The complicated history of simple scientific factsEvery now and then, the public gets a glimpse at what goes into the making of scientific consensus on an important question. No, we're not talking about the infamous climate change emailswe're talking about how science really comes to its conclusions, a process that involves a few hundred years of work.
By Chris Lee
| Last updated November 30, 2009 11:30 PM
Sometimes, even as a person pisses you off, they make a point that you can't ignore. In a recent forum discussion that I was involved in, scientists were accused of making pronouncements from on high. The argument was that scientists jump to a conclusion that seems desirable to them, and then treat it as an infallible truth.
Of course, my initial reaction was to pronounce that I, as a practicing scientist, never make pronouncements. But, looking at my articles from the perspective of someone who really knows absolutely nothing about scienceas a practice or as a body of knowledgeI can see how one could see little beyond a list of assertions. The truth is more complicated, of course, but it's a truth that science writers find challenging to convey. Science is impossibly broad, and the leading edge sits, precariously balanced, on a huge, solid, and above all, old
body of knowledge. To illustrate this problem, I am going to tell you the story about how the speed of light came to be the ultimate speed limit for the entire universe.
What I want you to remember from this story is that any new fact or change in our understanding sits upon generations of accumulated knowledge. Most of that knowledge is now trusted as "mostly correct," though some of it still lies in the "probably not too badly wrong" category. Sitting beneath that is a body of work stretching back some 6,000 years, some of which is still highly relevant.
My overall point is that, even if I were to extend each of my peer-reviewed articles by some 3,000 wordsI already get complaints about the length of some of my articlesI still would not have covered the science of an entire subject. By choosing a starting point for the knowledge described in an article, I really am pronouncing from on high that everything beyond that point is established, trusted knowledge, while everything after that point will be explained to some extent.So, how do we measure stuff anyway?
My arbitrary beginning for this storyand make no mistake, it is a story that leaves out any number of complicationsis Galileo. Apart from being a telescope builder extraordinaire, Galileo also had an important insight into the process of measurement. He saw that if he was on a moving boat and fired a cannon forward, he could measure the speed of the cannon ball and come up with a number. But, the poor guy on the receiving end of the cannon ballgiving up his life in the name of sciencewould, when making the same measurement, come up with a different answer.
Needless to say, a violent disagreement might ensue (provided the target survived the cannonball) over whose measurement was correct. Galileo saw that the difference between the two measurements was the speed of the boat. That is, the person receiving the cannon ball sees that it is moving a bit faster than Galileo because the target sees that the cannon that fired the ball was also moving. Once this extra speed is taken into account, agreement could be reached between different measurements, and Galileo could return to upsetting other people.
The key point that Galileo made clear was that measurements are always relative to some benchmark. We measure the speed of a car relative to the ground, and we measure the speed of stars relative to each other (including the Sun). This principle underlies a lot of modern physics, and it's so fundamental that we don't even give it a name when we teach it anymore.
But it turns out that this principle is, in fact, wrong sometimes. Showing how we know it's wrong and why we found out that it is wrong is what this story is really all about.Another arbitrary beginning: the story of light
Galileo was not the only person into optics and telescopes. Newton and Huygens both made huge contributions to our understanding of lightNewton demonstrated that white light contained all the colors of the rainbow, while Huygens created a model that explained the structure of the patterns light created after it had passed by a sharp edge.
But these two giants of science disagreed about what light actually was. Newton thought that light was a particle, while Huygens thought that light was a wave. Critically, all observed phenomena could be explained by both models, so both had their adherents and critics. Note, though, that this dispute happened a bit before 1700, but this issue remained unresolved until the middle of the 19th century.
That is not to say that no one cared or did anything about it. On the contrary, evidence for the wave theory of light accumulated and the particle theory of light had to be modified to accommodate the new findings; as it became more complicated, the number of people who supported it shrunk.
The straw that broke the camel's back when it came to support for light as a particle was Young's experiment that demonstrated that light, like water waves and sound waves, could be made to interfereone of the reasons this took so long is that Young needed a relatively modern light source to make his observations. In the meantime, an important question remained unanswered: if light was a wave, what was doing the waving?Yet another arbitrary beginning: the story of electricity
Off in a disregarded corner, people with names like Faraday and Gauss had begun to get interested in why, after you had rubbed a cat with a bit of amber, bits of paper would stick to both the cat and the amber, but not to each other. Equally interesting was why compass needles pointed north. Although these phenomena had been known for a long long time, no one had really investigated themor if they had, their findings had been lost. In any case, scientists got interested in static electricity and magnetism.
They discovered that some materials conducted electricity, that magnets could cause an electric current to flow, and that currents could be used to create magnets. The two were linked, but no one really knew how. Empirical laws were derived that allowed electricity and magnetism to be exploiteddynamos, electric motors, and alternators were all in the process of revolutionizing life, though their effects would take a while to percolate through society. But, despite the applications, the underlying principles remained obscurewe had laws, but no theory.
There were two problems with the laws developed for electricity and magnetism: first, they didn't shed any light on what electricity or magnetism were or why they were linkedthe concept of charge had been introduced, but no one knew what a charge might be. Second, they weren't predictive: that is, whenever anyone found a new magnetic or electrical phenomena, a new law was required.
That's where things stood until the late 19th century, when Maxwell decided to use some new-fangled math to describe electricity and magnetism. He found a common set of equations that described both phenomena and how they were linked to each other.
Maxwell's work didn't win instant acceptance. In the first place, it didn't do anything about the first problemMaxwell's equations offer no insight into the origin of electricity or magnetism, beyond the charge concept, anyway. Meanwhile, there were other theories floating around that were purely mechanisticthey solved the first problem, but failed to be predictive (or at least, accurately predictive). In addition, Maxwell's work introduced a series of new problems.The strands come together and cause headaches
One of the first things that Maxwell noticed was that if he combined his equations in a particular way, he got what is called a wave equationthat is, a particular form of mathematics that describe the movement of a wave. Essentially, he found that a collapsing magnetic field will create an electric field. Once the magnetic field had completely collapsed, the electric field could no longer grow and would begin to collapse. But the collapsing electric field would then generate a new magnetic field and the whole shebang would move.
These things looked pretty much like certain types of sound waves in a solid, but no one had seen them. Maxwell's genius was to see that this might be what light was. A few calculations showed that the speed of oscillation meant that we could never directly detect themand, for the record, we still can't.
A second problem was a little more subtle. A moving charge was known to create a magnetic field, while a static charge only had an electric field. But, as Galileo had worked out, movement had to be measured relative to something. That meant that while one observer might see a moving chargeand a corresponding magnetic fielda second observer might see only a static charge and no magnetic field.
There was no known way to get the two observers to agree. Although this problem was present before Maxwell had done his work, his work linked electricity and magnetism in such a way that scientists could no longer avoid dealing with it.Confusion reigns
These problems confused and upset a number of scientists. Maxwell had used new math, developed by Hamilton, that most scientists weren't familiar with. This made it hard for them to trust the results, and many felt that although it was a cute mathematical description, it couldn't be right. Others felt that it might be right, but it certainly couldn't be complete. A final group took a look at the implications and began to think about how they might change our view of the world.
The waves themselves created their own issues. The wave equation gave a speed for light. But, as we discussed earlier, speed is always relative to something, and, on that front, the wave equation was silent. This led to a lot wild speculation about what the speed of light might be relative to.
In the end, there were only a few choices. The speed of light could be relative to the medium through which it traveled, relative to the emitter, relative to the receiver, or relative to some absolute reference frame.
Each of these was tested. It turned out that, yes, the speed of light does change, depending on the medium, but not on the speed with which the medium was movingor at least, not in the way that Galileo would have expected. The speed of light was found to be independent of the speed of the emitter or the receiver, and attempts to find the absolute reference frame failed. The speed of light appeared to be constant, no matter what.Physics with consequences
Enter Einstein at the beginning of the 20th century. It should be pointed out that Einstein was well aware of the problems posed by Maxwell's theory of electromagnetism, but he was blissfully unaware of much of the experimental work being done. Instead, he played "let's pretend"say the speed of light is constant for all observers, what are the consequences? In particular, he wanted to know how Galileo's rules for getting observers to agree changed.
He found that getting observers to explain why they disagreed with each otherand, therefore, perform mathematical operations that would obtain agreementrequired giving up any notion of an absolute reference frame. As if that weren't confusing enough, a constant speed of light implied that time and space were somewhat interchangeable, and that energy and mass were two sides of the same coin.
Interestingly, as an object sped up, it observed time to pass more slowly. More importantly, if you extrapolated backwards, all objects had a rest mass, which was the minimum amount of mass it could have. Finally, and most important for this story, an object with a non-zero rest mass required an infinite amount of energy to get to a speed greater than that of light.Accepting the unpalatable
We may be good with it now, but no one really liked the idea that nothing could go faster than the speed of light. What sold Einstein's theory of relativity was that it explained how to obtain agreement between two observers looking at charges. That is, Einstein's theory, combined with Maxwell's theory, allowed one observer to see a magnetic field, one to see an electric field, and both to have a process by which they could reach agreement.
By itself, this might not have been enough to win over the scientific community, but, in the meantime, Hertz had both generated and detected waves predicted by Maxwell. Combined with the pure utility of Maxwell's formulation, scientists accepted the new theories relatively quicklyand the new math associated with it.
Accepting Maxwell's theory meant that Galileo's version of relativity had to be modified. Why did scientists choose Einstein's approach to doing so? For two reasons: first, there was now a large body of evidence that suggested that the speed of light was a constantno matter how you felt about Einstein's theory, Galileo's was certainly wrong. Second, it strengthened Maxwell's theory. Later on, a lot of experimental evidence placed it on the firmest of ground possible. We now treat it as a fact, and write science articles assuming that it is one.A full history, or pronouncements from on high?
As you can see, an explanation for why scientists accept a particular statement can involve a story that spans several hundred years and is almost never simple. You'll notice that the statement "nothing can go faster than the speed of light" fits this description exactly. The establishment of this statement as accepted fact involved at least three disparate fields of physics and relied upon several technical innovations, without which there would have been no experimental evidence to push our understanding forward.
Changes in theories are never overnight revolutions, nor do theories remain unaltered for long. Instead, acceptance of a theory is a matter of consensus, achieved over many years of work. No matter how ugly a theory, no matter how unpalatable its consequences, experimental and observational evidence is the final arbiter. This, in the end, is why we do experiments.
Terry Pratchet, in his Science of the Discworld
series, used the phrase "lies for children" to describe how we use simplifications to gently approach underlying scientific principles. In the case of "nothing can go faster than light," you are looking at another lie for children. There is an entire cottage industry of physicists who write papers that are devoted to using ideas developed by Einstein to circumvent Einstein's own conclusions. So, the latest conclusion from on high is something along the lines of "nothing in normal space can go faster than light, but if you can do funny things to space, you can go faster than light." A story 350 years in the making summed up in twenty-odd words.