[Physics FAQ] - [Copyright]
By Don Koks, 2019.
Herbert Dingle (1890–1978) is probably the best-known opponent of relativity. From the 1950s until the 1970s, he attempted to engage relativity specialists with an argument that, he insisted, showed that special relativity is incorrect. That argument was based on a misunderstanding of special relativity which was pointed out to him repeatedly by many physicists. But Dingle simply did not accept what they said. Even today, the Internet has various pages written by those who champion Dingle and repeat his words, usually using a few equations that are perhaps meant to look impressive, but which hide the very simple fallacy of Dingle's argument. What was his argument, and why is it wrong?
Even though Dingle actually wrote a book on relativity some decades before he decided to challenge the subject, he never understood relativity's two most important concepts. First, he didn't understand what an event is: he thought the word was used in relativity with its everyday meaning, such as the act of attending a concert. But this wasn't his real undoing. The main thing he did not understand was what is easily the most important concept of relativity, and which is taught in Relativity 101 classes: that relativity does away with the everyday idea that all observers agree on simultaneity. That is, relativity says that each inertial frame (all frames in this FAQ are inertial) has its own standard of simultaneity that is not shared by other frames. In other words, the events that one frame concludes are happening at the same time as some event A will, in general, differ from the events that another frame says are happening at the same time as A. There is no logical problem with these differing standards of simultaneity. The fact that they differ even resolves the Twin Paradox, and by extension (using Einstein's equivalence principle), it paves the way to the idea that time runs at different rates at different altitudes. This is then built in to general relativity: the fact that time runs at a different rate on GPS satellites than on Earth's surface must be accounted for in the GPS system, and is, very successfully.
This lack of agreement on the simultaneity of events allows two frames in relative motion each to conclude that the other is ageing slowly. Dingle thought it trivially obvious that two clocks in relative motion cannot each say that the other is ticking (i.e., ageing) slowly; and this is why he was adamant that relativity is incorrect. Indeed, if both clocks did agree on simultaneity, then it certainly would be a contradiction for them each to conclude that the other is ticking slowly. In that case, Dingle would have a point. But the clocks simply do not agree on simultaneity. Despite repeated attempts by many physicists to enlighten him, he never understood the central role that simultaneity plays in relativity.
For example, consider two observers, Alice and Bob, in relative motion and each carrying a clock. When they are momentarily next to each other, set each clock to display zero. They will then certainly agree that when one clock displays zero, the other clock does too. Now allow the observers to move apart. Set their relative speed to be such that relativity says that each observes the other to age slowly by a factor of 2. So, when Alice's clock shows 10 (seconds), Alice says "At this moment of my clock displaying 10, Bob's clock displays 5 (seconds). I don't see the number 5 yet, because it takes some time for the light to travel from Bob's clock to my eyes; but that is immaterial. I know that right now, his clock is displaying 5". [That, by the way, is the distinction between observing and seeing.] What does Bob say at that moment? He has a different standard of simultaneity, and he says "At this moment of my clock displaying 5, Alice's clock displays 2.5. I don't see the number 2.5 yet, because it takes some time for the light to travel from Alice's clock to my eyes; but that is immaterial. I know that right now, her clock is displaying 2.5". Each observer can say that the other is ageing slowly, because their standards of simultaneity are different. Is that strange? It's certainly not something that we grow up accustomed to in our everyday slow-moving world, and so we should not kid ourselves that it's obvious. But there is nothing illogical about it.
Because Dingle was simply unaware that simultaneity is observer dependent, no analysis of simultaneity entered his arguments. Thus in the above example, he would've implicitly assumed that Alice and Bob shared one standard of simultaneity. He would then have said "If Alice's clock displays 10 and she says Bob's clock is displaying 5, then Bob must conclude that when his clock displays 5, Alice's clock displays 10. So Alice says Bob is ageing slowly, but Bob says Alice is ageing quickly. This falsifies relativity." Dingle never managed to realise or accept that relativity says simultaneity is observer dependent, and it's clear that his followers remain likewise unaware. And yet, the concept follows easily from relativity's postulates, as taught in all Relativity 101 classes.
It's probably true that all arguments against relativity come down to the fact that those who advance those arguments don't understand that relativity says that two frames must disagree on simultaneity. Try it: the next time you wish to resolve any of relativity's well-known paradoxes, forget about length contraction and time dilation. Just recognise the differing standards of simultaneity of all relatively moving participants, and you will be well on your way to resolving the paradox.