There was a young woman named Bright,
Who traveled much faster than light.
She set out one day
In a relative way
And returned on the previous night.
Light is fast…really, really fast. It travels at 299,792,458 meters per second, or about 186,000 miles per second. If you could run at the speed of light, you could run around Earth almost 7 and a half times in one second.
The only thing that seems to be able to travel at the speed of light is light* itself. That’s because light has no mass. Anything with mass – including you and me – must forever travel slower than the speed of light. That might seem counter-intuitive; after all, the speed of light is just a number. Why can’t we keep accelerating until we eventually reach it?
Two very smart guys named Einstein and Lorentz showed that mega-strange stuff happens when you approach the speed of light. For one, the passage of time is altered. Whenever you move, time passes at a different rate for you than it does for somebody who isn’t moving. We don’t notice the difference in our everyday lives; however, if you could accelerate close to the speed of light the effect would be noticeable. The specifics are complicated (no surprise there), but suffice it to say that if you travel close to 299,792,458 meters per second, you’ll return to Earth to find that everybody else has aged more than you have.
Imagine a pair of twins: we’ll call them Alan and Bill. Alan boards a fantastically futuristic starship destined for Proxima Centauri, the nearest star to our Sun. Bill stays on Earth. Alan accelerates at one gee** for half the journey, then reverses his spaceship and spends the second half slowing down. When he reaches Proxima Centauri, he snaps a few pictures, then repeats the journey in reverse, finally returning to Earth.
The round trip takes nearly 12 years of Earth time. When Alan returns to Earth, Bill has aged 12 years. During the trip, however, Alan has reached about 95% of the speed of light. He has aged less rapidly than Bill. From his perspective, the round trip took only 7 years. The twins are now five years apart.
The faster Alan travels, the more dramatic the age difference will be. Supposing Alan could withstand a constant acceleration of 100 gees without turning into mush, he will reach 99.999% of the speed of light. From Bill’s perspective, Alan will return in about 8.6 years. To Alan, the trip will take only about 43 days. Alan will return in six weeks to find Bill (and everybody else on Earth) more than eight years older.
In a sense, traveling close to the speed of light is like traveling in time, but it’s a one-way trip into the future.
Mathematically speaking, if you could travel faster than light, the passage of time would be reversed. You would return to Earth before you left; in other words, you would have traveled (start reverb) BACK IN TIME! (end reverb)
It might be tempting to build a super-luminal back-in-time machine, but it just can’t be done. As I said before, no massive object can even reach the speed of light, let alone surpass it. Time dilation is not the only weird effect of traveling close to the speed of light. As you approach the light barrier, you require disproportionately more energy to make you go faster. You would require an infinite amount of energy to accelerate to the speed of light. Barring some undiscovered loophole in the laws of physics, humans will never, ever be able to reach the speed of light. Sorry to burst your hyper-dimensional bubble, Star Wars and Star Trek fans.
And so this humorous rhyme must fall into the category of Hypothetical Limericks With No Basis In Physical Reality. Still, it’s cute and it makes an important point about the bizarreness of relativity.
*The word light may refer only to the visible portion of the electromagnetic spectrum, but in this case I’m using it to refer to all forms of electromagnetic radiation: ultraviolet, infrared, microwaves, gamma rays, etc.
**One gee is the rate at which an object falls due to gravity near Earth’s surface (if you ignore complicating factors like air resistance). If Alan’s ship accelerates at one gee, he will feel a constant force equal to his own weight on Earth. In this way he could make the entire journey without suffering the ill effects of long-term exposure to weightlessness.
Nerdy Jokes 