A wish for 2013
Ken Tapping, January 2, 2013
One of the science highlights of 2012 was confirmation that the Higgs Boson exists. A wish for 2013 could be that we make some solid progress in finding the true nature of Dark Matter.
Whenever we measure the movements of stars in galaxies and how member galaxies move within their clusters, we encounter the same puzzle. There has to be more material out there than we can see.
How all the other bodies in our universe move is dictated by the force of gravity. Every body pulls at everything else with a force related to how much material it contains, that is, its mass. Therefore if we look at how something moves, we can calculate the size of the gravitational force acting on it, and the mass required to provide that attraction. However, when we use this technique to determine the masses of galaxies and clusters of galaxies, we end up with needing far more matter than we can see. By a large margin, there is more of this invisible material out there in space than there is of the ordinary visible kind, which we and our world are made of. This invisible material has come to be called “Dark Matter”, and finding out what it is has become important in learning how galaxies form and evolve. There is an ongoing, worldwide effort to actually see dark matter, so far with no success.
However, this is not as magical a situation as it might seem. Everything we have learned about the universe beyond the Solar System has be done through observing electromagnetic waves, such as light, radio waves, infrared, ultraviolet, X-rays and gamma rays. If something exists that does not produce or interact with electromagnetic waves we aren’t going to be able to see it, no matter how big a telescope we are using.
No self-respecting scientist is happy with introducing something that solves a problem but is otherwise undetectable and therefore unconfirmable. However, there is a possible solution, and it comes not from looking out into space, but using the Large Hadron Collider.
On the whole, our universe is fundamentally well behaved. This comes largely from its stable foundations: how things work down at the level of atoms and the particles of which they are made. This means to investigate things at the foundation level needs huge amounts of energy. The Large Hadron Collider is a tool that should meet most of those energy requirements. However, if you make a particle of dark matter, how do you know?
The trick is to keep careful track of where the energy you put into the process ends up. You accelerate a particle to a known energy, and then measure the energy of all the products of the reaction. If some of that energy went into making a particle of dark matter, it will seem as though that amount of energy has gone missing. If you then repeat the experiment over and over again, in different ways, and always end up missing the same amount of energy, you are probably on the track of dark matter. It’s not the most satisfactory way to search, but if what we’re looking for is invisible, we are stuck with it.
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