Joe Fitzsimons
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 Jan 20 comment Where's the best place to add weight to a Pinewood Derby car? @Mark This isn't possible without knowing more about the car. The change in potential energy is $m \Delta H$. To calculate the value of $\Delta H$ we need to know the position of the front axle as well as how low down the weight can practically be placed (the lower the better), we also need $m$ the mass added. Even then, although we can calculate the potential energy, this doesn't translate into a specific speed increase (even if it is all converted to kinetic energy). This is because the kinetic energy scales quadratically in velocity, so we need to know the velocity without the extra weight. Jan 20 comment What is the most efficient way to destroy the universe? Or simply refuse to accept the axioms of logic. Then you can insist whatever you want is true. Jan 20 comment What is the most efficient way to destroy the universe? This makes no sense. What does it mean to make the "strong force weak"? There are different mechanisms that give rise to either force and you can't simply invert their relative strengths at every point in space. Jan 20 comment Using delayed choice interference experiments as a computing device Note: In the case of these examples there is always interference, and it is simply measuring which pattern you get that gives the answer. However it is possible to break the schemes so that you get interference for the answer 0, and so the photon always comes in a particular mode, where as for 1 the coherence is deliberately destroyed, so you get the photon randomly in one of several modes. This is no longer deterministic computation, but can be made arbitrarily close to deterministic. It is worth noting,however,that theres no advantage to this and its more sensible to keep everything coherent Jan 20 comment Using delayed choice interference experiments as a computing device @vonjd: You won't see the pattern instantly, you are limited by the finite speed of light which means it takes a finite amount of time for the light to pass through the network, which will be proportional to the depth of the network. Jan 20 comment Can anybody provide a simple example of a quantum computer algorithm? "Now, a quantum computer isn't just a parallel processor, where you can give it a superposition of the configurations and get back f evaluated on all of them." Actually, it isn't -even- a parallel processor. Quantum parallelism is not the same as having an exponentially parallel classical computer, although an exponentially parallel classical computer can simulate a quantum computer efficiently. Jan 19 comment Where's the best place to add weight to a Pinewood Derby car? @Colin: I did so just before you posted that comment, so I guess you were already typing when I did the update. Jan 19 comment Where's the best place to add weight to a Pinewood Derby car? I've updated my answer below. Jan 19 comment Where's the best place to add weight to a Pinewood Derby car? Ah I see, well in that case there may be a small advantage to placing it towards the rear. I must admit, I somewhat misunderstood what a pinewood derby car is. I would imagine in that setting, however, that friction will play a much bigger role (and to a small extent aerodynamics). Jan 19 comment Where's the best place to add weight to a Pinewood Derby car? Ah ok, so in that case most of the handling concerns are not an issue, so it is less important where you place it but you will still maximize stability by placing it in this position. Jan 19 comment Where's the best place to add weight to a Pinewood Derby car? @Mark E: Actually there is only that effect if the finish line is on level ground. Jan 19 comment What is the statistical likelihood of getting a job as a theoretical physicist? Further, I agree entirely with @Matt that research grad students are also researchers. One difference, however, is that being a postdoc is a job in a way that being a graduate student isn't (whether funded or not). TP postdocs are very clearly professional theoretical physicists. Jan 19 comment What is the statistical likelihood of getting a job as a theoretical physicist? @Noldorin: I do not agree at all. It is an entirely objective fact that postdocs are researchers. They get paid to spend all day doing research. In TP this research is very usually independent, and while the direction of research may depend on the funding sources involved, this is true of all researchers. If you get a grant to study some area, then you are expected to do so whether a postdoc or a fellow of the Royal Society. Almost all postdocs have a number of papers to their name (and often very many) and many supervise students. I can't see any way in which they arn't proper researchers. Jan 18 comment What is the statistical likelihood of getting a job as a theoretical physicist? Wow,wow,wow. TP postdocs can legitimately call themselves theoretical physicists. Jan 18 comment Is there any thing other than time that “triggers” a radioactive atom to decay? @Lubos: Thanks. Jan 18 comment Is there any thing other than time that “triggers” a radioactive atom to decay? @Omega: nope, the particle in a box model isn't a good analogy here as there -are- hidden variables in that case: the position and momentum of the atom. Jan 18 comment Is there any thing other than time that “triggers” a radioactive atom to decay? @dmckee: I said it was -not- an intrinsic property, and I had hidden variables specifically in mind. As regards the increased probability, your point is exactly the reason for the disclaimer immediately following it. To explain it properly would require quite a technical discussion about nuclear structure and interaction cross-sections. Jan 18 comment Home experiments to derive the speed of light? I would imagine it simply uses the falloff in illumination to measure distance. That seems far easier than timing pulses. Nov 18 comment Home experiments to derive the speed of light? @Frédéric: At each hop there is routing overhead. When a packet expires at that hop the processing is different. Nov 18 comment Home experiments to derive the speed of light? @Frédéric: I'm not sure you can simply take the difference of two times in traceroute. It neglects the processing overhead which may be substantially different at each node.