Suppose you have a motion-detector that looks once per second and notices a change when the chair moves by 0.1m within a second and is completely blind to smaller changes. Then a chair moving at 0.09m/s won’t trigger it at all. Now suppose you add noise of amplitude +-0.01m. Then in most seconds you still won’t see anything, but sometimes (I think 1⁄8 of the time, if that noise is uniformly distributed) the apparent movement will be above the threshold. So now if you do some kind of aggregation of the detector output over time you’ll be able to tell that the chair is moving.
Yes, the cost of this is that above the threshold your performance is worse. You’ll need to take averages or something of the kind to make up for it. (But: when a detector has a threshold, it usually doesn’t give perfectly accurate measurements just above the threshold. You may find that even above the threshold you actually get more useful results in the presence of noise.)
Another example. Suppose you are trying to detect oscillating signals (musical notes, radio waves, …) via an analogue-to-digital converter. Let’s say its resolution is 1 unit. Then a signal oscillating between −0.5 and +0.5 will not show up at all: every time you sample it you’ll get zero. And any small change to the signal will make exactly no difference to the output. But if you add enough noise to that signal, it becomes detectable. You’ll need to average your data (or do something broadly similar); you’ll have some risk of false positives; but if you have enough data you can measure the signal pretty well even though it’s well below the threshold of your ADC.
[EDITED to add:] It may be worth observing that there’s nothing super-special about adding random stuff for this purpose. E.g., suppose you’re trying to measure some non-varying value using an analogue-to-digital converter, and the value you’re trying to measure is smaller than the resolution in your ADC. You could (as discussed above) add noise and average. But if you happen to have the ability to add non-random offsets to your data before measuring, you can do that and get better results than with random offsets.
In other words, this is not an exception to the principle Eliezer proposes, that anything you can improve by adding randomness you can improve at least as much by adding something not-so-random instead.
Suppose you have a motion-detector that looks once per second and notices a change when the chair moves by 0.1m within a second and is completely blind to smaller changes. Then a chair moving at 0.09m/s won’t trigger it at all. Now suppose you add noise of amplitude +-0.01m. Then in most seconds you still won’t see anything, but sometimes (I think 1⁄8 of the time, if that noise is uniformly distributed) the apparent movement will be above the threshold. So now if you do some kind of aggregation of the detector output over time you’ll be able to tell that the chair is moving.
Yes, the cost of this is that above the threshold your performance is worse. You’ll need to take averages or something of the kind to make up for it. (But: when a detector has a threshold, it usually doesn’t give perfectly accurate measurements just above the threshold. You may find that even above the threshold you actually get more useful results in the presence of noise.)
Another example. Suppose you are trying to detect oscillating signals (musical notes, radio waves, …) via an analogue-to-digital converter. Let’s say its resolution is 1 unit. Then a signal oscillating between −0.5 and +0.5 will not show up at all: every time you sample it you’ll get zero. And any small change to the signal will make exactly no difference to the output. But if you add enough noise to that signal, it becomes detectable. You’ll need to average your data (or do something broadly similar); you’ll have some risk of false positives; but if you have enough data you can measure the signal pretty well even though it’s well below the threshold of your ADC.
[EDITED to add:] It may be worth observing that there’s nothing super-special about adding random stuff for this purpose. E.g., suppose you’re trying to measure some non-varying value using an analogue-to-digital converter, and the value you’re trying to measure is smaller than the resolution in your ADC. You could (as discussed above) add noise and average. But if you happen to have the ability to add non-random offsets to your data before measuring, you can do that and get better results than with random offsets.
In other words, this is not an exception to the principle Eliezer proposes, that anything you can improve by adding randomness you can improve at least as much by adding something not-so-random instead.