why you can't explain qcd

Or maybe why I can't?

Quantum
Quantum
Quantum
Chromodynamics

[scolton]

The video really delivers on its promise, at the beginning of my shower I didn't understand QCD and at the end of my shower I still do not understand QCD.

[DannyBeans]

I like Feynman's opposite quote: "If I could explain it simply, it wouldn't be worth a Nobel Prize."

[tidenly]

"Angela Collier is my favourite YouTube science communicator"
- Albert Einstein

[jameslloyd]

I'm really glad that this professional science communicator was able to ensure I understood that I do not understand QCD.

[DuskoftheTwilight]

> sign up for a qcd lecture
> Ask the professor if it's really about qcd or if it's just qed
> They don't understand
> Prepare a half hour YouTube video about the difference between qcd and qed
> They laugh "it's a qcd lecture"
> Attend
> It's all qed

[janmelantu]

My favorite quote about quantum mechanics is from CGPGrey: “look, ‘spin’ and ‘whirl around’ don’t mean what you think they mean. In the quantum world, words mean NOTHING, there is only MATH.”

[powernade]

"You can't explain it to a six year old because it takes 4 years of undergrad and 4 years of grad school-"

Ok, so they must be at LEAST 8 years old. Got it.

[johngregor]

My mental model of the relative complexity:
QED: watching 2 or 3 billiard balls run into each other on a nice smooth pool table.
QCD: watching a writhing ball of spaghetti the size of the solar system and oh yeah, the spaghetti is moving at nearly the speed of light and is made up of super-powerful magnets.

[KMO]

“Albert Einstein catching strays from Dr. Collier is one of my favorite things about this channel.” - Mark Twain

[jeanf]

There is a reason for QCD : the subnuclear zoo.
In the late 60s the number of particles discovered using particles accelerators numbered in the hundreds. QCD brought that mess down to a handful.

[smuganimegirl]

"Those are cartoons. They're not math." Angela shoving a category theorist into his locker.

[kylehill]

I take notes on your videos. Let's start a podcast.

[sjorgen]

Watching at 2x speed so I can not understand QCD in less than 20 mins

[rokmedves]

Hi Angela! First of all, I loved the video! But I wanted to add on to what you said at the end -- that QCD can only be evaluated numerically (via lattice QCD). I just wanted to say that absolutely not! Most experimentally-relevant calculations of QCD are done completely analytically. It's literally what I did for my PhD and the ATLAS collaboration was even able to measure and compare to our analytical predictions.
I know this isn't well-known in the community, so here's a bit of background: The idea that QCD can't be computed perturbatively ("knowing which Feynman diagrams matter") is because the strong coupling alpha_s is very large (0.118 at 91.2GeV, compared to 0.007 of QED). However, since alpha_s runs, it gets smaller as you go to higher energies (asymptotic freedom, as you mentioned in the outro). By the time that you get to experimentally-relevant energies (i.e. 13.6 TeV at the LHC), alpha_s is again small enough to be able to treat the theory as being perturbative. In fact, at experimental energies QCD is basically just fancy QED. So, recap: At small energies you need lattice QCD to say anything tangible, but at high energies QCD is just a regular perturbative theory.
There is one additional complication however: The fact that colliders collide protons, which are low-energy QCD objects, and not quarks, which can be treated perturbatively. It turns out that this wrinkle gets handled by something called a "parton distribution function", which essentially connects quark/gluon cross-sections with proton cross-sections. In fact, because of this, your typical QCD experimental prediction pipeline looks as follows: 1) Experimentalists measure these parton distribution functions at other experiments, 2) A theorist computed the cross-section for some QCD process completely perturbatively/analytically, 3) They multiply their result with a parton distribution function. And voila, you've got yourself a phenomenological prediction for a proton-proton QCD experiment!

[anotheral]

I would like to propose that "Quantum Gastrodynamics" is a way better term for weak force flavor interactions.

[Matt_The_Hugenot]

I love relearning all this stuff. When I studied it 40+ years ago unitary symmetry was the model and quarks were still somewhat controversial. Now everything's changed and I'm learning from people half my age.

[TanyaLairdCivil]

"Doesn't that suck for Einstein?...People just make stuff up and they say Einstein said it."
-Albert Einstein

[iansanford]

The pain on your face as you got out the "they're called gluons because... they stick... things together... like glue" xD

[matthieuhenocque]

The more I listen to Dr Collier, the more I realize I don't know shit about fuck but also the more I enjoy realizing this about myself. Dr Collier is a superhero. Her power is knowledge. Her secret weapon is an mischievous smile.

Thank you so much.

[bronzedivision]

I'm extremely grateful to this video for showing me that a Saturn V plushy not only exists but is now also in my online shopping cart.