# What is time travel?

There is a natural progression in the fourth dimension of time from past, through present, and into the future which occurs at a rate of one second per second. But can the passage rate of change of time be altered? Can 1 second be experienced where 10 seconds have expired? Or minus 10 seconds? This is time travel:

To leave one moment in time and arrive at another, independent of the normal or background rate of time perception“.

This definition of time travel therefore excludes the idea that time travel has taken place if an object has been subjected to nothing more than the normal flow of time (you are now a few minutes into the future as compared to where (when) you were when you first started reading this page, but you have not travelled here independently from the normal time flow).

Consider the case of a canoeist on a moving river. If he drifts at the same rate as the water, he is travelling, but out of control. To be in control, he needs to be moving faster or slower than the speed of the water. Simply drifting along with the current is not paddling. A dead duck can drift. It is the same with time travel Ã¢â‚¬â€œ to be a time traveler, to be in control of your movement within time, we need to move slower or faster than the ambient time flow.

Time travelling is therefore a transport through time as a navigable dimension in much the same way that travelling through space takes you from one point in space to another.

Late for work

On Monday morning, along with hundreds of other commuters, I imagine, I was looking for my car keys.

“With you and your time travel, they’ve probably jumped forward in time or something.” said my wife, notably not helping me look for these elusive initiators of internal combustion.

“Why do you say that?”

“Because you’ll find them in the future!”

She was right, of course, that time had passed from the moment I lost them to the moment of finding them (my daughter had hidden them in her doll’s house), so they were in the future, but…so was I, and as such I don’t think that warranted time travel!

I’m going to go into the future and come back to the present when this will be the past.

Sounds great…except we all do this! It’s not time travel; just moving along with time.

Being in one moment of time and finding yourself in another (either into the past or future) needs a clarification on what it means to be present in a moment of time. Can a live witness of events from the past or future be considered as time travel? Or is a real interactive experience within that moment and of the physical objects and peoples therein, i.e. an exchange of information, required?

A nostalgic romantic evening: looking into the past.

You’re standing on a warm summer night with the girl/boy* of your dreams (* delete as appropriate) and looking into each other’s eyes. They are twinkling like the thousands of shining stars overhead.

“I can see the stars in your eyes, baby, but my heart is close to yours!”

When (s)he stops puking, you consider the more astronomic and temporal facets of the starlight overhead.

The closest star outside our solar system is Proxima Centauri, situated about 40 trillion km (40 with 12 trailing zeroes…) away. That’s 4.22 light years distant from us, or in other words, it takes the starlight travelling from Proxima Centauri 4.22 years to reach our eyes. This means that when we look at this star we see it as it was 4.22 years ago. Likewise, when looking at stars even more distant, we are effectively looking even further into the past (strangely, by differing amounts of time depending on the distance the light has travelled and the time is has taken to travel to us).

If we can see directly into the past in this way and glean information about that past – is this time travel?

Granted, we’re looking at the stars and seeing them as they were some time ago so effectively we’re looking into the past…but we’re not really *being* there (in the past) – we’re still standing in the viewpoint of the present. I’d argue then, that this is not time travel. Transposing this into the context of spatial navigation, it’s akin to standing in one place and looking at another and saying you’ve been there.

Super sprinter

Having failed at impressing your lover with time travel with the stars, you decide to show your prowess at athletic ability. In this case, this includes the ability to run faster than the speed of light. So off you go and run for a short while, stop, and then turn around. As the light coming from the running you catches you up and meets your eyes, you see yourself running towards you. That is you as you were a few moments ago; you are witnessing first hand an event as it occurred in the past. Ignoring the time dilation effects associated with moving at velocity, have you travelled back in time?

Again, I’d argue not; this is only symptomatic viewing – watching yourself run from the past is not much more than looking at an image or movie of yourself taken from some time ago. Perhaps it would be different if whilst you were running, you would be able to see (and communicate with) the future version of you who has already arrived and who has turned to face you.

As with the starlight example, there is often confusion between the speed of light and the concept of time travel. Note though, that the speed of light is important when considering time travel (I’ll write why that is, later!)

A picture speaks…

Your lover wants to see your baby photos. Thinking that you were a beautiful bouncing baby, you agree, dig out the photo album from the attic, blow off the dust and open the front cover.

The first photo is faded and brown, but there you are, dribble and all, looking sweet and cute in your nappy sitting on a blanket in the garden. Your lover lets out the obligatory “ooh’s”Â and “aahs’s”Â.

As you look more closely, you see the clouds in the sky behind you are moving and the flowers along the border are blowing in the wind. The faded brown of the photo melds into more life-like colours, and the flowers become more vivid. So vivid, that now you can smell them. The baby in the photo is…crawling! It reaches out and a butterfly is seen fluttering across the field of view. Now you can hear the baby babbling, and smiling, it’s looking up…where a pair of arms reach down and pick the baby up, pulling it out of view. The colours in the photo fade, and morph into a yellow brown. There is a photo of a dribbling baby sitting on a blanket in the garden.

Dribbling now at what happened, you look on the opposite page of the photo album. There is an envelope and you pull out a folder piece of paper. On the outside it reads “summer, 1971″Â and inside there is a pressed butterfly; the same as the one you saw in the photo moments ago.

In effect this was like watching an old cine movie, albeit from an alternative source, and it allowed the receipt of information from the observed moment in time (e.g. knowledge of the wind blowing, the smell of the flowers, etc.). But is this any different to the starlight or faster than light speed examples above?

You are still watching the past ‘as it happens’, in real time, as such, just as you could see what the stars were doing many years ago, or see yourself running in the past. But in those examples, the cause was purely a physical one in that the limit of the speed of light was basically used as a recording device.

What separates the photo album from the examples above is that the mechanism of looking into the past is entirely different; the same mechanism, presumably, might be able to be reversed and allow a view into the future – something which is not possible with the travelling light examples. The stars can’t be viewed as they will appear in the future (please don’t get me started about astrology!), just as running slower than the speed of light won’t allow us to see a future version of ourselves.

The baby photo acted more like a doorway, or a portal into the past, and in this respect if can be considered to be more of an instrument of time travel. The possibility of a two-way exchange of information also lends itself to the idea that a form of time travel has occurred.

But according to the definition of time travel above, is this time travel? What has travelled from one point in time to another? The viewer of the photo hasn’t really been taken from the present to another moment of time, but rather has been allowed a live witness.

Interaction is key. So is control. But how can you measure if you are really there at all? Perhaps that is more of a philosophical question! Is time travel possible anyway?

Summary
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What is time travel?
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A definition of what time travel is, with some examples of what it isn't.
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