If a distant solar system is millions of light years away and then we see it as it was millions of years ago, how can we see what it looks like now? Parul, age 13, Sri Ganganagar, India
What is the meaning of “now” 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 it takes time for light from a distant object in the universe to reach us.
Astronomers measure the vast distances of the universe in light years, the distance it takes light to travel in a year. If you use a telescope to look at a solar system that is, say, ten light years away – 95 trillion kilometers away – that means you see it as it was ten years ago on your watch.
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If a friendly alien lived in that solar system ten light years away and we passed a message to him, he would only receive the message in ten years. Our “now”, when we send the message, will be in their future. But if we get a message from them, our “now” will be in their past.
This seems to suggest that there is no way to see what is happening right now in a distant solar system. But we can look to the theories of relativity of the famous physicist Albert Einstein for an answer. These theories describe the relationship between objects and the space and time that surround them.
Einstein’s theories revealed something extraordinary. If you could leave Earth and travel very fast through space, close to the speed of light itself, then time for you would pass slower than time for someone you left behind on Earth.
Imagine traveling in space, going ten light years away and ten light years back, and leaving a twin sister on Earth. Time would pass differently for both of you while you were away. It would be 20 years for your sister. But for you, if you could reach 1% the speed of light, it would only take three years. When you came home, your sister would be 17 years older than you.
If you traveled within 0.1% of the speed of light, you would be back only a month older than when you left. Your sister would be nearly 20 years older than you.
It may seem like a trick, but we know it’s true. When very fast moving particles called cosmic rays strike atoms in the Earth’s atmosphere, they create particles called muons that are unstable and disintegrate. None of these muons should be able to reach the Earth’s surface. But we see them. As 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 have existed at your origin and destination at the same time, while others may have seen you travel in their time. Unfortunately, nothing with mass, such as a human or a spaceship, can travel at exactly the speed of light.
Time is not a fixed thing. It really has meaning only for you and for the way you see the world around you. Come to think of it, time is perhaps the greatest mystery in life.
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Jacco van Loon does not work, consult, own stock or receive funding from any company or organization that could benefit from this article and has not disclosed any relevant affiliations beyond their academic appointment.