photons

I've mentioned before that I'm answering the occasional question over at the Physics Stack Exchange site, a crowd-sourced physics Q&A. When I'm particularly pleased with a question and answer, I'll be promoting them over here like, well, now. Yesterday, somebody posted this question: Consider a single photon (λ=532 nm) traveling through a plate of perfect glass with a refractive index n=1.5. We know that it does not change its direction or other characteristics in any particular way and propagating 1 cm through such glass is equivalent to 1.5 cm of vacuum. Apparently, the photon…
In my post about how we know photons exist, I make reference to the famous Kimble, Dagenais, and Mandel experiment showing "anti-bunching" of photons emitted from an excited atom. They observed that the probability of recording a second detector "click" a very short time after the first was small. This is conclusive evidence that photons are real, and that light has discrete particle-like character. Or, as I said in that post: This anti-bunching effect is something that cannot be explained using a classical picture of light as a wave. Using a wave model, in which light is emitted as a…
A reader emailed me with a few questions regarding How to Teach Physics to Your Dog, one of which is too good not to turn into a blog post: What is a photon from an experimental perspective?... Could you perhaps provide me with a reference that discusses some experiments and these definitional issues? The short form of the experimental answer is "A photon is the smallest amount of light that will cause a detector to 'click.'" (For some reason, hypothetical light detector technology has never really advanced past the Geiger counter stage-- even though it's all electrical pulses these days, we…
Last week, Dmitry Budker's group at Berkeley published a paper in Physical Review Letters (also free on the arxiv) with the somewhat drab title "Spectroscopic Test of Bose-Einsten Statistics for Photons." Honestly, I probably wouldn't've noticed it, even though this is the sort of precision AMO test of physics that I love, had it not been for the awesome press release Berkeley put together, and this image in particular (grabbed with its caption): This is a nifty paper, and deserves a little explanation in Q&A format: Is this another New Scientist style "Einstein was wrong" paper? No. If…
There have been a bunch of stories recently talking about quantum effects at room temperature-- one, about coherent transport in photosynthesis , even escaped the science blogosphere. They've mostly said similar things, but Thursday's ArxivBlog entry had a particular description of a paper about entanglement effects that is worth unpacking: Entanglement is a strange and fragile thing. Sneeze and it vanishes. The problem is that entanglement is destroyed by any interaction with the environment and these interactions are hard to prevent. So physicists have only ever been able to study and…
Quantum mechanics is not my area of expertise. Really, I have no area of expertise. However, I think it is time to bring the whole photon thing back up. Yes, I know I was a little harsh before. Maybe I should start over. First, models. Yes models. I think science is all about models. Scientists build models that attempt to agree with observations. These models could be mathematical, physical, conceptual or numerical (like a computer program). For example, take Newton's Law of gravity (which isn't really a law). It says that the gravitational force between two objects has the…
This has been all over my inbox since the press release came out yesterday; it's been on slashdot (thanks Brian), it's been at space.com, and there's a mediocre writeup on Universe Today. What's the big news? Black Holes don't destroy information after all! What is this whole information thing, anyway? Take a look at all the normal stuff in the Universe: photons, protons, neutrons, and electrons, for example. They have lots of different properties each. They move around one another, they get bound and unbound from one another, they exert forces on one another, etc. They're aware of one…
So, what's the deal with this one? startswithabang.com reader Scott Stuart asks the following question: I was reading "The First Three Minutes" last night and came across an interesting section about blackbody radiation and energy density. The author states that as the universe expands, the number of photons running around (in the CMB, for example) is unchanged, but their wavelengths get stretched. The energy in a photon is, of course, inversely proportional to its wavelength, so the energy content of a photon decreases as its wavelength increases. That seems to mean that the total energy…