There is more mass in the universe, stuff generating gravity, than we can see. This observation lead to the conclusion that there must be “dark matter” in the universe – stuff out there that we cannot see but is exerting its gravitational effect. At least some physicists believe that this stuff may not be a new type of matter, but may be information itself.

How do we know how much “gravity” there is? Mainly by observing the movement of stars, but essentially any observation about the large scale movement in the universe must agree with our calculations about the effects of gravity. These calculations, based on Newton and Einstein’s general theory of relativity, are extremely precise and confirmed by observation. The first one to observe a disconnect between our theories and observation was in 1933, when Swiss astronomer Fritz Zwicky observed the movement of galaxies in the Coma Cluster. When object rotate about each other their momentum is like a force flinging them apart. Gravity has to balance that force with an inward attraction. We can therefore calculate how much gravitational force their needs to be to keep a galaxy cluster together, and Zwicky calculated that there wasn’t enough, but left the question as to how open.

Then in astronomer Vera Rubin made essentially the same observation of the Andromeda galaxy. She found that the stars in the galaxy rotate at about the same velocity regardless of how close they were to the center. Calculations predicted that the closer stars should be moving much faster than the farther stars. In fact, the stars at the outer portions of the galaxy should be flying apart. She concluded there must be dark matter throughout the galaxy creating enough extra gravity to hold the fast moving stars together. In fact, there must be more than 5 times as much of this dark matter than normal matter we can see. Most of the matter of the universe is invisible and mysterious.

Despite the fact that gravitational theory is pretty nailed down, some physicists proposed the alternative theory to that of dark matter that there was yet another tweak necessary to gravitational theory at the largest scales, so-called “Modified Newtonian Dynamics” or MOND. One of their main arguments is that there are some curious properties to alleged dark matter, if it existed. For example, Vera Rubin did not just observe that there was more gravity in Andromeda than necessary. She observed that there was exactly enough to make the rotation chart for the stars flat – stars rotate at the same speed regardless of distance. This could just be a cosmic coincidence, or there could be some other undiscovered reason that there is always just enough dark matter to cause this relationship. As one MOND proponent puts it:

In fact, there is no reason why normal and dark matter should conspire to mix in just the right way for the mass discrepancy to appear always below a fixed acceleration. This systematic, more than anything else, tells us we might be facing a failure of the law of gravity in the weak field limit rather then the effects of dark matter.

The problem is, observations have not verified MOND and it has fallen in favor, although some holdouts argue it has not been fully falsified either. Perhaps the single strongest argument against MOND, and one though to essentially confirm that dark matter exists, is the bullet cluster. Here two galaxy clusters are colliding. When they slam into each other, the gas clouds slow down due to their pressure on each other. As you can see in the linked picture, the pink in the middle is the observable gas clouds. The blue to either side (this is a composite picture) is the image using gravitational lensing of where the gravity is. So, the stuff with the most gravity (dark matter) seems to have passed right through each other (“collisionless”), while the visible baryonic (normal) matter slowed down in the middle. The galaxies, by the way, are also mostly collisionless and follow the dark matter, but most of the visible mass is in the gas clouds. Many take this one composite image as confirmation of dark matter, and it is powerful evidence, at least for the death of MOND.

Not all MOND proponents gave up, however, and some argue that there is some separation of visible and non-visible matter in the bullet cluster, but you still need MOND to explain the degree of gravitational lensing. This seems like a shrinking minority opinion, however.

All of this gets us to the current alternate hypothesis. Perhaps both camps are partly correct, at least in their statements about what is wrong. MOND proponents may be correct in their criticism of dark matter, and the bullet cluster and other evidence may, in fact, disprove MOND. In other words, both sides are wrong – perhaps the missing mass in the universe is something else, information. But wait, you are probably saying, information is just an abstract concept, not a real thing that can have mass. How is this possible?

The story begins with Elwood Shannon, considered the father of information theory and the digital age. He worked out how information can be transmitted over wires (pretty basic to our modern world), and even coined the term “bit” to refer to one unit of information. Essentially he demonstrated that the flow of information follows mathematical rules. Now, following math doesn’t mean that the math itself is a physical thing, but it is a connection between the abstract and the physical. By now you probably have guessed that quantum mechanics has something to do with this information theory, if for no other reason then that when mind-bending physics that challenges our concept of reality is proposed, it probably has something to do with quantum mechanics.

This all has something to do with the Landaur principle, which is the claim that when a bit of information is erased from the universe, it releases a tiny bit of heat. This principle has apparently stood up to testing in quantum experiments. So if information can release heat, then it has energy, which is also mass (remember Einstein).

Another physicist, John Archibald Wheeler, took this a step further and proclaimed that everything is information. Essentially information is a fundamental building block of the universe. This does square with quantum mechanics in that fundamental particles in their quantum state behave like mathematical functions – probability waves. Those waves of probability can even interact exactly like physical waves. So the idea that information is a physical thing is not absurd, at least in the context of quantum mechanics.

Dr. Melvin Vopson of the University of Portsmouth, in the UK, is now trying to bring all of this together, hypothesizing that perhaps there is not only a mass energy equivalence (E=MC2) but a mass-energy-information equivalence.

Vopson says, “He [Landauer] first identified the link between thermodynamics and information by postulating that logical irreversibility of a computational process implies physical irreversibility.”

Again – the physical world follows mathematical principles, therefore those mathematical principles are physical. I’m not sure I buy that, but I also don’t pretend to understand the underlying physics. More importantly, Vopson proposes an experiment to test it. You could, for example, weigh and empty hard drive. Then fill that hard drive with information and weigh it again. If information has mass, then the hard drive should weigh more. This would test his theory, but the problem is the amount of mass we are talking about is so tiny there is no way to currently measure it. In fact, designing and building a device capable of accurately measuring such a tiny amount of mass would be a huge project, on the scale of building a collider.

Perhaps this will be like gravitational waves – they were first proposed by Henri Poincaré in 1905, then calculated by Einstein in his General theory of Relativity in 1916. But they were not detected until 2015, more than a century later. Perhaps Vopson will have to wait a century for his hypothesis to be tested.