From: "Bill Hawkins" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Are there limits to the accuracy of clocks? Date: Thu, 30 Mar 2006 16:40:05 -0600 Message-ID: <[EMAIL PROTECTED]>
> > "While we have been discussing stuff that is about 10E-20 below > what we can measure, is it possible that the limit of resolution > of the measurement of time is determined by noise? Specifically, > thermal or Johnson noise in those 50 ohm impedances that we > use for our cabling." > > This didn't provoke any response, so maybe you think I'm crazy. > You could still be right, but let me explain. > > We are talking about the accuracy of clocks, which are basically > digital counters that are mostly immune to noise. You also need > an oscillator that generates something that a clock can count. > > Noise gets into it at the interface between the oscillator and > the clock. Assuming electronics (because mechanical oscillators > have too many sources of error) then something electronic has to > detect the periodic oscillations and turn them into pulses for > the clock's first counter/divider. Noise in the detector will > cause jitter in the clock. > > The problem can be reduced by increasing the signal to noise > ratio. Can amplifiers be made noise-free? Certainly not in > radio receivers. We are talking about accuracy better than > 10E-15. Johnson noise in 50 ohms is around 10E-8 volt at 100C > with a one MHz bandwidth. You would need a signal of around > 10E+7 volts to swamp the noise. > > Precision oscillators are cryogenic these days, which could > eliminate thermal noise. > > Truly random noise can be reduced by filtering or averaging > over time. The amount of time available depends on the use of > the clock. A time-of-day clock should have no error from noise. I'm not an expert in physics, but I have not perceived this uncertainty as noise such as Johnson noise, even if noise may be a good way to explain it. It is an uncertainty in measurement of time and space, such as if distances itself had a background noise (mind you, not temperature dependent). It's in the fabric so to say. If you figure out a way around it you'd probably come here to Stockholm after a few years, December time. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
