# Fermilab

## Inquiring Minds

You Wrote:

What is in the area between sub atomic particles? Does this area when accelerated give off more light or does it get darker? If you were to accelerate a flashlight would the electron flow through the filament slow down? Can we determine how fast something is traveling by the amount of light it releases? How does acceleration produce light?

Thank You
Tom

Dear Tom,

Wow, that's an interesting set of questions on the topic of "light emitted by particles." I tried to reorder your questions to provide a more coherent answer. The answers are not very simple, and I will try my best to present them as clearly as possible. Here we go:

Q: How does acceleration produce light?

A: Physicists accelerate electrically charged particles, increasing the energy of these particle. The particles can lose this "extra" energy by interacting with other particles (collisions) or by emitting light (radiation).

Conservation of energy (total energy remains the same) forbids that an elementary particle (an electron, for example) traveling at a constant speed (constant energy) through empty space (no forces, no collisions) emits a light particle (called photon). Once forces or collisions occur, a particle may emit a photon while satisfying the law of energy conservation.

Acceleration is nothing else but a force acting on a particle. If acceleration occurs, a particle may "eject" some of its energy by emitting a photon.

How much energy may a particle emit? The amount of energy in form of light that an accelerated particle may emit depends on the direction in which the acceleration occurs. One needs to distinguish between two types of acceleration: Acceleration in the same direction as the particle is flying (that is, "speeding up" the particle in its direction of motion, which can be achieved with electric fields) or acceleration perpendicular to the direction of motion (that is, forcing the particle to "fly a curve," which can be achieved with magnetic fields).

In the case of acceleration in direction of motion, only very little energy may be emitted. As a matter of fact, the forward acceleration provided by today's most powerful accelerators is not sufficient to create radiation. The perpendicular acceleration, created by, for example, the magnets in circular accelerator, allows for much more light to be emitted. This light, also called synchrotron light, is produced by all ring-shaped accelerators around the world.

Q: Can we determine how fast something is traveling by the amount of light it releases?

A: In principle, yes. In a circular accelerator, the more energetic the particles the more light is produced. I don't know how precise the results would be.

Q: What is in the area between sub atomic particles? Does this area when accelerated give off more light or does it get darker?

A: The area between subatomic particles is empty (vacuum, no other particles) but interspersed with force fields (the forces that keep the atom of falling apart). There is energy stored in these force fields, and parts of this energy are used to provide the light particles with energy. But physicists usually think of the particles, not the force fields, as emitting light. This, however, is a picture in which people think of atoms and such as particles. The reality is much more complex as atoms obey by the laws of quantum mechanics, and the subatomic particles obey laws of both particles AND waves. You can imagine that the true picture of what is happening is really "fuzzy." At this level of physics, you don't trust your visional intution but just the mathematical equations that describe the processes. To everyone's surprise, they can describe the emission of light perfectly well. The electromagnetic quantum theory, as a matter of fact, is one of the best tested and most successful theories.

Q: If you were to accelerate a flashlight would the electron flow through the filament slow down?

I don't see any reason why the electron flow should slow down. Once the speed of the flashlight becomes relativistic (close to the speed of light, but always less), all kinds of "weird" effects would have to be taken into account. The speed of the electrons in the flashlight, for example, has to remain below the speed of light, no matter how much you accelerate the flashlight and how close its speed will be to the speed of light. These effects are all described by the theory of special relativity.

For more details on this and other aspects of light, I refer you to the following excellent webpages: http://www.fnal.gov/pub/inquiring/more/light/index.html

Best wishes,

Kurt

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