# What is the role of the speed of light in time travel?

You’ve probably found that there are many references to the speed of light when reading about time travel. This brief articles hopes to explain the relationship between the two.

The speed of light is not just the speed that light travels – in some ways that can be considered to be a coincidence. The speed of light is the speed limit of the known physical universe, and is just shy of 300 million meters per second. This speed limit relates to everything, including the transfer of information. This might be counter-intuitive, but there is no such thing as “instant”! (And I’m not one to argue with Einstein!)

So how is this related to time travel?

Einstein’s theories show that time can dilate in a number of circumstances. Time dilation is where the passage of time occurs at a different rate in one situation than it does in another. For example, time passes more quickly for someone experiencing a lower gravitational acceleration than for someone who is subjected to high gravitational forces. Another example, and relevant to our discussion here, is that time passes more slowly for someone who is travelling at speed in comparison to someone who is stationary.

This means that if someone flew in a jet engine at high speed, his watch would register a shorter time of flight that someone who remained standing in the airport waiting for his return (this might explain why planes are always late! ðŸ˜‰

The greater the difference in relative velocities, the greater the effect of time dilation. For the velocities that we are easily able to acheive in everyday life, the effect of time dilation is very small (in the order of milliseconds.) However, if we could gain very high velocities, and travel at them for sufficient lengths of time to accrue the time differences, the effects can be noticeable.

This is of significance to the time traveller. If I travelled at a sufficiently high velocity, my experience of the passage of time is slower for that of someone waiting for me to come back. In practical terms, in my point of view, I’d fly for say 1 year, but someone waiting for me would have waited for a year and a month. In effect then, I have travelled 1 month into the future.

If then, I travel at a greater speed, the effects of time dilation are proportionally greater; I travel at twice the speed I did before for 1 year, and my expectant welcome committee would have waited for 1 year and 2 months.

It follows that the faster I travel, the further into the future I can transport myself. It also means that my journey doesn’t need to last as long – instead of travelling at high speed for 1 year, I can travel at a faster speed for 1 month. Or an hour. Or a second. There is greater efficiency in time travel at higher speeds.

And we know that the fastest speed we can travel is the speed of light! This is why achieving light speed is considered to be important in time travel.

There are two important things to note here…

The first is that in this way, time travel into the future is possible, but not the past.

The second is a possible time travel paradox – the so called twin paradox. I mentioned that I travelled at high speed for one year, whilst someone remains stationary on Earth. But relatively speaking…who’s to say that I wasn’t stationary, and it was the Earth-bound person who moved away at speed? In real terms, each of us would find that the other person has experienced more time than themself, and this is not possible!

Actually, the ‘solution’ to this quandary is in the means by which I gain high velocity. If my journey starts on the Earth, my velocity is zero in relation to my observer. I then accelerate to high velocity. Here then is the solution – acceleration brings about a further time dilation effect for me as the traveler (as well as time dilation by moving further away from Earth’s gravitational field). These differences would ensure that our relative experiences of time passage are different from each other.

As a side note – what would happen if the speed of light really could be exceeded?

I hope that this explains the relationship between the speed of light and time travel!

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