The Observer Effect in Quantum Mechanics

Transcript

First of all, I think it is, you know, there there are lots of different proposals on the table now for how to understand quantum mechanics, but I think it's fair to say that something that all of them share in common is that there was a certain fantasy that was very important to pre-quantum mechanical physics of at least the principled possibility of a kind of observation that was perfectly passive. Um um that you could by by by investing enough work reduce the degree to which you actually in a significant way physically interacted with something by measuring it and so on and so forth. something that's happened as a result of quantum mechanics that it doesn't look likely is ever going to be reversed because it's it's common to all ways of understanding quantum mechanics that I know of. That observation is is a sort of irreducibly violent process. Um um that having been said it doesn't you know that discovery I think or one way of telling the story is that that discovery produced all kinds of further extrapolations which turned out to be radically unjustified about the about the about the the impossibility of constructing a sort of old-fashioned scientifically realist picture of what was going on in the world.

And subsequent developments have shown that that's not right. Um um there is this fact about observation. There is this fact about making measurements. Um that that in the typical case you're not going to be able to reduce the disturbance to zero. But the disturbance is something you can understand in qualitative detail.

um um um the the the disturbance is something that that you can account for as part of a thoroughly objective realistic picture um of what's going on in these situations. So, as I say, as a historical matter, this was extrapolated um way way beyond what was warranted by by the actual situation. And people came up with all kinds of um goofy quasy mystical sounding attitudes, right? I mean I mean sort of the the idea that somehow through the act of observation we're creating reality or yeah or or something like that. You know what we are doing is you know you only it's true that you can only become sensually aware of stuff if in one way or another you knock into it. Okay.

Um um you might think that with enough work the knocking can be made as gentle as you would like and it turns out to be a surprise but something that's true that there are all kinds of circumstances in which the knocking can't be made as gentle as you like and uh uh and that's a fact about the way the world is. But it is, as it were, a thoroughly understandable mechanical fact about about the way the world is. Um, and it was often interpreted to be something much more than that. Much more. I'll give you an example.

So, and we're running short of time, so a quick example. I used to teach at Princeton and I uh had lunch with John Wheeler one day who's one of the all-time greats in 20th century physics and he didn't like inflation very much uh because he didn't accept its explanation for why the universe is so flat um and space is not curved and so I said to him well why do you think it's flat and he said oh it's because of an observation somebody will make two billion years from [Laughter]