I mean that literally. True, you aren’t a 7,000 ton assembly of wires and silicon, like the ATLAS experiment inside the Large Hadron Collider. You aren’t managed by thousands of scientists and engineers, trying to sift through data from a billion pairs of protons smashing into each other every second. Nonetheless, you are a particle detector. Your senses detect particles.
Your ears take vibrations in the air and magnify them, vibrating the fluid of your inner ear. Tiny hairs communicate that vibration to your nerves, which signal your brain. Particle detectors, too, magnify signals: photomultipliers take a single particle of light (called a photon) and set off a cascade, multiplying the signal one hundred million times so it can be registered by a computer.
Your nose and tongue are sensitive to specific chemicals, recognizing particular shapes and ignoring others. A particle detector must also be picky. A detector like ATLAS measures far more particle collisions than it could ever record. Instead, it learns to recognize particular “shapes”, collisions that might hold evidence of something interesting. Only those collisions are recorded, passed along to computer centers around the world.
Your sense of touch tells you something about the energy of a collision: specifically, the energy things have when they collide with you. Particle detectors do this with calorimeters, that generate signals based on a particle’s energy. Different parts of your body are more sensitive than others: your mouth and hands are much more sensitive than your back and shoulders. Different parts of a particle detector have different calorimeters: an electromagnetic calorimeter for particles like electrons, and a less sensitive hadronic calorimeter that can catch particles like protons.
You are most like a particle detector, though, in your eyes. The cells of your eyes, rods and cones, detect light, and thus detect photons. Your eyes are more sensitive than you think: you are likely able to detect even a single photon. In an experiment, three people sat in darkness for forty minutes, then heard two sounds, one of which might come accompanied by a single photon of light flashed into their eye. The three didn’t notice the photons every time, that’s not possible for such a small sensation: but they did much better than a random guess.
(You can be even more literal than that. An older professor here told me stories of the early days of particle physics. To check that a machine was on, sometimes physicists would come close, and watch for flashes in the corner of their vision: a sign of electrons flying through their eyeballs!)
You are a particle detector, but you aren’t just a particle detector. A particle detector can’t move, its thousands of tons are fixed in place. That gives it blind spots: for example, the tube that the particles travel through is clear, with no detectors in it, so the particle can get through. Physicists have to account for this, correcting for the missing space in their calculations. In contrast, if you have a blind spot, you can act: move, and see the world from a new point of view. You observe not merely a series of particles, but the results of your actions: what happens when you turn one way or another, when you make one choice or another.
So while you are a particle detector, what’s more, you’re a particle experiment. You can learn a lot more than those big heaps of wires and silicon could on their own. You’re like the whole scientific effort: colliders and detectors, data centers and scientists around the world. May you learn as much in your life as the experiments do in theirs.
4 gravitons 
Astronauts had reported that, sometimes, when in orbit, they saw sudden bright flashes that were quite intense. Probably energetic particles passed through their eyeballs(s).
Another case of a high energy particle physics “detection”.
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I always thought it was a settled case of Cherenkov effect in the eyeballs of specially trained astronauts.
Turns out that the jury is still out and that people put their heads into neutron beams to test the hypythesis. https://en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena
As far as borderline dangerous experiments go, that’s TikTok material before TikTok.
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There is a School of thought that originated mainly from Heisenberg and supported ( from different perspectives and with various nuances ), later, by Wheeler, and ( some) quantum information ( and Q. Gravity) theorists , that not only everything we know about the world has to do with interactions, but also, physics, at the fundamental level is , essentially, exchange of information (a kind of ” structural realism”, in a sense ).
Maybe this is related , also, with ideas about the emergence of spacetime and the classical world.
I’m not sure what to think of this. It’s an intriguing idea, that’s for sure, but what is , exactly, an interaction, without , at least, some notion of time ( and/ or locality ) ? I guess I’m rather
” attached” to the classical notions of spacetime / continuum etc, or, maybe , I’m not good at philosophy.
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I’m not too bothered by the notion of emergent time, conceptually (though the impression I get is that it’s a bit hard theoretically…there are plenty of examples of a space dimension emerging and fewer of time dimensions emerging). Conceptually, it’s just a matter of having a set of relations between events that puts them “on a line”, with each only relevant to “later” ones. It’s then “emergent” if this is only an approximate rule that gets violated on some scale.
Now, “the world is made of events” is probably a different claim from “the world is made of information” or “the world is made of interactions”. I think the latter are vague enough that it depends on how they get cashed out…for example, I could see “the world is made of information” being something like “the world is a set of regularities in sense data”, an extremely positivist perspective, but I could also see it being “the world is made of events and those events are discrete” which seems like it should be judged more as a physics claim.
I’m also not convinced that time isn’t just a matter of perspective, that we are each a “story” laid out in a particular way in spacetime and time is an organizing principle of those stories, not a feature of the world as such. It’s of course already true in relativity that some seemingly objective functions we attribute to time don’t hold (for example, the ability to order distant events unambiguously between observers).
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That interactions are necessary for anything to be perceived is not controversial.
The stronger claim, that they’re sufficient, fundamentally, for the ” construction” of all physics is something else.
From time to time, proposals for general principles appear, with various motivations.
Some of the proposed generalisations are proven fruitful , like the equivalence principle, some others are not so (e.g. the Machian principle) and some others , like holography, that are the subject of ongoing research , are still controversial – at least some versions of them.
The emergence of spacetime is one of the latter, in my opinion.
In GR, for general spacetimes, there is not a fundamental notion of time globally, as you said- but there is, locally, a causal relation between timelike or null separated events and , of course, we have the proper time that is defined for each timelike trajectory.
Physically realistic spacetimes ( e.g. that are globally hyperbolic) have , also, a well defined global arrow of time ( and some other related nice properties).
There are attempts, mainly in canonical quantum gravity ( probably that’s what you’re referring to)
that trying to construct a notion of emergent time. In – very different from each other- approaches there is, also, that very interesting dimensional ‘ reduction’ ( as in Ads/ CFT – the most prominent example- that you have mentioned in your blog several times ).
In that case, there is not exactly a full emergence, but only a ‘reduction ‘( as I understand it). Perhaps this is a first step.
Also , the Conformal field theory, that lives in the gravity- less flat timelike boundary of the Ads needs a notion of spacetime to be ‘ already there’- to be defined ( or , perhaps , this is not necessary? – I’m not sure about this, I need to learn more about this subject..)
Ok, I have some doubts, but these are huge, exciting topics, we’re only scratching the surface here..
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I think one can really describe certain cases in AdS/CFT and in M theory as emergence of a spacetime dimension. See this discussion under this post, particularly with Jacques Distler. There are certain kinematic regimes of the theory where there is a space dimension, but for general kinematics the same degrees of freedom cannot be described as space. That seems like exactly what one wants out of emergence. (That’s also the discussion where I get the example that time is tricky.)
CFT doesn’t exactly need the notion of spacetime to be already defined. It certainly doesn’t need the bulk spacetime to be already defined: that’s what all the research into reconstructing the bulk from entanglement is about, the idea that you can see that there ought to be a bulk spacetime purely from boundary properties. And for the CFT itself, you may want to think about it as defined in its own flat spacetime, but that’s only half true: the most productive way to think about a CFT is as defined by its OPE. The operators in the OPE are still defined at positions, but you can just think of it as an extra label that obeys certain relations: the notion of it as “place” is I think meaningfully secondary.
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Thanks!
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