MadSci Network: Physics

Re: What makes light 'go?' WHY does it 'travel?'

Date: Fri Aug 19 04:26:31 2005
Posted By: Ben Tordoff, Grad student, Lasers in nuclear physics, University of Jyvaskyla
Area of science: Physics
ID: 1124343677.Ph

the questions you have asked are very good and have plagued the physics 
community since the beginning of the age of reason. I think we are now at 
a place where we have a good understanding of what light is and how it 
works but this has only really come about very recently and an almost 
full description of the interaction of light with matter and itself comes 
from quantum electrodynamics. If you can wade through the maths involved, 
the answers to your questions are in there. Here's how it goes:

Light is generated by oscillating dipoles which means two charges of 
opposite sign separated by a distance d vibrating with a certain 
frequency f. As they vibrate the electric field between them vibrates 
with the same frequency and this causes a magnetic field to vibrate at 90 
degrees to the electric field (electric fields induce magnetic fields). 
These vibrating electric and magnetic fields are what we call light. What 
makes light go is these dipoles losing energy by radiating these fields 
in to space. They do this in all directions. The reason that they are 
expressed in sine waves is that this vibration is mathematically 
expressed as a simple harmonic oscillator and sine waves are easy to work 
with as their derivatives and integrals are also well known analytical 
expressions. The way that light is both a wave and a particle comes about 
when we include quantum mechanics into this description. Quantum 
mechanics splits these fields up into small pieces called photons. These 
photons have a characteristic energy E = hf, so the energy is directly 
proportional to the frequency of the oscillating dipole where the field 
came from. The particle nature of light becomes apparent when you try and 
change the energy of some particle for example by shining light on it. If 
the energy of the photon is less than the energy needed to cause a 
process to happen then it won't happen, sounds obvious. But if light is a 
wave then you're continuously adding energy to the system so it should 
eventually have enough to do the job. Also, the more light you shine on 
it, the faster it should happen as you give it energy quicker. This is 
not seen in practice and this is because everything is quantised, 
including the process you're trying to affect.   

Photons always travel in a straight line, that being the smallest 
distance between two places. This has some interesting consequences in 
general relativity but I won't go into that. If we want to think about 
what a photon looks like head on etc. we have to think about where the 
vectors that describe the size and direction of the electric and magnetic 
fields are pointing. This is called the polarisation state of the light. 
Polarisation actually talks only about the electric field, but as the 
magnetic field is always perpendicular to it then this implies magnetic 
field as well. The electric field vector when a photon is coming towards 
you can travel in a circle. It can spin both clockwise and anti-clockwise 
and this is called circularly polarised light but as it moves in space 
you're right, it draws out a spiral. You can also have linearly polarised 
light where the vector points directly at you as you look at the photon, 
so it looks like a dot. There is also vertically polarised light where it 
looks like a line when it comes towards you. 

As far as interference goes, this is an effect of phase difference 
between two photons and not affected by polarisation ie. what the size of 
the electric field is doing as a function of time, not direction.     

Finally to answer you're question about elementary particles, they will 
travel in a speed and direction which conserves angular momentum 
depending on how they got their energy. I'm assuming you mean things like 
protons and neutrons as elementary particles because anything smaller 
(like quarks)is in the realm of quantum chromodynamics and very 
complicated indeed. Angular momentum theory is also very advanced from 
the start but if you're interested here's a reference:

Angular Momentum by Brink and Satchler - graduate level
Optics by Smith and Thompson (chapter 1) - general light theory

I hope that helps,

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