Einstein determined light is by definition always moving

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Einstein determined light is by definition always moving

Published on Saturday August 11, 2012

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View of the earth's horizon as the sun sets over the Pacific Ocean as seen from the International Space Station.


By
Oakland Ross

Feature Writer


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Part 1: Century of breakthroughs

Part 2: Why light has just one speed

Part 3: E=MC²: A vicious cycle of energy, mass and speed

Part 4: Einstein discovered that gravity is not a force but a curvature

Part 5: Schrodinger’s cat and the mysteries of quantum mechanics

Part 6: The universe in a nutshell

By the time he was 16 years old, Albert Einstein was well on the way to reordering the cosmos.

He just didn’t know i
t yet.

It was not until 10 years later, at age 26, that he published what came to be known as the special theory of relativity, a wo
rk
that stood the universe on its head, revealing a domain of breakneck speeds, where almost nothing remains the same for
lo
ng and where truth itself is a relative notion.

No longer could measuring sticks or clocks be relied upon as absolute calculators of length or duration. In the new and
changeable world, it turns out that both rulers and the space they measure can extend or

contract according to
circumstance, while clocks and time can speed up or slow down, depending upon relative motion.

At the sluggish velocities we are accustomed to in our everyday lives, these variations are far too small for us to notice, b
ut
they do ex
ist.

“A frequent flier ages around one
-
thousandth of a second less than a counterpart on the ground after 40 years of weekly
Atlantic crossings,” writes Brian Clegg in
Light Years
, a book about relativity.

At velocities approaching the speed of light,
relativistic variations take over, and you simply cannot refute them, any more
than King Canute could turn back the tide.

At the speed of light, time and clocks seem to come to a complete halt, at least for certain observers, as do all other physi
cal
proce
sses. Meanwhile, measuring rods shrink to infinity, along with all other objects endowed with mass


again, for certain
observers.

This catch about “certain observers” is critical. If you were to move away from Prime Minister Stephen Harper at half the
spe
ed of light, the PM would perceive your watch to be moving more slowly than his, while you, looking back at his watch,
would see the opposite. His watch would seem to be moving more slowly than yours.

Meanwhile, each of you in your own time frame, would co
nsider nothing to have changed


and each of these contradictory
versions of reality would be accurate.

That’s relativity. As long as we are in motion with respect to each other, we each occupy a unique spatial and temporal
framework, a special niche in wh
at is now known as spacetime.

Or not “known” exactly, for the principles of relativity remain a mystery to most people.

Even today, more than a century after Einstein published his theory, its conclusions continue to baffle, mainly because they
are complic
ated but also because they so thoroughly contradict our daily experience of the world.

You can easily imagine a similar reaction greeting Nicolaus Copernicus after he concluded that our planet rotates around its
own axis and also revolves around the sun. A
fter all, our senses tell us that it’s the sun that is moving while we are standing
still.

But our senses are wrong. And it’s the same with relativity.

The central tenet of special relativity


the realization that drove its many stunning conclusions


act
ually occurred to
Einstein in his teens, and he wrestled with the idea for years.

Considered by his professors to be a backward and lazy student, Einstein was really a diehard individualist. He loathed the
rote, regimented approach to education favoured by

the schools where he studied. Besides, he had other matters to occupy
him.

He inhabited a laboratory of the mind and loved to conduct what he called
gedankenexperimente



thought experiments.

One of his favourite mental challenges involved transporting hi
mself aboard an imaginary ray of light so that he might
observe a second parallel beam travelling alongside him at an equivalent speed. He wondered what he would see. What
would a “stationary” beam of light look like?

Had Einstein chosen any other mode of
imaginary conveyance, this experiment would have involved no great difficulty.

It has been understood since the time of Galileo that motion is relative. Galileo himself proposed the example of a ship
loaded with various moving objects, both animate and ina
nimate. Their behaviour will be unaffected no matter whether the
ship is at rest relative to the shore or moving forward, just as long as the vessel’s motion is uniform.

In a similar fashion, two cars travelling side by side at the same speed along a strai
ght road will be at rest relative to each
other, even as they roar past a stationary observer.

The same goes for trains, planes, pedestrians, giraffes, hockey players and just about anything that has mass and the ability

to convey it through space


with o
ne exception. And that exception is light.

(Actually, a more accurate term than “light” would be “electromagnetic radiation,” which includes a range of wave
frequencies that the human eye perceives as light while also including other frequencies that are
invisible to humans, among
them ultraviolet and infrared waves.)

At least in theory, you could ride across an African plain clinging to the neck of a loping giraffe while leisurely observing

the
profile and stride of an adjacent giraffe, which would neithe
r gain on you nor fall behind. Relative to you, and to each other,
both giraffes are in a position of rest.

But this won’t work with a beam of light.

Even as a teenager, Einstein suspected this to be so, and his unease with conventional theories of light o
nly increased as
he began to appreciate the consequences of his intuition.

At the time


the closing years of the 19th century


scientists already understood a good deal about light. They knew that
it was extremely fast but also that its speed was finite.

They had even calculated light’s velocity with considerable accuracy.
Even so, they had no clear idea what it was.

In the 1820s, an English blacksmith’s son named Michael Faraday carried out some fascinating experiments with
magnetism and electricity that

started to put the scientific world on the right track in its pursuit of light.

Fifty years later, a remarkable Scottish scientist named James Clerk Maxwell went further. He discovered that a rapid,
shape
-
shifting exchange of electricity and magnetism yie
lded a self
-
propagating wave
-
like phenomenon that shimmered
through space with an almost effortless grace, but with one important constraint. The process operated at just one speed


a rate just shy of 300,000,000 metres per second.

As it happens, this val
ue precisely matched the speed of light.

At first, Maxwell marvelled at the seeming coincidence, but he later concluded it was no coincidence at all. This elegant
electromagnetic pas de deux did not merely resemble light: it
was

light.

“According to Maxwe
ll’s picture, light is a balancing act, a constant, self
-
creating marvel,” writes Clegg in his book on the
subject. “Electricity, when moving along, generates magnetism. Magnetism in motion generates electricity . . . Light was the
result of the interplay
of the two at just the right speed.”

The speed of light.

But there were problems, or there seemed to be.

In the Maxwellian view, light is a wave


a wave that oscillates rapidly between electricity and magnetism, but a wave
nonetheless. And a wave, it was
thought, requires a medium.

After all, there can be no surfing without water, and there can be no sound without air. By the same reasoning, it seemed
there could be no light without something for light to travel through.

But what?

To solve this conundrum,
scientists posited the existence of a pervasive substance that supposedly occupied the entire
cosmos. They called it the luminiferous ether


or just the ether, for short.

This putative element was deemed to perform two functions. It provided a medium thro
ugh which light waves could express
themselves on their journeys across the vastness of space, and it provided a fixed, omnipresent grid stretching through the
expanse of the universe


a system of consistent reference points against which light’s progress

could be measured.

Despite heroic efforts and ingenious experiments, however, no one could prove that the ether actually existed. In fact, all t
he
hard, direct evidence seemed to indicate that it did not. Yet the scientific establishment clung to the conc
ept, for its members
could not imagine how light could function without it.

But Einstein decided that the question was moot. Ether or no ether, light would behave the same way.

He continued to ponder his thought experiment, the one in which he imagined him
self to be riding a ray of light through the
heavens in tandem with a parallel beam. He wondered what that second shaft of light would look like when viewed from a
vantage point of relative rest.

Einstein’s eventual conclusion was as daring as it was simpl
e. Light, he decided, is much like the wicked. It knows no rest.

To use a contemporary analogy, light is a bit like the famously frenetic comedian Robin Williams, who is never
not

delivering
shtick. If he weren’t performing, he wouldn’t really
be
.

Much lik
e Williams, light exists only in the doing. Either it travels at 300,000,000 metres per second, constantly oscillating
between electricity and magnetism, or it is nothing. After all, light particles, or photons, are really pure energy. They pos
sess
no rest

mass. At rest, they would not exist.

“Can there be a ‘ray of light at rest’?” Einstein would later write. “That seems impossible.”

To a hypothetical observer travelling at the speed of light, a second beam would not stop oscillating, any more than Robin
W
illiams would cease making jokes in the presence of another human.

A second beam of light would behave exactly as though the observer were motionless.

In order for this manoeuvre to work, however, it is necessary that time stand still, at least for certain

observers, and this
condition represents a rude assault on the human senses, ruder even than the news


now universally accepted


that the
Earth circumnavigates the sun.

For a long while, even Einstein balked at the idea.

On a spring evening in 1905, he debated these questions with a Bern friend named Michele Besso. They spent hours in
concentrated discussion, a conversation that went long into the night. Something about that discussion nudged Einstein
forward.

“Next day,”
the great scientist would write later, “I came back to (Michele) again and said to him, without even saying hello,
‘Thank you. I’ve completely solved the problem.’ An analysis of time was the solution. Time cannot be absolutely defined . .
.


Instead, bot
h time and space are adjustable, slowing or contracting for certain observers in order to accommodate the
constant speed of light.

A short while later, Einstein submitted his paper on special relativity for publication, and the universe would never seem th
e
same.

And then he had another, even bolder idea.