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# Velocity: It's All Relative

We spend a lot of our lives getting from here to there. Everyday we slog from home to work and back again. Once a week we make the trek to the grocery store. A few times a year we take vacations across the state, the country or even across an ocean. Each trip, large or small, sets us in motion.

We get going.

We get up to speed.

But for all the time we spend hurtling through space in our cars, our trains and our jetliners, we never seem to care about what all this speed — this velocity — really means. The Universe, however, does care. That's because the world you experience is tightly linked to the speed you travel.

Velocity is one of the simplest ideas in physics. It's just the distance you cover in the time it takes to cover that distance. That means velocity is always a mixture of space measurements and time measurements. That may sound technical but you already have a lot of experience with what it really means.

Buffalo is about 60 miles away from where I live in Rochester. If I hop in my car and keep the speedometer — or, better yet, "the speed-o-meter" — fixed at 60 miles an hour, I'll get there in ... an hour.

We all do these kinds of calculations in our heads during morning drives or on longer cross-state journeys. But this easy road-trip familiarity with velocity, space and time obscures how entirely new and entirely radical experiencing high speeds is for human beings.

Think about this: Before the mid-1800s (and the invention of trains) no one, anywhere ever moved as fast you travel in the fast lane of the highway. At best, racehorses clock in at about 40 mph. And if you fell from a five-story building, (that's about 60 feet), you'd hit the ground doing about 45 miles per hour.

Thus, for most of human history the only folks who moved as fast as you do every morning on your commute were people falling to their death.

Gobbling up huge distances in small amounts of time is something fundamentally new for our species. It's allowed us to create crazy new ways of being human that never existed before. Things like eating fresh fruit from thousands of miles away or having long-distance relationships with lovers on a different coast.

There is no arguing with the fact that moving at ever-higher speeds has fundamentally rearranged human society. But if we go fast enough, the physics underlying reality itself change.

While your great-great-great grandpappy's head might have spun off if he roared through the sky at 500 mph (to say nothing of using Google to find the capital of Wyoming), it would be your head that'd be spinning if you had the direct experience of moving close to the velocity of light. That's because Albert Einstein's famous, mind-bending theory of relativity is really all about speed. What Al realized was that velocity is not just about measurements of space and time, it's what sets the behavior of space and time if your traveling fast enough.

Imagine that you know a pair of twins. Being twins, they are pretty much the exact the same age. Now imagine that one of twins climbs aboard a space ship. She blasts off and travels at 99.5 percent of light speed. Using the clocks and calendars on her spaceship, she measures her travel time to be 3 years. Then she lands back on Earth and goes looking for her twin sister. To her amazement, her Earth-bound sister is not 3-years older but 30!!!

What happened?

The flow of time for the space-traveling twin and the flow of time for the Earth-bound twin were not the same. Yes, that may seem wacky, but it's a law of nature that has been confirmed in laboratories a zillion times. The relative flow of time between people moving relative to each other depends on their relative speed. You don't see this difference in everyday life because it only becomes noticeable if you travel close to the speed of light, 670 million miles per hour.

See, velocity matters.

A thousand years ago most people traveled no faster than the speed their feet could carry them. That seems pretty alien to us these days. But if 1,000 years from now people routinely travel in spaceships that ride close to light speed, then their experiences carrying their own flows of time around with them will be even more divergent from our world than ours are from our ancestors of old.

And it all depends on the space you cover in the time you have.

You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Adam Frank was a contributor to the NPR blog 13.7: Cosmos & Culture. A professor at the University of Rochester, Frank is a theoretical/computational astrophysicist and currently heads a research group developing supercomputer code to study the formation and death of stars. Frank's research has also explored the evolution of newly born planets and the structure of clouds in the interstellar medium. Recently, he has begun work in the fields of astrobiology and network theory/data science. Frank also holds a joint appointment at the Laboratory for Laser Energetics, a Department of Energy fusion lab.

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