The young Einstein was a rebel, moving from job to job and scrambling to find a secure job. The great father figure in physics at that time was the famous Dutchman Hendrik Lorentz. A generation older than Einstein, and a picture of prosperous, upper-middle-class respectability, Lorentz had played a major role in the discovery of the electron, for which he received one of the first Nobel prizes in 1902.
He had come within a hair’s breadth of discovering special relativity, and yet always praised and encouraged the young upstart who scooped him. Some measure Lorentz’s greatness by his ability to recognize in Einstein an unusual and unconventional genius so very different from his own. In fact, Lorentz became one of Einstein’s earliest promoters, and generously helped him find positions that enabled him to continue his research. For his part, Einstein seems to have idolized Lorentz. He once wrote to a friend, “I admire this man as no other. I would say I love him”.
Decades later, shortly before his own death, Einstein voiced an extraordinary sentiment about his older colleague: “He meant more to me personally, than anyone else I have met in my lifetime.” Later in his career, Lorentz loomed over the world of physics as a wise and benevolent grand old man, perhaps the leading physicist of his generation. But historians have been less kind. In the aftermath of the relativity revolution, Lorentz has often been portrayed as a sad figure, with a mind mired in the comfortable past and simply unable to comprehend the dazzling world unveiled by Einstein’s theories.
The historian Thomas Kuhn wrote chillingly about older scientists who were left behind by scientific revolutions, and quoted the physicist Max Planck: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” For many people, Lorentz is perhaps the most prominent example of a great scientist who died clinging to his outmoded theories. His case provides extra evidence of the depth of Einstein’s reworking of our concepts of space and time: even a Lorentz, they say, could not make the revolutionary leap into the strange new world of relativity theory.
Today, however, as doubts about the foundations of Einstein’s theories multiply, Lorentz appears very differently. We now have more sympathy for his position, and even honour him for clinging to insights that time has rehabilitated. With Einstein, he is a hero in our story.
In particular, Lorentz helped begin a tradition of seeking deeper explanations of relativistic effects such as length contraction and time dilation. While Einstein simply derived these from the principles he assumed, Lorentz insisted that we press more deeply and uncover their causes. He was thus the founding father of what, for our purposes, we will call the minority interpretation.
The momentous debate between Einstein and Lorentz pitted two of the greatest physicists against each other. Their respect and affection for one another should not disguise how cutting their disagreement was. Both men had dedicated their lives to physics. If Lorentz proved correct, Einstein’s historic first discovery would be denied him. If Einstein triumphed, Lorentz’s whole approach to physics, his life-work, would be dismissed as old-fashioned, mechanical and metaphysical.
Einstein’s mainstream interpretation is dramatic. With a single sweep, it eliminates features of our world that seemed obvious and indispensable, and tumbles us headlong into a new world where distances and durations are not real properties. This has been the dominant view since the triumph of Einstein’s 1905 paper on special relativity. According to the minority interpretation first developed by Lorentz, however, each object does have a definite length of its own, but it varies with speed. That is lengths are real but variable properties of individual bodies. Similarly, an event such as the wink of an eye or a tennis match does have a definite duration, but the duration will dilate or shrink with speed. A tennis match on a large ship will really take longer than the same match would in a court at rest; a moving clock will really run more slowly. Thus the minority interpretation breaks the democracy among inertial measurements. It says that some measurements reveal the real distances and durations, while some instruments are distorted by the effects of their own high speeds and report merely apparent distances and durations.
Historically, Lorentz and other advocates of the minority interpretation were motivated by the following sorts of ideas. Just as water waves are disturbances travelling through water, they reasoned, light waves must be disturbances travelling through some very thin fluid filling all of space. They called this fluid the “ether”, which is Greek for flame or fire. Although there was no direct evidence for the existence of such an ether, it conveniently explained length contrac- tion. Just as a ship ploughing through water will feel a resistance that rises with speed, all objects that move in space are resisted by the ether. Since it is so thin, we are normally unaware of this, but at high speeds it would pile up against bodies and cause them to contract in the direction they are travelling in. A similar but more complicated argument explained time dilation as another effect of this resisting ether wind.
Thus the ether is important because it gave a physical explanation of length contraction and time dilation. Einstein and the mainstream interpretation simply deduce these effects from the mysterious constancy of the speed of light and the relativity principle, but do not explain them.
In fact, the minority interpretation has a very different view of the speed of light. It is well known that ordinary waves travel at the same speed in the same medium. Thus waves in water always travel at a characteristic speed. The reason is that each medium has a certain “bounciness” or elasticity that determines how quickly it pushes back when disturbed. Such a wave is, for one example, a push alternating downwards and upwards, so the degree of “bounciness” sets the speed of waves as they progress through the medium. In water, there- fore, waves from a high-power racing boat and from a small pebble dropped in a pond both travel at the same speed. The minority interpretation argues that light is just an ordinary wave that travels in the ether, and thus really always has, regardless of its source, the same speed relative to the ether.
But the peculiar thing about light is that measurements of its relative speed always give the same result. According to the minority interpretation this is mere appearance and not really true.
Actually, the speed of light relative to a spaceship does depend on how fast the spaceship is moving. If the spaceship is moving at half the speed of light, then a light beam racing ahead is gaining ground at only half the speed of light. The relative speed of light merely appears to be constant because of distortions due to length contraction and time dilation. Thus the minority interpretation removes the central mystery of Einstein’s theory by explaining the constancy of light’s relative speed, but it replaces it with the mystery of the ether.
Mainstream physicists have always been sceptical of the minority interpretation. They have great difficulty with these “real but variable” distances and durations. Since inertial movement is undetectable, passengers in a cabin below deck cannot tell how fast they are really moving, and likewise we cannot measure our real speed through the ether. Thus we cannot say how strong the ether wind is, and how much contraction it causes.
Physicists dislike properties that they cannot measure. The min- ority interpretation offers neat physical explanations but introduces unmeasurable and undetectable properties into physics.
There is a second, related reason why the mainstream never embraced the minority interpretation: it leads to no new predictions.
Although it is quite radical, Einstein’s theory is conceptually clean and very clear, whereas the minority interpretation is messy. It asserts the existence of real but unmeasurable lengths. It asserts the existence of the ether or some other cause of contraction and dilation, but provides no new or independent evidence for it. It asserts that these effects will coincidentally just match those predicted by Einstein, but seems to construct its theories just to produce this match. Physicists might accept this mess if the minority interpretation led to new ideas and made new predictions that would distinguish it from Einstein’s theory. But so far it has not.
Before we needed to explain length contraction and time dilation we believed that distances and durations were real and constant properties. Now we must choose between two interpretations of these observations:
• Majority interpretation: distances and durations do not exist as real properties of individual things (a shoe has no size)
• Minority interpretation: distances and durations do exist and vary with speed through the ether; they are real but variable properties of individual things (a shoe has a variable size) As we shall see, many other important consequences flow from this fundamental difference between the two interpretations.
The mainstream and minority interpretations lead to the same predictions. The mainstream interpretation is far more economical and cleaves closely to the results of measurement. The minority interpretation offers physical explanations and realistic pictures of the cause of length contraction and time dilation, but at the cost of introducing into physics unmeasurable properties and a ghostly, undetectable ether.
However, the debate between these two interpretations has heated up again in the past decade. In the following chapters we will explore the various advantages and disadvantages of these two interpreta- tions. Chapter 17 will outline new experiments that seem to favour the minority interpretation, and that have triggered a renewed assessment of it merits.
The minority interpretation is committed to real, physical lengths and therefore to real, physical length contraction, but not to any particular cause of that contraction. The ether is only one possible explanation of contraction.