Light seconds, light years, light centuries: How to measure extreme distances - Yuan-Sen Ting

在太空中如何測量距離? - Yuan-Sen Ting

譯者: 瑞文Eleven 林Lim 審譯者: Adrienne Lin

我們都知道光線是最快的，

Light is the fastest thing we know.

It's so fast that we measure enormous distances

它是如此之快， 所以我們藉由光通過所需時間，

by how long it takes for light to travel them.

來衡量遙遠的距離。

光在一年中傳播 約10兆(9.46×1012)公里，

In one year, light travels about 6,000,000,000,000 miles,

a distance we call one light year.

此距離稱為一光年。

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

給你一個概念，看看那有多遠：

the Moon, which took the Apollo astronauts four days to reach,

阿波羅號的太空人花四天才到達月亮，

is only one light-second from Earth.

那只是光一秒所走的距離。

Meanwhile, the nearest star beyond our own Sun is Proxima Centauri,

同時，與太陽最接近的恆星是比鄰星，

4.24 light years away.

距離4.24光年遠。

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

我們的銀河系直徑約十萬光年。

The nearest galaxy to our own, Andromeda,

距我們最近的星系「仙女座」，

is about 2.5 million light years away

大約250萬光年遠。

Space is mind-blowingly vast.

太空真是浩瀚無垠！

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

等等，我們如何知道 遙遠星體的距離呢？

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

畢竟，當我們仰望天空時， 只有兩維平面的視野，

If you point you finger to one star, you can't tell how far the star is,

如果你用手指著一顆星星， 你不會知道星星有多遠，

so how do astrophysicists figure that out?

那麼，天體物理學家 如何明白測量距離呢？

For objects that are very close by,

對於那些距我們很近的星體，

we can use a concept called trigonometric parallax.

我們可以使用三角視差的概念來測距。

The idea is pretty simple.

這個想法很簡單，

Let's do an experiment.

讓我們做一個實驗：

Stick out your thumb and close your left eye.

伸出你的拇指並閉上你的左眼。

Now, open your left eye and close your right eye.

現在，打開你的左眼後 再閉上你的右眼。

It will look like your thumb has moved,

拇指的位置看起來像是移動了，

while more distant background objects have remained in place.

而遠處背景中的物體仍然在原來位置。

The same concept applies when we look at the stars,

當我們觀察星星時也適用同樣的概念，

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

但恆星的距離遠超過你手臂的長度，

and the Earth isn't very large,

且地球也不是很龐大，

so even if you had different telescopes across the equator,

所以即使你在赤道兩端 用望遠鏡看同一顆星星，

you'd not see much of a shift in position.

它位置的變化也不明顯。

Instead, we look at the change in the star's apparent location over six months,

相反，若我們間隔六個月來看， 星星的位置就會明顯不同了，

the halfway point of the Earth's yearlong orbit around the Sun.

地球繞太陽軌道的兩側。

When we measure the relative positions of the stars in summer,

我們先在夏天測量一顆星星的位置，

and then again in winter, it's like looking with your other eye.

然後冬天再測一次， 就像你用另一隻眼睛看一樣。

Nearby stars seem to have moved against the background

較近的恆星在遠處 恆星與星系的襯映下，

of the more distant stars and galaxies.

位置似乎移動了。

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

但這種方法只適用幾千光年遠的星體，

Beyond our own galaxy, the distances are so great

出了我們的銀河系，空間是如此之大，

that the parallax is too small to detect with even our most sensitive instruments.

視差小到連最靈敏的儀器都無法檢測，

So at this point we have to rely on a different method

所以我們得運用不同的方法來測距，

using indicators we call standard candles.

利用參考指標，我們稱之為標準燭光。

Standard candles are objects whose intrinsic brightness, or luminosity,

稱為標準燭光的星體，

we know really well.

我們已很清楚其亮度，

For example, if you know how bright your light bulb is,

例如，如果你知道一個燈泡有多亮，

and you ask your friend to hold the light bulb and walk away from you,

你請朋友拿著燈泡走遠，

you know that the amount of light you receive from your friend

你看到的燈泡亮度，

will decrease by the distance squared.

將以距離的平方減少。

So by comparing the amount of light you receive

因此，以你看到的亮度

to the intrinsic brightness of the light bulb,

與燈泡原來的亮度相比較，

you can then tell how far away your friend is.

你就能知道你的朋友走多遠了。

In astronomy, our light bulb turns out to be a special type of star

在天文學，我們的燈泡 是一種特殊類型的星體，

called a cepheid variable.

稱之為造父變星。

These stars are internally unstable,

這類星體內部不穩定，

like a constantly inflating and deflating balloon.

就像一個不斷充氣和放氣的氣球。

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

由於膨脹和收縮導致它們的亮度改變，

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

我們可以透過測量脹縮週期 來計算出他們的亮度，

with more luminous stars changing more slowly.

越亮的星星變化的速度越慢。

By comparing the light we observe from these stars

經由比較這些恆星的亮度，

to the intrinsic brightness we've calculated this way,

與我們計算出的固有亮度，

we can tell how far away they are.

我們可以知道星體有多遠。

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

不幸的是，這仍然不是故事的結尾。

We can only observe individual stars up to about 40,000,000 light years away,

這樣我們還是只能觀察 約4億光年遠的單一恆星，

after which they become too blurry to resolve.

再遠，就因為太模糊而無法測距。

But luckily we have another type of standard candle:

所幸的是，我們有另一種類型的標準燭光：

the famous type 1a supernova.

著名1a型超新星。

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

超新星爆發，即巨型恆星爆炸， 是恆星死亡的方式之一。

These explosions are so bright,

這些爆炸是如此明亮，

that they outshine the galaxies where they occur.

在星系中一枝獨秀。

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

所以，即使我們看不到 星系中的單一個恆星，

we can still see supernovae when they happen.

當超新星爆炸時，我們仍然可以看到。

And type 1a supernovae turn out to be usable as standard candles

以1a型超新星作為標準燭光，

because intrinsically bright ones fade slower than fainter ones.

因為較明亮的星體 褪色較暗淡者慢一些。

Through our understanding of this relationship

透過我們對亮度和下降率關係的了解，

between brightness and decline rate,

我們可以利用這些超新星探測距離，

we can use these supernovae to probe distances

up to several billions of light years away.

高達數十億光年遠。

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

但是，為什麼測量 遙遠物體的距離那麼重要呢？

Well, remember how fast light travels.

好吧，記得光速很快吧！

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

例如，由太陽發射的光 只需要八分鐘就到達地球，

which means that the light we see now is a picture of the Sun eight minutes ago.

這意味著光我們現在看到的太陽 是8分鐘前的樣子。

When you look at the Big Dipper,

當你看到北斗七星時，

you're seeing what it looked like 80 years ago.

你看到的是它80年前的景象。

And those smudgy galaxies?

而那些模糊不清的星系？

They're millions of light years away.

它們在百萬光年遠，

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

發出的光線費時數百萬年才到達地球。

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

所以，宇宙本身在某種意義上 是一個內置的時間機器。

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

我們愈往前追朔， 就發現我們所探索的宇宙愈年輕！

Astrophysicists try to read the history of the universe,

天體物理學家嘗試讀取宇宙的歷史，

and understand how and where we come from.

並了解我們來自何處及如何到來。

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

宇宙正在不斷地以光的形式向我們發送訊息，

All that remains if for us to decode it.

接下來，就看我們是否能將訊息解碼。