A question many of us have asked ourselves is: where would you go if you had a time machine? ADVERTISEMENT ADVERTISEMENT Would you turn the dial back like Marty McFly and risk sabotaging your own future? Before you start packing your bags, a physicist has news that could ruin some Marvel movies: you may only be able to go one way, and not back. Hongsheng Zhang, a researcher at Jinan University in China, has shown that in a wide class of theoretical universes, traveling to one’s own past is impossible. The entire argument is based on fact. In relativity, the direction of light can never be at right angles to the direction of an object through time. Imagine space-time divided into layers through which light itself can travel. Anything that moves slower or faster than light must cross these surfaces in the same direction, like climbing a ladder that never allows you to go down. If you can only go up, you will never be able to return to the step where you started. The argument needs no exotic matter, no quantum effects, or assumptions about what fills the universe. “All the content is accounting about addresses,” Zhang told ScienceAlert. The two gaps Physicists call trips to the past “closed time curves”: routes through space-time that return to the same place and time where they began. Einstein’s equations allow them in several scenarios, such as near the core of a spinning black hole. Crucially, Zhang’s research, which is awaiting peer review, also reveals exactly how a time machine could achieve time travel, and claims there are only two ways. The first is the twist. Imagine rays of light that, instead of traveling side by side, spiral around each other like the strands of a rope. When that happens, clean light surfaces cannot form, so there is no one-dimensional ladder to keep you climbing. This is what happens near the center of a spinning black hole. Einstein’s theory says that a spinning mass drags space and time with it, an effect that NASA’s Gravity Probe B satellite measured around Earth in 2011. Near the core of the black hole, the drag is so strong that as it moves “in time” it bends into a circle. A traveler following that circle would see his watch move forward in time normally, but his journey would return to the moment it left. Physicist Brandon Carter demonstrated in 1968 that such loops exist inside spinning black holes. The second is the shape of the universe itself. The staircase may be perfectly constructed, but it may wrap around the sides of a building, so that the top step leads directly to the bottom, like the impossible stairs in an Escher drawing. You would keep going up and still end up where you started. Most of the time, the machines that physicists have designed on paper, including those built from wormholes, rely on this trick. The theorem has its limits, as Zhang acknowledges. “What it doesn’t do is tell us whether nature allows any of this. That remains open, and it’s the question I would most like to see answered,” Zhang told ScienceAlert. Destination: Tomorrow Travel to the future, on the other hand, is already well established. Einstein showed that moving very fast, or sitting in strong gravity, slows down the clock relative to that of others. Astronauts aboard the International Space Station return to Earth a few thousandths of a second younger than they would have otherwise been, according to the European Space Agency. The work is not resolved if our own universe allows a trip to the past. For now, however, anyone planning a trip may want to pack for tomorrow instead of yesterday… unless you have the Time Stone, of course.
