# Smallest possible spinning clock?

1. Oct 10, 2015

### jimgraber

The earth's rotation acts as a clock and defines a rather precise unit of time called the day.
We could go out in outer space and spin a marble and get a reasonable clock.

On the other hand, it is my understanding that you can't do this with an electron,
that is, you can't start an electron spinning and then tell time by looking at the electron.
Is this correct? Assuming it is, what is the smallest object you can use as your “spinning marble”?

Would an atom of tri-substituted methane, ie CHFClBr or flouro chloro bromo methane work?
What about a proton with its 3 quarks?

In short, what object qualifies as the smallest possible spinning clock?

Last edited by a moderator: Oct 10, 2015
2. Oct 11, 2015

### Staff: Mentor

I don't think that you could make them very small and still be a "reasonable clock". The smaller you make them the less angular momentum they have so the easier to disturb they become. A really small quantum scale one would be a bad clock since reading the clock would cause the clock to alter.

3. Oct 11, 2015

### Staff: Mentor

Heck, even the Earth isn't a very good clock, for that reason.

4. Oct 11, 2015

### Staff: Mentor

5. Oct 11, 2015

### jimgraber

This reply has made me think I should measure the size of my spinning clock in units of h-bar.

This then leads to Heisenberg's microscope and the observer effect. So the problem of reading the clock without destroying it may become the main issue. Perhaps I can only read my smallest possible clock once.

This is certainly an issue of quantum mechanics. Clearly if you want a continuously readable clock, you need to be able to put energy in as well as take it out. Google says a single atom laser has been experimentally realized, but I am not sure that qualifies as a spinning clock that you read by looking at its surface. Google also shows that computing the moment of inertia of hydrogen iodide is a common classroom exercise. I assume a HI+ ion would be something you could both spin and also tell one end from the other by scattering photons or electrons, but you would certainly also perturb your timepiece. I will have to think about this more.

True, but some pulsars are almost as good as the best atomic clocks.

Lots of points for making a clock out of a single electron! (or finding one.) This is a very good reference. Thank you.

Last edited by a moderator: Oct 12, 2015
6. Oct 11, 2015

### marcusl

A spinning system can be used to indicate intervals, that is, it functions like an oscillator, but a clock must, in addition, indicate absolute time. That's a harder task that a pulsar or other spinning object, by itself, does not do. Counting cycles of cyclotron resonance qualifies, but the electronics and readout are huge compared to the electron itself (in the case of earth, it's humans who originally counted days--also not a small clock). I think that the question of the smallest clock is still open.

7. Oct 12, 2015

### Staff: Mentor

I'm not sure if the question asks about the smallest complete clock. That would probably be a small part of an integrated circuit.
If you don't care about high precision, use a loop of 3 or 5 NOT (which is unstable and will send a pulse around its loop) and some basic frequency divider logic behind it. Every step allows to run a factor of 2 longer, so 30 steps convert 1 GHz to about 1 Hz, 10 steps more allow to track ~15 minutes, 15 more steps and you have one year. I guess your power source has to be much larger than the actual clock logic.

8. Oct 12, 2015

### marcusl

It has been suggested in the literature that mycoplasmic bacteria, which keeps time (it knows when to divide), is the smallest clock at 300 nm diameter...