2817: Electron Holes

Explain xkcd: It's 'cause you're dumb.
Revision as of 00:00, 3 November 2023 by 162.158.74.49 (talk) (Undo revision 327803 by 172.71.174.149 (talk) They aren't positrons. They're explicitly electron holes. Which are different. Perhaps describe positrons and why this *isnt* them...?)
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Electron Holes
They tried to report me to the authorities, but because I had the device they couldn't charge me.
Title text: They tried to report me to the authorities, but because I had the device they couldn't charge me.

Explanation

An electron hole is a quasiparticle denoting a lack of an electron. These are fundamental in the theory and design of semiconductors and discussed in many educational tracks regarding electronics engineering. Referring to a beam by what it seems to be doing in simplistic terms, is not typical terminology,[citation needed] otherwise we might refer to "shadow beams" instead of "destructively interfering photon beams" and a "nonmagnetic field" instead of a "magnetically shielded" space. It should be noted however, that the equivalent of a hole in the QED vacuum is a real particle, known as the positron. See the Dirac sea.

Wikipedia gives a good basic explanation of the concept of the "electron hole":

[A]n electron hole (often simply called a hole) is a quasiparticle denoting the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or crystal lattice the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, the absence of an electron leaves a net positive charge at the hole's location.

Holes in a metal or semiconductor crystal lattice can move through the lattice as electrons can, and act similarly to positively-charged particles. They play an important role in the operation of semiconductor devices such as transistors, diodes (including light-emitting diodes) and integrated circuits. If an electron is excited into a higher state it leaves a hole in its old state. . . .

In solid-state physics, an electron hole (usually referred to simply as a hole) is the absence of an electron from a full valence band. A hole is essentially a way to conceptualize the interactions of the electrons within a nearly full valence band of a crystal lattice, which is missing a small fraction of its electrons. In some ways, the behavior of a hole within a semiconductor crystal lattice is comparable to that of the bubble in a full bottle of water.

In this cartoon, the physicist is upset that the idea of the electron hole beam doesn't "make sense" – because a beam consisting purely of things that are "missing" doesn't seem possible; electron holes only exist in the context of a background field of electrons in which just a few are missing. Thus, an "actual" ray would have caused a travel of electrons in the opposite direction– yet the beam is still working to destroy her belongings (or at least create dramatic visual effects). Eventually she resorts to simply exclaiming "Stop it!", humorously more due to the beam being made of quasi-particles than because it's destroying her belongings.

The caption below the comic states that physicists, plural, were angry about this device, implying that this is not the first physicist whose lab he has interfered with. Considering his history of having silly hobbies and that he mentions it is his device in the caption, it must be Randall who managed to create this device. The physicists are also likely more angry that they are being attacked by quasi-particles somehow, instead of just being attacked by comparatively conventional weapons.

The title text plays on a double meaning of "charge". In the comic panels, "charge" refers to an electric charge. When the word is used with "authorities", it's an accusation. However, the title implies that his device can not only negate the ability to apply an electric charge (by recombining the applied electrons with his "electron holes"), but can also prevent the authorities from applying the legal sort of "charge" to him – perhaps by creating even-more-outlandish "prosecution holes".

Transcript

[Ponytail, who has her fists clenched and a black smoke cloud above her head, is standing in front of a desk, a beam of electron holes is being fired at a printer on the desk; the beam is shown reacting to the printer, dispersing lightning bolts and 'particles' but causing no obvious damage. There are little '+' signs distributed along the beam and in the circles around the printer, though they're much easier to see in the higher-resolution version of the strip that's displayed if one zooms in on the original comic page.]
Ponytail: This doesn't even make sense! They're quasiparticles, not real-
Off-panel voice: Pew pew pew
Ponytail: Stop it!
[Caption below the panel:]
Physicists got really mad about my device that fires a beam of electron holes.


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Discussion

I must admit... I'm not entirely convinced that one _couldn't_ build an electron hole beam. It would probably be called a quasibeam, but I think it could be done. 162.158.175.61 05:11, 19 August 2023 (UTC)

You could clearly do this if you fired a beam of physical material to carry the holes. You could also find a way to stimulate the production of holes at a distance, maybe by inducing static charge with electromagnetic emission. 162.158.62.192 00:49, 21 August 2023 (UTC)

Would an electron "vacuum" be an electron hole gun? Barmar (talk) 05:30, 19 August 2023 (UTC)

I think so! You’d create holes to pull the electrons, and the holes would travel away from the gun to the electron source as the electrons traveled to the gun. This is likely exactly what Randall is depicting. Could be wrong. 172.70.230.36 14:32, 21 August 2023 (UTC)

Is it worth mentioning that "pewpewpew" was one of the incorrect pronunciations of Perseids in 2814? Barmar (talk) 05:34, 19 August 2023 (UTC)

Foreshadowing! Note it there? 162.158.86.182 08:11, 19 August 2023 (UTC)

Objection, Your Honor! Don't know if electron quasibeams (see comment above) can be done, but the "a beam consisting of a lack of something is not possible" in the current explanation is too wide. Think of antisound devices! I think a "vacuum beam" going through normal gas pressure is very possible. And what about the Meissner effect, which could be seen as a beam of absence of a magnetic field? 198.41.242.211 07:25, 19 August 2023 (UTC)

Anti-sound just exactly(/sufficiently) compliments the sound you're trying to 'remove'. On its own, it is sound in its own right.
Sure, but it makes a region without sound from destructive interference. Beams take energy anyway. 172.70.230.36
Projecting a vacuum (which would, incidentally, quash sound except for that which travels round it or is caused by its creation/collapse) sounds like it needs a whole army of Maxwell's demons carrying their own 'portable hatches' to allow air molecules to leave the volume of the beam whilst batting away any that threaten to move into it. (That might be interesting to see, if noisy.)
This might work at absolute zero? For example, you could electrically charge the particles of a motionless gas and then pull them using a focused electric field and make a beam that emits vacuum. 172.70.230.36 14:32, 21 August 2023 (UTC)
Calling myself out for handwaving. I have no idea how to turn an electric field into a beam. It would be just like the electron hole gun. Maybe with EM. Does Randall conflate guns that fire in a straight line with electron guns that emit electrons which then behave like electrons rather than bullets or lasers? 172.70.114.216 05:56, 22 August 2023 (UTC)
The Meissner Effect is an interesting (practical) version of this. Upon decreasing the possibility of an interior field, an identical increase is detetected immediately outside of it (conserving flux across the whole system, or so it seems to be/makes most sense).
Which is not to say that there's no such thing as "a nothing", in the whole weird world of science, or variously vagues analogues to it (if you don't dig too deep, maybe). Some might suggest quantum vacuum decay might be the ultimate substantial 'nothing', but not that we know how to study it... let alone harness it. Yet! 172.71.242.5 10:23, 19 August 2023 (UTC)
Those aren't "beams of nothing" but rather "beams of things that cause another thing to be suppressed/expelled/cancelled" 172.69.247.43 14:49, 20 August 2023 (UTC)
I’m surprised nobody is mentioning an antimatter beam (edit: oh, positrons below, oops). This would eradicate matter in its brilliant path. But you could make a beam of nothing in an environment where the presence of something is controlled by another system, by engaging the system doing this. Shadows are maybe the most simple example of this. Shading a light source makes a beam of darkness. 172.70.230.36 14:32, 21 August 2023 (UTC)
For sure a beam of nothing can exist - it's called Reality TV. 172.70.90.164 09:57, 21 August 2023 (UTC)

When the positron was first predicted it was modeled as an electron hole, a gap in the hypothetical "sea" of negative-energy electrons filling all space. Sadly, AFAIK modern quantum field theory has done away with that idea, so while a gun that shoots a positron beam is theoretically possible, it probably wouldn't qualify as an electron hole beam. Hmj (talk) 08:03, 19 August 2023 (UTC)

I (separately) mentioned positrons in an edit summary just now. Of course, adding the two concepts together makes for an even more ridiculous thing... A 'positron hole beam'. You know all those positrons we (don't, in general terms) have floating around us? This now projects a concentrated lack of them! 172.71.242.5 10:23, 19 August 2023 (UTC)

I firmly believe the printer deserved it. It knows what it did. 162.158.19.95

I'm honestly surprised to see no mention of toner printers, or even conjecture on why it's a printer that the electron hole gun is being fired on. Maybe an another reason the physicist is upset is that he's messing up her currently-printing document? - Vaedez (talk) 11:51, 26 August 2023 (UTC)

Electrical current was defined as the flow of positive charge carriers before it was understood that the negative charge carriers (electrons) were what was moving. When talking about semiconductor physics, this became a problem because we’re very concerned about what particles are actually moving around, so the mathematical fiction of “hole flow” was invented so we wouldn’t have to use negative signs everywhere in the math. An electron hole is a property of p-type semiconductors, a place where electrons can move into, which can also be described as the nonsensical but more mathematically convenient flow of holes in the opposite direction. By analogy, if you had a children’s shape-sorting box, you could build a catapult that threw around the blocks, but you couldn’t build something that threw around the holes in the lid that the blocks fit into.

But... sure you could!? The blocks would function like pegs obstructing unaligned travel, & the "holes" could simply be panels that can only traverse the pegs when their holes are aligned. Just because a gap in a medium lacks one thing, doesn't mean it contains nothing, & the medium itself is necessarily still a thing. None of the math for a QED vacuum even works unless a space defined by masss-energy over time, is defined; there is no "nothing".
ProphetZarquon (talk) 15:31, 21 August 2023 (UTC)
Current sign is a convention, we could just as easily reverse the convention for proton and electron charge sign... that we haven't done so for one or the other is both a source of endless frustration, confusiong, and also, yet another XKCD comic 567: Urgent Mission - 172.70.211.114 19:15, 21 August 2023 (UTC)

Anode ray

A contributor wrote, "Here, the electron hole gun might refer to the anode ray tubes." An anode ray is a beam of positive ions; these are actual particles and not "quasiparticles", and therefore the comic does not refer to them. 162.158.86.243 07:05, 20 August 2023 (UTC)

Isn't a positive ion an atom missing an electron? Couldn't you make an anode ray tube that emits charged semiconductor dust? The quasiparticles are the missing electrons in the matter. 172.70.114.21 05:38, 22 August 2023 (UTC)
For sure you can make a beam of positively charged ions - and that's the first thing I thought of with the "electron hole gun".
But . . . the context pretty much says that the gun is sending the holes only, without any surrounding matrix. That is what is "impossible". 162.158.175.133 18:11, 22 August 2023 (UTC)
Holes always imply surrounding negative charges. They are, specifically, a way to discuss the flow of net positive charge in a sea of negative charges. If you work with them this becomes quite clear. 172.70.111.49 23:38, 27 August 2023 (UTC)

Who is firing the beam?

Are we so sure it's Beret Guy? Depending on how dangerous or destructive it is, it could be Black Hat? And is using the first person in the title text ("my") for a character like Beret Guy, instead of for a fictionalized Randall Monroe with weird hobbies, a departure from usual norms?

Quoting the article as of my writing, "It should be noted however, that a hole in the QED vacuum is a real particle, known as the positron. See the Dirac sea." That very linked Wikipedia article says that the Dirac Sea is not the QED vacuum, that they are equivalent but different models. Nitpicking (talk) 02:20, 21 August 2023 (UTC)

I could guess Cueball holding an anode ray gun a little. 172.70.115.31 05:42, 22 August 2023 (UTC)

I disagree with the punchline here. a layman might be angry about this hypothetical device, but a physicist would understand that an electron hole is indistinguishable from a positron. 162.158.158.118 17:24, 21 August 2023 (UTC)

It might depend on the kind of physicist; one who deals with semiconductors wouldn't use "electron hole" to refer to positrons. It's pretty clear that the beam in the cartoon is not a stream of positrons, which would be reacting with the electrons in the air and releasing high-energy photons. It's the difference between "fantastic behaviour of a thing that doesn't really exist" and "behaviour that violates the known properties of a thing that does exist". Perhaps the ST:TNG Enterprise needs to be swept periodically to remove all of the lowadekions it's picked up, fine, but it's a bad thing to take away all of the baryons. BunsenH (talk) 01:06, 22 August 2023 (UTC)
*"an electron hole is indistinguishable from a positron"* - oh, an electron hole is very, very distinguishable from a positron. They hold the same charge, so they are the same in that respect, but otherwise they have quite a lot of different characteristics. For one thing a positron has mass whereas an electron hole, not so much.
an electron hole in solid state has an effective mass. https://en.wikipedia.org/wiki/Effective_mass_(solid-state_physics). I renew my original point. Electron holes and positrons are the same thing. 162.158.158.91 16:56, 24 August 2023 (UTC)
If you shot out a beam of electron holes (however you are planning to manage that) and then followed it up by shooting a beam of positrons, I very well guarantee that you are going to be able to tell that the two are vastly different things. You might not even need any special equipment to do so . . . 162.158.174.196 18:16, 22 August 2023 (UTC)
My impression from the references given in this discussion is that the phrase "electron hole" can have two different meanings, in different areas of physics: semiconductors and Dirac field theory. I'm familiar with only the former, but the latter appears to be attested and to be equivalent to positrons. A bit like how "exothermic" means nearly opposite things between biology and physics/chemistry. BunsenH (talk) 20:43, 22 August 2023 (UTC)
They're not really different meanings. It's just that the properties of a hole depend on the background. 162.158.159.49 16:57, 24 August 2023 (UTC)