It was Einstein who first noted that we have two different definitions of mass.
- Mass is inertia. It's a measure of how hard it is to get something stationary to start moving or to get something that's moving to stop. F=ma.
- Mass is the source of gravity. Mass is attracted to other mass through the weakest force in the universe. F=Gm1m2 / R2
Jamie Farnes, an Oxford University physicist, thinks that some of the mass in the universe is negative. It has negative gravitational mass and negative inertial mass. So Farnes-stuff has a repulsive gravitational effect on itself, but it attracts ordinary matter. In the Farnes universe, negative mass is being continually created, popping out of the vacuum.
What's the motivation for making a radical new proposal? Well, 20 years ago, cosmologists realized that the conventional view isn't viable. On the one hand, they found that galaxies are held together by a force stronger than their gravitational mass can account for. On the other hand, they found that the universe is flying apart faster and faster, as if under the influence of some kind of negative gravity. To solve these two problems, the conventional wing of physical cosmology postulated that 70% of everything in the universe is "dark energy" while 25% is "dark matter", and only 5% is the matter we're familiar with. In order to explain the motions of the 5%, they have invented 95% of stuff that no human has ever seen, heard, or tasted, and that we know for a fact can't be made of electrons, neutrons, protons, or any of the exotic particles observed in high-energy accelerators.
So Farnes-stuff is no more crazy than the theory it purports to replace, and the advantage, says Farnes, is that it's just one thing-a single substance that can play the role of both dark matter and dark energy.
I was skeptical about dark energy and dark matter because they were invented out of the blue to rescue a failing theory. Three hundred years ago, people invented phlogiston to account for the properties of heat, and two hundred years ago there was the luminiferous aether to explain the properties of light. Today, we understand light and heat without the need for these fictional substances.
But I became less skeptical once I saw a video by Yale astrophysicist Priya Natarajan, describing two ways to locate dark matter in maps of the sky. (1) she looked for gravitational lenses-concentrations of matter that bend light from distant galaxies and distort the images, or even cause them to appear in two pieces (2) she reverse-engineered the gravity that binds galaxies together to locate the extra mass that would be needed to keep them from flying apart. Natarajan shows pictures in which these two maps coincide. In other words, two different ways to detect dark matter seem to agree.
The question I would like to see Natarajan and Farnes address is whether the same trick works for Farnes-stuff. Can gravitational lensing and the coherent force in clusters of galaxies be explained in a single map of where the negative mass is hiding?