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Is there any sound in space? An astronomer explains
How far can sound travel through space, since it’s so empty? Is there an echo in space? – Jasmine, age 14, Everson, Washington
In space, no one can hear you scream.
You may have heard this saying. It’s the tagline from the famous 1979 science fiction movie “Alien.” It’s a scary thought, but is it true? The simple answer is yes, no one can hear you scream in space because there is no sound or echo in space.
I’m a professor of astronomy, which means I study space and how it works. Space is silent – for the most part.
How sound works
To understand why there’s no sound in space, first consider how sound works. Sound is a wave of energy that moves through a solid, a liquid or a gas.
Sound is a compression wave. The energy created when your vocal cords vibrate slightly compresses the air in your throat, and the compressed energy travels outward.
A good analogy for sound is a Slinky toy. If you stretch out a Slinky and push hard on one end, a compression wave travels down the Slinky.
When you talk, your vocal cords vibrate. They jostle air molecules in your throat above your vocal cords, which in turn jostle or bump into their neighbors, causing a sound to come out of your mouth.
Sound moves through air the same way it moves through your throat. Air molecules near your mouth bump into their neighbors, which in turn bump into their neighbors, and the sound moves through the air. The sound wave travels quickly, about 760 miles per hour (1,223 kilometers per hour), which is faster than a commercial jet.
Space is a vacuum
So what about in space?
Space is a vacuum, which means it contains almost no matter. The word vacuum comes from the Latin word for empty.
Sound is carried by atoms and molecules. In space, with no atoms or molecules to carry a sound wave, there’s no sound. There’s nothing to get in sound’s way out in space, but there’s nothing to carry it, so it doesn’t travel at all. No sound also means no echo. An echo happens when a sound wave hits a hard, flat surface and bounces back in the direction it came from.
By the way, if you were caught in space outside your spacecraft with no spacesuit, the fact that no one could hear your cry for help is the least of your problems. Any air you still had in your lungs would expand because it was at higher pressure than the vacuum outside. Your lungs would rupture. In a mere 10 to 15 seconds, you’d be unconscious due to a lack of oxygen.
Sound in the solar system
Scientists have wondered how human voices would sound on our nearest neighboring planets, Venus and Mars. This experiment is hypothetical because Mars is usually below freezing, and its atmosphere is thin, unbreathable carbon dioxide. Venus is even worse – its air is hot enough to melt lead, with a thick carbon dioxide atmosphere.
On Mars, your voice would sound tinny and hollow, like the sound of a piccolo. On Venus, the pitch of your voice would be much deeper, like the sound of a booming bass guitar. The reason is the thickness of the atmosphere. On Mars the thin air creates a high-pitched sound, and on Venus the thick air creates a low-pitched sound. The team that worked this out simulated other solar system sounds, like a waterfall on Saturn’s moon Titan.
Deep space sounds
While space is a good enough vacuum that normal sound can’t travel through it, it’s actually not a perfect vacuum, and it does have some particles floating through it.
Beyond the Earth and its atmosphere, there are five particles in a typical cubic centimeter – the volume of a sugar cube – that are mostly hydrogen atoms. By contrast, the air you are breathing is 10 billion billion (1019) times more dense. The density goes down with distance from the Sun, and in the space between stars there are 0.1 particles per cubic centimeter. In vast voids between galaxies, it is a million times lower still – fantastically empty.
The voids of space are kept very hot by radiation from stars. The very spread-out matter found there is in a physical state called a plasma.
A plasma is a gas in which electrons are separated from protons. In a plasma, the physics of sound waves get complicated. Waves travel much faster in this low-density medium, and their wavelength is much longer.
In 2022, NASA released a spectacular example of sound in space. It used X-ray data to make an audible recording that represents the way a massive black hole stirs up plasma in the Perseus galaxy cluster, 250 million light years from Earth. The black hole itself emits no sound, but the diffuse plasma around it carries very long wavelength sound waves.
The natural sound is far too low a frequency for the human ear to hear, 57 octaves below middle C, which is the middle note on a piano and in the middle of the range of sound people can hear. But after raising the frequency to the audible range, the result is chilling – it’s the sound of a black hole growling in deep space.
注释：
tagline：n
表示"(产品或电影的)标志性语言"，如：Tagline: Time to save my family. 经典台词：是保卫我家的时候了。
Slinky: n
表示"一种弹簧玩具"
jostle: v
表示" 推；挤；争夺"，means "make one's way by jostling, pushing, or shoving"，如：The youths jostled against an old lady on the pavement. 那些青年人在人行道上推推搡搡挤着了一个老太太。
rupture: v; n
表示" （使）破裂"，means "seperate; state of being torn or burst open"，如：He'll rupture a muscle if he goes on dancing like that! 他要是继续那样跳舞，会跳得皮开肉裂！The negotiation came to a rupture. 谈判破裂了。
piccolo: n
表示" 短笛"，means "a small flute"，如：The girl was the piccolo player in the marching band. 那女孩是军乐队的短笛手。
proton: n
表示" 质子"，如：Proton and neutron form a charge doublet. 质子和中子构成电荷双重态。
Perseus: n
表示" [希神]珀尔修斯（达那厄和宙斯的儿子）; [天]英仙座"，means ""，如：Perseus rise in the northeast around midnight. 英仙座会在午夜从东北方向升起。
octave: n
表示" 八音度；倍频程"，如：Can you span an octave on the piano?你能按到钢琴的八个键吗？Each successive noise function you add is known as an octave. 每个你所叠加的噪声函数就是一个倍频。
中文简要说明：
我们从小所学的科普并没有错，太空中没有声音。因为声音是由原子和分子震动所传递的，但是！太空中也有许多地方有物质（例如尘埃、电浆等）在寂静的太空中他们也能成为传递声音的介质，因此「太空中的声音」仍然是科学家很感兴趣的议题，不管是我们在别的星球讲话的声音，还是外星瀑布的声响甚至黑洞的咆哮。
据《天文学》（Astronomy）报导，太空是真空环境，声音是物质震动传递的。在太空中，没有原子或分子来传递声波，所以没有声音。但即使太空足够真空，普通声音无法通过它传播，但实际上它不是完美的真空，其中确实有一些微粒漂浮其中，例如在地球大气层之外一定距离内，每毫升的空间内有5个氢原子。
科学家还是很好奇人类的声音在邻近行星金星和火星上会是什么样子。透过假设性的实验，火星通常是在冰点以下，大气稀薄且都是二氧化碳，金星更糟糕，其大气温度足以融化铅，而且也是有一层厚厚的二氧化碳大气层。
在火星上，你的声音会听起来会很尖锐且空洞，有点像迪斯尼卡通片的花栗鼠讲话。而在金星上，你讲话的音调会变很低沉，就像乐器贝斯（低音吉他）的声音。
原因在于大气的厚度，在火星上，稀薄的空气会产生高音，而在金星上，浓厚的空气会产生低音，科学家还模拟了其他声响，如土卫六上的瀑布的声音。
太空中的恒星附近的空间，因为辐射强大保持得非常热，那里的物质非常稀薄，处于电浆态，电浆是一种电子与质子分离的气体，而声波在电浆里传递的物理现象相当复杂，声波在这种低密度介质中传播得更快，而它们的波长更长。
2022年，美国太空总署（NASA）发布了太空中声音的壮观示例。它使用X射线数据制作了一段声音，代表了一个距离地球2.5亿光年的英仙座星系团（Perseus galaxy cluster）的黑洞搅动电浆的方式。黑洞本身没有发出声音，但其周围的扩散的电浆携带着非常长波长的声波。
黑洞搅动电浆的声音对人耳而言频率太低，无法听到，但在将频率提高到可听范围后，结果令人不寒而栗，彷佛一个黑洞在深空中咆哮一般。
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