Sabine Hossenfelder has gradually transitioned from critical written content about physics to YouTube videos, mostly short science news clips with the occasional longer piece. Luckily for us in the unable-to-listen-to-podcasts demographic, the transcripts of these videos are occasionally published on her organization’s Substack.
Unluckily, it feels like the short news format is leading to some lazy metaphors. There are stories science journalists sometimes tell because they’re easy and familiar, even if they don’t really make sense. Scientists often tell them too, for the same reason. But the more careful voices avoid them.
Hossenfelder has been that careful before, but one of her recent pieces falls short. The piece is titled “This Experiment Will Measure Nothing, But Very Precisely”.
The “nothing” in the title is the oft-mythologized quantum vacuum. The story goes that in quantum theory, empty space isn’t really empty. It’s full of “virtual” particles, that pop in and out of existence, jostling things around.
This…is not a good way to think about it. Really, it’s not. If you want to understand what’s going on physically, it’s best to think about measurements, and measurements involve particles: you can’t measure anything in pure empty space, you don’t have anything to measure with. Instead, every story you can tell about the “quantum vacuum” and virtual particles, you can tell about interactions between particles that actually exist.
(That post I link above, by the way, was partially inspired by a more careful post by Hossenfelder. She does know this stuff. She just doesn’t always use it.)
Let me tell the story Hossenfelder’s piece is telling, in a less silly way:
In the earliest physics classes, you learn that light does not affect other light. Shine two flashlight beams across each other, and they’ll pass right through. You can trace the rays of each source, independently, keeping track of how they travel and bounce around the room.
In quantum theory, that’s not quite true. Light can interact with light, through subtle quantum effects. This effect is tiny, so tiny it hasn’t been measured before. But with ingenious tricks involving tuning three different lasers in exactly the right way, a team of physicists in Dresden has figured out how it could be done.
And see, that’s already cool, right? It’s cool when people figure out how to see things that have never been seen before, full stop.
But the way Hossenfelder presents it, the cool thing about this is that they are “measuring nothing”. That they’re measuring “the quantum vacuum”, really precisely.
And I mean, you can say that, I guess. But it’s equally true of every subtle quantum effect.
In classical physics, electrons should have a very specific behavior in a magnetic field, called their magnetic moment. Quantum theory changes this: electrons have a slightly different magnetic moment, an anomalous magnetic moment. And people have measured this subtle effect: it’s famously the most precisely confirmed prediction in all of science.
That effect can equally well be described as an effect of the quantum vacuum. You can draw the same pictures, if you really want to, with virtual particles popping in and out of the vacuum. One effect (light bouncing off light) doesn’t exist at all in classical physics, while the other (electrons moving in a magnetic field) exists, but is subtly different. But both, in exactly the same sense, are “measurements of nothing”.
So if you really want to stick on the idea that, whenever you measure any subtle quantum effect, you measure “the quantum vacuum”…then we’re already doing that, all the time. Using it to popularize some stuff (say, this experiment) and not other stuff (the LHC is also measuring the quantum vacuum) is just inconsistent.
Better, in my view, to skip the silly talk about nothing. Talk about what we actually measure. It’s cool enough that way.
4 gravitons 
Does your comment on silly talk about nothing apply to Lawrence Krauss’s “Why there is a Universe out of nothing?” – https://www.youtube.com/watch?v=3SxowRIc20M
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Eh, that seems like it’s more about the philosophical “how do you get something from nothing” rather than the pop physics “properties of the quantum vacuum” thing. (“Seems like” because as someone who just admitted being podcast-allergic, I’m definitely not listening to Lawrence Krauss yammer on for an hour 😉 )
Fundamentally, I think for anything that you observe you can ask if there’s an explanation for it. “How does something come from nothing?” is a slightly more cutesy way to say “Is there an explanation behind all the other explanations?”
I don’t know if Krauss goes further into some sort of “the quantum vacuum is stuff and therefore not really nothing” story, so it could be he goes sillier there. It could be he’s saying something like “‘nothing’ is unstable”, which is closer to the truth. It’s not that the vacuum is full of bubbling particles, but that something that seems like empty space is not actually a stable state for the universe as a whole.
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Krauss definitely says that “when you put together quantum mechanics and special relativity it turns out that it must be that empty space is really a boiling bubbling brew of virtual particles that are popping in and out of existence”
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Yup, definitely the usual silly story then.
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Could you share a good pointer to the non-silly story?
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The Matt Strassler piece I link in the post (link again here) is pretty good.
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Matt
Previously, when you were talking about ” lies to children ” ( as an inadequate perhaps, but sometimes “necessary” means of doing science communication), I thought that you were just trying to be polite to other people (when doing this in YT etc..).
I think that you agree that bad metaphors and oversimplification are not doing a good job for communicating difficult or subtle points/ topics about physics.
Giving a simplified, yes, but accurate as possible “mental picture” is crucial (sometimes explanations incorporating simple scenarios or thought experiments are helpful), but unfortunately, it’s not often the case, especially in YT.
I have tried myself to contribute a little, doing a few comments on some of these videos ( I’m not talking here especially about Sabine’s “science news” YT videos to be fair:
When someone’s releasing videos at a regular basis , about lots of divergent topics, this unevenness in quality of content is almost unavoidable), but it’s a waste of time , nothing’s going to change anytime soon…
(…especially now, with the new AI tools we’ll see more of the same, I’m afraid 🙄).
I totally agree about the importance of thinking about measurements, observables, interactions when talking about physics ( even in cases that have to do with thought experiments or hypothetical scenarios).
Things that can be measured with detectors, observed/ recorded are “real”, this is what scientific realism is about, in my opinion.
Best wishes to everyone! 🙂
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Yeah, I do think “lies to children” have their place (and unavoidably use them myself). But we should avoid them when we can afford to, when we can express something just as clearly without them. (And sure, “what one can afford to” is a calculation that changes when you’re uploading a video every day 😉 )
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