Albert Einstein is often considered one of the most influential scientists of all time, with brilliant ideas about the laws of nature. However, his work has a very disappointing consequence, especially for science buffs who hope to one day travel to distant stars. His theory demonstrated that there is one fastest speed in the Universe: the speed of light. This means that the shortest possible round trip to the nearest star will take nearly a decade.
But how does it work? Even the most knowledgeable science enthusiasts often have the wrong, or at least incomplete, understanding of why you can’t go faster than light.
The manual’s answer
This ultimate speed limit is a curious fact and one that goes completely against our common intuition. After all, if you rush in your car and hit the accelerator, you’ll go faster. And while your car has a top speed, we know of faster things like fighter jets and bullets.
However, the acceleration eventually stops working. No matter how hard you try, you can’t go faster than ~186,000 miles per second (~300,000 km/h).
The most common explanation for this cosmic speed limit is that as an object goes faster and faster, its mass increases. And this explanation makes sense. After all, a mountain is harder to push than a pebble. If the mass of objects becomes infinite as they approach the speed of light, then it makes sense that you couldn’t break that speed barrier – it would take infinite energy to accomplish.
There’s a lot of merit to this explanation, and it’s often explained this way even in college physics courses. (Heck, even I’ve taught that once in a while.) But that’s not the better answer.
Everything travels at the speed of light
To understand the real reason why you can’t go faster than light, we need to learn a key idea from Einstein’s theory. While our common experience tells us that space and time are different things, he realized that they are more similar than different. Instead of space and time, there is one thing, called spacetime.
This idea is perhaps best understood by means of an analogy. Look at any world map. We can identify a location on the map by two numbers: latitude (the north/south number) and longitude (the east/west number). Although there are some minor differences (eg it is warmer or colder as you move north/south), there really is no difference between the two directions.
In space-time, it’s pretty much the same thing. Individuals can move in time or space, just as travelers can choose to move east/west or north/south.
Now for the key idea. One of Einstein’s teachers, a mathematician named Hermann Minkowski, examined Einstein’s theory of relativity and realized that at its deepest and most fundamental level, the theory said that any object simply traveled through space-time – partially through space and partially through time. . When Einstein’s theory was taken further, what was revealed was that all object moves through spacetime at only one speed – the speed of light.
To understand this more easily, let’s say you are in a large, flat place, like the Bonneville Salt Flats in Utah. Suppose further that you are in a car with the throttle locked so that it can only go at one speed, 60 mph (100 km/h).
Now let’s get behind the wheel and drive. If you’re going east at 60 mph, you’re not going north/south at all. Similarly, you could drive north at a fixed speed, but you wouldn’t move east/west at all. Or you can choose to drive northeast at 60 mph, thus traveling in the east and north directions at a speed of around 42 mph (69 km/h).
If you are moving at a fixed speed, the main thing is that you can move in any direction, but in no direction can you move faster than that fixed speed.
And it is identical in space-time. Objects move through spacetime at the speed of light. A stationary object does not move in space at all, so the object moves through time at the speed of light. Moreover, an object moving in space at the speed of light no longer has any speed to move in time.
So the absolute maximum speed at which an object can move individually in space or time is also the speed of light. Note that this idea also explains strange features of relativity, such as time slowing down for an object as its speed gets faster and faster. An object traveling more in space travels less in time.
The speed limit is a harsh reality
The reason it’s hard to travel through space at the speed of light is that you have to push the object to move in the direction of time to move further in the direction of space. And it turns out to be difficult. But, even if you succeed, you can’t beat this unique speed.
So that’s it. The real reason you can’t travel through space faster than light is that you’re still traveling through spacetime at the speed of light. The best you can do is to carry all your effort to move through space; but, once you have pushed all your movement in the direction of space, there is no more speed left. Just like the car that can’t go faster than 60 mph, you’ve reached the top.
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Does this mean that the day we finally venture into interstellar space, we will be doomed to travel no faster than light? A round trip to the center of the galaxy and back takes 50,000 years to travel at the speed of light. Can we do better?
Unfortunately, the answer is no, at least not without a new scientific discovery. Warp drive, hyperspace, and all the faster-than-light options of familiar science fiction aren’t real – or at least are unknown to our best understanding of the laws of nature. Note that scientists have already discovered unknown things, such as radio waves and radioactivity, so it is conceivable that one discovery could change everything. So I guess there is hope.
On the bright side, now that you know space-time, the next time your boss accuses you of sitting there, you can tell him he’s being unreasonable. You advance as quickly as possible.
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