A relatively brief introduction to modern physics

Looks good from what I got through, first four posts, I'll have to come back and get the rest when I'm sober and feel like learning.

Looking good, I love Physics.

The Wave-Particle Dual Nature of Light

In the 17th century there was much debate about the nature of light. Newton advocated a particle nature of light, while others argued that light was a wave. Early in the 19th century, a scientist named Thomas Young carried out an experiment which is now known as the double-slit experiment. At that time Young’s conclusions nullified Newton’s theory on the nature of light.

It is worth describing Young’s double-slit experiment here, as it will provide a startling insight into the nature of matter in the next section. His experiment was simple, and is easily reproducible. Essentially, it consisted of three flat pieces of cardboard, a powerful flashlight, and a wall to shine the flashlight towards. Each piece of cardboard had slits in them. The first piece had a slit towards its left side, the second piece had a slit towards its right side, and the final piece had two slits, one on the left side and one on the right side. When the piece of cardboard with the slit on the left side was placed between the beam of light and the wall, the wall had a single “streak” of light towards the left as expected. Similarly, when the cardboard piece with the slit on the right was placed between the wall and the beam of light, a streak of light towards the right was observed. If light truly has a particulate nature then when the third piece of cardboard is placed between the beam of light and the wall then we should see two streaks of light. Instead, what is observed is an interference pattern, a wave phenomenon.

Any amount of cash you have can be represented by an integer number of pennies. This is the fundamental American currency denomination. In 1900, Max Planck postulated that there is a fundamental energy denomination for light. Light can carry energy only in integer multiples of this fundamental energy denomination. Planck also postulated that the energy carried by light is proportional to its frequency.

The phenomenon that gave us insight into the particulate nature of light is known as the photoelectric effect. By the beginning of the 20th century the phenomenon had been well established, however there was no universal concurrence in the physics community as to what the consequences of the observations were. Once again, Einstein stepped in to enlighten the physics community. He would eventually be awarded the Nobel Prize for his explanation of the photoelectric effect.

In first year chemistry, you learn that metals have loose valence electrons. This is why they are such good conductors for electricity. This is also the basis of the photoelectric effect. When light strikes a metal, it dislodges some of the valence electrons. This in itself is not too puzzling. What baffled physicists was the fact that no matter what the intensity of the light was, the dislodged electrons moved with the same speed. Intensity of light only affected the number of electrons that were dislodged.
However, if the frequency (color) of the light was changed, the speed that the electrons were ejected at changed. What was more bewildering was the fact that below a certain frequency, no electrons were ejected at all despite the blinding intensity of the light.

Einstein postulated that light was quantized in little packets called photons. This simple assumption explained all the observations of the photoelectric effect. Instead of the light-wave being dispersed evenly throughout the entire metal, implying that the intensity (total energy) of light should directly affect the energy of an ejected electron, a single photon only carried a fixed amount of energy, and this is why an electron’s speed was invariable with intensity since only a single photon will strike an electron. It also explained why electrons weren’t ejected below a certain frequency. The energy carried by the photon was simply not enough to dislodge the electron at these low frequencies. With increasing intensity, more photons were fired at the metal; therefore more photons struck and dislodged more electrons. Hence, the particulate nature of light was revealed.

This is some of the last stuff we talked about in my last physics class. And that each point of a wave can be a source. Like with light any point on the wave can be a point light source. Electrons act like both particles and waves too methinks.

Electrons act like both particles and waves too methinks.

That's where the real weirdness starts. That will be my next post...either tonight or tomorrow. Depends on how bored I am.

Wave-Particle Dual Nature of Matter

Things get increasingly peculiar from here on out. Relativity forced us to reexamine our basic assumptions of time and space, but quantum mechanics is absolutely absurd. It would appear at the quantum level the universe acts in a way so obscure and contradictory to the familiar that even physicists have trouble making sense of it.

In 1923, Prince Lous de Broglie reasoned, roughly speaking, that Einstein, through special relativity, equated matter with energy, Planck related energy with waves, therefore matter may have wave-like properties as well. De Broglie’s theory was soon verified experimentally by the double-slit experiment. Rather than using a wall, however, a phosphorescent screen was used to track the end-locations of electrons. Even if a string of single electrons were fired they somehow found a way to interfere with each other, creating the interference pattern akin to waves. This was a remarkable discovery. Physicists were forced to conclude that matter exhibited wave-like properties in conjunction with particle properties more commonly associated with matter.

There is another interesting phenomenon associated with this experiment. What if you wanted to ‘look’ and see which slit an electron went through? One way to do this would be to shine light on the electron to see it – that is bounce photons off of it. The problem with this approach is that by doing this, you have modified the experiment. On macroscopic scales the energy carried by a photon is negligible, but when dealing with something as miniscule as an electron the energy carried by a photon could severely alter the trajectory of the electron. When this is done to the double-slit experiment, the resulting pattern on the phosphorescent screen is no longer an interference pattern, but it is a pattern we would expect if electrons only had particle properties. It’s as if the electrons know they are being watched and respond accordingly!

The logical question that follows from these realizations is how do these conclusions coincide with real world experiences. When de Broglie mathematically determined the wavelength of matter waves, he found that they are proportional to Planck’s constant. Since the constant is so small, the wavelength of matter waves are also consequently small.

The next question I suspect most people would have is what exactly is matter a wave of. The answer proposed by Max Born in 1926 is perhaps the most shocking conclusion of modern science. Born suggested that the electron wave is actually a probability wave of where the electron will be found. In other words, at the fundamental level, the universe intrinsically has a seed of randomness. This has interesting consequences on the philosophy of determinism, which I intend to discuss in the next section.

This means that if an experiment involving a fundamental particle of the universe such as an electron were repeated multiple times in an identical manner the results would vary with each experiment. Erwin Schrodinger defined an equation for the probability wave. Though quantum experiments can’t be reproduced identically, the probability wave created by iterated experiments can be mathematically modeled, tested and reproduced. The probability wave has been tested and reproduced with high-fidelity making Born and Schrodinger’s counterintuitive suggestions a valid scientific theory and a seemingly accurate description of the quantum world.

Following WWII, Feynman took quantum theory in a new direction. Richard Feynman’s perspective did not change the mathematics behind quantum mechanics but he provided a new way to analyze the probabilistic nature of matter. Feynman theorized that electrons did not travel through only one slit in the double-slit experiment, but through both slits simultaneously. Not only that, the electron traversed every possible trajectory it could take. It went smoothly through the left slit. It went zig-zagged through the right slit. It took a trip to the Andromeda galaxy and came back. Feynman assigned a number to each of these paths. When he averaged them out he found the exact same result as the probability wave of Born and Schrodinger. This drastic change of perspective alleviated the quantum world from the compulsory probability wave but proposed something perhaps more bizarre.

Once again, it is important to answer why these effects are not noticed on a macroscopic scale. Feynman showed for all particles larger than atoms his method of assigning numbers to paths allows all paths except one to cancel each other out, What we’re left with is the path that us macroscopic sentients are familiar with.