^{[1]}(The paper is linked to at the end of the article). McCulloch makes a number of controversial claims, but I think his idea on quantised inertia as an explanation of the EmDrive's 'anomalous' thrust is a very interesting connection.

In a nutshell, he postulates that inertia is an imbalance in the Unruh radiation

^{[2]}caused by the different distances to the Rindler horizon

^{[3]}forming behind an accelerating body and the Hubble horizon

^{[4]}around it. Only Unruh radiation with half-wavelengths equal to one of these distances (or harmonics thereof) is allowed, due to the 'one-way' nature of these horizons. This, he reckons, causes a wave pressure on the front of any accelerating object, which we observe as inertia (resistance to change in motion).

Since the Rindler horizon tends to infinity for negligible acceleration, there must be a certain minimum acceleration that can exist - when the Rindler horizon equals the Hubble horizon. Hence acceleration must be quantised and for every object, there must be a minimum inertia (or momentum) that is allowed. Acceleration can be zero, but then it must jump to this minimum quantum, where the Rindler horizon equals the Hubble horizon.

McCulloch then postulates that the tiny anomalous (unexplained) boost in speed that some unbound probes get during Earth fly-by maneuvers originates from this quantised jump when they go through the point of zero transverse acceleration (i.e. changing from positive to negative transverse acceleration). This is probably controversial because there is no proper acceleration in the probe's reference frame, as would be measurable by an accelerometer. But in defense, there is an exchange of momentum between Earth and the probe relative to the Sun, so that total momentum is conserved. He shows some curious agreements between his calculations and the observed anomalies.

McCulloch further conceptually relates this to the different wavelengths of standing waves that can exist inside a truncated cone when microwaves are injected into it and it undergoes a quantum of acceleration. He reckons this results in an imbalanced inertia (or momentum) of the photons, with the 'anomalous' EmDrive acceleration serving to balance the momentum. He further explains this as photons that accelerates somewhat in the wider end of the truncated cone and then decelerates towards the narrower end. He equates this "acceleration of photons" with a slight gain in "photon mass", which is of course very controversial.

Few scientists would buy his "photons that accelerate and gain mass" idea, but we know that a photon carries momentum proportional to frequency, so it could perhaps just be a frequency change. We also know that in any accelerated frame of reference, the observed speed of light is not 'c'. It is only 'c' if measured in a momentarily co-moving inertial observer's frame. So both of these claims may be just alternative interpretations of standard physics, perhaps expressed in imprecise language, scientifically speaking.

Nevertheless, his calculations again agree to the order of magnitude of thrust measured in EmDrive tests in many labs, including NASA. Maybe McCulloch is onto something.

Enjoy!

--oo0oo--

[1] https://www.technologyreview.com/s/601299/the-curious-link-between-the-fly-by-anomaly-and-the-impossible-emdrive-thruster/

[2] https://en.wikipedia.org/wiki/Unruh_effect

[3] https://en.wikipedia.org/wiki/Rindler_coordinates#The_Rindler_horizon

[4] https://en.wikipedia.org/wiki/Hubble_volume