Tuesday, February 5, 2013

It's all Relative

Hey everyone,


            I want to take this time to talk a little bit about the physics class I’m taking this semester, ‘Quantum Physics and Relativity’. As the course title implies, the class covers two different, and quite distinct, topics of physics. The first month or so of the class is devoted to the study of special relativity, which is a corrective theory of mechanics for situations in which classical Newtonian physics breaks down. The rest of the course from there surveys various areas of quantum physics. Also a correction to classical physics, quantum physics is a comprehensive treatment of the submicroscopic universe. It turns out that the laws of chemistry and physics are completely different at the quantum level. The quantum physics portion of the class will introduce the basics of quantum mechanics and then move into atomic, molecular, nuclear, and solid-state physics.

            The part of the class on special relativity, which we’re doing right now, is mostly what I’d like to discuss in this post, specifically the history behind the emergence of modern physics. Physicists generally take the time-threshold from classical physics to modern physics to be around the start of
the 20th century, although much of the foundational work that led to modern physics was conducted in the late 1880s and 1890s. Around this time scientists believed that we knew just about everything there was to know about our physical universe, with only a few mysteries left to solve. Galileo and Isaac Newton both contributed a vast amount of knowledge and discovery to mankind, and their theories and works make up what is generally referred to as classical physics (or Galilean/Newtonian physics). What these two men and many others discovered and developed was certainly ingenious, and the classical treatment of physics is quite valid in everyday situations that humans are accustomed to. If you’re doing a physics problem involving a car driving down the highway at 100 mph then by all means classical mechanics is valid and will lead you to the correct answer. However, some problems arise when we try to use classical physics to treat objects that travel at near-light speeds. The equations used in Newtonian mechanics theoretically allow for any desired speed to be achieved without bound, but we of course know that nothing in the universe can exceed the speed of light (the speed of light is 300 million meters/second; one light-year, the distance light travels in a year, is roughly 25 trillion miles). Newtonian mechanics also assumes that time is universal, which we now know not to be the case.

            In the study of waves, one learns that mechanical waves (water waves, seismic waves, sound waves, etc.) require a medium to propagate through (water waves require water and sound waves require air molecules). Electromagnetic waves, or light waves, on the other hand can propagate through empty space without a medium, but this was not always thought to be the case. Scientists prior to the 1900s thought that light waves similarly needed a medium, and they thought that this medium was this mystical, massless medium which they called ether. The finer details of all this gets pretty messy, but in 1887 the famous Michelson-Morley experiment failed to detect this “ether” (Interestingly, it just so happens that one of these physicists, Alfred A. Michelson, was a professor of physics at Clark University from 1889-1892.) This result uncovered many profound consequences and really revolutionized physics as we knew it, and is what led to the development of the theory of special relativity. The theory is based on the concept of reference frames. Two observers of an event will perceive information about that event differently if they’re in two different frames of reference. That is, if one observer is stationary but the other is moving, or if they’re both moving at different speeds and orientations, they will measure properties of that event differently (i.e. time, velocity, or applied force). These relativistic effects are negligible for events at speeds to which the human eye is used to but are quite prevalent at near-light speeds and in many electromagnetic phenomena, and some very puzzling things happen under these circumstances.


Two interesting effects are length contraction and time dilation. An object will appear to be shorter in a reference frame other than its own, and time actually passes by slower in different reference frames. And stemming from time dilation is the relativity of simultaneity, which demonstrates that two events which occur simultaneously in one reference frame (at the exact same time) are not simultaneous in any other reference frame (there is a famous thought experiment called ‘Einstein’s Train Paradox’ which beautifully demonstrates this notion.) In electromagnetic studies we see another interesting consequence of relativity. Any electric charge inherently has an electric field associated with it, and so an electric charge, moving or not, in some reference frame will always have an electric field around it. A charge in motion, however, is what defines an electrical current, and the presence of a current creates a magnetic field around that current. And so a charge that has no magnetic field around it in one reference frame does indeed have one, and all of the magnetic properties that come with it, in another frame.

Special relativity is extremely difficult to learn. Mathematically it is relatively (huh, no pun intended!) simple, but on a conceptual level it challenges most people’s normal, conventional way of thinking about the universe and at first glance often appears completely illogical. Things are equally strange and difficult to conceptualize in quantum physics, which I’ll be learning very shortly. Special relativity is designed only for Euclidean geometry, which assumes the structure of flat, three-dimensional space to which we’re all accustomed. General Relativity on the other hand makes the same corrections but applies them to non-Euclidean space, in which gravity distorts the geometry of the universe and causes a curvature of space and time. The general theory of relativity, developed by Albert Einstein and published in 1916, is truly one of the most amazing intellectual masterpieces of human thought and has since had much experimental evidence found in favor of it.

Relativity and other developments of modern physics left much of the scientific community in awe a century ago. This was a time when we realized that we were wrong about 95% of the things we thought we knew about the universe. It was of course an overwhelmingly frightening revelation, but, perhaps more importantly, it generated a great and powerful excitement and stimulated an almost Renaissance-like resurgence of scientific experimentation and discovery. What followed throughout the 1900s was glorious. Quantum mechanics exploded onto the scene in the 1910s, 20s, and 30s. And as I said, it certainly was quite unsettling to see our bank of knowledge crumble so rapidly towards the turn of the century. But it made for so unique a time, a time not quite like any other in human history. As Albert Einstein once said, “It was a marvelous time to be alive.”

Thanks as always for reading!

1 comment:

  1. The most important part of SR is the relativity of Simultaneity - it led directly to GR (See Einstein's 1907 first ever paper on GR).

    Have you seen the illustration for "Relativistic Bug Capture" in the Wikibook on Special Relativity? This would certainly help to teach your students. The Andromeda paradox is also central.