[chris (OP)]

A lady called up the Ask! The Naked Scientists radio show on Talk Radio 702, South Africa, today to ask about an experiment she'd seen regarding a spinning sphere at 600 million RPM (or "500,000 times faster than a washing machine", as  one source observed, presumably feeling they needed to provide a link to something familiar so readers could identify with the story!). This sphere, the woman said, "disappeared" at high speed. I asked her for the reference, which she sent to me.

The study is quite interesting; the group in question were "spinning" a particle using laser pulses to apply a torque.

https://www.nature.com/articles/ncomms3374

So why does the sphere vanish?

[evan_au]

Why did this particle spin at 600 million RPM?

It's due to the rotating electric field of a circularly-polarized laser.

The nanoparticle reverses the circular polarisation of some photons, transfering angular momentum to the nanoparticle. The frequency of the photons is reduced as a result.

The article only claims to have stable rotation at 5MHz = 300 million rpm (and sometimes they saw 10 MHz = 600 million rpm for short periods of time).

So not quite 600 million rpm An impressive gyroscope!
Just the thing you need so your smartphone compass isn't affected by nearby steel structures...

So why does the sphere vanish?

I think that this may be a version of the following statement, after being summarised in the press release, digested by the reporter and rewritten by the editor:

Decreasing the pressure further can lead to rotation rates of up to 10 MHz, although, at such rates, the particle is lost in a short period of time.

They explain this situation at the highest spin rates (ie lowest pressure/most intense vacuum) in sciency language as follows:

the low pressure particle loss might be due to the large inertial forces experienced by the particle at high rotation rates.

Rather than becoming shielded in an "invisibility cloak", I think what they are saying is that the particle disintegrated.

I imagine that a 4μm Vaterite particle spinning at 600 million rpm probably suffers some considerable stresses...

Correction pointed out by Bored Chemist...

[yor_on]

You know what they mean by this Evan?

" As the particle rotates faster, its orientation is stabilized by the gyroscopic effect and as such the particle experiences relative cooling. "

that they no longer use the lasers momentum? And that it then cools? Or??

[Bored chemist]

The article only claims to have stable rotation at 5 million rpm

No.
It claims "We show stable rotation rates up to 5 MHz"
A minute is longer than a second.

[evan_au]

" As the particle rotates faster, its orientation is stabilized by the gyroscopic effect and as such the particle experiences relative cooling. "

My simplistic reading of the article is that these nanoparticles have an effective temperature which is its total kinetic energy of the particle, including motion in various directions: x, y & z axes, as well as rotation.
- When you confine a particle within a beam, the particle will oscillate in these various directions.
- These various directions are coupled - if you constrain the motion in one dimension, it will move more in some other dimension, which could break it out of the confining beam
- So you want to cool the nanoparticle, reducing its total kinetic energy, increasing confinement time, and making it easier to study.
- They refer to other researchers using "active cooling",  which probably uses sensors to measure the particle position in x, y and z axes, and momentarily increase or decrease the laser power to reduce the particle's total kinetic energy.
- However, this paper uses the ability to spin the nanoparticle like a gyroscope, which gives it a degree of stability in x and y axes. This "cooling" helped keep the nanoparticle centered in the beam.
- They estimate a temperature of 40 Kelvin.

Of course, when the particle disintegrates at 600 million rpm, it is no longer at 40 Kelvin!

[yor_on]

ok, they reduce the lasers momentum due to the gyroscopic effect the particle impose on itself. But they used 'relative cooling' as a description of it which confused me. Either it cools or it doesn't.

[evan_au]

'relative cooling' as a description of it which confused me

Yes, it confuses me, too.

Maybe they mean "cooling relative to the same particle in a linearly-polarised laser beam"?
- A linearly polarised laser would not impart angular momentum to this nanosphere, so it wouldn't spin. It would then take on all the usual degrees of freedom.
- Inserting a quarter-wave plate optimised for the laser wavelength will cause it to spin, reducing the degrees of freedom.

They monitored light refracted from the transparent sphere, so they would be able to track movement of the sphere in
x, y and z directions. They also tracked Doppler shift in the refracted light, so they could also measure rotation (additional degrees of freedom).