Scientists have learned to measure stuff really precisely. So precisely that the uncertainties in some of their measurements are down to the level of parts per billion, which is like being able to measure the diameter of the Earth to within the thickness of a human hair. Some specific measurements do so much better than that that it boggles the mind.
Being able to measure things so precisely means that scientists can make serious tests of some of the basic assumptions we have about the universe we live in. For example, has the electron always had the same mass since the beginning of time? Or does it vary slightly? Has the speed of light always been the same?
The meaning of change
If these quantities, which we assume to be constant, change over time, then our entire structure of physics has some flaws buried deep down in the heart of it. It would be possible to correct those flaws but it means we are playing a much harder game if we are trying to understand the universe.
Because this question is critical to our deepest understanding of science, and also has pretty significant philosophical ramifications as well, scientists need to test all our basic assumptions.
Electrons and protons
I came across a couple of new papers published in Physical Review Letters that touch on this topic and thought I'd point out the results here because of the significance of this work to the foundations of physics and particle physics. I also find some of this work interesting because it sits in between particle physics and AMO (atomic, molecular, and optical) physics.
The first of the papers looks at the ratio of the electron mass to the proton mass. The proton has a mass 1836 times greater than the electron. (Actually, it is not exactly that-there are a bunch of extra decimal places -but it is this rounded number that is burned into my brain from a physics degree!) This ratio has been tested in a variety of ways over the years. The tests range from astronomical to geological, spanning vast time frames, but also laboratory tests looking at the emissions of light from atoms and molecules, where the frequency of emissions changes in time depending on the change in the mass ratio.
Previously, none of these tests had been "pure" in the sense that they were independent of any assumptions about how some parts of atomic, nuclear, astrophysical, or geological physics works. The new test compares the molecular vibrations of sulfur hexafluoride with the atomic vibrations of cesium. I can't get very far in telling this story without it becoming seriously technical so I won't give you the gory details. The end result, however, is that the electron/proton mass ratio is stable to less than a change of four parts in 100 trillion per year. That's pretty stable! Of course that test happens in a laboratory and was conducted over a timeframe of "only" two years. The physicists point out that this type of result needs to be compared with observations on cosmological timescales.
The tiny magnet that is an electron
The supposedly pure vacuum of empty space is actually a place teeming with activity. It's not that there is any matter floating around there, but the nature of quantum physics says that there is a bustle of particles temporarily popping in and out of existence, in a way such that you can essentially never detect those particles directly. However, if you put another particle in the vacuum, it is affected, very very slightly.
For example, put an electron in a vacuum and the strength of its magnetism is changed by about one part in a thousand. That is just due to the presence of other electrons and positrons (or other pairs of particles) popping into existence, messing with the electromagnetic fields around the electron and then disappearing again, like sly pranksters who'd sneak into your home, rearrange the cutlery drawer, and take off again leaving no other trace. They don't want to reveal their identities, just their presence.
The best measurement of the electron's magnetic moment, as it is called, remained the same for nearly 20 years, until 2006, when some new measurements and new theoretical calculations improved the value. Now a mere two years later, that record has been broken again. (We tangentially mentioned the 2006 record breaking in symmetry.)
The work was done in a device that has been around for most of a century, but scaled down to miniature size. A cyclotron (see the cover of this symmetry for the first one ever) was built to contain a single electron and measure its movements in a magnetic field extremely precisely. The result is three times more precise than the 2006 record and comes in at a mere 0.28 parts per trillion.
Is the electron a detector for dark matter?
So why do we care about this value? It is so precise that if it differs from theoretically predicted values, physicists could discover all kinds of new physics, from finding that the electron is actually made up of smaller components to maybe even finding signs of a dark matter particle. (Aha! It's the dark matter that rearranges my cutlery at night!)
The trouble with making any claims about new physics is that the theory needs to catch up and physicists need an independent measurement of other related properties of the electron. Still, with all these measurements getting more precise, perhaps we'll be seeing some pretty amazing particle physics discoveries from benchtop experiments to complement the discoveries imminent at the Large Hadron Collider.
