In Expanding de Sitter Space, Quantum Mechanics Gets Even More Elusive

In theory, a universe can come in any shape or size, but scientists prefer to think about three basic kinds of universes: one that’s expanding, one that’s collapsing, and one that stays the same.

理论上，宇宙可以有任何形状或大小，但科学家更喜欢考虑三种基本类型的宇宙：一种正在膨胀，一种正在坍缩，一种保持不变。

Out of these three simplified models, an expanding universe is the hardest for physicists to understand.

在这三个简化模型中，膨胀的宇宙是物理学家最难理解的。

Yet it’s exactly the one our real world most resembles.

但这正是我们现实世界最相似的地方。

When physicists calculate what’s going on with particles at the smallest, quantum levels in a universe that is collapsing or static, they can get their results to make sense.

当物理学家计算在坍缩或静态的宇宙中最小的量子水平上的粒子发生了什么时，他们可以得到有意义的结果。

Unfortunately for physicists, our real universe is not collapsing or static but expanding — being pushed apart by dark energy.

不幸的是，对于物理学家来说，我们真正的宇宙并不是在塌缩或静止，而是在膨胀——被暗能量推开。

When scientists try to make sense of quantum theory in an expanding universe, they are met with one confusing paradox after another.

当科学家试图理解膨胀宇宙中的量子理论时，他们遇到了一个又一个令人困惑的悖论。

In expanding space, physicists cannot square the world we experience with the way things work at the smallest levels.

在不断扩展的空间中，物理学家无法将我们所经历的世界与事物在最小层面上的运作方式联系起来。

Now physicists trying to make sense of how the quantum world works within our expanding universe are hoping to learn from an unexpected source: black holes.

现在，试图理解量子世界在不断膨胀的宇宙中如何运作的物理学家希望从一个意想不到的来源：黑洞中学习。

In 1915, Albert Einstein’s theory of gravity, called general relativity, introduced the idea that space and time are inextricably linked.

1915 年，阿尔伯特·爱因斯坦的引力理论（称为广义相对论）引入了空间和时间密不可分的概念。

Space and time react to the contents of the universe: If the universe is filled with matter, then over time, the attractive pull of that matter’s gravity should cause space to contract.

空间和时间会对宇宙的内容产生反应：如果宇宙充满了物质，那么随着时间的推移，物质引力的吸引力会导致空间收缩。

If the universe is filled with enough dark energy — or what in Einstein’s day was called a cosmological constant — then over time, its push should cause space to expand.

如果宇宙充满了足够的暗能量——或者在爱因斯坦时代被称为宇宙常数——那么随着时间的推移，它的推动力应该会导致空间膨胀。

When Einstein first wrote his general theory of relativity, he believed that our universe must be eternal and unchanging.

当爱因斯坦第一次写出广义相对论时，他相信我们的宇宙一定是永恒不变的。

In geometric terms, he believed that the universe should be infinite and flat, and that the forces that push and pull on space-time should exactly cancel out.

用几何术语来说，他相信宇宙应该是无限的、平坦的，时空的推拉力应该完全抵消。

But a Dutch physicist named Willem de Sitter was more open-minded.

但一位名叫威廉·德西特的荷兰物理学家的思想更为开放。

He realized that it was a natural consequence of relativity for the universe to evolve.

他意识到宇宙演化是相对论的自然结果。

In 1916-17, de Sitter published three papers exploring relativity’s possibilities.

1916-17 年，德西特发表了三篇论文，探讨相对论的可能性。

(In the process, he introduced many English speakers to Einstein’s theories, which, originally written in German, had been siloed off because of severed scientific communications during World War I.)

（在此过程中，他向许多说英语的人介绍了爱因斯坦的理论，该理论最初是用德语写成的，由于第一次世界大战期间科学通讯的中断而被封锁。）

Albert Einstein famously believed in an eternal and unchanging universe.

众所周知，阿尔伯特·爱因斯坦相信宇宙是永恒不变的。

Linda Hall Library of Science, Engineering and Technology

琳达·霍尔科学、工程与技术图书馆

De Sitter found that an empty universe — one devoid of matter, but which still has a cosmological constant — could take on one of just three shapes, depending on the sign of the cosmological constant: It could be flat, as Einstein predicted, positively curved, or negatively curved.

德西特发现，一个空的宇宙——没有物质，但仍然具有宇宙学常数——可以呈现三种形状之一，具体取决于宇宙学常数的符号：它可以是平坦的，正如爱因斯坦所预测的那样，可以是正弯曲的，也可以是负弯曲的。

If the cosmological constant is positive, then space-time is positively curved into what is now called de Sitter space.

如果宇宙学常数为正，则时空正向弯曲到现在所谓的德西特空间。

If the cosmological constant is negative, then space-time is negatively curved into anti-de Sitter space.

如果宇宙常数为负，则时空负弯曲到反德西特空间。

And if the cosmological constant is zero, then space-time is flat.

如果宇宙常数为零，则时空是平坦的。

You can tell the shape of the universe you’re in by observing how two otherwise stationary objects evolve as time ticks forward.

你可以通过观察两个原本静止的物体如何随着时间的推移而演变来了解你所处的宇宙的形状。

Imagine marking two dots on a balloon; if you blow up the balloon, the two dots will move away from each other as the balloon expands.

想象一下在气球上标记两个点；如果你吹气球，当气球膨胀时，两个点会彼此远离。

This is what happens in positively curved space-time.

这就是正弯曲时空中发生的情况。

In negatively curved space-time, stationary particles move toward each other, as if the balloon were deflating.

在负弯曲时空中，静止的粒子相互靠近，就像气球正在泄气一样。

In positively curved de Sitter space, space expands at an exponential rate.

在正弯曲的德西特空间中，空间以指数速率膨胀。

If you are an observer living within de Sitter space, this expansion creates a horizon beyond which it’s impossible to communicate.

如果你是生活在德西特空间中的观察者，这种扩张会创造出一个无法沟通的地平线。

If you try to send a message to someone beyond your horizon, the expanding space will ensure that it never makes it, almost like a current too powerful for a swimmer to overcome.

如果你试图向地平线之外的人发送信息，不断扩大的空间将确保它永远无法成功，就像游泳者无法克服的强大水流一样。

“It’s expanding so fast that there are parts of space-time which, if you wait forever, you will never be able to see,” said Monica Pate , a theoretical physicist at New York University.

纽约大学理论物理学家莫妮卡·佩特说：“它膨胀得如此之快，以至于时空的某些部分，如果你永远等待，你将永远无法看到。”

Anti-de Sitter space, on the other hand, acts like a box.

另一方面，反德西特空间就像一个盒子。

The edge of this box isn’t something you can touch — only light can reach it — but it bounds an anti-de Sitter universe like a picture frame.

这个盒子的边缘不是你可以触摸的东西——只有光可以到达它——但它像相框一样限制了一个反德西特的宇宙。

Everything is drawn back to the box’s center.

一切都被拉回到盒子的中心。

If you broadcast a message in anti-de Sitter space, or throw a rock, it will eventually boomerang back to you.

如果你在反德西特空间中广播一条消息，或者扔一块石头，它最终会飞去来器回到你身边。

“You can think of it as a constant” — not uniform, but persistent — “gravity everywhere,” Pate said.

“你可以把它想象成一个常数”——不是均匀的，而是持久的——“引力无处不在，”佩特说。

We don’t live in exactly any of these idealized universes; we live in a universe with an unbalanced amount of both matter and dark energy.

我们并不生活在这些理想化的宇宙中。我们生活在一个物质和暗能量数量不平衡的宇宙中。

However, our universe likely looked a lot like de Sitter space in the far past, during a period of rapid expansion called inflation.

然而，我们的宇宙很可能看起来很像遥远过去的德西特空间，当时是一个称为暴胀的快速膨胀时期。

After that, it looked flatter for a while, thanks to the presence of matter and light.

之后，由于物质和光的存在，它有一段时间看起来更加平坦。

But as space continues to expand today, and matter becomes sparser, the universe is looking increasingly like de Sitter space again.

但随着今天空间继续膨胀，物质变得越来越稀疏，宇宙看起来又越来越像德西特空间了。

“People think eventually we will live in pure de Sitter space, or something like it, for a very long time,” said Daniel Green , a cosmologist from the University of California, San Diego.

“人们认为最终我们将在纯德西特空间或类似的空间中生活很长一段时间，”加州大学圣地亚哥分校的宇宙学家丹尼尔·格林说。

Unfortunately, de Sitter space causes huge problems for physicists trying to understand the universe at the smallest scales.

不幸的是，德西特空间给试图在最小尺度上理解宇宙的物理学家带来了巨大的问题。

The issue, as is so often the case, lies with the strange rules of quantum mechanics.

正如经常出现的情况一样，问题在于量子力学的奇怪规则。

In quantum mechanics, there’s no such thing as certainty.

在量子力学中，不存在确定性这样的东西。

Because of random quantum fluctuations, even simple questions about where a particle is or how many particles there are in a small area don’t have well-defined answers.

由于随机的量子涨落，即使是关于粒子在哪里或小区域中有多少粒子的简单问题也没有明确的答案。

And yet, we need to probe those smallest bits of space to make sense of how the quantum world relates to our macroscopic experience.

然而，我们需要探索那些最小的空间，以了解量子世界与我们的宏观经验之间的关系。

The more precisely you want to probe, the more energy you need to use to overcome a background of quantum fluctuations.

你想探测得越精确，就越需要使用更多的能量来克服量子涨落的背景。

Essentially, this is why physicists use miles-long particle colliders that accelerate particles to enormous energies.

从本质上讲，这就是物理学家使用数英里长的粒子对撞机将粒子加速到巨大能量的原因。

In curved space, parallel paths don’t stay parallel.

在弯曲空间中，平行路径不会保持平行。

Willem de Sitter realized that something similar happens in a universe with a cosmological constant: A positive constant would cause objects to drift apart over time, and a negative constant would cause objects to come closer together.

威廉·德西特意识到，在具有宇宙学常数的宇宙中也会发生类似的情况：正常数会导致物体随着时间的推移而逐渐远离，负常数会导致物体靠得更近。

But there’s a limit to how much energy you can use for a single measurement.

但单次测量可以使用的能量是有限的。

Put too much energy in a small space, and you’ll create a black hole.

在狭小的空间中投入过多的能量，就会产生黑洞。

To make precise measurements without hitting this limit, physicists need to find another way to reduce the quantum fluctuations.

为了在不达到这个极限的情况下进行精确测量，物理学家需要找到另一种方法来减少量子涨落。

In flat space, physicists can do this by taking measurements from (effectively) infinitely far away — far enough away to shield their measuring devices from the fluctuations.

在平坦的空间中，物理学家可以通过从（有效）无限远的地方进行测量来做到这一点——远到足以保护他们的测量设备免受波动的影响。

In anti-de Sitter space, it’s even easier: Quantum fluctuations go to zero on the boundary of a boxlike universe, so you can make perfect sense of quantum measurements by setting up your experiment on the universe’s edge.

在反德西特空间中，情况甚至更容易：在盒状宇宙的边界上，量子涨落为零，因此您可以通过在宇宙边缘设置实验来获得完美的量子测量意义。

In de Sitter space, there’s a problem.

在德西特空间中，存在一个问题。

As you go farther away from the particles you’re measuring, quantum fluctuations don’t get any smaller.

当你远离正在测量的粒子时，量子涨落不会变得更小。

“Gravity is fluctuating, quantum mechanically, everywhere,” Green said.

“重力在量子力学中无处不在，”格林说。

“And there’s no place to shield yourself from that.” Without an accessible boundary to take a measurement from, it’s as if an experimenter in de Sitter space is always stuck inside their own experiment.

“而且没有地方可以保护自己免受这种影响。”如果没有可访问的边界来进行测量，就好像德西特空间中的实验者总是被困在自己的实验中一样。

“The whole machinery of quantum mechanics is built on the idea that there’s a quantum system, and then some big, giant experimentalist comes along and measures that system,” Green said.

格林说：“量子力学的整个机制建立在这样一个想法上：存在一个量子系统，然后一些巨大的实验家出现并测量该系统。”

In de Sitter space, where there’s no line between the quantum system and the observer, this machinery falls apart.

在德西特空间中，量子系统和观察者之间没有界限，这个机器就会崩溃。

The problems with de Sitter space get worse.

德西特空间的问题变得更加严重。

Much of physicists’ intuition stops being helpful in an expanding universe.

在膨胀的宇宙中，物理学家的大部分直觉不再有帮助。

Energy, for instance, is not conserved.

例如，能量是不守恒的。

“The expansion is literally pumping energy, changing the universe,” said João Penedones of the Swiss Federal Institute of Technology Lausanne.

“膨胀实际上是在泵送能量，改变宇宙，”瑞士洛桑联邦理工学院的若昂·佩内多内斯 (João Penedones) 说。

Even the concept of a particle is different.

甚至粒子的概念也是不同的。

We typically think of a particle as an object that has some location and moves through space.

我们通常将粒子视为具有某个位置并在空间中移动的物体。

“In de Sitter, there is no such thing,” said Manuel Loparco, a former graduate student of Penedones who is now at the University of Turin.

“在德西特，没有这样的事情，”佩内多内斯大学前研究生、现在都灵大学的曼努埃尔·洛帕尔科说。

The constant influx of energy in de Sitter space will eventually cause a particle to spread out or decay.

德西特空间中不断涌入的能量最终将导致粒子扩散或衰变。

Willem de Sitter realized that it was a natural consequence of Einstein’s theory of relativity for the universe to evolve.

威廉·德西特意识到宇宙的演化是爱因斯坦相对论的自然结果。

Photographic Archive/University of Chicago/Creative Commons

摄影档案馆/芝加哥大学/知识共享

In a paper first posted to the scientific preprint site arxiv.org in May 2025, Penedones and Loparco tried asking a simple question: What does a photon, or a particle of light, look like in exponentially expanding space?

在 2025 年 5 月首次发布在科学预印本网站 arxiv.org 上的一篇论文中，Penedones 和 Loparco 试图提出一个简单的问题：在指数膨胀的空间中，光子或光粒子看起来是什么样子？

The answer, which arose from careful mathematics, shocked them.

经过仔细数学计算得出的答案令他们震惊。

Photons, which are massless, could be made out of massive particles in de Sitter space.

无质量的光子可以由德西特空间中的大质量粒子制成。

The finding has strange implications.

这一发现具有奇怪的含义。

For example, if photons don’t have any mass, they should be stable, because particles can only decay into lighter things.

例如，如果光子没有任何质量，它们应该是稳定的，因为粒子只能衰变成更轻的物质。

But massive photons in de Sitter space could spontaneously decay into matter — which could then decay back into light again.

但德西特空间中的大量光子可能会自发衰变成物质，然后又可能再次衰变为光。

“We’re still trying to understand the physical implications of that,” Penedones said.

“我们仍在努力了解其物理意义，”佩内多内斯说。

These are the types of calculations that physicists are working to make sense of in de Sitter space.

这些是物理学家正在努力在德西特空间中理解的计算类型。

Their goal is to sort the hard technical problems from the hard conceptual problems, asking “What can we calculate; what can’t we calculate?” Green said.

他们的目标是将困难的技术问题与困难的概念问题区分开来，询问“我们可以计算什么；我们不能计算什么？”格林说道。

The hope, he said, is that this work will leave scientists “in a much better position to solve some of these other bigger problems, because we won’t be confusing them with the smaller, more tractable problems.”

他说，希望这项工作能让科学家“处于更好的位置来解决其他一些更大的问题，因为我们不会将它们与更小、更容易处理的问题混淆。”

Penedones still finds value in the challenge of trying to understand different versions of the universe — de Sitter, anti-de Sitter, flat — if only to better understand quantum theory.

佩内多内斯仍然认为尝试理解不同版本的宇宙——德西特宇宙、反德西特宇宙、平坦宇宙——的挑战是有价值的，哪怕只是为了更好地理解量子理论。

“De Sitter tells you that your intuition that you develop in flat space is not true in all spaces,” he said.

“德西特告诉你，你在平坦空间中形成的直觉并不适用于所有空间，”他说。

“That’s why it’s useful to do some things in de Sitter: to lose your prejudice.”

“这就是为什么在德西特做一些事情很有用：丢掉你的偏见。”

To try to make sense of the quantum mechanics of de Sitter space, some physicists have turned to black holes.

为了试图理解德西特空间的量子力学，一些物理学家转向了黑洞。

Black holes are ultra-dense objects from which light cannot escape.

黑洞是光无法逃逸的超致密物体。

While you cannot physically probe a black hole, physicists have studied their insides theoretically.

虽然你无法物理地探测黑洞，但物理学家已经从理论上研究了它们的内部。

And over the last few years, they’ve made a lot of progress.

在过去的几年里，他们取得了很大的进步。

Advancements in understanding black holes build on holography, the idea that the two-dimensional surface of the black hole somehow captures everything about the three-dimensional space inside it.

对黑洞理解的进步建立在全息术的基础上，全息术是指黑洞的二维表面以某种方式捕获了其内部三维空间的一切。

Physicists treat the volume of the black hole as illusory, like a hologram.

物理学家将黑洞的体积视为虚幻的，就像全息图一样。

Black holes have been useful settings for studying quantum gravity, since the extreme gravity of a black hole acts strongly even on the quantum scale.

黑洞一直是研究量子引力的有用设置，因为黑洞的极端引力即使在量子尺度上也会产生强烈的作用。

But in recent years, physicists have noted that black holes are also surprisingly similar to de Sitter space.

但近年来，物理学家注意到黑洞与德西特空间也惊人地相似。

Around a black hole, the region where light can no longer overcome the black hole’s gravitational pull forms what’s called a horizon.

在黑洞周围，光无法再克服黑洞引力的区域形成了所谓的视界。

In de Sitter space, a kind of horizon forms around an observer because space is expanding too quickly for light to reach them from beyond a certain distance.

在德西特空间中，观察者周围会形成一种地平线，因为空间膨胀得太快，光线无法从一定距离之外到达观察者。

If our universe continues to expand forever as physicists predict, then it will be as if we are trapped in a black hole; everything beyond our de Sitter horizon will always remain out of reach.

如果我们的宇宙像物理学家预测的那样继续永远膨胀，那么我们就像被困在黑洞里一样；我们德西特地平线之外的一切都永远是遥不可及的。

“We think of black hole cosmology as being sort of a warm-up problem for understanding quantum effects and cosmology,” said Tom Hartman , a physicist at Stanford University.

斯坦福大学物理学家汤姆·哈特曼说：“我们认为黑洞宇宙学是理解量子效应和宇宙学的一个热身问题。”

“So anytime we make progress on black holes, we go back and ask: Can we apply this to de Sitter?”

“因此，每当我们在黑洞方面取得进展时，我们都会回过头来问：我们可以将其应用于德西特吗？”

So far, when Hartman and others have tried to apply their advancements in understanding black holes to de Sitter space, they can’t seem to make sense of the results.

到目前为止，当哈特曼和其他人试图将他们在理解黑洞方面取得的进展应用到德西特空间时，他们似乎无法理解结果。

A black hole has a single horizon, whereas de Sitter space has many, centered on different observers.

黑洞只有一个视界，而德西特空间有多个视界，以不同的观察者为中心。

Without a single boundary to anchor physicists’ calculations, a de Sitter universe seems incapable of holding anything quantum at all.

如果没有单一边界来锚定物理学家的计算，德西特宇宙似乎根本无法容纳任何量子。

“There’s something empty about it,” Hartman said.

“这里面有些空洞的东西，”哈特曼说。

“Like if you try to formulate a quantum theory of de Sitter space, there’s some sense in which it wants to not have any states in it.”

“就像如果你试图建立德西特空间的量子理论一样，从某种意义上来说，它不希望其中有任何状态。”

This is a clear contrast to the world we observe, which is both full of quantum particles and increasingly de Sitter–looking.

这与我们观察到的世界形成鲜明对比，我们观察到的世界充满了量子粒子，而且越来越像德西特。

“The most likely answer is that we’re not interpreting that calculation correctly,” Green said.

“最可能的答案是我们没有正确解释该计算，”格林说。

Still, physicists are hopeful that holography will one day apply more generally to de Sitter space, and that it will answer some of our biggest questions about quantum gravity.

尽管如此，物理学家仍然希望全息术有一天能够更广泛地应用于德西特空间，并且它将回答我们关于量子引力的一些最大问题。

“That’s always been kind of believed, but I think in the last few years, it’s become more convincing,” Hartman said.

“人们一直相信这一点，但我认为在过去几年里，它变得更有说服力，”哈特曼说。

What other surprises de Sitter holds are yet to be seen, but the landscape seems to be ripe for insights, Loparco said.

洛帕科说，德西特还有哪些其他惊喜还有待观察，但洞察力似乎已经成熟。

“It’s really low-hanging fruit that we’re picking.”

“我们正在采摘的果实确实是唾手可得的。”

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获取发送到您的电子邮件收件箱的最重要新闻的摘要

Quanta Magazine moderates comments to facilitate an informed, substantive, civil conversation.

《Quanta》杂志对评论进行审核，以促进知情、实质性、文明的对话。

Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected.

辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。

Moderators are staffed during regular business hours (New York time) and can only accept comments written in English.

版主在正常工作时间（纽约时间）有工作人员，并且只能接受英文评论。