Date: June 26, 2010

Title: Dark Matter: Not Like the Luminiferous Ether


Podcaster: Rob Knop

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Description: Dark Matter is the mysterious substance that makes up most of the mass of galaxies and of galaxy clusters. We can’t see it directly; we only see it indirectly, because of the gravitational effects of its presence on the stars and gas around it, or on the light passing near it. Because it’s so elusive, many people instinctively want to reject the evidence for it. Indeed, it is sometimes compared to the luminiferous aether, a theory from the turn of the twentieth century invented to explain an apparent discrepancy between Newton’s mechanics and our theory of electromagnetism. The luminiferous aether does not exist, and those discrepancies were explained by the introduction of Einstein’s Special Relativity. However, Dark Matter is in fact not like the luminiferous aether; it’s not just there because we assume it is, and because we need it to explain discrepancies. Rather, we have direct and positive evidence that it exists.

Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. This semester, he’s teaching physics at Belmont University in Nashville, and next fall will join the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.

Today’s sponsor: “Between the Hayabusa homecoming from Itokawa and the Rosetta flyby of asteroid Lutetia, 13 June until 10 July 2010, this episode of 365 Days of Astronomy is sponsored anonymously and dedicated to the memory of Annie Cameron, designer of the Tryphena Sun Wheel, Great Barrier Island, New Zealand, a project that remains to be started.”


Dark Matter: Not Like the Luminiferous Ether

I am Dr. Rob Knop. This coming fall, I will be joining the faculty of Quest University in Squamish, British Columbia, where I will be teaching physics and related subjects.

Dark Matter. It’s the mysterious substance that makes up most of the mass of galaxies and of galaxy clusters. We can’t see it directly. It doesn’t emit any light, so we can’t observe it as we observe stars. It doesn’t absorb light, so we can’t even observe it the way we observe dust clouds blocking out the light of background stars and nebulae. We can only see it indirectly, because of the gravitational effects of its presence on the stars and gas around it, or on the light passing near it. Because it’s so elusive, many people instinctively want to reject the evidence for it. “Astronomers are inventing a fairy substance to explain problems in their data,” they say. “They have to invent something that we can’t even in principle see to match their observations to their theories. Why should we believe this?” It’s a fair question, but the truth is, the evidence for Dark Matter is rock solid today.

Often, when I give public talks about Dark Matter, somebody will compare it to the luminiferous ether. The implication of this statement is that Dark Matter is something we have invented, something that isn’t real, in order to explain something that we don’t understand. In fact, Dark Matter is not like the luminiferous ether at all. But, in order to convince you of this, I first need to tell you what the luminiferous ether is.

By the end of the 19th century, we had developed a complete theory of classical electromagnetism, summarized in Maxwell’s Equations. This theory described how magnetic and electric fields interacted, and how they interacted with electric charges and electrical currents. A simple manipulation of these equations predicts the existence of electromagnetic waves, or light– light is just an electromagnetic wave. The equations indicate that these waves move at a certain speed, the speed of light. The problem is, it’s not clear from the equations what that speed is relative to.

At the time, all the other waves that physicists were familiar with moved relative to a medium. Waves on the surface of the water move relative to the water. Sound waves are pressure waves in the air, that move relative to the air. If there is a wind in one direction, sound waves move faster in that direction than in the opposite direction, because they’re moving through the air. If the air itself is moving, it carries the sound waves along with it. With electromagnetic waves, though, it wasn’t clear what the waves were waving. What was the medium in which these waves existed? To answer this question, physicists hypothesized the luminiferous ether. This was a medium that permeated the Universe, and was the medium that light waves were thought to be waving, just as the surface of the ocean is the medium that water waves wave.

The luminiferous ether hypothesis provided testable predictions. Specifically, if there were a medium in the Universe that light was moving relative to, we should be able to detect our own motion relative to that medium by comparing the speed of light in different directions. If we are moving through the ether, it would be the same as if there were an ether “wind” blowing across and through the Earth. The speed of light in the direction of this wind would be faster than in other directions. Of course, it’s entirely possible that at the time we did this experiment, the Earth would be at rest with respect to the ether, in which case light would have the same speed in all directions. A better experiment would be to compare the speed of light in different directions at different times during the year. As the Earth orbits around the Sun, at different times during the year it is moving in different directions. As such, if at one time the Earth just happened to be at rest with respect to the ether, it would not be three or six months later.

And, so, physicists performed this experiment, the famous Michaelson-Morley experiment. This experiment showed that light moved at the same speed in all directions regardless of when during the year you made the measurements. This was an experiment that contradicted the prediction of the luminiferous ether hypothesis. Our modern view now is the view provided by Einstein’s Special Relativity, that the speed of light is the same in all reference frames, and there is no particular medium that is waved, and that light waves move relative to. The waving of light is the waving of the electromagnetic fields themselves, and the fact that every observer measures the same speed for them has a whole host of interesting consequences.

The luminiferous ether is left to the dustbin of history. It was an invisible substance invented to explain things that we couldn’t understand in our physics. The mechanics of Newton suggested that you could only specify a speed if it was relative to a specific reference frame, and so we invented the ether to be the thing that this speed was relative to. But the ether does not really exist, and the real answer is that the mechanics of Newton are incomplete, and must be modified by Relativity.

Let’s come back to Dark Matter. Astronomers have known since the mid twentieth century that there was something wrong with the dynamics of galaxies and of galaxy clusters. You can estimate the mass of a galaxy by measuring up all of the “luminous” matter, that is, all the matter that we can detect with telescopes. This includes all of the stars and gas and dust. Luminous matter need not be glowing as stars do; we can also detect atomic hydrogen gas with radio telescopes, and ionized plasma with X-ray telescopes, for example. If we measure all of the luminous mass in a galaxy, we can figure out how strong gravity is in a galaxy. We can also measure how fast the stars and gas in a galaxy are moving, or how fast individual galaxies move within a cluster of galaxies. The result is that things are moving too fast for how much gravity there is! They’re moving so fast that galaxies shouldn’t be held together, but should fall apart. Clusters of galaxies should all be unbound, and the galaxies should all be flying out of the clusters. Yet, we observe galaxies all over the place, and we observe galaxy clusters all over the place, and so they are evidently stable objects.

This discrepancy led to the hypothesis of dark matter. If there isn’t enough gravity from the mass that we can see to hold galaxies together, there must be more mass than we can see. There were many proposed forms of this dark matter, but most of them can be summarized as MACHOs and WIMPs. No, I’m not making this up, those are really what they were called. MACHOs, or “massive compact halo objects”, were proposed brown dwarfs, black holes, very dim stars, or other star-sized objects that weren’t emitting enough light to be included in our budget of massive objects. Indirect searches for MACHOs in our own galaxy have shown that there simply aren’t enough of these underluminous objects to make up the necessary missing mass to hold galaxies together, so we’re left with WIMPs, or “weakly interacting massive particles”.

WIMPs are particles similar to neutrinos, that have mass, but that don’t interact via the electromagnetic force. As such, they never emit nor absorb light. They will pass right through planets and stars as if there were nothing there. They form a sort of gas that permeates and surrounds galaxies. Today, we understand that something like 90 percent of the mass of galaxies and galaxy clusters is made up of these Dark Matter particles.

But hold on, you’re saying. You’ve just invented an exotic new form of matter, a form of matter we’ve never observed on Earth despite all of our high energy particle accelerators, that we can’t see directly, because of an accounting error between how much gravity you think galaxies have, and how fast you see things going. You may ask, aren’t you making the same mistake that was made with the luminiferous ether?

The answer is no. First of all, the luminiferous ether wasn’t a mistake. It was a hypothesis, a reasonable hypothesis, that made testable predictions. Those predictions were tested, and were not verified, so we know that the luminiferous ether was not real. Dark Matter is different. The hypothesis of an exotic Dark Matter made up not of normal protons, neutrons, and electrons also makes testable predictions. Those predictions have been tested, and have been verified. Dark Matter is real.

The oldest line of evidence for dark matter is the accounting error between the speeds of stars in galaxies and the amount of luminous mass. That could also be explained by saying that we don’t really understand how gravity works on the scales of galaxies, and that this accounting error is pointing us to a deeper and better theory of gravity. However, Dark Matter makes other predictions. For example, calculations of nuclear fusion in the very early Universe tell us what the ratios of the light elements ought to be in clouds of gas unpolluted by star formation. Measurements of those clouds have shown us that there can’t be enough baryonic matter– that is, matter made out of normal atoms– to make up the mass we know is in the Universe. The conclusion is that most of the mass of the Universe must be something different from baryonic matter. This is a second line of evidence that points to the reality of Dark Matter. A third is calculations of how structure should change from the early Universe to today; these calculations are able to remarkably well reproduce the general distribution of matter on large scales, but only if we include a dark matter component in the calculations.

Perhaps the best evidence for Dark Matter would be if we could find an object in the Universe where most of the mass was not located where most of the luminous mass was. This would be a smoking gun for exotic dark matter, by itself confirming Dark Matter’s existence even without the multiple other lines of reasoning that lead us to accept that it is real. This smoking gun came with measurements of the Bullet Cluster in 2006. Two clusters of galaxies ran into each other and passed through each other. Most of the luminous mass– the mass made up of protons, neutrons, and electrons– in galaxy clusters is in the form of plasma between the galaxies. When the two clusters passed through each other, the plasma in the two clusters interacted, getting caught up in the middle of the system. Non-baryonic dark matter particles, however, would just stream freely past each other, similar to what happens when you throw two handfuls of sand at each other. X-ray observations of the Bullet Cluster shows the plasma caught up in the middle of the cluster. However, observations of gravitational lensing of the light of background galaxies shows that most of the mass of the clusters is outside of the middle, having freely passed through each other. What we have is a situation where most of the mass of the galaxy is not where most of the luminous mass is. Hence, we have direct evidence for Dark Matter. This is a straightforward prediction of the Dark Matter hypothesis that has been verified by experiment.

Many people are philosophically uncomfortable with the idea that most of the mass in the Universe is made up of stuff that we’ve never directly seen, stuff whose identity is unknown to us. However, science has shown that unlike the luminiferous ether, Dark Matter is real. The luminiferous ether was a hypothesis that made predictions that weren’t verified. There are many lines of evidence pointing to the reality of Dark Matter, and today we know beyond the shadow of a doubt that Dark Matter exists.

End of podcast:

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