How Do We Know What We Know (2)?
Discounting the quark
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The established (and entrenched) view of anything: science, society, sociology, medicine, whatever, has a strong tendency to monopolize “the truth.” It is perfectly appropriate that a body of knowledge—built up over years, sometimes centuries, and supported by repeated experiment and empirical data—should be resistant to change. Arbitrarily changing viewpoints as a matter of whim or current fad would likely be quite unproductive. That said, the resilience of the accepted truth can act in the same way that blinders do on horses—it forces people to look only at what they are “supposed” to look at.
An example: when I was in high school, probably around 1967, as part of a physics lab class, I ran a Millikan’s Oil Drop experiment. This involved peering through a calibrated microscope while an oil drop squirted into a space gently dropped under the influence of gravity. By timing the rate of fall of the oil drop a number of key measurements can be allowed for: the viscosity of the air in the gap, the size of the oil drop, etc. The space is then zapped with some source of ionizing radiation so that oil drops, including the one being tracked, will themselves become ionized.
A high voltage positive electric field is then turned that, if the oil drop has picked up one or more free electrons, will cause the oil drop to rise. The rate at which the oil drop rises, accounting for viscosity, oil drop size, etc. and measured by timing through the microscope, is an indicator of the number of free electrons captured by the drop. By repeating for a number of different oil drops, the results can be graphed showing those oil drops with 1, 2, 3, 4, etc., captured electrons (oil drops with more electrons will rise faster that those with fewer for the same electric field strength). From this a value for e, the charge on an electron can be calculated.
Robert Millikan, after whom the experiment is named, won the Nobel Prize in Physics in 1923 in part because of this experiment and his calculation of e.
In my experiment, I recall that my graphed data points clustered reasonably well around the integers representing the numbers of electrons in the oil drops at 1, 2, 3, 4 etc. However, there was one point that seemed to show that one oil drop had captured 1/3 of an electron (or an electron with 1/3 of the normal charge). Clearly, according to my physics teacher, this was an experimental error and he told me to discard that data point. Which I did. While this was around the time that the concept of quarks was being initially discussed [1], such abstruse considerations had certainly not made their way to my high school in South Wales. It was only later, much later, that I found that the down, strange, and bottom quarks are considered to have -1/3 electron charge.
So, had I isolated a quark? Um, according to the Stanford Linear Accelerator Center (SLAC) probably not [2]. The interesting thing was the reaction of the physics teacher. Science is supposed to agnostically analyze the data to compare against the hypothesis, but here Mr. Edwards, good scientist and teacher as he was and quite contradictory to what he was teaching, told me to just pitch the data. While SLAC’s results indicate my result almost certainly was experimental error, it might not have been.
Indeed, in related news, famed physicist Richard Feynman (published in Surely You’re Joking Mr. Feynman!) noted that generations of physicists discounted their results in similar experiments when they conflicted with Millikan’s numbers and routinely eliminated those data points that were thought “too far off” from what was considered Millikan's canonical answer. It seems the actual value of e is not quite what Robert Millikan measured.
Such is the power of the received wisdom of the age.
FOOTNOTES
[1] By Murray Gell-Mann and George Zweig in 1964
[2] A study published by Stanford Linear Accelerator Center (SLAC) in 2007 of over 100 million oil drops in experiments tested between 1995 and 2007 noted only one possible fractional electron charge (at 0.3e) but considered it possibly to be due to “hydrodynamic coupling” with an nearby highly charged drop.
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