the average kinetic energy of the gas filling space will be low, the energy distribution of the particles is such that there will always be, every now and then, a few particles with very high energy hitting a solid object, dislodging an atom or two. It will take eons, but eventually the object will sublimate completely. In the end there will be no solids – or even complex molecules – left at any appreciable scale. --Lambiam 09:24, 26 June 2023 (UTC)[reply]

Atmospheric pressure effect on blood pressure readings

Does high and low atmospheric pressure distort the readings of blood pressure monitors towards higher or lower than normal values? That said, assuming that healthy blood pressure averages for a given age and sex group are actually fixed to some atmospheric pressure value they were measured at, would the increase or decrease of atmospheric pressure distort the normal blood pressure value range of a person being measured? Not seeking a medical advice, just curiosity. 212.180.235.46 (talk) 21:21, 25 June 2023 (UTC)[reply]

I'm not seeing anything in Blood pressure that says anything about altitude. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:00, 25 June 2023 (UTC)[reply]

"Blood pressure can also be affected by a sudden change in weather patterns, such as a weather front or a storm. A body — and blood vessels — might react to abrupt changes in humidity, atmospheric pressure, cloud cover or wind in much the same way it reacts to cold. These weather-related variations in blood pressure are more common in people ages 65 and older" - from mayoclinic. manya (talk) 05:30, 26 June 2023 (UTC)[reply]

I found this Heart Asia article which answers the question. They conclude "no", but I can't understand their reasoning:

blood pressure monitors measure relative or so-called gauge pressure, which means that measurements are performed with respect to the surrounding atmosphere.

That sounds to me as if the conclusion should be "yes, definitely". If they're measuring blood pressure relative to atmospheric pressure, they must give different readings when atmospheric pressure is different. It even sounds as if they're designed to do that. But then they say there is no reason to assume that altitude and/or lower barometric pressure will have any effect on their accuracy. Some part of the explanation is missing. Possibly the missing part is what manya found, which is that both the blood and the blood pressure monitor are affected by altitude, and thus they cancel out. Card Zero (talk) 05:35, 26 June 2023 (UTC)[reply]

If the blood-pressure homeostasis of an individual is relative to ambient pressure, the readings of blood-pressure monitors that measure relative pressure should not be affected. I did not manage to find sources that give a clear answer to the question whether this homeostasis is or is not relative to atmospheric pressure. --Lambiam 08:57, 26 June 2023 (UTC)[reply]

If I dive to the bottom of a swimming pool, my absolute blood pressure must increase, or it would be less than ambient pressure, leading to collapse of my circulatory system. Humans have no difficulty diving 3 metres. Blood pressure is measured as a gauge pressure, relative to ambient, and so does the control mechanism in the body too. It cancels out (and is the easiest way to measure). Gauge blood pressure is fairly low compared to ambient pressure in most of the body of most animals (giraffes excepted) and most of the time, what matters most is controlling the pressure difference across the walls of the blood vessels. Absolute pressure only matters in relation to decompression sickness.

Weather events could trigger the body to change the setpoint for the gauge pressure. PiusImpavidus (talk) 09:30, 26 June 2023 (UTC)[reply]

To emphasise PiusImpavidus's point, consider a SCUBA diver at, say, 40m in the sea where the absolute pressure is close to 5 atmospheres. With systolic pressure being 100–200 mmHg, that is about 0.2 atmospheres. Commercial saturation divers operate in the range 100–200m meaning that the human body will operate quite happily for extended periods at ~~200~~ 20 atmospheres. Martin of Sheffield (talk) 09:57, 26 June 2023 (UTC)[reply]

at 200 meters depth, it would be 21 atmospheres, not 200. Still, that's a 1000 times our "normal" blood pressure Rmvandijk (talk) 11:59, 28 June 2023 (UTC)[reply]

Quite correct, but as you point out massively more than blood pressure. Thanks for the correction, Martin of Sheffield (talk) 08:36, 29 June 2023 (UTC)[reply]