Why can't you go faster than light?

Over the past hundred years, scientists have pushed our understanding of the universe to some extreme conditions, for example, the world of the very small, the realm of very high velocities, and the frigid conditions near absolute zero. While each of us has developed an intuition about how the world works, it is very important to remember that this intuition only applies to a very limited set of conditions. For example, there is absolutely no reason to expect matter to act the same way in the center of the sun as it does here on Earth on a bright, sunny day.

That last statement is hard for some people to swallow, though, and judging by my email inbox, the extreme area that gives people the most difficulty is what happens when things go super fast. In 1905, Albert Einstein published his theory of special relativity. He predicts all kinds of mind-blowing things, for example, distance shortens and clocks slow down. He made another video about how clocks act at high speed. It turns out that all those seemingly crazy implications originate from a single cause, or maybe two if we take things slow. So first, let me tell you what this video is not.

He does not talk about the postulates that Einstein used to construct his intuition and he certainly does not derive the equations from it. Instead, this video attempts to tell you the key ideas that facilitate the development of a relativistic intuition. I hope to teach you why it is impossible to go

faster

than the speed of

light

. If you're not a physics fan, hearing that there is a maximum speed in the universe may surprise you, but it's true. And, if you're a groupie, you've probably heard that the reason you can't go

faster

than

light

is because mass increases when you accelerate.

It turns out that the explanation of mass changing as you go faster is wrong. I know that statement is going to confuse some people, including those with a fairly sophisticated understanding of relativity, but it's true. However, that leaves an open question. Why can't you go faster than the speed of light? It turns out that it is due to a combination of a deep, fundamental property of the universe and a fairly simple geometry. So let me explain how that all works. The first of the two crucial ideas is that Einstein taught us that space and time were not separate entities, but rather two components of a larger idea, called spacetime.

I'll give you a helpful visual way to think about this in a moment, but for now, trust me on this. Then we need to combine that idea with the observation that everyone sees that the speed of light is the same, no matter how fast they move relative to each other. Let's start with an analogy and then return to relativity. To understand the analogy, it is necessary to imagine a car driving on a huge flat surface. Additionally, you need to imagine that the car can only move at one speed, say 60 miles per hour. Or, so that the comments don't get filled with hate mail with metric snobbery, 100 kilometers per hour.

Now let's place a couple of arrows on the screen to point north and east. While we know the general speed the car is going, we don't know how much of it is heading east and how much is heading north. So let's take a closer look at that. The car can move completely in an easterly direction, meaning it has no movement in a northerly direction. Or, the car can move completely north and swim east. Or we can live dangerously and move northeast. In this case, we see that the car is moving in an east and north direction, with no direction getting all of the motion.

That's the central analogy and hopefully it's very clear. Now, let's introduce relativity. In relativity, we do not have east and north directions. Instead, we have space-time. Let's imagine that the horizontal direction is space and the vertical direction is time. Suppose then that there is a single, fixed speed at which we can travel through space-time. This turns out to be true, so it's not a ridiculous assumption. Therefore, we can mix these ideas with our previous analogy. An object can move vertically. In that case, they do not move through space and move entirely through time. That's probably what you're doing now.

You are sitting and watching this video, so your position in space does not change. However, you are experiencing time. You don't move through space, but you move through time. On the other hand, what happens when you start moving through space? That's a fancy way of saying you gain some speed. Well, we see here that what starts to happen is that as you start moving through space, you move less through time. And eventually, when you move only through space, you don't move through time at all. And this is basically what relativity says. As you move faster and faster, your clocks slow down.

And, as you get really close to the speed of light, your clocks almost stop. We have scientifically proven that this is what happens and I direct you to my video on time dilation so you can see one way we have proven it. So this brings us to our fundamental understanding of relativity. The reason we can't move through space faster than the speed of light is because we are constantly moving through spacetime at only one speed: the speed of light. If we do not move through space, we experience time the fastest; and if we start moving through space, we experience time becoming slower and slower.

Finally, since we move through spacetime at only one speed, that means that when we move through space only, there is no more speed to gain. We move through space at the speed of light and that's it. This observation was not made by Einstein. It was made by his mentor, Herman Minkowski. Minkowski was one of Einstein's mentors and was a better mathematician. Two years after Einstein's seminal 1905 paper, Minkowski appreciated the geometric foundations of special relativity and determined this profound and fundamental explanation for why we cannot travel faster than light through space. There are two last important points.

First, although Minkowski showed why the speed of light is the maximum speed through space, what he did not explain was why we move only at one speed through spacetime. To this day, no one really knows. It seems to be a fundamental property of space-time. Maybe it will take another person as smart as Einstein to solve that particular puzzle. The second point is more technical and I mention it only for true physics nerds. In my analogy, I connected space and time as being similar to east and north and there is a lot of merit in that. Going from movement through time to movement through space was like making a car go from north to east.

However, this analogy is also technically inaccurate. From a mathematical point of view, it uses the geometry of circles, while its own geometry is that of hyperbolas. I only mention this because I want you to know that my analogy is imperfect and you should not take it too far. Otherwise, you might come to a numerically incorrect conclusion and think you've made a new discovery. If you want to delve deeper into this, be sure to use complete and proper Minkowski mathematics. Still, even with the limitations I've mentioned, the central point is valid. The reason you can't move faster in space than the speed of light is because every object moves through spacetime at one and only one speed: the speed of light.

Once you've accepted that central idea and the fact that space and time are like two directions of spacetime, then all those seemingly strange observations of relativity just fall into place and special relativity makes a lot of sense. So I don't know about you, but I think this idea about relativity is the coolest thing ever. If you liked this video, be sure to like, subscribe and share it; Let's increase those numbers. And let me know what you think in the comments. See you next time and we will continue practicing physics.