光實際上是如何工作的？（How Does Light Actually Work?）

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The newly born universe buzzed and frothed with boundless energy.
Even after the raging furnace of the first few minutes had died away,
Temperatures universe-wide were more than a hundred million degrees.
For thousands of years, this primal heat burned - a cosmos of plasma, a super-hot mix of particles
and radiation.
Until one day it changed forever.
That day arrived when the universe was almost four hundred thousand years old, and had cooled
to about three thousand kelvin.
In this now comparatively tepid soup lone electrons met lone protons, and finally could
stick together - forming the first atoms.
But this was not all.
For as each electron and proton bound together, a small amount of energy was released.
A packet of energy that raced away at the cosmic speed limit, the speed of light.
A particle of energy born in the formation of a hydrogen atom.
A particle we call a photon, a particle of light itself.
This photon was far from the first.
But as the universe began transitioning from plasma into neutral gas, light could then,
for the first time, stream freely through its reaches.
And so, our photon's long, long journey began.
It headed out first into the universe’s dark ages
A time before the first stars burned, a time before the first galaxies formed.
In the eerie darkness, gravity pulled on mass to mold the first seeds of cosmic structure.
But the photon sped on, and noticed nothing.
Eventually, the first stars burst into life around it,
Massive and bloated, these ancient suns burned themselves out in a cosmic blink of an eye
as the first supergiant black holes grew rapidly between them as they eagerly devoured mass.
But the photon sped on, and noticed nothing.
The first galaxies began to assemble The sky lit up with the fires of uncountable
young stars across the cosmos as they began to fuse the initial hydrogen and helium atoms
into heavier elements.
But the photon sped on, and noticed nothing.
Millions steadily turned into billions of years,
And as galaxies grew and matured, eventually, the intense light of young stars began to
settle.
The photon’s journey could have potentially lasted forever into eternity
But after almost 14 billion light years of travel, a large spiral galaxy steadily came
into view Its destiny was set.
Near a small blue dot orbiting a small white star.
After crossing the last few thousand years, the photon collided with a piece of metal
Part of a telescope built by humans and orbiting near the planet Earth
The photon’s energy was completely absorbed, energising electrons, and registering on detectors.
But as the photon vanished from existence, its billions-year long journey complete - it
simply did not notice.
Because to the photon itself, the journey never took place.
13.8 billion years of cosmic time disappeared in an instant.
Yet how can this be?
Light has existed in the universe from its earliest moments,
And will continue to exist long after humanity and the stars are shred to dust.
But just how does it work?
And how can it seemingly last forever?
And perhaps most importantly - what even is it?
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Light had played a pivotal role since the cosmos´ very beginning.
In these earlier times, it had only existed for the very briefest moments,
Slamming into speeding particles before it had a chance to travel anywhere, one piece
of light dying as another was born However our parcel of light was born into
a very different, very transparent, universe.
Forged with the first true atoms from featureless plasma.
With the cosmic maelstrom of the Big Bang finally abated,
Our photon could begin its immense journey unhindered.
As it travelled, many generations of stars lead to our Sun, forming in the collapse of
an immense gas cloud, and billions of years after, humans began to walk on the surface
of our small rocky planet.
And so eventually, as our photon was a couple of thousand light years distant from Earth
- these humans began to wonder.
“All men by nature desire to know.
An indication of this is the delight we take in our senses…(and) above all others the
sense of sight…The reason is that this, most of all the senses, makes us know and
brings to light many differences between things.”
The ancient Greeks wondered if light emanated from the eyes, touching and feeling the world
around us.
But clearly, there are times when it is dark when we can’t see anything.
So, they concluded light must be something external, something captured by our eyes.
Islamic scientists began to unravel the properties of light,
Finding rules of reflection and the magnification properties of glass lenses.
Light was clearly a natural part of the universe around us.
But it took the coming of the scientific revolution, and a fight between two giants of science,
for light’s deep secrets to be finally uncovered.
The year was 1652, and Dutch physicist, astronomer, mathematician and all round genius Christiaan
Huygens was exploring optical phenomena.
He had noted how light travelled through lenses and bounced off mirrored surfaces, and was
particularly interested in the phenomenon of refraction - where the path of light is
bent when it passes from one medium to another.
Huygens noticed that light was often split into the colours of the rainbow by his instruments,
And sometimes strange patterns of dark and light would be produced.
Clear evidence, to him, that light was some sort of wave.
Some sort of travelling, oscillating phenomenon.
Oscillations can be found throughout nature, from planetary orbits to vibrating electrons,
But let’s start with a simple picture - think of a child on a swing.
As they swing, their position oscillates from one position to the next, then back again,
Just like a pendulum that drives the regular ticking and tocking of a grandfather clock.
When oscillations act in unison, but slightly out of step, waves are formed.
A stone thrown into a flat pond pulls the water down with it, but the water bounces
back.
This splash of water pulls on its neighbours, inducing them to oscillate, which, in turn,
pulls on their own neighbours.
These oscillations fan out across the pond as a steadily rippling pattern of waves.
Waves are everywhere, from sound waves coursing in the air, to ocean water waves driven by
the wind and moon.
Seismic shifts can generate violent and destructive earthquakes in our planetary crust,
Whilst similar waves ripple in the atmosphere of the Sun and other stars.
And so - light seemed to be a wave.
But this left a question.
If light is a wave, just what was doing the waving?
In Britain, Robert Hooke also reached a similar conclusion about the nature of light.
Hooke had observed light travelling through glass, and he realised that this picture of
wavey light could explain a lot of the phenomenon he had seen.
This was cutting edge science at the time.
But Hooke had a problem.
And that problem was a man.
There are no surviving portraits of Robert Hooke, and over the years a rumour passed
down the generations that this powerful man was to blame - having conveniently lost the
painting when taking over as head of the Royal Society in London.
For Robert Hooke had a powerful enemy, and that enemy’s name was Isaac Newton.
Since cleared of any wrongdoing in the absence of contemporary images of Hooke, there is
still little question that the two men were not friends.
For as well as his far reaching discoveries in mathematics and gravity,
Newton also had an interest in optics and the nature of light itself,
And he did not like what Hooke or Huygens had to say.
It was Newton who discovered that white light could be split into a rainbow by passing it
through a prism, And like Hooke he had kept musing on this.
But unlike Hooke, Newton did not conclude that light was some sort of wave.
To Newton, light consisted of “corpuscles”.
To Newton, light was made of tiny individual particles.
Newton’s focus was the phenomenon of diffraction, the fact that waves bend around a sharp edge.
He knew that sound, a wave in the air, bent as it travelled past sharp edges.
It was clear that you could eavesdrop on a conversation from around a corner without
being able to see the gossipers.
You could hear from behind an object but you could not see.
So, he reasoned, light simply could not be a wave.
And he didn't stop there.
Newton went further – much, much further.
He reasoned that light, as a stream of particles, would even feel the pull of gravity
In his book Opticks, published in 1704, he wrote
“Do not Bodies act upon Light at a distance, and by their action bend its Rays?”
And though on this he was right - he was not proved right for centuries.
So it was Newton's corpuscular theory of light that reigned supreme,
Due more to his weight of personality and scientific standing, as opposed to its ability
to explain the complex observations of light.
Over the years however, steadily, the tide began to turn away from Newton.
In 1800, polymath Thomas Young shone light through a pair of narrow slits,
And observed a pattern of interference on a background screen.
This wasn’t the first demonstration of interference, but it was the clearest.
How could Newton’s picture of light as particles explain Young’s observation of interference?
How could light, as tiny bullets passing through either one slit or the other, produce the
observed pattern?
Simply throwing a couple of pebbles into a still pond reveals that interference is naturally
produced by waves, either in water or in light.
Other observations of light supported its wave-like nature, including light´s polarisation
through a material called calcite.
But for centuries a big secondary question remained unanswered.
If light was a wave, what was doing the waving?
Our cosmic parcel of light was born when a proton captured an electron.
It sped out into the universe, powerful and energetic.
But as it travelled, and the universe expanded, it started to lose some of that energy.
The light, originally blue to our eyes, steadily morphed through the colours of the rainbow,
and into the red.
Soon it was joined by other energetic light, shining from countless billions of newly formed
stars.
There were different types of light too, light of exceptionally high energy - and light with
barely any energy at all.
The universe was awash.
Our light did not know that these would be invisible to human eyes,
For it would be many billions of years until eyes existed,
And indeed - these X-rays and radio waves, as we call them,
were unknown to us until the very end of the nineteenth century.
“In this new era, thought itself will be transmitted by radio.”
Guglielmo Marconi was at his father’s estate near Bologna in Italy.
He was still a young man, aged only 20, but his education had opened his eyes to an invisible
world.
In the decades before, the nature of light had steadily been unravelled
And Marconi was ready to use this new-found knowledge to change everything.
Staring at his equipment, Marconi was waiting to see a faint spark in the darkness.
With its bundle of wires and coils of his workshop, such a spark would not be surprising,
But the impetus for this spark was not in the equipment before him,
It was in similar equipment located several miles away.
Of course, the nineteenth century had seen the arrival of the telegraph,
Where electronic pulses are sent along wires that cross entire countries and continents.
But this needed wires to be strung through the air and under the oceans.
Marconi had no need for such wires connecting his equipment.
He would be sending messages not along bits of copper.
His messages would simply fly, completely unseen, through the air.
But how?
The answer lies with one of science's greatest geniuses.
The answer lies with James Clerk Maxwell.
When a young James Clerk Maxwell arrived at the University of Cambridge in 1850,
He was told that attendance at the 6 am church service was compulsory for all students.
The Scottish-born prodigy had long been a night owl and simply responded
“Aye, I suppose I could stay up that late”.
His name is writ large across the modern world.
But his crowning achievement was uniting two seemingly disparate phenomena and creating
something remarkable.
Electricity and magnetism had been known about since ancient times,
Seen in the strange attraction of rubbed materials, and mysterious stones that knew how to find
north.
But by the nineteenth century, it was becoming clear that these two are not truly distinct,
As experiments had revealed they were in fact implicitly entwined - a flow of an electric
current could generate a magnetic field, and a changing magnetic field could generate a
current in a wire.
But as Maxwell stared at these equations, he began to see a deeper picture.
Instead of separate relationships, he saw that electricity and magnetism could be united
into a single whole.
A united set of mathematics that encompassed all electric and all magnetic phenomena.
Maxwell’s vision would eventually be stripped back to four unique equations,
And the modern topic of electromagnetism was born.
But Maxwell’s great insight was not only concerned with electromagnetic complexity
- For he wondered about the simplest situation
of all - electromagnetism in the nothingness of a vacuum.
How does light travel through the emptiness of space?
He knew that electromagnetic fields filled all space, even in vacuums, but it was imagined
that in empty space these fields would be null - effectively not there.
But what if you plucked one of these fields, either electric or magnetic,
So that these fields were not zero at some location?
Maxwell pondered this question, using his equations to explore how the situation would
evolve And the answer was astounding.
Thinking about pinching the skin on the back of your hand.
What happens when you let go of your pinch?
Your skin sinks back to its unpinched self, Quickly if you are young, and somewhat slower
if you are older.
Maxwell’s equations told him that the electromagnetic pinch would evolve away back to zero,
But that was not the end of the story.
Pinching the electric field would generate a similar pinch in the magnetic field.
And the pinch in the magnetic field would generate a pinch in the electric field.
But that was not the end of it.
For the pinches in electricity and magnetism did not simply fade back to zero,
Instead, they oscillated, regenerating each other from one moment to the next.
And just like ripples on a pond, these oscillations travelled away as waves.
Maxwell realised these oscillations had the property of light.
Light, he realised, is a self-propagating electromagnetic wave.
But what caused the ripples?
What was the electromagnetic equivalent of the stone thrown in the pond?
He realised it was electric charges, something we now know as electrons.
As these charges jiggled and oscillate, they disturbed nearby electric and magnetic fields,
And these disturbances ripple away as electromagnetic radiations, what we call light.
He also realised the inverse must be true, as light entered the eye and fell on the retina.
The oscillations of the light must cause electrons in atoms in the eye to jiggle,
And it is this jiggling of electrons in the eye, sent as signals to be brain,
That we perceive as vision.
Finally Maxwell understood what it was that was waving, and what caused the waves - and
one more thing.
He knew that light had waves with a length of about a millionth of a metre - but his
equations showed no limitation on the wavelength of his electromagnetic waves.
And so he concluded that there must be light, with both long and short wavelengths, that
is invisible to the eye.
It would take two more decades for the answer to this puzzle to present itself, decades
in which Maxwell died of cancer at the age of only 48.
In 1886, Henrich Hertz, working at the University of Karlsruhe, was the first to find these
invisible waves.
Named Hertzian waves after their discovery, a new revolution had been born.
We now refer to these Hertzian waves as radio.
Hertz was very pleased with his discovery, but when asked about what practical use these
radio waves have, Hertz apparently responded, “Nothing, I
guess”.
It was these radio waves that only a few years later Marconi was using to send messages across
miles.
And then across counties, oceans, and all across the immensity of the globe.
In 1909, Marconi received the Nobel prize for his work on wireless telegraphy.
Hertz, however, died in 1894 at the youthful age of 36, never seeing the true promise of
his discovery.
The world was set to become full of invisible light as the twentieth century began, and
the mystery of light seemed settled.
That was, until 1905, and a remarkable year for one German patent clerk.
The universe continued to change and evolve as our parcel of light travelled.
The mixture of light joining it on its journey reflected that change, bursts of radio waves
and high-energy gamma rays becoming more and more frequent.
This energy surged through space, much of it flowing between the stars and into the
darkness, But some encountered lone atoms in the emptiness
of the void.
The low energy radio waves very gently shook and energised these atoms,
Like a calm ocean wave lapping at a sandy shore.
Just as we would expect from Maxwell’s picture of electromagnetic waves.
But the behaviour of the high-energy gamma rays was different.
They delivered their energy to the atoms with a violent punch that ripped electrons clean
away - not a lapping ripple, but an isolated smash.
The gamma rays hit the atoms not like waves, but like hard, little, energetic particles.
But how?
Could Maxwell be wrong?
Could, on some occasions, light be more like Newton’s vision and act like a particle?
And if so - just what would those occasions be?
“...for the present we have to work on both theories.
On Mondays, Wednesdays, and Fridays we use the wave theory; on Tuesdays, Thursdays, and
Saturdays we think in streams of flying energy quanta or corpuscles."
Alfred Nobel had made his fortune through his inventions and his businesses, especially
in the field of explosives and weapons.
And so, perhaps not unfairly, in an 1888 obituary in a French newspaper, he was called the “Merchant
of Death”.
This surprised Nobel, firstly as he was still very much alive,
But secondly, and more distressingly, because he realised what his historical legacy was
to be.
So, in his will, he decided to leave most of his fortune to a series of prizes,
Prizes that would honour those that have conferred the greatest benefit to humankind.
Across science, the Nobel Prizes are perhaps the most prestigious, the list of winners
replete with the giants of science over more than a century.
And in 1901, the inaugural Nobel Prize in Physics was awarded to German Wilhelm Rontgen,
for his discoveries about the nature of light.
For it was he who discovered the X ray.
Rontgen’s experiments with various materials found that only the densest could halt X-rays.
He even managed to convince his wife, Bertha, to place her hand into the beam,
after realizing his X-rays should stream through her flesh, but be partly blocked by her denser
bones - thus producing the first X-ray photograph.
Whilst it was suspected that X-rays were electromagnetic radiation with a wavelength much smaller than
visible light, it took several decades to conclusively show that this is the case - though
in the meantime, the medical application of X-rays to fix bones and save lives grew without
bounds.
And so this meant that by the early 1900s, Maxwell’s vision of electromagnetic waves
beyond the visible had been undoubtedly confirmed, all of light´s secrets uncovered - even the
electron having been discovered.
All that remained was to find the rest of the light we could not see, gamma rays, microwaves
and more - to fill out the last gaps on the electromagnetic spectrum and wrap up the story.
But, of course, if physics feels that its job is done, a rude shock is assuredly just
around the corner.
In Maxwell’s picture of light, it could be thought of as a continuous wave.
Scientists had found that when light crashed into most materials, it continuously dumped
energy that energized electrons, causing them to be emitted.
This was called the photoelectric effect.
By lowering the intensity of light, it took longer for energy to be deposited,
And it usually took longer for the electrons to begin to be spat out.
Usually.
For that was not what was observed when light was shone on certain metals – electrons
would seemingly be ejected instantaneously from the metal surface.
And the really confusing observation came from adjusting the colour of the light being
shone.
Blue light would result in very energetic electrons being emitted,
Green light resulted in less energetic electrons And red light produced no electrons at all.
This made no sense.
If all colours of light carry energy, why did red light fail to energize the electrons?
This mystery was solved, and new mysteries were born, in a very special year.
This was no ordinary year - it was a miraculous year.
For it was the year that Albert Einstein changed physics forever.
Most people are familiar with Einstein’s annus mirabilis.
1905.
The year he wrote down the special theory of relativity.
But that was just the beginning.
Einstein was awarded the Nobel Prize in Physics in 1921.
The citation noted the award was for his “services to theoretical physics”
But one topic, in particular, was singled out for recognition,
And it was not his work on relativity.
It was: “especially for his discovery of the law
of the photoelectric effect" When Einstein explored the photoelectric effect,
he had to radically revise Maxwell’s vision of light.
He realised that when light interacted with electrons, it could not do so as a continuous
wave of energy.
Instead, the energy must be concentrated and dumped into an electron as an instantaneous
packet.
Light, Einstein declared, must be quantized.
It must be chunks of energy.
It must interact like a particle.
Einstein went on to explain that each packet of energy was proportional to the frequency
of the light.
A packet of red light carries less energy than a packet of green light,
A packet of green light carries less energy than a packet of blue,
And so in experiments, the red light simply didn’t deliver enough energy for an electron
to escape.
This enigmatic packet of energy didn’t get its current name until 1926, when in an article
in the journal Nature, Gilbert Lewis coined the name photon.
Evidence for the particle nature of light swiftly grew,
And it was in 1923 Arthur Compton put together an important experiment - but one that relied
on a bizarre fact.
Light can push.
This might seem like a strange thing to say.
How can light, which has no mass, push?
But Maxwell’s equations showed that, in carrying energy, light also carries momentum.
You can easily buy a Crookes radiometer today as an executive toy for your desk,
Consisting of four vanes in an evacuated glass tube, one side black and the other side white.
When placed in bright sunlight, the vanes begin to spin,
Pushed, supposedly, by the momentum of nothing more than sunlight.
As ever, the physics of the Crookes radiometer is more complex than this simple explanation,
But the force of sunlight pushing on the vanes is real,
With visionaries imagining future humanity coursing amongst the planets solar sailing
on sunlight.
Compton´s experiment however was a little different - and groundbreaking.
In his experiment, Compton aimed a beam of high-energy X-rays at an atomic target, ripping
electrons from the outer parts of the atoms.
But when examining the rebounding X-rays and recoiling electrons,
Compton found that Maxwell’s picture of a wave of energy and momentum simply did not
work.
Instead, Compton had to treat the X-rays and electrons like colliding billiard balls.
For when an X-ray hits an electron, it delivers its energy and its momentum as a discrete
packet.
When an X-ray hits an electron, they definitely interact like hard particles.
Newton’s vision of particles of light was reborn!
Was this definitive proof that light was a particle?
Not quite.
There was still a mountain of evidence for its wave-like nature.
If anything, scientists were more confused than ever before.
Despite Maxwell’s picture of electromagnetic waves proving extremely powerful and successful,
These experiments in the early nineteen hundreds demanded that light must be a particle, not
a wave.
Was there even an answer to be found?
We began this story following a photon of light as it travelled across the universe.
Maxwell tells us that this photon was formed by the changing energy of an electron,
And that it vanishes when finally absorbed by electrons at its journey’s end.
But what happens in between?
Is this epic journey simply governed by fate?
Does the photon fly off in a random direction, careening randomly into an electron at some
point in its future?
This question is the next part of our story.
For in the early twentieth century, it was realised that this simply could not be the
case.
In the language of quantum mechanics, the photon’s journey has nothing to do with
chance.
“Not only is the Universe stranger than we think, it is stranger than we can think.”
We begin on a chilly morning in France in January 1793.
With the swish of a guillotine blade, the king, Louis the sixteenth, was no more.
Throughout this revolution, chaos reigned across France.
In the thick of the chaos, Victor-François, the 2nd duc de Broglie, battled for his king,
but eventually, like many others of the aristocracy, fled France for safety abroad.
The de Broglie’s eventually returned to their native France to shape the country after
the upheaval of the revolution, And after a series of statesmen, diplomats,
and writers, In 1892, Into the de Broglie family was born
the man who would change our understanding of everything.
His name was Louis Victor Pierre Raymond, 7th Duc de Broglie.
But in the annals of physics history, he is simply known as de Broglie.
In the early twentieth century, he bore witness to the birth of quantum mechanics,
And the growing confusion about whether light was a particle or a wave.
To de Broglie however, there was an obvious solution, though a counterintuitive one.
Light was neither and both at the same time.
It was clear that light, when it travelled, travelled as a wave, producing the effects
of interference and diffraction.
But when it interacted, it interacted like a particle.
It seemed to exhibit properties of being both a particle and a wave but was never really
either.
de Broglie’s remarkable insight was to realise that this was true, not only for light,
But for the entire quantum world.
Here, he claimed, there are no true particles and no true waves,
Everything, de Broglie told us, was some sort of quantum thing.
And so In his PhD in 1924, he claimed that electrons, which were clearly particles, should
ALSO exhibit wave-like properties, And in 1929, he received the Nobel Prize when
experiments bore out his predictions.
There has been significant philosophical discussion about this wave-particle duality in quantum
mechanics, But its observational consequences are incontrovertible.
A series of single photons or electrons sent through multiple slits still result in interference
patterns, And even large complex molecules have also
been shown to be both waves and particles - the largest yet tested being 2000 atoms
in size.
And so with quantum mechanics in hand, the quest was on to understand just how light
and electrons interacted.
With classical physics, the physics of Maxwell, electrons jiggled as electromagnetic waves
passed by.
Just like seagulls bobbing on a choppy ocean.
And by their jiggling, the electrons emitted their own electromagnetic waves, adding to
the mix.
But the quantum picture had to be different For the quantum world was one of quanta, and
particle reactions.
It didn´t take long for a solution to be found - and it was another scientific titan
- Paul Adrian Maurice Dirac - that began to crack the code.
In the late nineteen twenties, he was working to unite two of the greatest breakthroughs
in modern physics, The strange worlds of quantum mechanics and
Einstein’s special theory of relativity.
Dirac’s story has been told many times, his famous absentmindedness and lack of communication
skills, Quantum pioneer Niels Bohr went as far as
to call him “the strangest man”, But there is absolutely no doubt that Dirac
was a revolutionary genius.
And it was through his work on quantum mechanics that Dirac made his mark on scientific history.
To understand the modern view he helped to bring about,
We have to accept that everything is actually fields.
These fields are different to things like classical electric and magnetic fields.
In quantum field theory, there are electron fields, photon fields, fields for the various
quarks and more - A ripple in the electron field is an electron,
and a ripple in the photon field is a photon.
Think of an atom.
What do you see?
In our minds, we often have the picture given to us by Niels Bohr,
Of electrons orbiting a nucleus like a planet orbiting a star.
And when an electron jumps from a higher orbit to a lower orbit, a photon of light is emitted.
But when considering the quantum world, this is not quite correct.
In quantum field theory, we think of an orbiting electron as a vibrational pattern in the electron
field.
The higher energy orbit is one particular pattern, and the lower energy orbit is another.
In the language of physics, the electron field and the photon field are coupled together,
And jumping between the higher and lower orbits, the electron field generates a vibration in
the photon field.
Quantum field theory has grown to become arguably the most successful theory of our world to
date, describing almost everything in our universe across 24 quantum fields, corresponding
to the various possible interactions of the Standard Model.
And so, simple - everything is fields, and the fields interact.
But, of course, as they often do in the quantum world - things are about to get a lot stranger.
Many of the great minds of quantum mechanics were involved in this move towards strangeness,
But perhaps the most well-known is a man from Far Rockaway with a broad Brooklyn accent,
A man named Richard Feynman.
Born in 1918, he started his career as part of the Manhattan Project, and was recommended
by Oppenheimer himself for Berkeley, in a now famous letter sent in 1942:
“He is by all odds the most brilliant young physicist here, and everyone knows this…I
may give you two quotations from men with whom he has worked.
Bethe has said that he would rather lose any two other men than Feyman from this present
job, and Wigner said, "He is a second Dirac, only this time human."
Though his 1985 autobiography “Surely You're Joking, Mr. Feynman!”
was an eye-opener for many.
Not only regarding his numerous contributions to science,
But also, his extroverted personality and complex private life, including a penchant
for strip clubs.
These aspects did not fit the stereotypical vision of a professor.
Indeed, Murray Gel-mann, another giant of quantum mechanics once even quipped of Feynman:
“[Feynman] was a great scientist, but he spent a great deal of his effort generating
anecdotes about himself.”
And yet, when it came to thinking about the quantum world, for many physicists Feynman
changed everything.
Whilst Feynman´s quip that that no one truly understood quantum mechanics may have been
true, Feynman himself certainly understood the depth
of the mathematics that underlies it.
This gave him the insights to think about the true nature of light and how it interacts.
And it all starts with a solitary electron.
In the electromagnetics of Maxwell, the charge of the electron results in an electric field
surrounding it.
And a charge in an electric field feels the presence of the electric field,
In this situation, there must be energy in the interaction – but how much?
The problem was, every time Feynman tried to calculate the amount of energy,
The answer came out to be the same – infinity.
So, Feynman did something quite radical, He threw away the classical notion of the
electric field as defined by Maxwell.
In the acceptance speech for the award of his Nobel Prize in 1965, Feynman said:
“I suggested to myself, that electrons cannot act on themselves,
they can only act on other electrons” And a new picture of the interaction of light
and electrons emerged.
The best visual representation of this interaction is the diagram named after Feynman himself.
The Feynman Diagram.
They are often a complicated mixture of lines, wiggles, and loops,
But at their heart, Feynman diagrams describe all of the possible interactions in quantum
mechanics.
To pick apart a Feynman diagram it is best to start with the simplest of interactions.
The interaction between electrons and light.
Feynman diagrams represent an interaction over space and time.
Lone electrons trace out straight-line paths through space-time, a path known as its world-line.
The electron is really just a vibration in the quantum electron field,
And with no interactions it happily traces a simple straight-line path.
We also know, however, that the electron field can couple with the photon field,
And when this happens, the vibrations in the electron field change.
In an atom, the electron jumps from a high-energy orbit to a low-energy orbit,
But for a free electron, conservation of momentum means that the electron changes direction.
If we imagine this over space-time, the world-line of the electron possesses a distinct kink,
And this occurs where and when the photon, usually depicted as a wavey line, is emitted.
This structure, this junction - is known as a vertex,
And it is the basic lego piece for building all Feynman diagrams.
Full Feynman diagrams are more than a single vertex, they usually combine several distinct
pieces.
The emitted photon from one electron is eventually received by another electron.
Two vertices are joined together to give the complete interaction,
Two kinked electron paths joined with the wiggly line representing the photon.
But what governs the coupling between the electron field and the photon field?
This is related to the charge on the electron, and one of nature’s constants, the fine-structure
constant.
This is electromagnetism, and the exchange of the photon between two electrons is the
electromagnetic force in action.
And so, in Feynman´s view, we wave goodbye to the electromagnetic field.
In its place we have two electrons interacting through the exchange of a photon - and when
huge numbers of these photons are exchanged, it approximates the classical force,
Even though at its heart, this electromagnetic force is a quantum phenomenon.
And it’s not just electromagnetism, but it is also true for the fundamental weak and
strong nuclear forces.
For the strong force, it is gluons instead of photons that are exchanged between quarks,
And for the weak force, it is via the exchange of massive particles known as the W and the
Z, But at their heart, all these forces can be
presented by a combination of Feynman vertices.
Feynman, however, had one even stranger card yet to play when it came to light.
He had said that one electron acts upon another, And this happens through the exchange of a
photon, Producing the complete Feynman diagram of
the interaction.
But does this mean that an electron fires out a photon at random?
Does this photon stream out into the universe with only a remote chance of being absorbed
by another electron?
The answer, counterintuitively - was no.
Feynman told us that the photon is only passed between two electrons that have agreed on
the exchange.
But there is something odd happening here.
If we are in the middle of the photon’s journey,
Its emission from one electron occurred in the past, whilst the absorption of the photon
by the other electron is going to occur in the future.
So, when did the electrons communicate and agree to exchange the photon?
How did they even know of each other’s presence?
It clearly cannot be via the electromagnetic force,
As this is precisely what the exchange of the photon actually is.
So, what is the solution?
As with a lot of quantum mechanics, whilst the mathematics just works,
The interpretation, the question of what is really happening, is the biggest challenge.
And so Feynman, with his supervisor John Wheeler, put a mind-bending possibility on the table.
The suggestion is something we now call the transactional interpretation.
They said that the two electrons handshake their acceptance of exchanging the photon,
But that this handshake is taken through time, With one electron messaging from the past,
and the other from the future.
This might sound ridiculous - but it completely fits with the mathematics of quantum mechanics.
So, on a dark night when you gaze at a distant star,
An electron in your eye and an electron in the atmosphere of that star
Spoke to each other through time and agreed to exchange the photon you see.
And going even further - for the lonely photon we met at the beginning of our story,
Two electrons separated by an immensity of space
Shook hands over billions of years of time, billions of light years of space,
And agreed that the photon should undertake its cosmic journey.
The world of quantum mechanics never disappoints.
And yet - there is one final, even stranger mystery to unfold about light,
and our lonely photon in the blackness of space.
As it travels over its many billions of lightyears, Just what does it experience?
We have followed our photon over many billions of years.
Eventually, at journey’s end, the universe it inhabits is very different to the one of
its birth.
Yet there is a disconnect.
For whilst this photon was almost as old as the universe itself, it remained eternally
youthful.
Galaxies formed in the void, stars were born, lived and died, whole superclusters splintered
and collapsed.
And the photon missed it all.
Because to the photon, time itself meant nothing.
This might seem a strange thing to say - the photon clearly had an existence in time.
But with the coming of Einstein, and his special theory of relativity, it was realised time
was actually flexible.
Time was relative, dependent upon who or what was actually measuring it.
And light - light takes this idea to the extreme.
“What would the universe look like if I were riding on the end of a light beam at
the speed of light?”
In the middle of the seventeenth century, Ole Romer was baffled.
Working at the Paris Observatory, Romer was peering at Io, one of the bright moons of
Jupiter.
Like clockwork, the moon orbited the giant planet in just over forty-two hours.
Vanishing from view as it ducked in and out of Jupiter’s shadow.
Except there seemed to be something odd about this cosmic clock!
Romer noticed that the timing of Io’s eclipses drifted.
Romer realised that the timing of the eclipse of Io was somehow tied to the Earth’s orbit,
Changing from earlier to later and back again when the Earth was either closest to or furthest
from Jupiter.
And it was then Romer realised the culprit was light,
And in particular its speed.
He reasoned that the drift in Io’s eclipses must be due to a finite speed of light.
As the Earth moved in its orbit, the distance to Jupiter changed,
And the change in time was because light had to traverse these differing distances.
His initial estimate was fast, very fast - two hundred and twenty thousand kilometres every
second.
And eventually, more accurate measurements tied the speed of light to almost three hundred
thousand kilometres per second.
But just what was this speed relative to?
It had been the belief for centuries that there existed a medium, the aether, that carried
light waves.
Surely, therefore, light’s speed was relative to this medium?
From Plato to Newton, this aether had long been suggested as a solution to various questions
in physics, but never firmly detected - experiments in search of evidence had failed time and
time again.
And so it was that in 1905, during his annus mirabilis, Einstein rang the final death knell
for this invisible medium.
Special Relativity.
The truth was that it was the speed of light was the universal absolute and invariant,
Measured to be the same value for all observers across the cosmos.
A lot has been written about special relativity, and although much of it seems confused and
paradoxical, there is a simple central message at its heart.
Particles with mass, such as electrons, chart out their own time as they travel through
space-time.
Imagine two clocks sitting at the same location, synced to show exactly the same time.
Now take these clocks on two separate journeys, speeding up and slowing down.
In Newton’s view of the universe, of absolute time, if you were to bring the clocks together
agajn and compare their times they would have remained synchronised.
But not in Einsteins.
The relative motion of the two clocks will have influenced their relative passage of
time.
And as they traced out different paths through space-time,
When they reunite, their times will now be out of sync.
This mind-bending aspect of relativity seems too strange to be true,
But numerous experiments have shown this to be the way the universe works,
From globe-trotting atomic clocks to high-speed particles in accelerators, time is definitively
relative.
But what does this mean for light?
Light had taken a central place in Einstein’s new vision of the cosmos - everyone in space-time
should measure the speed of light to be precisely the same value.
But in demanding this, something else had to give,
And so space and time themselves had to bend - become flexible and rubbery to accommodate
the consistency of the speed of light.
Indeed one immediate consequence of this Einstein’s insights was that light would feel the existence
of gravity, And as it travels through the universe, light's
path would be deflected by the presence of mass.
Newton’s claim from two centuries prior reborn.
Indeed, experiments have borne this out again and again - with the results becoming more
and more accurate.
Massive objects, such as stars and galaxies, can even behave as gravitational lenses, magnifying
distant baby galaxies in the very early universe and revealing the presence of dark matter.
The beauty of these natural telescopes is clear in deep space images - such as the first
revealed by the James Webb Space Telescope.
And so the flexible nature of space and time had truly seen the end of Newton’s view
of a rigid universe.
But what about light?
What did this mean for its experience of space and time?
Travelling at the fastest speed possible in the universe, the effects of relativity become
extreme.
Very extreme.
All distances shrink to zero.
As does the time taken to cover these zero distances.
And so, for photons, no matter how far they travel across the universe, not an instant
of time will tick by.
Even though this light may have existed in time and space for many many years or light
years, Even though it would have been clearly formed
by one electron in one location and vanished when absorbed by an electron in another,
The space-time distance between these two events would be exactly zero.
To the photon, it is born and dies at precisely the same moment.
To the photon, it is as if it never existed at all.
We began this story by following a photon from its creation just after the beginning
of time, To its ultimate destruction in the detector
of a telescope orbiting our planet today, And yet the photon itself saw nothing of this.
Not the intense light of stellar birth, Or the catastrophic explosions that came with
stellar death, Or the formation of planets and eventual rise
of life on our own pale blue dot.
The photon noticed nothing.