The Pauli Exclusion Principle

ETA: Since this is my most popular page ever, how about I skip the sci-fi and jump straight for what you came looking for?

The Pauli Exclusion Principle simply states that a fermion cannot exist twice in a quantum state.  At a more basic level (also known as, the level I’m at and I’m a fifth year chemistry student), two electrons with the same spin cannot be in the same quantum state.   They have to be of opposite spins.

Pauli Exclusion Principle Example

Figure 1 is hydrogen. Yeah, it’s pretty boring.  One electron in the 1s orbital.  That electron could be pointing up or down (ie, of either spin), but by convention we write it upwards.

Figure 2 is helium… sort of.  In this example, two electrons are in the 1s orbital.  However, both electrons have the same spin.  That is what the Pauli Exclusion Principle says we cannot do.

Figure 3 is also helium.  Two electrons in the 1s orbital however, the second is pointing downwards, thus signifying a spin in the other direction.  That is the correct way of drawing the 1s orbital.

(The electrons can also be drawn on top of the line.  In fact, I had one chem prof who wanted them drawn that way and a different chem prof who wanted them to be drawn through the line.  Clearly, I like the second chem prof more.)

Now, back to the regularly scheduled tv discussion.


Last Thursday, Fringe made a grave mistake in science.

Yes, Fringe is all about mistakes in (fake) science.

This mistake, however, holds the singular claim (for this show) of pissing me off.

The mistake?  Nina Sharp was explaining to Olivia about why the two universes cannot be brought together – one will inevitably be destroyed.

Yes, that’s a pretty common theory and it always shows up in science fiction.  In fact, I have absolutely no problem with that – it’s effing theoretical physics so do whatever the hell you want.

Just don’t mutilate quantum mechanics whilst you do so.