The Science Book

A PARADIGM SHIFT 217

principle behind the emission
is fairly easily described in
modern terms (energy supplied by
the radiation is absorbed by the
outermost electrons in the metal’s
surface atoms, allowing them to
break free). The puzzle was that the
same materials stubbornly refused
to emit electrons when illuminated
by longer wavelengths, no matter
how intense the light source.
This was a problem for the
classical understanding of light,
which assumed that intensity,
above all, governed the amount of
energy being delivered by a light
beam. Einstein’s paper, however,
seized on the idea of “quantized
light” recently developed by Max
Planck. Einstein showed that if the
beam of light is split into individual
“light quanta” (what we would
today call photons), then the energy
carried by each quantum depends
only on its wavelength—the shorter
the wavelength, the higher the
energy. If the photoelectric effect
relies on interaction between an
electron and a single photon, then it
does not matter how many photons

bombard the surface (that is, how

intense the light source is)—if none

of them carries sufficient energy,

the electrons will not break free.

Einstein’s idea was rejected by

leading figures of the day, including

Planck, but his theory was shown

to be correct by experiments

conducted by American physicist

Robert Millikan in 1919.

Special relativity

Einstein’s greatest legacy was born

in the third and fourth 1905 papers,

which also involved an important

reconceptualization of the true

nature of light. Since the late

19th century, physicists had

faced a crisis in their attempts to

understand the speed of light. Its

approximate value had been known

and calculated with increasing

accuracy since the 17th century,

while James Clerk Maxwell’s

equations had demonstrated

See also: Christiaan Huygens 50–51 ■ Isaac Newton 62–69 ■ James Clerk Maxwell 180–85 ■ Max Planck 202–05 ■
Erwin Schrödinger 226–33 ■ Edwin Hubble 236–41 ■ Georges Lemaître 242–45

The grand aim of all science

is to cover the greatest

number of empirical facts by

logical deduction from the

smallest number of

hypotheses or axioms.

Albert Einstein

that visible light was just one

manifestation of a wider spectrum

of electromagnetic waves, all of

which must move through the

universe at a single speed.

Since light was understood

to be a transverse wave, it was

assumed that it propagated

through a medium, just as water

waves travel on the surface of

a pond. The properties of this

hypothetical substance, known as

the “luminiferous ether,” would

give rise to the observed properties

of electromagnetic waves, and

since they could not alter from

place to place, they would provide

an absolute standard of rest.

One expected consequence of

the fixed ether was that the speed

of light from distant objects should

vary depending on the relative

motion of source and observer.

For example, the speed of light

from a distant star should vary ❯❯

Electrons are

ejected from the

surface of sodium

only by certain

wavelengths of

light. Einstein

showed that this

phenomenon can

be explained if light

travels as individual

quanta, or photons.

No matter how many

of them there are, if

the photons are of the

wrong wavelength,

they will not eject

electrons.

Sodium

Light photons

Electrons ejected

from the surface