My link to a video of a discussion between physicist Bernard Carr and Robert Lawrence Kuhn generated a request for me to to clarify what was being said about the possible role of consciousness in quantum measurements. With me, you have to be careful about what you wish for because as so often happens, my attempts to explain difficult physics concepts leads to multi-part posts because of all the subtleties involved. I hope that readers will think and discuss each part and clarify it in their minds before moving on to the next section.
Since this is a tricky topic, before I give my views, let me state my background in this area so that you can judge for yourselves whether to give any credibility to my opinions. I have worked all my professional life in the area of quantum physics, and thought and read about the measurement problem a lot and have even taught about it as part of quantum mechanics courses. But I have not published any papers in this particular area of quantum mechanics. I also apologize in advance for some oversimplifications that I will make in order to make the subject more intelligible to people without a background in quantum mechanics or even physics. I will also, where appropriate, include the technical terms for various processes. It is not important that you know this jargon. I only include it so that people who read other articles that use those terms will have a better idea of what is being talked about.
To start off, let us postulate that the universe can be split up into two categories: a microscopic world (roughly of the order of atoms and smaller) and a macroscopic one of large objects. How we describe how things behave in the two worlds is quite different. The behavior of entities in the microscopic world is governed by the laws of quantum mechanics. That world is not accessible to our senses but has to be probed with sophisticated instruments and so our knowledge about it is necessarily indirect. The behavior of the macroscopic world is governed by classical physics (such as Newtonian and Einsteinian dynamics) and much of it is accessible via our senses.
The macroscopic (or classical) world can be roughly defined as such that at any given time, entities are presumed to have specific properties such as an exact location is space and an exact velocity, even if we do not actually know them. The classical laws are such that those two things, combined with knowledge of the forces acting on the entities, completely determine their subsequent behavior. As a result, they behave deterministically and in principle if not in practice, we can predict with 100% accuracy where they will be at a later time. This is called classical determinism. So if I flip a coin, the outcome of whether it will lands heads or tails was 100% determined by the type of coin, how it was tossed, and the environment. Since we do not have perfect knowledge of those conditions, we cannot predict the outcome, even though it was completely fixed by the toss.
The microscopic (or quantum) world is very different. The equivalent of a coin toss is the creation an elementary particle that could have two different properties. We usually discuss it in terms of objects (say electrons and photons) that can rotate in two different ways about an internal axis (like the Earth spinning): clockwise (we can call it ‘spin up’) or counterclockwise (we can call it ‘spin down’). (To be precise, the objects are not thought to be actually rotating. They have a property that has the dimensions of rotational motion so ‘spin’ is an appropriate metaphor to describe it.) When we measure the spin (the equivalent of looking at the flipped coin) we always find it to be either spin up or spin down, just like the coin is always found to be either heads or tails.
So far, so good. We are on firm ground, I hope
The catch is that the popularly accepted interpretation of the laws of quantum mechanics (known as the Copenhagen model) that is used to describe the microscopic world say that prior to the measurement of its spin, the entity was in both states and that it was the act of measurement that forced it into one or the other state. We cannot predict the state into which it will be forced but it is not completely arbitrary. The outcomes are determined statistically, depending on the experimental set up, with fixed probabilities of being found spin up or spin down. Thus the best we can hope are probabilities, not certainties. This is called quantum determinism, to distinguish with classical determinism.
To expand on this difficult idea, the laws of quantum physics say that entities (such as the electron or the photon) do not have a specific location and velocity either. The most information that we can get about them is what is called the wave function which only gives us the probabilities that the entity will be is a given spin state (or location or velocity) if we decide to measure any of those things. If we repeat the identical experiment (the equivalent of repeating the coin toss), sometimes we will find one spin state (or location or velocity depending on what we choose to measure) and sometimes another. But again, it is not completely random. If we know the wave function, then we can predict the probability that we will find each state. The difference with the classical coin case is that in that case, each individual result (heads or tails) existed before we looked at it but in the quantum case the result does not come into existence until we make the measurement. If we applied that same idea to the classical world, it would be like saying that the coin flip outcome is both heads and tails until the moment when we look at it when, by some mysterious mechanism, the coin abruptly goes into the heads or tails state, which is what we always find.
In short, in the classical case, we lack knowledge of something that already exists but in the quantum case, the result itself does not exist until we measure it.
This is what boggles people’s minds. And it was this particular feature of quantum mechanics that Einstein found unsatisfactory, though he acknowledged that quantum mechanics was the best theory that we had so far. Even during his lifetime, the successes of quantum mechanics in explaining so much had convinced him and pretty much everyone that it was the best way we had to describe the microscopic world and the evidence in support of quantum mechanics has only increased since then.
Next: Attempts to bridge the gap between the microscopic and macroscopic descriptions of the world.
it’s a noble effort at communication with the uneducated, but on first read, i find myself feeling questions i don’t even know how to articulate. i’ll try again later.
Thank you Mano for taking up this challenge. Though not a Physicist, I read, and watch videos, about Quantum Physics quite lot while still being confused.
With regard to the example of the coin toss, it seems to me that it is very much analogous to the quantum observation problem. We have a situation where there is a binary choice, the result of which we cannot know in advance. This is the reason coin tosses are used in many games, eg cricket, to decide which side will have the first move.
It seems to me that, although this seems counter intuitive, we would be justified in saying that the coin is indeed both heads and tails, until the actual state is revealed by observation. If as you say the electron is not literally spinning, but has some characteristic which we call “spin”, why can we not say that all instances of binary outcomes are indeterminate until we observe the outcome.
Perhaps I am not getting the fundamental idea here, but I am glad you are making an attempt to enlighten us.
Mano, I have physics background. (I have taken QM courses as undergrad and grad student)
Based on my experience, I think your explanation accurate and well written.
enkidu @#2,
You raise an interesting point. The problem is that the laws of classical mechanics are 100% deterministic, in that if we have complete knowledge of the initial conditions, the final outcome is completely preordained. It has to be heads or tails. The indeterminacy in predicting the outcome exactly is due to our lack of that complete initial knowledge. The classical laws do not permit the coin to be in a superposition of states at any time during its motion.
In quantum mechanics the story is different. The solutions to Schrodinger’s equation (the quantum equivalent of Newton’s laws of motion) have the superposition of states and indeterminacy of outcomes built into the structure, so that even complete knowledge of the initial conditions does not give us 100% accuracy in predicting the outcome.
In short, the laws of classical physics do not allow us to insert indeterminacy at any point (other than due to lack of complete knowledge of the initial state) while the laws of quantum physics do not allow us to get rid of the indeterminacy even if we had complete knowledge of the initial state.
couldn’t a physicist -as a thought exercise- insert indeterminacy into classical physics, just to see what it would look like?
Presumably you’re going to be doing Bell’s Inequality before too long, since that’s one of the earlier things that really drives home that the rules are fundamentally different.
One thing that always bother me is the fact that, at the macroscopic level at least, statistics apply to populations so that you can model populations but you cannot determine the characteristic of a single individual from the model.
Since quantum physics uses statistic to model behaviours why can you extend the model to a single entity instead of a population and not assume that the behaviour is deterministic but that the model is not complete for each individual entity since we cannot observe the initial states (and therefore complete the model)?
I just want to say at this point thatI am going to lift the three comment limit to posts because I can see that there is going to be a lot of back and forth on this topic as people seek clarifications.
Bébé @#5,
There is indeterminacy in classical physics. But it is classical indeterminacy due to lack of complete knowledge of the initial states.
Quantum indeterminacy only comes in when the entities involved obey quantum laws (like Schrodinger’s equation) and not classical laws like Newton’s laws of motion.
We have to start from the basic laws of motion and see what we get. We cannot start with one set of laws then insert features that arise from another set of laws. To try to do so would not make any sense.
Jean @#7,
Is it the case that you meant to write ‘can’t’ in the first line of the second paragraph instead of ‘can’? That would make more sense to me.
Mano @10: I could be wrong, but I think Jean really meant ‘can’. They seem to be asking how we can (or why we do) assign what seem to be statistical predictions to a single particle.
The answer is in part in a complement to Bell’s theorem; the Kochen-Specker theorem. In a somewhat oversimplified nutshell; you can’t assume that a particle has properties with definite values (i.e. ‘initial values’) before they are measured. In other words, measurements don’t just reveal pre-existing values.
Rob is right, that is what I meant.
To take the analogy of the coin flip, it’s like saying that the model is a statistical probability of 50% head and 50% tail which is what you get from statistical analysis of numerous coin flips. And from that you say you cannot predict the outcome of a single throw and assign it randomness. Of course, for the coin flip there is the deterministic aspect of Newtonian physics behind the actual coin flip. For quantum physics, it is assume that the statistical model is all there is. What I am suggesting is that the model is not complete and that you cannot assume that the quantum result is not deterministic.
Moreover, there is no quantum experiment where the observer (whatever the means) is not part of the experiment since the whole universe is part of the quantum field. And any experiment is therefore different from any other. So the model might need to take a whole universe to be able to make a prediction and might need to be done outside the universe (whatever that means).
I know that sounds like meaningless and uninformed pseudo-philosophical nonsense but so does some of what “serious scientists” declare when they talk about reality being created through quantum consciousness or some similar garbage.
It seems to me that it’s like if we were in a book and we only were able to see and know about a very small speck of ink from a dot on an ‘i’ and we needed to extrapolate that to come up with Moby Dick. Or is it War and Peace?
Thanks Jean.
I now understand better your concerns. The lack of completeness of quantum mechanics was something that Einstein also believed so you are in good company!
These are important questions. I think (hope?) I address them in Part 2 (that I just posted) and also in some of the later posts to come.
Unrelated to your post -- but it amuses me that science (evidence based , empirical in nature) has to lean on “In principle” but not in practice (Yeah the evidence says we got it wrong , but thats just in practice , In principle we got everything right!)
Deepak @#14,
That is not how science works. Practice is vitally important. But there are only a few cases where we can exhaustively test theories. If theories hold up in the cases we can test, we believe that the theory will also work in areas we have not tested unless we have reasons to be skeptical.
If we find counter evidence in the form of a failed test, we do not dismiss it by saying that we got it right ‘in principle’. We look for reasons why it failed. That sequence of prediction, test, and refinement is how science progresses.
@Mano Singham
You are replying to something different though -- The amusement is not that Science is absolute or that Science determines “truth” or that theories are complete or that theories cant be changed or tweaked.
Its an observation that says even if you run a set of experiments , and the values are not exactly as you predicted , you can get away with “In theory we could have got it exactly right, even though in practice we almost never do so” for a system which prides itself on evidence and its predictions. I wish I could have answered so in my exams. In theory I could have got this answer correct ,but in practice made silly mistakes -- where is my A+ grade ?
(Note amusement , not disagreement)
Deepak @#16,
The sentiments you are describing is what students sometimes say (because they are often told that the theory they are testing is true) but is not accepted practice among scientists. If an experiment turns up contradictory results, they do not say they could have got it right in theory. What they will say is that the theory predicts a result that they did not get. That means that either the theory needs to be revised or that something went awry with the experiment. If the theory is such that we have a lot of confidence in it, it is standard practice to first look for reasons why the results were not in agreement. But we never glibly dismiss the negative outcome (and try to “get away with it” as you say) by saying that “in theory we could have got it exactly right”.