Category Archives: Physics
Time Travel
A nit: if “strength of gravity” means the value of g, then it’s incorrect. The amount of dilation is due to the depth in your gravitational well (the gravitational potential), which is important if you compare two planets with each other. Since the force varies as 1/r^2 and the potential as 1/r, it’s possible to contrive a planet whose mass and size are such that gravity (g) is weaker, but you are “deeper in the well” and your clock runs slower (or the opposite). If you are talking about a single planet then the distinction doesn’t matter, but the details do. You don’t want to misapply the model because of a vague description such as this.
At the end he tells us that 24k miles will slow you be about 5 nanoseconds, but you may already have known that.
Two For the Price of One
A while back I put up a post explaining that you can add photons together under the right circumstance. The other day at work someone pointed me to an interesting doubling paper: High-efficiency frequency doubling of continuous-wave laser light (arXiv link)
Second harmonic generation (SHG, or doubling) is a nonlinear process, meaning it has a nonlinear dependence on the strength of the electric field. In other words, it becomes more efficient as you go to higher intensity. Naively, if you need two photons interacting at the same time, the odds of having two right where you need them is higher if you have more photons in your volume.
The higher intensity here is achieved with a resonant cavity, comprised of a mirror and the doubling crystal. The left-hand side of the cavity is the mirror which is mounted on a piezoelectric transducer (PZT) so the cavity length can be tuned to give you a standing wave inside the cavity, made up of an integral number of wavelengths of the light.
The crystal is periodically poled potassium titanyl phosphate crystal (PPKTP), and the right-hand surface is curved and has a dielectric coating to make it highly reflective at both the 1550 nm pump wavelength and the 775 nm output wavelength. Clever! This way you don’t have to worry about additional losses from reflection off of the crystal surface; the other surface is antireflection coated, but that’s never going to give you 100% transmittance.
Since the mirror is reflective at 1550 nm, these photons will bounce multiple times before leaving, so the power can build up. This is shown schematically with the dotted line in a loop, but the actual light profile will be a bowtie shape, so the intensity is even higher inside the crystal. However, the mirror is antireflection coated for 775 nm, so once the light is doubled it leaves. The input beamsplitter to the cavity is dichroic, so it only reflects at 1550 nm and the 775 nm light is free to pass through it.
With this they got 95% conversion efficiency with an input of 1.1 Watts, and think they can bump it to 98% with 1.3 Watts.
Splish Splash
Humble drops of water levitate their way to stardom
First the researchers increase the strength of the field, which flattens the floating drops into discs. They then turn the drops into stars by tuning the field to the resonant frequency of the drops – or exact multiples of that frequency. Using a particular multiple produces a star with the corresponding number of spikes.
Getting Rubbed Out
A step-by-step explanation of the delayed-choice quantum eraser experiment.
Not That Kind of Atomic Wristwatch
No, this is not one of the pretenders that link up to NIST’s atomic time via a radio signal.
$12,000 watch has its own built-in atomic clock
I link to this article because it actually mentions USNO, but there’s the original, which mentions it’s made (or will be made) in Switzerland, meaning this is probably not just a Symmetricom CSAC that’s been marked up with a counter and a display attached, but it’s undoubtedly the same technology.
This watch actually points to a problem in timekeeping, that there are two elements one must worry about: telling the time, and disseminating the time. Having a great clock is not particularly useful if you can’t transfer the information to anyone, so there is a dual, usually parallel effort to improve clocks and to improve time transfer. Time transfer can’t lag too far behind timekeeping or else there’s no point in pushing the boundaries.
Here we have the time transfer problem in reverse. If the input is the stem and you have to look at a display (or listen to a voice) to get the time, it is going to be limited to the feature of not gaining or losing a whole second over some long interval. Which goes out the window because you have to reset it when you change the batteries. The watch really doesn’t require or exploit its precision, so why? It’s really nothing more than an expensive trophy, while some pretty incredible technology is basically wasted. And an analog display? I’d want a digital one that showed the time to better than a second.
However, this does point out the ridiculousness of an episode of Person of Interest from last season, where a very rich guy™ supposedly had a watch that kept time to the nanosecond. 1 second in 1000 years is roughly a part in 10^10, so that’s not even a microsecond per day.
Known Knowns, Known Unknowns, etc.
Physics: What We Do and Don’t Know
Cosmology and elementary particle physics span a range from the largest to the smallest distances about which we have any reliable knowledge. The cosmologist looks out to a cosmic horizon, the farthest distance light could have traveled since the universe became transparent to light over ten billion years ago, while the elementary particle physicist explores distances much smaller than an atomic nucleus. Yet our standard models really work—they allow us to make numerical predictions of high precision, which turn out to agree with observation.
Up to a point the stories of cosmology and particle physics can be told separately. In the end, though, they will come together.
More Fun with Lenz's Law
I’ve linked to videos showing the effect before, but it’s still cool. I notice he spins the magnet — that gives an even faster change to the magnetic field, and enhances the braking beyond what simply dropping it would do. I looked at the effect with a coil, by measuring the induced voltage some time back.
Physics in the News
Dry ice bombs were in the news, with two having gone off at LAX. A bit of a surprise that these haven’t been on the radar much, seeing as they don’t have a chemical signature that could be sniffed out, and have no moving parts. When they blow is not really controllable, but the mechanism is simple — the ice sublimates and pressure builds up. Since a mole of an ideal gas wants to take up 22.4L at STP, while the solid takes up negligible volume. Since the volume is restricted, the pressure builds as more gas accumulates, until the container fails. Boom.
It’s noted that such devices are illegal, but when I ran across a story about a teen getting in trouble for doing this in his backyard a while back, the law was stated (or perhaps paraphrased) in such a way that it would make any carbonated beverage illegal. I have to hope most laws are written better than that. It’s rare, but soda bottles can explode, and small amounts of dry ice in containers can be interesting. Context, i.e. the amount of material, matters, much like how the dose makes the poison.
Quantum Crosswords
Uncertain Principles: Quantum Crosswords: My TED@NYC Talk
[W]hen we look around us, we see waves in water and particles of rock, and they’re nothing alike. So, what bizarre thought process could make physicists think they’re the same? The answer is surprisingly familiar. We were led to the dual nature of the universe through the same process you follow to solve a crossword puzzle.
I don’t mean dictionaries full of words sorted by length. I’m talking about the puzzle as a whole—those theme clues that run all the way across the grid, and contain a five-word phrase or a dreadful pun. You can’t look those up, and you won’t guess them. Instead, you piece them together a letter at a time from the simpler clues that cross them. If all of those other crossing words fit together in a satisfying way, you can be confident that you’ve also got the right the theme answer.
I think this is an excellent analogy. Like the jigsaw puzzle analogy, it conveys that the bit of science one proposes has to fit in with other science (whether part of a model that’s not being replaced and/or the evidence we have gathered) so if your “word” is the wrong length, it doesn’t matter that it matches that you know that “a” is the third letter. It has to fit in with what we observe to be true. (This, by the way, is where a lot of crackpots fail — they’re so fixated on one specific area of science that they ignore all the other bits with which a new theory must be consistent.)
One could take this a step further and note that you always do the crossword in pencil, because at any time you can get to a part that shows all of the other words around them are wrong, even though they made sense when you first filled them in. But the more the words interlock and are consistent, the more confidence you have that you’re right.
Can’t wait for the book to come out.