内容来源：https://www.quantamagazine.org/gravity-seems-holographic-what-does-that-mean-for-reality-20260925/

内容总结：

近一个世纪以来，物理学界始终被一个根本性矛盾所困扰：描述宏观引力的广义相对论与支配微观世界的量子力学，在数学框架上难以调和。然而，过去三十年间，一个名为“全息原理”的理论构想逐渐浮出水面，不仅为统一这两种理论提供了可能，更暗示我们所体验的三维空间或许只是一种更深层现实的投影。

这一原理的核心主张令人震惊：任何一个空间区域内的全部物理信息，都可以完全编码在其边界表面上。换言之，你无需窥视盒子内部，仅通过测量其表面上的点，就能推断出内部发生的一切——包括气体分子的运动乃至黑洞的碰撞。这意味着，填充盒子的“体积”与覆盖盒子的“面积”在信息量上是等价的，这直接挑战了我们对几何逻辑的基本认知。清华大学理论物理学家巴泰克·捷克将此比作脑部CT扫描：全息原理暗示，仅通过拍摄大脑表面，就足以重建其内部每一个褶皱、血管和神经元，而无需真正“看进去”。

这一看似荒诞的构想，其根源可追溯至引力本身的独特性质。与电磁力不同，引力荷——即质量——恒为正，不存在负质量。因此，空间弯曲的方式唯一地记录了内部的物质分布，不会像电荷那样因正负抵消而混淆信息。20世纪70年代，贝肯斯坦和霍金关于黑洞熵的计算首次为这一图景提供了关键线索：黑洞的熵与其事件视界的面积成正比，而非体积。斯坦福大学物理学家伦纳德·萨斯坎德由此在90年代进一步提出，黑洞本身就是一幅全息图，其内部的一切均可从外部描述。

但真正让全息原理获得坚实数学基础的，是1990年代末诞生的“AdS/CFT对偶”。该理论设想了一个具有特殊曲率的反德西特空间，其无限延展的时空可被想象为容纳于一个有限“雪花玻璃球”之内。令人震惊的是，数学上证明，这个球内部的引力理论与球表面上的共形场论完全等价。布兰迪斯大学的布莱恩·斯温格指出：“两者非但不对立，实则相互交织，一个从另一个中涌现。”这一对偶关系之出人意料，用哥伦比亚大学物理学家塞巴斯蒂安·米泽拉的话说，其震撼程度堪比“发现国际跳棋的每一步都能转化为合法的篮球动作”。

然而，这一理论是否适用于我们真实的宇宙，物理学家们意见分歧。批评者强调，我们的宇宙正在加速膨胀，属于德西特空间，其曲率方向与反德西特空间相反，并不存在一个可以将全息图投影其上的边界表面。但全息原理的支持者则从“蚂蚁视角”出发：身处雪花玻璃球深处的蚂蚁难以察觉整体曲率，因此反德西特空间与德西特空间对局域观察者而言或许并无本质区别。鉴于黑洞证据的普遍性以及AdS/CFT对反德西特空间的数学保证，我们宇宙遵循同样全息规律的可能性不容忽视。

若我们确实生活在一幅全息图中，这意味着什么？物理学家对此态度审慎。萨斯坎德坦言：“我不试图回答那个问题，那超出了我的职责范围。”这种克制源于物理学的一贯目标：并非解释“何为真实”，而是寻找能以数学关系做出可靠预测的简洁概念。不过，宾夕法尼亚大学的维杰·巴拉苏布拉马尼安仍勾勒出三种可能：其一，量子表面是真实的，我们体验的空间如同水面看似连续实则由分子构成，正如游戏角色误以为像素构建的世界是实在的；其二，引力体积是真实的，全息描述只是一种数学巧合；其三，两者皆非终极实在，背后存在一个更深刻、尚未可知的理论。

目前，多数全息理论研究者倾向于第一种可能，即“量子比特造就万物”的图景：空间如同屏幕像素，距离与影响的关系被颠倒——并非空间分隔导致物体互不影响，而是物体间缺乏关联才显得空间上遥远。但萨斯坎德等人近年对德西特空间的全息模型探索表明，终极理论可能更为激进：所有维度或许都蜷缩于一个没有大小的量子点中，时间本身亦从中涌现。

尽管尚无实验证据，这一构想因其恰到好处的激进与简洁，正吸引着越来越多理论物理学家的目光。正如玻尔在物理学革命前夕所言，这种理论或许“激进得刚刚好，也简单得刚刚好”——足以触及真理的轮廓。

中文翻译：

引力似乎是全息的。这对现实意味着什么？

引言

大约十年前，我刚成为一名物理学记者，在最初的几个月里，我不断碰到一串令人费解的字符：AdS/CFT。我被彻底吓住了，决定干脆无视它。

但我没法一直把头埋在沙子里。很快我就了解到，这串字符是引力和量子力学这两个看似水火不容的领域之间一个惊人联系的缩写。更离奇的是，这种“反德西特/共形场论”对偶关系暗示，引力消除了体积与面积之间的区别。这个更宏大的思想被称为全息原理，而它现在在我看来，是过去30年里理论物理学中最深刻的构想。

理论物理学家倾向于用脚投票，而AdS/CFT引发了一场蜂拥而至的追随潮。20世纪90年代末关于这一主题的三篇奠基性论文已被引用数万次，使它们成为数字时代迄今为止被引用最多的理论物理学著作。在我采访研究全息的物理学家时，他们常常显得由衷地震惊，并用“神奇”和“不可思议”这样的词来形容它。而且，全息还带来了一个被广泛接受的答案，解决了物理学中最著名的谜题：与斯蒂芬·霍金的论点相反，黑洞并非无法逃脱的监狱。

但即便报道了全息领域的众多进展，并与相关物理学家进行了无数次对话，我仍然感到困惑。我听说过，全息暗示引力和量子力学是同一回事，空间可能是一种幻觉。我也听到有人把全息既描述为数学事实，又描述为思辨性的异想天开。于是我试图对这些天马行空的想法进行三角定位，弄清楚全息原理究竟对我们的宇宙意味着什么。

证据

在你所选的任意空间区域——实际上是时空区域，但为了便于想象，我在本文中将全程略去时间维度，物理学家也常这么做——外面套一个盒子。全息原理断言，无论盒子内部发生了什么——从气体分子四处碰撞到黑洞互相撞击——你只需反复测量表面上的点，就能破译盒子里的全部内容。

暂停片刻，想想这个断言有多么令人瞠目。你根本无法看到盒子内部。然而全息说，你可以在完全不接触内部的情况下，准确了解盒子中每一处正在发生的事。仅观测表面就足够了。在这个意义上，装满一个盒子所需的东西的量，等同于覆盖它所需的油漆的量。这违反了逻辑和几何学。它要求我们抹去平方米与立方米之间的类别差异。它让人想起全息图像尽管是平面的却显得有深度，只不过全息图里的鸟和真实的鸟是同一回事。

中国清华大学的理论物理学家巴特克·捷克通过将其与大脑CT扫描作比较，凸显了这一原理的力量。CT扫描利用X射线窥视器官内部，并从数百到数千张截面图像中重建它。全息暗示你可以做到这一点——在三维中重建每一道褶皱、每一根血管和每一个神经元——而无需真正看到内部。仅仅拍摄大脑表面不知怎的就足够了。

为什么会有人认真对待如此牵强的概念？它植根于思想实验和数学，似乎可以追溯到一个力：“引力的奇迹，”捷克说。

一个多世纪以来，科学家们就知道引力不同于其他力。想象一个充满电荷的盒子，电荷代表电磁力——另一种基本力。盒子里的东西由电荷和它们产生的电场组成，电场同样穿过外表面。如果你试图仅凭观察表面来推断什么样的电荷排列产生了这个场，你会遇到麻烦。因为正电荷会中和负电荷，不同的排列可以看起来一样。如果你没有观测到场，那可能意味着内部没有电荷——也可能意味着正电荷的效应恰好完美抵消了负电荷的效应。从表面上，你分辨不出区别。

对于引力，质量扮演电荷的角色。它使周围的时空弯曲，而且它总是正的。不存在负质量，所以你总是可以从盒子表面的时空弯曲推断出内部唯一真实的物质排列。“直觉上，这就是全息之所以合理的原因，”加拿大滑铁卢 perimeter 理论物理研究所研究量子引力的物理学家洛朗·弗雷德尔说。

但全息真正开始发挥作用，是在你把量子力学错综复杂的细节纳入考量之后。最早的线索出现在20世纪70年代，当时雅各布·贝肯斯坦和斯蒂芬·霍金计算了黑洞的熵——熵通常是衡量一个物体内部能容纳多少东西的指标。他们用量子理论预测黑洞在吞噬粒子时会如何增长。令人困惑的是，当他们设想向黑洞添加粒子时，发现熵的增长与表面积同步——而非如你所预期的那样与体积同步。

斯坦福大学的物理学家伦纳德·萨斯坎德在20世纪90年代基于他们的结果提出，黑洞就是一个字面意义上的全息图，内部发生的一切都可以从外部观察到。在某种意义上，内部是多余的。“我觉得这有点疯狂，”萨斯坎德说，“但我认为在所有可能性中，这是最不疯狂的一个。”（诺贝尔奖得主杰拉德·特霍夫特和佛罗里达大学盖恩斯维尔分校的物理学家查尔斯·索恩大约在同一时间得出了类似结论。）

我一直觉得这个黑洞熵的论证很有说服力，因为只要你往任何一块空间里放入足够多的质量，它就能变成黑洞。尽管黑洞以怪异著称，但它们是空间的代表性样本。它们只是有一种把空间更奇特的性质推到前台的本事。所以如果黑洞是全息的，而任何空间区域都能变成黑洞，那么按照这个论证，连你正坐着的房间也应该是全息的。“这完全是普遍适用的，”萨斯坎德说。

琳达·A·西塞罗/斯坦福新闻社

这个论证有着坚如磐石的普遍性，但我也听到物理学家将全息描述为一个思辨性的想法，与现实的联系并不确定。于是我打电话给爱丁堡大学希格斯理论物理中心的物理学家莱瑟姆·博伊尔，希望听到一种不同的观点。他没有让我失望。

博伊尔不否认贝肯斯坦和霍金关于黑洞的发现，但他确实质疑全息的解读。他怀疑，在一个空间区域周围放置一个表面的行为——就像黑洞形成时发生的那样——会产生两种不同的熵。一种熵告诉你内部能容纳多少粒子——那确实取决于体积。表面的存在给了你第二种“纠缠”熵。内部的粒子与外部的粒子共享一种量子联系，即纠缠；表面越大，穿过它的纠缠就越多。纠缠熵取决于面积，而非体积。博伊尔认为，它们不是同一回事。

“这似乎是对所发生之事的一种不那么神秘、更脚踏实地的解读，”他说。

但全息得益于其背后有第二个在概念上更无懈可击的发现：AdS/CFT。

AdS/CFT要求我们想象一个与我们自身不同的宇宙，它的弯曲方式使得其无限广阔的空间可以被想象为装在一个有限的雪花玻璃球里。这听起来可能要求很高，但数学家——以及像M.C.埃舍尔这样有数学头脑的艺术家——对此完全游刃有余。这种几何结构被称为反德西特（AdS）空间。

除了其奇特的曲率之外，反德西特雪花玻璃球的内部与我们的宇宙非常相似，充满了电子和原子。更重要的是，它也会对物质产生涟漪般的响应，从而提供引力的效果。与此同时，雪花玻璃球的表面是一个自成一体宇宙。它同样布满了量子粒子，但它是刚性的，所以无法对粒子做出反应：没有引力。这个表面世界完全由一种被称为共形场论（CFT）的量子理论所支配，在这种理论中，物理规律不会随着你放大或缩小而改变。

20世纪90年代末那三篇轰动性的论文表明，在数学上，这两个理论世界（AdS内部和CFT表面）是相同的。这就是AdS/CFT对偶。与黑洞熵的论证一样，体积和表面是等价的。但与黑洞论证不同的是，AdS/CFT本质上是关于引力和量子力学的一个数学事实，没有替代解读。即便是怀疑论者也觉得这确实令人惊讶。“我不知道有任何平庸的方式能解释它，”博伊尔说。

AdS/CFT不可否认的信息是，至少在这个特殊的雪花玻璃球中，引力的规律和量子力学的规律在暗中描述着同一场游戏——尽管这两个理论之间有着众所周知的对抗。“它们远非对立，实际上是交织在一起的，”布兰迪斯大学的物理学家布莱恩·斯温格尔说。“一个从另一个中涌现。”

这一对偶关系令人震惊。我觉得它好比发现了一种方法，能把任何跳棋走法转换成合法的国际象棋走法：这到底为什么会行得通？当我把这个比喻说给哥伦比亚大学研究量子理论数学结构的物理学家塞巴斯蒂安·米泽拉听时，他告诉我这还不够戏剧化。“这是个好比喻，”他说，只不过“它更像是跳棋和篮球。”

但雪花玻璃球的全息性质能告诉我们关于我们现实的任何东西吗？在这一点上，物理学家们意见不一。怀疑论者强调，我们的宇宙与雪花玻璃球恰恰相反。宇宙的加速膨胀意味着我们生活在一个向外弯曲的空间中，方向相反——一个德西特空间。因为我们的空间不会弯回自身，它没有可以将全息图投影其上的边界表面。所以没有什么理由认为AdS/CFT与现实世界有任何关系。

然而，最专注的全息研究者采取一种地面视角。一只生活在雪花玻璃球深处的蚂蚁无法轻易探测到任何曲率，因此无法区分反德西特空间和德西特空间。所以他们论证说，或许对一个空间成立的东西，或多或少也应该对另一个空间成立。（如果你好奇的话，我们就是那些蚂蚁。）

虽然两种论点都有道理，但我倾向于全息研究者。黑洞提供了有趣但间接的证据，表明所有类型的空间都是全息的。而AdS/CFT对偶基本上保证了反德西特空间——恰好是物理学家理解得最透彻的空间——是全息的。我们的宇宙以一种完全不同的方式运作，这种概率有多大？完全有可能，但我不会押注于此。我认真对待这样一种可能性：引力使每一种空间，包括我们的空间，都是全息的。

那么，生活在一个全息图中对我们意味着什么？

意义

我发现大多数物理学家不愿推测空间的全息性质与“本体论”——关于何为真实的大写的真理——之间的联系。

“我不试图回答那个问题，”萨斯坎德说。“那超出了我的能力范围。”

我认为这是一种审慎的回应，忠实于物理学的终极目标——这个目标并非如我常忍不住以为的那样，是解释何为真实。相反，物理学家寻求找出少数几个用数学关系表达的简单概念，它们能在许多不同情境中做出可靠预测。引力是一个强大的概念，因为它适用于坠落的苹果、涨落的潮汐和绕行的行星。全息是这一传统的又一步，是面积与体积之间的一种等价关系，至少对某些空间成立。

“物理学家构建模型，”捷克说。而全息模型竟然能够被构建出来，这本身就令人兴奋。

但我渴望一些更直觉、不那么审慎的东西。我想知道，如果我们生活在反德西特空间中（我们并不），或者如果物理学家发展出了德西特空间的全息理论（他们还没有），全息对我们意味着什么。当我以这种方式提出问题时，宾夕法尼亚大学研究全息的物理学家维杰·巴拉苏布拉马尼安爽快地列出了几种可能。

如果我们的宇宙最终只有一种本质（而非巴拉苏布拉马尼安所说的可能存在多种等价本质），那么有三种选项：量子表面是真实的，引力体积是真实的，或者别的东西是真实的。

第一种解读——表面是真实的——在花时间研究AdS/CFT的物理学家中最为流行。他们怀疑我们体验到的空间像水一样虚幻。如果你足够仔细地观察那光滑透明的液体，它会分解为四处弹跳的分子——“真实的东西”。类似地，如果你足够仔细地观察我们的宇宙，你会发现它比表面看起来更空。在这种情境中，我们将类似于电子游戏中的角色。我们周围3D游戏世界中看似庞大的建筑和树木，实际上只是平面屏幕上闪烁的像素。

“我们被愚弄了，以为宇宙中的东西比实际存在的更多，”弗吉尼亚理工大学的理论物理学家曹查尔斯说。你可以“把整个三维世界压缩到二维中”。

这种视角在一个名为“it from qubit”（万物源于量子比特）的研究纲领中比比皆是，该纲领假定我们周围的空间（“it”）由量子信息单元（量子比特）组成。这些量子比特将构成我们现实的真正织物，就像屏幕像素构成电子游戏角色的物理现实一样。

这种解读的深刻含义在于，它颠覆了距离与影响之间的正常关系，东北大学研究全息的宁鲍说。我们通常认为两个事物之所以不相互影响，是因为空间将它们隔开：外星恒星的耀斑很遥远，这就是为什么它们不会让地球停电。但it from qubit暗示我们把因果关系搞反了。或许空间之所以似乎将两个事物隔开，恰恰是因为它们不相互影响。想象一个电子游戏中的太阳从电子游戏中的树后面经过。太阳的像素直接接触树的像素，然而树并没有燃烧起来。这是因为太阳的像素独立于树的像素。它们的独立性使太阳离树“很远”。

然而，这种对偶是双向的。全息原理把世界的两幅图景放在同等地位上。那么为什么引力体积不能是真实的东西呢？全息研究者回避这种解读，因为他们对空间——即便是反德西特空间——还没有完整的量子把握。但这只是我们的无知，巴拉苏布拉马尼安说。某种关于空间和物质的直接量子理论，比如弦论，必定存在，而那可能就是基本的描述。

如果是这样的话，3D游戏世界就是真实的，它只是看起来像是由2D像素构成的。全息将是一个数学巧合。在这种情境中，“现实是你拥有所有[三]个维度。只是恰好它们有某种[全息的]描述，”巴拉苏布拉马尼安说。

然后是第三扇门，核选项：内部体积和表面积都不是真实的。引力和量子力学都是一个更锐利、更真实、完全未知的理论的粗糙草稿。冒着过度延伸电子游戏类比的风险，你可以论证电子游戏世界和屏幕像素都不是“真实的”，两者都只是电子在游戏机和电视中物理流动的复杂方式的反映。在这种情况下，全息告诉你屏幕像素如何与游戏世界中的物体相关联，但它与电子的本质无关。“真正的理论是别的东西，”巴拉苏布拉马尼安说。

在这一点上，我赞同it from qubit解读的一个更极端的变体——主要是跟随那场蜂拥的潮流。我押注量子比特是真实的东西，但它们并不存在于像平面屏幕这样熟悉的东西上。

物理学家们几十年来一直试图将AdS/CFT对偶扩展到适配德西特空间——它没有明显的屏幕——但收效有限。近年来，他们开始变得更加有创造力。萨斯坎德和其他团队在全息德西特模型上取得了进展，这些模型与AdS/CFT截然不同。这些宇宙不是把一个体积压缩成一个面积，而是似乎把所有维度塞进一个单独的量子点。我想象一堆量子像素全部共存于一个点上，而不是散布在一个屏幕上。这可能很难可视化，但我们已经接受了丢弃一个空间维度的想法。把其余的也丢掉又有什么不同呢？

巴拉苏布拉马尼安怀疑，即便是这种激进的模型也走得不够远。爱因斯坦的理论将空间与时间融合在一起，所以如果空间的三个维度从一个无空间的点涌现出来，那么时间也应该从某种无时间的东西中涌现。不知怎的，我们以及我们所体验的一切，存在于一个没有大小、不眨眼的点之中。物理学家距离构建这种形式的功能性理论还差得很远，更不用说找到我们宇宙如此运作的硬证据了。但套用尼尔斯·玻尔的话——在一个物理学家们正在寻找下一个大突破的更早时代——这种理论在我看来恰好足够激进——也恰好足够简单——以致可能是对的。

英文来源：

Gravity Seems Holographic. What Does That Mean for Reality?
Introduction
In my first months as a physics journalist nearly a decade ago, I kept running into an inscrutable string of characters: AdS/CFT. Thoroughly intimidated, I decided to just ignore it.
But I couldn’t keep my head in the sand for long. I soon learned that those characters are shorthand for a surprising connection between the seemingly inharmonious worlds of gravity and quantum mechanics. And even more bizarrely, this “anti-de Sitter/conformal field theory” correspondence suggests that gravity eliminates the distinction between volume and area. This broader idea is known as the holographic principle, and it now strikes me as the most profound proposal in theoretical physics in the last 30 years.
Theoretical physicists tend to vote with their feet, and AdS/CFT sparked a stampede. The three foundational papers on the topic in the late 1990s have garnered tens of thousands of citations, making them by far the most highly cited theoretical physics works of the digital era. In my interviews with physicists who study holography, they often seem genuinely stunned, and reach for words like “magical” and “miraculous” to describe it. And it doesn’t hurt that holography led to a widely accepted answer to the most famous puzzle in physics: Contrary to what Stephen Hawking argued, black holes are not inescapable prisons.
But even after covering numerous developments in holography and having countless conversations with the physicists involved, I still felt confused. I had heard that holography suggested that gravity and quantum mechanics are one and the same, and that space might be an illusion. I had also heard holography described both as a mathematical fact and as a speculative flight of fancy. So I tried to triangulate these wild ideas and figure out what, exactly, the holographic principle implies about our universe.
The Evidence
Put a box around any region of space (space-time, really, but I’m going to drop time throughout this essay for ease of visualization, as physicists often do). The holographic principle asserts that no matter what’s going on inside — from gas molecules pinging around to black holes colliding — you can decipher the entire contents of the box just by repeatedly measuring points on the surface.
Pause for a moment to reflect on how outrageous this assertion is. You can’t see into the box at all. Nevertheless, holography says that you can learn exactly what’s happening everywhere in the box without any access to the interior. Observing the surface alone is enough. In this sense, the amount of stuff that fills a box is the same as the amount of paint that covers it. That’s a violation of logic and geometry. It asks us to erase the categorical difference between square meters and cubic meters. It recalls how holographic images appear to have depth despite being flat, except the bird in the hologram is the same as an actual bird.
Bartek Czech, a theorist at Tsinghua University in China, highlights the power of the principle by comparing it to a CT scan of the brain, which uses X-rays to look inside the organ and reconstruct it from hundreds to thousands of cross-sectional images. Holography implies that you can do that — reconstruct every fold, vessel, and neuron in three dimensions — without actually looking inside. Simply photographing the surface of the brain somehow suffices.
Why would anyone entertain such a far-fetched notion? It’s rooted in thought experiments and math, and it appears to trace back to one force: “a miracle of gravity,” Czech said.
Scientists have known for more than a century that gravity is different from the other forces. Imagine a box filled with electric charges, representing one of the other fundamental forces, electromagnetism. The stuff in the box consists of the charges and the electric field they create, which also passes through the outer surface. You will have a problem if you try to infer what arrangement of charges generates the field by looking at the surface alone. Because positive charges neutralize negative charges, different arrangements can look the same. If you observe no field, it could mean there’s no charge inside — or it could mean that the effects of the positive charges are perfectly blocking the effects of the negative charges. From the surface, you can’t tell the difference.
With gravity, mass plays the role of charge. It bends space-time around it, and it is always positive. There is no negative mass, so you can always infer the one real arrangement of stuff inside from the warping of space-time at the surface of your box. “Intuitively, this is why holography is plausible,” said Laurent Freidel, a physicist studying quantum gravity at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
But holography really starts to bite only after you take the intricate details of quantum mechanics into account. The first clues came in the 1970s, when Jacob Bekenstein and Stephen Hawking calculated the entropy of black holes — typically a measure of how much stuff fits inside an object. They used quantum theory to predict how a black hole would grow as it swallowed particles. Perplexingly, as they imagined adding particles to the black hole, they found that the entropy grew in lock step with the surface area — not the volume, as you would expect.
Leonard Susskind, a physicist at Stanford University, built on their result in the 1990s and proposed that the black hole was literally a hologram, that everything happening inside can be observed from the outside. In some sense, the interior was superfluous. “I thought it was a little bit crazy,” Susskind said, “but I thought it was the least crazy of all the possibilities.” (Gerard ’t Hooft, a Nobel laureate, and Charles Thorn, a physicist at the University of Florida in Gainesville, came to similar conclusions around the same time.)
I’ve always found this black hole entropy argument compelling, because any patch of space can become a black hole if you put enough mass into it. Despite their reputation for weirdness, black holes are representative examples of space. They just have a way of bringing space’s stranger properties to the fore. So if a black hole is holographic, and any region of space can become a black hole, then, the argument goes, even the room you’re sitting in should be holographic. “It’s completely general,” Susskind said.
Linda A Cicero/Stanford News Service
This argument has a rock-solid universality, but I’ve also heard physicists describe holography as a speculative idea with an uncertain connection to reality. So I called up Latham Boyle, a physicist at the Higgs Center for Theoretical Physics at the University of Edinburgh, hoping for an alternative view. He did not disappoint.
Boyle doesn’t dispute Bekenstein and Hawking’s black hole findings, but he does question the holographic interpretation. He suspects that the act of putting a surface around a region of space — as happens when a black hole forms — creates two distinct types of entropy. One entropy tells you how many particles can fit inside — and that really does depend on the volume. The existence of the surface gives you a second, “entanglement” entropy. Particles inside share a quantum connection, known as entanglement, with those outside; the bigger the surface, the more entanglement crosses it. The entanglement entropy depends on the area, not the volume. They’re not, Boyle posits, the same thing.
“That seems like a less mystical, more down-to-earth interpretation of what’s going on,” he said.
But it helps holography that there is a second, more conceptually airtight finding behind it: AdS/CFT.
AdS/CFT asks us to imagine a universe that is not like our own, one that curves in such a way that its infinite expanse of space can be pictured as fitting inside a finite snow globe. That might sound like a big ask, but it’s one that mathematicians — and mathematically minded artists such as M.C. Escher — are perfectly comfortable with. This geometry is known as anti-de Sitter (AdS) space.
Other than its peculiar curvature, the interior of the anti-de Sitter snow globe is a lot like our universe, filled with electrons and atoms. More importantly, it also ripples in response to that matter, providing the effect of gravity. The snow globe’s surface, meanwhile, is a universe of its own. It’s also populated with quantum particles, but it’s rigid, so it can’t react to the particles: no gravity. This surface world is ruled exclusively by a type of quantum theory known as a conformal field theory (CFT), where the rules of physics don’t change as you zoom in or out.
The blockbuster trilogy of papers in the late 1990s showed that, mathematically, these two theoretical worlds (the AdS interior and the CFT surface) are the same. This is the AdS/CFT correspondence. As with the black hole entropy argument, the volume and surface are equivalent. But unlike the black hole argument, AdS/CFT is essentially a mathematical fact about gravity and quantum mechanics with no alternative interpretation. Even skeptics find this genuinely surprising. “I don’t know of any mundane way to explain it,” Boyle said.
The undeniable message of AdS/CFT is that, at least in this special snow globe, the rules of gravity and the rules of quantum mechanics are secretly describing the same game — despite the storied antagonism between the two theories. “Far from being opposed, they’re actually intertwined,” said Brian Swingle, a physicist at Brandeis University. “One emerges from the other.”
The correspondence came as a shock. I think of it as akin to discovering a way of converting any checkers move into a valid chess move: Why on Earth would that work? When I ran that picture by Sebastian Mizera, a physicist at Columbia University who studies the mathematical structure of quantum theories, he told me it wasn’t dramatic enough. “That’s a good analogy,” he said, except “it’s more like checkers and basketball.”
But does the holographic nature of the snow globe tell us anything about our reality? On this point, physicists disagree. Skeptics emphasize that our universe is the opposite of a snow globe. The accelerating expansion of the cosmos implies that we live in a space that curves outward, in the opposite direction — a de Sitter space. Because our space does not curve back in on itself, it has no boundary surface where you can project the hologram. So there’s little reason to think that AdS/CFT has anything to do with the real world.
The most dedicated holographers, however, take a ground-level perspective. An ant living deep inside the snow globe can’t easily detect any curvature, and therefore can’t tell the difference between anti-de Sitter and de Sitter space. So perhaps what’s true of one space, they argue, should more or less hold for the other. (And in case you were wondering, we’re the ants.)
While both arguments have merit, I lean toward the holographers. Black holes provide intriguing but circumstantial evidence that all types of space are holographic. And the AdS/CFT correspondence essentially guarantees that anti-de Sitter space — which happens to be the space physicists understand best — is holographic. What are the odds that our universe works in a totally different way? It absolutely could, but I wouldn’t bet on it. I take seriously the possibility that gravity makes every kind of space, including ours, holographic.
And so what would it mean for us to live in a hologram?
The Meaning(s)
I found that most physicists are hesitant to speculate about the connection between the holographic nature of space and “ontology” — the capital-T truth about what’s real.
“I don’t try to answer that question,” Susskind said. “That’s beyond my pay grade.”
This strikes me as a prudent response, one that stays true to the ultimate goal of physics, which is not, as I am often tempted to think, to explain what is real. Rather, physicists seek to identify a few simple concepts, expressed in mathematical relationships, that make reliable predictions in many different situations. Gravity is a powerful concept because it holds for falling apples, sloshing tides, and orbiting planets. Holography is another step in that tradition, an equivalence between area and volume that holds at least for certain spaces.
“Physicists build models,” Czech said. And it’s exciting that holographic models are even possible to build.
But I craved something more intuitive, less prudent. I wanted to know what holography would mean for us if we lived in anti-de Sitter space (which we don’t), or if physicists developed a holographic theory of de Sitter space (which they haven’t). When I framed the question in that way, Vijay Balasubramanian, a physicist who studies holography at the University of Pennsylvania, gamely laid out a short menu of possibilities.
If our universe ultimately has just one nature (as opposed to multiple equivalent natures, which Balasubramanian said is possible), then there are three options: The quantum surface is the real thing, the gravitational volume is the real thing, or something else is the real thing.
The first interpretation — the surface is real — is the most popular among physicists who spend their time studying AdS/CFT. They suspect that the space we experience is as illusory as water. If you look closely enough at the smooth, clear liquid, it resolves into ricocheting molecules — the “real thing.” Similarly, if you were to look at our universe closely enough, you’d find that it’s emptier than it seems. In this scenario, we would resemble characters in a video game. The apparently bulky buildings and trees of the 3D game world around us would actually be pixels flickering on a flat screen.
“We are fooled into thinking that there is more stuff in the universe than there actually is,” said Charles Cao, a theorist at Virginia Tech. You can “compress all of the three-dimensional world into two dimensions.”
This perspective abounds in the research program called “it from qubit,” which posits that the space around us (“it”) is made up of quantum units of information (qubits). These qubits would make up the true fabric of our reality in the same way that screen pixels make up the physical reality of the video game characters.
The profound implication of this interpretation is that it flips the normal relationship between distance and influence, said Ning Bao, who studies holography at Northeastern University. We typically imagine that two things don’t influence each other because space separates them: Flares from alien stars are far away, and that’s why they don’t knock out power on Earth. But it from qubit suggests we have it backward. Perhaps space seems to separate two things precisely because they don’t influence each other. Consider a video game sun passing behind a video game tree. The sun pixels touch the tree pixels directly, yet the tree does not burst into flames. This is because the sun pixels are independent of the tree pixels. Their independence is what makes the sun “far” from the tree.
The correspondence goes both ways, however. The holographic principle puts the two pictures of the world on equal footing. So why can’t the gravitational volume be the real thing? Holographers shy away from this interpretation because they don’t have a full quantum handle on space — even anti-de Sitter space. But that’s just our ignorance, Balasubramanian said. Some direct quantum theory of space and matter, such as string theory, must exist, and that could be the fundamental description.
If that were the case, the 3D video game world would be the real one, and it would merely seem as if it were made of 2D pixels. Holography would be a mathematical coincidence. In this scenario, “the reality is you’ve got all [three] of these dimensions. It just so happens that they have some [holographic] description,” Balasubramanian said.
And then there’s door number three, the nuclear option: Neither the interior volume nor the surface area is real. Both gravity and quantum mechanics are rough drafts of a sharper, truer, completely unknown theory. At the risk of stretching the video game analogy, you could argue that neither the video game world nor the screen pixels are “real,” and that both are just reflections of the complicated ways that electrons physically flow through the game console and television. In this case, holography tells you how the pixels of the screen relate to the objects of the game world, but it has nothing to do with the nature of the electrons. “The actual theory is something else,” Balasubramanian said.
At this point, I subscribe to a more extreme variation of the it from qubit interpretation — mostly just following the rumble of the stampede. I’d bet that the qubits are the real things, but that they don’t live on anything as familiar as a flat screen.
Physicists have tried to stretch the AdS/CFT correspondence to fit de Sitter space — which has no obvious screen — for decades, with limited success. In recent years, they’ve started to get more creative. Susskind and other teams have made progress on holographic de Sitter models that differ radically from AdS/CFT. Instead of squashing a volume into an area, these universes seem to cram all the dimensions into a lone quantum point. I imagine a bunch of quantum pixels all coexisting in one spot, rather than spreading across a screen. That might be hard to visualize, but we’re already accepting the idea of dropping one dimension of space. Why should tossing the others be so different?
Balasubramanian suspects that even this kind of radical model doesn’t go far enough. Einstein’s theory fused space with time, and so if the three dimensions of space emerge from a spaceless point, then time should emerge from something timeless. Somehow, we and everything we experience exist within an unblinking dot of no size. Physicists are nowhere close to constructing a functional theory of this form, much less finding hard evidence that our universe works this way. But to paraphrase Niels Bohr, during an earlier era when physicists were seeking the next big thing, this sort of theory strikes me as just radical enough — and just simple enough — to be right.