You can measure time by looking at a clock. You can measure space by watching athletes run from one point to another.
Intertwine those measurements of space and time and you have a competitive race. Have spectators watch the race from different perspectives and you also have another example of Einstein’s theory of relativity.
Albert Einstein theorized, quite accurately, that time and space are relative rather than absolute. Even though they are inextricably linked, space and time are perceived differently by sentient beings. That’s because an object’s movement and how it experiences time is relative to other things around it.
For example, a motorcyclist who speeds alongside the track will see those athletes moving slowly, while a spectator sitting still in the stands will perceive the runners to be moving faster. However, in the end, the finishing time of the race winner will be almost exactly the same for the motorcyclist and stationary fan, assuming they hit “start” and “stop” on their stopwatches at the precise moments.
Another component of Einstein’s theory explains this phenomenon: at 186,000 miles per second, the speed of light will always be the same in a vacuum. This rule is important because of the third factor in his theory: nothing travels faster than light. (Well, almost nothing; scientists believe some things are faster without disturbing the theory.)
Those three factors largely underpin how we see and interact with space and time, or more simply “space-time.” The interplay of it all is fascinating, even if your brain has to stretch a bit to put it all into context. But get ready to stretch your powers of reasoning a bit more. Try placing your understanding of space-time within the theater of not just Earth but also the universe.
How does it all work when thinking about our lives as part of everything that exists?
A Time Lag Reveals the Universe’s History
Einstein’s theory that nothing can move faster than the speed of light has significance in space for several reasons. For one, it prevents humans from exploring deep into the galaxy. Because only photons can travel at light speed, humans are stuck to speeds slower than light and thus would not live long enough to complete a journey to the far reaches of space.
Although that might seem like a barrier, in other respects, the relationship between space and time has allowed us to better understand the galaxy and its origins, according to Vatche Sahakian, professor of theoretical physics and cosmology at Harvey Mudd College in Claremont, California.
“When two points in space are to be communicated with each other, there is always a timeline,” Sahakian said. “The time it takes for an object to speak to another is the speed of one light signal to another. When observing the world around you, everything is communicated with a time lag.” On Earth, that timeline is perhaps most evident in the time elapsed between seeing a flash of lightning and hearing thunder. “That gives you a feeling of how you do not have access to information instantly,” he said.
This interplay is more pronounced when we look at the moon. “It takes 1.3 light seconds for the light of the moon to reach us,” Sahakian said. “Whenever you’re looking up, it’s 1.3 seconds old. If the moon was to explode, it would take 1.3 seconds for us to see it no longer existed.”
Considering that humans won’t access the far reaches of space anytime soon, the time lag between event and perception has given us a unique opportunity to see the history of the universe, Sahakian said. “The deeper you look into space you’re also looking into the past.”
A Wormhole Would Help, Only if We Could Create One
New revelations about that past occur seemingly all the time. Telescopic advancements continue to lead to astonishing findings about other galaxies, no matter how distant.
Last year, scientists studying a galaxy far larger than our Milky Way, one with more than a trillion stars, found that its light was emitted just 1.5 billion years after the Big Bang — 1 billion years earlier than previously estimated. Sure, what’s a billion years when the universe itself is estimated to be 13.8 billion years old? Well, the update on the galaxy’s age is a helpful piece of a puzzle that scientists hope will eventually explain how massive galaxies formed and died in the early universe. This particular galaxy had a suppressed star formation that means it was dying.
Would it be possible to ever receive information faster than the speed of light, so we could see what’s going on in the universe closer to our time? That would require a wormhole, Sahakian said, a tear in spacetime that could give us a shortcut to explore and observe the deeper reaches of space. While that’s an exciting plot device in movies, such a wrinkle in time is not known to exist.
“We know the mathematical theory to create them but what we need to do that doesn’t exist,” said David Mattingly, an assistant professor of physics and astronomy at the University of New Hampshire. “We need a different kind of matter and would have to have different kind of properties.”
Our inability to shorten space travel might disappoint fans of science fiction movies such as “Interstellar,” which convincingly built a plot around a wormhole, Mattingly said, but there’s much to explore in the universe within the limitations of our space-time continuum. “We have a lot of practical work to do just to colonize and explore our own solar system,” he said. “There are lot more planets in space that we will be busy exploring.”
Gravity Influences Spacetime, to an Extent
For that matter, scientists still have to determine how certain longstanding theories apply to spacetime while others don’t. Einstein’s theory of relativity holds up in our solar system but Newtonian physics, which govern the laws of motion, doesn’t, Mattingly said.
Imagine if a baseball fell to Earth from a certain point in the atmosphere. The laws of gravity inform how it will accelerate at a certain and measurable pace, Mattingly said. Now, release the baseball from the center of the solar system with the laws of gravity unchanged, the ball takes longer than expected. “Well, you could say gravity gets weaker at larger distances, but is there something I can’t see that is pulling my baseball?”
Indeed, the baseball’s motion was influenced by a mysterious pull, Mattingly said. Things don’t move the “right way” as you move farther away from the gravitational spots between the Earth and its sun and our solar system. So, to explain the behavior of stars in other galaxies, scientists chalk up any gravitational force to dark matter, which has never been observed but is estimated to comprise more than 80% of all matter in the universe.
“And that’s what we don’t know about,” Mattingly said.
Newton and Einstein expect us to find out.