Scientists Say: Spacetime

Spacetime (noun, SPAYS-TIME)

Spacetime is a model that combines two ideas that most people think of as totally different: space and time.

In this model, our universe has four dimensions. The first, second and third dimensions are those of space — length, width and height. They describe where an event occurs. The fourth dimension is that of time. It describes when an event occurs.

The concept of space and time being interwoven can seem weird. But this model explains phenomena we see in the universe.

For one thing, it explains why objects’ speed can affect how they experience the passage of time. We do not notice this in our daily lives, because the difference is so small. But when something moves very fast through space, it moves more slowly through time. The clock literally ticks more slowly for it. This is called time dilation.

The spacetime model also explains why objects’ masses can affect time.

Physicists often imagine spacetime as a “fabric.” Extremely massive objects can “dent” this fabric, causing other objects to fall toward them. That’s the force we experience as gravity. But because space and time are interwoven, massive objects don’t just “dent” space. They also “dent” time. The result is that time slows down near massive objects, where gravity’s pull is strongest.

This idea is explored in the 2014 film Interstellar. Astronauts in the film explore a distant planet near a black hole. The black hole exerts a strong gravitational pull that slows the passage of time on the planet. For every hour astronauts spend on this planet, seven years pass on Earth.

You don’t have to travel to a distant planet to witness the interplay between time and space, though. The nature of spacetime impacts our daily lives. For example, it affects the satellites needed to run much of our modern tech. 

Satellites orbiting Earth travel through more space each second than we do. This difference in speed causes a tiny difference in how satellite clocks measure time. They tick about 7 microseconds — that is, 7 millionths of a second — more slowly each day than those on Earth.

Gravity affects satellite clocks as well. The lower gravity they experience in orbit causes those clocks to gain 45 microseconds each day.

When these effects are combined, the net time difference between clocks on Earth and in orbit is 38 microseconds. Engineers take this into account when setting satellite clocks. That way, when satellites get into orbit and start experiencing time differently, they’ll stay in sync with clocks on the ground.

In a sentence

Ripples in spacetime — also known as gravitational waves — allowed researchers to unlock secrets of the oldest black holes in the cosmos.

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