Testable proposals for the measurement problem

In a previous post, I made the statement: “Currently, there are no known experimental conditions that can distinguish between different interpretations of quantum mechanics.” Well, that is not exactly true. Perhaps one can argue that no experiment has yet been performed that conclusively ruled out or confirmed any of the interpretations of quantum mechanics. But, the fact is that recently, there has been some experimentally testable proposals. Still, I’m not holding my breath.

Recently, seeing one such proposal, I remembered that I also knew about another testable proposal made by Lajos Diósi and Roger Penrose. The reason I forgot about that is probably because it seems to have some serious problems. At some point, during a conversation I had with Lajos, I told him I have a stupid question to ask him: does quantum collapse travel at the speed of light? His response was: that is not a stupid question. So, then I concluded that it is not something that any of the existing collapse models can handle correctly. In fact, I don’t think any such model will ever be able to handle it in a satisfactory manner.

Thinking back to those discussions and the other bits and pieces I’ve read about the measurement problem, I tend to reconfirm my conviction that the simplest interpretation of quantum mechanics (and therefore the one most likely to be correct) is the so-called Many Worlds interpretation of quantum mechanics. However, the more I think about it, the more I believe that “many worlds” is a misnomer. It is not about many worlds or many universes that are constantly branching off to become disjoint universes.

Perhaps one can instead call it the “multiple reality” interpretation. But how would multiple realities be different from “many worlds”? That fact is that these realities are not disjoint, but form part of the unitary whole of a single universe. These realities can be combined into arbitrary superpositions. What more, these realities are not branching of to produce more realities. The number of realities remains the same for all time. (There are actually an infinite number of them, but the cardinality of the set remains the same.) The interactions merely change the relative complex probability amplitudes of all the realities. 

Anyway, just thought I should clear this up. I don’t see myself ever writing publication on this topic.

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The state of physics

One of the quirky things about me is that I don’t believe things I don’t understand. As a result of that, I’ve had a long turbulent relationship with the notion of black holes. See the thing is, for the longest time, I couldn’t understand how an event horizon can form if the time becomes frozen when the infalling matter approaches the point where the event horizon should form.

While I was grappling with this existential aspect of black holes, the rest of the world happily proceeded to invent wormholes, Hawking radiation, singularities, and eventually the information paradox. Together with event horizons, none of these ideas have entered the realm of establish scientific fact, which requires observational confirmation.

Eventually, I read somewhere that the reason an event horizon can form even though the time becomes frozen is because the location for the event horizon with and without this additional matter implies that the matter would past the point where a new event horizon would form in finite time. So, now I understand it and I believe that event horizons can form. But we are not done yet. What about the interior beyond the event horizon? It is still frozen in time. Where does the singularity come from? I still don’t believe that part, perhaps because I still don’t fully understand it.

In all this, the importance of the scientific method should be emphasized. Even if I don’t understand something, I would believe it if it has been observed. While event horizons may be difficult to observe directly, the singularity inside the black hole is completely impossible to observe. For that reason, it can never be part of our scientific understanding.

This year, the Nobel committee announced that the Nobel prize is award for work on black holes. Half of it goes to two people that inferred the existence of a massive black hole at the centre of the milky way galaxy based on the orbits of stars close to the centre. This work is based on scientific observation and therefore satisfies the requirements imposed by the scientific method.

The other half of the Nobel prize is awarded to Sir Roger Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity.” If I understand correctly, the award is based on the Penrose–Hawking singularity theorems. (Hawking did not share the Nobel prize because he passed away.) So what is meant by “a robust prediction” here?

Sir Roger Penrose

Sir Roger Penrose is a formidable person. During his lifetime, he has produced a remarkable collection of ideas that range over diverse fields. The originality and complexity of these ideas give evidence to Penrose’s uniquely creative intellect. However, these ideas are of a mathematical nature and they show very clearly that Penrose is primarily a mathematician. Many of these ideas have never been confirmed by scientific observations. This lack of scientific confirmation includes the work on singularities in black holes for obvious reasons explained above.

It now brings me to the question, why would the Nobel committee decide to award a prize for “a robust prediction,” instead of something that has been confirmed in a scientific manner? The answer is probably related to the current state of physics. If we look at the work that was awarded recent Nobel prizes in physics, one can see that there must a problem. The problem is that progress in fundamental physics is slowing down or has come to a complete stop. There simply is nothing else to be awarded a physics Nobel prize anymore.

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The problem with the measurement problem

It is not really a topic I want to discuss. In fact, I don’t think it is worthy of including under my Demystifying Quantum Mechanics series. However, since even physicists don’t seem to get it, it is necessary to clarify a few things.

So the argument seems to go that even of one were to consider a completely mixed quantum states with equal probabilities for different outcomes then a measure would convert this mixed state into one with only one outcome and zero for all other outcomes. This transformation is then interpreted as a quantum collapse and the fact that this process is not understood is called the measurement problem.

The problem with this interpretation of the situation is just that: it is an interpretation. So it falls under the general topic of interpretations of quantum mechanics. Currently, there are no known experimental conditions that can distinguish between different interpretations of quantum mechanics. As such it is not physics, because it is not science. It falls under philosophy. As a result, it would not be possible to solve the so-called measurement problem.

Just in case you are wondering whether this measurement scenario can be interpreted in any other way that does not involve collapse, the answer is yes. The obvious alternative is the Many World interpretations. In terms of that interpretation the mixed quantum state describes the different probabilities for all the different world in which measurement are to be performed. If one would restrict the quantum state to any one of these worlds (or realities) then it would have 100% probability for a specific outcome even before the measurement is performed. Hence, not collapse and no measurement problem.

So, yes indeed, the measurement problem is a pseudo-problem. It is not one that can (or need to be) solved in physics.

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Entanglement

Demystifying quantum mechanics VIII

Everything is no more or less in place to discuss one of the most enigmatic phenomena found in quantum mechanics: entanglement. It is sometimes called the quintessential property of quantum mechanics. 

We have discussed the fact that quantum mechanics introduces the concept of discrete entities that carry full sets of degrees of freedom, and which I called partites. Then we learned about the paradox introduced by Einstein, Podolski and Rosen (EPR) and how it led to the understanding that nature does not have a unique reality. Although it also allows that interactions could be nonlocal, we saw that such non-locality is not in agreement with our understanding of special relativity. The final ingredient that we need to explain quantum entanglement is the concept of a superposition. We can deal with that here.

The term superposition is a fancy way of saying that we are adding up things. Superpositions are also found in classical optics. There, one can observe interference effects when two waves are superimposed (added on top of each other at the same location). What makes the situation in quantum mechanics different is that the things that are added up in a quantum superposition can consist of multiple partites (multiple combinations of discrete entities) and these partites (discrete entities) do not have to be at the same location. Since each entity carries unique properties, as described in terms of the full set of degrees of freedom, the different terms in the quantum superposition gives complete descriptions of the state in terms of the set of discrete entities that they contain. 

Each the terms in the superposition can now be seen as a unique reality. The fact there are more than one term in the superposition, implies that there are multiple realities, just like the EPR paradox showed us. One can use the many-world interpretation to try to understand what this means.

Entangled entities
Artist impression of entangled entities

There are now different effects that these superpositions can produce. In some cases one can factorize the superposition so that it becomes the product of separate superpositions for each of the individual partites. In such a case one would call the state described by the superposition as being separable. If such a state cannot be factorized in this way, the state is said to be entangled.

What is the effect of a state being entangled? It implies that there are quantum correlations among the different entities in the terms. These correlations will show up when we make measurements of the properties of the partites. Due to the superposition, a measurement of just one of these partites will give us a range of possible results depending on which term in the superposition ends up in our measurement. On the other hand, if we measure the properties of two or more of the partites, we find that their properties are always correlated. This correlation only shows up when the state is entangled.

Some people think that one can use this correlation the communicate instantaneously between such partites if they are placed at different locations that are far apart. However, as we explained before, such instantaneous communication is not possible.

This discussion may be rather abstract. So, let try to make it a bit simpler with a simple example. Say that we form a superposition where each term contains two partites (two discrete entities). In our superposition, we only have two terms and the properties of the partities can be one of only two configurations. So we can represent our state as A(1) B(2) + A(2) B(1). Here A and B represent the identities of the partites and (1) and (2) represent their properties. When I only measure A, I will get either (1) or (2) with equal probability. However, when I measure both A and B, I will either get (1) for A and (2) for B or (2) for A and (1) for B. In other words, in each set of measurements, the two partites will have the opposite properties, and this result is obtained regardless of how far apart these partites are located.

The phenomenon of quantum entanglement has been observed experimental many times. Even though it is counterintuitive, it is a fact of nature. So, this is just one of those things that we need to accept. At least, we can understand it in terms of all the concepts that we have learned so far. Therefore, it does not need to be mysterious

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Physics employment

Recently, I heard somebody talking on the radio about ways to start a day with positive energy. The person suggested all sorts of things, ranging from the things to eat or not to eat, the kinds of exercises to do and also even meditation. It occurred to me how fortunate I am that I don’t need to waste all that time on getting positive energy. When I wake up and I ask myself “what am I going to do today?” The answers is “physics!!!” and there I get all the positive energy that I need. It is my profession, my purpose and my passion.

Then I read the lamentations of Peter Woit in a blog post on the job situation in theoretical high energy physics. So, while it may be great to have a passion for physics, it is not a given that one can make it your profession. Indeed, I can remember that for as long as I’ve been in this field, the job situation was challenging. 

Students at University of Toronto
Students at University of Toronto

Part of the problem is the way that physics as a profession is being practiced. One typically starts as a student studying physics, but what are the career expectations? Most physics students apparently expect to become physics professors at universities. Well, if you look at the number of students compared to the number of faculty positions in physics, then it is obvious that such expectations are quite unreasonable. Moreover, if every physics professor produces scores of physics PhD’s during his or her career, then obviously there would be a huge oversupply of physics PhD to replace that professor.

So where do these PhD’s go to work? First they become postdocs. The ideal postdoc is a person that basically runs the research program for a professor. They come up with the ideas of what to investigate and they even supervise the professor’s PhD students. But what are the professors doing then? They travel and give talks, raising their profile, building their networks, and increasing their impact. Some of them don’t even touch any research. It’s all about fame and glory. The postdocs basically become cheap labor to produce the content on which these professors are riding their ego trips. More than once, when I’ve asked such “eminent researchers” questions after their talks at conferences, I’ve discovered that they don’t really understand what they are talking about.

So what can be done? Firstly, the poor students studying physics need to understand this situation and be realistic about their expectations. Other career choices include teaching (in schools, not universities) or industry. The latter represents the idea of an “industrial physicist.” However, in this case there is a different form of competition. The industry is better geared for engineers, for obvious reasons.

Another thing. When you decide to do physics, please do it for the right reasons. If physics is you passion and will remain your passion for the rest of your life, by all means proceed. Somehow you’ll find a way to live out your passion. But, if you want to do physics because you want to show off how bright you are, then rather join Mensa and leave physics to those that are passionate about physics. And, if you want to do physics because you want to be famous, like Einstein and those guys, rather consider a career as a rock star or a movie star. Very few physicists ever become really famous, contrary to what they may think.

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