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 what it looks like now? Parul, 13, Sri Ganganagar, India
What is the meaning of “now” and how does our “now” relate to the “now” elsewhere?
Nothing can travel faster than the speed of light: 300,000 kilometers per second. This means that it takes time for the light from a distant object in the universe to reach us.
Astronomers measure the vast distances across the universe in light years, the distance it takes light to travel in one year. If you use a telescope to look at a solar system from, say, ten light-years away – 95 trillion kilometers away – that means you’re seeing 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 sent them a message, they wouldn’t receive the message until ten years later. Our “now,” when we send the message, will be in their future. But if we receive a message from them, our “now” will be in their past.
This seems to suggest that there is no way we could see what is happening right now 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.
Travels in time
Einstein’s theories revealed something extraordinary. If you could leave Earth and travel very fast through space, close to the speed of light itself, time for you would pass more slowly than time for anyone you had left behind on Earth.
Imagine you traveled through space, went ten light years away and ten light years back, leaving behind a twin sister on Earth. Time would have passed differently for both of you while you were gone. For your sister it would be 20 years. But for you, if you managed to reach within 1% of the speed of light, only three years would pass. 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 return 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 hit atoms in the Earth’s atmosphere, they create particles called muons that 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 fall apart until they reach us.
If you were able to travel at the speed of light, no time would pass for you. Your “now” would be the same as the “now” in the distant solar system or galaxy, because you would be there immediately. You would exist at your origin and at your destination at the same time, while others may have seen you travel through 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 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.
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Jacco van Loon does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.