Albert Einstein’s most happy thought

Albert Einstein’s most happy thought

SDSS J1038+4849, a gravitational lens imaged in 2015 • NASA/ESA (Public Domain)

Originally published 30 November 1987

It was, said Ein­stein, the “hap­pi­est thought of my life.”

The young physi­cist was sit­ting at his desk in the patent office at Bern when this thought occurred to him: If a per­son fell freely from the roof of his house, he would not feel his own weight. While he was falling it would be — at least in his imme­di­ate sur­round­ings — as if there were no gravity.

For exam­ple, if our plum­met­ing per­son released an apple from his hand, the apple would not fall; or rather, since apple and human are falling togeth­er, the apple would stay near the hand. If our observ­er could see no part of his envi­ron­ment except the apple, he might rea­son­ably con­clude that he is at rest in a place where there is no grav­i­ty — that is, where apples don’t fall.

Of course, his inevitable col­li­sion with the ground would dis­abuse him of his fal­la­cy. And the vio­lence of the impact might sug­gest that Ein­stein’s hap­pi­est thought was not so pleas­ant after all.

But the thought was indeed a hap­py one, for it led Ein­stein to a new the­o­ry of grav­i­ta­tion, called gen­er­al rel­a­tiv­i­ty. In the the­o­ry, space and time are bound up togeth­er in a con­tin­u­ous fab­ric of “space­time.” The pres­ence of mat­ter imparts a local cur­va­ture to space­time, in much the same way that a man stand­ing on a tram­po­line caus­es the orig­i­nal­ly flat fab­ric to sag. It is the cur­va­ture of space­time that gives rise to the effect we call gravity.

A beautiful theory

So beau­ti­ful is the the­o­ry that physi­cists have gen­er­al­ly believed it to be true, even though it is dif­fi­cult to test. In ordi­nary expe­ri­ence, dif­fer­ences between the pre­dic­tions of Ein­stein­ian rel­a­tiv­i­ty and New­ton’s the­o­ry of grav­i­ty are immea­sur­ably small. But on the scale of the uni­verse the dif­fer­ences can be con­sid­er­able. Many of the grand­est con­cepts of con­tem­po­rary sci­ence — the Big Bang, black holes, the fate of the uni­verse — are based upon gen­er­al relativity.

One of the more intrigu­ing pre­dic­tions of gen­er­al rel­a­tiv­i­ty is the grav­i­ta­tion­al lens.

In Ein­stein’s the­o­ry, light rays fol­low curved paths in curved space­time. Very mas­sive objects, such as galax­ies or clus­ters of galax­ies, can sig­nif­i­cant­ly alter the cur­va­ture of space­time in their neigh­bor­hoods. These regions of “warped” space­time can act like lens­es, bend­ing light rays and form­ing curi­ous­ly dis­tort­ed images of dis­tant sources of light.

In recent months, astronomers have made a num­ber of obser­va­tions that seem to con­firm the idea of a grav­i­ta­tion­al lens. In sev­er­al cas­es, they have observed close pairs of vir­tu­al­ly iden­ti­cal quasars. (Quasars are very bright sources of light far out in space, pos­si­bly cor­re­spond­ing to the bright nuclei of young galax­ies.) It is wide­ly assumed that the paired quasars are actu­al­ly mul­ti­ple images of the same dis­tant object, formed by the lens­ing effect of a galaxy or galax­ies lying between us and the quasar.

Even more excit­ing is the dis­cov­ery of “lumi­nous arcs” in space.

In Jan­u­ary of this year, astronomers Roger Lyn­ds and Vahe Pet­rosian serendip­i­tous­ly dis­cov­ered two per­fect­ly geo­met­ri­cal arcs of light, each about a quar­ter of a cir­cle, that seemed to lie near clus­ters of galax­ies bil­lions of light-years from Earth. Noth­ing like them had been observed before. At first, they seemed to defy con­ven­tion­al explanation.

Now, the two astronomers have become con­vinced that the arcs are the effects of grav­i­ta­tion­al lens­es. In each case, the grav­i­ta­tion­al influ­ence of a clus­ter of galax­ies is bend­ing the light from an even more dis­tant galaxy that lies behind the clus­ter, smear­ing the light into a cir­cu­lar arc, in the same way that a glass lens (say, a clear glass decanter of water) can dis­tort the image of a can­dle flame. If this expla­na­tion of the arcs holds up, it will be a stun­ning cor­rob­o­ra­tion of Ein­stein’s theory.

Einstein’s ring

A more spec­tac­u­lar con­fir­ma­tion of gen­er­al rel­a­tiv­i­ty may be forth­com­ing. If a dis­tant light source (a galaxy, for exam­ple) were lined up pre­cise­ly with an inter­ven­ing mass of appro­pri­ate sym­me­try (a galaxy or clus­ter of galax­ies), the light of the dis­tant source would be dis­tort­ed into a com­plete cir­cle. What one might then see in space is a galaxy or clus­ter of galax­ies enclosed with­in a ring of light!

It would be excit­ing to dis­cov­er such a ring, if for no oth­er rea­son than the breath­tak­ing beau­ty of the effect. What is at stake is not so much the con­fir­ma­tion of a par­tic­u­lar the­o­ry of physics, as con­fir­ma­tion of the abil­i­ty of the human mind to under­stand the order­ing prin­ci­ples of the universe.

Ein­stein once said that the most incom­pre­hen­si­ble thing about the uni­verse is the fact that it is com­pre­hen­si­ble. There is sure­ly no deep­er mys­tery in nature than the con­gru­ence of the mind and the world, a con­gru­ence so remark­able that a patent clerk/physicist sit­ting at his desk can be led by an appar­ent­ly friv­o­lous thought about a man falling from his roof to a com­pre­hen­sive the­o­ry of space, time, and gravity.

The the­o­ry of gen­er­al rel­a­tiv­i­ty, at the moment of its incep­tion, had no obser­va­tion­al foun­da­tion. What com­pelled the atten­tion of physi­cists was the beau­ty of the the­o­ry, its inter­nal con­sis­ten­cy, and the grand­ness of its vision. It seemed almost incon­ceiv­able that so mar­velous an inven­tion of the human mind might not be in har­mo­ny with the world.

It is hard to imag­ine a more dra­mat­ic demon­stra­tion of the mys­te­ri­ous con­ver­gence of the­o­ry and nature — and of the per­spicu­ity of Ein­stein’s hap­pi­est thought — than the dis­cov­ery in space of that most improb­a­ble sight, a galaxy wreathed in a cir­cle of light.


With the improve­ment in astro­nom­i­cal imag­ing in the decades since this essay was pub­lished, astronomers have dis­cov­ered many exam­ples of grav­i­ta­tion­al lens­ing, pro­vid­ing stun­ning con­fir­ma­tion of Ein­stein’s ideas on grav­i­ty and space­time. ‑Ed.

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