# Fusion power thirty years away

Economical power generation through fusion has always been "thirty years away", but this time it feels like it really may be just thirty years away. With machines such as ITER, the Z Machine, and the National Ignition Facility, which may be able to far surpass the break-even point, where more energy is produced by fusion than is put into the machines, the prospects look pretty good for commercial fusion power in the foreseeable future. Even inertial electrostatic confinement, which works by smashing accelerated particles instead of heating them to unimaginable temperatures as in the more mainstream approaches, seems promising as a source for fusion power generation, with showstopper flaws apparently having been solved recently.

It's a pity that fusion research has been chronically underfunded, though, given how expensive the research is but with the huge potential payoffs. Ironically, this is also the reason why it's so much harder to pursue promising approaches that could be much cheaper than the mainstream approaches.

# Fusion supporting fission

Practical power generation with controlled fusion is one of those things that has always been "just thirty years away", in contrast to uncontrolled fusion that goes boom from over fifty years ago. But it turns out that controlled fusion does not even have to reach its breakeven point, where more power is generated by fusion than is consumed, to be useful as researchers at the University of Texas at Austin have devised a way to use fusion to reduce nuclear waste.

While atoms such as uranium or plutonium can break apart all by themselves, they're much more likely to split apart when hit by a neutron. Fission in nuclear bombs or nuclear reactors takes advantage of neutrons emitted from splitting atoms, where a dense enough blob of fissionable elements will cause a chain reaction and generate a lot of energy. In fact, one of the issues in nuclear reactors is to keep the neutron emissions low enough lest a meltdown occurs. But nuclear fuel in nuclear reactors eventually reach the point that they are no longer useful for generating energy, as not enough neutrons are emitted to split the still plentiful fissionable atoms. Even worse, the fissionable atoms make nuclear waste radioactive.

What the researchers from the University of Texas at Austin did was to devise a way to use fusion to promote fission. Using a room-sized tokamak, neutrons are emitted from the fusion reaction in the tokamak. These neutrons bombard nuclear waste and promote fission in the material. Not only does this generate energy from the nuclear waste, it also turns much of the fissionable atoms into non-fissionable ones, which greatly reduces the amount of radioactive nuclear waste. The researchers have yet to build an actual system for reducing nuclear waste, though, so it still remains to be seen whether there will be an unforeseen caveat that renders their method impractical.

This method of reducing nuclear waste and generating energy is an interesting reversal of the relationship between fusion and fission in thermonuclear warheads, which use fission to cause a fusion reaction. On the other hand, they aren't so different after all as both end up using neutrons from fusion to enhance fission, since most thermonuclear weapons use the fission-triggered fusion only to force more fission to occur.

# More tests for Lorentz invariance

Lorentz invariance is an essential part of special relativity, which basically specifies that speeds of objects observed by one observer are seen by different observers such that the speed of light always ends up the same. Another way to interpret it is that Lorentz invariance specifies the laws of physics are exactly the same for any non-accelerating observer, no matter what speed they're moving at. Applying this to Maxwell's equations for electromagnetism, this means that the speed of light would have to be the same no matter who the observer is.

Lorentz invariance has become a cornerstone of modern physics, with tons of experiments for special relativity confirming the principle. So it's always interesting to hear about possible new tests that might disprove it. Researchers from Indiana University are proposing experiments to test whether gravity works the same way it does with antimatter as it does with matter. By extending the Standard Model of particle physics, they describe the possible ways in which Lorentz invariance could be violated by the different gravitational influences on matter and antimatter, with possible seasonal changes as the Earth revolves around the Sun.

It is one more way to confirm special relativity that has not been tested before, given that it's likely the proposed experiments will only confirm Lorentz invariance with its track record, even if the headline for the ScienceDaily article makes it sound like there has already been observations of Lorentz violations. Not much is known about the gravitational behavior of antimatter, given how incredibly difficult it is to collect enough of it and keep it from annihilating long enough, and it will be tremendous news if such observations are actually made. However, I do wonder what analyses of previous gravitational free fall experiments that take season into account would suggest, and I also wonder what previous experiments that very indirectly show the gravitational equivalence of matter and antimatter imply for the proposed tests.

# 137

In physics, there are quite a few fundamental constants that describe our universe. The speed of light in a vacuum $c$, the vacuum permeability $\mu_0$, the Planck constant $h$, and the elementary charge $e$ are examples of such constants.

# Big bounce with loops

I'm a sop for articles about fundamental physics. The October issue of Scientific American has an article about how loop quantum gravity might indicate that the universe didn't start with the Big Bang; instead, the Big Bang might actually be a Big Bounce, where the universe started expanding again after a collapse which erased all trace of its previous existence. It appears that at extremely high energy densities, gravity would no longer be an attractive force and instead becomes a repulsive force.