Newsletter: Look into the sky – you are now looking into the past!

The Newsletter is back this week with a little bit of physics to give you something to think about!

Consider this: every time you open your eyes to look at something, you’re literally looking back in time. Since light travels incredibly fast, but not infinitely fast (300,000km per second), whenever you look at something, you are seeing it as it was when the light that now reaches your eyes bounced off that something.

Of course, in everyday life this is a rather pointless fact (nothing changes much in a billionth of a second, right?), but have you ever thought about what you are seeing when you look to the sky?

For example, you see the moon as it was 1.28 seconds ago, and the Sun as it was 8 minutes and 20 seconds ago. In fact, the closest star to the Earth (apart from the Sun) is Proxima Centauri, and that one is more than 4 light-years away. Moving a bit farther, the Eagle Nebula is 7,000 light-years away, and astronomers believe that a supernova (a huge star exploding at the end of its life) has already blasted away this entire region. Go look at it with a telescope, however, and you’ll see something like this (these are the famous Pillars of Creation):

Isn’t it strange to think that you can take, right now, a (very) powerful telescope and see something like this “live”… and yet, at the same time, be fairly sure that this beautiful scenery has already disappeared?

But we are just getting started! The core of our own galaxy, the Milky Way, is about 25,000 light-years away. And the closest galaxy to ours, Andromeda, is more than 2.5 million light-years away: we see Andromeda as it was before the Homo erectus had set foot on Earth. Think about this, in reverse: might there be an advanced civilization somewhere else, watching dinosaurs roaming the Earth, right now?

[The Milky Way and the Andromeda Galaxy]

Some of the brightest objects in the sky are the so-called Quasars: supermassive black holes at the cores of galaxies, actively feeding off stars and gas. These emit incredibly intense bursts of electromagnetic radiation, and the closest one is about 2.5 billion light-years away. The funny thing is that some of these are farther than 4.5 billion light-years away… and the Earth is ‘just’ 4.5 billion years old, which means we can actually look at stuff that was already shining before the Earth even existed!

So what’s the farthest we can see? That would be around 13.8 billion light-years away: the very edge of the observable universe. That light was emitted when the Universe was barely a few hundred thousand years old, and is reaching us just now. What is even weirder: the place where that light was emitted from is now around 46 billion light-years away, so we can’t even see it! And how might this be possible? Well, this has to do with the fact that the Universe is not something static, but is actually expanding, and doing so faster and faster… however, we’ll talk about this in a future Newsletter!

For now, and to finish off this Newsletter, allow me to end with a picture of the Crab Nebula.

This Nebula is the result of another supernova. What’s curious about this one, is that the explosion was seen from Earth on the 4th of July of 1054 (the Chinese were the first ones to spot it), and remained visible on the night sky for more than two years… despite it being more than 6,500 light-years away!

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