So far it is only possible to travel forward in time, and only by fractions of a second.
Imagine there are two identical twins; the first travels into space in a high-speed rocket, while the other remains on Earth. When the first twin returns he finds he has aged less than his Earth-bound twin.
This happens because of something called ‘time dilation’ where, at high speeds, the faster something travels, the slower time passes for it.
This effect is seen in global positioning system (GPS) satellites. As GPS works by precisely timing signals sent from the satellites to receivers on Earth, it is vital for the clocks involved to have a nanosecond (1 billionth of a second) accuracy. However, the satellites are travelling fast enough that time dilation occurs on the nanosecond scale, meaning the satellites are travelling through time at a slightly different rate to the receiver on Earth.
If we could develop technology to travel fast enough to take advantage of time dilation, then it could be considered to be a time machine.
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