What is Time Dilation ?

We are going far too slowly for any noticeable change to take place.

Even if we go at high enough speeds to bring about a large slowing down of local time we wouldn't notice because our own body clocks would also be running just as slowly.

The speed of light is very close to 300,000 km per second (186,300 miles per second)

It isn't until we get to speeds that are a large fraction of the speed of light that any change in the flow of time becomes apparent.

However, at speeds very close to that of light the effect grows in magnitude very rapidly indeed until time almost comes to a standsti

This slowing down of clocks due to high speeds is called time dilation and has a precise mathematical relationship

We can see from the graph that at “low” speeds there is only a small change in time dilation (i.e. the flow of time doesn't change very much), but at speeds over about 75% of the speed of light the effect of time dilation is quite dramatic. Even at the "low speed

of 10% of the speed of light (300,000 km per second, or 186,300 miles per second) our clocks would slow down by only around 1%, but if we travel at 95% of the speed of light time will slow down

to about one-third of that measured by a stationary observer. Note that at zero percent of the speed of light there is no time dilation at all.

Also, while we can get as close to the speed of light as our technology allows, it's impossible to actually reach a speed of 100% of the speed of ligh

So, when we move, at whatever speed, time slows down relative to a stationary observer.

But note that, for example, the occupants of a rocket travelling at very high speeds would still experience time passing normally.

However, if they could see out to an Earth-bound clock it would appear, to them, to be running too quickly.

If an Earth-bound observer could see a clock inside the rocket it would appear to be running too slowly.

This is why the theory is called "relativity", it is because time is relative to whoever is observing it at a particular speed

It's sometimes said that time would run backwards. Special relativity tells us that this is simply not possible.

The universe has a speed limit of just under the speed of light, and it has a clever way of stopping us from breaking it. As we go faster our apparent mass (i.e. as measured by an external observer) increases in proportion to our speed

In fact our mass seems to increase at the same rate as time slows down (in a similar way to the graph seen earlier). We know from everyday experience that the heavier (i.e. more massive) an object is the more energy is needed to move it

If we try to move an object at 0% of the speed of light we will find that it has the mass we expect it to have. However, the mass of the object will appear to increase in ever increasing proportion to our speed

As our speed goes ever higher so the apparent mass increases, and so does the energy required to move it. At the speed of light it would take infinite energy to move any mass.

Since it's clearly impossible to obtain infinite energy we can never quite reach the speed of light (but we can get as close as our energy supply, and technology, will allow).

Note that the occupants of any rocket travelling at very high speeds will not be aware of any increase in mass, just as they wouldn't be aware in any change in the rate that time passes.

It's only when they measure the mass of stationary observers that they will see that there has been a change in mass -- the astronauts will perceive that

One of the strangest consequences of time dilation is the so-called twin paradox. In this "paradox" one identical twin is sent at very high speed out into space.

Because he or she is travelling at a very high speed all the clocks on board the rocket, including the body clock of the twin, slow down in accordance with the principle that "moving clocks run slow