Saturday, July 07, 2012

The Ether

And as imagination bodies forth

The forms of things unknown, the physicist’s pen

Turns them to shapes and gives to airy nothing

A local habitation and a name…………………………………. Not Quite W. Shakespeare

We are, of course, talking about the Higgs particle – apparently found now, not quite a half-century since Peter Higgs lent them his name and gave to them their local habitation. To the uninitiated, the oddest thing about them is their ephemerality. Their work is everywhere, lending mass to this, that and everything, but it took decades and a ten billion dollar machine to find them. How could that be?

That question is intimately bound up with the nature of that “airy nothing” mentioned earlier. It is sometimes said that Einstein killed the ether – the notion that space was pervaded by an invisible fluid that transmitted light – but the long term effect of his theory and quantum mechanics was to replace that space filling fluid with a different one that lived in space-time, the quantum field theoretic “vacuum.”

The picture that quantum field theory gives us of the vacuum is far from empty, and in fact it swarms with activity, with so-called virtual particles winking in and out of existence at an incredibly frenetic pace.

How could we not notice them, then? The answer to that is purely quantum mechanical. The virtual particles don’t have the energy or momentum to exist on their own. They have to somehow “borrow” it from the vacuum, and the vacuum is a very strict lender – what you borrow, you must give back, and the more you borrow, the less time you are allowed to keep it. To notice the virtual particles, you need a very quick, fine and delicate probe. One such is the electron bound to a hydrogen atom. Quantum field theory turns out to predict with exquisite mathematical precision just how much the orbit of that electron is affected by the virtual particles, and the effect that has on the spectral lines that that atom emits. The calculation of that “Lamb shift” and its measurement were the first truly solid evidence of the reality of that busy vacuum picture. That happened sixty plus years ago, and plenty of further evidence has appeared since.

One of the many particles believed to live in that vacuum was the Higgs. Its signature is everywhere. Much of our understanding of how other particles behave is inferred from the postulated properties of the Higgs, but the purported protagonist kept out of sight. How so? Well, he had a few characteristics that made him hard to see. One is that he decays very quickly. Another is that he is very heavy, so to promote him from virtual particle living on borrowed energy supplied by the vacuum to real article means that you need to pay off that huge loan that he got from the vacuum – and you need to be darn quick about it. That’s what the ten billion dollar machine is for, to provide enough energy in a small package (a collision between two quarks, each themselves embedded in a different colliding protons) to pay off Higgsy’s vacuum loan and set him free. It’s a very short lived reality, though, as he almost instantly decays into a bunch of other particles. It’s those decay products that lived long enough to testify that the Higgs lived at all.