Seeing Ghosts in Space

One of the best layman’s responses I’ve seen to Intelligent Design’s “problems with Evolution” argument is

Every time you hear them say the word “evolution,” substitute the word “gravity.”

This statement is concise and doesn’t get into the nitty-gritty biological details. It points out the absurdity of the ID argument by drawing a parallel to something we all can understand: gravity. We all can see an apple falling from a tree; we all can (sort of) see the Moon orbit the Earth; we can all feel gravity’s effects when we ride a roller-coaster (well, all the time really). The above statement basically says that we can in fact see evolution happening, just as clearly as we see the effects of gravity. No serious biologist doubts that evolution is real just as no serious physicist doubts that gravity is. The devil is in the details.

What I always found ironic about that statement is that biologists actually know more about the workings of evolution than physicists know about the nature of gravity. Ask ten different physicists to explain the nature of gravity and you might get less than ten different answers only if more than one admits that they don’t really know.

Among the fundamental forces in nature, gravity is the rogue oddball. It doesn’t want to play by the same rules as everyone else. For decades, physicists have been searching for a grand unification theory that will unite all the fundamental forces under one umbrella. Many consider it the last great frontier of science. Michael Faraday made the first move in this direction when he showed the relationship between the electric and magnetic forces. Over the years physicists have been able unite them all, except for one: gravity. They have been able to quantize them all, except for one: gravity. It just wants to be different—so different that astronomers have discovered a type of matter that seems to be impervious to all the fundamental forces, except for one.

Dark matter is a funny animal. We know it’s there because we can see it’s gravitational effect on regular matter. You see, galaxies don’t have enough regular matter in them to keep them from flying apart. So there must be some other type of matter out there (in huge quantities, it turns out) that’s supplying the extra gravitational force to keep things together. Yet, apart from gravity, this dark matter doesn’t seem to interact with regular matter. It doesn’t come together to form atoms, or molecules or stars. It’s just there.

That’s why many physicists feel that it might not exist at all. Perhaps there’s another explanation for this observed gravitational effect. One of the more popular alternative explanations is MOND (Modified Newtonian Dynamics). MOND says that the laws of gravity (i.e. Newton’s and Einstein’s formulae) are wrong and that at galactic distances, gravity is proportionately stronger than the reciprocal of the square of the distance. These hypotheses have recently been dealt a blow by a new discovery made by astronomers around the world.

This discovery relies on the hypothesis that dark matter only weakly interacts with regular matter. One way to think about this is that dark matter only interacts with regular matter gravitationally. This would be like a movie ghost that walks through walls but is gravitationally bound to the earth. (However, most movie ghosts that can walk through walls can also fly, so perhaps ghosts aren’t made of dark matter after all.) But there’s another school (we don’t know who’s right—another mystery of dark matter) that says that dark matter can interact with regular matter if there’s a direct collision between particles of dark and regular matter. These collisions, however, should be so rare as to be practically ignored. Imagine an atom as filling an area the size of a soccer stadium, with a soccer ball sized object at roughly mid-field, several marble sized objects racing around the stands, and all the empty space in between filled by a powerful force field holding it all together. Now imagine trying to hit this atom with a pistol shot. If the bullet is affected by the force field, then hitting the atom is like hitting a stadium-sized target. However if the bullet is made of some dark material which is completely unaffected by the force field, then the atom becomes a soccer ball sized target—almost impossible to hit.

Therefore if two large objects (such as galaxy clusters) should collide, then the regular matter from the objects should interact, slowing down each object’s momentum, while each object’s dark matter, which doesn’t interact, should initially overshoot the collision before being brought back into the fold by gravity, like a stretched spring. If this indeed happens, then the effect should be observable using a technique called gravitational lensing. This method works because light is affected by gravity. Therefore light passing near a massive object—say, the dark matter from a galaxy cluster—will be bent (according to General Relativity) and focused, just as if it were passing through a lens. We should be able to observe this effect from earth, and we did! Furthermore, this observation is incompatible with MOND.

Does this finally prove that dark matter exists? Not quite. Can there still be another explanation for what we’re seeing? Yes, but that is becoming increasingly remote. Are we any closer to actually knowing what dark matter is? No, but I think we’re headed in the right direction.

And if that doesn’t satisfy you, then you can always just chalk it up to the designer.