For those confused, it’s worth noting the difference between observed as a layman concept and as a quantum mechanical one.
In QM, to observed is to couple the observer to the “system” being observed. Think of it like “observing” your neighbour, over a fence using a BB gun. When you hit flesh, you know where your neighbour is. Unfortunately, the system has now been fundamentally changed. In a classical system, you could turn down the power, until your neighbour doesn’t notice the hits. Unfortunately, QM imposes fundamental limits on your measurements (heisenburg and his uncertainty principal). In order to observe your neighbour accurately, you need to hit them hard enough that the will also feel it and react differently.
QM behaves in a similar way. Initially, the system is just a single particle, and is not very restrained. This allows it to behave in a very wave like manner. When you observe it, the system now includes the whole observation system, as this coupling propagates, more and more atoms etc get linked. The various restraints cause an effect called decoherence. The system behaves ever more like a classical physical system.
In short, a quantum mechanical “observer” is less sneaky watching, and more hosing down with a machine gun and watching the ricochets.
Thanks! I’ve never fully grasped the concept and this really helps.
I’ve always heard it that observing was actually “measuring” and still wasn’t sure why that would impact anything but chalked it up to the quantum world being other-worldly.
Honestly physicists don’t actually know what measuring is either. We don’t know when exactly the system is considered “measured” in the chain of entanglement, this is called the Measurement Problem.
Answers range from “shut up don’t think about it” to “there’s an infinite amount of universes split from each other for each quantum event!”.
We know how it works, we just don’t yet understand what is going on under the hood.
In short, quantum effects can be very obvious with small systems. The effects generally get averaged out over larger systems. A measurement inherently entangled your small system with a much larger system diluting the effect.
The blind spot is that we don’t know what a quantum state IS. We know the maths behind it, but not the underlying physics model. It’s likely to fall out when we unify quantum mechanics with general relativity, but we’ve been chipping at that for over 70 years now, with limited success.
We know how it works, we just don’t yet understand what is going on under the hood.
Why should we assume “there is something going on under the hood”? This is my problem with most “interpretations” of quantum mechanics. They are complex stories to try and “explain” quantum mechanics, like a whole branching multiverse, of which we have no evidence for.
It’s kind of like if someone wanted to come up with deep explanation to “explain” Einstein’s field equations and what is “going on under the hood”. Why should anything be “underneath” those equations? If we begin to speculate, we’re doing just tha,t speculation, and if we take any of that speculation seriously as in actually genuinely believe it, then we’ve left the realm of being a scientifically-minded rational thinker.
It is much simpler to just accept the equations at face-value, to accept quantum mechanics at face-value. “Measurement” is not in the theory anywhere, there is no rigorous formulation of what qualifies as a measurement. The state vector is reduced whenever a physical interaction occurs from the reference point of the systems participating in the interaction, but not for the systems not participating in it, in which the systems are then described as entangled with one another.
This is not an “interpretation” but me just explaining literally how the terminology and mathematics works. If we just accept this at face value there is no “measurement problem.” The only reason there is a “measurement problem” is because this contradicts with people’s basic intuitions: if we accept quantum mechanics at face value then we have to admit that whether or not properties of systems have well-defined values actually depends upon your reference point and is contingent on a physical interaction taking place.
Our basic intuition tells us that particles are autonomous entities floating around in space on their lonesome like little stones or billiard balls up until they collide with something, and so even if they are not interacting with anything at all they meaningfully can be said to “exist” with well-defined properties which should be the same properties for all reference points (i.e. the properties are absolute rather than relational). Quantum mechanics contradicts with this basic intuition so people think there must be something “wrong” with it, there must be something “under the hood” we don’t yet understand and only if we make the story more complicated or make a new discovery one day we’d “solve” the “problem.”
Einstein once said, God does not place dice, and Bohr rebutted with, stop telling God what to do. This is my response to people who believe in the “measurement problem.” Stop with your preconceptions on how reality should work. Quantum theory is our best theory of nature and there is currently no evidence it is going away any time soon, and it’s withstood the test of time for decades. We should stop waiting for the day it gets overturned and disappears and just accept this is genuinely how reality works, accept it at face-value and drop our preconceptions. We do not need any additional “stories” to explain it.
The blind spot is that we don’t know what a quantum state IS. We know the maths behind it, but not the underlying physics model.
What is a physical model if not a body of mathematics that can predict outcomes? The physical meaning of the quantum state is completely unambiguous, it is just a list of probability amplitudes. Probability captures the likelihoods of certain outcomes manifesting during an interaction, although quantum probability amplitudes are somewhat unique in that they are complex-valued, but this is to add the additional degrees of freedom needed to simultaneously represent interference phenomena. The state vector is a mathematical notation to capture likelihoods of events occurring while accounting for interference effects.
It’s likely to fall out when we unify quantum mechanics with general relativity, but we’ve been chipping at that for over 70 years now, with limited success.
There has been zero “progress” because the “problem” of unifying quantum mechanics and general relativity is a pseudoproblem. It stems from a bias that because we had success quantizing all the fundamental forces except gravity, then therefore gravity should be quantizable. Since the method that worked for all other forces failed, this being renormalization, all these other theories search for a different way to do it.
But (1) there is no reason other than blind faith to think gravity should be quantized, and (2) there is no direct compelling evidence that either quantum mechanics or general relativity are even wrong.
Also, we can already unify quantum mechanics and general relativity just fine. It’s called semi-classical gravity and is what Hawking used to predict that black holes radiate. It makes quantum theory work just fine in a curved spacetime and is compatible with all experimental predictions to this day.
People who dislike semiclassical gravity will argue it seems to make some absurd predictions in under specific conditions we currently haven’t measured. But this isn’t a valid argument to dismiss it, because until you can actually demonstrate via experiment that such conditions can actually be created in physical reality, then it remains a purely metaphysical criticism and not a scientific one.
If semi-classical gravity is truly incorrect then you cannot just point to it having certain strange predictions in certain domains, you also have to demonstrate it is physically possible to actually probe them and this isn’t just a metaphysical quirk of the theory of trying to make predictions to things that aren’t physical possible in the first place and thus naturally what it would predict would also be physically impossible.
If you could construct such an experiment and its prediction was indeed wrong, you’d disprove it the very second you turned on the experiment. Hence, if you genuinely think semi-classical gravity is wrong and you are actually following the scientific method, you should be doing everything in your power to figure out how to probe these domains.
But instead people search for many different methods of trying to quantize gravity and then in a post-hoc fashion look for ways it could be experimentally verified, then when it is wrong they go back and tweak it so it is no longer ruled out by experiment, and zero progress has been made because this is not science. Karl Popper’s impact on the sciences has been hugely detrimental because now everyone just believes if something can in principle be falsified it is suddenly “science” which has popularized incredibly unscientific methods in academia.
Sorry but both the “measurement problem” and the “unification problem” are pseudoproblems and not genuine scientific problems but both stems from biases on how we think nature should work rather than just fitting the best physical model to the evidence and accepting this is how nature works. Physics is making enormous progress and huge breakthroughs in many fields, but there has been zero “progress” in the fields of "solving the measurement “problem” or quantizing gravity because neither of these are genuine scientific problems.
They have been working at this “problem” for decades now and what “science” has come out of it? String Theory which is only applicable to an anti-de Sitter space despite our universe being a de Sitter space, meaning it only applies to a hypothetical universe we don’t live in? Loop Quantum Gravity which can’t even reproduce Einstein’s field equations in a limiting case? The Many Worlds Interpretation which no one can even agree what assumptions need to be added to be able to mathematically derive the Born rule, and thus there is also no agreed upon derivation? What “progress” besides a lot of malarkey on people chasing a pseudoproblem?
If we want to know how nature works, we can just ask her, and that is the scientific method. The experiments are questions, the results are her answers. We should believe her answers and stop calling her a liar. The results of experimental practice—the actual real world physical data—should hold primacy above everything else. We should set all our preconceptions aside and believe whatever the data tells us. There is zero reason to try and update our theories or believe they are “incomplete” until we get an answer from mother nature that contradicts with our own theoretical predictions.
People always cry about how fundamental physics isn’t “making progress,” but what they have failed to justify is why it should progress in the first place. The only justification for updating a theory is, again, to better fit with experimental data, but they present no data. They just complain it doesn’t fit some bias and preconception they have. That is not science.
