Vacuum literally means "nothing", but such a vacuum does not exist. The definition of vacuum in physics is a state of complete nothingness except time and space. In other words, a state where there is only time and space and nothing else is a vacuum.

真空，從字面上來理解就是“什么都沒有”，不過這樣的真空是不存在的。物理學(xué)上對真空的定義是，除了時空之外，完全虛無的狀態(tài)。也就是說，只有時空，其他什么都沒有的狀態(tài)就是真空。
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So does such a vacuum exist?

那么這樣的真空是否存在呢？

Ancient humans were very curious about vacuum, but the level of science and technology at that time was greatly limited, and people's understanding of vacuum was mostly at the level of thought experiments, and even philosophical thinking was needed to understand vacuum.

古代人類對真空就很好奇，不過當(dāng)時的科技水平受到很大限制，人們對真空的認(rèn)知更多的只停在思想實驗層次，甚至需要哲學(xué)思想去理解真空。

Humanity's real understanding of vacuum began in the 17th century. In 1641, physicist Torricelli conducted such an experiment, which was very simple.

人類對真空真正的認(rèn)識開始于17世紀(jì)。1641年，物理學(xué)家托里拆利做了這樣的實驗，實驗很簡單。

He filled a one-meter-long tube with mercury, blocked the opening of the tube with his hand, and placed it upside down in a tank filled with mercury. He then found that the length of the mercury in the tube dropped to 76 centimeters, and a 24-centimeter vacuum state appeared at the top.

一根一米長的管子注滿水銀，然后用手堵住管子的開口，倒放在裝滿水銀的槽里。之后他發(fā)現(xiàn)管里的水銀長度下降到76厘米，最上端出現(xiàn)了24厘米的真空狀態(tài)。

In fact, this is the earliest barometer. So is the vacuum environment 24 cm above the water pipe really a "vacuum"?

其實這就是最早的氣壓計。那么水管上面24厘米高的真空環(huán)境果真是“真空”嗎？

Now we know that the local area 24 centimeters high cannot be a vacuum, because although it seems that there is nothing, the universe is full of electromagnetic waves of various energies, which can easily penetrate obxts. So the environment 24 centimeters high will not be a vacuum, various electromagnetic waves will pass through there, and there are neutrinos that we cannot see, they are everywhere.

如今我們知道，那24厘米高的局部不可能是真空，因為雖然看起來什么都沒有，但由于宇宙中充滿了各種能量的電磁波，它們能輕易穿透物體。所以那24厘米高的環(huán)境不會是真空，各種電磁波會穿過那里，還有我們看不到的中微子，它們無處不在。
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So, if all electromagnetic waves, neutrinos and other microscopic particles are removed, will the remaining environment be a vacuum environment?

那么，如果把所有的電磁波，中微子等微觀粒子全部去除掉，剩下的環(huán)境就是真空環(huán)境了嗎？
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It is not. Although from a macroscopic perspective, it seems that there is only time and space and nothing else, if we continue to magnify the so-called vacuum and reach the microscopic world, we will find something different.

同樣不是。雖然從宏觀上來看，好像只有時空，其他什么都沒有了，但如果我們把所謂的真空不斷放大，到了微觀世界，我們就會有不一樣的發(fā)現(xiàn)。

Because at the quantum level, the vacuum is far more complex than we imagine. Even if we can remove all matter from the vacuum, or even make the vacuum temperature reach absolute zero, the vacuum will not be an absolute vacuum.

因為在量子層面，真空遠比我們想象的更復(fù)雜。即便我們能去除真空中的一切物質(zhì)，甚至讓真空的溫度達到絕對零度，真空也不會是絕對的真空。

Quantum field theory, which is based on quantum mechanics, believes that the so-called vacuum is actually filled with different types of quantum fields at all times, and each quantum field corresponds to a fundamental particle. Normally, the quantum field is in the ground state, which is also the most stable state, like a calm sea.

基于量子力學(xué)發(fā)展起來的量子場論認(rèn)為，所謂的真空，其實時刻充斥著不同類型的量子場，每種量子場都對應(yīng)著一種基本粒子。通常情況下，量子場處于基態(tài)，也是最穩(wěn)定的狀態(tài)，就像風(fēng)平浪靜的大海一樣。

However, according to the uncertainty of quantum mechanics, the uncertainty of time and energy must not be less than a constant, which means that the quantum field in the ground state must fluctuate, and the more time, the greater the energy fluctuation. Just like the temperature cannot reach absolute zero and the speed cannot exceed the speed of light, the quantum field cannot be completely in the ground state, it must be disturbed and become an excited state.

不過根據(jù)量子力學(xué)的不確定性，時間和能量的不確定性必須不小于一個常數(shù)，這意味著處于基態(tài)的量子場一定會存在波動，而且時間越多，能量波動就越大。這就像溫度不可能達到絕對零度，速度不可能超越光速一樣，量子場也不可能完全處于基態(tài)，它一定會受到擾動從而成為激發(fā)態(tài)。

At a certain moment, the fluctuation of the quantum field may be very large, so that a pair of virtual particles and antiparticles can be derived out of thin air in the vacuum. This does not violate the law of conservation of energy, because the derived virtual particles and antiparticles will be annihilated and disappear instantly.

在某一瞬間，量子場的波動可能非常大，這樣真空就能憑空衍生出一對正反虛粒子，這并不違反能量守恒定律，因為衍生出來的正反虛粒子會瞬間湮滅消失。

The reason why they are called "virtual particles" is that we cannot directly observe their existence. They are somewhat different from real particles. Strictly speaking, virtual particles are not particles.

之所以被稱為“虛粒子”，就是因為我們不能直接觀測到它們的存在，與真實存在的粒子有些不同，嚴(yán)格意義上來講，虛粒子并不是粒子。

You may ask: Since we can't see virtual particles, how can we be sure of their existence?

你可能會問：既然看不到虛粒子，怎么確定它們的存在呢？

Although we cannot see them, we can confirm the existence of virtual particles based on their interaction with real particles. The specific process will not be described in detail here.

雖然看不到，但我們可以根據(jù)它們與真實存在的粒子發(fā)生作用，進而確實虛粒子的存在。具體過程這里就不詳述了。

In general, the vacuum is not empty, and the vacuum also has energy, which is the so-called "vacuum zero-point energy". There are vacuum fluctuations in the vacuum at all times, which looks like a calm sea surface suddenly becoming turbulent.

總的來說，真空不空，真空也擁有能量，也就是所謂的“真空零點能”，真空中時刻存在真空波動，看起來就像是平靜的海面突然變得洶涌澎湃一樣。

Not only does the vacuum have energy, but the amount of energy it has is also relative. In other words, there are different levels of vacuum, some vacuums are relatively empty, while others are not so empty.

真空不但擁有能量，而且擁有的能量多少還是相對的。也就是說，真空也有等級之分，有些真空相對來講比較空，而有些真空就不那么空。

The famous physicist Casimir once conducted such an experiment in 1948. He placed two thin metal sheets close to each other in a vacuum. Since the metal sheets were not charged, they would not attract or repel each other.

著名的物理學(xué)家卡西米爾曾經(jīng)在1948年做過這樣的實驗，在真空中讓兩張薄金屬片相互靠近，由于金屬片不帶電，所以它們不會相互吸引或排斥。

However, when the metal sheets were close enough, something strange happened. The two metal sheets actually began to move closer to each other, as if gravity was generated between them.

不過，當(dāng)金屬片靠得足夠近的時候，奇怪的事情發(fā)生了，兩張金屬片竟然開始相互靠近，好像兩者之間產(chǎn)生了引力一樣。

This is actually the Casimir effect. It is not that the metal sheets attract each other, but because the two metal sheets are close enough, the fluctuations on the inner side will be restricted, and only the fluctuations with shorter wavelengths can exist. The fluctuations on the outer side of the metal sheet will not be restricted at all.

這其實就是卡西米爾效應(yīng)。并不是金屬片相互吸引，原因在于，由于兩張金屬片靠得足夠近，內(nèi)側(cè)的波動會受到限制，只有波長更短的波動才可以存在。而金屬片外側(cè)的波動不會受到任何限制。

Therefore, the fluctuations on the outside will be more numerous and more violent than those on the inside, and more virtual photons will be derived from the outside, which will in turn squeeze the metal sheets and bring the two metal sheets closer to each other.

于是外側(cè)的波動就比內(nèi)側(cè)的波動更多更劇烈，外側(cè)衍生出來的虛光子就會更多，進而擠壓金屬片，讓兩張金屬片相互靠近。

So is there any way to turn virtual particles into real particles that we can observe? So that we can get energy out of nothing? In theory, it is indeed possible to turn virtual particles into real particles, but it is not possible to get energy out of nothing.

那么有沒有什么辦法讓虛粒子變成我們能觀測到的真實粒子呢？從而可以無中生有獲取能量？理論上確實能讓虛粒子變成真實的粒子，但并不能憑空獲取能量。

For example, in the Casimir effect mentioned above, we can continuously pull one of the metal sheets back and forth, so that the two metal sheets continue to move closer and then farther away, and repeat this action. In this process, some virtual particle pairs that should have been annihilated may not be able to find their "other half" due to the constant movement of the metal sheet, and thus become real particles.

比如說，上面所說的卡西米爾效應(yīng)，我們可以來來回回地不斷拉扯其中一片金屬片，讓兩張金屬片不斷靠近然后遠離，一直重復(fù)這個動作，在和這個過程中，某些本應(yīng)該湮滅的虛粒子對，可能會因為金屬片的不斷移動找不到自己的“另一半”，從而變成真實的粒子。

This process cannot come out of nothing, because we also put in energy!

這個過程并不能無中生有，因為我們也付出了能量！

Similar to this is the "Hawking radiation" proposed by Hawking. Virtual particle pairs will be derived near the event horizon of a black hole. Normally, they will annihilate instantly, but under certain accidental circumstances, a virtual particle will be swallowed by the black hole, and the other virtual particle will escape into space and become a real particle. This process is manifested as the evaporation of the black hole. The black hole continues to evaporate in this way and eventually disappears!

類似的還有霍金提出的“霍金輻射”，黑洞事件視界附近會衍生出虛粒子對，通常情況下它們會瞬間湮滅，但某個偶然情況，某個虛粒子被黑洞吞噬，另一個虛粒子就會逃逸到太空成為真實的粒子，這個過程表現(xiàn)出來的就是黑洞的蒸發(fā)，黑洞就是通過這種方式不斷蒸發(fā)，最終消失的！