The concept of time is crucial in describing our environment. The passage of time enables
us to quantify object change and mobility. At first glance, one could think that with
a single clock for the entire universe, time is absolute.
Time isn’t as simple as we might believe. We tend to work by the clock, arriving at
9 a.m. and leaving at 5 p.m., and our perception of time does not vary during that time, though
it seems to slow down before the weekend. At the turn of the 20th Century, a discovery
radically shattered our previous understanding of relativity. Time is no longer absolute,
but relative. Each item has its own time and clock, and
two previously synchronized clocks on different trajectories across space can meet again and
show different times. What is time dilation? Time dilation is the
seemingly strange phenomenon of time passing slower for an observer who is moving relative
to another observer, depending on their relative motion or positions in a gravitational field.
This is how it works. Time is a relative concept. That, as absurd as it sounds, is a result
of Einstein’s theory of relativity. We’re used to speed being relative in everyday
life; for example, a car traveling at 60 mph (97 km/h) relative to a stationary observer
would be observed as moving at 120 mph (193 km/h) by a motorist heading in the opposite
direction at the same speed. This same phenomenon also affects time. Depending
on an observer’s relative speed or position within a gravitational field, time passes
at a different rate for that observer than for another observer.
This effect, known as time dilation, is only noticeable under certain circumstances, although
we are constantly subjected to it at a low level.
Time dilation is a consequence of Einstein’s relativity, in which time is not as absolute
as it may appear; the rate at which it passes varies for observers in different frames of
reference. Einstein’s starting point was that light
has the same measured speed irrespective of the observer’s motion. This seemingly innocent
assumption leads to the inevitable conclusion that moving clocks run slowly.
This phrase is frequently used to describe time dilation, but it’s a little deceptive
since it places too much emphasis on clocks, which are only relevant in the sense that
we use them to measure time. However, rather than seeing time dilation
as a clock property, we should consider it an unexpected reality about space and time.
The theory of relativity is divided into special relativity and general relativity, and time
dilation features in both. The principle that the speed of light is the
same for all observers is crucial in special relativity.
According to Boston University physicist Andrew Duffy, one of its consequences is that two
observers, moving at constant speed relative to each other, measure different times between
the same events. However, the effect is only detectable at speeds approaching the speed
of light. Ten years after he proposed special relativity,
Einstein expanded his theory to include gravitational effects in general relativity. However, time
dilation in this hypothesis is determined by the strength of the local gravitational
field rather than the speed of travel. We currently live in a modest gravitational
field on the Earth’s surface; therefore, we’re already experiencing time dilation
without even realizing it. Additionally, the strength of the effect changes when we move
up and down within Earth’s field. Gravity is weaker on the top floor of a tall
building than on the ground floor; therefore, the time dilation effect is similarly weaker
higher up. The further you are from Earth’s surface, the faster time passes.
Although the effect is too small to perceive with human senses, the time difference between
different altitudes can be measured using extremely accurate clocks.
A spectacular example of gravitational time dilation is the region around a black hole,
which has stronger gravity than Earth. NASA has considered what would happen if a
clock was placed 10 kilometers from a black hole with the mass of the Sun. The clock would
take roughly an hour and ten minutes to show a difference of one hour when viewed through
a telescope from a safe distance. One of the most perplexing consequences of
time dilation is the twin paradox. In this hypothetical experiment of twins,
one identical twin lives on Earth while the other twin travels to a faraway star at speeds
approaching the speed of light. When they reunite later, the traveling twin
has aged significantly less than the one who stayed on Earth, thanks to special relativistic
time dilation. The seeming “paradox” arises from the
erroneous belief that the situation is symmetrical. In other words, you could say that the traveling
twin is stationary relative to the Earthbound twin, implying that the Earthling would have
aged less than the traveling twin. However, this isn’t the case because the situation
isn’t symmetrical. When special relativity talks about relative
motion, it means motion in a straight line at a constant speed. That is not the case
in this illustration. Because the twins are together at the beginning
and end of the journey, the traveling twin must accelerate from a standstill to top speed,
then turn around and head back in the opposite direction before finally decelerating to a
stop. These phases of acceleration and deceleration
introduce general relativity because they have effects similar to those of a gravitational
field. When the math is done to account for this
acceleration, it turns out that, like time travel, the spacefaring twin does age more
slowly than the Earthbound twin.