Every day, we check the clock to keep track of time. We rely on it to know when to wake up, when a train will arrive, or when to meet someone. But have you ever wondered whether time actually moves at the same rate everywhere?
Modern physics says no.
The passage of time can be affected by both speed and gravity. That is why scientists need extremely precise atomic clocks to measure time. Cesium-133 plays a particularly important role in these clocks.
Why Is Cesium-133 So Special?
Cesium-133 is an atom used to establish the scientific definition of time around the world.
An atomic clock measures the frequency of microwave radiation associated with a specific transition between energy states of cesium-133 atoms. This process is extremely stable and regular, making it possible to measure time with remarkable precision.
In other words, scientists do not define time based on the ticking of an ordinary clock. Instead, they rely on an extremely stable process found in the nature of atoms.
How Long Is One Second, Anyway?
This is where the story gets even more interesting.
In the International System of Units (SI), one second is defined as the duration of 9,192,631,770 periods of radiation corresponding to a specific transition between energy levels of the cesium-133 atom. Simply put, scientists have connected the unit of time to an extremely stable atomic phenomenon found in nature.The advantage is that time can be defined not by an old mechanical clock, but by an extremely regular and reproducible measurement based on nature.Think about it—the one second that seems so short to us is based on an incredibly precise count of more than 9 billion oscillations at the atomic level.
But Can Gravity Also Change Time?
Yes.This is where Albert Einstein’s General Theory of Relativity becomes especially important.
According to this theory, gravity is not simply understood as a force that pulls objects downward. Mass and energy affect the structure of spacetime.One consequence of this is known as gravitational time dilation.
Time Runs More Slowly in Strong Gravity
Where gravity is stronger, a clock can appear to run more slowly compared with a clock farther away from the source of gravity.For example, gravity can be stronger near a very massive object. Under extreme conditions, this effect becomes particularly significant around a black hole.
A Clock Runs Relatively Faster in Weak Gravity
In contrast, a clock in a weaker gravitational field can run slightly faster than one in a stronger gravitational field.We cannot notice this difference in everyday life, but extremely precise atomic clocks can measure it.
Is This Just Physics From Textbooks? Absolutely Not!
GPS, which is used on your phone, is a great example of how deeply Einstein’s physics is connected to our everyday lives. GPS satellites orbit Earth at a considerable altitude and travel at very high speeds. As a result, their clocks are affected by two different relativistic effects.
Effect of gravity: The satellite is farther from Earth, where gravity is relatively weaker. This affects its clock in a way that makes it run faster compared with a clock on Earth’s surface.
Effect of speed: The satellite is moving at a very high speed. According to Special Relativity, high speed affects the clock in the opposite direction, causing it to run more slowly.When the two effects are combined, GPS satellite clocks run about 38 microseconds per day faster than clocks on Earth’s surface.
So If the Difference Is So Small, Why Does It Matter?
The answer is GPS.GPS satellites orbit far above Earth’s surface and travel at very high speeds. As a result, their clocks are affected by two different relativistic effects.On one hand, being at a greater altitude means the gravitational field is relatively weaker, causing the satellite clock to run faster. On the other hand, the satellite’s high speed causes its clock to run more slowly according to Special Relativity.Over time, this can create a very large error—potentially reaching the kilometer scale per day.
Time Is More Than Just the Hands of a Clock
This entire story helps us understand something fascinating.
To us, time may simply mean the seconds, minutes, and hours shown on a clock. But in modern physics, time is a deeper scientific concept connected to both motion and gravity.On one hand, a cesium-133 atom helps us establish an extremely precise unit of time. On the other hand, Einstein’s theory of relativity tells us that clocks can run at different rates under different conditions.And the most interesting part is that this is not merely a theoretical idea.The GPS technology used in our smartphones has to account for these relativistic effects in order to work correctly.
In other words, Einstein’s physics is not just something found in textbooks—it is also present in the modern technology we use around us every day.
