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  • Light is the fastest thing we know

  • It’s so fast that we measure enormous distances by how long it takes for light to travel them.

  • In one year, light travels about 6 trillion miles,

  • A distance we call one light year

  • To give you an idea of just how far this is,

  • the Moon, which took the Apollo astronauts four days to reach, is only one light second from Earth

  • Meanwhile, the nearest star beyond our own Sun is Proxima Centauri, 4.24 light years away

  • Our Milky Way is on the order of 100,000 light years across

  • The nearest galaxy to our own, Andromeda, is about 2.5 million light years away

  • Space is mind-blowingly vast

  • But wait, how do you know how far away stars and galaxies are?

  • After all, when we look at the sky, we have a flat, two-dimensional view

  • If you point your finger to one star, you can’t tell how far the star is,

  • So how do astrophysicists figure that out?

  • For objects that are very close by, we can use a concept called trigonometry parallax

  • The idea is pretty simple. Let’s do an experiment.

  • Stick out your thumb and close your left eye.

  • Now open your left eye and close your right eye

  • It will look like your thumb has moved while more distant background objects have remained in place

  • The same concept applies when we look at the stars

  • But distant stars are much, much farther away than the length of your arm

  • And the Earth isn’t very large, so even if you had different telescopes across the equator,

  • You’d not see much of a shift in position

  • Instead, we look at the change in the star’s apparent location over 6 months,

  • The halfway point of the Earth’s yearlong orbit around the Sun.

  • When we measure the relative positions of the stars in summer and then again in winter, it’s like looking with your other eye.

  • Nearby stars seemed to have moved against the background of the more distant stars and galaxies.

  • But this method only works for objects no more than a few thousand light years away.

  • Beyond our own galaxy, the distances are so great that the parallax is too small to detect with even our most sensitive instruments.

  • So at this point we have to rely on a different method using indicators we call standard candles.

  • Standard candles are objects whose intrinsic brightness, or luminosity, we know really well.

  • For example, if you know how bright your light bulb is, and you ask your friend to hold the light bulb and walk away from you,

  • You know that the amount of light you receive from your friend will decrease by the distance squared

  • So by comparing the amount of light you receive to the intrinsic brightness of the light bulb,

  • You can then tell how far away your friend is.

  • In Astronomy, our light bulb turns out to be a special type of star called a Cepheid variable.

  • These stars are internally unstable, like a constantly inflating and deflating balloon.

  • And because the expansion and contraction causes their brightness to vary,

  • We can calculate their luminosity by measuring the period of this cycle,

  • With more luminous stars changing more slowly.

  • By comparing the light we observe from these stars to the intrinsic brightness weve calculated this way,

  • We can tell how far away they are.

  • Unfortunately, this is still not the end of the story.

  • We can only observe individual stars up to about 40 million light years away,

  • After which they become too blurry to resolve

  • But luckily we have another type of standard candle

  • The famous Type 1a Supernova

  • Supernovae, giant stellar explosions are one of the ways that stars die.

  • These explosions are so bright that they outshine the galaxies where they occur.

  • So even when we can’t see individual stars in a galaxy,

  • We can still see supernovae when they happen.

  • And Type 1a supernovae turn out to be usable as standard candles,

  • Because intrinsically bright ones fade slower than fainter ones.

  • Through our understanding of this relationship between brightness and decline rate,

  • We can use these supernovae to probe distances up to several billions of light years away.

  • But why is it important to see such distant objects anyway?

  • Well, remember how fast light travels.

  • For example, the light emitted by the Sun will take 8 minutes to reach us,

  • Which means that the light we see now is a picture of the sun 8 minutes ago.

  • When you look at the Big Dipper, youre seeing what it looked like 80 years ago.

  • And those smudgy galaxies?

  • Theyre millions of light years away.

  • It has taken millions of years for that light to reach us.

  • So the universe itself is in some sense an inbuilt time machine.

  • The further we can look back, the younger the universe we are probing.

  • Astrophysicists try to read the history of the universe and understand how and where we come from

  • The universe is constantly sending us information in the form of light.

  • All that remains is for us to decode it.

Light is the fastest thing we know

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B1 中級

TED-ED】光秒、光年、光世紀。極端な距離の測り方 - 袁泉亭 (【TED-Ed】Light seconds, light years, light centuries: How to measure extreme distances - Yuan-Sen Ting)

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    Go Tutor に公開 2021 年 01 月 14 日
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