Virtual particles and the Nobel Prize

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The 2016 Nobel Prize in Physics was recently awarded “for theoretical discoveries of topological phase transitions and topological phases of matter“. The following animation shows one aspect of this research:

Vortex (left) and antivortex (right) emerging from the spins of atoms (arrows) in a thin sheet of magnetic material. Credit: Brian Skinner
A vortex-antivortex pair. Credit: Brian Skinner

Picture a thin sheet of magnetic material, with each arrow representing a single atom and the direction of its “spin”. At the lowest energy, all the spins line up in the same direction. Add some energy, and you can get a “vortex” (left) and “antivortex” (right), which exist in a pair, remaining bound together.

But add even more energy and there is a critical level where the vortex and antivortex can separate. This is named the “Kosterlitz-Thouless transition” after two of the Nobel Prize awardees. It is a phase transition, meaning an abrupt change of state like the melting of ice into water at around 0°C or the evaporation of water into steam at around 100°C. (My summary is based on a very readable introduction.)

The vortex and antivortex almost have the appearance of being literal concrete particles moving to the left or right, however it is clear from the animation they are only emergent from patterns of atoms spinning around. There are many examples of such “virtual” or “emergent” particles in physics, which leads us to an intriguing video by MinutePhysics. (Speaking of abrupt transitions!)

The video describes virtual particles such as an electron “hole” which is simply a gap in an otherwise densely packed sea of electrons. It also describes emergent properties such as electrons behaving as if they had very different mass, charge, or spin, in certain circumstances. Hopefully you will enjoy the physics, or in the very least the spinning Lego models.  :)

Gravitational waves detected!

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Physicists are very excited, because the first ever direct detection of gravitational waves has just been announced! The signal matches the prediction for two black holes colliding.  This will likely mean a Nobel Prize for someone. This is a tremendous scientific achievement, representing a vast global collaboration between scientists, advanced technology, government funding, and simple good luck.

The signal lasted for just 1/5 of a second, but scientists have extracted an impressive amount of information from it. This video plays the “chirp” which was detected, converting the gravitational wave signal to sound so you can hear it. The video repeats the chirp 8 times, half of those scaled to a higher frequency where human hearing is more sensitive.

The LIGO detectors have two 4km long pipes housing laser light for detecting gravitational waves. This is the Hanford, Washington instrument
The signal was picked up by the two “LIGO” detectors in the United States. These have two 4km long pipes at right angles, housing laser light which measures the miniscule expansion and contraction of space caused by a passing gravitational wave. This is the Hanford, Washington instrument; the other is in Livingston, Louisiana.

But understand that calling it “sound” is metaphorical, for instance when someone gave a demonstration by playing a cello on Australian Broadcasting Corporation (ABC) TV. A gravitational wave is a ripple through the “fabric” of space itself and travels at the speed of light, whereas a sound wave transmits via air molecules bumping together and travels a million times more slowly. It should also be clarified that the gravitational waves would have been emitted for a far longer period than 0.2 seconds, it’s just they were too weak to be detected by us.

Gravitational waves are a consequence of general relativity, and were first predicted by Einstein in 1916. Though not an area of my research so far, I have looked in-depth at the measurement of distances in relativity, which is somewhat related. I look forward to learning and sharing more.