Roger Penrose - Do We Understand Spinors? | Eric Weinstein

In this Portal podcast clip, Eric Weinstein discusses the extent to which we understand the illusive spinor with Sir Roger Penrose.

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This episode of The Portal was initially released on the 25th of January, 2020.

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"This object has fascinated me my entire life."

"It appears everywhere in the universe and it's not universally known that it's even there."

#RogerPenrose #ThePortalPodcast #EricWeinstein

[CaesarCapone]

"... When I went to Dirac's course." Just kind of throws it out there. I mean, he's Penrose, but still neat hearing him say that.

[M.-.D]

So incredible to see Professor Penrose win the Nobel Prize.

One of the greatest minds.

[JimFarrand]

I love the fact that, the very next thing that came after "people won't understand *that*" was an explanation involving taking the square root of a klein bottle.

[devalapar]

It seems like Roger Penrose understands what Spinors are.

[ericcorrea]

Great setup for a podcast. The lighting is perfect, the mics sound crisp, and the chairs look comfy.

[thomasedward]

EW got me into the blues. I have no idea what these gentlemen are talking about but I watched the whole thing. Why? Because EW got me into the blues. So I will continue to watch...listen....and maybe something will stick.

[rudypieplenbosch]

Watch the videos of EigenChris to start understanding spinors, he explains them very well, of course not the higher dimensions ones these two are sometimes talking about, its a good introduction.

[Adam-uiyn]

I came here knowing nothing about spinors, I left knowing even less.

[Blendletan]

There are four different definitions of a Spinor:

1)Physicists use spinor to mean a representation of the Spin Group
2) Mathematicians use spinor to mean a smooth section of the Spinor Bundle
3) Both will use the term loosely to refer to any number of objects associated to the Spin group
4) When asked by a layperson, everyone will say a spinor is something which somehow magically changes when rotated a full 360 degrees. And then they demonstrate a system made up of two parts, a rigid body and a ribbon, and when the rigid body rotates but not the ribbon the ribbon gets a twist.

Every single time I want to point out that if you rotate the WHOLE system, including the ribbon, of course it would be unchanged.

So if we don't understand Spinors, it's at least partly by choice. If you can't be bothered to distinguish 1) and 2) above, then you have no chance of getting started. If you refuse to look at dimension 2 where Spin Structures are Theta characteristics, which have been studied since the 1840s, you're trying to make things harder then they need to be. People just love doing the stupid cup dance, and they love lying and saying "some things aren't invariant if you rotate them by 360 degrees". Because it sounds like magical thinking.

[franciserdman]

good comment at start - spinor dimension in terms of the spacetime it is in's dimension, d --> spinor_dimension = 2^( d / 2), so a 2-d spacetime would have a 2-d spinor, a 4-d spacetime would have a 4-d spinor, a 10-d spacetime would have a 32-d spinor, a 100-d spacetime would have spinors with 1, 125, 899, 906, 842, 624 dimensions, and so forth.

[JimbeauxGo]

I was on a Zoom call recently with Sir Roger Penrose. This is the world's greatest mathematical physicist, but he is so incredibly modest - as you will see here - even though he was just awarded a Nobel Prize. I loved hearing him reminisce about working with Einstein, Dirac, and his most famous student, Stephen Hawking. I mention his modesty because, in this video, he is forced to carry on a conversation with possibly the world's greatest ego. Take note that Sir Roger never goes to the outer limit of obscure mathematical phraseology, but that he gets his point across, even while dealing with the constant interruptions. Weinstein (pronounced like "Einstein" he always reminds us), goes constantly to his storehouse of verbal obfuscation with highbrow mathematical terminology.

As the English would say: these two are "like chalk and cheese!"

[joaothomazini]

It is the most interesting conversation which i couldn't understand much of it.

[mitchellhayman]

The way Rodger said that, I finally understand.

[johnclawed]

I wish they elaborated a bit more because I almost understood some of that.

[juan-fernandogomez-molina]

Complex issue! In simple terms, Spinors are a linear representation of rotations in n dimensions. Complex numbers rotate when we multiply by them (e.g. multiplication by i rotates a vector 90 degrees) so two-component complex column vectors (i.e. and spinor) works similarly. Spinors were first applied to describe the interaction of the spin of a particle with anelectromagnetic field by Pauli (1927) and later by Dirac (1930). Pauli used "spin matrices" (3 2x2 complex matrices with i, -i, 1 or 0. Spinor works when we want to represent combinations of sequential rotations and manipulate them using an algebra (e.g. SO(3) etc). They are really hard to understand in higher dimensions

[stevenwilson]

I only started watching videos on spinors recently. I thought they were pronounced spine-ors. Having now grasped the basic of it a lot of quantum physics and field theory now actually starts to make sense.

[lastchance]

Although I'm sure virtually none of us would understand anything, I would love to see Sir Roger and Ed Whitten have a long form conversation.

[ancestralrocha]

Let the man speak damnit, in the end he was going to explain it

[seanmichael-jbif]

The Philippine wine glass trick is actually a method of concentration exercise taught in Pentjak Silat King fu.

[radwizard]

Im fascinated by spinors. Glad he did this discussion. Found Dirac’s book on Spinors at the UC library. Very thin blue book, but heavier than the Sun when it comes to the knowledge. Blew me away.

Edit:
UC as in University California. They might have digitized it by now.

Also has a quick review in Garrett Sonczyk book.