Thursday, July 15, 2010

Radio in a vacuum

I was asked recently how radio waves travel in a vacuum.  I said "It's simple..."  and I stopped right there. it's not actually simple. It's actually a huge conceptual problem.  I understand it, but it's something that requires an understanding of physics.My biggest problem is trying to explain it in a concise way.  I decided to contact Paul over at the Blog Engineering Radio. Somtimes every engineer needs a consult. (thanks Paul)  Here was his response.
"Light, RF and all other EM energy travel through vacuums because they depend on the electromagnetic field, which is universally present. One might say that the EM field ends at the edge of the universe, or so the theory goes, because no one really knows for sure. Since space is mostly a vacuum, this allows us to see stars, transmit telemetry to space craft using radio waves, and listen for extra terrestrials since EM energy can transit through those area with little or no degradation."
Paul is right of course. If you read his blog you'd expect nothing less. So let's dig into some of the details. First the basic facts: In a vacuum, radio waves travel at the speed of light, or roughly 186,000 miles per second. The speed of light in a vacuum is C = 3.00 x 108 m/. It's the top speed except in science fiction. This is the speed which all electromagnetic radiation travels at in a vacuum: light, radio, microwaves etc. In a vacuum all electromagnetic waves obey the inverse-square law.  It's power density is proportional to the inverse of the square of the distance from the source. But if the radio waves are propagating through a medium other than the vacuum, their speed will be less than C and vary to the medium. But the medium always diminishes it.

The problem after that is most lay people imagine it to be like a sound wave.  This is easier to grasp but is somewhat inaccurate. Sound waves move through matter. They vibrate air and are also travel only at the speed of sound which is actually variable. It travels faster in water than in air, that's 1,484 m/s compared to 343 m/s. That's a 70% different. But that vibration is traveling through matter and is bound by it. This was  first predicted by mathematical work done in 1865 by James Clerk Maxwell. I wrote about him previously.  Even then he predicted that it moved like a light wave. In 1887 Heinrich Hertz generated those waves in a laboratory confirming Maxwell's theory.

According to Einstein the photon is the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force. He believed mostly in a particle-based model. Photons have no mass, do not decay, carry no energy but spin and can be polarized. Their momentum is proportional to the frequency of their vibration. This is contrary to the wave-model. I am much more comfortable with the wave model, as are most despite it's short comings. Modern quantum mechanics fail to completely reconcile the two ideas.  Today they proceed with an idea called wave-particle duality. Isaac Newton originally described light particles as wave packets. He wasnt exactly right but we do consider photons to be "discrete bundles."  The energy of light is a discrete function of frequency or E = nhv. link

That is where it stops making sense for most people. Defining a vacuum is difficult. we've struggled with it since the ancient Greeks. Just because a vacuum contains no matter does not mean that it's truly a vacant space. It's full of electromagnetic fields, gravitational effects, radiation pressure, vacuum energy etc.  After that it's all Poynting vectors and the Lorentz Invariant. It becomes quite literally philosophical.  It is a wave or a particle?  Do waves exist or are they an artifact of the of the relative movement of the observer?  Is it just an excitation of the vacuum state?  In truth I don't think we have a definitive answer. I recently read that radio waves can be made to travel faster than light. link And that does not make this any easier to understand. In the end being succinct of being complete eon this specific topic.