As a physicist, I understand what science is about. I have a good understanding of the scientific method and what science has achieved. But, unlike many other physicists it seems, I also know about the limits of science.
So, recently, I saw this “pledge for science” where people are asked to add their support to say that they put their trust in science. Unfortunately, I could not find a fully worded statement of this pledge to understand exactly what is meant by it. What does it mean to put your trust in science?
To be honest, I think I know where this is coming from. With all the anti-vaxxers, followings on the heals of global warming denial, and all those kinds of trends and misinformation that is being spread via social media, it is not surprising that some reaction would follow from the scientific community. However, one needs to guard against an over-reaction.
Science does not have a clean track record. It is unfortunately responsible for several serious problems in our world today. Take for instance global warming. It does not take much to realize that in as far as it is caused by human activity, it is with the aid of scientific development that this human activity is able to cause global warming.
Another example is weapons of mass destruction. Through scientific investigation humanity achieved the point where it can cause unprecedented death and destruction. Not exactly a highpoint in human cultural achievement. Once this door was opened, nothing could close it again. Forever, humanity will have this sword having over its head.
One can proceed to list other negative effects of scientific development such as pollution and the hole in the ozone layer, but I think the message is clear by now. An unconditional trust in science is a very dangerous thing. Instead, one should rather support an effort to get people educated and informed, not only about science and the scientific method, but also about other aspects of culture. For instance, if people have better knowledge of history, they would have a better understanding of how ignorance can lead to terrible things.
Let me emphasize then, I do not support an unconditional pledge to put my trust in science. In fact, it is a dangerous thing to put one’s unconditional trust in any specific thing on this earth.
The world is in the grip of a pandemic. In many ways, it tests the systems that have come into existence through the cultural development of humanity. To some extent, it may even be testing its survivability.
If humanity were still in the process of biological evolution, the outcome of the test would simply be determined by the survival-of-the-fittest criterium. But humanity has moved beyond mere biological evolution. It has developed culture.
Culture provides a mechanism that can circumvent the threats posed by the environment, even when humans are not the “fittest.” A culture can adapt much faster than the genome. The pandemic is testing the fitness of culture as a mechanism for survival.
The world consists of many different cultures. Each culture provides various systems, including those responsible for government, finance, education, health and so forth. All these systems must work together to provide the protection for the people of these cultures during times such as these.
We live in a time where the cultures of the world have started to merge. The information obtained through research and the technologies that are developed are shared among all the cultures. The benefit of cooperation is obvious. For any single culture to survive, it is necessary that all of humanity survives.
While the picture of the world in terms of cultures may give a positive view of humanity’s prospects, it does not reveal the negative side. For this picture to prevail, these cultures need to be healthy. Here lies the problem.
A culture transcends the mechanism of biological evolution in that it cares for the individual members of the culture. The golden rule “do unto others as you would have them do unto you” forms the foundation for civilization. It allows people to live and work together to enable the development of culture and to achieve the great works that are associated with it.
At some point, it was decided that the golden rule is not enough. Individuals need more protection. So people introduced the notion of “human rights” and even call them “fundamental.” The idea makes sense, but begs the question of a culture that is at least moderately civilized. Within the context of the golden rule the concept of human rights can work.
But the world contains many communities where the cultural development is either lacking or has decayed to the point where the golden rule is not part of culture. In such situations, human rights provides a platform from which a community can be terrorized. The individual is elevated above the community. The result is a general breakdown of culture and a return to the survival of the fittest scenario.
Examples of this breakdown becomes prevalent. People oppose the attempts of governments to contain the spread of the virus. In some cases, these situations become violent. It threatens the culture’s ability to protect.
Human rights have a place in culture, but it must be placed in balance with the protection of the community. The individual human is not more important than all of humanity. How this balance should work and be implemented is not clear to me at the moment. But I’m sure the thinkers of our time can come up with some ingenious ideas.
One of the most iconic quantum phenomena is quantum teleportation. But the reason why it is so iconic has nothing to do with the idea behind “beam me up, Scotty.” In quantum teleportation, it is only the state of matter that is being transferred and not the matter itself. Usually, it is the state of light (a photon) that is being teleported. Quantum teleportation is iconic because it involves a mechanism that reveals a truly quantum nature.
How does it work? The state to be teleported is represented by photons that are specially prepared for the purpose. You can think of some light source that produces photons having specific properties that represent their state. We shall label one such photon as A. The resource that will mediate the teleportation process is a different bunch of photons representing an entangled state. This entangled state consists of a pair of entangle photons, which we label as B and C, respectively. To perform the process of teleportation, all we need to do is to make a joint measurement of photons A and B. It is the nature of this joint measurement that makes the process of quantum teleportation possible. The information that we obtain from this measurement tells us what transformation to perform on C to reproduce the state of A. Sometimes, we would not need to make any transformation. The state of C would already be that of A.
So, let’s look a little more carefully at the nature of the joint measurement. What do we mean by a joint measurement? To understand what it means, we need to discuss the state of photon A . There are many different possible states that this photon can have. All such states are collected into a set that we call a Hilbert space. Any of the states in this set can be represented as a superposition of a small set of states that we call a basis. One way to determine the state of a photon is the measure how much of each of these basis elements are required to make up the state of the photon. Such measurements are called projective measurements.
To understand joint measurements we just need to generalize our understanding of projective measurements a bit. What the measurement instrument in a teleportation experiment sees is not just A, but A and B together. The Hilbert space for the combination of the states of these two photons consists of all the combinations of all the states from their respective Hilbert spaces. One can produce a basis for the combined Hilbert space by combining the elements of the respective bases. There are different ways to do that, including some that would cause the elements of the combined basis to be entangled states. That is the key for quantum teleportation. One needs to make projective measurements of the combined state in a basis where the elements are themselves entangled.
Why would projective measurements in terms of an entangled basis cause teleportation? This mechanism is what makes teleportation an amazing process. It involves the multiple-reality nature of the quantum world. The entangled resource state can be interpreted in terms of such multiple realities. What joint measurements are doing to knit these multiple realities together with those presented by the input state A. But the latter is just one state (one reality), therefore, in the ideal case, only one of the realities of the resource state will survive the measurement process, the one where C has the same state as A. In a less ideal case, bits and pieces of A will be distributed over different realities. In that case, one can reconfigure the different realities with the aid of a unitary transformation on C, such that A becomes associated with just one reality in which C would then have the same state as A. The outcome of the joint measurement would tell us which unitary transformation to perform to achieve the necessary reconfiguration.
How does one make projective measurements in terms of an entangled basis? That is challenging, but people have identified at least two ways to do that. The first process and the one most often used is the Hong-Ou-Mandel effect. It is accomplished with the aid of a beamsplitter, causing a quantum interference effect. If two photons are observed simultaneously from the two output ports, then it signals the detection of a special entangled state called a Bell state, which implies a successful teleportation. The benefit of this method is that it does not require any unitary transformation of C.
Another way to perform a joint measurement is with the aid of the inverse of a process that would produce entangled states. In quantum optics, most entangled photon states are produces with the aid of a nonlinear optical process called parametric down-conversion. The inverse process is parametric up-conversion (also called sum frequency generation). While down-conversion converts a single incoming photon into two photons that are entangled to maintain energy and momentum conservation, the up-conversion process takes two incoming photons and combine them into one photon. A successful up-conversion implies a projection unto an entangled state to maintain energy and momentum conservation. Therefore, it can also be used for quantum teleportation. However, the process of up-conversion is very inefficient.
It is argued that the current impasse in fundamental physics is at least partially caused by the fact that there does not exist a credible understanding of the process of quantum collapse. This argument begs the question of those interpretations of quantum mechanics that incorporate quantum collapse. Therein lies a dilemma: interpretations of quantum mechanics are generally non-scientific – except for a few special cases – interpretations of quantum mechanics are not falsifiable. Therefore, even if we were able to come up with a mechanism for quantum collapse, it would not form part of our scientific understanding because it would not allow falsification.
However, there are interpretations of quantum mechanics that do not involve quantum collapse. They present a possible way out of this dilemma. Whatever reason one would have to introduce the notion of quantum collapse in the first place, must somehow be reproduced in those interpretations of quantum mechanics that do not explicitly contain it. In other words, the observable phenomena that suggest a collapse mechanism, must somehow be reproduced in interpretations without quantum collapse. The explanation of any mechanism that would reproduce such phenomena would therefore provide a kind of mechanism for quantum collapse without quantum collapse. In this case, the falsification takes on the lesser form of a retrodiction where the observed phenomena are explained in term of a successful theory. That successful theory is standard quantum mechanics. In other words, what we’ll do is to use standard quantum mechanics without introducing quantum collapse to explain those phenomena that seem to require quantum collapse.
If we do not want to introduce quantum collapse we inevitably employ an interpretation of quantum mechanics that does not involve quantum collapse. The one that we use is the so-called many worlds interpretation. However, we first need to review it to remove some misconceptions. In effect, we’ll use a modified version of the many worlds interpretation.
The many worlds interpretation is often associated with a multi-verse that is produced by the constant branching of a universe due to the quantum interactions that take place in that universe. This notion is misleading. Taking a good hard look at the quantum mechanics formalism, one should realize that it does not support the idea of a multi-verse produced by constant branching. Instead, there is just one universe, but with an infinite multiple of “realities” that are associated with the infinite number of elements in the basis of the Hilbert space of this universe. The number of these realities never change – there is no branching. Instead, the basis always consist of an infinite number of discrete elements. The only effect of quantum interactions is to change their relative probabilities. So, the many worlds (or the multiple realities) correspond to the terms in the superposition of all these basis elements forming the state of the universe. This state evolves in a unitary fashion that incorporates all the interactions that take place in the universe and thereby produces a constant variation over time in the coefficients of the terms in the superposition.
With this picture in mind, we can consider what it means to have quantum collapse, or to understand any physical process that seems to suggest quantum collapse. As a first example of such a physical process, we use a historically relevant example presented by Albert Einstein during the 5th Solvay conference, which was held in Brussels in 1927. [For a transcript of the discussions at the 5th Solvay conference, see G. Bacciagaluppi and A. Valentini, Quantum Theory at the Crossroads, Reconsidering the 1927 Solvay Conference, Cambridge University Press (2009); arXiv:0609184.] At this conference, which played a prominent role in the development of quantum mechanics and the Copenhagen interpretation, Einstein described a scenario where an electron propagates toward a screen on which it is registered as a single point of absorption on the screen. He then presented two possible ways to view the process that takes place, exemplifying the problem of maintaining energy conservation without introducing action at a distance. This example is an apt demonstration of one of the key problems with the notion of quantum collapse.
Before we deal with the understanding of this process in the context of multiple realities, we first need to remove an unfortunate tacit assumption that we find in the discussions of such experimental scenarios. It mentions a particle. What particle? Or perhaps one should ask what is meant by the term particle? Does is refer to a localized lump of matter (or a dimensionless point) traveling on a world line? Or is it a more abstract notion associated with a finite mass and a discrete charge, without localization? I suspect, that the former is implies in these discussions, because it would help to explain the localization of the observed absorption. However, what we see is a localized absorption and not a particle. The notion of something that is itself localized is not the only possible explanation for the localization of an absorption process. The latter can simply be the result of a localized mechanism for the absorption process. In the scenario, the screen is assumed to be a photographic material, presumably consisting of little silver crystals that can register the electron.
So, a slightly different picture from that which was put forward by Einstein emerges. The electron is now represented by a wave function (not a particle) that propagates toward the screen. The screen contains numerous little crystals that can register the electron. However, assuming that the wave function represents only one electron, one would find only one such absorption event (otherwise we’ll have the problem of violating conservation principles). In terms of multiple realities, many of these crystals could serve to perform the absorption process. Each reality would correspond to a different crystal receiving the electron.
But how does the situation within a particular reality manages the localize the electron wave function without causing action at a distance? Remember that the different realities are associated with the different basis elements in the Hilbert space. The basis of the Hilbert space is not unique. One can define infinitely many different basis, each of which is related to any other basis by a unitary transformation. In general, there is nothing special about any particular basis. In other words, the separation of the wave function of the universe into a superposition of different realities can be changed via unitary transformation into another set of realities, each of which is a combination of the previous set of realities. However, when it comes to a specific interaction process, such as the absorption of an electron by a specific crystal, then there is a special basis that would clarify the process. This basis corresponds to the measurement basis of the crystal.
To understand what this measurement basis is, one can determine what electron wave function would have been radiated by the crystal if the absorption process is inverted as the adjoint process. At the fundamental level, all processes are invertible. It is just that the probability for the required initial conditions to produce a specific inverted process may be vanishingly small. We can nevertheless assume that these required conditions exist so that it would produce the radiated wave function. The conjugate of the radiated wave function would describe the ideal measurement basis element for the absorption of an electron by this crystal. In effect, nature transforms the basis (the realities) into the measurement basis for the absorption of electrons by the crystals. All the different realities now correspond to different basis elements, each of which is associated by the absorption of the electron by a different crystal.
Obviously all these basis elements are localized at the their associated crystals. This localization is accomplished purely through a unitary transformation without any funny action at a distance. The way that the unitary transformation accomplishes this localization is through constructive and destructive interference among the elements of any other basis that may initially have represented the multiple realities.
The different realities in the localized measurement basis have different coefficients associated with them in the superposition. Some of these coefficients would be larger than others, indicating which crystal is most likely to absorb the electron. It may be that one specific reality has a coefficient that completely dominates. In that case, although there are multiple realities, one specific reality would dominate. This dominant reality may be the one that we perceive as the single reality in our experience.
Hopefully, this understanding gives a feasible picture of the process whereby quantum collapse seems to take place. More can be said about this topic, but since this discussion has already been rather long, I’ll postpone further discussions for another day.
Let there be no doubt about the matter: I am far from being an avid gardener. Previous attempt in this direction has led to uncontrollable laughter. However things are looking better than it had for a while.
Much of the improvement in my garden is thanks to my gardener who keeps a hand on plants and makes sure the weeds don’t take over. It is not that much that needs to be done, but somebody needs to do it and that person is not me.
Then of course there’s the rain. I tend to use water sparingly due to the dry climate and my fear that our taps may run dry at some point. Fortunately, it has rained copiously a few weeks ago and the garden just loved it. Everything is green and there are flowers everywhere.
It is a pleasure to be in such a garden. Sometimes I would take my book and sit there reading. If only I can do something about all the bugs.