Neutrons have short, mysterious lives

"By convention sweet is sweet, bitter is bitter, hot is hot, cold is cold, colour is colour; but in truth there are only atoms and the void."

So said Democritus of Abdera, a Greek philosopher who lived from about 460 BCE to 370 BCE. We have only fragments of his writings as he was dismissed by other Greek philosophers and his work was not preserved except by the writing of others.

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Yet it is an incredibly insightful statement and easily thousands of years ahead of its time. Democritus came up with the theory of atoms long before modern science revitalized the concept.

He even went so far as to describe how atoms could join together through a hook-and-eye mechanism and why acids taste sour - acids have pointy atoms which poke the tongue.

That, of course, is not the reason acids taste sour. Rather our sense of a sour taste is the result of molecular and ionic interactions. But he understood the universe is composed of atoms and therefore not infinite in either direction. Democritus' atoms were tiny indestructible particles beyond our capacity to observe.

Or at least they were until the last decade of the 19th century.

Henri Becquerel observed mysterious rays emitted from a sample of pitchblende. Through careful work he and his co-workers, Pierre and Marie Curie, were able to show the rays emanated from the radioactive decay of atoms. Uranium, radium, and polonium were all isolated from the pitchblende and their decay measured.

When talking about radioactive decay, scientists often use the term "half-life."

It arises out of a first-order differential equation and is the time it takes for exactly half of something to turn into something else.

It doesn't have to be radioactive. For example, prescription drugs have a half-life in the human body which measures the rate at which the drugs are utilized and discarded.

But the radioactivity of uranium-238 gives it a half-life of around 4.5 billion years which means right now there is only half as much U-238 on Earth as there was at its beginning. For the uranium-235 isotope, the half-life is shorter - at 700 million years - leaving only about one per cent of the original concentration of U-235 on the planet.

At almost the same time as Becquerel was making his observations, J.J. Thompson was able to peal electrons out of metal and fire them across a glass tube.

While not sure what he had, he was able to measure its charge-to-mass ratio and show it was a sub-atomic particle.

Over the next decade, Rutherford formulated the solar system model of the atom with the nucleus composed of positively charge protons surrounded by a halo of orbiting electrons - Thompson's particles. Democritus was right, but he was one layer too high. It is not atoms which are the fundamental building blocks, but protons and electrons.

Except this presented a puzzle. If protons all have positive charges and like charges repel each other, how did the nucleus hold together?

At the same time another puzzle emerged as mass spectroscopy was invented and some atoms of the same element were found to have different masses.

In 1932, James Chadwick discovered the neutron - a sub-atomic particle similar to the proton in mass and energy but with no charge. With the exception of hydrogen, all atomic nuclei are made up of a combination of protons and neutrons. The more protons present, the more neutrons required to stabilize the nucleus.

And with the really heavy elements, their nuclei still fall apart which is why uranium is radioactive and has a half-life.

But in 1951, the structure of sub-atomic particles became a little murky. As far as scientists have been able to measure neither protons nor electrons have a measureable lifetime.

The protons and electrons which coalesced from the big bang are still with us today.

Not so for the neutron. A free neutron has a lifespan of 14 minutes and 39 seconds. Or a lifespan of 14 minutes and 47 seconds.

This discrepancy is a result of the two different methods for measuring the lifespan of the neutron, but no one is sure why it exists.

In the first method, neutrons are essentially captured in a bottle and counted after a period of time. Several laboratories have taken this approach giving the 14:39 lifespan. The error bars on this value are relatively small.

In the second method involves a beam of neutrons which are injected into a detector and the protons produced by decay are counted. The results have equally small error bars.

In both cases, the scientists are convinced what they are seeing is real differences in time but are not sure why.

One possible and intriguing explanation is the discrepancy may arise from dark matter.

This possibility is spurring on efforts to measure the lifetime of neutrons even more accurately as it may help us to understand atoms and the void.

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