The Curvature of Spacetime: Newton, Einstein, and Gravitation

  • Harald Fritzsch
Columbia University Press: 2002. 341 pp. $29.95, £20
Credit: DAVID NEWTON

When Daniel Defoe's fictional castaway Robinson Crusoe finally collected his thoughts on 30 September 1659, on the lonely island where he would remain for 28 years, 2 months and 19 days before returning to England in 1687, the year Newton's Principia was published, his first action was to write down in the notebook he had providentially saved from the wreckage a debit–credit account of his state in two columns, Evil and Good. Physicists on their island, no less remote, despite being linked to the outer world by mobile phone and Internet, might well set down a similar debit–credit sheet. Knowing so much, they might start with just one column, headed Good, but eventually comes the need to list in an Evil column a few of the many things we don't understand.

Harald Fritzsch's The Curvature of Spacetime is a time-travel dialogue set in 1996 between three men: Isaac Newton, aged 45 after completing the Principia; Albert Einstein at 51, riding the triumph of relativity; and an imaginary modern expert, Adrian Haller. They meet in various German locations familiar to Einstein, and then at Caltech, where Einstein had spent time in the 1930s. As an expository device, the dialogue form is quite successful. It lets Haller teach Einstein and Newton (and us) the current status, experimental and theoretical, of particle physics, drawing the reader into exchanges of view and conflicting ideas more readily than conventional exposition would allow.

The format is problematic historically, however. We find that Newton apparently knows everything about physics up to about 1890, yet is fogged and slow on the uptake about Einstein's work. Newton had faults — Fritzsch deliberately passes over certain 'disagreeable' aspects of his character — but slowness of uptake was not one of them. And is it pedantry to object to Haller's “reminding” Einstein (in a book containing no mention of Ernest Rutherford) that in 1896 “Henri Becquerel found out that the atomic nucleus of uranium is unstable”? This must have been quite a feat before α-particles, β-particles and γ-rays were known, the electron was discovered, or Rutherford had proposed the nucleus.

Taken literally, Fritzsch's title is misleading. Much of the book is about particle physics, rather than gravity or spacetime. However, this expansion benefits the content; it reveals how far physics has come since Einstein and shows off Fritzsch's considerable gifts as an expositor.

Both Newton and Einstein puzzled over the meaning of mass. Particle physics has failed to illuminate them: the large unexplained range of masses of elementary particles from neutrinos to quarks has only complicated matters still further. Reading Fritzsch on these issues, including his account through Haller of how the Large Hadron Collider at CERN, the European Laboratory for Particle Physics, will (we hope) reveal the Higgs particle and explain mass, leads to the deeper question of what constitutes explanation in physics.

One of Newton's discoveries, often wrongly credited to Galileo, was that in physics, mass fulfils two functions. According to the law of acceleration, mass is the receptacle of inertia; the inverse-square law of gravity makes it the source of gravitation. The masses in these equations are said to be 'equivalent'. Equivalence is often said to be incomprehensible in Newton's physics but explained in Einstein's. Einstein's falling elevator made inertial and gravitational accelerations indistinguishable; this indistinguishability became a principle; curved spacetime followed. And so, Fritzsch has Einstein saying that gravitational and inertial accelerations are not only proportional to each other, but “completely identical”.

But surely something is wrong. Complete identity means, in ordinary language, no difference. If there is no difference between inertia and gravity, why does physics need a gravitational constant? Einstein is credited with removing unnecessary concepts — he eliminated the ether, and consolidated accelerations. His true gift, however, was the unexpected extension of concepts — mass turned into mass-energy, and equivalence became a principle that applies to other phenomena besides mass. His last statement on mass leaves Newton's mystery intact. Distinguishing 'inert mass' from 'heavy mass', Einstein wrote in Out of my Later Years (Philosophical Library, 1950): “That these two radically different definitions lead to the same value for the mass of a body is, in itself, an astonishing fact.”

Fritzsch's narrative centres on Einstein. Each chapter head has a quotation from Einstein, and Newton rather improbably seems in awe of him. To find Einstein's true greatness requires an act of distancing. Examining the 23 chapter quotations as an entity shows Einstein not to be necessarily all wise, but certainly to be a most unusual person. Consider this: “All I really want is to sit back and find out how God created this world. It is His thought that I am trying to understand — not the spectral lines of this or that element. I really could not care less about things like those.” One may disagree. Great discoveries can come from tiny discrepancies — witness the Lamb shift and quantum electrodynamics. But it is hard to think of anyone but Einstein who would have said that.