TUESDAY, AUGUST 3, 2021
On its face, it never made sense: Walter Isaacson is a highly experienced journalist and an acclaimed biographer. Unsurprisingly, he's also a very good teller of stories.
In Chapter Six of his biography of Albert Einstein, he describes a "eureka moment"—the "eureka moment" in which Einstein "took one of the most elegant imaginative leaps in the history of physics."
Isaacson's chapter deals with Einstein's special theory of relativity, which Einstein formulated in 1905, when he was just 26. The eureka moment occurred in early May of that year, on a beautiful spring day in Bern, as Einstein was walking and talking with Michele Besso, "the brilliant but unfocused engineer" who was his best friend.
By all accounts, Isaacson is describing a key moment in the history of physics—indeed, in the intellectual history of the twentieth century. What "key insight" had suddenly come to Einstein that day? Isaacson starts his account like this:
Only five weeks elapsed between that eureka moment and the day that Einstein sent off his most famous paper, "On the Electrodynamics of Moving Bodies." It contained no citations of other literature, no mention of anyone else's work, and no acknowledgments except for the charming one in the last sentence [to Besso]...
So what was the insight that struck him while talking to Besso? “An analysis of the concept of time was my solution,” Einstein said. “Time cannot be absolutely defined, and there is an inseparable relation between time and signal velocity.”
More specifically, the key insight was that two events that appear to be simultaneous to one observer will not appear to be simultaneous to another observer who is moving rapidly. And there is no way to declare that one of the observers is really correct. In other words, there is no way to declare that the two events are truly simultaneous.
In Isaacson's paraphrase, Einstein's key insight was this:
Two events that appear to be simultaneous to one observer will not appear to be simultaneous to another observer who is moving rapidly. And since there is no way to say that one or the other observer is right, there is no way to declare that the two events are truly simultaneous.
There's no way to declare that two events are truly simultaneous! Remember, this is Isaacson's paraphrase of Einstein's "key insight"—but as we'll see tomorrow, it's a perfectly reasonable account of what Einstein actually said in the source upon which Isaacson is relying.
There's no way to declare that two events are simultaneous! This principle is generally known as "the relativity of simultaneity." According to Isaacson (and many others), it's the key insight—the eureka moment—which suddenly came to Einstein on that spring day in Bern.
Within five weeks, Einstein had sent off his most famous scientific paper, On the Electrodynamics of Moving Bodies. In Isaacson's words, it was that key insight which lay at the heart of "one of the most elegant imaginative leaps in the history of physics."
As far as we know, no one disagrees with Isaacson's general portrait of this historic event. The problem arises when Isaacson further explains Einstein's "key insight"—though, in fairness to Isaacson, he's working directly from Einstein's own work when he offers his account of this important matter.
On what is Isaacson relying? As he continues, Isaacson doesn't quote from Einstein's work in the famous technical paper he sent off in June of 1905. Instead, he sensibly turns to a different source as he tries to explain Einstein's key insight.
Sensibly, Isaacson turns to the account Einstein offered in Chapter IX of Relativity: The Special and the General Theory, the book he wrote for general readers in 1916. (For the text of that book, click here.)
As he continues, Isaacson presents Einstein's account of the relativity of simultaneity—but he explains it the way Einstein did in that 1916 book, the one Einstein wrote for non-specialists. That said, there is a problem here::
Unfortunately, the account Einstein offered in that book has, at least on its face, never quite seemed to make sense.
Isaacson offers a reasonably faithful rendering of the explanation Einstein presented in Chapter IX of his book. This is what Isaacson writes as he continues from the passage posted above:
Einstein later explained this concept using a thought experiment involving moving trains. Suppose lightning bolts strike the train track’s embankment at two distant places, A and B. If we declare that they struck simultaneously, what does that mean?
Einstein realized that we need an operational definition, one we can actually apply, and that would require taking into account the speed of light. His answer was that we would define the two strikes as simultaneous if we were standing exactly halfway between them and the light from each reached us at the exact same time.
But now let us imagine how the event looks to a train passenger who is moving rapidly along the track. In a 1916 book written to explain this to nonscientists, he used the following drawing, in which the long train is the line on the top.
"Einstein later explained this concept using a thought experiment involving moving trains?" As becomes clear two paragraphs later, Isaacson is referring to the explanation Einstein offered in his 1916 book.
At this point in his exposition, Isaacson reproduces the drawing Einstein used in that book. The drawing appears in Chapter IX, The Relativity of Simultaneity, in Einstein's extremely brief historic text.
That chapter is only three pages long. You can see the entire chapter, and the drawing Einstein employed, by just clicking here. The rudimentary drawing does purport to show a railroad train and two distant lightning strikes.
From here, Isaacson proceeds to explain this important concept involved in this matter almost exactly as Einstein did. He faithfully describes the thought experiment Einstein employed—a thought experiment involving that fast-moving train and that pair of lightning strikes.
As he proceeds, Isaacson is faithful to Einstein presentation. There's only one problem:
On its face, Einstein's presentation didn't seem to make sense. On its face, it didn't seem to make sense back in 1916, and it still doesn't today.
Tomorrow, we'll consider the presentation Einstein offered in his 1916 book. Isaacson worked directly from that presentation in his deeply-researched 2007 book. Eight years later, Nova followed suit, in an hour-long PBS program, Inside Einstein's Mind.
Tomorrow, we'll look at what Einstein said in his presentation. Quite reasonably, Isaacson thought that Einstein was saying this:
The key insight was that two events that appear to be simultaneous to one observer will not appear to be simultaneous to another observer who is moving rapidly.
It's reasonable to think that Einstein was saying that—that he was offering that account of his key insight. That's what Einstein appears to be saying in his 1916 book.
There's only one problem with the explanation Isaacson (and Nova) derived from Einstein's presentation. On its face, their explanation of this key principle had never seemed to make sense.
On its face, Einstein's presentation for general readers didn't seem to make sense in 1916. It still didn't make make sense in 2007, or in 2015, but neither Isaacson nor Nova seemed to notice this fact.
By now, more than a hundred years have passed since Einstein published his 1916 book. On its face, the presentation he made in Chapter IX of his book still doesn't seem to make sense.
It's one of the most important moments in the history of physics—in the intellectual history of the twentieth century. But more than a hundred years later, on the highest levels, no one seems to have noticed that Einstein's account of this historic moment didn't quite seem to make sense.
Two events which appear to be simultaneous to one observer will not appear to be simultaneous to another observer who is moving rapidly?
In most cases, though not in all, that will surely be true. But that will also be true for most observers who aren't moving rapidly—indeed, for most observers who aren't moving at all!
Isaacson accepted Einstein's presentation at face value. Eight years later, Nova followed suit.
On its face, that presentation had never made sense! Tomorrow, we'll journey back in time to 1916 to see what Einstein wrote.
Tomorrow: On its face, this didn't make sense
Thursday or Friday: As seen on PBS