# How Things Work: Time Dilation

With over a century since its debut in scientific literature, time travel may be more of an everyday reality than most people think.

By definition, time dilation is the difference in time measurement between two observers, where one moves with respect to the other. Time dilation has been experimentally proven.

Albert Einstein was the first scientist to claim that time runs at different rates depending on the relative velocity between two observers. In 1905, Einstein published a paper titled *The Electrodynamics of Moving Bodies* in which he put forth his “Special Theory of Relativity.”

According to this theory, the speed of light and the form of physical laws are the same for all non-accelerating observers. In other words, the speed of light and the relationship between force, mass, and acceleration are the same for all observers moving at constant velocity.

However, the only way this can happen is if an observer’s space and the time measurements of a system depend on the system’s velocity relative to that observer. This means that two observers can measure the time interval between the same two events and come up with different time measurements for these events, as long as one of the observers is moving at constant velocity with respect to the other.

To understand time dilation at a more technical level, one must first understand the concept of a reference frame. A reference frame is an imaginary coordinate system that specifies the location and time measurements of events with respect to a fixed origin. It can also be thought of as an imaginary map.

In space-time physics, every person (or observer) has his or her own reference frame in which he or she is the origin. Thus, in space-time physics, a person assigns spatial and time coordinates to events based on his or her position.

Having now defined reference frame, time dilation can be thought of as the difference between time measurements of two reference frames that are moving at different velocities with respect to one another.

Another way to think of time dilation is in terms of flight. When two people synchronize their clocks to read the same time, this synchrony remains as long as the two people remain at rest with respect to one another. However, if one person boards an airplane and flies a certain distance, that person’s clock will run at a slower rate than the person on the ground.

Researchers used this same scenario in the 1970s to test Einstein’s theory of time dilation. In this experiment, they used atomic clocks, which are very accurate clocks that can measure time to billionths of a second. One clock remained on the ground, and the other clock flew on a jet at 600 miles per hour. As predicted by Einstein’s Special Theory of Relativity, the clocks ran at different rates. The airplane clock ran billionths of a second slower than the ground clock.

Time dilation also occurs in the detection of muons. Muons are particles created in the atmosphere by cosmic rays. They fall constantly down to Earth at around 200,000 miles per second. The muons decay as they fall, and scientists are able to predict how many muons will decay after a certain amount of time has passed in the laboratory (or Earth) reference frame.

However, because muons move so quickly with respect to the laboratory reference frame, time passes slower in the muon reference frame than in the laboratory rest frame. Thus, more muons survive than predicted in the laboratory reference frame according to this frame’s clock.

The physical explanation for time dilation is still under research, but it has been proposed that particle physics is the reason for time dilation.

The Basic Particle Model says that every particle consists of smaller particles that orbit each other at the speed of light. That is, each of these particles has an “orbital speed” that equals the speed of light.

According to Einstein’s Special Theory of Relativity, the speed of light remains constant in all reference frames. Consequently, when a particle moves horizontally, the total speed of the constituent particles with respect to the rest frame is still equal to the speed of light. The difference, though, is that when the particle is moving horizontally, the total speed of the particle is made up of “orbital speed” and “horizontal speed” components, rather than just an orbital speed component as is the case when the particle is at rest.

This leads to the orbiting particles having a longer period with respect to the rest frame. The period is the amount of time that it takes one of the particles to complete one full orbit.

The time difference between the two reference frames is equal to “gamma,” or the Lorentz factor. The Lorentz factor is a mathematical expression that accounts for the relative velocity between the reference frames and the speed of light.