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Interesting Fact

January 2024

Interesting Fact: Don’t Just Stand There: Get Movin’

Every minute, you travel over 12,000 miles in space.  That’s just while standing still.  And thanks to the miracle of relativity, you’re not even aware of it. That’s because speed (velocity) is always measured relative to something else.  If you are standing on earth looking up into the heavens, it appears to you that everything else is moving while you are standing still.  But to someone on another planet or star, it would appear that you are moving and they are standing still.

Any speed you measure has to be relative to something that you are calling “stationary”.

For instance, a car speedometer tells you the speed of the car relative to the surface you are driving on. That would normally be the road, but maybe it is the deck of a ship that is steaming across the sea. Even standing still, the car is sitting on a planet that is rotating, and orbiting the Sun – the Sun is orbiting the galactic center and the whole galaxy is moving through the Virgo cluster of galaxies etc. So the car is only “stationary” relative to the surface of the earth.

Imagine that you are on a train, traveling at a steady speed of 50 miles per hour (mph). Your physics textbook on the table in front of you. Now, you and the textbook (and the train) are all moving at the same speed. To an outside observer standing next to the train tracks, you and the book are each rushing by at 50 mph. But, from your point of view, the book isn’t moving at all. That is, it’s not getting closer to or farther from you. The following are all true:

  1. You are moving at 50 mph relative to the observer next to the tracks.
  2. You are not moving, relative to the textbook.
  3. The book, the train, and yourself are not moving at all, relative to each other.
  4. Relative to the train, the ground is moving by at 50 mph.
  5. Relative to the ground, the train is moving by at 50 mph.
Bullet train in motion relative to the earth.

Sometimes people will try to impress others with how fast something is going.  An example is jet airplanes flying in close formation.   The announcer says they are only feet apart will moving at 400 miles per hour, or some such.  But relative to others in the formation, they are traveling at close to zero mph.  That is the important comparison, since they are fixated on maintaining exactly zero mph relative to the lead airplane.

The Blue Angels in tight formation.
The Blue Angels in another tight formation.

The same can be said of astronauts in space.   The ISS orbits the earth at a speed of 17,150 mph, yet the astronauts sense very little movement unless they look out the window. And even then, since that speed is relative to the earth, and the earth is so far away (approximately 220 statute miles), the apparent motion is minimal, akin to floating very slowly and gently.

Astronaut Ellen Ochoa peers out window of International Space Station
(Courtesy NASA)

You can see the same effect with racing cars (or horses, or anything else).  From  the stands they are moving at 200 mph, while only a few inches from each other. But for the drivers, the relative motion is as close to zero as they can make it.

Indianapolis 500

Why is relative motion important?

Here’s an example from aviation.

During WWII, B-29’s over Japan encountered the jet stream, fierce winds above 25,000 feet that added or subtracted as much as 250 mph to an aircraft’s speed relative to the ground.

B-29 Superfortress

The B29 had the state-of-the-art bomb targeting system- the Norden Bombsight. The bomb system was an analog computer with 40+ parameters to tune, like wind velocity, the temperature outside, and even the earth’s curvature. The bombardiers (crew members who operated the device) would peek into their lens, mark the target, and set the parameters. The device would take care of the rest. The system worked flawlessly in Europe

But against Japan the jet stream pushed the bombers over the target too fast for the Norden bombsight to compensate. And flying against the jet stream on the return to base, the speed relative to the ground was so slow that the airplanes were sitting ducks or might even run out of fuel.

That is why all airplanes need two speedometers: one for airspeed (velocity through the air) and another for ground speed (velocity over the earth beneath the plane).  In some cases, the jet stream exceeded the speed of the aircraft, causing it to actually track backwards with reference to the ground.

What about relative motion in space?

Earth is revolving on its axis, then around the sun, then around the center of the galaxy, etc. But how can we know how fast it is moving in space? We can’t. The laws of physics say it’s meaningless (impossible) to make a distinction between what is moving and what is not. However, if something is accelerating, unlike velocity, everybody in the Universe will agree that it is accelerating, but not by how much.

This is what happens when we say the Sun rises or the Sun sets. We say this because we tend to think earth is stationary and the sun is moving, just because our surroundings do not change but the sun appears to.

Einstein demonstrated this with trains. Suppose you are on a train that is stopped and another train is stopped beside it.  If the other train begins moving forward, it is impossible for you to know whether that train is moving forward or if your train is moving backward.

Trains at station. Apparent motion is relative to the observer’s position.

Likewise, imagine you’re in a space suit, somewhere far from any other objects–so far away, you might as well be the only thing in the universe. Are you moving, and if so how fast? With nothing to compare your speed to, you can’t tell. Some people in history thought of space as a background against which you can be said to move or not move. But Galileo and Einstein, in their different ways, rejected this idea. They argued that since there’s no way to measure absolute motion (velocity relative to space itself), there’s no good way to define it, and so there’s no such thing.

Astronaut Bruce McCandless on spacewalk. Is he moving?

We can only define velocity relative to another object. That object can be real or imaginary. It might just be a coordinate system we’ve invented. But it’s a free choice. There are infinitely many other coordinate systems we could have chosen to define our velocity relative to. If we chose one, such as the earth, and said “this is the still point relative to which everything else moves” it would just be an arbitrary human choice, a whim, and nothing directly to do with the real nature of motion in the universe. This is the viewpoint of Newtonian mechanics, based on Galileo’s ideas, and of Einstein’s relativity.

So the next time you get a speeding ticket, consider going to court and arguing that you weren’t actually moving at all. And because the law you were cited under probably did not identify a reference framework for measuring your speed, you are not guilty.  Just hope you get a judge who has a degree in astrophysics. And a sense of humor! 🙂

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