… but I won’t, if only because I don’t have a slo-mo camera as good as the one these guys have (it’s at 2500fps with a Phantom Flex).
Category Archives: Physics
Juuuust a Bit Outside
Short version: How big is a platinum atom ends up with an answer that’s pretty big. I had tweeted a response that I had once seen an answer that was “the mass of the frog is 10^24 kg”, which is a similarly egregious answer. Being able to judge answers like this is tied in with solving so-called Fermi problems — the solutions to which require reasonable estimations and approximations.
I explained the grading policy we had when I was teaching in the navy included having physically realistic answers. If you got something this far off, or a negative number for a scalar, you lost points for not grasping the concept well enough. In the navy that was a little easier to enforce, because our material was grounded in a specific application — that of running a nuclear reactor for propulsion (and other energy requirements) of a boat or ship. But even for the more basic introductory material, we used realistic numbers for whatever we could and discussed reasonableness of answers.
That’s probably a little harder in an academic setting, using a textbook written by someone else who might not adhere to some standard of reasonableness in writing up examples for each topic and dozens of questions at the end of each chapter. But it is something you could be disciplined enough to observe in class and then hold the students to some standard. Still, it’s not unreasonable to expect that students know that a human is around two meters tall and small but macroscopic objects are going to have in the vicinity of Avogadro’s number of atoms in it (give or take a few orders of magnitude), and things like that.
As for the rest of my tweet, I said that a 10^24 kg frog made of silicon that was 10^18m in diameter would probably not gravitationally form itself into a sphere. If the frog has Avogadro’s number of silicon atoms in it, that’s a hundred million atoms in each dimension, meaning the frog would be 10^26 meters in each dimension — bigger than our galaxy — yet have a mass similar to that of earth (6 x 10^24 kg). Since we’re dealing with an inverse square law, an object of equal mass but 10^19 times larger than the earth (around 10^7 meters across) will have a surface gravity 10^38 times smaller. Pretty much negligible.
Master of My Domain
Making magnets speak: the Barkhausen effect
There are two important aspects to this “snapping” of a domain wall past a defect. First, it explains how iron can stay magnetized after the applied field is shut off: the domain walls try and return to their original position, but they routinely get “stuck” on a defect, leaving the metal partially magnetized. Second, this “snapping” results in a tiny spike in the magnetic field produced by the metal — and these tiny sharp changes in the magnetic field are what we are hearing in the Barkhausen effect!
Quantum Loopholes
Demolishing Heisenberg with clever math and experiments
There is, however, an end run around Heisenberg’s uncertainty principle. If you choose to measure things that aren’t paired, the precision with which we measure these properties is limited only by how good our measuring stick is.
IOW, this is not a matter of violating the HUP, just finding a way around it.
Silent Night
Did you catch everything? There were noctilucent clouds, halos, moondogs, and a brief lunar corona.
But my favorite is the phenomenal oncoming storm starting three minutes in.
When Half a Drop is Just Enough
Water-hating knife slices droplet in half
Cutting a drop of water in half may sound like the kind of impossible task given to heroes of folk tales. You don’t need a magic knife, though – just one that really, really hates liquid.
Famous or Infamous?
Backreaction: 10 effects you should have heard of
It actually goes to 11.
Hey Photon, That Black Hole Makes You Look Rather Svelte
Physicists’ Use of Black Hole Studies to Measure Photon Mass Published in Physical Review Letters
“Ultralight photons with nonzero mass would produce a ‘black hole bomb’: a strong instability that would extract energy from the black hole very quickly,” said Pani, the paper’s lead author. “The very existence of such particles is constrained by the observation of spinning black holes. With this technique, we have succeeded in constraining the mass of the photon to unprecedented levels: the mass must be one hundred billion of billions times smaller than the present constraint on the neutrino mass, which is about two electron-volts.”
Writing Press-Release Checks Your Physics Can't Cash
A Clock that Will Last Forever
Imagine a clock that will keep perfect time forever, even after the heat-death of the universe. This is the “wow” factor behind a device known as a “space-time crystal,” a four-dimensional crystal that has periodic structure in time as well as space.
Bold prediction.
Let me say at the outset that I don’t implicitly trust any press release, especially one that gets quantum entanglement wrong or explains it way too vaguely (“an action on one particle impacts another particle” No!), as this one does, so it’s possible this wasn’t fully vetted by the scientists involved.
But there are other reasons to think they are overselling the experiment here. Let me say at the outset that I find the proposal intriguing; it’s not the physics that is in question, and the claims in the press release are not present in the paper. It’s those promises, of what we’ll be able to do with the experiment, that give me pause. Namely:
Imagine a clock that will keep perfect time forever, even after the heat-death of the universe.
Ok, yeah, about that. It might be fair to claim an atom, or possibly a molecule, will survive the heat death of the universe, but a macroscopic device? The device forms a quantum-mechanical oscillator with an ion trap, requiring a certain configuration of electric and magnetic fields, i.e. this space-time crystal is not a physical crystal. Somehow I doubt that the equipment running it will last forever.
If we lose the expectation that this will last a super long time, we still have the idea that it will be a perfect clock, right? Why is this supposed to be perfect?
The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state.
Because the space-time crystal is already at its lowest quantum energy state, its temporal order – or timekeeping – will theoretically persist [for a long time]
It’s true that a quantum mechanical ground state can persist without violating any laws of thermodynamics, and the ground state of a system has a frequency that is infinitely narrow — excited states have a width that is dictated by the uncertainty relation \(Delta{E}Delta{t}>hbar/2 \) but a ground state has an infinite lifetime. Thus, no time uncertainty.
However… (you knew this was coming)
The paper shows that the rotation frequency of the ions in the crystal depends on the magnetic field you apply to it. That magnetic field will not have a perfectly precise value — it will have fluctuations in it, which means that the oscillation frequency is not going to be a delta function — there will be uncertainty.
Not only that, but how do you count the oscillations and discern the phase? That introduces error into any clock — the perturbation of measurement. In most atomic clocks you have a transition at some frequency, and the excited state does have some width to it, which is why long-lived transitions are used whenever possible — it means the transition will be narrow — but the proposal for this clock is to measure where a particular ion is by shining a spatially narrow laser on it. So they aren’t leveraging the infinitely narrow state; I don’t think they can. The mental picture I have is that it would be like counting a wheel’s rotation by painting a spot on its rim and counting how many rotations you have. The problem is that any ion is going to have an inherent location uncertainty, and the laser will add to that because the spot will likewise have a spatial extent. So even if that’s small, it won’t vanish — there will be a measurement uncertainty introduced, on top of the frequency uncertainty from the magnetic field. Not perfect.
Go ahead and blame me for being the reason we can’t have nice things that are perfect and last beyond the heat death of the universe.
The March of the Metro Gnomes
Yay for mechanical coupling, which is enough of an effect to drive these into synch as long as they are all naturally oscillating close to the same frequency. This same effect is/was used in clock shops — pendulum clocks hung on a wall would similarly synchronize, giving the illusion that they must all be wonderfully precise clocks, to all be ticking at the same rate and in phase like that.
Spoiler alert: nothing dramatic happens in the last minute of the video — they just tick away. It’s tempting to try a cadence (There she was, just a-walkin’ down the street…), but the ticks are a bit fast.