Question: Is time travel possible for humans? Jasmine, age 8, Canberra, Australia. 

Hi Jasmine.

I wish! In books and movies, our favourite characters can use “time-turners” and treehouses to travel through time. Unfortunately, it isn’t that easy for people in real life. Let’s look at why.

First, there are two types of “time travel”: going back in time, and going forward in time.

Traveling to the past

As far as we know, traveling back in time is impossible. Even sending information back in time is difficult to imagine, because it can change things that have already happened, which should be impossible.

Say you broke your arm falling off the monkey bars. What if you could travel back in time and tell yourself to not go on the bars? If you were successful, you’d never fall and break your arm. But then you would have no reason to travel back in time. So what does this mean for your arm? Did it break, or not?

If thinking about this makes your head hurt, you’re not alone.

Time traveling is a confusing idea for most people. That’s because when we think of time, we think about it as going in a straight line, with one thing happening after another.

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If we could travel back in time and change something that happened before, we would then change the order of that line. This would mean breaking a rule called “causality”.

Causality is the rule saying that a “cause” (your actions, for instance) happens before an “effect” (the result of your actions). In our monkey bar example, the cause is falling, and the effect is breaking your arm – which happens because you fell.

Causality is one of the unbreakable rules of the universe. Breaking it would have nasty consequences for the universe and all of us. Experts think that because the universe has this rule, traveling to the past must be impossible otherwise the rule would be broken all the time.

Read more: national interest