Time dilation in physics

Important concepts of Time dilation

Did you watch Interstellar movie?
I guess most of you did watch it. You might have wondered why such a difference in time existed in the movie. One hour on the planet was equivalent to seven years on Earth. It is a bit of an exaggeration to say this, but such a difference in the passage of time is true. Do you know that gravity distorts space and time, i.e., time passes differently for a person when he’s standing on a star when observed from Earth, or time passes relatively slower for a person standing on International Space Station than for a person standing on Earth, or time passes differently for a moving person when observed by a stationary person, this phenomenon is called time dilation. This article gives comprehensive information on this fantastic concept in physics called time dilation in space.

Spacetime concept is a path-breaking idea of Albert Einstein

Time dilation: Introduction

Time is a fascinating concept in physics. Many movies have been made on an imaginary idea called time travel. Movies like Interstellar and Ender’s Game have given us a glimpse of how time passes differently for different points in space. Time dilation is a concept in physics that explains the difference in time measured between two clocks. This difference occurs in two forms. In the first form, when person A travels at a certain velocity relative to person B, the two observe the change in the passage of time with respect to each other. In other words, moving bodies appear to move slowly for stationary bodies. In 1905, Albert Einstein proposed this phenomenon in his theory of special relativity. In the second form, time dilation can be observed when there is distortion in space due to gravity. Spacetime can be imagined as a cloth where all the objects, from massive stars to tiny hummingbirds, are embedded. These objects create a curve in the fabric of spacetime. Massive objects like black holes, neutron stars create more distortion on this spacetime fabric. As a result, the clock of person A standing on these heavy objects runs slower than person B observing from the earth. Albert Einstein first described this gravitational time dilation concept in his theory of general relativity.

Time dilation: A brief history


Before Albert Einstein proposed the theory of special relativity, everyone assumed that time is absolute. Newton proposed his work on classical mechanics. He presumed that time and space are absolute, and their passage for a body is independent of its surroundings. It was widely accepted among scientists until Maxwell proposed his work on electromagnetism in the 19th century. According to Maxwell, light propagates in the form of waves and has a constant speed of 186,000 miles/sec. But any wave needs a medium to propagate successfully. Similar to sound waves travelling through the air medium, it was initially thought that a hypothetical, ubiquitous medium existed called Aether.

The idea of Aether medium

At that time, many scientists conducted experiments to validate the Aether concept. In 1881, A. Michelson conducted one such famous experiment to calculate the relative motion of Earth and Aether using an interferometer. It wasn’t until 1887, Michelson and Edward Morley repeated the same experiment with higher accuracy; they couldn’t provide enough proof as there was no discernible change in diffraction patterns, and the results were contradictory to each other. .
Galileo proposed his transformation theory which gives information on the transformation of co-ordinate axes of two reference frames where they move at a constant relative motion from one another. Unfortunately, this theory failed as it didn’t work when the velocity approached the velocity of light. It wasn’t until Lorentz introduced Lorentz transformations and explained important concepts like the doppler effect and aberration of light. Gradually, the concepts of time dilation and length contraction were derived.

Einstein theory of relativity


In 1905, Einstein published his ground breaking paper called special relativity. He introduced the concept of spacetime, the principle of relativity, and the constant speed of light and dismissed the idea of Aether. Einstein conducted several thought experiments, such as magnet and conductor, lightning strikes experiment etc., radically changed the way we see the world. Similarly, in 1907, Einstein first came up with the idea called gravitational time dilation in his theory of general relativity.

Time dilation is real and many tests have proved it
Time dilation is real and several studies proved this

Time dilation: explanation in simple words

Time dilation due to relative velocity:

Let us consider a situation. If person A is sitting on a train wants to measure the time taken by light to reach from one point to another. He has a flashlight and a stopwatch with him. He turns on the flashlight and uses the stopwatch to calculate the time taken by the light to reach the other end.
Now imagine person B standing at a train station with the same apparatus. He uses the stopwatch to calculate the time period. For this person, the light beam travels with an inclination. Unfortunately, we must take the train’s speed into account in this case. So, the relative velocity comes into the picture. As the speed of light remains constant, we can derive the time dilation expression from Lorentz transformations.

t = t₀/√(1-v²/c²)

where: t = time observed by person B who sees the train as moving,

t₀ = time observed by person A(stationary)

v = the speed of the train

c = the speed of light in a vacuum

We can say from this basic time dilation equation that the person who’s stationary and sees the moving clock perceives that his clock runs faster than the person who’s actually moving and is stationary to the clock frame of reference. if you’re traveling at a speed closer to the speed of light, your clock would show it takes less time to reach your destination as it would on Earth. Space and time get all distorted as you approach the speed of light. This point is where the special theory of relativity can be easily misunderstood and forms the basis for the twin paradox. Two twins, about same age, one travelling nearly the speed of light heading for a distant location in space, observers that she aged lesser than the second twin back at home. Both the twins see the other as moving and think they would age less due to time dilation; this logical contradiction is resolved by doppler effect and relativity itself.

Time dilation due to gravity:

Gravity is not a force. Simply, it is a curvature in spacetime. Bodies with massive masses such as stars, neutron stars, black holes etc., create a curvature in spacetime itself. For example, a person standing on Earth observes a person on a star; clearly, the star exerts more gravity than Earth and, thereby causing more curvature in spacetime, sees that his clock runs faster than the person on the star. Here, time itself is bending, and the slowing effect in time is relative.

Gravitational time dilation
Gravity bends space and time

Time dilation: Experimental proofs

Even though Einstein predicted time dilation in his theories, they are fundamental to many theories of physics today. It’s crucial to test this theory’s accuracy with utmost caution and importance.
In 1971, an experiment was conducted with highly accurate atomic clocks aboard commercial airlines. This experiment was known as the Hafele-Keating experiment. Two commercial planes set about their journey in opposite directions; the plane moving in the direction of rotation of the Earth (inertial frame of reference) had its clock running slower(as it is running faster) than the other plane moving in the opposite direction. This phenomenon first confirmed the time dilation due to relativity.
Two commercial planes set about their journey in opposite directions; the plane moving in the direction of rotation of the Earth (inertial frame of reference) had its clock running slower(as it is running faster) than the other plane moving in the opposite direction. This phenomenon first confirmed the time dilation due to relativity.
The clocks on the plane moved faster than the clocks on earth as earth’s gravitational field causes curvature in spacetime. This proved gravitational time dilation. Funny fact is that your feet age slower than your head due to gravity.
A few years ago, German scientists conducted tests on particle accelerators to verify the effect of time dilation. The results of the experiment successfully corroborated with Einstein’s predictions about time dilation.

Time is a fascinating concept in physics. Though Einstein’s work provided us with many answers, there’s a lot to learn and understand.

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