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Journal Club: Friction of the vacuum could slow the rotation of pulsars

Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates. Credit: Wikimedia commons/User:Mysid, User:Jm smits CC BY-SA 3.0

Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates. Credit: Wikimedia commons/User:Mysid, User:Jm smits CC BY-SA 3.0

A vacuum is by definition a space entirely devoid of matter, so one might naturally assume that objects in a vacuum do not encounter friction. However, quantum physics implies that the vacuum is not actually completely empty, but is rather filled with ghostly particles wavering in and out of existence. Now astrophysicists suggest that analyzing rapidly spinning dead stars might reveal whether or not “quantum vacuum friction” exists. The scientists detailed their findings June 1 in the Astrophysical Journal.

The uncertainty principle suggests that the universe can borrow energy from nothing for a short amount of time in the form of “virtual” particles that fleetingly blip in and out of existence in a vacuum. Those particles can potentially exert an influence on their surroundings. Previous research suggested that friction from such virtual particles should slow down spinning objects. The recent findings suggest that one could detect such quantum vacuum friction by looking at rapidly spinning dead stars known as pulsars.

“The most important implication of these findings is the possibility of an effect of quantum vacuum on macroscopic objects,” says physicist Carlo Rizzo at the University of Toulouse III Paul Sabatier in France, who did not take part in this research.

When massive stars die in gigantic explosions known as supernovae, the blasts can compress the remains of these dead stars into extraordinarily dense lumps of neutrons known as neutron stars. Rotating magnetic neutron stars can give off incredibly powerful pulses of radio waves, earning them the name “pulsars.”

Pulses from pulsars are seen each time the dead stars complete a turn. As such, astronomers can very precisely measure the rate at which a pulsar rotates. This can reveal whether or not a pulsar’s rate of spin is changing.

The researchers suggest that a pulsar’s magnetic field should induce virtual particles to generate magnetic fields. The pulsar’s magnetic field should then encounter resistance from the total magnetic field of the virtual particles. This resistance should brake the dead star’s rotation ever so slightly and cause it to warm up. “We believe that given the reasonableness of quantum vacuum friction for the phenomenology of pulsars, one is led to believe it could indeed be an energy loss mechanism in nature,” says study co-author José de Araujo, an astrophysicist at Brazil’s National Institute for Space Research.

There are, though, other ways that neutron stars might slow down. For instance, standard models of neutron stars suggest they slow their spinning by losing energy through what is known as classical magnetic dipole radiation.

However, the researchers say that the effects of quantum vacuum friction can in principle be easily distinguished. Quantum vacuum friction should lead pulsars to decelerate less than classic magnetic dipole radiation would; it should also lead to a faster rate of change in the way a pulsar’s magnetic poles tilt, and should lead to weaker surface magnetic fields over time.

“We were really excited and surprised when we found that quantum vacuum friction and classic magnetic dipole radiation could lead to very different predictions for some physical quantities,” says study co-author Jonas Pereira, an astrophysicist now at Brazil’s Federal University of ABC.

However, Adam Burrows, of Princeton University and not involved in this research, is skeptical that the effects will be seen any time soon, due to the unknowns and complexity of pulsars. “Currently, things are just too messy to do precise work on, or extract precise numbers and geometries for, the pulsar environment.”

Future research should look for evidence from pulsars “to constrain or even falsify quantum vacuum friction,” says study lead author Jaziel Coelho, an astrophysicist at Brazil’s National Institute for Space Research. Coelho would also like to understand the consequences of the heat associated with quantum vacuum friction, and the extent to which it influences the surface temperature of a pulsar—or perhaps other classes of stars.

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