原文来自:The Black Hole Information Paradox Comes to an End | Quanta Magazine

The Most Famous Paradox in Physics Nears Its End

In a landmark series of calculations, physicists have proved that black holes can shed information, which seems impossible by definition. The work appears to resolve a paradox that Stephen Hawking first described five decades ago.

In a series of breakthrough papers, theoretical physicists have come tantalizingly close to resolving the black hole information paradox that has entranced and bedeviled them for nearly 50 years. Information, they now say with confidence, does escape a black hole. If you jump into one, you will not be gone for good. Particle by particle, the information needed to reconstitute your body will reemerge. Most physicists have long assumed it would; that was the upshot of string theory, their leading candidate for a unified theory of nature. But the new calculations, though inspired by string theory, stand on their own, with nary a string in sight. Information gets out through the workings of gravity itself — just ordinary gravity with a single layer of quantum effects.

This is a peculiar role reversal for gravity. According to Einstein’s general theory of relativity, the gravity of a black hole is so intense that nothing can escape it. The more sophisticated understanding of black holes developed by Stephen Hawking and his colleagues in the 1970s did not question this principle. Hawking and others sought to describe matter in and around black holes using quantum theory, but they continued to describe gravity using Einstein’s classical theory — a hybrid approach that physicists call “semiclassical.” Although the approach predicted new effects at the perimeter of the hole, the interior remained strictly sealed off. Physicists figured that Hawking had nailed the semiclassical calculation. Any further progress would have to treat gravity, too, as quantum.

That is what the authors of the new studies dispute. They have found additional semiclassical effects — new gravitational configurations that Einstein’s theory permits, but that Hawking did not include. Muted at first, these effects come to dominate when the black hole gets to be extremely old. The hole transforms from a hermit kingdom to a vigorously open system. Not only does information spill out, anything new that falls in is regurgitated almost immediately. The revised semiclassical theory has yet to explain how exactly the information gets out, but such has been the pace of discovery in the past two years that theorists already have hints of the escape mechanism.

“That is the most exciting thing that has happened in this subject, I think, since Hawking,” said one of the co-authors, Donald Marolf of the University of California, Santa Barbara.

“It’s a landmark calculation,” said Eva Silverstein of Stanford University, a leading theoretical physicist who was not directly involved.

You might expect the authors to celebrate, but they say they also feel let down. Had the calculation involved deep features of quantum gravity rather than a light dusting, it might have been even harder to pull off, but once that was accomplished, it would have illuminated those depths. So they worry they may have solved this one problem without achieving the broader closure they sought. “The hope was, if we could answer this question — if we could see the information coming out — in order to do that we would have had to learn about the microscopic theory,” said Geoff Penington of the University of California, Berkeley, alluding to a fully quantum theory of gravity.

What it all means is being intensely debated in Zoom calls and webinars. The work is highly mathematical and has a Rube Goldberg quality to it, stringing together one calculational trick after another in a way that is hard to interpret. Wormholes, the holographic principle, emergent space-time, quantum entanglement, quantum computers: Nearly every concept in fundamental physics these days makes an appearance, making the subject both captivating and confounding.

And not everyone is convinced. Some still think that Hawking got it right and that string theory or other novel physics has to come into play if information is to escape. “I’m very resistant to people who come in and say, ‘I’ve got a solution in just quantum mechanics and gravity,’” said Nick Warner of the University of Southern California. “Because it’s taken us around in circles before.”

But almost everyone appears to agree on one thing. In some way or other, space-time itself seems to fall apart at a black hole, implying that space-time is not the root level of reality, but an emergent structure from something deeper. Although Einstein conceived of gravity as the geometry of space-time, his theory also entails the dissolution of space-time, which is ultimately why information can escape its gravitational prison.

The Curve Becomes the Key

In 1992, Don Page and his family spent their Christmas vacation house-sitting in Pasadena, enjoying the swimming pool and watching the Rose Parade. Page, a physicist at the University of Alberta in Canada, also used the break to think about how paradoxical black holes really are. His first studies of black holes, when he was a graduate student in the ’70s, were key to his adviser Stephen Hawking’s realization that black holes emit radiation — the result of random quantum processes at the edge of the hole. Put simply, a black hole rots from the outside in.

 

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本文转载自:

Does Time Really Flow? New Clues Come From a Century-Old Approach to Math.

The laws of physics imply that the passage of time is an illusion. To avoid this conclusion, we might have to rethink the reality of infinitely precise numbers.

If numbers cannot have infinite strings of digits, then the future can never be perfectly preordained.

Dave Whyte for Quanta Magazine

Strangely, although we feel as if we sweep through time on the knife-edge between the fixed past and the open future, that edge — the present — appears nowhere in the existing laws of physics.

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这个一体机制作的真优良。主机为树莓派4B,配备显示器、鼠标键盘、甚至还有编程器件和学习中常见的电子元件。

这种一体机非常适合初学电子电路的学生使用。

这个设备官网地址为:https://www.letscode.cn/ ,但是售价不菲啊。我觉得这个设备成本最多也就是一千块钱。看来STEAM教育的设备利润率很高啊。

今天在阅读彭罗斯的《通向实在之路》这本书时,看到了33.1节,才知道我一直思考的时空,是“斯奈德-席尔德时空(Snyder-Schild spacetime)”。

本处内容摘录自《通向实在之路》在线电子书,原链接:点击访问

斯奈德-席尔德时空(Snyder-Schild spacetime)出自以下两个论文

这两个参考文献l来自下面这篇文章,专门讨论时空结构:【HEP-TH-9506171】

The Small Scale Structure of Space-Time: A Bibliographical Review (文件大小423KB)

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本文转载自知乎:一文读懂量(xiang)子(ai)纠(xiang)缠(sha)

英文原文1:Entanglement Made Simple

英文原文2:Your Simple (Yes, Simple) Guide to Quantum Entanglement


量子纠缠及其“多世界”诠释都带有一种神秘而迷人的光环。然而,这些都是,或者都应该是科学观点,它们都有实实在在的具体含义。在下面这篇文章中,我们将尽可能简单明了地为大家解释一下量子纠缠和多世界的概念。

纠缠:从经典迈入量子

量子纠缠经常被看作量子力学才独有的现象,但事实并不是这样。实际上,我们可以首先通过思考一个简单的非量子(或者“经典”)现象来考察纠缠,这是一种比较反传统的做法。这样可以让我们绕开量子论中纠缠的怪异之处来体会量子纠缠的精妙。

一个系统由两个子系统组成,纠缠发生在我们对系统的状态有部分了解的情况下。我们将子系统称之为c-on。“c”的意思是“经典的”,为了便于理解,我们把c-on看作蛋糕。

这里我们的蛋糕有两种形状,正方形或者圆形。那么两个蛋糕的总状态就有4种,它们分别是(方,方)(方,圆)(圆,方)(圆,圆)。下面两个表格给出了在四个状态中找到某一个状态的概率。

当我们不能通过一个蛋糕的信息来判断另一个蛋糕的状态时,我们称这两个子系统是独立的。我们的第一个表格就具有这种特性。即使我们知道第一个蛋糕是方的,我们仍然不知道另一个的形状。类似的,第二个子系统的形状并不能告诉我们关于第一个子系统形状的任何有用信息。

另一方面,如果一个蛋糕的信息可以增加我们对另一个蛋糕的认识,我们就说这两个蛋糕是纠缠的。第二个表格中的情况就表现出高度的纠缠。在这种情况中,如果我们已经知道第一个蛋糕是圆的,那么我们就知道第二个蛋糕一定也是圆形的。如果第一个蛋糕是方形的,第二个也是。当我们知道了第一个蛋糕的形状我们就能确定另一个蛋糕的形状。

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