## Archive for September, 2012

### 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.

### There's More Than One Way to Crack an Egg

Cracking Eggs 101

[S]cience has finally come up with an explanation for what comes naturally to most home cooks: Eggs crack best around their equators, says MIT mechanical engineer Pedro Reis, because of their geometry. He and a young colleague, Arnaud Lazarus, have just published a paper in Physical Review Letters demonstrating a link between an eggshell’s geometry (it belongs to a class of shapes known as ovoids) and a mechanical property called rigidity—the quality that, along with strength, determines how much force an object can withstand before it cracks.

The Infinity Set

The set includes four pieces:
Cantor fork :: now you can pin a single kiwi seed. Twice in a row.
Recursive spoon :: it will never let you spill a drop of soup. Ever.
Koch knife :: to delicately cut hair-thin slices out of an egg. A raw egg.
The Infinity Set :: the set includes itself. As a subset.

One keyword is “contest” so I don’t know if this is simply an artistic concept or a product that will appeal to the geek crowd. I want to use the Koch knife to cut a Möbius strip of bacon.

### The Gender Bias of Physics

A couple of posts relating to gender issues in physics that I have run across recently.

Scientists, Your Gender Bias Is Showing

To test scientist’s reactions to men and women with precisely equal qualifications, the researchers did a randomized double-blind study in which academic scientists were given application materials from a student applying for a lab manager position. The substance of the applications were all identical, but sometimes a male name was attached, and sometimes a female name.
Results: female applicants were rated lower than men on the measured scales of competence, hireability, and mentoring (whether the scientist would be willing to mentor this student). Both male and female scientists rated the female applicants lower.

*Sigh* For whatever reason I’m having trouble accessing the actual article, so I don’t know if they show how much worse this might be than in general (assuming that there is gender bias elsewhere, and I’m pretty sure there is), and if there is an age component, i.e. is this more of a problem with older folks, who might soon be removing themselves from being part of the problem. However, that’s a small and faint hope, having observed some of the attitudes displayed in some corners of the internets and in the blogohedron, where presumably the age bias might be in the other direction. I recognize that certain types of change might occur on generational time scales, but it’s 2012, and we (well, women, actually) are still dealing with crap like this.

The second post deals with wondering why women stay, and why they drop out of the physics pipeline.