June 1, 2018 | David F. Coppedge

Dark Matter Hunt Comes Up Empty Again

Physicists face a philosophical quandary. Something they deeply wish to believe in does not appear to exist.

What happens when your theory requires something but you can’t find it? That’s the situation with dark matter. Scientists around the world have built ultra-sensitive detectors deep in mines to look for “weakly interacting massive particles” (WIMPs) that would theoretically explain why galaxies and galaxy clusters move the way they do. Dark matter is also an essential ingredient for the leading big bang theory. But the most sensitive test ever cannot find it.

Photograph of dark matter. Frame and white background provided for contrast.

Experimental results from the XENON1T dark matter detector limit the effective size of dark matter particles to 4.1 x 10-47 square centimeters — one-trillionth of one-trillionth of a centimeter squared — the most stringent limit yet determined for dark matter as established by the world’s most sensitive detector.

The results, presented Monday in a seminar in Italy at the Gran Sasso Underground Laboratory (LNGS), were produced using an active target volume of 1,300 kilograms of Xenon, the first search for dark matter that has monitored the equivalent of one ton of xenon for an entire year.

We now have the tightest limit for what is known as ‘the WIMP-nucleon cross section,’ which is a measure of the effective size of dark matter, or how strongly it interacts with normal matter,” said Ethan Brown, a member of the XENON Collaboration, and assistant professor of physics, applied physics, and astronomy at Rensselaer Polytechnic Institute. “With these results, we have now tested many new theoretical models of dark matter and placed the strongest constraints on these models to date.

Physicists don’t like to say that dark matter is not there. They prefer saying they have placed tighter limits on where it could be hiding. Space.com makes the same dodge. Nature is more blunt: “Dark matter detector draws a blank.

The world’s largest experiment intended to detect weakly interacting massive particles (WIMPs) has come up empty-handed after collecting data for nearly a year. XENON1T is located 1.4 kilometres underground at the Gran Sasso National Laboratory in central Italy. The experiment looks out for the tiny flashes of light that should be given off when WIMPs — a popular candidate for dark matter, which is thought to make up 85% of the Universe’s matter — collide with atoms in 1,300 kilograms of cold liquid xenon. On 28 May, researchers from the XENON1T collaboration reported at seminars held simultaneously at Gran Sasso and at CERN, Europe’s particle-physics laboratory in Geneva, Switzerland, that no such flashes were detected. The data suggest that WIMPs — if they exist — interact even more weakly with ordinary matter than previously thought.

This is like saying that ‘ghosts, if they exist, are faster at escaping from our glances than previously thought.’ The hunt is in a vicious cycle:

  1. Dark matter must exist.
  2. Build a more sensitive detector.
  3. No dark matter found.
  4. Return to Step One.

Any particle as small as 4.1 x 10-47 square centimeters is, for all practical purposes, non-existent. That’s almost a trillionth of a trillionth of a trillionth of a trillionth of a square centimeter! An electron (10-16 cm) is inconceivably gigantic by comparison. This doesn’t mean that the proposed particle is that small, but its “effective size” or ability to interact with normal matter makes it that small, practically speaking. WIMPs were thought to be quite large, actually (massive), but the “collisional cross section” (size of the interaction target) has now been so narrowed by this latest search that a successful collision would be like hitting a bull’s-eye on earth much smaller than an electron with a dart thrown from another galaxy. And yet these same theoretical particles are proposed to exert so much gravity that they hold galaxy clusters together and keep the big bang from disrupting the fabric of spacetime.

Will physicists and cosmologists ever quit their so-far vain attempt to find the mysterious unknown stuff? Quitting would be a huge blow to modern physics. It would mean admitting that the Standard Model is incomplete or, at worst, wrong. A scientific revolution may be in the making.

Update 6/02/18: Two physicists at the Harvard-Smithsonian Center for Astrophysics, one of them astronomer Avi Loeb, are assigning properties to dark matter. According to Space.com, they are speculating whether dark matter particles carry an electric charge. Perhaps they should discover the particles first. “The thought that dark matter could somehow be electrically charged seems too bizarre to be anything but science fiction,” reporter Chelsea Gohd admits. The two astronomers base their speculation on one possible interpretation from an instrument called EDGES that measured a particular absorption spectrum in the cosmic background radiation. Still, they confess, “The nature of dark matter is one of the biggest mysteries in science, and we need to use any related new data to tackle it.”

We’ve been watching this hunt for years, curious to see how it turns out (e.g., 20 Jan, 2002, 23 July 2007, 9 Jan 2017; search on “dark matter” for more). We’ve used the case of the Mysterious Unknown Stuff (MUST) that must be there as a test of empiricism vs theory. So far, empiricism is winning. Don’t even ask about dark energy. That stuff is even more mysterious, and physicists don’t even have a theory of what it could possibly be. It’s like watching serious scientists in our own time trying to defend belief in ghosts (30 Aug 2016).

The search recalls physicists’ vain attempts to look for phlogiston as the cause of combustion, or caloric as the carrier of heat. Those two historical non-detections of the 18th century led to scientific revolutions of their own (the oxygen theory, and the mechanical theory of heat). Some of the champions of those occult substances went to their deaths without admitting they were wrong. Are we seeing another case of it now?

We cannot rule out a successful detection. Physicists did find the Higgs boson, didn’t they? Well, at these levels of constraint, any seemingly empirical results become so theory-laden, it’s difficult to ascertain whether they found a real particle or another version of the theory that they can keep believing in. Does that recall a situation in biology? Darwinism, perhaps?



Categories: Astronomy, Cosmology, Physics

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