My previous post was *awfully* long, so I must assume many of my readers may have started to read it, but… Well… Gave up halfway or even sooner. 🙂 I added a footnote, though, which is interesting to reflect upon. Also, I know many of my readers aren’t interested in the math—even if they understand one cannot really appreciate quantum theory without the math. But… Yes. I may have left some readers behind. Let me, therefore, pick up the most interesting bit of all of the stories in my last posts in as easy a language as I can find.

We have that weird 360/720° symmetry in quantum physics or—to be precise—we have it for elementary matter-particles (think of electrons, for example). In order to, hopefully, help you understand what it’s all about, I had to explain the often-confused but substantially different concepts of a *reference frame *and a *representational base *(or representation *tout court*). I won’t repeat that explanation, but think of the following.

If we just rotate the *reference frame* over 360°, we’re just using the same reference frame and so we see the same thing: some object which we, vaguely, describe by some *e*^{i}^{·θ} function. Think of some spinning object. In its own reference frame, it will just spin around some center or, in ours, it will spin while moving along some axis in its own reference frame or, seen from ours, as moving in some direction while it’s spinning—as illustrated below.

To be precise, I should say that we describe it by some *Fourier* sum of such functions. Now, if its spin direction is… Well… In the other direction, then we’ll describe it by by some *e*^{−i}^{·θ} function (again, you should read: a *Fourier *sum of such functions). Now, the weird thing is is the following: if we rotate *the object itself*, over the same 360°, we get a *different *object: our *e*^{i}^{·θ} and *e*^{−i}^{·θ} function (again: think of a *Fourier *sum, so that’s a wave *packet*, really) becomes a −*e*^{±i}^{·θ} thing. We get a *minus *sign in front of it. So what happened here? What’s the difference, *really*?

Well… I don’t know. It’s very deep. Think of you and me as two electrons who are watching each other. If I do nothing, and you keep watching me *while turning around me*, for a full 360° (so that’s a rotation of your reference frame over 360°), then you’ll end up where you were when you started and, importantly, you’ll see the same thing: *me*. 🙂 I mean… You’ll see *exactly *the same thing: if I was an *e*^{+i}^{·θ} wave packet, I am still an an *e*^{+i}^{·θ} wave packet now. Or if I was an *e*^{−i}^{·θ} wave packet, then I am still an an *e*^{−i}^{·θ} wave packet now. Easy. Logical. *Obvious*, right?

But so now we try something different: *I *turn around, over a full 360° turn, and *you *stay where you are and watch *me *while I am turning around. What happens? Classically, nothing should happen but… Well… This is the weird world of quantum mechanics: when I am back where I was—looking at you again, so to speak—then… Well… I am not quite the same any more. Or… Well… Perhaps I am but you *see *me differently. If I was *e*^{+i}^{·θ} wave packet, then I’ve become a −*e*^{+i}^{·θ} wave packet now.

Not *hugely* different but… Well… That *minus *sign matters, right? Or If I was wave packet built up from elementary *a*·*e*^{−i}^{·θ} waves, then I’ve become a −*e*^{−i}^{·θ} wave packet now. What happened?

It makes me think of the twin paradox in special relativity. We know it’s a *paradox*—so that’s an *apparent *contradiction only: we know which twin stayed on Earth and which one traveled because of the gravitational forces on the traveling twin. The one who stays on Earth does not experience any acceleration or deceleration. Is it the same here? I mean… The one who’s turning around must experience some *force*.

Can we relate this to the twin paradox? Maybe. Note that a *minus *sign in front of the *e*^{−±i}^{·θ} functions amounts a minus sign in front of both the sine and cosine components. So… Well… The negative of a sine and cosine is the sine and cosine but with a phase shift of 180°: −*cos*θ = *cos*(θ ± π) and −*sin*θ = *sin*(θ ± π). Now, adding or subtracting a *common *phase factor to/from the argument of the wavefunction amounts to *changing *the origin of time. So… Well… I do think the twin paradox and this rather weird business of 360° and 720° symmetries are, effectively, related. 🙂

**Post scriptum**: *Google *honors Max Born’s 135th birthday today. 🙂 I think that’s a great coincidence in light of the stuff I’ve been writing about lately (possible interpretations of the wavefunction). 🙂