You've probably seen a lot of headlines claiming that quantum mechanics is "weird", "weird" or "creepy". At best, it's "not intuitive" and "no one understands it". The fullness of the article will be about that the problem with quantum mechanics is not weird. The problem with quantum mechanics is chaos. Our author is Sabine Hossenfelder. The strangeness of Quantum Mechanics and the possibility of this strange relationship between Saturn's Moon Hyperion and how Newtonian Mechanics and Quantum Mechanics are intertwined can sometimes be overlooked.
The Weirdness of Saturn's Hyperion Moon
Saturn has 82 moons. This is one of them, his name is Hyperion. Hyperion is about 200 kilometers in diameter and its movement is chaotic. It is not the orbit that is chaotic, but the direction of the satellite in that orbit.
It takes about 3 weeks for Hyperion to orbit Saturn once and about 5 days to rotate on its axis. But the direction of the axis rotates erratically every few months. And this rolling is technically chaotic.
Even if you measure Hyperion's position and orientation with the highest precision, you cannot predict what the orientation will be one year from now.
Hyperion It is a huge headache for physicists. Not so much for astrophysicists. Hyperion's motion, if not predicted, can be understood by general relativity, or a good approximation. With Newtonian dynamics and Newtonian gravity. These are all theories without quantum properties.
Physicists refer to such theories without quantum properties as "classical".
But Hyperion is a headache for those who think that quantum mechanics really is nature's way of working.
Because quantum mechanics predicts that Hyperion's chaotic motion shouldn't take longer than about 20 years. But it took much longer. So, quantum mechanics has been falsified.
It has been known since the 1950s that quantum mechanics does not accurately reproduce the dynamics of classical, chaotic systems. The example with the particular moon of Saturn comes from the 1990s.
If you remember, in quantum mechanics we describe everything with a wave function. There are not only wave functions for particles. There is a wave function for everything in quantum mechanics. The change of the wave function over time looks like this Schrodinger equation You calculate with The Schrödinger equation is linear, and the products of the wave function appear in it.
There is no chaos in systems with linear equations like this. To have chaos you need nonlinear equations.
But quantum mechanics was supposed to be a theory of all matter. So we should be able to use quantum mechanics to describe large objects, right? If we do this, which we should. Just find that the motion of these large objects agrees with classical non-quantum behavior.
This is called the "correspondence principle", a name that goes back to Niels Bohr.
But if you look at a classical chaotic system like this moon of Saturn, the prediction you get from quantum mechanics is only "Ehrenfest TimeIt is consistent with the estimation you get from classical Newtonian dynamics for a given time known as ”. During this time, you can actually use quantum mechanics to study chaos. This is what quantum chaos is all about.
But after Ehrenfest's time, quantum mechanics gives you an estimate that doesn't agree with what we're observing. He would have guessed that Hyperion's orientations would not roll, but instead would blur until they were so blurry that you wouldn't notice any rolling. Basically, chaos disappears into quantum uncertainty.
Isn't it possible that physicists have known about this problem for 60 years and ignored it? In fact, they didn't exactly ignore it. They offered a similar explanation.
Hyperion may be very far away from us and not much is happening there, but it still interacts with dust and light, or rather, quanta of light called "photons." Each of these are really small interactions, but there are a lot of them. And the satellite needs to be added to the Schrodinger equation.
What these little interactions do is mix them with the satellite's environment, dust and light.
This means that every time a grain of dust hits the moon, it changes some of the satellite's wave function very slightly, and then the two are related. This correlation is entanglement.
And these little bumps slightly shift the peaks and troughs of the parts of the wavefunction. This is called "decoherence" and this is exactly what the Schrodinger equation predicts.
And this equation is still linear, so all these interactions don't solve the problem of the prediction not agreeing with the observation.
The solution to the problem comes in step 2 of the argument. Physicists now say, okay, we have this wave function for the moon with these many entangled dust grains and photons.
But you don't know exactly what that dust is, where it is or what the photons are doing, etc. we don't know.
So we do what we always do if we don't know the exact details:
We make estimates of what the details might reasonably be, and then average them.
And this average agrees with what classical Newtonian dynamics predicted. Physicists say, everything is fine! But there are two problems with this explanation.
The first is that it forces one to accept that in the absence of dust and light, a satellite will not follow Newton's law of motion.
Okay, well, in that case you can say you can't see the satellite either, so all we can say is that it might be true.
The more serious problem is that averaging is not a physical process. It does not change the state of the satellite.
It's still in one of those fuzzy quantum states entangled with dust and photons, you don't know exactly which one.
Let me use an analogy to see the problem with the argument. Let's take the classic chaotic process of rolling a dice. The result is an integer from 1 to 6, and if you average many shots, the average value per shot is 3.5.
Exactly what result you get is determined by many small details, such as the positions of air molecules, surface roughness, and the movement of your hand.
Now suppose I am writing a model for the die. My model says the result of rolling the dice is 1 or -2 with probability 106/99. Wait, you say there's no way to roll the dice it will give you minus 99. Look, the average is 3.5, I say everything is fine. Will you accept this?
Probably not. Obviously, for the model to be correct, one must not only get the average truth, but each possible individual outcome must also agree with the observations.
And rolling a dice yields no more than minus 99, as a large, fuzzy rock entangled with lots of photons agrees with our observations of Hyperion. Okay, but what happened to the collapse of the wave function?
The Schrodinger Equation at Work
When we make a measurement, the wavefunction changes in a way that the Schrödinger equation does not predict. What happened to this?
Exactly! In quantum mechanics, we use the wave function to make probabilistic predictions.
Let's say an electron has a 50% probability of hitting the left or right side of the screen. But then when we measure the electron, let's say we know it has 100% probability.
This means we need to update the wave function from 50-50 to 100-0 after a measurement. More importantly, what we call "measurement" in quantum mechanics doesn't actually need to be done by a measuring device.
I know it's a strange nomenclature, but in quantum mechanics a "measurement" can only happen by interacting with many particles. Like dust grains or photons.
This means that Hyperion is, in a sense, constantly "detected" by all these tiny particles.
And updating the wave function is indeed a nonlinear process. This neatly solves the problem: Hyperion rolls unevenly in its orbit.
But here's the thing. This only works if the collapse of the wavefunction is a physical process. Because you really have to change something about that fuzzy quantum state of the satellite to match the observations.
But today the vast majority of physicists think that wave function collapse is not a physical process. Because if it did, then it would have to be instant everywhere.
Take the example of the electron hitting the screen. The wave function is propagated when it is displayed. But when the particle appears on one side of the screen, the wave function on the other side of the screen should change immediately.
Likewise, when a photon hits the moon from one side, the satellite's wave function should immediately change from the other side. This is what Einstein called "spooky action at a distance".
It would exceed the speed of light. So physicists said that measurement is not a physical process. We only account for the information we have acquired.
And if we update our knowledge of another place, nothing propagates faster than light.
But the example of Hyperion's chaotic motion tells us that we need measurement collapse for it to actually be a physical process. Without it, quantum mechanics just doesn't accurately describe our observations. But then what is this process? Nobody knows.
Ref: Sabine Hossenfelder