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I'm a post-doctoral researcher with a wide range of interests. My career is in complex systems science (or maybe cybernetics) and the origins of life, but I also have research interests in

  • the foundations of statistical mechanics and its relationship to information theory
  • Earth systems science
  • non-equilibrium thermodynamics in general

I'm also generally interested in the foundations of quantum mechanics and in black holes, though I wouldn't say I'm an expert on those things.

It's probably worth noting that despite the fact that my research is in physics-related areas, all my degrees are in other subjects. If I occasionally seem to start talking in an alien language, this is probably why.


Aug
29
revised Space translation of operators, states, and particle densities
LaTeXified, homework tag
Aug
29
comment Would this pump water up? and if so, how far?
@Adrian the argument requires you to step back a bit from thinking about osmosis. The point is that any engine that can convert heat at a single temperature into work is a perpetual motion machine of the 2nd kind. It doesn't matter whether it uses osmosis, magnets, antimatter, quantum entanglement or anything else. This allows you to see that there must be something wrong in your reasoning, but it doesn't necessarily make it obvious were the error is. In this case it's the assumption that the air will draw water out through the membrane without evaporating it, as I explained in my answer.
Aug
29
comment What prevents this magnetic perpetuum mobile from working?
+1, very nice. I guess an intuitive version of this answer is that although the magnets are being accelerated in the way shown in the question, there is also a torque on the magnets, making them want to twist around to have their south poles facing the big magnet's north pole.
Aug
29
reviewed Approve Why does a ball bounce forever?
Aug
28
revised What exactly does the holographic principle say?
rolled back to a previous revision
Aug
28
comment Why does boiling water in the microwave make a cup of tea go weird?
Please don't do that, it's really dangerous! You're superheating the water, as user28161's answer says, and it's possible for this to result in all the water boiling at once when you put the tea bag in, sending boiling hot water up into the air.
Aug
27
comment Would this pump water up? and if so, how far?
@Adrian ok, so the thing with heat engines is that you need two different heat reservoirs, one at a higher temperature than the other. A Stirling engine will only run if you heat one side up. The maximum possible efficiency of the engine depends on how much hotter the hot side is. It's called the Carnot efficiency and is given by $1-T_C/T_H$. Taking heat from the environment at a single temperature and converting it into work is impossible, because then the $T_H=T_C$ and the efficiency is zero. A machine that tries to break this rule is called a perpetual motion machine of the second kind.
Aug
27
comment Would this pump water up? and if so, how far?
[...continued] The energy for the work can't have come from gravitational potential, because all the water that moved up has moved down again, and vice versa. Similarly, it can't have come from any osmotic pressure or vapour pressure gradients, because all the salt concentrations are the same in the final state as they were initially, and none of the water has evaporated. All the machine's components are back in their initial state, so none of them can have given a net amount of energy - which means that heat in the environment is really the only option.
Aug
27
comment Would this pump water up? and if so, how far?
@Adrian imagine we start the machine with lots of water in the bottom reservoir, then let it run until lots of water is in the top reservoir, then let the water run back down, powering a water wheel. Now the machine is back in its initial state, but some work has been done. Where did the energy for that work come from? Either it came from nowhere, in which case this is a perpetual motion machine of the first kind, or it came from heat in the environment, in which case it's a perpetual motion machine of the second kind. [to be continued...]
Aug
27
comment Do bifacial solar panels operate at lower temperature than normal solar panels?
The idea that "convection is the dominant heat transfer mechanism and so the differences in optical absorption and emission aren't relevant" sounds a bit weird to me: convection may be the dominant thing removing heat from the cell, but it's absorption of thermal radiation that's putting the heat there in the first place. The less optical radiation you absorb, the less convection has to carry away. At least, that's how it seems to me without knowing much about the specifics of the cells you're talking about.
Aug
25
comment Would this pump water up? and if so, how far?
@Adrian that dictionary definition does not go into enough depth to get the concept across. Please read up on "perpetual motion machines of the second kind." These are machines that take energy from the environment in the form of heat and turn it into work. They are just as impossible as perpetual motion machines of the first kind, which is what your link describes. In general it is a bad idea to rely on dictionary definitions for physics concepts - you should check Wikipedia at the very least, and preferably also a good text book.
Aug
25
answered Would this pump water up? and if so, how far?
Aug
25
comment Would this pump water up? and if so, how far?
@Adrian if you left your machine running, with the water from the top reservoir running back down to the bottom, would it ever stop? If the answer is no then it is a perpetual motion machine. From your description it sounds like this is what you expect.
Aug
25
comment Would this pump water up? and if so, how far?
A quick sanity test for this sort of idea is this: "if it worked, could I use it to construct a perpetual motion machine?" In this case, yes, you could - all you need to do is let the water flow back down from the top reservoir to the bottom again, via a waterwheel, and you'd have an endless source of work without putting any energy into the system. This means that your idea breaks the first or second law of thermodynamics somewhere along the way, and the only remaining task is to figure out where exactly this happens...
Aug
25
comment What are some predictions from string theory that say some crystalline materials “will end up in one of many lowest-energy ground states?”
The third law of thermodynamics is more of a guideline than an actual law anyway. Lots of things can have multiple ground states, and the third law just says that for those things that do have a single ground state, we can define the entropy as zero in that case. So while breaking the second law would be a death sentence for any theory, breaking the third isn't really that bad. (Breaking the first law is pretty bad too, but general relativity seems to have gotten away with it.)
Aug
24
revised Entropy and Information
unfortunate typo
Aug
24
answered Entropy and Information
Aug
24
comment Why is the sky not purple?
@AlejandroMezcua any light sensor will have a spectral response curve. We want digital cameras to show things the same colours as we see them, so they are designed to work as similarly to our eyes as possible. This means they do actually have response curves similar to the ones shown in the answer. You can see some actual examples here: maxmax.com/spectral_response.htm
Aug
24
comment Motivation for preservation of spacetime volume by Lorentz transformation?
This question seems tangentially related to physics.stackexchange.com/questions/31534/… , in that an answer to one of them might shed some light on the other.
Aug
24
comment Motivation for preservation of spacetime volume by Lorentz transformation?
Here is a random thought that may or may not be helpful: as a metaphor, given any smooth dynamical system, we can always find a coordinate system such that phase space volume is preserved, at least in a local area surrounding some given point. Maybe the way to look at this question is to say that if boosts are invertible then there must exist some coordinate system such that the area is preserved. Having established this, we can just assume we're using such a coordinate system and proceed as before - does your proof work if you do that?