Dr. Alejandro Jenkins is a post-doctorate researcher studying high-energy physics at Florida State University. His work was featured on the cover of the January 2010 Scientific American. He participated in this interview as part of the accompanying article on quantum immortality. He is currently working on the application of quantum field theory to the physics of elementary particles, atomic nuclei and cosmology.
Quantum mechanics includes some of the most difficult concepts for laypeople (this one included) to understand. Why do you think that is?
In our day-to-day experience we always see, for example, that a marble moves along a single, well-defined path. Cats are always either dead or alive. These are features of what we call "classical physics." But we know for a fact that things aren't that way at the atomic and sub-atomic level. An electron, for example, takes all possible paths from point A to point B. It can spin in one way and in another, simultaneously. The various possibilities interfere with each other in a mathematically well-defined way.
There is an analogy to this sort of thing even in the "classical," day-to-day world that we are used to: for example, waves in a water tank don't take a well-defined path from point A to point B, and the waves moving one way can interfere with the waves moving the other way.
The thing that is bizarre and confusing about quantum mechanics is that when we go and measure where an electron is, or how it's spinning, the answer is not fuzzy at all. We always find the electron in one particular location, and spinning in one particular way. So what happened to that interference of all possible alternatives that I just talked about? Let me call this the “problem of observation.”
The math of quantum mechanics, which tells us how the various possibilities interfere with each other, is not really that complicated. In fact, the equations of quantum mechanics are usually simpler to solve than those that show up in classical physics, such as the equations that describe how moisture moves in the atmosphere, or how three planets interact gravitationally with each other, which can be immensely complicated and can be solved only approximately, if at all.
In practice, even theoretical physicists tend not to think too much about the problem of observation. Stephen Hawking likes to say that every time he hears of Schrödiger’s cat, he’d like to reach for a gun. They are generally happy to solve the equations of quantum mechanics and compare their results with experiments, which some have called the “shut up and calculate” approach.
The successes of quantum mechanics are numerous and spectacular. The transistor, for example, on which all of modern computer technology depends, is a quantum device. Chemistry —with its countless applications— is now understood to be essentially an application of the laws of quantum mechanics. Quantum physicists built the atomic bomb.
But those equations and applications are not something that a layman can appreciate. You need years of training to be able to work them out. Instead, non-scientists tend to be drawn to the problem of observation, which we physicist have not finished wrapping our minds around, even today. This tends to make quantum mechanics sound mystical or obscurely philosophical to laypeople, even though it’s the science behind something as concrete and useful as a transistor.
The Copenhagen Interpretation and many worlds are just two of the various "interpretations" of QM and its ramifications for understanding the world around us. Which interpretation do you think is the best supported and why?
The “Copenhagen Interpretation” is not, to my mind, really an interpretation at all. It's just an ad hoc prescription that allows physicists to, well, “shut up and calculate,” without worrying about the murky conceptual issues surrounding the problem of observation.
According to the Copenhagen Interpretation, the act of making a measurement or an observation irreversibly disrupts the evolution described by the equations of quantum mechanics. It forces what you look at to pick one out of the countless possibilities that were interfering with each other before. For example, before observation the electron has no well-defined position. After observation, it does. The equations of quantum mechanics only tell us the probability that it will be found to be located at a given place.
The fact that quantum mechanics predicts only the probabilities of the outcomes of observations is something that made Einstein, for example, very unhappy. It's in this context that he made the famous quip about refusing to believe that God plays dice with the Universe (even though Einstein was not religious in any conventional way and appears to have used "God" more as a colorful expression for the logic behind physical laws).
Although much of what Einstein said about trying to fix this problem turned out to be wrong, he correctly suspected that quantum mechanics was giving only probabilities (not certainties) because we had missed something important about what happens when you go and observe something.
Hugh Everett, the originator of the "many-worlds interpretation," realized that an observation or a measurement is really a physical interaction between the observed and the observer. The person in the lab is just another part of the physical system, along with what’s under the microscope. The process of observation affects all involved. Our inability to keep track of all of the corresponding effects is what limits us to computing probabilities.
I personally think that this sounds very logical. But it seems to lead to some bizarre conclusions. In Everett’s interpretation, all those interfering possibilities (the electron taking all possible paths and spinning in all possible ways, Schrödinger’s cat being both dead and alive, etc.) never really go away. This seems to imply that this particular world, in which I'm writing this to you right now, is just as real as another in which I'm not because I was killed this morning by an asteroid. Hence the name: “many-worlds interpretation.”
Some physicists also believe that this interpretation of QM implies that there’s no concept of time at all in the fundamentals laws of physics. As sentient creatures, we remember the past but not the future. In the Copenhagen Interpretation, the basic difference between the past and the future is that the past is definite (because it involves things already observed) but the future is not. But in the many-worlds interpretation, the indefiniteness of QM never goes away. It therefore seems that the flow of time might not exist in that interpretation
Needless to say, these are troubling features for a scientific theory to have, since as scientists we can directly study only one world, and all of our experiments take place in time.
Nonetheless, Everett’s work, which was totally ignored when he first published it, has slowly but steadily gained support among theoretical physicists. It has also led to a large body of work on what is called the theory of "quantum decoherence," which builds on the many-worlds interpretation in order to explain how classical physics emerges from the underlying quantum laws.
The idea behind quantum decoherence (see the work of Dieter Zeh and Wojciech Zurek) is that once the observer has interacted with the observed, the observer also interacts with its surroundings, and that this complicates things dramatically. It complicates things so much, in fact, that the observer can no longer keep track of the full quantum state. The loss of this knowledge leads to the impression of a wave-state collapse and to the emergence of a world governed by classical physics, where quantum interference is no longer present.
Do you believe that the phenomena described by QM, in particular wave-state collapse and the uncertainty principle, are subject to the law of causality?
The Heisenberg uncertainty principle is actually a mathematical property of waves, which can be corroborated even in classical physics (for example, in waves in water). It says that the more precisely you specify the frequency of a wave, the less precisely you can specify its position, and vice-versa. What Heisenberg did was apply this to things like electrons, which before him were not thought of as being wave-like. He was certainly correct, and the wave-like nature of subatomic particles has been amply confirmed by experiments.
But, again, the problem is that whereas a wave doesn’t have a well-defined position (think of ripples on the surface of a pond), when you go look for an electron, it always shows up at a specific point in space. In the Copenhagen Interpretation, there is no direct causal connection between the quantum state of the electron before observation and the actual outcome of the observation.
What disturbs me most is that the Copenhagen Interpretation sees observation as a one-way affair, in which the observer stands outside of the rules of QM, and it’s the observer’s decision to look at a particle that makes it adopt a definite position. I find the many-worlds interpretation superior in that it recognizes that the act of observation is a two-way process, in which the observer and the observed interact, affecting both.
The Copenhagen Interpretation is not fully causal in the sense that nothing explains the precise outcome of the observation (also known, technically, as “wave-state collapse”), which is random. But in the many-worlds interpretation this becomes the question of why we perceive ourselves to live in this world, rather than in another, and this seems to me just as problematic from the point of view of causality. I suspect that the concept of causality, which arose in the world of classical physics, might just not be well suited for QM.
Your work on cosmological inflation was featured in the January 2010 Scientific American cover. What can you tell me about that work and how it relates to QM and its interpretations?
In the early 1980's, Alan Guth (now a professor at MIT) proposed the so-called theory of "cosmic inflation" in order to solve several puzzles about the structure of our universe. One of them is that the distribution of galaxies in our universe is very homogeneous, but by the time galaxies formed, the universe was already so big that it would have been impossible for the various parts of the universe to “find out” what the others were doing, since information cannot travel faster than the speed of light. How, then, did the galaxies "know" to distribute themselves homogeneously?
Guth's solution was to postulate that our universe started out as a very tiny patch of space-time (a billionth of the size of a proton), which then inflated exponentially to a size of a few centimeters, before continuing to expand at the more leisurely pace that we enjoy today. The homogeneity of our universe is a consequence of the fact that it started out as something small enough to be homogeneous in the first place.
Not only did this theory solve this and other long-standing cosmological puzzles, it also made other predictions that were confirmed by precise astrophysical measurements years later. Today most cosmologists accept inflation as a basic part in the picture of the early history of our universe.
But just like our universe came from a small space-time patch that inflated exponentially, other patches could have inflated into universes in their own right. We can't see them because, once the exponential growth got going, not even light was fast enough to accommodate communication between the inflating patches.
If you combine this idea with other speculative theories about high-energy physics, it seems plausible that those other universes would not have the same physical laws as ours. For example, they might have different forces between particles, or the same forces as we do but with different intensities, and fundamental particles might have different masses.
People call this collection of inflationary “pocket universes” the multiverse. It's not really the same thing as the “many worlds” of Everett's interpretation of QM. In inflation, the other universes are out there in space, but we would have to move faster than light to be able to reach them. In Everett's picture, those other worlds aren't “out there” in a spatial sense. They only appear in the equations of QM.
Many physicists think that even if the inflationary multiverse existed, it's a waste of time to think about the other universes, because it's impossible to communicate with them. Others, however, have suggested that doing so could be useful when it comes to explaining why the laws of physics in our universe are as they are. For example, explaining why the electron weighs as much as it does, and not a little more or a little less.
It has been suggested that these observations have an "anthropic explanation." The idea is that most of the universes in the multiverse would not be able to form any kind of conscious, intelligent life. Only a very few universes would have the right conditions for intelligence to grow in them. Then it wouldn't be a surprise that we happen to find ourselves in one of those rare universes congenial to intelligent life, just like we humans happen to find ourselves on one of the rare planets with conditions congenial for organic evolution.
In our article for Scientific American, we explore whether this "anthropic explanation" holds up to more careful scientific scrutiny. The answer is that in many cases it doesn't: it looks like the laws of physics could be significantly different from the ones we've measured here, without there being any obvious reason why intelligent life would be impossible.
What kind of life might we find in a universe composed of "sigma carbon?"
"Sigma carbon" is a term that we made up to describe an atom in a possible, alternative universe, whose nucleus would be made of neutrons and particles called “Sigma-minus” (instead of neutrons and protons as in our universe).
Since that nucleus would have six units of electric charge, chemically it would behave very much like the carbon that we see in our world. (Chemical properties are largely determined by the electric charge of the nucleus of a given atom).
Therefore, we argue, "sigma carbon" could make up life forms very much like the ones we see around us, based on the chemistry of organic compounds.
You used computers to model other universes and their physical laws. That could be considered "demonstrative" evidence of other universes. Can you envision being able to observe or obtain direct evidence of a different universe?
In the case of the inflationary multiverse, the fact that information cannot travel faster than the speed of light prevents us from having any communication or interaction with other universes. Any science fiction fan will know that the solution to this is to build a “warp drive” engine, but everything we know (or think we know) about physics so far suggests that this isn’t possible.
That said, I do think that whether your fundamental theory of physics predicts a multiverse or not is an important scientific question.
Let's say that your favorite theory of high-energy physics predicts a multiverse (in fact, they generally do, and not because people want them to). Suppose that it turns out that out of all the universes in that multiverse that can accommodate intelligent life, the vast majority look nothing like our own. That wouldn't make your theory sound very convincing.
It would be like throwing a coin into the air one thousand times and getting only heads. If your theory is that the coin is fair, you'll have a very hard time convincing me, even though it’s possible that a fair coin will give a thousand heads in a row.
What do you think of the Elitzur-Vaidman bomb-tester?
This is a thought-experiment in which the laws of quantum mechanics are used to determine with certainty that a bomb is not a dud, without having to actually detonate it. That the laws of quantum mechanics make such a device possible is certainly true. Also, an equivalent setup (without actual bombs, of course!) has been shown to work in the lab.
It's true that this experiment shows that one can experimentally deduce the counter-factual that the bomb might have exploded, which would be impossible in classical physics. If you want, you can phrase this as deducing that there is a parallel universe in which the bomb exploded. But I'm personally disinclined to say that you've detected that parallel universe, because the same result could be phrased in the language of the Copenhagen Interpretation. You could just say that you've determined that the bomb is an actual, working “measuring device” (i.e., a live bomb).
What do you think of the theory of quantum immortality?
As far as I understand it, this is the idea that in the many-worlds interpretation there’s always some world in which a conscious being (like myself) continues to exist. Take the world I mentioned earlier where I was killed this morning by an asteroid. Let's say there's always another world in which I survive. Presumably the asteroids could keep coming, but there would always be some worlds in which I continue to exist. This seems logical enough if you buy the many worlds interpretation. But it seems, frankly, like idle speculation. I personally can't think if any important scientific consequence for this idea.
Some people have phrased this as QM making it impossible to ever actually kill yourself. Everett seems to have believed this. He led an unhappy and tragic life, despite a variety of important accomplishments, and his own daughter committed suicide, leaving behind a note saying that she continued to exist in other worlds. In this world, Everett, who was an alcoholic and a chain-smoker, died of a heart attack at the age of 51.
Is Quantum immortality falsifiable? What type of experiment might be able to test it?
Those other worlds only appear in the equations and their existence is certainly not falsifiable. Needless to say, this should give scientists pause, since we claim that our job is to understand and make predictions about things we can observe. But I personally don’t think that this makes the notion of parallel worlds a scientific non-starter.
Back in the 19th century, several very influential scientists (Ernst Mach, for instance) insisted that atoms should not be invoked in scientific theories, because they were too small to be seen. But physicists kept talking about atoms, because they were useful in order to explain other things that could be directly observed. Today we know that atoms do exist.
For me then, the real question is how those other, parallel worlds fit into the logical structure of the theory, and whether the theory as a whole makes valid predictions.
It seems likely to me that the many-worlds interpretation will turn out to be the only one in which the laws of QM are logically complete and consistent, and the existence of those other worlds might perhaps have some conceptual consequences analogous to those I mentioned for the inflationary multiverse. But I'm not so sure that quantum immortality will turn out to be a useful or meaningful scientific concept.
If Quantum immortality is true, how should we describe the past and the present? What do QM and your work tell us about the structure of time and the past and future?
A number of theoretical physicists have argued that the many-worlds interpretation implies that there is no concept of time at all in the fundamental laws of physics. The universe simply is, and the flow of time is an illusion created by the fact that each of us is conscious only of a very small part of the universe, which each of us happens to define as the “self” and its surroundings. That is, time derives from our own insensitivity to the full quantum state of the universe. This insensitivity is what is technically called entropy, and for us time seems to flow in the direction of increasing entropy.
John Wheeler (who had supervised Hugh Everett’s doctoral thesis at Princeton University) and Bryce DeWitt (one of the earliest advocates of the many-worlds interpretation within mainstream theoretical physics) wrote down an equation that expresses the supposed timelessness of the fundamental quantum laws. The meaning and scientific relevance of the Wheeler-DeWitt equation, needless to say, remains controversial.
Interestingly, the notion that there’s no fundamental difference between observer and observed, and that the sense of self and of the flow of time are illusions that can be overcome by considering reality as a whole, agree with certain reflections that you can find in Hindu and Buddhist scriptures, and a little more recently in the work of Arthur Schopenhauer. Ricardo Castañeda, a professor of psychiatry at NYU, was the first to bring this similarity to my attention.
I consider myself a hard-nosed rationalist, so I don't want to give the impression that I take a scientific idea more (or less) seriously just because it sounds like what older mystical or philosophical thinkers wrote. If these ideas turn out to have merit, I think it would just be a case of certain conclusions following from careful thought, regardless of whether its background was mystical, philosophical, or scientific.
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