A Crash Course in Relativity and Quantum Mechanics


By Bradley Stockwell

In this post, I’m going to attempt to highlight the basics of the two most successful theories in all of physics, relativity and quantum mechanics. Relativity governs objects on astronomic scales; planets, stars, galaxies and so forth, while quantum mechanics governs objects on subatomic scales; particles smaller than an atom. I’m also going to address why while these theories have been proven beyond a doubt, when combined together they are completely incompatible. A theory that successfully combines the two, a theory of quantum gravity, aka “the theory of everything”, is the holy grail of physics because it holds the key to the origins of our universe.

Relativity describes gravity and the universe in a beautifully simplistic way. Think of nothing more than a ball sitting atop a blanket stretched out between two people. The tautness of the blanket between the two people always remains constant. In the language of physics, this is the gravitational constant. The blanket is the fabric of the universe, called spacetime and as the name suggests, not only is it made of space, but also time. This is why the blanket analogy isn’t exactly accurate, because technically spacetime is four dimensional; three dimensions of space and one of time. However the analogy makes the concept easier to comprehend. The ball represents an object in space. The more massive an object, the more it depresses, or distorts the blanket. The distortion is what is known as a gravitational field. The more massive an object, the greater its gravitational field and ability to attract less massive objects. Say a large ball was placed on the blanket with some smaller ones. The smaller balls would naturally gravitate towards the larger one, like planets in our solar system to the sun.

Now one logical question probably came to mind when visualizing this, what keeps those smaller objects from falling completely into the larger one? Or what keeps our planets in orbit instead of falling into the sun? The answer is angular momentum. Objects in space are always moving. If it wasn’t for the gravitational pull of more massive objects, these objects would travel in straight lines. It is this inertia that keeps them in orbit instead of being pulled in completely. Think of a skateboarder riding the walls of an empty bowl-shaped pool. The skateboarder’s momentum keeps him glued to the walls and if he were to stop, he’d roll into the center of the pool. The planets are doing the same thing. They are riding the walls of the depression the sun makes in the fabric of spacetime.

Gravity keeps everything very orderly and very predictable because it obeys certain laws. This is how astronomers are able to predict the timing of cosmic events with amazing accuracy. Isaac Newton was the first to recognize these regularities and calculate them. He believed gravity was the attractive forces between objects in space. This theory survived for over two hundred years until Einstein realized gravity’s close connection to time. Gravity was not an attraction, but a distortion in the fabric of space, spacetime. And when this fabric is distorted, not only is space distorted but so is time. Time is relative to the observer and how distorted spacetime is at his, or her position. Time moves slower the closer you are to an object of mass because the distortion of spacetime is greater. This is called gravitational time dilation. For example, if you were to stand atop Mount Everest, time would move faster (an unperceivably small amount) for you than say someone standing at sea level because those standing at sea level are closer to the gravitational source (the earth) and deeper within the ‘gravity well’. This has been measured in experiments with atomic clocks at differing altitudes and clocks on GPS satellites have to be regularly adjusted because of this. So technically your head ages faster than your feet.

Gravitational time dilation however only applies to relatively stationary objects. Since you atop Mount Everest and the person standing at sea level are both standing on Earth, you both are traveling through spacetime at the same speed. If you or the other person begins to move, another form of time dilation needs to be applied to receive a correct calculation called relative velocity time dilation. According to relative velocity time dilation, the faster an object moves through spacetime, relative to another object, the slower time moves. Time dilation of relative velocity is much stronger than that of gravity. This can be observed with astronauts aboard the International Space Station. According to gravitational time dilation, because of their higher altitude, the astronauts should age faster than humans on Earth. But because they are moving much faster than humans on Earth, they actually age slower—about .007 seconds per six months. As strange as this all sounds, not only has this effect been proven by atomic clocks, but a measurable difference in brain activity has also been observed.

Gravity and time can be counteracted by velocity and this is a key component to the wackiness of quantum mechanics.  As I said previously, according to relativity, the faster an object moves through spacetime relative to another object, the more time slows down for that object because the effects of gravity are less until time and gravity become completely irrelevant at the speed of light. To understand this better, let’s go back to the blanket and balls analogy. Think about one of the smaller balls in orbit stuck at the same speed going around the larger ball. Gravity and angular momentum regulate this orbit and speed. But again, gravity is not a strong force; it can be beaten. Let’s say you take another ball and skip it across the surface of the blanket. The effects of gravity, which include time, are less on the ball you threw than the idly orbiting one. Time, compared to the idle ball, moves slower for the one you threw because gravity doesn’t have the same grasp on it. If you were somehow able to throw a ball at the speed of light it would travel so fast that it would catch up to the point before you ever threw it, like a jetfighter catching its own sound waves. However unlike the jetfighter which can break the sound barrier and outpace its sound, it is impossible for the ball to outpace the speed of light, at least as far as we know. At the speed of light the ball is stuck in the light barrier; its future, present and past existences become piled on top of one another and it has now entered the strange world of quantum mechanics. The ball exists at every time in every place. This is what existence is like within an atom because everything within it moves at, or nearly at the speed of light. This is where the term quantum mechanics comes from. The position and time of particles on a subatomic scale can’t be predicted to an absolute certainty. They can only be ‘quantized’ into approximate probabilities.

Let’s go back to our theoretical ball traveling at the speed of light across our spacetime blanket. I’m going to say this ball is a particle of light, a photon, because a photon is massless and only massless particles are able to travel at the speed of light. The reason being, if the ball had any mass it would distort spacetime and therefore has to play by the rules of gravity and consequently time. As we learned in my previous post, a photon is timeless. It is timeless because it has no mass and without mass, time and gravity have no grasp on you. Even if the ball had massive (pun intended) amounts of energy, it could never reach the speed of light because the energy needed would require an infinite amount of mass. As we also learned in my previous post, mass equals energy. The more energy required, the more mass required so the two counteract each other.


Light from a star being bent by the curvature of spacetime around the sun so that it appears to be in a different position. Einstein’s Theory of Relativity was proven by comparing the known positions of stars with their positions behind the sun during a solar eclipse. Light travels over the contours of spacetime which are created by massive objects within it. The star’s light bends around the distortion the sun’s mass creates in spacetime and appears to be in a different position when viewed from Earth.

However there are places in the universe where even massless matter, like light, has to give into gravity and the worlds of relativity and quantum mechanics come crashing together. Those places are called black holes. Although light can escape the grasp of gravity, it still has to travel along the contours of spacetime that gravity creates. Imagine our theoretical photon ball traveling across the dips and dents of the blanket created by the mass of the other balls. Now imagine if it were to encounter a hole in the blanket; there would be no escape. A black hole, the afterbirth of a collapsed massive star, is something so massive it completely rips through the fabric of spacetime. This is where the theory of relativity fails. The most dreaded symbol in physics is an infinity sign. If your calculations end in infinity, it means your theory has failed. When calculating the gravitational field of a black hole using relativity it ends in infinity; an impossible amount of mass has to be compressed into an impossibly small area, called a singularity. A spacetime singularity is where the quantities normally used to measure gravitational fields become so strong they curve in on themselves. They become looped much like the wacky world of quantum mechanics. Naturally you’d think a combination of the two, a theory of quantum gravity, would solve this. However it produces an answer that is infinity plus infinity for an infinite amount of times. It is an infinitely bigger failure than relativity at predicting the mechanics of a black hole. String Theory is the closest we’ve come to solving this, but it has yet to be definitively proven. If we could find a successful theory of quantum gravity, we’d most likely discover the origins of our own universe. The reason being, our universe has been continually expanding for the last 13 billion years from a singularity much like the one found inside a black hole. Our universe could be the answer for what’s inside a black hole—well at least one black hole. Pretty trippy to think about, right? Until next time, stay curious my friends!


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