Most of us have heard that the speed of light is the absolute upper speed limit of the universe. This fact was realized by Albert Einstein over 100 years ago, as he wrestled with what came to be known as his Special Theory of Relativity. He made an absolutely brilliant breakthrough, after struggling for several years with the dilemma of what would happen if we were able to race through space at velocities approaching the speed of light.... sometimes called relativistic speeds.
Einstein published his famous Special Relativity paper in 1905, at the tender age of 26, after having anguished for some 10 years over the paradox of travel near the speed of light. His special theory has a number of brilliant insights, but I'll focus here on just one of them: that the speed of light truly is an absolute upper speed limit. Nothing, not even light itself, can go faster. Here's a simple example of why that's true, and I'll use the TV series “Star Trek” as an illustration.
As millions of “Star Trek” viewers know, the USS Enterprise cruised through interstellar space, going “where no one has gone before,” and often at warp speed—that is, faster than light. Cap'n Kirk (or Cap'n Jean Luc Picard, in the follow-on series) would often command Scottie to move post haste to the next star system, let's say at warp factor 3—in other words at 3 times the speed of light. In critical circumstances, Kirk would even order a speed of warp factor 8! Sounds good, but those are impossible velocities. Here's how Einstein's theory demonstrates why.
Let's suppose that the Enterprise is temporarily based on Planet Zenon, where it is undergoing an extensive refurbishing. While the overhaul is going on, another (slower) starship, the USS Intrepid, leaves Zenon, headed for Earth, at 60% the speed of light, but they have inadvertently left behind an important document. Not to worry, says Cap'n Kirk; as soon as the Enterprise is ready to fly, we'll chase after the Intrepid at warp factor 3, catch up with them, and deliver the document well before they get back to Earth.
Unfortunately, however, overhaul work on the Enterprise goes excruciatingly slowly. It's now four years later when Kirk's ship finally departs, at 3 times the speed of light. At that rate it requires them only one year to catch up with the slower Intrepid, which has been tootling along for five years, but has only gotten three light years away. Thus they meet in space five years after the Intrepid had departed—five long years of travel for the slow Intrepid, but just one for the speedy Enterprise. Still sounds OK. But...
I won't go in to all the details by using Einstein's complex equations of his special theory. Let me simply leap to the weird result that his theory predicts. From the perspective of the crew on the Enterprise (going at 3 times the speed of light), they left at year four and caught up with the Intrepid at year five. But if you use Einstein's relativistic equations, you get the impossible situation that, from the Intrepid's perspective, they were met by the Enterprise one year before (yes, before!) the Enterprise had even left Planet Zenon!
I realize I've thrown around some possibly confusing numbers here, but the bottom line is, if faster-than-light speed is proposed, Einstein's special theory predicts an effect (the meeting with the Enterprise) before its cause (the finish of the rehabbing of the Enterprise). It's like saying that I celebrated my first birthday a year before I was born, or an arrow hits the target before the archer draws the bow! Clearly this is impossible. It's like time running backwards. We cannot have effects happening before the cause.
Scientists
have many times conducted experiments that verify Einstein's Special
Theory of Relativity. The theory is right. In other words, the speed
of light is an absolute upper speed limit. If it wasn't,
things going faster than light would literally turn clocks backwards.
Sorry, Cap'n Kirk, you can't do warp 3... and especially warp 8. But
Star Trek was science fiction, after all.
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