h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I read about some nuclears giving off positrons when falling apart.
Now, how come they do, and how do they do it?

I was also thinking about this theory about antimatter being matter moving "backwards" in time.
If that's true, wouldn't common matter be able to change direction in time, and in doing so becoming antimatter? This would however lead to some complications as this would result in a paradoxal crash as the "backwards" and "forward" moving matter meet, and turn into energy, and thereby preventing the normal moving matter from changing direction thus avoiding the antimatter explosion, which means the explosion would happen, and so on...
This however made me wonder, in what direction, of time, do the nuclears that give off antimatter move?

Anti-matter was first proposed by Dirac, and resulted from his modification of the Schrodinger equation to take into account special relativity. At that formulation (which is admittedly the only one which I have any familiarity) there is no mention of backwards motion in time.

who is to say we aren't moving backwards in time at the moment... (see hawking - "a brief history of time"...

sure I read something about quantum mechanics equations being time independant so that they work regardless of the flow of time. A somewhat hazy memory though.

Yeah... Once you are in a time stream you are unable to say which way it is actually going... (well we, humans, are).

Quantum mechanics equations *can* be time independent sure but then you are modelling a static system. If you want your osciallator or whatever to actually interact with something else then the two systems must perturb one another *then* you need to use time dependent equations.
That's when it starts to get *really* tricky

Whether we are flowing 'backwards' or 'forwards' in time. Surely every particle will have its antiparticle which ever way time is flowing.

I think the positron was first postulated because during the solution of a quantum equation for an electron it was noticed that the solution was quadratic which meant two solutions an so another particle was possible but with opposite charge. This was after observed, the positron.

Positrons are nothing unusual, they often emitted from nuclei as one form of beta-radiation.

Imagine a nucleuse decaying thus:

Unstable nucleus has x protons and y neutrons. To make it stable a proton decays to a neutron. Giving x-1 protons and y+1 neutrons.

This means the nucleus has one less positive charge and so this is balanced by the emittance of a positron

1p+ ------> 1n +1 e+

BTW, I'm aware of a theoretical 'lightest element' which has been named positronium.

This consists of an electron and a positron orbiting one another.

Has positronium actually been observed?

*wonders whether emittance is a real word *

But that's just a question of labelling. Why not just call whatever direction in time we're heading "forward"?


(Ictoan, you might be thinking of "microscopic reversibility" which says that fundamental processes must have the same forward rate as backwards rate...but there are conditions for, this. I'll look them up when I get to my office)

might be. It was a bit in Roger Penrose's Road To Reality book so I may well have completely mis-remembered it! Or it could be the quadratic equation thingy. I probably shouldn;t have said anything with such a sketchy memory of it!

Would positronium be stable?

Orcus, you're right on the money - when Dirac formulated the Schrodinger equation in the "Lorentz" frame of reference (e.g. taking special relativity into account), he got a new operator that had the quadratic form you describe.

A quick search of Physical Review Letters online reveals that positronium has been observed/synthesized:

Phys. Rev. Lett. 93, 113401 (2004)
Investigations of Positronium Formation and Destruction Using 3 gamma /2 gamma Annihilation-Ratio Measurements

W. E. Kauppila, E. G. Miller, H. F. M. Mohamed*, K. Pipinos, T. S. Stein, and E. Surdutovich
Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48202, USA

Received 18 June 2004; published 9 September 2004

Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.

©2004 The American Physical Society

URL: http://link.aps.org/abstract/PRL/v93/e113401

As long as they don't collide with any other matter yes.

Antiprotons, positrons and the like are not inherently any less stable than their counterparts, it's just when they interact with their opposites that fireworks happen.

I think that a small breaking of symmetry is proposed to have occured near to the time of the big bang and this caused a slight excess of matter over anti-matter. This caused the more or less complete annihalation of all antimatter and so we now have a matter universe.

Of course it is still possible to make small quantites of anti-particles but they don't last long.

Ah, cheers Arnie

Gamma rays are emitted!

That gives a pretty stunning demonstration of the violence of anti-matter to matter reactions

One intresting thing I got from talking to a couple of my proffesors is that there may still be equal antimatter and matter. It depends on how the Higgs Boson behaves if it means matter and antimatter repel by gravitional force (not accepted but a possibility) then we see lots of matter here and somewhere else there is lots of matter

Surely, if 1-positronium is possible (1e + 1p) then 2-positronium (2e + 2p) would also be possible... So then you have a whole periodic table of "nucleus less" atoms/elements... ... Intriguing

Possibly but then the wavefunctions might be different for something without a massive nucleus, not sure.

"As long as they don't collide with any other matter yes.
Antiprotons, positrons and the like are not inherently any less stable than ..."

Er, I meant the element positronium, not the positron particle. I know that's stable 'cos Asimov made his robots' brains out of them
I guess it isn;t too stable given the above paper!

The difference between 2 positronium and helium is that there is no equivalent to the nucleus. In the current/basic periodic table, all the positive charge is in the nucleus.

2-positronium would be really interesting - it would seem to be like the "plum-pudding" atomic model that was once proposed. You'd have everything all jumbled up together...

Presumably you can also throw it the other way around. Why not a proton and an anti-proton as a binary system...