No One Will Believe You 

How will you search for something you do not know at all? And if you should meet with it, how will you know that this is the thing that you did not know? 
Plato, Meno 

According to a wellknown story, Max Planck once tried to discourage Albert Einstein from working on a new theory of gravity, with the words 

As an older friend I must advise you against it, for in the first place you will not succeed, and even if you succeed, no one will believe you. 

The remark is amusing, but the attribution to Planck is indirect at best. Although one often finds those words presented as an actual quote, even with the claim that it was written by Planck to Einstein in 1913, it is not to be found in Einstein’s published correspondence. More careful authors say that Planck spoke the words when he and Nernst met with Einstein in Zurich in 1913. But how do we know this? 

Planck and Nernst did indeed travel to Zurich in the spring of 1913 to meet with Einstein, hoping to persuade him to accept a position in Berlin, but no contemporary record of Planck’s famous advice exists. In 1950, 37 years after the famous meeting took place in Zurich, a young mathematician named Ernst Straus became Einstein’s assistant at the Institute for Advanced Studies in Princeton. Almost 30 years later, in 1979, Straus wrote a letter to Abraham Pais, who was then working on his biography of Einstein. In this letter, Straus told Pais that Einstein had told him (Straus) about the advice that Plank had supposedly given Einstein in 1913. 

If Planck actually said those words, it was presumably in jest, or at least halfjest. It would be an odd thing for Planck, while attempting to recruit Einstein to work in Berlin, to seriously tell Einstein that he was wasting his time. Einstein had been publishing on the subject of a relativistic theory of gravity for some time, and this was known to be the focus of his research during these years. If Planck believed Einstein’s efforts were unwise or misguided, why would he be trying to get Einstein to come to Berlin so that he could continue that very work? In the letter written by Planck and Nernst at this time (12 June 1913), proposing Einstein for membership in the Prussian Academy, they devoted over a full page to Einstein’s work on special relativity and his contributions to the application of the quantum hypothesis to questions of specific heats and molecular motions, as well as his work in classical statistical mechanics. Planck then gently chided Einstein for “overshooting the target” in his speculation on the light quanta (referring to work for which Einstein was later awarded the Nobel prize), and then added the sentence 

At the moment he works intensively on a new theory of gravitation; with what success, only the future will tell. 

This appears to be the only documented statement that Planck made to express his view (at this time) of Einstein’s work on general relativity. It certainly isn’t an enthusiastic endorsement, but neither does it indicate that Planck was trying to discourage Einstein from pursuing this line of research. The letter of recommendation concludes by saying 

Even though we, naturally, cannot guarantee the future…[we are] convinced that the entire world of physics will consider Einstein’s joining the Berlin Academy of Sciences as an especially valuable gain for the Academy. 

Although Planck may not have been overly optimistic about Einstein’s work on the gravitation theory, he may never have actually uttered the famous “advice”. Perhaps Einstein was just making a little joke when he described the meeting to Straus. It’s also possible that Straus misremembered what Einstein told him. 

Incidentally, Straus was born in Germany in 1922 and his family immigrated to Palestine in 1933. Later he got his PhD at Columbia in New York, before becoming Einstein’s assistant. Straus worked in various areas of pure and applied mathematics, but is perhaps best known today for the ErdosStraus conjecture in number theory, according to which every fraction of the form 4/n can be expressed as a sum of just three (or fewer) unit fractions. 

Whether or not Planck actually uttered the famous advice, the quote is often useful when considering notoriously vexing subjects that have been studied, written about, and debated for many years. It is almost certainly a waste of time to grapple with such subjects, for the twofold reason given in that quote: The likelihood of discovering the truth is extremely low, and even if the truth were found, it would likely not be accepted. One example is the foundations of quantum mechanics, about which there have always been widely differing views, held with great certainty by their respective proponents. Even if someone actually discovered “the truth”, they surely would not be believed. However, quantum foundations are interpretational, so it’s not surprising that alternate views would persist indefinitely, lacking any empirical test. 

In the case of general relativity, Planck might have feared that no strong empirical test was possible, since it might differ from Newton’s theory (if at all) only by inappreciable amounts. In that case it is certainly plausible that many people would decline to adopt the new theory, no matter how logically compelling it was. Einstein had already in 1911 predicted the deflection of light in a gravitational field, but his prediction (at that time) was identical to what Newton’s theory would predict for ballistic particles of light, as had already been computed by Soldner and Cavendish. Of course, Newton’s theory of gravity was not relativistic, so it was subject to falsification (at least in principle) in other ways. The only theoretically viable competitor to general relativity was the scalar theory of Nordstrom, which predicted no light deflection at all. 

Planck succeeded in luring Einstein to Berlin, and the theory of general relativity was completed at the end of 1915, by which time Einstein had identified the three famous empirical tests that would discriminate his theory from its competitors: light deflection (at twice the Newtonian value), precession of orbits, and (most fundamentally of all) gravitational redshift. The precession of Mercury’s orbit was in good agreement with his theory, and the 1919 eclipse expedition results were interpreted as providing support for his theory as well. (The observations certainly ruled out Nordstrom’s theory, because there was clearly significant deflection.) Observations of gravitational redshift proved to be extremely difficult and troublesome, so definitive results were not available for decades, but the precession of Mercury and the eclipse results, combined with the theoretical strength of Einstein’s spacetime curvature model, were enough to persuade most people within just a few years of the completion of the theory. So, in this instance, Planck’s alleged predictions (“you won’t succeed” and “no one will believe it”) both turned out to be spectacularly wrong. 

In his acceptance letter to the Prussian Academy in December 1913, Einstein may have expressed some internal uncertainty and insecurity about whether his work on the gravitation theory would turn out to be worthwhile: 

When I reflect upon the fact that each working day demonstrates to me the weakness of my thinking, then I can only accept the high distinction intended for me with a certain trepidation. But what encouraged me to accept the election was the thought that all that can be expected of a person is that he devote himself with all his might to a good cause; and I do feel capable of that. 

At the same time (according to Kollros), he privately told a friend “the Berliners are betting on me as on a prime laying hen, but I am not certain that I can still lay eggs!”. 
