Time as a Physical Quantity

What is time in physics?

Time is one of the fundamental quantities in physics, defined as the interval between two events.

More generally, you can think of time as what allows us to perceive change and the progression of events.

In the physical world, time is measured using clocks, which track the duration of repeated periodic phenomena. The unit of time is the second.

But beyond being a measure of change, as we’ll explore, time plays a central role in our understanding of the laws governing the universe. Many questions about it still remain unanswered.

The unit of time: the second

The second (s) is the unit of time, and until 1967, it was defined as 1/86,400 of a mean solar day, based on Earth’s rotation.

However, since Earth’s rotation isn’t constant, a more precise definition was needed.

Today, the second is defined based on the atomic properties of cesium-133.

Specifically, one second equals 9,192,631,770 oscillations of the radiation emitted during the transition between two energy levels of the cesium-133 atom.

Measuring time: from sundials to atomic clocks

Our ability to measure time has seen remarkable advancements throughout history.

From the early civilizations that used the movement of the Sun to build basic sundials, we progressed to mechanical clocks, then quartz clocks, and finally atomic clocks, which use atomic oscillations to measure time with unparalleled precision.

Today, cesium-based atomic clocks are so accurate that they gain or lose less than a second every 60 million years.

Yet, research continues. Optical atomic clocks, and potentially future nuclear clocks, promise even greater accuracy.

These clocks will be able to measure time with an error of just 0.01 picoseconds per day, a level of precision that could revolutionize both physics and technology.

To put this in perspective, such a clock would be off by only about one second over the entire 13.8-billion-year lifespan of the universe.

Time in classical mechanics

In classical mechanics, as described by Isaac Newton in the 17th century, time is considered an absolute quantity.

Newton proposed that time flowed uniformly throughout the universe, regardless of the observer or the environment.

This view, often referred to as Newtonian time, imagines time as a river flowing steadily, independent of space.

This concept worked well to explain everyday phenomena and the mechanics of objects, but it proved inadequate as modern physics progressed.

Einstein's revolution: time and relativity

The 20th century brought a major breakthrough in our understanding of time with Albert Einstein’s theories of relativity.

According to the theory of special relativity (1905), time is not absolute but relative, it depends on the motion of the observer.

A famous example of this is the twin paradox: a twin traveling in space at speeds close to the speed of light ages more slowly than their sibling who remains on Earth. This occurs because, according to relativity, time dilates for objects moving at high speeds relative to a stationary observer.

The theory of general relativity (1915) added another layer of complexity. Einstein showed that time is not only affected by speed but also by gravity.

Near large masses, like planets or stars, time passes more slowly. This phenomenon, known as gravitational time dilation, has been confirmed by numerous experiments, including those using satellites orbiting Earth.

With general relativity, Einstein unified time and space into a single entity known as spacetime.

In modern physics, time is considered the fourth dimension, alongside the three dimensions of space (length, width, and height). In other words, events in the universe don’t just happen in three-dimensional space but in a four-dimensional spacetime continuum. The curvature of spacetime, caused by the mass of objects, affects both how objects move and how time flows.

Time and thermodynamics: the arrow of time

Beyond relativity, another important aspect of time in physics is tied to thermodynamics and the concept of entropy.

While most fundamental physical laws are time-symmetric (meaning they remain the same even if time were reversed), entropy introduces a clear distinction between the past and the future.

Entropy, which measures the amount of disorder in a system, tends to increase over time, in accordance with the second law of thermodynamics.

This gives time a preferred direction, often referred to as the arrow of time: the past is different from the future because entropy increases.

Quantum time

In quantum physics, the nature of time becomes even more puzzling.

While in classical theories, time is treated as a continuous variable, in the subatomic world, particles seem to follow probabilistic laws that blur the concept of time.

Some physicists have even speculated that time itself might be quantized, existing in discrete "packets" like energy (discrete variables).

However, we currently lack a complete quantum theory of time that unites general relativity with quantum mechanics.

Time: a fundamental and mysterious quantity

Despite all the advancements we’ve made in understanding and measuring time, its true nature remains one of the greatest mysteries in physics.

The question "what exactly is time?" remains beyond the reach of modern science to fully answer.

Is it a fundamental dimension of the universe or merely a construct of our minds? Is time continuous or discrete? These questions continue to intrigue both philosophers and scientists and may require a deeper physical theory to answer, one that unites gravity and quantum mechanics.

In conclusion, time remains at the core of modern physics, shaping our understanding of the cosmos, but many questions still remain unanswered.