What is relativity?

By: Robert Lamb

Conditions inside a speeding train illustrate a great deal about relativity.
Conditions inside a speeding train illustrate a great deal about relativity.
Peter Cade/Iconica/Getty Images

Relativity is like a triple-scoop ice cream cone; most of us just can't gobble it down in one bite, not without experiencing some serious brain freeze. So let's tackle the topic one scoop at a time. We'll start with the version of relativity that dates back more than four centuries: Galilean relativity.

Yes, this scoop of cosmic gelato originates with famed Italian astronomer Galileo Galilei, and it breaks down like this: Any two observers moving at constant speed and direction will obtain the same results for all mechanical experiments.


Let's say the experiment in question is nothing more complicated than throwing a Ping-Pong ball down the aisle of a train. As long as the speed and direction are constant, the Ping-Pong ball would behave exactly the same whether the train's creeping along at a snail's pace or barreling down the tracks. As long as the train isn't jerking around due to speed or directional changes, there's absolutely no difference inside the train car.

Outside the speeding train, however, it's a different story (or frame of reference).

To the individual aboard the speeding train -- let's say it's traveling at 100 miles per hour (161 kilometers per hour) -- the ball appears to move at regular speed. To the individual standing by the tracks, the ball (assuming he or she could see it) would appear to move with the speed of the train, plus the speed with which it was thrown.

How fast is that ball really traveling? Let's say you threw it at a mere 5 miles per hour (8 kilometers per hour). If we added the speed of the train to it, we'd get a total speed of 105 miles per hour (169 kilometers per hour) -- a calculation known as a Galilean transformation. Aboard the train, it wouldn't feel like 105 miles per hour if it bounced up and hit you in the chest. Relative to the outside however, that's the speed it would be traveling.

Now here's where it gets tricky: What if you were to shine a flashlight up the aisle of the train? Would the light waves travel 100 miles per hour faster than the speed of light? Not so, according to physicists Albert A. Michelson and Edward Morley. In 1879, the two Americans conducted a groundbreaking experiment to measure the speed of light. As it turns out, light travels at a constant speed of 186,000 miles per second (300,000 kilometers per second). It can't travel any faster by any means, breaking the concept of Galilean relativity.

Luckily, Albert Einstein stepped in to fix things in 1920 with his theory of special relativity.

Special Relativity and General Relativity

The theory of general relativity better enables us to study distant stars.
The theory of general relativity better enables us to study distant stars.

Let's heap a second scoop onto the relativity cone -- a lovely taste of Black Forest courtesy of German-born physicist Albert Einstein. As we just mentioned, Galilean relativity, even after it got a few tweaks from Newtonian physics, was broken. Scientists learned that light travels at a constant speed, even on a speeding train.

Therefore, Einstein proposed the theory of special relativity, which boils down to this: The laws of physics are the same in all inertial frames, and the speed of light is the same for all observers. Whether you're in a broken-down school bus, a speeding train or some manner of futuristic rocket ship, light moves at the same speed, and the laws of physics remain constant. Assuming speed and direction are constant and there wasn't a window to peer through, you wouldn't be able to tell which of these three vessels you were traveling in.


But the ramifications of special relativity affect everything. Essentially, the theory proposed that distance and time aren't absolute. 

Now it's time for the third ice cream scoop, and it's another hefty helping from Einstein. Let's call it German chocolate. In 1915, Einstein published his theory of general relativity to factor gravity into the relativistic view of the universe.

The key concept to remember is the equivalence principle, which states that gravity pulling in one direction is equivalent to acceleration in another. This is why an accelerating elevator provides a feeling of increased gravity while rising and decreased gravity while descending. If gravity is equivalent to acceleration, then it means gravity (like motion) affects measurements of time and space.

This would mean that a sufficiently massive object like a star warps time and space through its gravity. So Einstein's theory altered the definition of gravity itself from a force to a warping of space-time. Scientists have observed the gravitational warping of both time and space to back this definition up.

Here's how: We know that time passes faster in orbit than it does on Earth because we've compared clocks on Earth with those on orbital satellites farther from the planet's mass. Scientists call this phenomenon gravitational time dilation. Likewise, scientists have observed straight beams of light curving around massive stars in what we call gravitational lensing.

So what does relativity do for us? It provides us with a cosmological framework from which to decipher the universe. It allows us to fathom celestial mechanics, predict the existence of black holes and chart the distant reaches of our universe.

Explore the links on the next page to learn even more about cosmology.

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More Great Links

  • Fowler, Michael. "Special Relativity." Galileo and Einstein. March 3, 2008. (Sept. 2, 2010)http://galileoandeinstein.physics.virginia.edu/lectures/spec_rel.html
  • "Gravitational Lensing: Astronomers Harness Einstein's Telescope." Science Daily. Feb. 24, 2009. (Aug. 9, 2010)http://www.sciencedaily.com/releases/2009/02/090220172053.htm
  • Knierim, Thomas. "Relativity." The Big View. June 10, 2010. (Sept. 2, 2010)http://www.thebigview.com/spacetime/relativity.html
  • Lightman, Alan. "Relativity and the Cosmos." NOVA. June 2005. (Sept. 2, 2010)http://www.pbs.org/wgbh/nova/einstein/relativity/
  • "Relativity." Worldbook at NASA. Nov. 29, 2007. (Sept. 2, 2010)http://www.nasa.gov/worldbook/relativity_worldbook.html
  • Ryden, Barbara. "Special Relativity." Ohio State University Department of Astronomy. Feb. 10, 2003. (Sept. 2, 2010)http://www.astronomy.ohio-state.edu/~ryden/ast162_6/notes23.html
  • Wright, Edward. "Relativity Tutorial." UCLA Astronomy. Sept. 4, 2009. (Sept. 2, 2010)http://www.astro.ucla.edu/~wright/relatvty.htm