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Pulsars Ride Neutrino Rockets

by John G. Cramer

Alternate View Column AV-222

Keywords: pulsars, neutron stars, spin down, neutrinos, cyclotron radiation
Published in the January-February-2023 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 09/06/2022 and is copyrighted ©2022 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

The first pulsar was first discovered in late 1967 by Cambridge University graduate student Jocelyn Bell.  She noticed a peculiar blip in the strip-chart data from the new Interplanetary Scintillation Array, the radio telescope that she had recently participated in building.  She had the nagging feeling that she has seen something similar a few days earlier at the same sky position.  Searching through the many meters of strip-chart recorder paper that the radio telescope generated, she found similar blips.  There were repeating radio pulses coming from the same fixed position in the sky.  She had discovered the first pulsar.  It was a Nobel-Prize-winning discovery (but not for her).

Initially, Bell and her thesis supervisor, Prof. Anthony Hewish, seriously considered the possibility that they might be detecting the repeating signals from a high technology alien civilization.  The Hewish group even used "LGM-1" (for little green men) to name signal they had discovered.  However, now that they understood what to look for, the group soon found similar repeating radio pulses coming from several other positions in the sky.  It was ultimately concluded that the pulses were not signals from little green men, but rather periodic radio bursts from a rapidly rotating neutron star that was beaming out radio waves along its magnetic poles.  The radio beam swept the galaxy as the neutron star spun on its rotational axis, like the light beam of a lighthouse.  The observed radio blips picked up by the Cambridge antenna were made when the pulsar's radio beam swept past the antennaís spot on the Earth every few seconds.  Bell and her colleagues were observing the radio emissions of a rapidly rotating neutron star.

When a normal star burns up all the fuel consumed in the nuclear fusion processes, it goes into gravitational collapse and produces a supernova explosion.  If the remnant left behind by that titanic explosion has a mass of 5 solar masses or more, the gravitational force is so strong that the collapsing star forms a black hole.  If the stellar corpse has a mass of 2.5 solar masses or less, the repulsion from the nuclear force between neutrons comes into equilibrium with the attraction of gravity, and the collapsing object becomes a neutron star.  (Don't ask what happens if the collapsing star's final mass is between 2.5 and 5 solar masses; nobody knows!)

The neutron star that forms in a supernova has a radius of around 10 kilometers (the size of a small town), a density of about 1017 kg/m3 (100 trillion times the density of water), and a spin rate that can be as high as 716 rotations per second at first, but gradually slows down to one rotation per second or less as the neutron star ages and sheds it initial angular momentum over a time of a few hundred thousand years or so.  The pulsar that Bell discovered, now given the name PSR B1919+21, is located in our galaxy at a distance of 978.5 light years from Earth, rotates once every 1.3373 seconds, and has a mass of 1.4 times that of our Sun.  

Since that 1967 pulsar discovery, radio astronomers have been very busy.  They have found over 3,300 more pulsars, and that number is still growing.  Typically, these pulsars have super-strong magnetic fields of 1010 tesla or more, the result of concentrating the magnetic flux trapped in the conducting stellar interior as the parent star collapses.  The angular momentum of the parent star is also preserved during the collapse.  This leads to tiny rotational periods as fast as a few milliseconds for "new" pulsars.  As we said above, older pulsars are known to "spin down" as they age and eventually reach rotational periods measured in seconds.  Some of the pulsars are isolated single stars, while others share a binary orbit with another star.  Such binary pulsars have even been used to observe the rotational slowing down of the binary system as it radiates gravitational waves and loses energy and angular momentum.

Another of the peculiarities of pulsars is their very high average linear velocity.  While most Population-I stars (like our Sun) are observed to have velocities around 30 km/s, the average velocity of pulsars is more than an order of magnitude greater, around 450 km/s or 0.15% the speed of light.  Five known pulsars even have velocities exceeding 1,000 km/s (0.33% c).  In the cases of the relatively recently formed Crab pulsar (950 years old) and Vela pulsar (about 10,000 years old), the velocity direction is known to be roughly aligned with the spin-axis of the pulsar.

This anomalously large value of the pulsar velocity raises many questions:   Why is it so large?  Is it the result of a brief "kick" from the supernova as it was forming the pulsar?  Or did some unknown process produce continuous acceleration?  Did the acceleration come and go in an initial burst, or is it still going on?

Most theoretical attempts to explain such large pulsar velocities have focused on a brief "kick".  They assume that the supernova that formed the pulsar was asymmetrical and that, on a time scale of 0.1 to 10 seconds, it boosted the pulsar to its observed high velocity.  These models assume asymmetrical matter ejection or asymmetrical neutrino emission due to fluid instabilities in the rotation of the collapsing parent star.  Another approach suggests that the acceleration is related to the super-strong magnetic field of the neutron star (1011 to 1012 tesla is needed), implementing a mechanism that creates the velocity boost either through neutrino oscillations or electromagnetic radiation.

All such attempted explanations up to now have had problems.  A large supernova explosion asymmetry, which must occur for every pulsar and result in a "kick" large enough to account for the observed final velocity, seems unlikely.  For the other theories magnetic fields of at least 1011 tesla are required, while most pulsars with velocities exceeding 300 km/s have magnetic fields of less than 5 x 108 tesla.  Thus, the high pulsar velocity remains an astrophysical mystery.

In a June-2022 paper published in The Astrophysical Journal a Chinese team of astrophysicists, Zheng Li, et al, suggest a new pulsar acceleration mechanism: the neutrino rocket.  Their model ties pulsar acceleration to its spin-down rate and supplies a mechanism by which a pulsar can be continuously accelerated along its spin axis.  The work seems to supply the missing puzzle-piece for understanding why pulsars have such high velocities.

At face value, obtaining rocket action from emission of neutrinos seems impossible, even ridiculous.  Neutrinos, very low-mass leptons with no electric charge, are typically produced only in very low-probability nuclear reactions involving the weak force or in slow weak-interaction decay processes like beta decay.  As a rule of thumb, neutrinos are very hard to make and very hard to detect.  However, in the bizarre neutron star environment, where several solar masses of neutrons are jammed in together at nuclear density so that they form a superfluid medium, unlikely scenarios can become rather likely.

When a charged particle like an electron undergoes rapid circular motion, it emits a form of electromagnetic radiation called cyclotron radiation (see my column #176 in the March-2015 issue of Analog).  Invoking electro-weak unification, the Li paper argues that when neutrons undergo the high acceleration of very rapid circular motion, as would be the case in superfluid vortexes in the interior of a neutron star, they should analogously produce weak-interaction cyclotron radiation, radiating neutrino plus anti-neutrino pairs that carry off energy and angular momentum.  Because of the parity violation in weak interactions, the emitted neutrino and anti-neutrino are emitted in the same direction.   That direction is backwards along the spin axis of the neutron star, so that the pulsar should be accelerated  forward along that axis.  This weak-interaction cyclotron radiation is made possible by the large population of neutron superfluid vortexes that should be present within the neutron star.  The neutrino-radiation process removes energy and angular momentum from the neutron star, contributing to the spin-down of pulsars, a well-observed but poorly explained phenomenon.

When neutron stars are formed, the angular momentum of the collapsing star system must remain unchanged, so that the leisurely rotation of the parent star becomes the rapid millisecond-level rotation period of the newly collapsed nuclear-density object.  Shrinking the radius increases the spin rate of the neutron star, like an ice dancer pulling in her arms to spin faster.

Relatively new pulsars like the Crab and Vela have rotation period measured in milliseconds.  As the pulsar ages, its rotation period grows due to angular momentum lost to electromagnetic radiation (including the radio beam of the pulsar) and to cyclotron neutrino emission.  At first, electromagnetic radiation drives the spin down, but as the rotation slows, the neutrino emission comes to dominate the spin- down process.

In the Li paper, the authors show data of the correlation between rotation period and spin-down rate, that rate rising as the rotation period grows.  They also show a rough growth in velocity as the pulsar rotation period becomes longer, i.e., as continued acceleration boosts the velocity.  They can fit this dependence and other correlations with their neutrino rocket model.  We note that the predicted acceleration of the pulsar is not very large, but it has a large effect on the velocity because it always pushes in the same direction and acts for many hundred-thousands of years.

This, then, is currently the best explanation of how and why the observed pulsars have such high velocities.  They are all converting their enormous stored angular momentum to linear propulsion and accelerating in the process.

So, we have a grandiose new way of building rockets, using neutron stars as the driving engines, accelerating as they shoot neutrinos out their backside along their spin axis.  Could an advanced civilization exploit this mechanism for their own interstellar purposes?  Perhaps.

Gregory Benford and Larry Niven have recently given us Bowl of Heaven (2012), Shipstar (2014), and Glorious (2020), a trilogy of hard SF novels describing a star-scale interstellar spaceship powered by a moving star, created by advanced aliens and being colonized by space-faring humans.

Something similar could be done with a neutrino-rocket-propelled neutron star.  The enormous bowl structure of the Benford-Niven shipstar could be replaced by a planet in orbit around the neutron star, brought along for the ride as the neutrino rocket accelerates the entire system.  The acceleration isnít large, so any interstellar neutrino-rocket trips would have to be generational, on the scale of many thousand years.  Steering, control, and perhaps boosting neutrino acceleration might be problems, but not beyond the reach of the imagination of our best hard SF writers.  Any takers out there?

John G. Cramer's 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: or .

SF Novels:  editions of John's hard SF novels Twistor and Einstein's Bridge are available online at:  and .  
John's new hard-SF novel Fermi's Question, sequel to Einstein's Bridge, will be available soon from Baen Books.

Alternate View Columns Online: Electronic reprints of 222 or more of "The Alternate View" columns written by John G. Cramer and previously published in Analog are currently available online at: .


Pulsar Discovery:
A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott, and R. A. Collins, "Observation of a Rapidly Pulsating Radio Source", Nature 217, 5130 (1968), doi:10.1038/217709a0.

The Neutrino Rocket Hypothesis:
Zheng Li, Qiu-He Peng, Miao Kang6, Xiang Liu, Ming Zhang, Yong-Feng Huang, and Chih-Kang Chou, "Neutrino Rocket Jet Model: An Explanation of High-velocity Pulsars and Their Spin-down Evolution", The Astrophysical Journal 931, 123 (2022), doi: 10.3847/1538-4357/ac6cdd.

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 This page was created by John G. Cramer on 11/19/2022.