Time seems to follow a cosmic ticktock rhythm. But it doesn’t. It can vary for different observers depending on your speed through space. Time is relative.

Einstein predicted in his Special Theory of Relativity that time is relative. In other words, the rate at which time passes depends on your frame of reference.

Time moves slower for a moving clock than for a stationary one. The rate at which time passes depends entirely on your speed and acceleration at any given moment.

The idea of time traveling has been a popular topic for science fiction for decades. Franchises like Star Trek or Doctor Who make it look simple, but in reality, time travel is extremely complicated, and not all scientists believe it is possible.

An object in motion experiences time dilation, meaning that time moves more slowly when one is moving than when one is standing still.

A great example is astronaut Scott Kelly. When he spent nearly a year aboard the International Space Station in 2015-16, his twin astronaut Mark Kelly aged a little faster than Scott.

This becomes extremely obvious at speeds approaching the speed of light.

Let’s say you have been moving through space, at the speed of light, for around three years, according to your clock. During this time, the clock on Earth was moving much faster. In fact, for people on Earth, more than sixty years have passed. You should only be a few years older, but everyone on Earth would be much older.

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You personally wouldn’t notice time passing any differently. In your own rest frame, time passed at one second per second. It is only when you compare clocks with your friends on Earth and you see how much they have aged that you notice things being weird.

This time dilation may sound very theoretical, but it does have practical applications as well.

If you have a GPS receiver in your car, the receiver attempts to find signals from at least three satellites to coordinate your position. The GPS satellites send out timed radio signals that the receiver listens to, trilaterating its position based on the travel time of the signals. The challenge is, the atomic clocks on the GPS are moving and would therefore run faster than atomic clocks on Earth, creating timing issues. So, engineers need to make the clocks on a GPS tick slower.

In General Relativity, the flow of time doesn’t only depend on your velocity but the local gravitational field as well.

To Einstein, time is the “fourth dimension.” Space is described as a three-dimensional place that has length, width, and height. Time gives space another dimension — direction. Einstein’s theory of relativity unified space and time into a single fabric – space-time.

When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Scientists have proven both the general and special relativity theories using GPS satellite technology that has very accurate timepieces on board. The effects of gravity, as well as the increased speed of satellites above the Earth relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day.

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Long story short, astronauts on the International Space Station age more slowly due to the spacecraft’s high orbital speed. This means that when they return to Earth they are very, very slightly younger than they would have been—as if they’ve traveled into the future.

The effect, however, is very small. It would take more than 100 years on the ISS, to warp ahead by just one second.

As for gravitational time dilation, it occurs because objects with a lot of mass create a strong gravitational field. The gravitational field is really a curving of space and time. The stronger the gravity, the more spacetime curves, and the slower time itself proceeds.

But an observer in the strong gravity experiences time as running normally. It is only relative to a reference frame with weaker gravity that his time runs slow.

Time moves differently depending on where you are in a gravity field. For example, a clock on top of a tall mountain will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger.

So, time only has meaning near where you are. Most are at a loss to even describe what time is. Time in physics is operationally defined as “what a clock reads”.

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