High-Velocity Charged Particles Emit Cherenkov Radiation in Quantum Vacuum

According to new research published in the journal Physical Review Letters, charged particles traveling through empty space can emit Cherenkov radiation by interacting with quantum vacuum.

Macleod et al analyzed the properties of vacuum Cherenkov radiation in strong laser pulses and the magnetic field around a pulsar. Image credit: Gerd Altmann.

Macleod et al analyzed the properties of vacuum Cherenkov radiation in strong laser pulses and the magnetic field around a pulsar. Image credit: Gerd Altmann.

It has long been known that charged particles, such as electrons and protons, produce the electromagnetic equivalent of a sonic boom when their speeds exceed that of photons in the surrounding medium.

This effect, known as Cherenkov radiation, is responsible for the characteristic blue glow from water in a nuclear reactor.

According to Albert Einstein, nothing can travel faster than light in vacuum. Because of this, it is usually assumed that the Cherenkov emission cannot occur in vacuum.

But according to quantum theory, the vacuum itself is packed full of ‘virtual particles,’ which move momentarily in and out of existence.

These particles are usually not observable but, in the presence of extremely strong electric and magnetic fields, they can turn the vacuum into an optical medium where the speed of light is slowed down so that high velocity charged particles can emit Cherenkov radiation. This is totally unexpected in a vacuum.

“This is a very exciting new prediction because it could provide answers to basic questions such as what is the origin of the gamma ray glow at the center of galaxies?” said University of Strathclyde’s Professor Dino Jaroszynski.

“Also, it provides a new way of testing some of the most fundamental theories of science by pushing them to their limits.”

Professor Jaroszynski and colelagues found that in extreme conditions, such as found at the focus of the world’s most powerful lasers, and the huge magnetic fields around neutron stars, this ‘polarized’ vacuum can slow down gamma rays just enough for Cherenkov emission to occur.

This means that the highest energy cosmic rays passing through the magnetic fields surrounding pulsars should predominantly emit Cherenkov radiation, vastly in excess of other types such as synchrotron radiation.

“Quantum electrodynamics is one of the best tested theories in physics, with extraordinary agreement between theoretical predictions and experimental data,” said Alexander Macleod, a researcher at the University of Plymouth.

“But this agreement has only been verified in the weak-field regime. Vacuum Cherenkov radiation offers a new way to test whether it survives in the strong-field limit.”

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Alexander J. Macleod et al. 2019. Cherenkov Radiation from the Quantum Vacuum. Phys. Rev. Lett 122 (16): 161601; doi: 10.1103/PhysRevLett.122.161601

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