Tuesday, 19 January 2010

Radiation From Within

Can you classify the Sun and a steel ball in the same category? What do you think? I think so. I know the Sun isn't metalic, but it is shiny... right? Right? Historically, this is the mother of Quantum Physics. The very beginning. Plank did well.

Anyways, the Sun and the steel ball can be categorized as... A Black Body! Do think of it the wrong way, think of black body as a scientific object. Let's dissect the word first, then we can check other fun properties.

Black: it's a shade in our "colour" scale that as my grade 8 art and grade 9 science teachers put it, "Absorb all light". White is different. It reflects all light. So if a perfectly black object absorbs all light, it is unable to reflect any light. Using this defintion of black, we can then define Black Body as an object that absorbs all light directed at it without emitting any light back (reflecting light).

The Sun can be considered a black body (Before I go any further, there is no such thing as a perfect black body in our known universe. There is only what we can approximate to be black bodies.). But you ask, why are we able to see white light emitted? This is what puzzled physicist Maxwell Plank. Not only that but with old models, we would have the Ultraviolet Catastrophe. So what changed?

Maxwell looked at the Sun (not for very long though) and thought that maybe what is being seen, was thermal energy. Energy, in the form of heat, was being converted to electromagnetic energy (light). So he figured out that the old models were wrong, and that his new formula and idea of light packets (or quanta) would help explain the empirical (experimental) data (as shown in the picture).

This new model indicated that at certain temperatures, there was a maximum intensity for a given wavelength. As the temperature increases, the maximum intensity of the wavelength increases. If you take the integration of this curve, you get infinity. The area under the curve is the energy per unit volume.

This was the problem with the old model. When you integrated the curve from 0 to infinity (wavelengths) it would say that you get infinite energy! Most of this energy was in the ultraviolet spectra. This was cause for concern! How could something forever be emitting UV radiation. This did not get picked up by health critics because they did not know of the adverse affects of UV radiation at the time.

So in all, a black body is something that emits thermal energy in the form of light and light like radiation. Oh and by the way... The steel ball, it is considered a black body at about 1000 K or 1273 degrees Celsius.

Sunday, 10 January 2010

Bell's Theorem

*For this post, you will need to read the "Spukhafte Fernwirkung" posted on July 10th, 2009 in order to understand this post more. I have read about the topic and find it somewhat complex. I will do my best to explain it.*

Before we get started, lets look at the EPR paradox or the Einstein-Podolsky-Rosen Paradox. In simple terms, there are 3 parts to the paradox. In terms of a system:

  1. Without disturbing it, if we can predict a physical value with certainty, then there exists a physical reality that goes with the quantity.
  2. Measurements of particle at place A cannot instantaneously disturb another particle at place B because nothing goes faster than the speed of light.
  3. Any complete theory in physics must be able to predict all elements of reality.

As shown in the previous posts, Entanglement has the ability of instantaneously transmitting information from one particle to another (for example electrons, whether one is spin up or spin down). Using the logic in the paradox, Einstein insisted that Quantum Theory (More specific, Entanglement) was an incomplete theory.

John S. Bell, a theoretical physicist came up with a theorem that coincides with the EPR Paradox. It states that "No physical theory can produce the all same predictions of quantum mechanics."

This is easier to understand if I set up a scenario. A pion is a subatomic particle which, when it decays, produces 2 photons that move in exactly opposite directions. They are entangled, since they were produced by the samepion. Now we want to measure multiple properties of the photons. Due to Heisenberg's Uncertainty Principle, we can only take one measurement of the photon accurately. So here is the conundrum, if we measure one propertie of photon 1, we automatically know with accuracy the same property about photon 2. Are we able to measure a property of photon 2 with great accuracy then? If photon 1 has a measured spin in the x-direction, we know the spin for photon 2 to be the opposite, in the same direction. But can we measure the y or z direction of the spin?

According to Heisenberg, we can only know one thing really, really well. So this would break down the Uncertainty Principle. Bell setup a assumptions for his inequality to work:
  1. Logic is valid
  2. There is a reality separate from its observations.
  3. Information cannot travel faster than light
There is an invalid argument because the we do know that information can travel faster than light (contrary to Einstein's belief). Scientists believe that the statement of "Logic is valid" could be wrong. We do not have the right mathematics to see if our statements are true.