Key Takeaways on What Space-Time Is
Einstein’s theory of general relativity views time similarly to width, height, and length. As such, both time and space can be curved by gravity. This effectively means that time is slower the closer you are to an object with a strong gravitational pull.
Space-time compression and space-time differ from each other. Space-time compression focuses on how the world seems smaller now that we have more access to it, not time in space.
Space-time vs. Earth time depends on where you are in space. If you're near an event horizon, time will be much different than on Earth.
Days sometimes may feel like mere hours when times are good, and the moments may barely tick by in a dull day, but the passage of Earth around the sun hasn’t changed in an easily measurable way since humans first started using sundials.
But about half a century after Big Ben was built in London, marking the center of Greenwich Mean Time, Albert Einstein’s theory of general relativity changed the way that scientists think about time. Suddenly, as his theory suggests, the way people experience time might change depending on their location. Humans rocketing through space age more slowly than those trying to stay forever young on the planet’s surface.
“Einstein’s general theory of relativity requires that we can treat space and time as the same thing,” says Lia Medeiros, an astrophysicist at the University of Wisconsin-Milwaukee’s Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics. “My entire career started because I thought time slowing down was the craziest thing and I wanted to understand it.”
What Is Space-Time?
Humans see life mostly in a three-dimensional world, and while we experience time — the fourth dimension — as it passes, we tend to see it differently from the other three dimensions, which deal with space.
But Einstein’s theory of general relativity views time similarly to width, height, and length. As such, both time and space can be curved by gravity.
“What we experience as gravity is actually the curvature of space-time,” Medeiros says.
This effectively means that time is slower the closer you are to an object with a strong gravitational pull. Medeiros uses an example to illustrate this — if one twin lives in the penthouse suite of a tall skyscraper, and the other in the basement of the same building, the basement dweller would age a little slower than the high-living sibling, all else being equal.
Space-Time Compression: Misunderstood
When it comes to space-time compression, it isn’t exactly related to the way that space bends time, or to Einstein’s theory of relativity. The idea of space-time compression has more to do with the way that the world seems like a smaller place in the modern era due to advances in air travel, the internet, and other things that make the world seem like a smaller place.
Read More: Spacetime’s “Brownian Motion” Could Spell The Death of Dark Matter
How Astronauts Experience Time Differently
Following Einstein’s theory, there is no such thing as one day in space.
“We can’t define a stationary observer in space,” Medeiros says.
How you experience time relative to others on Earth all depends on where you are in space. For starters, astronauts in space won’t feel like things seem to pass faster — they will still experience an hour the same way they would on Earth. It’s just that when they return to Earth, less time will likely have passed for you than for someone who stayed on the surface.
But for the sake of argument, let’s take an astronaut on the International Space Station (ISS). The space station is farther from Earth, meaning it experiences less gravitational pull. Relative to someone on Earth, the astronaut would, in theory, age a little faster than someone on the surface, since they are much higher even than the penthouse suite of a skyscraper.
But this doesn’t happen. In fact, astronauts who spend time orbiting Earth typically age a little less than if they had stayed on the surface. This is mainly because speed slows time down.
So, as astronauts speed around the planet, they are actually moving at a slower pace of time than people on the surface. The difference isn’t much, though. Some calculations show that an astronaut who spends six months on the ISS might age just a fraction of a second less than someone on Earth’s surface, for example.
Space-Time vs. Earth Time
Similarly, the way time flows in space compared to someone on Earth depends on where you are in space. The most extreme example of this would be if you were in space near an event horizon — the point from which you could not escape the pull of a black hole.
To an outside observer, someone crossing this threshold would seem to slow down so much in time that they would be around forever. The unlucky explorer crossing the event horizon would probably experience a rather abrupt end, however.
“The amount of life that I will experience is actually quite short, but if you are watching me fall into a black hole, you’ll get really bored,” Medeiros says.
Similarly, you would experience time differently than someone else, depending on how fast you were going. On the extreme end, someone who managed to move at the speed of light would essentially not age compared to someone who is more or less stationary. This is why photons don’t age, Medeiros says — they move at the speed of light.
Read More: Will Time Ever Stop: Could There Be an End to the Future?
Article Sources
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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
