If a distant solar system is millions of light years away and we see it as it was millions of years ago, how can we see it as it is now? Parul, 13 years old, Sri Ganganagar, India
What does “now” mean and how does our “now” relate to the “now” somewhere else?
Nothing can travel faster than the speed of light: 300,000 kilometers per second. This means that light from a distant object in the universe takes time to reach us.
Astronomers measure the vast distances across the universe in light-years, the distance it takes light to travel in one year. Using a telescope to look at a solar system at a distance of, say, ten light years – 95 trillion kilometers away – means you see it on your watch as it was ten years ago.
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If a friendly alien lived ten light years away in this solar system and we beamed a message to them, they wouldn’t get the message for ten years. Our “now” when we send the message will be in its future. But when we receive a message from them, our “now” will be in their past.
This seems to indicate that there is no way we can see what is happening in a distant solar system. But we can look to the famous physicist Albert Einstein’s theories of relativity for an answer. These theories describe the relationship between objects and the space and time around them.
travel in time
Einstein’s theories revealed something extraordinary. If you could leave Earth and travel through space very quickly, almost at the speed of light itself, then time would pass more slowly for you than for someone you left behind on Earth.
Imagine that you had set out to travel through space ten light years away and ten light years back, leaving a twin sister on Earth. Time would pass differently for both of you while you were gone. 20 years would pass for your sister. But it would only take you three years if you managed to get to 1% of the speed of light. When you came home, your sister was 17 years older than you.
If you were traveling within 0.1% of the speed of light, you would come back only a month older than when you left. Your sister would be almost 20 years older than you.
This may seem like a trick, but we know it’s true. When very fast-moving particles called cosmic rays collide with atoms in Earth’s atmosphere, they create particles called muons, which are unstable and fall apart. None of these muons should be able to reach the earth’s surface. But we see them. Because their time runs slower than ours, they don’t disintegrate before they reach us.
If you could travel at the speed of light, time would not pass for you. Your “now” would be the same as the “now” in the distant solar system or galaxy because you would be there instantly. You would exist at your starting point and your destination at the same time while others might have observed you during your journey. Unfortunately, nothing with mass, like a human or a spaceship, can travel at exactly the speed of light.
Time is not fixed. It only really matters to you and the way you see the world around you. When you think about it, time is perhaps life’s greatest mystery.
This article was republished by The Conversation under a Creative Commons license. Read the original article.
Jacco van Loon does not work for, advise, own any interest in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations other than her academic appointment.