quote from my physics textbook

[Smike]

..The particles like electrons and protons which make up matter have a special property. It is that the amplitudes fro two identical particles to arrive at exactly the same point in space-time subtract: they add up in opposite phase.

I defy anyone to find any meaning in the second sentence.

(It's at the start of a paragraph about Fermions)

 

[geni]

I defy anyone to find any meaning in the second sentence.

(It's at the start of a paragraph about Fermions)

Fro?

Other than that it appears to be talking about treating the particles as waves.

 

[Brian the Snail]

It's talking about the fact that fermions must have an antisymmetric wavefunction, though you wouldn't guess it from the sentence if you didn't know it already.

I suggest you get a new textbook.

 

[rppa]

It must be talking about their wave function. This is probably a discussion of what in the wave function of fermions leads to the Pauli exclusion principle, which says that you can not have two at the same point in space with identical quantum numbers. (That is why electrons in atoms come in pairs with opposite spin).

My QM education is far in the past, so I can't remember the details about what mathematical property of fermion wave functions leads to exclusion, but that's what this passage seems to be about.

 

[69dodge]

First, you need to know what "amplitude" means in quantum mechanics. Then, it makes perfect sense.

Of course, I am too lazy to explain here what "amplitude" means in quantum mechanics ...

Ok, basically, amplitudes are complex numbers, a bunch of which you add together to find out how likely something is to happen. Or subtract, as the case may be.

Your book seems to be talking about Feynman's path integral approach to QM. What book is it, BTW?

 

[Smike]

Having done a little more research, I think it's talking about that two Fermions can't hit each other as they would have the same amplitude, so the probabilty would be 0. Maybe.

The book is "Advancing Physics A2".

It's the one in the middle here

 

[CurtC]

It is that the amplitudes fro two identical particles to arrive at exactly the same point in space-time subtract: they add up in opposite phase.

Sometimes when making a post on this board, I first write something, maybe incomplete, then come back and finish the thought later. After I post it, sometimes I notice that my changes didn't flow correctly with the text that was already there. I suspect that's what happened to this author. "It is that the amplitudes of two identical particles which arrive at exactly the same point in space-time subtract..."

 

[Mercutio]

Write to the publishers or authors. With any luck, you can help the book be clearer in the next edition. I have had publishers ask me to keep track of errors...in one case, my students found over 2 dozen errors in a statistics book. Most were fixed by the second printing (printing, not edition).

 

[Tez]

Having done a little more research, I think it's talking about that two Fermions can't hit each other as they would have the same amplitude, so the probabilty would be 0. Maybe.

The book is "Advancing Physics A2".

It's the one in the middle here

yeah it is a cruddy sentence, and without the surrounding paragraphs I can't even be sure what they were trying to say. Obviously something to do with the exclusion principle - which holds that identical fermions cannot occupy the same quantum state ("point in spacetime" is a poor and inadequate allegory for this I guess).

Who are the authors? Its published by IOP and definitely should be better than that. Send them an email, or, if you rather, send me a copy of the whole page and contact details for the author/publisher and I'll hash it out with them.

 

[phildonnia]

My guess at the meaning: something like this:

Even though an electron has a probablility X of being at a certain point, and another electron has the same probability X, you can't add these probilities in the normal fashion to determine the probability that they will both be at the same point. The probabilities are actually complex, and 180 degrees out of phase with each other. So if you add them, you get zero. This is why you never find two electrons at the same place.