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How can light be matter or not?
Question Date: 2013-04-24
Answer 1:

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.

Answer 2:

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:

click 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 that one.

-Yours in Science,

Answer 3:

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.

Answer 4:

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!

Answer 5:

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 Electromagnetic Spectrum.

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 frequencies.

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!

Best wishes,

Answer 6:

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.

1) http://en.wikipedia.org/wiki/Matter
2) http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Answer 7:

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 following:

1 (the main reason). Photons have no rest mass, so only have energy while in motion. Traditionally, matter has energy (mass) even when not moving.

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 photon's wavelength.

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 energy?).

Answer 8:

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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