A. Glaser and R.J. Goldston of Princeton University have conducted research into the risks of nuclear materials associated with proposed fusion power plants being used in the productions of nuclear weapons [1].

The authors identify three main scenarios

1. Clandestine production of weapon-usable material in an undeclared facility.
2. Covert production of such material in a declared facility.
3. Use of a declared facility in a breakout scenario, in which a state begins production of fissile material without concealing the effort.

In the reserach the authors made a quantitative assessment of the risk of nuclear proliferation, that is using the materials produced in fusion plants for weapons.

They show that it is not feasible to build a small-scale nuclear fusion system capable of producing weapons within a couple of years, in a clandestine manner. In essence such a plant could not remain invisible due to the power consumption and dissipation. Such a plant should be quite easy to spot.

This is very different to nuclear fission plants, which can be much smaller in size and use far less power. Such fission plants’ power consumption is similar to lots of industrial processes and so a clandestine fission production of weapons could be hidden in an industrial setting.

The second scenario is more plausible, but it would be very easy for inspectors to identify materials being used for weapon production in any declared open fusion plant. Again, this is not quite so easy in fission plants.

The last scenario proposed is breakout; weapon-usable material is produced very quickly and without concealment. The hope being the producers can get a weapon ready before anyone can stop them. The minimum period to produce any weapon-usable material in a fusion power plant would be one to two months, as estimated by the authors.

It is also easier and safer to stop a fusion plant than a fission plant once in operation. There are lots of other supporting infrastructure needed in fusion like the power input and cooling towers. All these could be interrupted with no risk of nuclear contamination.

In all, fusion power stations would be safer and have less risk for nuclear proliferation than existing fission technologies.

References

[1] A. Glaser and R.J. Goldston, Proliferation risks of magnetic fusion energy: clandestine production, covert production and breakout, Nucl. Fusion 52 04, 2012.

The speed of neutrinos has been measured to be consistent with neutrinos travelling at the speed of light by the ICARUS detector at the CNGS beam. This is in stark contrast to the results of OPERA [1].

The expected time of flight difference between the speed of light from CERN to ICARUS and the actual position of the vertex of the LAr-TPC events has been neatly analysed. The result is compatible with the simultaneous arrival of all the 7 events with the speed of light and not compatible with respect to the result reported by OPERA [1].

M. Antonello et al. [2]

References

[1] T. Adam et al. [OPERA Collaboration], arXiv:1109.4897.

[2] M. Antonello et al. arXiv:1203.3433v1 [hep-ex]

Scott Aaronson has offered $100,000 to anyone that can show that viable quantum computers are fundamentally impossible.

The question is not a practical one, there seems many technical difficulties that have prevented the primitive quantum processors being scaled up to working quantum computers. The question is deeper than this and asks is our understanding of quantum mechanics right?

What is true is that the scaling up of quantum computers cannot be forbidden by some obvious or trivial reason within quantum mechanics. So the bet is to come up with some convincing reason why practical scaled up quantum computers cannot be realised.

Links

Scott Aaronson’s blog

IEEE Spectrum

Scott Aaronson’s homepage

Professor Peter Higgs has been awarded the the Edinburgh Award 2011. The award is organised by the City of Edinburgh Council. The recipients are nominated by the citizens of Edinburgh for their outstanding contribution to the city.

It was back in 1964 that Higgs developed the theory of spontaneous symmetry breaking in the context of particle physics. The Higgs boson is an important, but as of yet undiscovered component of the standard model of particle physics. CERN in 2011 narrowed down the possible mass of the Higgs boson and it is hoped that a proper discovery will be made this year.

The Edinburgh University news report can be found here.

Professor Higgs website can be found here.

The BBC news report can be found here.

We all remember the hype and controversy that followed after the OPERA collaboration at CERN released their report saying that they measured neutrinos travelling faster than the speed of light. The report can be found here.

The OPERA collaboration how identified a rather mundane possible solutions to this…

CERN has released a statement to confirm that the OPERA collaboration has identified a feature of its experiment that could explain its puzzling superluminal-neutrino discovery – a faulty optical fibre. The collaboration is now investigating this, and one other potential source of error, and it plans to carry out new experimental runs in May.

James Dacey is multimedia projects editor for Physics World

There now appears two potential faults with the experiment. First is a faulty optical cable which transmits data in the experiment. Second there maybe issues with the way the GPS systems were synchronised giving an over estimation of the distance travelled by the neutrinos.

Looks like Jim Al-Khalili will not have to eat his boxer short live on TV. Unless he wants to that is!

See the Physics World report here.

See the CERN news report here.

The UK has overtaken the US in terms of the quality of physics-research output, according to a new report carried out by Evidence, which is owned by information-services provider Thomson Reuters. The report, Bibliometric evaluation and international benchmarking of the UK’s physics research, states that the UK is now second to Canada when ranked on the quality of research papers, measured as the average number of times that such papers are cited.

Michael Banks, news editor of Physics World.

For a relatively small country the United Kingdom of Great Britain and Northern Ireland (to use the full name) does rather well in the impact of science research.

You can read the full Physics World news report  here.

For a closed system we have energy conservation and this is deeply tied into the notion of the physics being unaltered under shifts in time. A salient point here is that we have fixed a frame in which to measure the energy. It is not true that the energy is the same measured in any inertial frame, what is true is that it is conserved.

A simple example

Let us consider a free particle of mass \(m\) in one dimension. Let us pick some coordinates \((x,t)\). With respect to this frame we have that the kinetic energy

\(E = \frac{1}{2 m}p^{2}\),

where the momentum \(p \) is given by \(m v = m \dot{x} \). The dot is the derivative with respect to time. (I am using the usuall abuses of notaion here, this should not confuse)

Now let us consider a Galilean transformation

\(x’ = x {-} ut\)
\(t’ =t\).

Then we calculate how the momentum transforms

\(p = m \frac{d}{dt}\left( x’ – ut \right) = m \left( v’ +u \right)= p’ + mu\).

Or \(p’ = p – mu\).

This is exactly what you would expect. So on to the energy…

\(E = \frac{1}{2m}p^{2} = \frac{1}{2} \left(p’ + mu \right)^{2}\)
\(= E’ + \frac{1}{2}mu^{2} + up’ = E’ – \frac{1}{2}mu^{2}+ up\)

so

\(E’ = E + \frac{1}{2}mu^{2} -up\).

Clearly \(E’ \neq E\)

.

Recap

Energy conservation should not be confused with energy being observer independent, it clearly is not.

It is now folklore that general relativity is well tested and that there are no experiments that disagree with the predictions. This goes back to the early days, Einstein calculated the perihelion of Mercury   accurately in 1915  and Eddington in 1919 proved that the bending of light around the Sun is in agreement with general relativity.

Since then there has been many different experiments aimed at testing different aspects of the theory. These include detailed analysis of the time delays of messages to spacecraft all the way to studies of binary pulsars.

What I was not aware of is just how accurate general relativity is.

The Eötvös Experiment

One of the founding pillars of general relativity is the weak equivalence principle. It basically says that the passive gravitational mass is the same as the active inertial mass. This idea is much older then general relativity and Newton was the first to do experiments testing this.

Eötvös used a two equal masses of different composition on a torsion balance to test this principle. More details can be found here. The Eötvös parameter is defined as

\(\eta = 2 \frac{|a_{1}-a_{2}|}{|a_{1}+ a_{2}|}\),

which is the fractional ration of the accelerations of the two masses. If gravity couples differently to the different materials then this should show up as a non-zero value of this parameter.

Eötvös was able to get this parameter down to \(10^{-9}\), so clearly very small.

The Eöt-Wash group at Washington using modern techniques have brought this value to \(\eta \approx 10^{-13}\).

Local Lorentz Invariance

General relativity also requires that locally we have Lorentz invariance. The breaking of Lorentz invariance would imply some universally preferred rest frame. One way to test this is to look at the speed of light. So let us define

\(\sigma = c^{-2} -1\).

Units have been picked here so that the “usual speed of light” is one. So in general relativity \(\sigma =0\) locally.

Examine very carefully the energy levels of atoms and how this changes due to our orientation in the Universe one can test Lorentz invariance of the electromagnetic sector.

Such test give \(\delta \approx 10^{-22}\).

There has been a bit of interest in examining the potential for Lorentz violating in extensions of the standard model. These tend to have motivation from quantum gravity where it is expected that local Lorentz invariance will be broken.

Other tests

Other tests, both direct and indirect have been preformed and all give good agreement with general relativity. This includes:

• The Shapiro time delay.
• The Nordtvedt effect
• The Hulse–Taylor binary pulsar and its decay in orbital period.

This is both reassuring and frustrating for theoretical physics. The lack of experimental direction on what replaces general relativity at the quantum level has, in my opinion, not helped the quest for quantum gravity. But that is another story.

For more details of the experimental tests of general relativity see [1,2].

References

I won’t give references to the original material, see the following for details:

[1] Clifford M. Will. The Confrontation between General Relativity
and Experiment. Living Rev. Relativity, 9, (2006), 3.

[2] S G Turyshev. Experimental tests of general relativity: recent progress and future directions. Phys.-Usp. 52 1, 2009.