Time in physics

Time tells us when things happen.

Long ago, people used the sun. They watched the sun move in the sky. They also watched the stars.

People used shadows to tell time. A shadow falls on the ground. The sun makes the shadow move. This helped farmers work.

Later, people made clocks. Some used gears and wheels. Others used a swinging weight. This weight is a pendulum.

Now we have very special clocks. They use tiny parts of an atom. These clocks are very, very good. They stay right for a long time.

Time started with the big universe. It began a long time ago. We use time to learn about space.

In physics, time is what a clock reads. It is a way to measure how things change.

Long ago, people looked at the sky. They watched the sun and the stars. They used the sun to mark the day. Farmers used the sun for thousands of years. Some used a gnomon to track shadows. A gnomon is a stick that casts a shadow.

Later, people made mechanical clocks. These used gears and wheels to show the time. Galileo found that a swinging weight, called a pendulum, moves in a steady way. This helped people make better clocks.

Today, we use atomic clocks. These clocks are very, very accurate. They use the tiny parts of a caesium atom. These clocks help us keep time all over the world. Scientists also study how time and space work together. This idea is called spacetime. Some thinkers once believed time was the same for everyone. Now we know that time can run at different rates. This is part of the theory of relativity. Time began about 13.8 billion years ago with the Big Bang.

In physics, time is defined by what a clock reads. It is a way to measure how things change and move.

Scientists treat time as a fundamental quantity, just like mass or length. It can be combined with other numbers to study motion or energy. Timekeeping is a huge task that helps us keep records of our world. Without measuring time, we could not understand how the universe works.

Long ago, people used nature to mark the passing hours. They watched the sun rise and set every single day. Farmers used the sun for thousands of years to know when to work. Some used a gnomon, which is a stick that casts a shadow. By measuring the shadow's length, they could tell the time. Mesopotamian astronomers even watched the stars with their naked eyes 3,500 years ago.

History shows how our tools for time became much better. In 1583, Galileo Galilei discovered that a swinging pendulum moves in a steady way. He even used a water clock to time a rolling ball. Later, Isaac Newton thought time was like a universal clock for everyone. In the 1300s, Richard of Wallingford built a mechanical clock using gears. Eventually, people made small pocket watches that families could use at home.

Today, we use incredible technology to keep time very accurately. The standard unit for time is the second. Since 1967, the second is defined by a caesium 133 atom. We use caesium atomic clocks to keep this standard. These clocks are so precise they are used for science and satellites. One modern time standard is accurate to within one second every 30 million years.

Our ideas about time have changed a lot over the years. Most people used to think time was the same for everyone. Now, thanks to Einstein, we know about the theory of relativity. This theory says time and space are joined together as spacetime. Rates of time can actually run differently depending on how you move. Scientists believe time itself began about 13.8 billion years ago during the Big Bang.

In physics, time is defined by its measurement. This means that time is essentially what a clock reads. Scientists treat time as a fundamental quantity, much like mass, length, or electric charge. It is a scalar quantity, which means it is a value that does not depend on direction. Time can be used mathematically to understand other important concepts. For example, it helps scientists calculate motion, kinetic energy, and time-dependent fields. Timekeeping is a complex mix of science and technology. It also serves as a vital foundation for all human recordkeeping.

Before modern clocks existed, people relied on natural physical processes to mark time. These processes were understandable to every civilization. For example, ancient people watched for the first appearance of the star Sirius. This helped them predict the yearly flooding of the Nile. Others observed the regular cycle of night and day. Farmers used the position of the sun in the sky to mark their work. They watched the sun rise at dawn and reach its highest point at noon. To do this, they often used a gnomon, which is a simple stick that casts a shadow. By measuring the length of this shadow, they could determine the time of day.

Humanity has developed many different ways to track the passage of time. In ancient Mesopotamia, astronomers recorded the stars using only their naked eyes over 3,500 years ago. This changed in 1609 when Galileo Galilei began using a telescope. This new tool allowed for much more precise observations. Eventually, mechanical clocks began to appear. In the 1300s, Richard of Wallingford built a complex mechanical clock that worked like an astronomical orrery. As technology improved, clocks used ratchets and gears to display time. These machines grew smaller over time. Eventually, they became personal pocket watches that families could own. Later, pendulum clocks became common in the 18th and 19th centuries.

Today, our most accurate timekeeping relies on atomic physics. In the International System of Units, the standard unit of time is the second. Since 1967, the second has been defined by the radiation of a caesium 133 atom. Specifically, it is the duration of the transition between two hyperfine levels of the atom's ground state. This is measured using a caesium atomic clock. These clocks became practical for scientific use after 1955. The current worldwide time standard, known as UTC, is an atomic time standard. It is incredibly precise. Its accuracy is about 10⁻¹⁵ seconds. This means it would only lose one second every 30 million years.

Our scientific understanding of how time works has shifted significantly over the centuries. For a long time, scientists like Galileo and Isaac Newton believed time was absolute. Newton described time as a universal clock that flows steadily for everyone. He believed time was a linear parameter that moved the same way regardless of external factors. However, Albert Einstein changed this with his theory of relativity. Modern physics shows that time and space are actually merged into a single fabric called spacetime. In this view, time is a coordinate rather than just a background timeline. Rates of time can actually change depending on how fast an object is moving.

There are even theoretical limits to how small a measurement of time can be. Scientists discuss a concept called Planck time. This is the smallest time step that is theoretically observable. It is approximately 5.391 × 10⁻⁴⁴ seconds. This number is much smaller than anything our current clocks can measure. On a much larger scale, scientists look at the beginning of time itself. According to the Big Bang theory, time began with the universe about 13.8 billion years ago. This connects the tiny measurements of atomic clocks to the massive history of the entire cosmos.