Within Newtonian dynamics, inertia is a fundamental property of matter, observed as the resistance of an object to be accelerated (an object will remain at constant velocity unless acted upon by an external force). This is also known as the inertial mass of an object, and is equivalent to the gravitational mass. Within Newtonian dynamics, it is simply a given property, and there is no explanation for its source, like many other free parameters of the Standard Model.
A recent discussion examines this idea in light of a novel theory about the origin of inertia. The theory in question is that of Physicist Mike McCulloch, who has described inertia as originating from an interaction with quantum vacuum of an accelerating object. There is a theoretical prediction that accelerating objects will observe thermal radiation from the vacuum where observers at rest relative to the accelerating frame will not observe such thermal emission from the vacuum. This is known as Unruh radiation, and is a sister effect to Hawking radiation, in which black holes thermalize.
Unruh radiation has never been observed, but it appears in quantum physics. In quantum theory, empty space can be described as being filled with a quantum field. A vacuum, in this view, is simply the lowest possible energy state for these fields. In most cases empty space looks like a vacuum as we’d expect, but for an accelerating observer the field has an observed energy. As a result, an accelerating observer would be heated by quantum particles known as Unruh radiation. McCulloch argues that when an object accelerates it interacts with Unruh radiation, which causes the object to resist a change of motion. Thus inertia is an effect of acceleration rather than an inherent property of matter.
The Unruh effect in standard quantum theory is extraordinarily small. If you accelerated a trillion times greater than Earth gravity, you’d only see a thermal temperature of 40 billionths of a degree above absolute zero. Furthermore, since Unruh radiation comes from all directions, it couldn’t create the effects of inertia on its own
But rather than be deterred by this, McCulloch adds other effects into the mix. Since the observable universe is finite, the wavelengths of Unruh radiation is limited, and combined with a cosmic Casimir effect and a bit of information theory, can somehow produce the effect of inertia .
McCulloch’s theory describes an anisotropic force produced by Unruh radiation around an accelerating object due to a Hubble-scale Casimir effect. In the Casimir effect, a force is produced between two closely spaced parallel plates due to the exclusion of certain wavelengths of zero-point fluctuations in the vacuum between the plates. McCulloch describes a similar type effect experienced by an accelerating object, where the cosmological horizon and a dynamical Riddler horizon act as the “plates” which disallow certain wavelengths of the Unruh radiation behind the accelerating object. There will therefore be a net force from the Unruh radiation opposing the direction of acceleration, effectively creating a resistance to acceleration, or inertia.
McCulloch’s model has been in the works since 2008, but it has become popular in recent years due to its connection to the EMDrive. It is the device that (according to its proponents) can create a thrust without any traditional propellant which could revolutionize space travel and take us to the stars. The EMDrive has created quite a stir among the general public because of the tremendous possibilities if it succeeds. Meanwhile, scientists have noted that even the best experimental results can’t be distinguished from background noise, and that such a device would violate basic physics. McCulloch argued that the effect was not only real, but that it could be explained in the context of his model.
DARPA invests in propellant-free rocket theory
Physicist Mike McCulloch plans to use a $1.3 million grant from the federal agency DARPA to prove his quantized inertia theory is more than just a spark plug for heady debates on online physics forums. McCulloch believes his ideas about quantized inertia and Unruh radiation can inspire the creation of a rocket engine that turns light into thrust without the assistance of a chemical propellant.
Chemical rockets are very expensive because of the explosive propellant they need, so this new kind of thruster would be much cheaper and safer as it would only need a source of electrical power to accelerate the core of a thruster. The research is being funded through DARPA’s Nascent Light-Matter Interactions (NLM) programme, which aims to improve the fundamental understanding of how to control the interaction of light and engineered materials.
Engineers at DARPA, the Defense Advanced Research Projects Agency, think McCulloch might be onto something. “There is increasing global activity in space,” Mike Fiddy, program manager for the Nascent Light-Matter Interactions program in DARPA’s Defense Sciences Office, told UPI. “DARPA is seeking to deepen our understanding of how to move objects around in more energy efficient and versatile ways.”
McCulloch thinks imbalances in Unruh radiation can be used to generate a more energy efficient thrust. “Uhler radiation is a kind of radiation that you see when you accelerate,” McCulloch, a professor of physics at the University of Plymouth in England, told UPI. “When you accelerate, a horizon radiation appears behind you, and the radiation emanates from this horizon the way Hawking radiation is emitted by the horizon of a black hole.”
“One definition of quantized inertia is that the force we know as inertia is caused by a gradient in this Uhler radiation,” he said.In previously published papers, McCulloch has used his quantized inertia theory to explain galaxy rotation without the presence of dark matter, as well as the thrust achieved by the EmDrive.
The EmDrive was NASA’s attempt at developing a propellant-less rocket engine. According to the EmDrive’s inventors, the engine musters up a bit of thrust by bouncing microwaves from one end to the other of an unevenly-shaped container, creating a difference in radiation pressure and generating drive — although a study earlier this year questioned whether it worked at all.
“I believe that the EmDrive is a manifestation of quantized inertia,” McCulloch said. He said a different set of experiments may produce more powerful QI-powered thrusts.
Before the DARPA grant, announced this month, is used to build experiments, it will fund more theorizing. “The first thing the money will allow me to do is hire a postdoc,” McCulloch said. With the help of a postdoctoral researcher, McCulloch plans on building out and filling in his quantized inertia theory. “We’re going to try to develop a numerical model to make the theory fully predictive,” he said.
After 18 months of theory-building, the grant will help fund experimental teams in Germany and Spain, which will build a pair of thrust-producing experiments. “One such experiment is a shielded laser loop, and another uses asymmetrical mirrors and laser light,” McCulloch said.
If the experiments succeed, as predicted by McCulloch’s theory, researchers will look for ways to enhance the thrusts. Throughout the different phases of research, McColloch hopes to continue to use his ideas to explain and understand observable astronomical data.
“I think that the strength of the theory is that it explains a lot of things on different scales — both at the cosmological level and the level of the laboratory,” he said.
McCulloch’s theory purports to explain cosmological phenomena more accurately than the Standard Model, which relies on the existence of dark matter. Coincidentally or not, he’s faced pushback from some physicists. “I have gotten a lot of resistance from people who believe in dark matter,” McCulloch said. “They don’t like it at all, as many scientists and universities receive a lot of money to build expensive machines looking for the stuff.”
But even if mainstream physicists are reluctant to engage with McCulloch’s ideas, DARPA was interested enough to open the coffers. They might be willing to do so again, should the right idea come along.
“The broad interest in understanding more about how electromagnetic waves and matter interact will continue to stimulate new hypotheses and theories,” Fiddy said. “When these new ideas are relevant to DARPA’s mission and are testable, the agency may well support them.”