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seen Jul 7 '13 at 22:47

May
19
comment Can we measure “wavefunction” of quantum particles?
The no-cloning theorem prohibits the copying of an unknown quantum state. Making multiple copies of the same state is required for many experimental systems. If I want a spin-up particle, and I build a system that always generates spin-up particles, then I have made multiple copies of the same state.
Mar
16
comment Is energy always proportional to frequency?
I agree that X is much easier to measure than Y because, for instance, it has a lower mass (X=atom, Y=cat). But you should be careful about saying that something "cannot be measured in principle", which means that it is impossible to elucidate the desired quantity from the experiment on theoretical grounds. Such a statement requires a qualitative, not quantitative, difference. It was long thought that atomic and molecular matter-wave interference would be impossible. I agree that we shouldn't expect a cat-ter wave interferometer any time soon.
Mar
5
comment Is energy always proportional to frequency?
In other words, a cat does have a fantastically large frequency.
Feb
10
comment What would happen if Large Hadron Collider would collide electrons?
"Electrons (as far as we know) don't break apart (and not at these low energies)": I'm scared to imaging what "high" energies are! E.g.: slac.stanford.edu/cgi-wrap/getdoc/slac-pub-7436.pdf
Feb
10
comment Time-reversal symmery and topological insulators
@annushka, Does this answer your question? If so, please mark it. =)
Feb
10
comment Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?
@John, if we're discussing the Faraday effect, it's completely incorrect to say that there's no interaction. The whole point is that you can use materials to couple DC magnetic fields and optical fields. If you restrict yourself to a vacuum, you miss most of the interesting phenomena in electromagnetism and I'd be out of a job.
Feb
10
comment Is a hard drive heavier when it is full?
@OBrien, I mean "much greater than". The energy (or energy barrier) needs to be much greater than the thermal energy to prevent temperature-induced changes in the bits. Since we're thinking in terms of an exponential process, we can satisfy $E>>kT$ with $6 >>1$.
Feb
5
comment Time-reversal symmery and topological insulators
It is an interesting topic, and one that makes the "feel" of statistical mechanics very different that other areas of physics. I think many physicists would disagree with the following statement, but I think of the difference between microscopic and macroscopic reversibility as an example of an emergent phenomena.
Feb
5
comment Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?
The Faraday effect in particular is an off-resonant excitation, hence it's only dispersive. Dichroism is a related effect in which the light polarization it changed or rotated through absorption -- this is what I mean by "relaxation". Incidentally, dichroism is an excellent example where the light becomes aligned with the magnetic field: one ciruclar polarization is absorbed and the other one remains. "Information" about the original state is lost. To distinguish these effects, I'm referring to the way that light can be dissipitated, absorbed, or incoherently scattered.
Jan
27
comment Travel direction of lightning
that's one of my favorite youtube videos.
Jan
26
comment Have we managed to make a perfect vacuum?
@user44430, UHV is mostly limited by H2, not N2 or O2. This is because (1) most pumps (turbopumps, ion pumps, etc.) preferentially pumps heavier molecules and (2) the hydrogen load is from the walls of the vacuum chamber. Air leaking into a vacuum chamber is negligible in most systems.
Jan
26
comment Have we managed to make a perfect vacuum?
In most UHV (ultra high vacuum) systems, including mine, the main component is by far hydrogen molecules (H2). They're light and very difficult to pump out. For that reason, many pumps in the UHV range focus solely on pumping hydrogen (e.g., titanium sublimation or non-evaporable getters).
Jan
21
comment What can the D-Wave quantum computer do?
@Lagerbaer: I think D-wave has a fantastic PR department, for exactly the same reason. They make a lot of incorrect claims and hype their system, which is exactly what a PR department is supposed to do. As a scientist, I find the whole thing highly disturbing.
Jan
20
comment Temperature in space
Temperature is NOT a measure of kinetic energy. It is a measure of how much energy is required to change the energy of a system. Truth be told, temperature can be somewhat subtle in many situations, as the answers clearly demonstrate in this one.
Jan
13
comment Can all quantum superpositions be realized experimentally?
Beware of question like, "Can ... be realized experimentally?" Do you mean that, theoretically, can an experiment realize it? I would argue that's not the same as, "Can an experiment realize ..."? (More below on my answer.)
Jan
13
comment Rutherford's Gold Foil Experiment
As the older professors say: things are easy for us youngin' grad students. I can go to dinner while I acquire data, unlike poor Marsden and Geiger.
Jan
12
comment Does a quantum phase transition have latent heat?
Some do, some don't. Just like classical phase transitions, quantum ones can be first order (with latent heat) or second order (without) -- or something else. Most of the traditional examples that leap to my mind are second order.
Jan
12
comment Why are so many forces explainable using inverse squares when space is three dimensional?
Feynman had an interesting comment: nature tends to be described by laws that can be understood or mathematically expressed in qualitatively different ways. Take a look at Ch. 8 of lecture 2 of the Messenger Series (research.microsoft.com/apps/tools/tuva/#data=3|||).
Jan
12
comment Alkali atom in oscilating electromagnetic field
The rotating wave approximation can be confusing. I originally learned it in the context of a spin-1/2 particle in a magnetic field, where the coordinates of the spin and real space match. It's a little more confusing in general problems, where the real space and spin space coordinate systems are nontrivially related.
Jan
9
comment Where does energy for high and low tides come from?
This is why we always see the same face of the moon: there are no longer tides on the moon. Tidal forces lead to friction, which locks the precessional and rotational rates.