What Would Happen if you Went the Speed of Light?

Even space has a speed limit. Sorry officer.

Feb 04, 2021

Light moves crazy fast: 2.99e8 meters per second, to be exact. Or 670,600,000 mph – 186,000 miles per second. Zoom. At that speed, the trip from our last newsletter, New York to LA, could be done 75 times every second. At the speed of light, you could get to Mars in 4.5 minutes, compared to the current time of ~6 months. Now we’ve briefly touched on this in a past newsletter, but we don’t currently have technology to move people that fast, though we have made other things go pretty close. Actually, come to think of it, you propel things at the speed of light every time you hit a light switch. When you do this, your lightbulb emits photons, which are the only particles that can reach this speed. These photons can go this fast because they don’t have any mass. This is where things get kind of hairy, but first, we’ll start with an equation that is used the world over to scare and/or enlighten physics students.

Let’s get a better understanding of what this means. E is the energy of an object, m is its mass, and c is the speed of light. For our discussion, what this equation and theory say is that to reach the speed of light, you either need to have infinite energy or no mass. This is a bit confusing but it’s why we said photons can reach this speed – because they have no mass. If you or I were to get in a rocket ship and keep accelerating, we could asymptotically approach light speed – meaning that we can keep getting closer, but we’ll never get there. Similar to the idea that if you start with the number 1 and cut it in half, you get 0.5. Halve it again and you get 0.25. Continue this process and you’ll continue to get closer to zero, but you’ll never actually reach it. This is what happens if we were to try to get to the speed of light: we can never actually get there.

Humans have successfully accelerated particles to 99% the speed of light, but again, they can never quite reach it, because these particles have mass. It requires much less energy to get them up to this speed than, say, a spaceship, but they both have the same problem getting to 100%. I know that this isn’t very intuitive. I’m thinking right now, “if I’m in a car going 99.999% the speed of light, couldn’t I just step on the gas a tiny bit more? What happens that’s stopping me?” Think of it like you’re redlining your car. At the very end of your speedometer, odds are you’re already floored on the gas, and as a result the car is literally incapable of getting itself faster. It simply cannot produce the required energy to go that final little bit faster. It’s like that, but there is no known energy source that can give us enough gas to get to light speed. As far as space travel goes, we haven’t gotten close. We’ve topped out at around 0.064% the speed of light.

So far this hasn’t been very inspiring. Let’s take a brief detour away from reality and explore what it would be like to be going that fast, and if it’s a practical method of interstellar travel. First, what would you see? Everything that you can see right now is actually the light reflecting off of it. How light reflects off of surfaces is what determines their color, shine, etc. When it’s dark, you can’t see anything because there is no light being reflected and coming into your eyes. Therefore, when something visual happens, like a light switching on, or someone waving to you, that information travels from its source to your eyes at the speed of light. This is why if the Sun were to suddenly flicker out, we wouldn’t know for 9 minutes because that’s how long it takes for light from the Sun to get to Earth.

Fun fact: gravity works the same way. Gravitational waves (information) also travel at the speed of light. As a result, given the same scenario of the sun disappearing, we would continue to orbit like nothing happened for 9 minutes. Earth would operate as normal. Then, after the time elapsed and the information arrived, the lights would go out and Earth would slingshot tangentially straight…forever. Pretty scary.

But here’s my point: if you were traveling at the speed of light, could you see things? Light coming from the stars behind you would never reach you, since you and the light would be moving at the same speed. We often see this depicted in space movies as the stars leaving streaks behind as you enter hyperspace, but in real life, would any of that actually happen? [1]

The first thing we would see is an extremely small “tunnel” of light in front of us. This would happen because, at this speed, we would be running into the photons in front of us, so only the photons that are directly in front of our eyes would actually be registered. Additionally, there would be a doppler shift. Similar to when a car passes by you honking their horn, or an ambulance with its siren wailing, and the sound’s pitch gets higher and higher as it approaches, and lower as it speeds past and gets farther away. At this point we would be going so fast that a similar phenomenon would happen with light. As you get faster and faster, all of the light in front of you would shift to blue, and all the light behind would shift to red. Then, once you hit light speed, the light would all fade out of the spectrum that our eyes can see. In short, as you speed up, your vision tunnels into a combination of blue and red shift, until it all fades out of view when you reach light speed. [2]

At this point, you’re stagnant when compared to photons, so you don’t see anything, which raises a very confusing question. Can you see the instruments inside of your spaceship? They are traveling the speed of light with you, and the light they’re emitting is also traveling the speed of light… so are the photons going twice the speed of light? The answer is no, because of math. And as is usually the case with math, it yields a less than intuitive result. No matter what, photons will always travel the speed of light, regardless of how they start off. If your car is traveling 50 mph, the photons coming from your headlights aren’t going the speed of light + 50 mph, they’re just going the speed of light. This starts to get into the whole “we can’t yet reconcile general relativity and quantum mechanics and what if the smartest physicists ever were just kind of…wrong?” Let’s not think about that. At our current level of understanding, that’s how it works, and that’s what you would see if you could get yourself going that fast.

So, at this point the only thing we can definitely say is that the headlights on the spaceship would not work as planned. But what are the implications of going this fast? Is it a practical way of getting around the stars? Upon an initial analysis, if we (and our spaceship) were going the speed of light, we would have infinite energy. This is per our discussion earlier. So, unfortunately, if we were to hit something it would be bad. Let’s discuss an extreme case: a pebble. Imagine the smallest piece of rock you can think of, like the size of a grain of sand. If this was sitting in space (which very many pieces of debris and rock often do) and we hit it, we would collide with infinite energy. Which theoretically would wipe out the entire universe, because, you know, infinity is wicked huge. Even at 50% the speed of light us, our spaceship, and more importantly the pebble, would be instantly vaporized. Assuming our spacecraft weighs about 10,000 kg (weight of the SpaceX Dragon capsule), we would have a kinetic energy of 1.12e20 Joules (112,000,000,000,000,000,000 Joules). If we were to collide with, say, a planet, we would impact with the energy of 100 million nuclear bombs. Therefore, I argue, going this fast is only worth it if you can be sure that you won’t hit something. There is a whole lot of empty space in…space… but there are also unfortunately enough things out there to make this a real risk. And if you’re going that fast, you don’t have a whole lot of ability to maneuver around them.

Would I ever want to go this fast? No. And neither should you. I’m all for chasing your dreams and stuff but this one is the only one that I’ll say is impossible. Either that or Einstein is stupid; your call. Thanks for reading and I’m proud of both of us for getting through an admittedly intense topic. Happy February!

It's Not Rocket Science

Discussion about this post