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One particle’s trek suggests that ‘spacetime foam’ doesn’t slow neutrinos

The nearly massless particles appear to travel at virtually the speed of light

SPEED TEST A neutrino blasted from a bright galaxy known as a blazar (illustrated) along with a flare of light reveals that neutrinos travel at roughly the speed of light.

An intergalactic race between light and a bizarre subatomic particle called a neutrino has ended in a draw.

The tie suggests that high-energy neutrinos, which are so lightweight they behave as if they’re massless, adhere to a basic rule of physics: Massless particles travel at the speed of light.

Comparing the arrival times of a neutrino and an associated blaze of high-energy light emitted from a bright, flaring galaxy (SN Online: 7/12/18) showed that the neutrino and light differed in speed by less than a billionth of a percent, physicists report in a paper posted July 13 at arXiv.org.

Massless particles — including the particles of light known as photons — consistently move about 300,000 kilometers per second, while massive particles move more slowly. Although neutrinos have mass, their heft is so infinitesimal that high-energy neutrinos travel at a rate effectively indistinguishable from that of light.

Some theories propose that a “spacetime foam” might slow particles of very high energies. The idea is that spacetime on extremely small scales is not smooth, but foamy. As a result, high-energy particles could get bogged down, as if moving through molasses. That effect could cause a significant difference between the speeds of the neutrino and the associated light, which would build up into a delay over the 4-billion-light-year trip from the neutrino’s home galaxy to Earth. But since the flare of light was spotted around the same time as the neutrino, there’s no evidence for such a discrepancy.

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