Comment by cshimmin | Hacker News Reader

Comment by cshimmin | Hacker News Reader

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Loop is definitely a technical term. I guess I shouldn't have quoted it? It refers to a Feynman diagram that looks like this:

https://en.wikipedia.org/wiki/One-loop_Feynman_diagram

In quantum field theory, perturbative physical processes can be expressed as the sum of all possible diagrams compatible with the initial and final state of interest. Each diagram corresponds to an integral, and diagrams that have loops are special because they are underconstrained by the initial/final conditions. Hence, you are left with an additional integral over every possible combination of momenta that can be exchanged within the loop. These integrals are usually hard or impossible to compute and may require weird mathematical techniques (like dimensional regularization). So, yes, (theoretical) particle physicists talk about loops a lot!

"Lend" here is total b.s. that I made up. The point is that the gluons can't directly interact with the Higgs boson, so they use their collisional energy to create some massive top quarks that can.

So a little from column A, a little from column B. Got it!

In all seriousness, I'm just interested in how much is being obscured when folk like Cox do a TV show, obviously written for those interested but not involved like myself. I've no idea what anything in your paragraph meant past the word "In" (yes, the first word), but I can definitely picture a loop, and get that there is a something which is transmitted / moves / traded / otherwise-not-at-its-origin within that loop, and trying to understand what that something is will leave me drooling into the carpet.

Still, I'm glad I asked. Thanks!

If anyone's looking for a really solid "accessible to a layperson but you're still going to have to put in some work" series on this stuff, I wholeheartedly recommend PBS Space Time on YouTube. It's paced like your standard 10-15 minute light educational fare, but the core is serious physics.

Here's their 7-part series on Quantum Field Theory: https://www.youtube.com/playlist?list=PLsPUh22kYmNBpDZPejCHG...

An excerpt from part 5, "The Secrets of Feynman Diagrams":

The overall interaction described by a set of Feynman diagrams is defined by the particles going in and the particles going out. These are the particles that we actually measure. We know their properties - for example, their energy and momentum. And they obey Einstein's mass-energy equation. We say that these particles are on the mass shell, or just on shell. They sit on the shell structure you get when you plot Einstein's equation of energy, momentum, and mass. On the other hand, everything that happens between the ingoing and outgoing tracks has questionable reality.

Each possible diagram that results in the same ingoing and outgoing particles is a valid part of the possibility space for that interaction. The particles that have their entire existence between vertices within the diagram but don't enter or leave are called virtual particles. Their correspondence to anything resembling real particles is debatable. They are also, by definition, unmeasurable. Otherwise, they'd be one of our ingoing or going particles. These particles do not obey mass-energy equivalence. So they are off shell. These particles aren't even limited by the speed of light or the direction of time, which leads to all sorts of fun.

Nice.

And thanks for the link.

While you're explaining individual words here (!) can you help me understand that use of "virtual" in high energy physics?

"Virtual particles" implies they're somehow not real, but I assume the theory says they do have a real physical existence in some sense -- they have an influence on the real particles. Are they "virtual" just because they're extremely hard to isolate, so all our practical experiments are indirect, or are they different in principle from the normal, "real" particles that our experiments work with? Is it a qualitative or just a quantitative difference?

It’s been a long time (and this was never my strongest area), but in case someone who really knows what they’re talking about doesn’t get back to you, it’s basically a particle who may or may not exist based on quantum uncertainty, but in practice IIUC it’s more like an excitation of the underlying quantum field.

Both real and virtual "versions" of a type of particles are described as excitations of the same quantum field; the difference is that only the real particles can be detected experimentally, whereas the virtual ones are used to model interactions between (real) particles and thus only appear in calculations.

Thanks, that helps, I think...!

Would it be roughly correct to say that virtual particles by definition always vanish (mutually annihilate, etc, whatever) before the measured outcome of an experiment, therefore they definitely cannot be directly observed? But they're an integral part of the model, therefore if the model is correct, they really do have a real physical existence (in some complex quantum sense)?

I do not believe so. Generally speaking, mathematical models often include what I call "scaffolding," which is something that, while being part of the model, does not represent anything real. (Different models are likely have different scaffolding, and some future theory might do away with virtual particles as a mechanism of interaction.)

IIRC, those are also called quasiparticles.

As I understand it, ’virtual’ particles are those that are not observed ”on shell” (i.e. going into or coming out of a specific interaction), and hence whose existence cannot be affirmed if not by the effect their evanescente presence had upon what did actually come out, or rather, the statistics of what comes out of many identical inputs interacting.

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