#### Transcript of The Relativity of Spacetime

**Time is a variable, **

not a constant.

**Relativity**

This is called length contraction.

The counterpart to length contraction is time dilation.

As objects approach the speed of light, distances are shortened in the direction of motion.

Length contraction is very exciting because it means that the most distant objects in the universe are suddenly a lot closer...

...we just need a spaceship that can travel close to the speed of light!

Just as distances shorten at extremely high speeds, contrary to intuition, time stretches out.

If we had a spaceship that could travel at 95% the speed of light, we could zip on over to Proxima Centauri, our closest neighboring star, and the distance would shrink from 4.2 light years to 1.3 light years.

While this may seem like it turns everything we know about the universe upside down, it has been repeatedly verified by scientific experiment.

Einstein's Theory of Special Relativity very elegantly explains how the universe works, but it has some surprising implications.

In our spaceship, a trip to Proxima Centauri would take 1.4 years.

During our trip, time would pass just as we expect, but the people on Earth would observe something very different.

When we returned from visiting our next-star neighbor, we would find that more than three times as much time had passed on Earth.

The effects of time dilation are more extreme the faster the speed and greater the distance.

If we could speed our spaceship up to 99.99999999% the speed of light, we could travel to the Andromeda Galaxy, three million light years away, in just 50 years.

But at the end of our 100-year round trip intergalactic journey, we would come home to a very different world, because 6 million years would have passed on Earth!

While time dilation sounds like science fiction, it is embedded in the very fabric of our universe.

On Earth, we can ignore special relativity because everything moves at speeds much less than the speed of light, but if we ever want to see what's out there, we cannot escape time dilation.

**Traveling through space means traveling through time.**

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