Wednesday, January 11, 2017

Step Aside, WIMPs!

It seems the search for particles of dark matter has come up short once again, leading some scientists to question whether we should be looking for particles at all. Two of the world's most massive detector projects—China's PandaX-II collaboration and the US's LUX group—have ended up empty-handed in their search for weakly interacting massive particles (or WIMPs), long considered one of the most plausible explanations for our galaxy's surprising rotational behavior.

In a new paper from the LUX lab (Large Underground Xenon experiment), slated for publication today in Physical Review Letters, the collaboration of scientists report that their detector found no trace of particles that match the expected behavior of WIMPs. The experiment involved keeping a close eye on a tank of xenon buried nearly a mile underground. That kind of shielding—along with a 70,000-gallon water tank surrounding the xenon—is necessary to prevent cosmic rays or solar radiation from leaking in and creating a false signal. Among the only things that can penetrate that far into the ground are neutrinos and, theoretically, particles of dark matter.

This is the interior of LUX's water shielding tank—the metal canister at the center of the image holds both liquid and gaseous forms of xenon, which acts as the detector's scintillator material.
Image Credit:Wikipedia user Gigaparsec, (CC BY 3.0)

If the galaxy truly is filled with massive particles streaming through us at all times, as WIMP theories of dark matter predict, they ought to interact at least occasionally with ordinary matter. When that happens, we should see a physical reaction occur seemingly out of nowhere—an atom of xenon suddenly accelerating as though it's been struck by something highly energetic, producing UV radiation in the process. But that hasn't happened—the LUX collaboration's enormous tank of liquid xenon has remained, for the most part, quiet and dark.

This comes just a few months after a similar null-result report from the PandaX-II collaboration, short for (P)article (AND) (A)strophysical (X)enon detector. Yes, the acronym's a bit of a reach—but just look how cute their logo is!

Image Credit:PandaX Dark Matter Experiment

PandaX's detector has the curious honor of being located in the deepest lab in the world, China Jinping Underground Laboratory, nearly 8,000 feet underground. The rock from which the lab's space was carved is almost pure marble, which means extremely low signal contamination rates—you can dig as far down as you want, but if you accidentally set up shop near a vein of radioactive minerals, your data's going to be full of noise.

Going to all that trouble to find a quiet place has paid off, though—the extremely low levels of background radiation have allowed scientists to place some of the tightest constraints yet on the possible properties of particle dark matter. If it's out there, we're closing in on a firm idea of what it must look like, just based on process of elimination.

The hope is that soon, detector projects like LUX and PandaX will either finally spot the WIMPs we've been searching for, or they'll rule out their existence entirely with a thorough enough search. Either possibility would, in its own way, come as a huge relief—direct detection experiments aren't cheap, and some scientists wonder if having multiple countries running nearly-identical experiments simultaneously is the best use of often scant research funds.

Alternative possibilities to WIMP dark matter abound, and although each has its own problems, ideas like Modified Newtonian Dynamics—which posits that gravity works differently at large enough scales—are gaining traction again, partly driven by the continued stream of no-signal results from direct detection experiments.

Whatever the true nature of this mystery turns out to be, it will almost certainly come as a surprise.

—Stephen Skolnick
Title suggested by Eran Moore Rea


  1. PART 1

    Visualize the Complex Number Plane of mathematics; with its horizontal x-axis representing space-time and called the "real" axis, plus its vertical y-axis representing imaginary time. When Max Planck originated the idea of quanta to solve the ultraviolet catastrophe, I'm sure that idea (like so-called "imaginary" time) was initially thought of as a mathematical trick. Albert Einstein thought differently about quanta, and developed his explanation for the photoelectric effect. So it appears entirely possible that imaginary time and the Complex Number Plane will find practical application in the future, at which point they'll cease being mathematical trickery and analytic continuation. Imaginary time will be a real, large-scale thing and since it's known that time and space are always united as space-time, imaginary time will undoubtedly be united with imaginary space: with the word imaginary being only a poorly chosen adjective, and a relic from history. Travel to the right along the x-axis could include things like forward travel in time and General Relativity's "pushing" gravity, which is the result of positive (spherical) curvature of space-time. Travel to the left along the x-axis could be called antigravity and would include things like backwards travel in time* and Newtonian "pulling" gravity, which would be the result of negative (saddle shaped) curvature of space-time. This 180° reversal of direction might be termed the "complex" axis - and since antigravity is equated with dark energy, dark energy receives a gravitational interpretation. I think "complex gravity" is more exact than the terms "antigravity" or "dark energy". If real gravity is involved in ordinary matter's mass-production**, antigravity would conceivably be involved in the mass-production of other matter called "dark" (which would not be WIMPs, sterile neutrinos, axions or any particles that travel forwards in time).

    * John Cramer's 1986 proposal of the transactional interpretation of quantum mechanics (TIQM) says waves are both retarded (they travel forward in time) and advanced (they go backward in time). Also, the existence of both advanced and retarded waves as admissible solutions to Maxwell's equations was explored in the Wheeler–Feynman absorber theory. The waves are seen as physically real, rather than a mere mathematical device. Einstein's equations say that in a universe possessing only^ gravitation and electromagnetism, the gravitational fields carry enough information about electromagnetism to allow the equations of Maxwell to be restated in terms of these gravitational fields. So there are "advanced" (or complex) gravitational waves going back in time.

    **"Spielen Gravitationfelder in Aufbau der Elementarteilchen eine Wesentliche Rolle?” ["Do gravitational fields play an essential role in the structure of elementary particles?"] by Albert Einstein - Sitzungsberichte der Preussischen Akademie der Wissenschaften, [Math. Phys.], 349-356 [1919] Berlin.

  2. PART 2

    ^ Einstein's paper titled "Do gravitational fields play an essential role in the structure of elementary particles?" was written prior to the discovery of the nuclear forces (modern science attributes the stability of atoms and their particles to the strong nuclear force, not the gravitational force). However, it seems to imply to modern science that the 2 nuclear forces are not fundamental but, like the matter they're associated with, are products of gravitational-electromagnetic interaction (a coupling which produces e.g. the mass of the weak nuclear force's W and Z particles). This agrees with theories in which the role of the mass-bestowing Higgs field is played by various couplings (see M. Tanabashi; M. Harada; K. Yamawaki. Nagoya 2006: "The Origin of Mass and Strong Coupling Gauge Theories". International Workshop on Strongly Coupled Gauge Theories. pp. 227–241). By the way - if matter's composition is a gravitational-electromagnetic coupling, and if both gravitational and electromagnetic waves can travel forwards and backwards in time, then all matter has the innate ability to defy modern physics and journey into the past.

    One way of determining if dark matter belongs to a higher dimension would be to measure its gravitational effects in space dimensions (see "A Brief History of Time" by Stephen Hawking – Bantam Press 1988, pp. 164-165). In three dimensions, the gravitational force drops to 1/4 if one doubles the distance. In four dimensions (4th-dimensional hyperspace), it would drop to 1/8 and in five dimensions (5th-dimensional hyperspace) to 1/16. The positive direction on the x-axis (representing the 3 space dimensions of real space-time) is in continuous contact with the negative direction on x (the 5th space dimension of complex space-time). Therefore, real gravity is perpetually amplified by complex gravity. Using Professor Hawking's figures, the amplification equals ¼ x ¼ ie doubling the distance in 5 space dimensions causes gravity to become 1/16 as powerful. It is not ¼ x -¼ since numbers have the same property regardless of direction on the Complex Number Plane (they increase in value). To conserve this sameness, the second one must be +¼ if the first one is +¼. Alternatively, the gravity's strength is reduced 4 times and this number is multiplied by another 4 to reduce it 16 times overall. In the 4th space dimension/2nd time dimension represented by the imaginary axis, this y-axis is half the distance (90 degrees) from the real x-axis that the complex x-axis is (it's removed 180 degrees). So gravitational weakening from doubling distance in 4 space dimensions = (reduction of 4 times multiplied by another reduction of 4 times) / 2, for an overall reduction of 8 times to a strength of 1/8. Only 5 space dimensions can exist – along with real time, imaginary time and complex time.