Expert: Daniel Mazur - 8/3/2010

Question
QUESTION: Does all electromagnetic radiation/photons
travel at the speed of light? I have heard that
electrons within atoms do this. If all this was so,
I wonder if there is something within electragnetic
radiation and atoms (such as the nucleus) which
isn't going so fast which helps to balance
things and keeps us from hurtling into
space at this speed. Gravity must play a part
in keeping us grounded in both our bodies
and environment, so that even though there is a lot
going on we can still feel stationary.  I look forward
to hearing your feedback on this. Thanks.

ANSWER: Dear Gena
Yes, all photons travel at the speed of light. Remember that speed of light in vacuum (the universal constant c) is the same for all photons (EM radiation), whereas speed of light in materials - air, water, glass, solids,... - is smaller than the one in vacuum and it is generally different for different frequencies. This is the reason for prisms splitting white light into rainbows, for example.
All this has little to do with electrons in atoms. Electrons never move at the speed=c. In atoms they occupy certain orbitals and our description of their existence there is in terms of standing electron waves (we call them wavefunctions) - not EM waves!!! Different orbitals have generally different energies associated with them and a transition of an electron from one orbital to another is accompanied by a sudden change of its energy. This sudden jump of an electron to a more or less energetic orbital comes from absorption or emission of a photon, respectively. The energy of the photon equals precisely to the difference between the energies of the orbitals.
What keeps the electrons in those precisely defined orbitals is the Pauli Exclusion Principle. It says that each electron must have a different wavefunction. The wavefunction is a function of position and momentum (corresponds to energy, vaguely speaking). Electrons far apart can have the same energy. Electrons in a very confined space as in atoms must have all different energies and this is where the orbitals get created. It is a very rigid and balanced structure (all by itself, gravity plays no role) and all behavior of matter stems from it.
Daniel

---------- FOLLOW-UP ----------

QUESTION: Are you saying that electrons within atoms don't travel at the speed
of light and if so is there another part of the atom that does? Also, is the
electromagnetic radiation that people, animals, and objects emit at a slower
speed of light since it involves matter? Thanks so much for your time and
patience in answering my questions.

ANSWER: Hi,

yes, I am saying, that electrons within atoms (actually even anywhere else) don't travel at the speed of light c. Electrons have a non-zero rest mass (see Special Theory of Relativity) and therefore may never travel at the speed c. Only photons (inside atoms or outside) may travel at the speed of light c. No other part of an atom can move at the speed c either, because all the parts have non-zero rest mass.

When light travels through matter, it is slower than c due to the matter's index of refraction n. Once the light leaves matter (i.e. gets radiated out), it travels at the speed c again.

Daniel

---------- FOLLOW-UP ----------

QUESTION: When people, animals, and objects emit radiation wouldn't this
radiation go into the air and travel at a slower speed of light since
air is a material (or considered to have matter)? Where does the
light, or radiation go when it gets radiated out and could this vary?
Thanks.

Answer
Hi Gena,

yes, the light travels in air at a slightly slower speed than in vacuum. According to http://en.wikipedia.org/wiki/Speed_of_light , the speed in air is about v = c/1.0003 .

Where does radiation go? As soon as a photon (particle of radiation) leaves its place of origin, it moves in a straight line until it bumps into another particle. This collision results in a disappearance of the photon and (more or less immediate) creation of another photon. This new photon may have the same energy (wavelength) as the previous one or different. In the (sub)atomic scale, this is what ALWAYS happens. When you consider not one photon at a time but many, new laws of group (!) behavior emerge. This group behavior of photons may look differently in different situations, but it is all based on that one process of photons absorbed and emitted by other particles, which I described above.

Daniel