| MadSci Network: Physics |
Hi James,
This is a rather difficult question, as it hinges on your definition of "observed". It would perhaps be closer to philosophy than to science. Nonetheless, I'll give you some background on the issue, and describe how the electroweak force was "observed".
The electroweak force is a unified force that manifests as an electromagnetic force and a weak force at "standard" conditions in the universe around us. The electromagnetic force is easily measured and observed with high-school lab equipment. The weak force is a force that happens mostly inside an atomic nucleus [1]. This means it cannot be "directly" observed with the same ease one would observe an apple or the magnetic force. However, it manifests in the decay of elementary particles, such as neutrons, but also in mesons and in leptons. The decay of a wide variety of particles can be described in the framework of the weak interaction. This can be seen as evidence of the weak force, and given the vast amount of different interactions and decay times that can be used to verify the theory, this experimental evidence is rather solid.
Physicists strive for elegance and maximum predictive power in theories. Hence, they are not content with having four (electromagnetic, strong, weak and gravitational) forces describing the Universe when it might be possible to unify those forces. Unification here means that a single theory has as much descriptive power as multiple other theories. An example would be electricity and magnetism: it is possible to describe moving current, static electricity and magnetism using a single theory, the electromagnetic theory, that is formulated using Maxwell's equations [2]. Here, current, static electricity and magnetism are limiting cases of the broader framework of Maxwell's equations.
It turns out that under certain circumstances, the electromagnetic and weak force should result in the same behavior. In other words, you can't tell them apart, and they are the same force, just like the electromagnetic force, that can manifest as an electric and as a magnetic field. However, these "circumstances" involve enormously high temperatures, far higher even than the temperature in the Sun. With particle accelerators becoming ever more powerful, it became possible to recreate these circumstances on Earth. The electroweak interaction [3], that we see under our more normal as electromagnetic and weak interactions, also predicted a phenomenon called neutral currents [4]. Now, this is observable, albeit with some difficulty. Scientists attempted to verify the theory by testing this prediction, and this is what they did. In 1973, a team from CERN found experimental results consistent with the electroweak theory. This is an important step in accepting a theory: not only does it describe what we already know, it also makes falsifyable predictions. If one tests these falsifyable predictions, and the theory survives, one can say to have proven the theory. In that sense, the electroweak force is an observed, proven theory, although it is at the limit of what we can measure.
An example of a theory that has not yet fully passed this last test is the standard model [5], that unifies not only electroweak force, but also the strong force and gravity. The theory has predicted several particles that have later all been observed. However, one important aspect of the theory is the existence of the Higgs boson, in essence the "thing" that causes matter to have mass. This particle has not yet been observed, as we currently do not have an experiment that is powerful enough to produce the circumstances under which it could be observable.
Summarizing, the unification of electromagnetic and weak force into the electroweak force can be seen as an observed, verified theory rather than a mathematical postulated. The theory has produced new, falsifyable predictions which have been tested and found true. I hope this answers your question.
Regards,
Bart Broks
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