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# PSA: Our galaxy is going to collide with another...

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...in 4 billion years.

Yea I got you, you were probably really afraid for a second and frantically clicked this blog to find out if you really HAD to do your physics homework tonight or if it would all be pointless since the end of humanity could be just over the horizon.

Well sadly for you it is not, but since you're already reading you might as well find out what will happen when it does.

I was of course reading The Cosmic Cocktail again by Katherine Freeze, since that is all that I do now, and came across this little tid-bit while reading about red-shifting. Red-shifting is a term used by really smart people to prove that the universe is expanding. You see since we know that the frequency of light equals c (the speed of light) times wavelength. Since we know that the speed of light is constant (3 x10^8 meters per second) if we see light with a different frequency we know that the wavelength must have changed.

Think of it like this, if you see a fire truck as it approaches the pitch of the siren is deeper, this is because the speed of sound is constant but the wavelength is being pushed closer and closer together as the fire truck approaches you making a shorter wavelength and deeper pitch. When the fire truck passes you on the other hand the wavelength is being stretched out so the pitch is higher.

The same principle applies to light waves and galaxies! When we examine the light waves reaching earth we find that almost all of them have a low frequency and shift more to the red spectrum of light, therefore they have a longer wavelength, showing that these galaxies emitting light are getting farther from us proving the universe is expanding!! I did say almost.

The one exception is the Andromeda Galaxy, the light from this galaxy is actually blue shifted, it has a higher frequency and a shorter wavelength than normal. This means, like the fire truck approaching us, that the Andromeda Galaxy is actually getting closer! Even the Fire Department can't save us from this apocalyptic event though.

Luckily for us the space inside our galaxies is so great that the Earth would probably not even be affected by the galactic wrestling match going on around it. Also on the positive side, this battle won't be taking place for almost 4 billion years! Even better, by that time our sun will have reached the red dwarf stage by then and already engulfed Earth!

So do not fret Earth dwellers, our planet is safe for now!

Maybe not 4 billion years from now though...

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