Name: 引力波背景終於被發現了嗎？ (Was the Gravitational Wave Background Finally Discovered?!?)
Uploaded: 2025-02-24T01:14:58.000Z
Duration: 17 min 3 s

感謝歌劇院對公共廣播公司的支持。

A few weeks ago the world's gravitational wave astronomers announced something pretty wild.

幾周前，世界引力波天文學家宣佈了一件非常瘋狂的事情。

關係子句

The moderately confident detection of pervasive ripples in the fabric of space-time that presumably fill the cosmos.

在時空結構中探測到普遍存在的漣漪，據推測這些漣漪充滿了整個宇宙。

Detected by watching for subtle connections between the signals from rapidly spinning cores of dead stars in our galactic neighborhood.

通過觀察銀河系附近快速旋轉的死亡恆星核心發出的信號之間的微妙聯繫而探測到的。

In other words, the gravitational wave background has probably been detected using a pulsar timing array.

換句話說，引力波背景很可能是利用脈衝星定時陣列探測到的。

The likely detection of the gravitational wave background is huge.

可能探測到的引力波背景是巨大的。

Several channels have gotten to this news before us, but in our defense we did an episode on the slightly more tentative detection two years ago.

在我們之前，已經有多個頻道報道了這一新聞，但為了保護我們的利益，我們在兩年前做了一期節目，報道了這一略帶試探性的檢測結果。

Now the detection is on firmer footing and the basics have been thoroughly covered by ourselves and others, so we can dig a little deeper.

現在，偵查工作已經有了更堅實的基礎，我們自己和其他人也已經對基本知識進行了全面介紹，是以我們可以再深入一些。

Today I want to talk a bit about what it took to spot the gravitational wave background and then more about what it tells us about our universe.

今天，我想先談談發現引力波背景的過程，然後再談談它告訴了我們關於宇宙的更多資訊。

First up, let's take a moment to appreciate the awesomeness of this achievement.

首先，讓我們花點時間來欣賞一下這項成就的神奇之處。

We started with a deceptively simple idea.

And by we I mean humanity, but specifically the incidence of humanity named Albert Einstein.

我說的 "我們 "指的是人類，但更具體地說，是人類中的一個名叫阿爾伯特-愛因斯坦的人。

The following thought popped into his head one day.

有一天，他突然想到了下面這個問題。

For a man falling from the roof of a house, there is no gravitational field.

對於從房頂墜落的人來說，不存在引力場。

This became the equivalence principle, which basically states that the feeling of weightlessness you have while falling is the same as the weightlessness in the absence of gravity, and the feeling of heaviness when accelerating is the same as that when stationary in a gravitational field, at least as far as the laws of physics are concerned.

這就是 "等效原理"，其基本原理是：墜落時的失重感與無重力時的失重感相同，加速時的沉重感與在重力場中靜止時的沉重感相同，至少就物理定律而言是如此。

From the equivalence principle and one more axiom that the speed of light is constant for all observers, an inevitable chain of reasoning led Einstein to the general theory of relativity, which explains gravity as being due to the warping of space and time.

從等效原理和 "光速對所有觀測者都是恆定的 "這一公理出發，一連串不可避免的推理將愛因斯坦引向了廣義相對論，該理論將萬有引力解釋為空間和時間的扭曲所致。

The equations of GR give us so much more than gravity.

全球定位系統方程為我們提供的遠不止引力。

They predict that gravitational fields slow clocks and deflect light, reveal the inevitability of black holes, and also predict that the fabric of spacetime should carry waves.

他們預測引力場會減慢時鐘和偏轉光線，揭示黑洞的必然性，還預測時空結構應攜帶波。

Gravitational waves were the last great prediction of general relativity to be experimentally verified, and that happened only in 2016 when LIGO spotted the spacetime ripples caused when a pair of black holes spiraled together and merged over a billion light years away.

引力波是廣義相對論中最後一個得到實驗驗證的偉大預言，直到2016年，當LIGO發現一對黑洞在10億光年外螺旋合併時產生的時空漣漪時，引力波才得到驗證。

LIGO did this by measuring the literal stretchy and squishing of space using what amount to a pair of ultra-precise rulers a few kilometers in length, set at right angles to each other.

LIGO 通過使用一對長度為幾千米、互成直角的超精密尺子來測量空間的實際伸縮情況。

In the eight years we've been doing this, we've observed the gravitational waves resulting from the final in-spiral of pairs of black holes and or neutron stars.

在我們從事這項工作的八年時間裡，我們觀測到了成對黑洞和中子星最終內旋產生的引力波。

These have masses from a few to a few tens times that of the sun.

它們的品質是太陽的幾倍到幾十倍。

Our earth-based facilities were built to be sensitive to these, because we knew there should be lots of these sources of gravitational waves.

我們在地球上建造的設施對引力波非常敏感，因為我們知道應該有很多這樣的引力波源。

As waves, gravitational waves have wavelengths.

An observatory will be sensitive to wavelengths that have a similar size to the detector arm, to its rulers.

天文臺對波長的敏感度取決於探測器臂和尺子的大小。

And in-spiraling stellar corpses generate wavelengths roughly equal to their orbital period times the speed of light, which is a few kilometers in the last seconds of that in-spiral.

螺旋內的恆星屍體產生的波長大致等於其軌道週期乘以光速，也就是螺旋內最後幾秒鐘的幾千米。

The larger the orbit, the longer the wavelength.

The gravitational waves produced by binary stellar-mass black holes when they're further apart should be visible to the Laser Interferometer Space Antenna, LISA, with its 2.5 million kilometer arms of its laser-connected spacecraft, at least after it launches.

雙恆星品質黑洞相距較遠時產生的引力波，至少在脈衝光干涉儀空間天線（LISA）發射後，其250萬公里的脈衝光連接航天器臂應該可以看到。

But there are also gravitational waves that stretch for light-years, waves that no human-built device could hope to detect because we can't build galactic-scale rulers.

但是，還有綿延光年的引力波，這些引力波是人類製造的設備無法探測到的，因為我們無法制造銀河尺度的標尺。

However, by happy chance, the galaxy has obliged and provided us with a network of natural rulers, the pulsars.

然而，幸運的是，銀河系為我們提供了一個天然統治者網絡--脈衝星。

與其說是標尺，不如說是時鐘。

Pulsars are rapidly rotating and processing neutron stars whose jets sweep past the Earth, resulting in blips of electromagnetic radiation that repeat with extreme regularity, sometimes several hundred times per second.

脈衝星是一種快速旋轉和處理的中子星，其噴流掠過地球時會產生電磁輻射，這種輻射極為規律，有時每秒重複幾百次。

Those ridiculously fast ones are called millisecond pulsars, and they are the most precise clocks in the universe, natural or unnatural.

那些快得離譜的脈衝星被稱為毫秒脈衝星，它們是宇宙中最精確的時鐘，不管是天然的還是非天然的。

但我們想要的是一把尺子，而不是一個鐘。

However, with the conveniently constant speed of light, a clock becomes a ruler if we just measure the travel time of light.

然而，由於光速恆定，如果我們只測量光的傳播時間，時鐘就成了一把尺子。

If a gravitational wave passes by the stream of incoming signals from a pulsar, it will stretch and compact the space between those pulses.

如果引力波經過脈衝星傳入的信號流，就會拉伸和壓縮這些脈衝之間的空間。

Measuring the change in pulse arrival time measures the gravitational wave.

測量脈衝到達時間的變化可以測量出引力波。

This can, in principle, be used to spot individual gravitational waves.

原則上，這可以用來發現單個引力波。

Several international collaborations have now been watching dozens of pulsars for over 15 years, using many of the largest radio telescopes on Earth.

15 年多來，一些國際合作組織利用地球上許多最大的射電望遠鏡觀測了數十顆脈衝星。

These pulsar timing arrays don't yet have a sure signal from a single gravitational wave, but essentially all of these teams agree that their data reveal something that's arguably even cooler.

這些脈衝星定時陣列還沒有一個確定的引力波信號，但基本上所有這些團隊都同意，他們的數據揭示了一些可以說更酷的東西。

All of spacetime, across the pulsar network but probably across the universe, is a bit wibbly wobbly.

整個脈衝星網絡乃至整個宇宙的時空都有點搖擺不定。

They claim detection of the stochastic gravitational wave background, the jumbled overlap of the many many very weak but very long wavelength gravitational waves that must originate from across the known universe.

他們聲稱探測到了隨機引力波背景，即許多非常微弱但波長很長的引力波的雜亂重疊，這些引力波一定來自整個已知宇宙。

We don't know what creates the background yet.

我們還不知道是什麼創造了背景。

It could be echoes from the inflationary epoch, which kickstarted the big bang.

這可能是引發大爆炸的暴脹時代的回聲。

Or it could be universe-wide phase transitions from right after that.

也可能是在那之後的全宇宙相變。

It could be cosmic string collisions in which fissures in spacetime tangle and split.

這可能是宇宙弦碰撞，時空裂縫在其中糾結和分裂。

Or it could be the frolicking of galactic gigawales in galaxies far far away.

也可能是銀河系的巨鯨在遙遠的星系中嬉戲。

也許不是這樣，但我們可以期待。

Most likely however, this gravitational wave background results from binary black holes.

不過，這種引力波背景最有可能是由雙黑洞產生的。

Although in this case it's not from the individual tens of solar mass black holes seen by LIGO, we're probably seeing the reverberating tremors caused by binary pairs of behemoth supermassive black holes in the hearts of galaxies.

雖然在這種情況下，它不是來自 LIGO 看到的單個幾十個太陽品質的黑洞，但我們看到的可能是星系中心的雙對巨型超大品質黑洞引起的震盪。

These SMBHs with masses of millions to billions that of our sun are close to my own heart as a researcher, so I'm more than a little excited about what we can learn about them, and I'm going to spend some time talking about them today.

作為一名研究人員，這些品質相當於我們太陽數百萬到數十億品質的超大品質塊與我的心息息相關，是以我對我們能瞭解到它們的情況感到非常興奮，今天我將花一些時間來談談它們。

But first, let's start with an analogy to get a better picture of all this craziness.

但首先，讓我們先打個比方，以便更好地瞭解這些瘋狂的事情。

Take the surface of a still lake, and very rapidly stir it at one point with a pinpoint.

在靜止的湖面上，用針尖快速攪動一點。

The expanding ripples are like the gravitational radiation detected by LIGO.

膨脹的波紋就像 LIGO 探測到的引力輻射。

Now, instead of one pinpoint spiral, stir the surface of the lake with many, many, I don't know, tree trunks or something, but much more slowly.

現在，不是一個針尖大小的螺旋，而是用很多很多，我也不知道是樹幹還是什麼的攪動湖面，但速度要慢得多。

The entire lake is now covered in a jumble of very low frequency ripples that aren't distinguishable from each other.

現在，整個湖面上佈滿了雜亂無章的頻率極低的波紋，彼此無法區分。

This is similar to what the stochastic gravitational wave background should look like if it's caused by binary supermassive black holes.

這類似於如果隨機引力波背景是由雙超大品質黑洞引起的，那麼它應該是什麼樣子。

LIGO is tiny compared to the resulting spacetime ripples.

與由此產生的時空漣漪相比，LIGO 微不足道。

Both of its arms are affected to the same degree by these light-years-long oscillations, and so it doesn't notice their passage.

它的兩隻手臂受到這些長達一光年的振盪的影響程度相同，是以它不會注意到它們的經過。

But the relative distances to pulsars are affected by these enormous waves, and so they should cause observable shifts in the timing of their pulses as we observe them.

但是脈衝星的相對距離會受到這些巨波的影響，是以它們會導致我們觀測到的脈衝時間發生可觀測到的偏移。

These ripples are messy, apparently random or stochastic.

這些波紋雜亂無章，顯然是隨機的或隨機的。

So how can we be sure that we're even seeing gravitational waves?

那麼，我們如何確定我們看到的是引力波呢？

After all, there are various reasons why the rate of a pulsar's signal may change.

畢竟，脈衝星信號的速率會發生變化的原因是多方面的。

Pulsar rotation rates can slow down or speed up, and the travel time of their signals to us can be affected by more than gravitational waves, for example passing through a region of ionized gas slows the radio light.

脈衝星的旋轉速度可以減慢或加快，它們向我們發送信號的時間也可能受到引力波以外的影響，例如穿過電離氣體區域會減慢射電光速。

But all of these things should affect each pulsar individually, or at worst affect groups of pulsars that are in one particular direction.

但所有這些都會對每顆脈衝星產生單獨的影響，或者在最壞的情況下對處於某一特定方向的脈衝星群產生影響。

However, gravitational waves cause the pulse rates of pulsars across the galaxy to change in ways that are correlated with each other.

然而，引力波會導致整個銀河系脈衝星的脈衝率以相互關聯的方式發生變化。

Imagine signals travelling to us from different pairs of pulsars.

想象一下，信號從不同的脈衝星對向我們傳播。

Those signals could be travelling together if the pulsars are near each other on the sky, or they could be travelling to us from opposite directions on the sky, or the signals could be travelling at right angles to each other, or they could be situated in between those extreme cases.

如果脈衝星在天空中彼此靠近，這些信號可能會一起傳播，或者它們可能從天空中相反的方向向我們傳播，或者信號彼此成直角傳播，或者它們可能位於這些極端情況之間。

Any gravitational wave that makes up part of the gravitational wave background will also be travelling through the galaxy in some direction relative to both of those pulsar signals.

任何構成引力波背景的引力波，也會以相對於這兩個脈衝星信號的某個方向穿過銀河系。

In some cases, that relative direction will cause both pulse rates to be affected in the same way, correlated, and in some cases they'll be affected in opposite ways, or anti-correlated.

在某些情況下，這種相對方向會使兩個脈衝頻率受到相同的影響，即相關；而在某些情況下，它們會受到相反的影響，即反相關。

For example, you would get a correlated pulsar timing shift if both pulsar signals are surfing the same gravitational wave, or if 180-degree separated pulsar pairs encounter a gravitational wave moving at right angles to both of their signals.

例如，如果兩個脈衝星信號都在衝浪同一個引力波，或者如果相距 180 度的脈衝星對遇到一個與它們的信號都成直角運動的引力波，就會產生相關的脈衝星時移。

You'd get an anti-correlated shift if the pulsar signals are travelling at right angles to each other because of the way gravitational waves ultimately stretch and squish space at 90 degrees as they pass.

如果脈衝星信號的傳播角度是直角，就會產生反相關的偏移，因為引力波最終會以 90 度拉伸和擠壓空間。

The correlation, or anti-correlation, due to a single gravitational wave is extremely difficult to pick out from all the sources of noise.

單個引力波引起的相關性或反相關性很難從所有噪聲源中分離出來。

However, if you look at enough pairs of pulsars for enough time, you expect to see a statistical correlation in what we call the pulsar timing residual, that's the amount of deviation from the very precise expected arrival time of these pulses.

然而，如果你觀察足夠多的脈衝星對，觀察足夠長的時間，你就會發現我們所說的脈衝星時間殘差，也就是這些脈衝與非常精確的預期到達時間的偏差量，存在著統計上的相關性。

This is the Helling's-Downs curve, it's the theoretical correlation between pulsar timing residuals for pairs of pulsars as a function of their separation on the sky.

這是海靈氏-下降曲線，是一對脈衝星的脈衝星計時殘差與它們在天空中的距離之間的理論相關性。

Pulsars with little separation should be highly correlated.

相距甚遠的脈衝星應該高度相關。

Pulsars with 180-degree separation should be somewhat correlated.

相距 180 度的脈衝星應該有一定的相關性。

Pulsars with 90-degree separation should be anti-correlated.

相距 90 度的脈衝星應該是反相關的。

Okay, so how are the real pulsars behaving?

好吧，那麼真正的脈衝星表現如何呢？

Well, this is the result published by the Nanograv Collaboration.

這就是 Nanograv 合作組織公佈的結果。

This is for every combination of pairs for 67 pulsars observed over the last 15 years.

這是過去 15 年中觀測到的 67 顆脈衝星的每一對組合。

And it's very consistent with our Helling's-Downs curve.

這與我們的海靈氏下降曲線非常吻合。

Nanograv claims that this is from 3.5 to 4 sigma depending on the statistical analysis used.

Nanograv 聲稱，根據所使用的統計分析，這一數值在 3.5 到 4 個西格瑪之間。

That means it's not quite a slam-dunk 5-sigma detection in that the apparent correlation could still have popped out of the random noise by a 1 in thousands chance.

這就意味著，這並不是一個完美的 5 西格瑪檢測，因為表面上的相關性仍有可能以千分之一的概率從隨機噪音中跳出來。

But it's looking increasingly likely that the correlations are real.

但現在看來，這種相關性越來越有可能是真實的。

The same results have been observed by other pulsar timing array experiments with varying degrees of confidence.

其他脈衝星定時陣列實驗也觀測到了同樣的結果，但可信度各不相同。

Now we can start looking into what we can learn about the universe from this observation.

現在，我們可以開始研究從這次觀測中我們能瞭解到宇宙的哪些資訊。

But before we do that, let's pause for a moment to appreciate how crazy this achievement really is.

但在此之前，讓我們先靜下心來欣賞一下這項成就到底有多瘋狂。

Remember that we started with a simple thought experiment about the experience of someone falling off a roof.

請記住，我們是從一個簡單的思想實驗開始的，這個實驗是關於一個人從屋頂上掉下來的經歷。

That what-if scenario led us all the way to actual observation of galactic-scale spacetime ripples.

這種 "假如 "的設想讓我們一路走來，最終觀測到了星系尺度的時空漣漪。

I've been talking about binary supermassive black holes because that's what the Nanograv team thinks that this is.

我一直在談論雙超大品質黑洞，因為 Nanograv 團隊認為這就是雙超大品質黑洞。

The type of timing correlation that was observed is what you'd expect from many, many sources of gravitational waves that are a. powerful and low frequency, b. randomly distributed across the cosmos, and c. randomly polarized so no preferred direction for the stretching and squishing of space from any given wave.

所觀測到的這種時間相關性是你從許許多多的引力波源中所能預料到的，這些引力波a. 強大而低頻，b. 隨機分佈在宇宙中，c. 隨機極化，是以任何給定的波對空間的拉伸和擠壓都沒有首選方向。

Any such population of sources should give you this characteristic curve.

任何此類數據源都會呈現出這種特徵曲線。

Well they do potentially fit the requirements, but just as importantly, we know they should exist.

它們確實有可能符合要求，但同樣重要的是，我們知道它們應該存在。

We know that every galaxy contains a huge black hole at its center, and we know that bigger galaxies are made from smaller galaxies combining, and that bigger galaxies have bigger black holes.

我們知道每個星系的中心都有一個巨大的黑洞，我們也知道更大的星系是由更小的星系組合而成的，更大的星系有更大的黑洞。

It only stands to reason that there are a good number of binary supermassive black holes out there, even if we haven't directly detected them yet.

是以，即使我們還沒有直接探測到它們，但也可以推斷出有大量的雙超大品質黑洞存在。

Any other source of gravitational wave background like this is much more speculative, and we do talk about those in our previous episode.

任何其他類似的引力波背景源都更具推測性，我們在上一集中也談到了這些引力波背景源。