# The Theory of Time Dilation and the Experiments Done to Prove It

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## Introduction

The Theory of Time Dilation was first introduced in 1905 by Albert Einstein as part of his Special Theory of Relativity. It explains that time actually slows down when objects are moving near the speed of light compared to stationary ones – but how exactly does it work? How can we prove that time dilation exists, and how can we see the effects on objects close to the speed of light? We’ll discuss this using simple examples and experiments so you can understand exactly what time dilation is and why it’s important in our universe today.

## Albert Einstein's Theory of General Relativity

In his Theory of General Relativity, Einstein gave us a new way to think about gravity. According to Einstein, gravity isn't just a force; it's a result of how things move through space-time. Simply put, everything from baseballs to planets curve space-time when they move around in it. We observe those curves as gravity. The most simple example: If you look at a spinning record (or any object on which friction slows its movement), you see that it seems to be curved. What you're seeing is a slowing down of time near the spinning object. To understand why, let's take another look at what special relativity tells us about moving objects: For example, if I have an atomic clock on Earth and one on my spaceship speeding away from Earth, I'd find that my ship clock was running slower than mine. This shows up as time dilation; clocks run slower when they move away from me because their local passage of time (measured in meters per second) is slower than mine. Or said another way, it takes longer for events to occur for observers traveling relative to me--they experience more time passing between two events because the distance traveled has increased during that period of observation.

## The Michelson–Morley Experiment

In 1887, Albert Michelson (1852–1931) and Edward Morley (1838–1923) conducted an experiment that was intended to measure Earth’s motion through space. As they hoped, they found Earth’s motion; however, they also found that light traveled at different speeds depending on whether it was traveling with or against Earth’s motion. What did that mean? If a body is moving relative to another body, then all physical properties must be affected by their relative movement. For example, if we were standing still and observed someone walking past us on a sidewalk, we would see them get shorter in our field of vision as they walked past us because their feet are moving towards us faster than their head is.

## The Signac Effect

In 1908 a French scientist by the name of Albert A. Michelson created an experiment that proved time dilation did indeed exist (this is now known as The Sagnac Effect). His setup was fairly simple, he took a rotating table covered in mirrors with some light source inside. He then made a beam splitter with half transmitting light out one side through some more mirrors, then back into another beam splitter; half reflected at 90 degrees into some photodetectors. He then mounted a small mirror on the output edge of his beam splitter so it acted like a piston moving at whatever speed his spinning table was going. After running hundreds of tests changing all sorts of things about his system, he found what we would today consider obvious: objects close to massive bodies are affected differently than those further away from them.

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## What is an Example of Time Dilation?

An example of time dilation is when two people on a train move away from each other at high speeds, where one observer sees their clock as moving slower than another observer. If one person leaves Earth for an interstellar trip at speeds greater than 186,000 miles per second (300 million meters per second), he will observe Earth’s clocks as ticking more slowly than his own. The discrepancy will be noticeable after only a few years, with seven years before every second on his watch passes on Earth also passing by on his watch. From his perspective, time appears to move slower because he is moving faster. Scientists call that time dilation.

## Examples from Day-to-Day Life

n 1956, Ives and Stilwell were able to measure time dilation in special relativity using an atomic clock that traveled with a commercial passenger flight. The anatomic clock consists of two separate elements. The first is a lump of atoms--cobalt-60 or cesium-133, for example--that keep track of standard time units as they decay at regular intervals. The second element is a sensor that detects when those regular intervals occur; by measuring how long it takes for radiation given off by these decaying atoms to reach sensors on Earth, physicists can calculate a set amount of elapsed time. When an atomic clock travels between reference frames, however, its actual distance from Earth changes slightly depending on where in its orbit around Earth it happens to be at any given moment.

## How it can Apply in Daily Life

The theory behind time dilation is that as a person goes faster, objects around them will go slower. This means that astronauts who are experiencing high-speed travel experience time more slowly than people on Earth. And so when they come back home, they have aged less than their friends or family members who stayed behind. Experiments were conducted in 1971 by Donnie McClure, who was traveling with a watch synchronized with a clock in Houston, Texas, just before liftoff. During his trip, he had traveled at speeds close to 1/10th of the speed of light. Once he returned, upon checking his watch against his clock in Houston, it had recorded 2% less time than for him being gone. This experiment has been replicated multiple times since then.

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