Time dilation is a concept that often receives a lot of media attention due to its counter-intuitive nature. But what actually is it and is it true?

### Einstein’s Postulates

Time dilation is a consequence that arises when one explores the theory of Relativity. The fundamental aspect of this theory boils down to two postulates:

a) “All the laws of physics are the same in every inertial frame of reference”

b) “The speed of light in a vacuum is constant in all inertial frames of reference”

Frame of reference just means perspective or viewpoint. So two different people will have two different perspectives, or frames of references. By inertial we mean something that doesn’t accelerate. So someone that is standing still or travelling at a constant velocity will be in an inertial frame of reference. However, if someone is on a train that is accelerating, they are in a non-inertial frame of reference.

So as long as someone is in a inertial frame, all the conventional laws of physics hold, such as Newton’s mechanics, energy conservation, etc. No surprises there.

### A Simple Though Experiment

However the second postulate is the one with much more far-reaching consequences and is the basis of time dilation. To understand why it is so significant, we can consider a simple thought experiment. Consider a person standing on a train travelling at a constant velocity ‘v’. If they throw an object forwards at some speed ‘s’, a person standing outside the train will observe the object to have a speed of ‘v + s’.

In other words, the speed of the object they measure is equal to the speed of the train plus the speed at which the ball was thrown. This is obvious and easy to understand from our everyday experience. However, the second postulate says that this would not be true if the object was light. The person inside the train and outside will both measure the speed of light to be the same!

This is already something that is hard to accept as it disagrees with what we observe in our everyday life. The reason why it’s true boils down to some complicated maths and theory, so we will accept it as a fact for now and see what the consequences are.

Imagine a different thought experiment, where now a person is standing on a rocket (moving at a constant speed) with a torch, underneath a mirror. If they shine the torch onto the mirror, we can expect that they will see the light travel vertically upwards, bounce off the mirror, and return by travelling vertically downwards.

Now consider this same event as observed by a person standing on a planet outside the rocket. Since the rocket was originally moving horizontally, they will not only see the light move vertically when it is turned on, but also horizontally. Therefore from their perspective, the light travels a different distance to what is observed by the person on the rocket.

But now remember the second postulate – both these observers will observe the same speed of light! So the two people observe light travelling different distances but with the same speed. How can this be possible? Well from the basic equation of distance = speed x time, we can see immediately that the two observers must have a different experience of time. So simply by virtue of motion, the person standing on the rocket will experience a different time to the person outside.

This is time dilation!

### Time Dilation in Real Life

Once we accept the fact that the speed of light in a vacuum is constant in all inertial frames, we are forced to accept that time dilation occurs. We are used to thinking about time as a clock that just ticks in the background at the same rate for everyone, unaffected by the world. However that is just not true, each person has a personal clock that ticks at a different rate depending on their relative velocity.

One may naturally wonder then why people that are travelling in real life don’t (seem to) experience a different time to people who are standing still. This can easily be understood if one calculates the factor by which the time changes for the two frames of reference mathematically. It turns out that for any speed that is not close to the speed of light, this effect is negligible. If we keep in mind that light is able to travel around the earth’s equator about seven times in a second, we can see how fast that is!

So then why is it important if we can’t observe it? Well, we may not be able to personally observe the effect of time dilation, however it does affect many things around us. GPS clocks, for example, go out of sync overtime because the satellites are travelling at a different velocity to us on Earth. So even though this effect is small it can accumulate over time if it is not accounted for, and the clocks around the world will go out of sync with the ‘actual’ time.

Another interesting effect is in muon decay. Muons are a type of fundamental particle that enter the atmosphere through cosmic rays. However, since they only have a lifetime of a few microseconds, they should only be able to travel a few meters before they decay. Keeping in mind that the surface is about ten kilometres away from where they enter the atmosphere, it doesn’t make sense at first why we are able to detect them.

However, because they are travelling very fast, their time is dilated so a few micro seconds in their perspective is much longer for us, allowing them to travel much further than they’d otherwise be able to. As a result, we can observe them even on the surface.

These effects become significant in pretty much any physical phenomenon when the speeds become high enough, and the equations must be adjusted to include a relativistic correction. As a result, it is clear to see that this idea is not just science fiction but very real.

### Possibility of Time Travel

One final example we can think about is the possibility of time travel.

Suppose a person leaves Earth on a rocket that can travel at half the speed of light and circles back around after 10 years in their frame of reference. However, from the perspective of people on Earth, 11.5 years would have passed. In other words, the person would have travelled 1.5 years into the future! This effect can become much more significant as the speed becomes closer to the speed of light, for example if they travel at 99.99% the speed of light, just 1 year for them is 71 years for us! In other words, travelling into the future is not science fiction, but is something that is physically possible if such speeds could be reached (which is a very big if!).

### Conclusion

Time dilation and time travel is one of the most intriguing and possibly counter-intuitive ideas in Physics, often getting mentioned in popular science fiction shows, science magazie, and so on. But now, you understand the theory behind where it all comes from!

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