Mike Nicholson

There was once a time when physics made sense and seemed to explain everything. This time started when Sir Isaac Newton, probably the greatest physicist of all time, proposed that the same force that caused an apple to fall before him also controlled the orbit of planets in the heavens. His equations unified gravity by assuming that the distances in space and rate at which time passed remained constant regardless of where any observer was or how fast they were moving.

This Newtonian model of physics, which was more than adequate to plot our course the moon, fit upon a single sheet of paper and seemed to explain everything… well, almost everything. A few things still seemed a bit out of place: for instance, the orbit of the planet Mercury did not quite match the predictions made using Newton’s equations. Mercury, being so very close to the sun, though, was difficult to observe and most scientists presumed that future discoveries would resolve this problem and reinforce Newton’s equations.

This was until nearly 200 years later in the 1880s when experiments began to find that the velocity of light remained constant regardless of the velocity of the observer or the source of light generation. This defied the Newtonian model in that light, like any other tossed object, should retain the velocity of the generator in addition to its velocity just as a tennis ball thrown from a moving car retains the velocity of the car in addition to that of the throw and can really hurt a pedestrian that it hits.

To phrase this in easier terms, imagine that you are in a space ship traveling at 1000mph zooming by your friend who is standing on the moon. You throw a tennis ball from your space ship at 30mph in the same direction that you’re traveling. To you, the tennis ball appears to move away from you at 30mph. To your friend the tennis ball appears to zip by at 1000mph plus the additional 30mph from your throw making 1030mph. On your next pass you accelerate to 99% of the speed of light. This time you shoot a laser beam. You see this beam of laser light travel away from you at the speed of light, while your friend sees it travel not at 199% of the speed of light like in the tennis ball example, but at exactly the same speed of light that you observe.

So confounded by this apparent impossibility, scientists repeated these experiments for many years until a young patent clerk named Albert Einstein realized the solution. The reason that all observers, regardless of their velocity, observed light proceeding at the same speed was because it really was traveling at the same speed. Light was not the problem, but instead our assumption that distance and time remained constant were fatally flawed.

He proposed, instead, an altered set of Newton’s equations. These equations allowed for light to always remain the same velocity by causing rate of progression in time and the distances across space to warp in order to balance out the math at very high velocities.

These only slightly more complex equations explained why when you shoot the laser beam from your space ship while traveling at 99% of the speed of light, both you and your stationary friend see the light travel at the same speed: time for you has slowed down greatly and the distances you’re crossing in space have become shorter to you relative to your friend on the moon!

The first implication of this change in perspective is that two observers can no longer always agree about the simultaneousness of events. For instance, if you were again on your spaceship traveling at the same 99% of the speed of light, and your friend on the moon had rigged up two light bulbs, one behind him and one in front of him, to turn on at the exact same time as when you pass him, you would see the light bulb that you were traveling towards turn on first even though he would see them illuminate at the same time! Although this may seem to allow an event to occur before its cause, it does not. Since nothing can travel faster than the speed of light, we cannot travel at a high enough speed such that the light bulb we were traveling towards turns on before we see him toggle the light switch.

The next, and most famous, implication is that energy is unified with matter. This reveals the famous equation: E = mc2. The way that we’re prevented from exceeding the speed of light is encapsulated in this equation. As we attain higher and higher velocities, our mass increases to compensate. As we approach the speed of light our friend on the moon would see our mass increasing such that it’d reach infinity just as we reach the speed of light. Since our engine can only produce a finite amount of power, or energy per time, we’d never actually be able to reach the speed of light.

This equation can be used outside of relativistic (extremely high velocity) settings too. For example, as we compress a simple spring the deformation in the metal causes it to increase very slightly in mass in a way that can be modeled with the exact same equation; however since we’re not adding very much energy, this increase in mass is too small to easily observe. Every time energy increases so does mass and conversely every time energy is released mass decreases.

So now they had it. Special relativity gave the world the required physics to predict the energy yield of atomic bombs and understand why light always appears to move at the same velocity to everyone, everywhere. But, the orbit of Mercury still remained unexplained. This would take the same man, Albert Einstein, another ten years to uncover with his theory of general relativity. This theory is a whole different story, though.