Science Seen Physicist and Time One author Colin Gillespie helps you understand your world.

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WIMPed Out?

The mystery of the missing WIMPs is heading for a crisis. It’s looking like the reason we can’t find them is they don’t exist.

Most people never heard of them. What is a WIMP? It is Dark Matter by another name. It stands for Weakly Interacting Massive Particle. They must be weakly interacting because no-one’s ever seen one interacting. That is to say, they may be imaginary. And they must be massive because that is the main reason for imagining them.

We know that Dark Matter exists. We know there’s five times more of it than matter we can see. So the observable universe holds some 150 trillion trillion trillion trillion tons of it; that’s a lot of matter to be missing. We know it is affected by gravity and so it tends to hang out in galaxies. Or maybe it is galaxies that hang out with dark matter? And we know nothing more. We don’t know that Dark Matter’s made of WIMPs.

So the hunt is on for missing matter of some kind. That most of the universe’s matter is missing is no longer news. That so much should be missing is a huge problem, both literally and conceptually. The central question is: What is it? And, as always, why should we care? Of course we could ask the same questions about the matter that we see. We don’t know what it is either (though readers of Time One may have ideas). But because we see it we know where it is, we give its bits names and smash them into smaller bits to try to understand it so we can make it do more good things for us.

Ever since establishing the awkward missing-matter facts, physicists have favoured the idea that there must be a new kind of particle with so much mass that gravity attracts them into galaxies where they do nothing more than hang around. Hence WIMPs.

WIMPs have theoretical credentials. The Standard Model needs a whole new set of particles to prop it up. Superstring theory provides them (and calls them supersymmetric). It predicts about the right number of tons of WIMPs. The Large Hadron Collider was supposed to make many more of them.

After trillions of collisions, the LHC has found no WIMPs, indeed no supersymmetric particles at all. The pressure’s on. As I mentioned in The Supersymmetry Calamity a year ago, the atmosphere at CERN was already being described as ‘panic’.

Of course if WIMPs are real we don’t need to manufacture them. They are all around. More than two xenon1t_outercryostatdozen different experiments have been trolling for them. All produce the same result. Nada. The latest and perhaps last try is known as XENON1T. It should start looking for WIMPs next month deep in an Italian mountain called Gran Sasso d’Italia. If XENON1T too finds none, maybe even the most dedicated fan of WIMPs will abandon them.

My money, as readers will recall, says that the missing matter is invisible black holes that were created in the first moments of the universe. That bet started to look even better with last week’s news that LIGO had detected the collision of two thirty-solar-mass black holes. Black holes were long ago written off as Dark Matter candidates because theory says such medium-sized black holes should not exist. And now our first direct black-hole detection finds not only one but two of them.

That’s two of them bumping into each other! Should we be rethinking more than WIMPs? All this seems to be heading for new physics. And new physics is good for us all; it builds a foundation for new science and it is the best tonic for the health of the economy.

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