Is Time Travel Possible For Humans?

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Question: Is time travel possible for people? Jasmine, 8 years old, Canberra, Australia.
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Hello Jasmine.
I wish! In books and films, our favorite characters can use "time-turner" and tree houses to travel through time. Unfortunately, it's not that easy for people in real life. Let's see why.
First, there are two types of "time travel": time travel and time travel.
Travel back in time
As far as we know, time travel is impossible. Even sending information back into the past is difficult to imagine, as this can change things that have already happened, which should be impossible.
Suppose you broke your arm and fell off the monkey bars. What if you could go back in time and tell yourself not to go to the bars? If you were successful, you would never fall and break your arm. But then you would have no reason to go back in time. What does that mean for your arm? Did it break or not?
If your head hurts, you are not alone.
Time travel is a confusing idea for most people. This is because when we think of time, we assume that it runs in a straight line and happens one by one.
If we could go back in time and change something that happened before, we would change the order of that line. This would mean breaking a rule called "causality".
Causality is the rule that says that a "cause" (e.g. your actions) occurs before an "effect" (the result of your actions). In our example of a climbing pole, the cause falls and the effect breaks your arm - which happens because you fell.
Causality is one of the unbreakable rules of the universe. Breaking it would have dire consequences for the universe and all of us. Experts think that because the universe has this rule, it must be impossible to go back in time, otherwise the rule would be broken all the time.
Travel to the future
If it is impossible to go into the past, can we go into the future in time?
Technically speaking, we are already moving ahead in time because time is passing. Every second we travel a second into the future. But that happens to everyone, so it's not really time travel, is it?
Believe it or not, two people can feel the time at different speeds. Time goes by for someone who moves quickly, unlike someone who stays silent. This is a very complicated idea called "time dilation".
Someone who flies from Sydney to Melbourne will feel that time has passed faster than someone who is waiting for him at the airport without moving throughout the flight time. Why don't we notice this difference?
This is because you have to move much, much faster than an airplane before you notice any time dilation. Even if you were to fly all the way around the world, time would only feel a billionth of a second different than for someone who stayed at home.
The only way scientists know time dilation is through amazingly accurate experiments that they have measured.
Unfortunately, this still cannot help us with "time travel". If you had flown around the world for more than four million years, the local people would only have experienced a second more than you!
How fast can we go
So if it's all about speed, the answer has to be to drive faster, right? If you could drive fast enough long enough, hundreds of "human" years could pass by on your trip, which means you feel like you're traveling into the future!
Unfortunately, a speed that is fast enough would be close to the speed of light, which is the fastest speed that can achieve anything. Light moves with an hour of about a billion kilometers - that's very, very fast.
The fastest human-made thing is NASA's Parker Solar Probe, a spacecraft that was sent to the sun in August 2018. But as fast as it is, it's only 0.064% as fast as the speed of light. So light is more than 1,000 times faster!
All of this means that we have a long, long way to go if people want to visit the future.
Looking back at the past
Ok, so we can't travel through time. But we can look into the past every night.
Light has a fixed speed, as we have just learned. It's really, really fast, but things in the universe are so far apart that it takes a long time for light to reach us from distant stars and planets.
When light comes from the sun, the light we see actually left the sun eight minutes and twenty seconds ago. This means that we see the sun as it was eight minutes and twenty seconds in the past. By the way, remember to never look directly into the sun, as this can damage your eyes.
The closest galaxy to our Milky Way is the dwarf galaxy Canis Major, 25,000 light years away. So the light takes 25,000 years to come here!
When we look at this galaxy through a telescope, we actually see it as it was more than 25,000 years ago. Although we cannot travel through time ourselves, we can look up to the sky every night and see the past.
Lucy Strang, PhD student, University of Melbourne
Jacqueline Bondell, Education and Public Relations Coordinator, Swinburne University of Technology
This article is republished by The Conversation under a Creative Commons license. Read the original article.
Image: Reuters
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