More to light than meets the eye!
Light is strange stuff indeed. Unless you have a scientific
background, chances are you have not wasted too much time
wondering what light is made of. Surely, you might think, it
is what emanates from objects like the Sun, electric light bulbs,
torches, candles, fires and so on and, whatever it is made of, enters
our eyes and we ‘see’ things. When light bounces off an object,
it carries with it into our retinas information about the shape and
colour of that object. But what is light itself ultimately made of?
I have already described how the light from an object that is
moving away from us becomes redder due to a stretching of the
light’s wavelength. This implies that it is not made up of physical
material that we can touch. In fact, we are taught at school that it is
just periodic, oscillating waves of energy, like sound waves or the
ripples on the surface of a pond when a stone is thrown in. All the
experiments you would have done in school science labs would
have probably confirmed this. Light waves reflect off mirrors, get
focused through lenses and are split up into the colours of the
rainbow, known as the visible spectrum of light (when sunlight is
passed through a prism).
Some of these experiments with light can be a lot of fun, and
I recall enjoying building a box camera as a child and trying tounderstand how rays of light passing through a tiny pinhole in the
side of the box could still produce the (upside down) image on the
back. Fortunately for those with a lower boredom threshold than
your average physicist, it turns out that light is nowhere nearly as
boring and straightforward as we are led to believe at school. In
fact it is so weird that all science teachers sign a secret document
in which they vow never to divulge the true nature of light to
the innocent and unsuspecting children. By the time the children
grow up they will either be totally disinterested or would simply
refuse to believe that something as familiar to our everyday life as
light could hide so much mystery and yet be so fundamental to
the workings of the Universe.
OK, I admit that sinister secret covenants signed by school
teachers sounds like something out of a Roald Dahl story. Of
course there is no global conspiracy to hide the true nature of light,
but I am serious when I say that there is more to light than meets
the eye!
Sound is a simple example of a wave. An object is said to
make a sound if it sets the air molecules around it vibrating. These
molecules collide with others nearby setting them in motion and so
on all the way into our ears. The air molecules inside the ear then
set the ear drum vibrating and our brains translate this vibration
into something we know as sound. But at no point can we say
that a material ‘substance’ has travelled from the object making
the sound to our ears.
Light is much more than this. In Chapter 6 I will reveal how
light is fundamental to the very nature of space and time. The
physicist David Bohm summed it up when he said that “when
we come to light, we are coming to the fundamental activity in
which existence has its ground”. For now, and for the purpose
of discussing black holes, let us investigate what light actually
comprises.
Isaac Newton firmly believed, based on his famous
experiments with prisms, that light was composed of a stream of
tiny particles he called corpuscles. This, he claimed, was obvious
since light most certainly did not behave like sound waves. Light
rays always travelled in straight lines (the bending of light due to gravity was a long way from being discovered) and cast sharp
shadows. Sound waves worked their way around obstacles and
could easily bend round corners. Sound waves need a medium
to travel through; a material substance made up of atoms which,
by oscillating, carry the energy and frequency of the sound. This
is why the posters advertising the cult film Alien carry the, quite
correct, caption ‘In space, nobody can hear you scream’, since in
space there is no air to carry the sound waves. Light, on the other
hand, is not at all like this and clearly has no difficulty travelling
through empty space.
For these reasons Newton was convinced that the particle
theory of light was correct. But not everyone was convinced, and
it took over a century for clear proof to emerge that Newton’s
theory was not the whole story. At the beginning of the nineteenth
century Thomas Young discovered that the reason light did not
appear to bend round corners was because the effect was so small.
The wavelength of light is so short compared with that of sound
that the amount of bending, called diffraction, is hard to detect.
Nevertheless, Young achieved this by sending light through very
narrow slits and showed that, when it hit a screen on the other
side, it formed a row of light and dark fringes in a way that would
be impossible to explain if light were composed of particles. Such
interference fringes, as they are known today, are explained in
every physics textbook as being due to the way the peaks and
troughs in the light waves from the two slits reinforce and cancel.
We have all observed this effect at school, with varying degrees of
excitement, in ripple tank experiments.
So was Newton wrong? Is light a wave rather than a stream of
tiny particles after all? In the late nineteenth century it appeared
that Young’s interpretation of light as a wave was put beyond any
possible doubt when the Scottish physicist James Clerk Maxwell
developed a set of equations which showed that all the light we
see is a form of electromagnetic radiation, which we know now
includes other forms such as radio waves, microwaves and x-rays,
as well as infrared and ultraviolet radiation. Light, it turned out,
was made up of a combination of electric and magnetic fields,
vibrating at right angles to each other, that could travel through empty space. So light was a wave after all. Was this the end of the
story?
Far from it. Enter Albert Einstein, who won his Nobel prize in
physics due to a paper he wrote in 1905—which, amazingly, had
nothing to do with his theories of relativity. The paper explained
something called the photoelectric effect and proved that Newton
was not entirely wrong after all. Light at its most fundamental
level is made up of tiny entities called photons.
So what of Young’s interference fringes? And what of
Maxwell’s electromagnetic waves? What on earth is going on
here? Surely light must make up its mind what it is made of:
waves or particles?
There have been numerous books explaining what is going
on. It turns out that light is indeed schizophrenic. Sometimes we
see it behave like a periodic wave and other times like a stream of
particles. It depends on what type of experiment we do! If you
don’t like this, then tough. I told you light was weird. The theory
that describes the rules for the behaviour of light is known as QED,
which stands for quantum electrodynamics, and was developed
by, among others, the American physicist Richard Feynman in the
late 1940s. QED, as its name suggests, is itself derived froma much
broader theory in modern physics called quantum mechanics which
describes the behaviour of not just light but all matter and energy
at its most fundamental level (the level of atoms and smaller).
Quantum mechanics was developed in the mid-1920s by a
number of European physicists, including Einstein. It describes
things like how a single atom can be in two different places
at the same time, and how tiny particles can spontaneously
appear out of nowhere then quickly disappear again. The world’s
top physicists all agree that if anyone is not uneasy with what
quantum mechanics tells us about the world we live in then he
or she has probably not really understood quantum mechanics.
Despite this it has been the single most successful and important
scientific discovery of the twentieth century. Quantum mechanics
underpins the whole of modern chemistry and the whole of
modern electronics. Without it we would not have been able
to understand the structure of crystals, or invent the laser or the silicon chip. Without an understanding of the rules of quantum
mechanics there would be no televisions, computers, microwaves,
CD players, digital watches and so much more that we take for
granted in our technological age.
We will leave our discussion of light, for now, with a quote
from Einstein from 1951 (four years before he died):
“All these fifty years of conscious brooding have brought me no nearer
to the answer to the question ‘What are light quanta [photons]?’
Nowadays every Tom, Dick, and Harry thinks he knows it, but he
is mistaken.”
Having given you this brief introduction to the nature of light
I can begin to discuss the properties of black holes.
Light is strange stuff indeed. Unless you have a scientific
background, chances are you have not wasted too much time
wondering what light is made of. Surely, you might think, it
is what emanates from objects like the Sun, electric light bulbs,
torches, candles, fires and so on and, whatever it is made of, enters
our eyes and we ‘see’ things. When light bounces off an object,
it carries with it into our retinas information about the shape and
colour of that object. But what is light itself ultimately made of?
I have already described how the light from an object that is
moving away from us becomes redder due to a stretching of the
light’s wavelength. This implies that it is not made up of physical
material that we can touch. In fact, we are taught at school that it is
just periodic, oscillating waves of energy, like sound waves or the
ripples on the surface of a pond when a stone is thrown in. All the
experiments you would have done in school science labs would
have probably confirmed this. Light waves reflect off mirrors, get
focused through lenses and are split up into the colours of the
rainbow, known as the visible spectrum of light (when sunlight is
passed through a prism).
Some of these experiments with light can be a lot of fun, and
I recall enjoying building a box camera as a child and trying tounderstand how rays of light passing through a tiny pinhole in the
side of the box could still produce the (upside down) image on the
back. Fortunately for those with a lower boredom threshold than
your average physicist, it turns out that light is nowhere nearly as
boring and straightforward as we are led to believe at school. In
fact it is so weird that all science teachers sign a secret document
in which they vow never to divulge the true nature of light to
the innocent and unsuspecting children. By the time the children
grow up they will either be totally disinterested or would simply
refuse to believe that something as familiar to our everyday life as
light could hide so much mystery and yet be so fundamental to
the workings of the Universe.
OK, I admit that sinister secret covenants signed by school
teachers sounds like something out of a Roald Dahl story. Of
course there is no global conspiracy to hide the true nature of light,
but I am serious when I say that there is more to light than meets
the eye!
Sound is a simple example of a wave. An object is said to
make a sound if it sets the air molecules around it vibrating. These
molecules collide with others nearby setting them in motion and so
on all the way into our ears. The air molecules inside the ear then
set the ear drum vibrating and our brains translate this vibration
into something we know as sound. But at no point can we say
that a material ‘substance’ has travelled from the object making
the sound to our ears.
Light is much more than this. In Chapter 6 I will reveal how
light is fundamental to the very nature of space and time. The
physicist David Bohm summed it up when he said that “when
we come to light, we are coming to the fundamental activity in
which existence has its ground”. For now, and for the purpose
of discussing black holes, let us investigate what light actually
comprises.
Isaac Newton firmly believed, based on his famous
experiments with prisms, that light was composed of a stream of
tiny particles he called corpuscles. This, he claimed, was obvious
since light most certainly did not behave like sound waves. Light
rays always travelled in straight lines (the bending of light due to gravity was a long way from being discovered) and cast sharp
shadows. Sound waves worked their way around obstacles and
could easily bend round corners. Sound waves need a medium
to travel through; a material substance made up of atoms which,
by oscillating, carry the energy and frequency of the sound. This
is why the posters advertising the cult film Alien carry the, quite
correct, caption ‘In space, nobody can hear you scream’, since in
space there is no air to carry the sound waves. Light, on the other
hand, is not at all like this and clearly has no difficulty travelling
through empty space.
For these reasons Newton was convinced that the particle
theory of light was correct. But not everyone was convinced, and
it took over a century for clear proof to emerge that Newton’s
theory was not the whole story. At the beginning of the nineteenth
century Thomas Young discovered that the reason light did not
appear to bend round corners was because the effect was so small.
The wavelength of light is so short compared with that of sound
that the amount of bending, called diffraction, is hard to detect.
Nevertheless, Young achieved this by sending light through very
narrow slits and showed that, when it hit a screen on the other
side, it formed a row of light and dark fringes in a way that would
be impossible to explain if light were composed of particles. Such
interference fringes, as they are known today, are explained in
every physics textbook as being due to the way the peaks and
troughs in the light waves from the two slits reinforce and cancel.
We have all observed this effect at school, with varying degrees of
excitement, in ripple tank experiments.
So was Newton wrong? Is light a wave rather than a stream of
tiny particles after all? In the late nineteenth century it appeared
that Young’s interpretation of light as a wave was put beyond any
possible doubt when the Scottish physicist James Clerk Maxwell
developed a set of equations which showed that all the light we
see is a form of electromagnetic radiation, which we know now
includes other forms such as radio waves, microwaves and x-rays,
as well as infrared and ultraviolet radiation. Light, it turned out,
was made up of a combination of electric and magnetic fields,
vibrating at right angles to each other, that could travel through empty space. So light was a wave after all. Was this the end of the
story?
Far from it. Enter Albert Einstein, who won his Nobel prize in
physics due to a paper he wrote in 1905—which, amazingly, had
nothing to do with his theories of relativity. The paper explained
something called the photoelectric effect and proved that Newton
was not entirely wrong after all. Light at its most fundamental
level is made up of tiny entities called photons.
So what of Young’s interference fringes? And what of
Maxwell’s electromagnetic waves? What on earth is going on
here? Surely light must make up its mind what it is made of:
waves or particles?
There have been numerous books explaining what is going
on. It turns out that light is indeed schizophrenic. Sometimes we
see it behave like a periodic wave and other times like a stream of
particles. It depends on what type of experiment we do! If you
don’t like this, then tough. I told you light was weird. The theory
that describes the rules for the behaviour of light is known as QED,
which stands for quantum electrodynamics, and was developed
by, among others, the American physicist Richard Feynman in the
late 1940s. QED, as its name suggests, is itself derived froma much
broader theory in modern physics called quantum mechanics which
describes the behaviour of not just light but all matter and energy
at its most fundamental level (the level of atoms and smaller).
Quantum mechanics was developed in the mid-1920s by a
number of European physicists, including Einstein. It describes
things like how a single atom can be in two different places
at the same time, and how tiny particles can spontaneously
appear out of nowhere then quickly disappear again. The world’s
top physicists all agree that if anyone is not uneasy with what
quantum mechanics tells us about the world we live in then he
or she has probably not really understood quantum mechanics.
Despite this it has been the single most successful and important
scientific discovery of the twentieth century. Quantum mechanics
underpins the whole of modern chemistry and the whole of
modern electronics. Without it we would not have been able
to understand the structure of crystals, or invent the laser or the silicon chip. Without an understanding of the rules of quantum
mechanics there would be no televisions, computers, microwaves,
CD players, digital watches and so much more that we take for
granted in our technological age.
We will leave our discussion of light, for now, with a quote
from Einstein from 1951 (four years before he died):
“All these fifty years of conscious brooding have brought me no nearer
to the answer to the question ‘What are light quanta [photons]?’
Nowadays every Tom, Dick, and Harry thinks he knows it, but he
is mistaken.”
Having given you this brief introduction to the nature of light
I can begin to discuss the properties of black holes.
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