Time machines are a favorite ploy in science fiction stories. Climb into one and zap, you’re in another century.
Entertaining, sure, but a real possibility? It does seem far-fetched, to say the least. But what most people don’t realize is that time travel is actually a common, everyday experience. It all has to do with a core tenet of Einstein’s theory of special relativity (called time dilation).
In Einstein’s crazy world, time is not rigid. Per the great physicist, time does not go by at the same rate for everyone. Imagine, for example, that you are sitting in a chair and I walk by you at a steady pace. (And assume we are both wearing identical super-accurate watches.) Per Einstein, you see my watch running a tiny bit slower than your watch. Why? Because I am moving with respect to you. Time slows down with motion.
Now at the speed I am walking, this slowing of time is a very, very small effect. That’s why we don’t notice it. But it is real. And according to Einstein’s seminal theory, the effect becomesdramatic at speeds approaching the speed of light. Say, for example, I somehow speed by you at an incredible 580 million miles an hour. This is 87 percent the speed of light. Because of my tremendous speed, you now see my watch running at only half the rate of your watch. That is, for every second that is ticked off on your watch, only a half second ticks off on mine!
Hard to believe? It does sound crazy. We can only imagine how Einstein must have felt when he first thought of it. It took tremendous guts for him to just propose such a thing! But today, we have all kinds of evidence; from atomic clocks on airplanes, rockets and satellites, from the measured lifetimes of subatomic particles, and from numerous laboratory experiments that Einstein was right! Time is relative; time does in fact slow down with motion; and in just the amount Einstein’s formula predicts. Einstein’s crazy universe is our universe.
Into the Future
OK. So what does this have to do with time travel? Imagine it is the year 2950. Let’s say you are now the one who is moving. Say you take a trip into outer space in your hot new rocket-car at 87 % the speed of light. Because of your great motion, time for you passes at half the rate as time for the rest of us back here on Earth.
Say it takes you 5 years round-trip to travel out to a distant star and return to Earth. (This is 5 years rocket time.) To you time appears to be running normally. Every 24 hours you mark off one day. And every 365¼ days, you mark off a year on your rocket calendar. Recall you left Earth in the year 2950. So when you arrive back on Earth, your rocket calendar says it is the year 2955. And indeed you and everything else on the rocket have aged 5 years. But here on good old Earth, time is not running slow, like time on your rocket. So when you return, 10 years have elapsed here on Earth. (This is 10 years Earth time). In fact, everyone on Earth has aged 10 years and Earth calendars now say it is the year 2960.
When you alight from your rocket-car and step onto solid ground, to you it is the year 2955. After all, you have only been away 5 years (in your rocket time). But it is now the year 2960 on Earth. So in effect you have traveled 5 years into the future! And, theoretically, if you traveled fast enough, you could arrive back on the Earth some thousands of years into the future For example, according to Einstein’s formula if your rocket traveled at a speed of 99.99999 % the speed of light for just one year, you would arrive back on Earth 2236 years later!
But we must be careful here. According to special relativity, this time travel is a one-way trip “You may be able to buy a round-trip ticket to (outer space),” wrote relativists Edwin F. Taylor and John Archibald Wheeler in Spacetime Physics, “but you get only a one-way ticket to the future.” So you can (and do) travel into the future, but you cannot travel into the past.
Everyday Time Travel
We don’t have the technology (yet) to accelerate macroscopic objects like rockets and people to so-called relativistic speeds; but even at the relatively slow every day speeds we experience, the effect is still there. Say you go to work or school today, but your room mate stays home sick in bed. You drive your car to your destination, and then return in the evening. Time has run a tiny bit slower for you than for your room mate. Why? Because you were in motion in your car, while your room mate was at rest in her bed. So (big drum roll here) you arrive back at home a tiny bit into the future. Any time you are in motion, you experience this effect. In fact, every time you leave a place, the slowing of time due to your relative motion means that you return to that same place a tiny fraction of a second into the future. We are all time travelers!
