Can you travel back in time

Wells' novel The Time Machine , people have been intrigued by the idea of time travel. Is it just fiction or is it real? What do physicists say? This opens up the possibility that points in time are similar to points in space — destinations to which we can travel. Can we really travel in time? Physics may allow for time travel but perhaps logic does not? Philosophers who have thought about time travel are most worried by the idea that it would lead to paradoxes, i. The most famous one is the so-called grandfather paradox.

Consider the following scenario: You possess a time machine that could take you back to the past. You also passionately hate your grandfather, a truly horrible man who committed many awful crimes. You pack your gun and travel back to when your grandfather was 15 years old. This is before he committed the crimes, but also before he had met your grandmother. You lurk in the bushes in front of his house. Another popular theory for potential time travelers involves something called cosmic strings —narrow tubes of energy stretched across the entire length of the ever-expanding universe.

These skinny regions, leftover from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them. The approach of two such strings parallel to each other, said Gott, will bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.

Mathematically, you can certainly say something is traveling to the past, Liu said. However, some scientists believe that traveling to the past is , in fact, theoretically possible, though impractical. GPS satellites orbit around Earth very quickly at about 8, miles 14, kilometers per hour. This slows down GPS satellite clocks by a small fraction of a second similar to the airplane example above.

GPS satellites orbit around Earth at about 8, miles 14, kilometers per hour. Credit: GPS. However, the satellites are also orbiting Earth about 12, miles 20, km above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground. The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time. If scientists didn't correct the GPS clocks, there would be big problems.

GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. For decades, physicists have been studying and debating versions of this paradox: If we could travel back in time and change the past, what would happen to the future? His work, published in the journal Classical and Quantum Gravity last week, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

Physicists have considered time travel to be theoretically possible since Einstein came up with his theory of relativity. Einstein's calculations suggest it's possible for an object in our universe to travel through space and time in a circular direction, eventually ending up at a point on its journey where it's been before — a path called a closed time-like curve. Still, physicists continue to struggle with scenarios like the coronavirus example above, in which time-travelers alter events that already happened.

The most famous example is known as the grandfather paradox: Say a time-traveler goes back to the past and kills a younger version of his or her grandfather. The grandfather then wouldn't have any children, erasing the time-traveler's parents and, of course, the time-traveler, too. But then who would kill Grandpa? A take on this paradox appears in the movie "Back to the Future," when Marty McFly almost stops his parents from meeting in the past — potentially causing himself to disappear.

To address the paradox, Tobar and his supervisor, Dr. Fabio Costa, used the "billiard-ball model," which imagines cause and effect as a series of colliding billiard balls, and a circular pool table as a closed time-like curve.