Universe sandbox 212/8/2023 ![]() “The only difference is that we as humans cannot decide to move forwards and backwards in time,” Medeiros says.īecause we don't have control over it, it’s hard for our human brains to conceptualize the fourth dimension of spacetime. But we are also always moving through a fourth dimension: time. We can move through the three dimensions of space at will, passing up and down, side to side, or forward and backward. The equations are very beautiful and elegant.” Throwing spacetime for a curve “Once you combine them, everything kind of follows naturally. “If you allow both space and time to change in a connected way, then everyone agrees that light moves at the speed of light,” Holz says. It’s only when you combine the different measurements from the different frames of reference of space and time that all the observers will agree on the result, suggesting that space and time are inextricably linked. A similar thought experiment, with the photon clock tipped on its side, shows that objects are more compact along the direction of the train's motion, an effect called length contraction. If the speed of light is constant regardless of the frame of reference, then to the person on the train, the photon clock next to them will tick more quickly, while to the person on the platform, that same clock on the train will tick more slowly. The puzzle Einstein faced becomes apparent if you imagine two photon clocks, one sitting stationary on the platform, and the other whizzing by in the train. From another-on the platform-it follows a stretched-out zig-zag path instead. Not only will the photon bounce up and down, it will also move forward with the train.įrom one frame of reference-on the train-the photon follows the shortest possible path, a straight line. To a person standing outside the moving train, on the platform, however, the photon will seem to move in a different way. Traveling at the speed of light, the photon should bounce at regular intervals, like a steadily ticking clock.Ī person standing on a moving train with this photon clock will see the photon moving up and down in a line. Imagine a single quantum of light, a photon, bouncing up and down between two mirrors that are facing each other. The key to understanding how this works is the universal speed of light. What Einstein realized is that something similar happens with time: Different people will experience the passage of time differently, depending on their frame of reference. Zoom out even farther, and another observer floating in space will perceive the person standing outside the train as in motion as well, spinning along with the Earth while in orbit around the sun, which in turn is flying through the galaxy. But an observer standing outside the train will perceive both of those people as in motion, chugging along at the speed of the train. Two people standing together on a moving train will perceive one another as stationary. How a person experiences the world depends on their individual frame of reference. It all starts with the concept of different frames of reference. And spacetime grew out of that thought experiment.” “Einstein took the question ‘What if the speed of light is just the same to everyone?’ seriously. “Spacetime is a necessary consequence of the fact that all observers measure the same value for the speed of light,” says Scott Hughes, a professor of physics at the Massachusetts Institute of Technology. Among other things, the theory combined space and time into a single entity that he called spacetime. In 1905, building on existing experimental and theoretical work, Einstein published the theory of special relativity. “But if you want to make a general statement about how the universe works, then you really need to view them as one object.” A matter of perspective “For common-day experience, as well as most experiments, space and time being separate is totally fine,” says Daniel Holz, a professor of physics and astrophysics at the University of Chicago.
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