Friday, June 28, 2013

Science Friday: Einstein's Special Relativity 1

One of the things on my bucket list is to understand an equation in quantum physics called Schrödinger's equation before I die.  For anyone who thinks Wikipedia is for lightweights, tell me what you think of this page.

Off and on for years I have tried to find an entry point, but I usually don't even get through the starting point of quantum physics, Max Planck in 1900.  Schrödinger proposed his equation in 1925. I would love to grasp those 25 years in physics, let alone get beyond them. Dare I share one of my novel starts (I've started dozens)--a quantum physicist who has an accident in which his brain damage leads him to struggle to relearn some of what he formerly knew so well, only to achieve the barest of success.

I found a delightful book recently by Alan Lightman called, The Discoveries. Not that I have made much progress in it either. But he has some of the most groundbreaking scientific essays of the twentieth century in it, and he gives nice introductory essays.

I feel almost ready to write one "chapter" in those groundbreaking first 25 years.  In 1905, Einstein published four groundbreaking papers, including one that introduced his idea of special relativity. I thought perhaps I could summarize that paper slowly here.
"On the Electrodynamics of Moving Bodies," Albert Einstein, 1905.

Einstein starts by noticing that the famous theories of James Clerk Maxwell (1831-79) lead to some inconsistencies. Maxwell was a Scottish physicist who seemed to establish that light was a wave and at least established that electricity and magnetism were manifestations of the same basic phenomenon. His "electromagnetic theory" was the greatest achievement in physics since Isaac Newton (1642-1727) and paved the way for the developments of twentieth century physics (not to mention the radio, television, and cell phones).

The first paragraph of Einstein's essay points out some of the problems Maxwell's theory had left. For example, science at that time had two different explanations for the current created in a wire around a magnet, depending on whether you moved the magnet through the wire or the wire over the magnet. If you moved the magnet through the wire, science said that the magnet generated an electric field around it that caused current in the wire. But if you moved the wire over the magnet, science did not say there was an electric field over the magnet. Instead, it said that an electromagnetic force was created in the wire. The current generated, however, was exactly the same.

[Note: I do not really grasp the relevance of this particular example at this point]

In the second paragraph of the introduction, Einstein sees a solution to these sorts of anomalies in the reconciling of two principles that were already accepted but that seemed contradictory.

First, there was the "principle of relativity" that had been established three hundred years earlier by Galileo.  Physical laws operate the same in any "inertial frame of reference," that is, in any collection of items moving together at a constant velocity. The recent laws of electromagnetism set down by Maxwell were no exception: "the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good." (72).

The second postulate on which Einstein will base his theory is the "theory of light constancy."  "Light is always propagated in empty space with a definite velocity c which is independent of the state of motion  of the emitting body" (72). In other words, light will move at 300,000,000 meters per second whether it shines from a spaceship moving 100,000 meters per second or from someone with a flashlight in your back yard. Einstein himself was not the first to suggest this constancy.

Einstein was keen to resolve the apparent contradiction between these two postulates. How is it that light does not move faster off a moving truck than it does from someone standing on the side of the road? They are two different inertial frames of reference and so, relative to each other, the velocity of the truck should add on to the velocity of light relative the ground.

Maxwell's equations worked for "stationary bodies." That is to say, they worked within the framework of a single inertial frame of reference. Einstein's goal in this essay is to present a "simple and consistent theory" that will work for bodies moving in relation to each other, to present a theory relating to the "electrodynamics of moving bodies."

As a side benefit, he aims to show that the notion of a "luminiferous ether" is superfluous. Since Maxwell had seemed to show that light was a wave, the question had arisen as to what sort of a medium the wave moved through.  Water waves moved through water.  What did light waves move through?  At the time, physicists assumed there must surely be some sort of invisible medium through which light moved, something they called the "ether."

But experiments had failed to show any ether (e.g., the Michelson-Morley experiments). The ether gave a sense of absolute space--there would be something at rest at every point of the universe. Einstein's theory ends up negating the notion of "absolute stationary space" or absolute rest in space.


Anonymous said...

Thanks for not letting my brain rot during these two weeks off from school! Light is cool stuff -it's God stuff. Light moves at a constant speed in a vacuum, but does it change its speed moving through stuff? What stuff was there on day one of God's creation? And how much of a third dimension did that stuff have? Was it different than the light from the lit-up things God made on day four? The light from day four had the atmosphere to contend with, yet there is no line of demarcation between the heavens of outer space and the sky with our atmosphere in it. Also, if light in effected by stuff, then how can it travel in a straight line? For instance, if light is being beamed out to earth from a distant star 100 light years away. The light I'm seeing today was emitted from that star some exceedingly long time ago. What is holding that light in a presumably straight line towards me from where it left off? And where is that star now? Is that why nobody will know when the end will come? Susan

Ken Schenck said...

Light doesn't go through everything, but it seems to move at the same speed no matter where it's moving. I'm not sure whether it moves entirely in a straight line because, from one point of view it travels as photon particles. Others may speak more informed...

Anonymous said...

Gosh, you know what? To be totally honest, those were my feel-out-this-arena questions.
Ok. So, say light travels at ten miles per hour, and is leaving an explosion towards me that occurred four hours away from me if I travelled there at the speed that light travels. That means I will see the initial explosion about four hours after it actually occurred. Correct?
Light actually travels at about 669,000,000 miles per hour. And say there were explosions in the heavens that occurred three years away in travel time, travelling at the speed of light. That means three years after the initial explosion, I would just start seeing that explosion from Earth. I wonder if that is why no one will know when the end will come (because of that God-built delay factor in the slowness of the speed of light). "There will be signs in the sun, moon and stars. On the earth, nations will be in anguish and perplexity at the roaring and the tossing of the sea. People will faint from terror, apprehensive of what is coming on the world, for the heavenly bodies will be shaken. At that time they will see the Son of Man coming in a cloud with power and great glory. When these things begin to take place, stand up and lift up your heads, because your redemption is drawing near" (Luke 21:25-18). Thoughts? Thanks. Susan