Sorry for the delay, but I’ve been busy trying hard to keep this as simple as possible and Friendster isn’t helping. Now, back to where we stopped, Jason believed that an overview on the ether concept was not necessary (and yes, complicated) in our discussion on relativity but I believed otherwise. Why bother about history and the outdated theories? Well, if we do not bother, it’ll ignore the creative aspect of science and thus give a false impression of how science develops, to quote the author a physics reference text. Besides, it’ll make the long storytelling less of a story and more of a lecture. Plus, I think it’ll help in understanding the whole ’story’.
Ok, on with the storytelling. Towards the end of the 19th century, it was believed that physical theories are more or less complete. Newtonian mechanics and Maxwell’s theory of electromagnetism predominates. There were a few quirks left unresolved, but it was believed that they would eventually be explained using existing principles. Oh boy, they were so far from the truth.
Newtonian mechanics were essentially the same from Galilean times, albeit with some improvements. Basically, the laws of physics are the same for all inertial reference frames. This is known as the principle of relativity. Newton also strongly believed in the concept of absolute time, a fundamental quantity separate and unrelated to the 3 spatial dimensions of space. This was, in part due to his religious beliefs, as if time is not absolute, the question “Whence did we come forth?” would simply become null as everybody would have their own ‘personal time’ and everybody would have their own date of coming into being. Then, whose personal time is right in determining when “then there was light”?
Now what does that hodgepodge the laws of physics are the same for all inertial reference frames mean?
Basically, in these frames, all objects obey the law of inertia (hence the name inertial reference frame). For example, in a car being stationary or moving at constant velocity, everything in the car is at a standstill until you apply a force. When the car drives over potholes, the car (and everything in it) is now a noninertial reference frame, as things in the car acquire motion without a force being applied to it. It’s as simple as that. And, in all these reference frames, the laws of physics are the same. This means that in their own reference frames, by using the same set of physics equations, they can accurately predict the changes in their surroundings. (Go refer to Jason’s blog for a more detailed explanation)
At the moment, just focus on the inertial reference frames, as the special theory of relativity is only ’specific’ to them, hence the name ’special theory of relativity’. Now, as long as two frames are in uniform motion (where they don’t accelerate nor decelerate), they’re both inertial reference frames. This is where the fun starts.
Classical mechanics dictate that when train A moves at 7km/h relative to you, and train B is moving at 9km/h relative to you, both moving in the same direction, again relative to you, you would observe train B overtaking train A at 2km/h. Same thing applies if train A is stationary relative to you, and train B goes at 2km/h relative to you. In both cases, train B overtakes train A at a speed of 2km/h, relative to you.
Maxwell’s EM theory came up with an absolute value for the speed of light, c! As you should know, c is approximately close to 300,000km/s (I shall assume this value henceforth for simplicity). Now, let’s go back to our A and B objects. From the classical perspective, if train B moves at 150,000km/s relative to you, the observer and train A moves at speed c, 300,000km/s relative to train B, doesn’t that mean that train B will be overtaken by train A at a speed of 150,000km/s, relative to train B? And from your point of view, who is stationary relative to both of A and B wouldn’t that mean that train B is moving at a speed of 450,000km/s? If train B is a light wave, then wouldn’t it mean that Maxwell is wrong, relative to the observer? But in the first place, if light is a wave moving at a speed of 300,000km/s, what is that speed relative to?
Hence the search begins to find the ‘absolute reference frame’, where light travels at speed c relative to it. In essence, this is actually in violation of the principle of relativity. As the principle dictate that the laws of physics are the same in all reference frames, that implicitly implies that there is no such thing as an ‘absolute reference frame’, where the laws of physics would be in their ‘truest’ form. But we’ll leave that ‘violation’ for later. Now, light actually exhibits both wave and particle properties (described in physics by the innocuous name of wave-particle duality). Analogous to the principle of sound waves having a fixed speed in the medium in which it is traveling in, it was proposed that light propagates through the universe in an all-permeating medium called the luminiferous ether (also spelled aether). It was presumed that relative to the ether, light travels at speed c. So problem solved?
Not so. By proposing the ether medium, they added more questions to the situation, naturally. Firstly, the medium must be pretty vacuous or else we would have noticed the presence of the medium in our everyday lives (by impeding our motion). And the other thing is that since light propagates so quickly through the medium, the ether must be very stiff (analogous to the sound wave that travels faster in solids than air). So it must be pretty vacuous but yet extremely stiff, two contradictory properties. But if we abandon the ether concept, we must be able to answer the question "Light travels at speed c relative to what?" Enter the alternative.
Remember that the laws of physics are the same for all inertial reference frames? We can apply this and say that Maxwell’s equations should hold in all inertial reference frames. And that also means that the speed c is the same in all reference frames, even if they’re moving relative to each other! That’s an even bigger headache no? So the physicists at that time decided to stick with the ether even as the concept appears to violate the principle of relativity.
This was essentially the dichotomy physics was in at the turn of the 20th century. Classical mechanics obeys the relativity principle but Maxwell’s equations do not. To try to solve the problem, it was proposed that the speed of light is c relative to the ether, just like the speed of sound moves at 330 m/s in air. Maxwell’s equations would have to be modified for the motion of different reference frames. So the question of Earth’s motion relative to the ether comes into play, and that is what the Michelson-Morley experiment aims to find out. To their surprise, their experiment was an abject failure, as they detected no change in the speed of light when the earth is moving in different directions as it moves around the Sun!
In the wake of the experiments, our genius Einstein was wondering for about a decade with this question in mind: "What would I see if I run along a light wave at speed c?" This gedankenexperiment, or ‘thought experiment’ would later serve its purpose in showing him the way to go about the dichotomy in Physics at the turn of the 20th century.
P.S.: This was assembled together with minimal reference to text, and largely from memory so please point out any errors in my text. Thanks! And if you do have questions, however stupid they may seem, do post them. After all, a Physicist once noted in his book, if our human experience isn’t so limited, in the sense of not being able to move at relativistic speeds, relativity would have been a natural part of our lives and Einstein would have merely been stating the obvious. For the sake of clarity, km/h, km/s and m/s are the solidus notation for the units kilometers per hour, kilometers per second and meters per second respectively.
Everything I need within arm’s reach, with space to spare. On a related note, by this month’s end, I’ll be ready to dump my A-level Mathematics books for once and for all! I can almost taste the sweetness of triumph now, having conquered all of what it contains. Any buyers? They’re still in more or less pristine condition and they’re original books, so they’ll cost a bit. Soon, there’ll be more space on my desk, for something else to occupy. Perhaps time to relearn Physics or something. I still have plenty of areas to improve in that subject, so I’d better start ASAP.
