###### By Don Lincoln**, **Fermilab

## When it comes to modern science, things changed radically in about 1900, when quantum mechanics and relativity began to become understood. Relativity simply blows people’s minds. Clocks slow down, rulers shrink, and, in the right circumstances, a young man could lay to rest his aged twin. How?

### The Birth of Modern Science

A sensible date for the birth of modern science is 1543, when Nicolaus Copernicus’s published his book *De revolutionibus orbium coelestium*, or *On the Revolutions of the Heavenly Spheres*. From there, the development of the scientific method took off, with such luminaries as Sir Isaac Newton and James Clerk Maxwell working out the rules of motion, gravity, and electromagnetism.

By the late 1800s, we had a very good understanding of what we now call the classical world. Certainly, some of the calculations were complex, like a detailed understanding of the motion of the Moon around the Earth, and some of the phenomena were counterintuitive, like how blowing across the top of a paper can lift it.

Things changed rapidly with the theory of relativity coming into the picture. Many people don’t like Albert Einstein’s ideas on relativity. And yet, his ideas have been proven again, and again, and again. It’s not simply a matter of theoretical musings. Clocks really do slow down. Rulers really do shrink.

This article comes directly from content in the video seriesThe Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.

### Theory of Special Relativity

To begin with, relativity isn’t a new thing. We’ve known about simple forms of relativity since the time of Galileo, back in 1600 or thereabout. At a deep and fundamental level, relativity isn’t about clocks ticking at different speeds or people disagreeing with each other over the length of a ruler. Basically, all relativity is, is about how different people see the same situation. A sort of a conversation between two raging egomaniacs who both absolutely insist that they are the center of the universe.

Let’s take an example in order to gain a better understanding. Suppose there are two people, Alice and Bob. The two of them are located basically in the same spot, standing back to back. They are moving away from one another at 10 miles per hour. Both of them agree that the direction that Alice is looking is the positive direction and, of course, the direction Bob is looking is the negative direction. They call the direction Alice is looking the positive x direction.

### Position Zero

Alice insists that the world revolves around her. Accordingly, she is at the center of the universe, or what a mathematician or physicist would call position zero or, more mathematically, x equals zero. Time moves as time has a habit of doing. But she’s not moving. Thus, we would say at the time we started this imaginary scenario, Alice says her location is zero and the time is zero. An hour later, she says that she’s still at position zero, but time is now plus an hour. An hour after that, she is at position zero, but now time is plus two hours.

Now, we could ask Alice where Bob is. Since he’s behind her, which is the negative direction, and he is moving away from her, his position gets more and more negative. When we started this thought experiment, according to Alice, Bob is at both position and time zero. After an hour, she says that he is at position negative-10 miles, at time equals one hour. After a second hour, she says that Bob is at position negative-20 miles at time equals two hours.

### How Different People See the Same Situation

Relativity is about how different people see the same situation. What does Bob say? Well, Bob says that she’s just wrong. It is he that is at the center of the universe, and Alice is moving. Remember that they agreed on a direction. So, according to Bob, Alice is moving in the positive x direction. When they started, they actually agree on both location and time. Bob says that Alice is at position equals zero at time equals zero. After an hour, Bob says that time is now plus an hour and, according to him, he’s still at location zero.

However, he thinks that Alice is at location plus-10 miles. An hour later, the time is now plus two hours and he’s still at location zero according to him. Alice, on the other hand, is now at plus-20 miles.

And that’s the most important feature of relativity. Alice says she’s stationary and that Bob’s position is constantly getting more and more negative. Bob says that he’s stationery and Alice’s position is getting more and more positive. And they agree about time.

### Einstein’s Theory of Special Relativity

So, that’s relativity, or at least what we call Galilean relativity. Now Einstein’s theory of special relativity is not all that different at the big picture level. The equations are surely somewhat different, although they look pretty similar. But there are a couple of qualitative differences.

The first and biggest difference is that Alice and Bob experience time differently. This is very easy to prove using the Pythagorean Theorem and by assuming that the speed of light is the same for all observers. This last assertion is the bit that is hard for people to accept.

The second big difference is that when a person observes a moving object, it is shorter than the same object, observed by the same person, if that person sees the object stationary with respect to them. Anyone who has ever taken an introductory physics class, would’ve observed this to be true, even though one doesn’t realize it when they are it.

Thus, the bottom line is that moving objects are shorter than stationary ones.

### Common Questions about Modern Science and the Theory of Relativity

**Q: What would be a sensible date for the birth of modern science?**

A sensible date for the birth of modern science is 1543, when Nicolaus Copernicus’s published his book *De revolutionibus orbium coelestium*, or *On the Revolutions of the Heavenly Spheres*.

**Q: What is relativity?**

All relativity is, is about how different people see the same situation. A sort of a conversation between two raging egomaniacs who both absolutely insist that they are the center of the universe.

**Q: According to Einstein’s theory of special relativity, what does a person observing a moving object see?**

According to Einstein’s theory of special relativity, when a person observes a moving object, it is shorter than the same object, observed by the same person, if that person sees the object stationary with respect to them.