Another particle, called an antineutrino, is also involved, but that need not concern us here because it is almost massless. The only reason that any neutrons still exist is because, within a few minutes after the hot big bang that made the universe, some neutrons stuck themselves to protons. The strong neutron-proton binding force changes the energy balance — not by much, but enough to stabilise the neutrons. Had the Great Designer done it the other way round, with protons about 0.
Under these circumstances, isolated protons would turn into neutrons rather than the other way around. Some protons would be saved by attaching to neutrons. But hydrogen, the simplest chemical element, does not contain a stabilising neutron; hydrogen atoms consist of just a proton and an electron. In this backward universe, hydrogen could not exist. Nor could there be any stable long-lived stars, which use hydrogen as nuclear fuel.
Heavier elements such as carbon and oxygen, made in large stars, might never form either. Without stable protons there could be no water and probably no biology. The universe would be very different. The fact that the universe we know, including our own existence within it, hinges so delicately on the precise value of the neutron-to-proton mass ratio has led to heated debate among scientists.
Was it just a lucky fluke that the laws of physics turned out this way? Or does it suggest something more profound? Scientists are disinclined to believe in luck, so there has been a surge of interest in the multiverse theory, according to which our universe, with its neutron-to-proton mass ratio of 1.
Other universes will have different ratios and possibly only a tiny fraction will contain water and stars that go on to form atoms like carbon, from which life may arise. The size of the nucleus of an atom is about , times smaller than the size of the atom. The motion of tiny unseen particles, such as the molecules in a gas, produces heat, and more heat is generated by hotter particles. Hotter particles of a given mass move at faster speeds, with greater thermal energy and thermal velocity.
At a given temperature, particles with smaller mass move at faster speeds. The temperature determines the average speed of a large number of particles, but there are a smaller number of particles that move faster and slower than the average speed. The Maxwell speed distribution specifies the fraction of gas particles moving at a particular speed at any given temperature and particle mass.
Moving gas particles produce outward gas pressure, which holds our atmosphere above the ground. The Sun is so hot inside that there are no atoms, just protons and electrons separated from their atomic bonds.
Such an electrically neutral collection of charged particles is known as plasma. The motion of the protons inside the Sun support the star, and protons closer to the Sun center have to be hotter and move faster to support greater amounts of overlying material. This measurement, together with a concurring one made using a different technique that was published 2 in Science in September, has been known to experts since last year.
Physicists use two main techniques to measure the size of the proton. One relies on how electrons orbit atomic nuclei. Because some electron orbits pass through the protons in the nucleus, the size of the protons affects how strongly the electrons bind to the nucleus. The second technique involves hitting atoms with a particle beam and seeing how those particles scatter off the nuclei. About ten years ago, it seemed that both spectroscopy and scattering experiments had converged on a proton radius of 0.
But in , a new twist on spectroscopy cast uncertainty on this idyllic consensus. At the Paul Scherrer Institute PSI in Villigen, Switzerland, physicists created exotic hydrogen atoms by replacing the electrons with muons, an elementary particle that is similar to electrons but times more massive.
Because muons spend more time inside the proton, their energy levels are affected much more strongly than are those of the electrons. The team measured a proton radius of 0. Randolf Pohl, who led that muonic hydrogen measurement and is now at the Johannes Gutenberg University in Mainz, Germany, has collaborated on other muonic experiments that have confirmed this value.
For a while, researchers hoped that the discrepancy might reveal a previously unknown difference in how electrons and muons behave — something that could have upset the established quantum theory of electromagnetic phenomena.
More recently, however, improved spectroscopy experiments using ordinary hydrogen found a shrunken proton , suggesting that muons were not so special after all.
The prospect of a revolution in physics began to fade. Those efforts culminated with the Science paper 2. After spending eight years perfecting a spectroscopy technique, the team behind that work found a radius of 0.
But more-traditional spectroscopy experiments done at Sorbonne University in Paris continued to disagree with this result 4. And no one could explain why the scattering technique had pointed to a larger proton.
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