###### By Emily Levesque, University of Washington

## Albert Einstein published his work on special relativity—and three other ground-breaking research papers—in 1905, at the age of 26. Contrary to legend, his theory didn’t quite erupt from nowhere out of an obscure patent office. Einstein had been researching and publishing for years and acknowledged the work of fellow physicists like Max Planck and Hermann Minkowski as important influences. Still, the theory was seen as a great and triumphant scientific leap.

### Time Dilation

When we consider time, with respect to Einstein’s special theory of relativity, a strange implication crops up. Imagine a baseball pitcher on the back of a moving train moving 40 miles an hour. If we give clocks to our pitcher and another person watching the train, perfectly tracking how they experience time, they’ll each see their own clock as working normally. But if they check each other’s clocks, things get strange: the clock on the train will run ever so slightly slower, reflecting that in the train’s frame of reference time has actually slowed down.

At the speed of a train this effect is infinitesimal, but in frames of reference moving at much faster speeds—say, approaching the speed of light—time will slow down dramatically. This phenomenon is known as time dilation, and while it may seem strange, it’s a crucial detail of special relativity that keeps time and physics working perfectly, even for a person in a very fast-moving frame of reference.

### Inertial Frame of Reference

Einstein’s fellow physicists, many of whom had also been grappling with the puzzling contradictions of mechanics and electromagnetism, were quick to accept his theories. His solution was simple, elegant, and worked perfectly. Almost. Einstein’s new theory, and his new explanations for how space and time worked across different frames of reference, still made one important assumption.

Crucially, relativity states that the laws of physics must stay the same everywhere, but today we call Einstein’s work from 1905 special relativity. In special relativity, Einstein kept this idea simple: he imagined the laws of physics hard at work in any inertial frame of reference; in other words, in any place that didn’t have a net force acting on it.

Remember our pitcher standing on the train? Let’s give him a break; imagine him walking back through the train, sitting down, and pouring himself a glass of water. Even though the train is moving 40 miles an hour, from our pitcher’s perspective everything inside the train is sitting still, and pretty much normal. He can walk to his seat, sit comfortably in his chair, and the water he pours drops straight into his class, just like the laws of physics say it should. The train is moving smoothly, and he’s in an inertial frame of reference.

### Acceleration Due to Gravity

But what happens if the train suddenly speeds up to 80 miles an hour? Suddenly, the pitcher stumbles on the way to his seat, or gets thrown back into his chair, or the water he was pouring into the glass winds up all over the place. This happens because the pitcher’s frame of reference isn’t inertial anymore: the train accelerated, and he felt that as a sudden change in the forces acting on him. The laws of physics haven’t changed, but we have introduced a new effect: the acceleration of the train.

For Einstein’s theory of relativity, going from special relativity to general relativity—making the laws of physics the same in all frames of reference, and not just inertial frames of reference—meant contending with a crucial and ubiquitous source of acceleration. It’s an acceleration we’re all experiencing right now, holding my feet on the floor and keeping all the things around me from floating away. It’s acceleration due to gravity.

This article comes directly from content in the video seriesGreat Heroes and Discoveries of Astronomy.Watch it now, on Wondrium.

### General Relativity and Spacetime

Scientists have known the math of gravitational attraction since Isaac Newton, and while Einstein’s special theory of relativity did a great job of reconciling mechanics and electromagnetism, he knew it didn’t quite work for gravity. Special relativity had explained the relationship between space and time. General relativity, which Einstein developed between 1907 and 1915, expanded this work to describe the relationship between space, time, and gravity.

We imagine this today by invoking the concept of spacetime, with space and time inextricably linked to make up the fabric of our universe. In special relativity, we imagine spacetime as flat, with something like light traveling neatly along straight lines. But in general relativity, when we introduce gravity, spacetime becomes curved. An object with a lot of mass, like a planet or star, will affect spacetime thanks to the effects of its gravitational field, warping the fabric of the universe and appearing to bend the path that light takes as it travels, changing its apparent position and travel time.

### Einstein’s Other Discovery: Photoelectric Effect

Tying together gravity, space, and time is today considered one of Einstein’s greatest discoveries, and he made many. Oddly enough, his Nobel Prize in 1921 actually had nothing to do with relativity. Instead it mentions another discovery he made in 1905, the photoelectric effect. This is the emission of electrons caused when light hits a surface, similar to what Kent Ford came across while designing the image tube in the 1960s, which he and Vera Rubin used to discover dark matter.

Today, we’re still testing and exploring the implications of general relativity. Although it might not seem to have all that much to do with astronomy considering that we’ve been talking about pitchers and glasses of water and speeding trains. Still, on a cosmic scale the interaction between space, time, gravity and light—the primary tool astronomers have at our disposal for studying the universe—is a cornerstone of how we explain the way the universe works and we have Einstein to thank for it!

### Common Questions about Einstein’s Theory of General Relativity

**Q: Which fellow physicists did Albert Einstein acknowledge****as important influences?**Albert Einstein had been researching and publishing for years and acknowledged the work of fellow physicists like Max Planck and Hermann Minkowski as important influences.

**Q: What did Albert Einstein’s special theory of relativity**did not work for?While Albert Einstein’s special theory of relativity did a great job of reconciling mechanics and electromagnetism, he knew it didn’t quite work for gravity.

**Q: What did Albert Einstein**win the Nobel Prize for?Albert Einstein‘s Nobel Prize, in 1921, mentions a discovery he made in 1905, the photoelectric effect.