|How can light be matter or not?|
|Question Date: 2013-04-24|
That's a really great question. In fact this
question caused a great debate among physicists
during the time of Isaac Newton. Newton thought
that light was a stream of little particles and
other physicists such as Christian Huygens thought
that the universe was filled with tiny particles
called aether particles and that light was just a
wave moving through the aether.
To make a long story short, it turns out
they're both kind of wrong... and they're both
kind of right. In some situations light sort of
acts like a stream of particles and in others it
acts sort of like a wave. Today many physicists
(myself included) like to say that light is
neither a particle or a wave... it's just light.
So, some might say that because light can carry
energy from the sun to the earth it must be
matter. Others might say that because you can't
hold a lump of light in your hand light isn't
matter. It all depends on what you mean by the
word 'matter.' But the important thing to
remember is that light doesn't care what you call
it, it will still act the same way no matter what.
That's why in physics we try not to worry too
much about what things are called and focus on
understanding how things actually behave.
Both your class, and the entirety of science is
having this debate. First, some precision. Photons
are not in light, they are light. Light is made up
of photons, so one photon is like one unit of
light. Additionally, photons aren't really matter,
although this depends on how you define things.
Traditionally people say that for something to be
called matter, it has to have mass and photons
don't really have mass... although this also
depends on how you define it.
I'll cut to then end, and then explain the
controversy. Light is a form of electromagnetic
radiation, which is a form of energy. This energy
always comes in distinct units, which we call
photons. In this way, light is like a particle,
but it still doesn't have mass, so it's not really
matter. The way these particles behave though, is
very strange. Sometimes they behave more like a
wave, like a ripple moving across a pond.
Sometimes they behave more like actual solid
objects, like bullets being shot from a gun. And
the craziest thing is that whether they behave one
way or the other depends largely on whether or not
anyone is watching them at the time. I cannot
explain this behavior... actually no one can. All
we can do as scientists is observe that it is the
case and describe it using mathematical laws. If
you want to know more about how we know this to be
true, there is a famous physicist by the name of
Richard Feynman who explains it quite nicely here:
here to watch video
Now the video is a little old, but it's a good
one and the laws of physics, as we know them, have
not changed since those days.
Now I said that whether light has a mass or not
depends on your definition, let me explain that.
As I said, photons, and thus light, is energy. But
there is a famous equation E=mc2 that
tells us that the mass and energy are perfectly
equivalent. E is energy, m is mass and c is the
speed of light in a vacuum.
You can convert mass into energy and energy
into mass. As a matter of fact, that's what the
sun does every day, it converts some of its mass
into energy, which we see as light. And so there
is a perfectly reasonable way to talk about the
"mass" of a particle of light. All you do is
convert its energy to mass, although
conventionally we people try not to do that these
days. And so... light is energy, but energy and
mass are equivalent. And if you're asking whether
light is a particle or a wave... no one in the
universe, that we know of, has a good answer to
-Yours in Science,
This is a fantastic question! The topic of
photons can be a very confusing topic even to
scientists studying their behavior. Let’s start
off by defining what matter is. Matter is
generally something that has mass (it has weight)
and volume. For example, a bus is made of matter
because it is heavy and takes up space. Now, what
is a photon? A photon is generally considered to
be a “particle” of light, but this particle is
very special. A photon particle does not have any
mass (because you cannot “weigh” light), so it is
not considered to be matter.
Scientists call a particle of light a photon
because it can carry and pass on energy like
matter can, but it also sometimes acts like a
wave, as light should. Since a photon is a
particle that has very unique abilities that
normal matter does not have, we can say that light
is not matter.
This is a very deep question at the heart of
quantum mechanics, which is the field of science
describing matter at a very small scale. There is
a theory called the "wave-particle duality" that
says at a quantum scale, we can't really use what
are called "classical" words such as "wave" or
"particle" to describe objects. Instead,
quantum-scale objects exhibit properties of both
particles and waves. In quantum mechanics, the
wave-like behavior of a particle is encoded in a
mathematical construct called a "wavefunction"
which is "evolved" under an equation called the
"Schrodinger equation." The particle-like behavior
is associated with measurements, where the
particle is forced to be found in some localized
region of space. This is all closely related to
the "Heisenberg Uncertainty Principle," which
states that we can never simultaneously know both
the exact position and exact momentum of a
particle. I realize this is a lot of jargon, but
the main idea to get from this is that at the
quantum scale (where things are very very tiny),
light behaves a little like both waves and
particles. I hope this helps!
This is a very interesting question. I will try
to give you some insight:
No, light is not matter, light is energy, but
matter and energy are related,
and in certain situations, at very high energies
such as inside a star or
particle collider, matter can be converted into
energy and vice versa.
Photons have no mass, but matter has mass.
Photons are "quanta" (tiny bits) of
electromagnetic energy. Light is very interesting
in that it behaves either
like waves or like particles, depending on how you
measure it. Photons travel
all the way across the universe at the speed of
light! Matter cannot ever
travel at the speed of light.
So, we know that light cannot *be* matter, at
least not in the form we know it
(gross matter - like rocks, plants, animals,
people, and stuff).
The term "light" is usually used to mean the
small portion of all
electromagnetic radiation that we can see with our
eyes, but there is a
tremendous range of electromagnetic radiation that
goes from very long
wavelengths (radio frequency waves) to extremely
short, high energy waves
(gamma rays). This range we call the
Light that you can see is not in matter, but
all living matter contains
energy. You are warm, right? So your body emits
heat energy (we call this
infra red radiation) and electrical energy because
of all our brain and nerve
impulses. If you look at a person using an infra
red camera, you can see the
warmer and cooler areas of their body.
All planets contain heat energy, too, and we
can measure their heat output by
observing them in infra red, microwave, or radio
But at extremely high temperatures and
energies, matter and radiation become
interchangeable. Matter can be converted into
energy, such as in nuclear
fusion. Einstein predicted this in his famous
equation E = mc2 where m stands
for mass and c is the speed of light, and I write
c2 to mean "c squared".
This takes place inside stars, and that is how
they shine - by converting some
of their matter to energy
Energy can be converted into matter in very
extreme conditions, such as in the
very early universe (we're talking within the
first one to three minutes after
the so-called Big Bang). At first, the universe
was all radiation. At a
certain moment it cooled down so that protons
could condense out of the
radiation, and shortly afterwards it cooled enough
so that electrons could
condense. But at that time of extreme heat and
light (trillions of degrees,
unimaginable!) matter AND antimatter condensed out
of the radiation field! So,
for a few more minutes, there was an equilibrium
reaction going on, of matter
and antimatter condensing out of the radiation
field, and then recombining
into pure energy, because (maybe you have heard
this) matter + antimatter
annihilate each other into pure energy (photons).
Then, a short time later, the universe cooled
enough so that this equilibrium
reaction stopped. The last bit of matter and
antimatter formed out of the
radiation, and mutually annihilated each other.
Luckily for us, the balance
between matter and antimatter was not equal, and
there was extra matter left
over from this last annihilation, and so that is
why we are here today!
Here is another bit of 'extreme' information
that you can tell your class: If
you could magically transform yourself so that you
could ride on a photon
traveling across the universe, you would see that
all the space in front of
you would be contracted down to no distance at
all, and you would not
experience the passage of time! Of course you
cannot really do this, because
you are made of matter.
I hope this helps you win the debate!
Matter is usually defined as something that has
both a rest mass and a volume. Photons have
neither of these so they are not considered
matter. When something is moving very fast, close
to the speed of light, it starts to gain extra
mass because of relativity. This is why photons,
which move at the speed of light, have mass. But
if it were possible for a photon to be at rest, it
would have no mass. By the way, in Einstein's
famous equation E=mc2 which says that
mass and energy are the same thing, the m in the
equation is actually the rest mass plus this extra
mass due to relativity.
The word "matter" is imprecisely defined, but
traditionally, photons are not matter and light is
not matter, despite the fact that they do have
matter-like properties. The reason why photons are
not normally considered to be matter are the
1 (the main reason). Photons have no rest mass,
so only have energy while in motion.
Traditionally, matter has energy (mass) even when
2 (secondary reason). Photons are bosons, which
means that they aren't subject to the exclusion
principles that prevent more normal matter from
being crammed into each-other. For example, no two
electrons in the universe can have the same
quantum numbers, which includes their locations in
space, so in order to have multiple electrons in
the same space, they all have to have different
energy states. Photons aren't like that; you can
cram as many identical photons into whatever
volume you like so long as it's larger than the
Now, all of this may fall apart when we learn
what dark matter is. This is where the imprecision
in the definition of matter comes in, as we really
don't have any solid idea yet of what dark matter
consists of or what its properties are.
Problems with the definition also come up when
you start delving deep into quantum mechanics
(most of the matter of a proton is also contained
in a type of boson called a gluon), or general
relativity (are black holes matter or are they
Thanks for the great question; I am glad that
your class is having a scientific debate! This is
a tricky question, and I hope that I can help.
You are correct that light is made out of
photons. However, photons are a bit different than
matter. Sometimes people will say that photons are
"particles" of light, but these light "particles"
(photons) are much different than the particles of
matter that make up all of the objects around you.
One major difference between photons and matter
particles (for example, atoms or electrons) is
that photons are entirely made of energy, while
particles have what is known as a "rest mass,"
meaning they have a mass when they are not moving.
Light on the other hand can never stop moving; in
fact, all light moves at exactly the same speed
(light speed: 300,000,000 m/s !)
Please let us know if you have any further
questions about light, or if any other interesting
topics get raised during your class debate!
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