Part-2: The Post-War Years
It has been stated that we might define the start of the post-war years precisely at 05:29:45, local time in Socorro, New Mexico, on July 16, 1945, if linked to the testing of the first atomic bomb. Despite the negative connotations associated with this starting point, science in the initial post-war era appeared to offer the promise of a better tomorrow and instilled a belief that it might soon unlock the secrets of the universe. Whether this optimism in fundamental science is still present in today’s world might be questioned, although it is not really the focus of this set of discussions. However, the issues as to whether quantum mechanics has provided a true description of the working of our universe is tabled as a valid point of debate.
In contrast to the 1950’s style marketing of quantum mechanics, the transition into the second half of our review of quantum theory involves many incremental developments, which we might initially characterise in terms of the ‘particle model’. In many ways, Dirac’s equation for the electron had provided the foundations for a more sophisticated atomic model. In 1927, Jordan and Pauli had developed a mathematical model for free photons, which we might simply described in terms of a quantized electromagnetic theory. The electromagnetic ‘force’ was also known to play a part in the interaction between electrons, protons and photons, which would eventually lead to the development of quantum electrodynamics (QED). On this basis, it was initially assumed that science had some of the key components of an atomic particle model, even though it was realised that the electromagnetic force, in isolation, was inadequate to explain the stability of larger atoms. As a result, it was speculated that some additional ‘components’ were still needed to account for a very strong attractive force within the nucleus of larger atoms. In 1932, Chadwick discovered the neutron, after which the proton-neutron model of the nucleus was proposed in conjunction with the idea of a strong nuclear force. However, even with this additional piece of the puzzle, it was realised that the idea of radioactivity, discovered in 1896 by Henri Becquerel, also needed a place within this developing particle model. Eventually, the idea of yet another interaction, i.e. the weak nuclear force, was proposed to complete the makeup of what were initially described as the 4 basic forces of nature, i.e.
- Strong Nuclear
- Weak Nuclear
Later, it was discovered that radioactivity was linked to the disintegration of a neutron, which then produced another particle called the neutrino. Although Pauli had forwarded a somewhat tentative hypothesis, as early as 1929, the existence of the neutrino was not confirmed until 1956. Of course, with hindsight we might realise that the developing atomic model turned out to be a ‘Pandora's Box’ of particles, such that the initial model of protons, electrons and photons had to be extended, first to include neutrons and neutrinos, then extended again to include muons, pions, kaons, lambda and sigma particles and the omega-minus particle. Then, in 1955/56, the anti-proton and the anti-neutron were ‘observed’ and the theoretical model extended, yet again, to include ‘sub-particles’ known as quarks and gluons plus W-Z bosons. So it continued, until the particle model encompassed a ‘vast horde’ of particles whose existence was so fleeting that even experimental physicists would begin to question the fundamental nature of their existence. However, it seemed that such issues were not a barrier to theoretical physicists, who then went on to hypothesise ‘virtual’ particles, which were even further removed from empirical verification. Today, the particle model might be said to include a bewildering array of particles, mostly hypothetical, such as X-bosons, axions, photinos, squarks, gluinos, magnetic monopoles, dilatons plus the now famously elusive Higgs particle, which is assumed to explain the mass of every particle, even though we might still have to question the ‘substance’ of any sub-atomic particle. As such, we might realise that the ongoing development of quantum theory, in the post-war years, would continue to become increasingly entwined with the rationale underpinning the atomic particle model. As such, it might be said that quantum theory not only represents the foundation on which much of 20th century science would come to rest, but also on what the 21st century still rests. However, while many still question some of the underlying assumptions of quantum mechanics, it has provided a successful method of calculation and, in many ways, proved to be a pragmatic approach to solving many of the pressing problems within the field of particle physics and the subsequent development of many technologies:
- Quantum theory predicted anti-matter
- Formed a better understanding of radioactivity
- Helped underpinned the nuclear power program
- Supported the development of semiconductors and superconductors.
However, fragmenting the discussion of quantum physics into such specific topics is not necessarily helpful to somebody trying to understand how all the various ‘mechanisms’ of quantum theory fit together within the ‘big picture’ of science. In this respect, we shall first focus on how quantum mechanics continued to develop, as a science, in the post-war years, after which we might be in a better position to consider some of the wider philosophical debates that still surround this subject to this day. However, before expending too much time and effort on trying to understand any more details linked to the post-war developments of quantum physics, many people might want to ask a simple and straightforward question:
What is the current status of quantum theory?
To which the simple and straightforward answer might be ‘useful, but not proven ’. It can be said to be ‘useful’ in that it has undoubtedly provided some profound insights into the quantum realm, especially in the field of particle physics. However, it is ‘not proven’ for the reason that there are still too many nagging doubts that the insights it has provided are limited to describing the effects emerging from an unobserved quantum reality, where the process at work remain essentially unexplained.
OK, but what has led to this state of affairs?
Unfortunately, this is where the simple and straightforward Q&A stops and where we must start to try and understand the details of some of the post-war developments in quantum theory. As has been previously outlined, the pre-war years established many of the foundation principles of quantum mechanics (QM) and relativistic quantum mechanics (RQM) in terms of concepts and mathematic plus an initial philosophical interpretation. However, in so doing, it might be said that a change took place in the underlying methodology of science relating to the dependence of physics on mathematics, such that the tangibility of physical reality came to be questioned. Therefore, before embarking on the next phase of discussions, it might be worth establishing some reference to the semantics underpinning the status of modern quantum theory:
- A ‘conjecture’ is a proposition, which while unproven, is thought to be true.
- A ‘hypothesis’ is an individual empirically testable conjecture.
- A ‘theory’ may be a collection of hypotheses that provide a coherent explanation.
By way of a footnote, a conjecture need not necessarily be thought to be true by everybody and, in fact, in the wider world beyond theoretical physics, the number supporting an unproved conjecture or hypothesis may actually be quite small. In this context, the development of quantum physics has always been based on a mixture of conjecture, hypothesis and theory, which has not been fully subjected to empirical verification. However, in practice, empirical verification of a hypothesis extending into the quantum realm has often proved difficult, or even impossible, within the remit of any known scientific methodology. As such, the initial assumption of the validity of a given conjecture, especially if supported by some mathematical model, may often persist in the absence of any immediate method to prove or disprove it.
Can a specific example be given to clarify such concerns?
If we initially restrict the example to pre-war quantum mechanics, as already discussed, we might question the status of deBroglie’s hypothesis regarding the duality of particle-waves. Normally, the wave-particle duality is described in terms of a single ‘object’ that has the attributes of both a particle and a wave. However, according to deBroglie’s pilot wave theory, both the particle and the wave exist as real and distinct physical entities; where the pilot wave guides the motion of a point particle. This interpretation was the first example of a hidden variable theory, even though it might be said to only be unproven conjecture or hypothesis. Subsequently, this idea was taken up and modified into the Bohm interpretation, which remains an open-ended attempt to interpret quantum mechanics as a deterministic theory that avoids the problematic notion of the wave-function collapse. However, as indicated, this interpretation remains hypothetical in as much as there is still no obvious way to empirically prove, or disprove, the existence of the pilot wave. In contrast, the description of a composite matter-wave packet, localised in space, appears to require a superposition of waves, which are subject to dispersion and generally described as mathematical entities in as much that they appear to propagate with superluminal velocity – see ‘Matter Waves’ and ‘Time Evolution of a Matter Wave’. So, at face value, it would appear that the issue of the quantum wave collapse is also conjecture, which is then extended to include the idea of an ‘entangled quantum state’ prior to the collapse of a mathematical wave function, which describes an ‘entity’ that may have no existence in objective reality. As such, science of the ‘physical’ transcends or descends depending on your worldview into a discussion of metaphysical or mathematical objects, which by 1935, Schrodinger appears to have accepted:
|“I am long past the stage where I thought that one can consider the wave function as somehow a direct description of reality.”|
This inability to prove, or disprove, a theory based on any level of empirical verification allowed others to argue that the wave function exists outside the objective reality of ‘physical spacetime’ in a more abstracted form of ’configuration space’. Within the scope of such definitions, we might see one of the fundamental reasons why the description of quantum physics has become so dependent on mathematics and subject to philosophical interpretations. For example, the following quote is by David Albert, a professor of philosophy who also has a B.S. in physics and a doctorate in theoretical physics:
|“..the space we live in, the space in which any realistic understanding of quantum mechanics is necessarily going to depict the history of the world as playing itself out…is configuration-space. And whatever impression we have to the contrary (whatever impression we have, say, of living in a three-dimensional space, or in a four dimensional space-time) is somehow flatly illusory.”|
This position appears to be arguing for the ‘reality’ of the mathematical description of configuration space. In contrast, Bradley Monton, who is an associate professor at the University of Colorado with a doctorate in philosophy argues for a different viewpoint:
|“I maintain that quantum mechanics is fundamentally about a system of N particles evolving in three-dimensional space, not the wave function evolving in 3N-dimensional space…If (David Albert’s) claim were true, this would be the most radical revision of people’s everyday understanding of the world ever engendered by science, far surpassing any other scientific revolution in our worldview. It is this radically revisionary nature of the wave function ontology hypothesis that leads me to assign a low prior probability to the hypothesis; the evidence would have to be very strong to lead me to accept the hypothesis.”|
However, at this stage, the key point to highlight is that both these opposing interpretations are based on the hindsight afforded by the last 50 years of post-war developments in quantum theory. As such, the goal of the following discussions is primarily focused on trying to understand what quantum physics can and cannot tell us about the fundamental structure of the universe.