# Particle and anti particle

1. Sep 14, 2005

### kiru

Hi friends,
What is the need for the existence of an antiparticle for a particle?

2. Sep 14, 2005

### EL

One of them is to preserve causality. (See e.g. Peskin and Schroeder Chapter 2)

3. Sep 14, 2005

### mathman

That sounds like a philosophical rather than a scientific question. Physical theory predicts their existence and experiments confirm it. They aren't "needed".

4. Sep 19, 2005

### dextercioby

If the classical fields describing (when quantized) "particles" are elements of an associative star-algebra, then there are 'antiparticles' in the theory, too.

Daniel.

5. Sep 19, 2005

### Haelfix

Any field theory that respects CPT invariance must include anti particles.

6. Sep 20, 2005

### dextercioby

The free real scalar field (which can account for a description of $\pi^{0}$ if one neglects inner quark structure) is CPT invariant, but there's no distinction between particles and antiparticles. See pages 128 pp. 130 from Griffiths' text on elementary particles.

Daniel.

7. Sep 24, 2005

### jhmar

That sounds like a philosophical rather than a scientific question. Physical theory predicts their existence and experiments confirm it. They aren't "needed".

This is the wrong way around. Read the introduction to any particle physics primer and it will tell you that the theory is constructed to match observations; only then can the theory be used to predict, where the prediction is in error the theory is adjusted. That is why QT is called a 'mathematical prediction theory'; to become a 'scientific theory' it should have an 'interpretation' and it does not at present have anything like a complete interpretation.

8. Sep 24, 2005

### mathman

to jhmar

Dirac predicted the existence of the positron before it was confirmed by experiment.

9. Sep 24, 2005

### tariq5

I like Positron Emission Tomography Scan ( Pet Scan).

10. Sep 25, 2005

### jhmar

Dirac predicted the existence of the positron before it was confirmed by experiment.

First Dirac constructed a theory based on a number of unexplained experimental observations, the theory predicted the existance of positrons. Experiments precede prediction theory see-
http://news.bbc.co.uk/2/hi/in_depth/uk/2000/newsmakers/2094374.stm

In order to understand 'the need for anti-particles' we need to know what patricles are and how they are constructed; prediction theory does not answer such questions. Marcus Veltman points out in 'Facts and Mysteries in Elementatary Particle Physics' that in many cases we know only the computations, we have no explanation (in words) for what happens between the start and finish of particle interactions, we only know how to compute (i.e. predict) the result.

Last edited: Sep 25, 2005
11. Sep 25, 2005

### mathman

I got this from looking up "positron history" using Google.
--------------------------------------------------------------------------------
Positron

A positron is the antimatter equivalent of the negatively charged electron. A positron is equal to the electron in mass, but has a positive charge. In 1828, English physicist Paul Dirac advanced an equation that incorporated both quantum physics and the requirements of the theory of special relativity to provide a complete description of the electron. The equation resulted in a particle, however, that could be positively or negatively charged. On this basis, Dirac predicted the existence of the positron.

In 1932, American physicist Carl Anderson observed a new kind of particle in his cloud chamber. A particle too faint to be a proton or alpha particle entered the chamber, and then curved toward the negative area of the magnetic field that was around the chamber. It's velocity and mass indicated was the same as the electron but it swerved toward the negative pole and therefore had to carry a positive charge. Anderson realized he had discovered Dirac's antimatter particle to the electron, the positron (e+ ).
--------------------------------------------------------------

http://www.bookrags.com/sciences/physics/positron-wop.html

The above is the source. Note that Dirac predicted the positron in 1928 and it was discovered in 1932.

12. Sep 25, 2005

### ZapperZ

Staff Emeritus
This is very odd. I don't know what "experiment" you are claiming that preceded the prediction of antimatter, but even according to the site you are citing, it says

It is commonly accepted that the Dirac equation was the first theoretical prediction for antimatter, in this case, the positron. There have been no experimental evidence, nor predictions, before that. What you are quoting only strenghten this view.

The neutrinos were also never observed experimentally first either. It was predicted to preserver conservation laws in beta decays.

Zz.

13. Sep 26, 2005

### jhmar

This is very odd. I don't know what "experiment" you are claiming that preceded the prediction of antimatter

The article begins with-
'Experiments had shown that classical physicists could not explain the behaviour of atoms'.
Dirac devise a mathematical theory to match these experiments and that theory predicted (numerically) anti-matter.
Dirac's theory predicts, it does not tell us 'why' or 'how' so it is common practice to call the first discovered particle of any new class, 'the particle' and its mathematical opposite then becomes the 'anti-particle'. (We have no scientific definition that allows us to separate particle and anti-particle ). We know what entities of the particle/anti-particle are mathematically opposed, but we do not know what they are or why they exist, we only know what they do.
Numerous particles are predicted before discovery but, these predictions are always based on mathematical theory devised to match previous observations.

Last edited: Sep 26, 2005
14. Sep 26, 2005

### ZapperZ

Staff Emeritus
Yes, but there's two problems with using this to justify your idea (i) it contradicts the quote later in the article and (ii) Dirac was trying to solve for a different problem regarding the atom, and NOT trying to solve for the existence for some yet unpredicted entity.

There have been many instances where the theoretical attempt at solving a phenomenon leads to the prediction of OTHER observations. This is just the case here. There have been ZERO hints at the existence of anti-matter before Dirac incorporated special relativity in QM. His work in doing this was to solve other problems in atomic physics at that time, NOT to explain the existence of antimatter. Your source clearly indicate as such.

Zz.

15. Sep 26, 2005

### dextercioby

It's highly interesting that negative energy solutions were obtained when Schrödinger, Gordon, Klein, Fock, and de Donder solved the

$$\left(\Box+m^{2}\right)\Psi (\vec{r},t)=0$$

for complex $\Psi$ equation back in 1926.

Daniel.

Last edited: Sep 26, 2005
16. Sep 26, 2005

### Haelfix

Hey Dexter, yea I suppose you are correct although it is a bit mincing words. Lets just say its a necessary but not sufficient condition.

CPT invariance guarentees any antiparticle to have the same decay width and mass as its partner.

But fundamentally and conceptually speaking anti particles are guarenteed by special relativity and quantum mechanics. Namely the existance of an SMatrix, and the lorentz invariance thereof imply a creation and annihilation operator algebra. Now in order for such a system to satisfy certain fundamental properties of QM (eg in the presence of some conservation law given some symmetry generator like say the charge operator) we need in general to impose a species doubling in the creation and annihilation operator algebra. Only in the case where conserved current is identically zero, are we left with the particular solution where particles are identical to their antiparticles. And again here, CPT invariance guarentees that we don't have some mass degeneracies or any such thing that would break the simple picture.