I been thinking a lot about wave function collapse and entanglement after watching all of Professor Susskind’s “Theoretical Minimum”. He explains it to his students but I don’t think they actually understand just how weird the concept he is trying to explain is.

First of all the entanglement thing and ‘wave function collapse’. if you haven’t taken your quantum physics courses since graduate school back in the 1980’s things are different now than they used to be. Now sure if you watch some random physicist on television talking about quantum physics he will tell you about the collapse of the wave function but that is old. It doesn’t work anymore. I think everyone knows by now about quantum physics and wave function collapse it’s pretty simple. Everything is waves, the little electron the elephant and everything in between. And you can state some mathematics that describe it all, and it is a wave function dirac’s or schrodinger’s equation. Now if you have a two slit experiment an electron doesn’t go through one slit or the other, it goes through both. Because it is a wave. But when it hits the back of the detector it becomes a particle. That is the “wave particle duality” and this also is called a wave function collapse because we measure it and this measurement means it will be somewhere not all spread out.

This means that there is something different and special in the world. There is one world, the microscopic and one large world the macroscopic and when they touch something magical happens, suddenly there is no wave it becomes macroscopic we can see it. If you want to be kooky about it you can think about the conscious act of observing means the wave function collapses. Ok, this is all 1930’s 1940’s style of thinking. For a long time ti was completely acceptable to think about the wave function this way. You have a wave, it is probabilistic and then you measure it, then the wave collapses and hey what happened to quantum physics, well on the microscopic quantum physics exists but on the macro side it doesn’t. Weird, where did the wave function go? It’s a real puzzle and for a while scientists were ok about this, let us just leave it at that, we have so much more to discover! Don’t worry be happy let’s smash some atoms and maybe discover new particles. This was feynman’s thinking don’t worry about it, we haven’t discovered the standard model yet. But then by about 1980, or 1990 physics was stuck. We figured out quarks, we figured out the forces, electromagnetic weak strong gravity, we had all the particles, all the forces, and could describe things down to 11 or more decimal places.

And then scientists started to worry again about the wave function collapse. And I think today the thinking is there is no such thing as wave function collapse it doesn’t exist at all. The thinking is that not only is the particle you are measuring a quantum mechanical object, the apparatus you are measuring it with is also a quantum mechanical object. So if you have a complete description of the measuring device you no longer specify a “collapse” this is merely lazy thinking. The wave continues to exist, the dot on your detector and the measuring apparatus have become entangled into a new object, and that entire object can be described by this quantum wave function.

So you might think ok the measuring apparatus and the particle we are measuring are both waves now, but what if we have another observer that looks at the two interacting objects, doesn’t he now collapse the two things? Well that’s one way to think about it, but what really happens is his second observer can also be described by entangled particles and the act of him measuring the apparatus and the particle now means all three are entangled and described by a quantum wave function. And because there is no such thing as something “outside the universe” this means there is never any such thing as wave function collapse. Everything is waves and represented by quantum effects.

Of course this does not make any sense in our every day experience we don’t seem to see this wave function we only see the macroscopic effects. Why is that? And here it seems that nature becomes statistical. The only reason that that macroscopic world is so full of quantum weirdness and the big macro world is not is because statistics. If you have a large amount of particles that can take a large number of states, it is more likely that this large number of particles will go from a state of more organization into less organization in other words the reason we seem to have a world that is fixed and follows a timeline and is not all wobbly like quantum theory would say is because entropy means that what is likely to happen will happen and that is what we observe.

Now let me get to entanglement. It’s often said that entanglement means that there is “spooky action at a distance” but let us quit thinking in terms of wave function collapse and think instead of entanglement and measuring apparatus. You have particle a, and an entangled particle b. B is taken far away and measured. If it is in an UP state, then we know that a will be in a DOWN state. But conversely if it is in DOWN than the other particle will be UP. However, these particles aren’t actually at any time UP or DOWN they are entangled which means . . . well one way to think about it is that never make up their minds to be UP or DOWN, they are both UP and DOWN.

So we measure particle B far away. Is it actually UP? No, what has happened is that the particle has become entangled with the observer that is far away. It is both UP and DOWN and both have been measured. A is also measured and it entangles the observer at point a. Both particles are far away and both are UP and both are DOWN and both have been measured. But there is no collapse of the wave function. The old thinking is that particle B was measured a long distance away and the act of measuring it changed the local particle A so that they would match. But the new thinking is that the observer or device measuring B becomes entangled with particle B, while the observer or device measuring A becomes entangled with particle A. There is no longer “spooky action at a distance” because you no longer have wave function collapse. Another way to think about it is to say if you have schrodinger’s cat in a box it is both dead and alive. Observer A watches the cat and declares it to be dead or alive. But you draw a box around the box which includes not just the cat but also observer A. Now observer A sees both the cat alive and the cat dead. And if observer B watches observer B he will see something but until he becomes entangled with the box containing observer A and the cat they exist in both ways. And then you can draw a larger box around observer B ad infinitum.

so let us go back to the original entangled particle a and particle b and let’s bring the observer of particle a and particle b back together. They agree that they measured something opposite. But at no time did the wave function actually collapse.

Sometimes to get your head around this scientists talk about the many world hypothesis. In one world you measured particle A as up and in another world you measured it as down. And both universes exist at the same time. When we come back and look purely at the macroscopic world we can use the quantum wave function along with statistics to figure out how the wave functions will probably go.

It bugs me that modern popular science figures still talk about the wave function collapse and about the “spooky action at a distance.” Modern physics doesn’t have either of these things and life is much, much weirder than this more simplistic model. The copenhagen intepretation was invented back in the 1920’s it’s time I think for the public to get a better idea about how much weirder quantum physics is than we thought it was back in the 1920’s. Quantum decoherence is a lot more interesting than a wave function collapse.

It’s funny you posted this just now. I’ve puzzling over it since I just read Sean Carroll’s argument last week that many worlds is likely correct. Not quite clear from above if you are in the many worlds/Everett camp or not. Or if you just want to deprecate the wave collapse version. Just curious about your many worlds sympathies.

http://www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/

just read http://www.preposterousuniverse.com/blog/2014/06/30/why-the-many-worlds-formulation-of-quantum-mechanics-is-probably-correct/ I think I agree with it.

Nathan if I get a chance ill write an example of quantum decoherence showing why entropy is required to understand the transition from quantum to macro . many worlds is lazy thinking you shouldn’t do it.

nathan try googling “quantum decoherence”. I’m not real fond of “many worlds” it simplifies what is going on way too much. Many worlds ignores why you need the idea of entropy to describe macroscopic physics.

What’s my position? My position is that the more I know about quantum mechanics the less I understand. I keep thinking if I can use the psi function and ket operators and calculate the probability density it will eventually “click” but all that happens is I think the universe is some kind of math and simple ideas like “many worlds” are just words and reality is math.