[drran]

It's false information. It's debunked multiple times. "Watching" means "placing or turning on an additional detector", which ruins interference pattern. To develop intuition, look at double slit experiment made at macro scale using walking droplet.

[JohnDeHope]

I'd really appreciate a few links to this. I am on a youtube binge lately watching quantum dynamics lectures for lay people. The constant talk about "watching" and "observers" drives me crazy. I refuse to believe anything has anything to do with whether or not anybody is "watching" anything. Bouncing photons off of it? Sure I can see how that would affect all sorts of things. But if the bouncing photons alter what's going on (and it seems they cause as much quantum decoherence as anything else might) then that's not the same thing as just "watching". That word implies to me "there are ambient photons or some other field or background that does not alter the experiment, and I am merely observing that background to get an idea what's going on within the experiment". If the experiment cannot be conducted without interference within an ambient background of photons flying around, or within some other medium we passively non-destructively-to-the-experiment observe, then we cannot currently be said to be "observing" anything at all with respect to that experiment. Let's just say that, can't we? If we can't not alter the relevant circumstances of what's going on, then let's just admit that, without using faulty and improper language to "describe" what's happening. Can somebody help me out with this?

[Viliam1234]

You make a good point. The problem is interacting with the particles, e.g. the photons bouncing off them. Don't poke the particle -- it only interferes with itself. Poke the particle -- now it is entangled with your measurement device, that means zillions of other particles, which changes the interference patterns.

But it is more complicated that this. Consider https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_t... -- if you only poke the particle in a "parallel universe", that also changes its pattern in this one.

At the end, quantum physics says there are multiple "versions" of how the particles move, and those versions interfere with each other. By interacting with each other, particles become entangled, which means that the "versions" of their states are no longer calculated independently, but together.

There are disagreements of what exactly this means: some people believe that reality only has multiple "versions" on a microscopic level, but when the entangled configuration becomes large enough (how large? no one knows), the parallel computation collapses, one of these "versions" is randomly selected to become the actual reality and the remaining ones disappear. Other people believe that multiple "versions" is the whole story; that observing the outcome means that you (being composed of particles that follow the laws of physics) also become entangled with the particles in the experiment, and now there are multiple "versions" of you, each observing a different outcome.

So, I'd say you got it half-right. Yes, it is about "observation = interaction"; and "observer" is just a shortcut for "the thing that poked the experimental particle, optionally also a display connected to that thing, optionally also a person observing the display". (That is, you could also have a completely impersonal "observer", e.g. a machine that measures the particle but no one is looking at its display.) But quantum physics is a different thing than mere classical physics where you correct for photons being actual things that hit the measured particle. It means there actually are multiple outcomes, which then interfere with each other, at least on the microscopic level.

[drran]

If you want to develop intuition, look at this video: https://www.youtube.com/watch?v=nsaUX48t0w8 .

Please note that physics as science in general is advancing constantly, but mainstream physics advances in sprints. Currently, walking droplets and Pilot Wave Theory are not good enough to be included into mainstream physics.

[jacknews]

Exactly, there's no such thing as passively 'watching' a system at the quantum level.

In order to obtain the information you wish to 'observe', you need to interact with it.

That interaction 'collapses the wave function', or rather, you become part the wave function, but seeing it 'from the inside' looks different compared to from the outside.

[jabirali]

I love the “bouncing droplet” experiments. There are some cool videos of it on YouTube for those that want an intuitive macroscopic analogy to how quantum mechanics work.