Einstein Relativity Locks Past and Future Into Static Block Universe

Sabine Hossenfelder

Because Special Relativity makes simultaneity dependent on observer motion, past and future events remain as physically real as the present across a static four-dimensional space-time.

AI-assisted summary of Does the Past Still Exist? — original source. Check the original for full context.

Source published: 2022-07-23

Key ideas

Special Relativity demonstrates that the concept of 'now' is observer-dependent, as relative motion alters the perceived order and simultaneity of spatially separated events. (8:06)
Because planes of simultaneity tilt across different reference frames, linking distant events across observers leads directly to the 'block universe' model, where past, present, and future exist simultaneously. (11:43)
Attempts to escape the block universe via preferred reference frames or quantum indeterminism either contradict empirical evidence or fail to dismantle the enduring physical existence of the past. (12:46)

Detailed notes

## Background Published on YouTube by physicist Sabine Hossenfelder on July 23, 2022, this video explores the profound physical and philosophical consequences of Albert Einstein's concept of space-time. The video's description explains how treating time as a dimension leads to the 'block universe' model, where the past, present, and future all exist simultaneously, challenging our everyday perception of time.

## TL;DR The video explains how Hermann Minkowski and Albert Einstein unified space and time into a four-dimensional continuum, demonstrating that the constancy of the speed of light makes the concept of 'now' entirely observer-dependent. Through the relativity of simultaneity, events in one observer's past can be simultaneous with another observer's present, linking all moments of time together. This logical chain establishes the 'block universe' model, where the past, present, and future exist eternally and statically. While alternative concepts like presentism or quantum indeterminism attempt to challenge this, they fail to easily dismantle the reality of a persistent past.

## Atmosphere & style The video has an intellectually stimulating yet highly accessible atmosphere, driven by Sabine Hossenfelder's signature dry wit and clear, structured explanations. Complex physics concepts are demystified using intuitive thought experiments and clean visual diagrams, making deep philosophical questions feel approachable. The tone is cheeky but authoritative, blending rigorous scientific logic with lighthearted quips that keep the viewer engaged through mind-bending revelations about the nature of reality.

## Bulletin - [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) Special Relativity demonstrates that the concept of 'now' is observer-dependent, as relative motion alters the perceived order and simultaneity of spatially separated events. - [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) Because planes of simultaneity tilt across different reference frames, linking distant events across observers leads directly to the 'block universe' model, where past, present, and future exist simultaneously. - [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) Attempts to escape the block universe via preferred reference frames or quantum indeterminism either contradict empirical evidence or fail to dismantle the enduring physical existence of the past.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=0s) Classical physics treated time as an absolute, universal parameter, but Hermann Minkowski revealed it functions as a geometric fourth dimension inherent to Maxwell's equations. - [02:09](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=129s) In space-time diagrams, changing velocity corresponds geometrically to rotating one's trajectory, with the speed of light representing an absolute 45-degree cosmic speed limit. - [02:58](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=178s) Einstein's Special Relativity relies on four postulates: light speed is finite, invariant, unbeatable, and all observers' viewpoints (coordinate frames) are equally valid. - [03:28](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=208s) In theoretical physics, an 'observer' does not mean a conscious being, but simply an arbitrary reference coordinate system used to describe physical reality. - [05:05](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=305s) Because sensory information and light take time to travel, defining an objective 'now' at a distance requires an operational measurement procedure rather than subjective perception. - [06:05](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=365s) Einstein's two-mirror thought experiment establishes an operational definition of simultaneity by measuring when reflected light pulses return to a midpoint observer. - [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) The relativity of simultaneity arises because an observer in relative motion sees the mirrors moving toward and away from the light pulses, causing reflection events that are simultaneous in one frame to occur at different times in another. - [11:10](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=670s) For any two causally disconnected events (separated by a space-like interval), an inertial reference frame always exists in which both events occur at the exact same instant. - [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) Chaining relativistic simultaneity across different observers connects past and future events (like a past dinner and a future morning) via distant phenomena, demanding equal physical existence for all points in time. - [12:15](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=735s) The resulting 'block universe' implies that four-dimensional space-time does not progress or change from an external viewpoint; it simply exists statically. - [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) Radical presentism avoids the block universe by claiming only the observer's immediate point exists, but this requires adopting an extreme, solipsistic denial of distant reality. - [13:16](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=796s) Introducing a preferred universal 'slicing' of space-time to restore an absolute present either conflicts with empirical tests of Special Relativity or introduces an unobservable, unphysical assumption. - [13:56](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=836s) Quantum wave-function collapse cannot establish an absolute 'now' because it cannot be directly observed or used to transmit information faster than light. - [14:23](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=863s) Even if quantum indeterminism prevents the future from being predetermined, it leaves the block universe's conclusion intact regarding the past: the past remains as real as the present.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=0s) The Mystery of Time and the Move to Four Dimensions

Human intuition treats time as a steady, universal current that flows at the exact same pace for everyone and everything. We perceive an absolute "now" dividing an unchangeable past from an open future: half an hour ago you were doing something else, right now you are watching a video, and what happens next feels like a matter of choice. In physics, however, time behaves entirely differently. As Sabine Hossenfelder jokes when introducing the physicist who revolutionized our understanding of it: “The person who figured this out was none other than Albert Einstein. I know. That guy again. Turns out he kind of knew it all.”

Historically, classical physics treated time as a universal parameter—a shared cosmic clock labeling how much time passes and defining what everyone everywhere means by "now." The first major break from this view came from Hermann Minkowski. Minkowski realized that James Clerk Maxwell’s equations of electrodynamics make far more mathematical sense if time is treated not as an external parameter, but as a dimension. In regular space, rotating an object (like a ball) leaves its physical properties unchanged. Similarly, Maxwell’s equations possess an inherent symmetry: they remain invariant when a spatial direction is mathematically rotated into time. By unifying space and time into a single four-dimensional continuum—space-time—Minkowski revealed that the behavior of electric and magnetic fields stems directly from the geometry of space-time itself.

```mermaid flowchart LR accTitle: Evolution of Time in Physics accDescr: Diagram tracing time from an absolute classical parameter to Minkowski space-time.

subgraph Classical["Classical View"] A["Universal Parameter"] -->|"Universal Clock"| B["Absolute 'Now'"] end

subgraph Relativistic["Minkowskian View"] C["Maxwell's Equations"] -->|"Rotational Symmetry"| D["Time as a 4th Dimension"] D --> E["4D Space-Time"] end

Classical -.->|"Overturned by"| Relativistic ```

- In classical physics, time is a global parameter: one second passing for an observer on Earth is assumed to equal one second passing for another person or the clouds above. - Hermann Minkowski showed that the Lorentz invariance of Maxwell's equations is a geometric property of 4D space-time rather than a quirk of electromagnetism. - Detailed discussions of these concepts are expanded upon in Hossenfelder's book, *Existential Physics*.

---

[02:09](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=129s) Space-Time Diagrams and Einstein's Four Postulates

Visualizing four dimensions is impossible for human intuition, but physics simplifies this by using a two-dimensional space-time diagram, where the horizontal axis represents a single dimension of space and the vertical axis represents time. An object at rest simply traces out a vertical straight line as time elapses. An object moving at a constant velocity creates a tilted straight line; thus, changing your velocity corresponds geometrically to rotating your trajectory in space-time. The ultimate cosmic speed limit is the speed of light, which by standard convention is drawn as a straight line at a 45-degree angle. While space allows movement forward, backward, left, right, up, and down, time permits movement in only one direction: we cannot make a U-turn, and as Hossenfelder quips, there are no driveways available for awkward three-point turns.

Once time becomes a dimension, the time coordinate is merely an arbitrary coordinate label. Setting axes on space-time is as flexible as drawing arbitrary grid lines on a flat sheet of paper. To understand what this physical reality means, Einstein based his theory of Special Relativity on four core assumptions:

1. The speed of light in a vacuum is finite. 2. The speed of light is always the same (invariant for all observers). 3. Nothing can travel faster than the speed of light. 4. All observers' viewpoints are equally valid.

Hossenfelder clarifies that within theoretical physics, an "observer" is not a conscious entity, an ant, a tree, or a dolphin, nor is it what the FBI would call an observer. Rather, it simply refers to a theoretical coordinate system: “So Einstein’s assumption basically means any coordinate system should be as good as any other for describing physical reality.”

---

[05:05](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=305s) Operationalizing "Now" and the Gedankenexperiment

Because light has a finite speed and nothing travels faster than light, you cannot technically verify what is happening right now elsewhere in the universe. Even the screen in front of you is viewed via light that took a tiny fraction of time to reach your retinas. Relying on touch does not solve the problem either: biological nerve signals travel so slowly compared to light that if an unseen hand snatched your phone away, it would take several microseconds for the tactile information to register in your brain. While René Descartes retreated to radical skepticism with *“I think, therefore I am,”* physics requires an operational definition—a concrete measurement procedure—to define the present.

To construct an objective "now," Einstein devised a famous *Gedankenexperiment* (thought experiment). Imagine you stand centered between two mirrors positioned at equal distances to your left and right:

```mermaid flowchart TD accTitle: Einstein's Simultaneity Thought Experiment accDescr: Sequence showing how an observer establishes simultaneity using mirrors and light pulses.

A["Place mirrors at equal distances to left and right"] --> B["Emit two photons simultaneously in opposite directions"] B --> C["Photons bounce off mirrors and reflect back"] C --> D{"Do photons return at the same time?"} D -- "No" --> E["Adjust position until return times match"] D -- "Yes" --> F["Midway point (t/2) marks the simultaneous reflection ('Now')"] ```

Because the speed of light is constant, if both light pulses return to you simultaneously, you know you are located precisely midway between the mirrors. Consequently, at exactly half the total round-trip time, the photons must have struck both mirrors simultaneously. This operational recipe allows an observer to calculate precisely when an event occurred at a distance, establishing their personal plane of "now."

---

[08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) The Relativity of Simultaneity

The operational definition of "now" breaks down when relative motion is introduced. Suppose an observer named Alice zooms past in a spaceship at a high, constant velocity from left to right. In your stationary space-time diagram, Alice's path is a tilted line. Alice executes the identical mirror experiment inside her own frame: she positions mirrors to her left and right, sends out two photons, and notes that they return to her at the exact same time. Halfway through their travel time, Alice concludes that both photons bounced off her mirrors simultaneously.

From your perspective on the ground, however, Alice's conclusion appears completely incorrect. Because her ship is moving to the right, her right-hand mirror is actively receding away from the outgoing photon, meaning that light pulse has to travel a longer distance. Conversely, her left-hand mirror is rushing forward to meet the left-bound photon, so it strikes that mirror earlier. Alice only perceives them returning simultaneously because on the rebound journey the geometry reverses, perfectly canceling out the difference in travel times.

```mermaid flowchart LR accTitle: Observer Perspectives on Simultaneous Events accDescr: Contrast between the stationary observer's view and Alice's moving view regarding photon reflection.

subgraph Ground["Your Frame (Stationary)"] G1["Left mirror rushes toward photon"] --> G2["Left bounce happens FIRST"] G3["Right mirror recedes from photon"] --> G4["Right bounce happens LATER"] end

subgraph Ship["Alice's Frame (Moving)"] A1["Speed of light is invariant"] --> A2["Both mirrors are equidistant"] A2 --> A3["Both bounces happen SIMULTANEOUSLY ('Now')"] end ```

Who is correct? Under Einstein’s fourth postulate, both coordinate systems are equally valid: > “So you’re both right. Or, to put it differently, the notion of ‘now’ depends on the observer, it is ‘observer-dependent’ as physicists say. Your ‘now’ is not the same as my ‘now’.”

This phenomenon is known as the **relativity of simultaneity**. In daily human life, relative speeds are far too low compared to light speed for this effect to be detected, so ordinary conversation about what happened "just now" works fine. But at relativistic velocities, shared agreement on the present completely disappears.

- If two events are causally disconnected (separated by a space-like interval, having a slope shallower than 45 degrees on a space-time diagram), no signal traveling at or below light speed can travel between them. - For *any* two causally disconnected events, there exists an inertial reference frame where those two events happen at the exact same instant. - One simply has to imagine the mirrors passing through the two events and a theoretical observer cruising directly through the middle at the specific velocity required for the light rays to strike both events simultaneously.

[11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) The Logic of the Block Universe

To see how Special Relativity locks all moments of time into existence, consider an event from your past—such as eating a rich cheese dinner—and an event later in your timeline, such as waking up feeling terrible the morning after. Now identify an event completely causally disconnected from both, such as a supernova detonating in a far-off galaxy. Because simultaneity is relative, there will always be a class of reference frames and observers for whom the supernova is simultaneous with your cheese dinner, and another set of observers moving at a different velocity for whom that exact same supernova is simultaneous with your miserable morning after.

```mermaid flowchart LR accTitle: Simultaneity Chaining to the Block Universe accDescr: Flowchart illustrating how a distant supernova links past and future events across different observer frames into a block universe. subgraph EarthEvents ["Observer's Worldline"] E1["Cheese dinner (Past)"] E2["Morning after (Present)"] end subgraph DistantSpace ["Distant Galaxy"] SN["Supernova event"] end E1 ---|"Simultaneous for Observer A"| SN SN ---|"Simultaneous for Observer B"| E2 SN -.-> BU["Transitive Existence: All times exist in the same way"] ```

When you chain these perspectives together under Einstein’s assumption that the laws of physics are invariant across all inertial observers, the philosophical consequences become unavoidable. If you accept that something exists “now” even when it is located far away in space, consistency demands that this standard of existence applies to all observers. Because observer frames tilt and overlap across space and time, every event in the past, present, and future becomes linked through planes of simultaneity. As Sabine Hossenfelder emphasizes: “Another way to put it is that all times exist in the same way.” This complete four-dimensional structure of space-time is known as the block universe. Describing the static nature of this reality, she remarks, “It’s just there. It doesn’t come into being, it doesn’t change. It just sits there.”

- **The Relativistic Chain of Simultaneity:** If Event A (cheese dinner) is simultaneous with Event X (supernova) for Observer 1, and Event X is simultaneous with Event B (morning after) for Observer 2, then denying the physical reality of either event creates an arbitrary contradiction. - **The Block Universe:** Past, present, and future are equally real; time does not "flow" or progress from the perspective of the four-dimensional manifold as a whole.

---

[12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) Objections, Slicing, and Quantum Mechanics

Accepting a static block universe can feel deeply counterintuitive, but escaping its conclusions within modern physics is notoriously difficult. One alternative is radical presentism: insisting that only things existing in the immediate “here and now” are real. If you refuse to attribute existence to anything you cannot directly see or experience at your exact coordinates, the block universe argument falls apart, though Sabine notes that adopting such extreme solipsism is likely even harder to swallow. Another potential workaround is mathematically partitioning space-time into preferred surfaces of simultaneity, a technique known as slicing. Sabine quips that “unfortunately it has nothing to do with pizza,” before pointing out the fundamental issue: if this preferred slice had any observable consequences, it would directly contradict Special Relativity and the mountain of empirical evidence supporting it. If the slice has no physical consequences whatsoever, one can postulate it, though she remarks, “Though I’m not sure why you would want to.”

```mermaid flowchart TD accTitle: Potential Escapes from the Block Universe accDescr: Diagram evaluating strict presentism, space-time slicing, and quantum indeterminism against the block universe. BU["Block Universe Challenge"] --> Opt1["Strict Presentism: Only 'here and now' exists"] BU --> Opt2["Preferred Slicing: Define a privileged 'now'"] BU --> Opt3["Quantum Mechanics: Indeterminism & Wave-function collapse"]

Opt1 --> R1["Counter-intuitive: Denies existence of anything unobserved"] Opt2 --> R2["Either conflicts with Special Relativity or has zero observable effect"] Opt3 --> R3["Collapse is unobservable; past remains locked in existence"] ```

Turning to quantum mechanics does not dismantle the block universe either. Quantum theory remains compatible with Special Relativity; while the measurement update—the collapse of the wave function—appears to occur instantaneously across space, it cannot be observed directly or harnessed to send signals faster than light, meaning it cannot establish an objective physical standard of absolute simultaneity. Some physicists and philosophers argue that because quantum mechanics is fundamentally indeterministic, the future cannot be pre-determined or exist yet, requiring a moving boundary of “now” between an open future and a fixed past. Sabine acknowledges this possibility, but stresses that it does nothing to rescue the past from eternal reality: “So, yeah, it’s true. For all we currently know, the past exists the same way as the present.”

- **Strict Presentism:** Restricts existence exclusively to the observer's immediate spatial and temporal point, avoiding the block universe at the expense of common sense. - **Space-Time Slicing:** Arbitrarily choosing a universal time coordinate across space-time adds an unobservable preferred reference frame, violating the spirit of relativistic symmetry without empirical backing. - **Wave-Function Collapse:** Occurs faster than light mathematically, but because quantum measurement outcomes cannot transmit faster-than-light signals, no observable simultaneity slice can be established. - **Growing Block Interpretation:** Even if quantum indeterminism prevents the future from being real ahead of time, the past remains completely real under relativistic simultaneity arguments.

---

[14:50](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=890s) Quantum Mechanics Course on Brilliant

Closing out the video, Sabine jokes about having pivoted from discussing Special Relativity into yet another analysis of quantum mechanics. In tandem with these conceptual discussions, she highlights her dedicated interactive course on quantum mechanics hosted on Brilliant.

The course is designed to guide learners step by step through the foundational concepts of the quantum world, including superpositions, quantum entanglement, Heisenberg's uncertainty principle, operator non-commutativity, and Bell’s theorem. Learners can continue building their mathematical and physical intuition on the platform through follow-up modules covering quantum objects, quantum computing, and linear algebra. Because the material is presented through interactive questions and challenges, users can test and confirm their comprehension in real time. Sabine invites viewers to support the channel by visiting `brilliant.org/sabine`, noting that anyone can get started for free and that the first 200 people to sign up using the link receive a 20 percent discount on an annual premium subscription.

- **Featured Course Topics:** Superpositions, entanglement, the uncertainty principle, non-commutativity, and Bell’s theorem. - **Advanced Paths on Brilliant:** Quantum objects, quantum computing, and linear algebra. - **Offer Details:** Accessible via `brilliant.org/sabine`, free to start, with 20% off an annual premium subscription for the first 200 users.

## Notable quotes > [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) For all we currently know, the past exists the same way as the present. — Sabine Hossenfelder

> [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) The notion of ‘now’ depends on the observer, it is ‘observer-dependent’ as physicists say. Your ‘now’ is not the same as my ‘now’. — Sabine Hossenfelder

## Mentioned Albert Einstein, Bell's theorem, Brilliant, Existential Physics, Hermann Minkowski, James Clerk Maxwell, René Descartes, Sabine Hossenfelder, Special Relativity

## Video description To try out our new course (and many others on math and science), go to https://brilliant.org/sabine. You can get started for free, and the first 200 will get 20% off the annual premium subscription.

Albert Einstein taught us that space and time belong together to a common entity: space-time. This means that time becomes a dimension, similar to space, and has profound consequences for the nature of time. Most importantly it leads to what has been called the block universe, a universe in which all moments of time exist the same way together. The future, the present, and the past are the same, it is just our perception that suggests otherwise.

💌 Sign up for my weekly science newsletter. It's free! ➜ http://sabinehossenfelder.com/ 👉 Support me on Patreon ➜ https://www.patreon.com/Sabine 📚 Buy my book ➜ https://amzn.to/3HSAWJW

0:00 Intro 00:58 Space-time 02:09 Space-time diagrams 03:40 Special Relativity 05:05 The Relativity of Simultaneity 10:36 The Block Universe 12:46 The if's and but's 14:50 Sponsor Message

[Source](https://www.youtube.com/watch?v=GwzN5YwMzv0)

中文简报

爱因斯坦相对论将过去与未来锁定于静态块状宇宙由于狭义相对论使同时性取决于观察者的运动，在静态的四维时空中，过去和未来的事件与当下同样具有物理实在性。该视频解释了赫尔曼·闵可夫斯基与阿尔伯特·爱因斯坦如何将空间和时间统一为四维连续体，并表明光速不变性使“现在”这一概念完全依赖于观察者。通过同时性的相对性，某位观察者过去发生的事件可与另一位观察者的当下同时发生，从而将所有时间点联系在一起。这一逻辑链条确立了“块状宇宙”模型，即过去、现在与未来都是永恒且静态存在的。尽管现在论或量子不确定性等替代概念试图挑战这一理论，但它们难以轻易破除过去持久存在的实在性。狭义相对论表明“现在”的概念取决于观察者，因为相对运动会改变空间上分离事件被感知到的先后顺序及同时性。 (8:06) 由于同时性平面在不同参考系间会发生倾斜，在不同观察者之间关联遥远事件将直接推导出“块状宇宙”模型，即过去、现在和未来同时存在。 (11:43) 试图通过优越参考系或量子不确定性来摆脱块状宇宙的尝试，要么与经验证据相悖，要么无法破除过去在物理上的持久存在。 (12:46)

Reading summary

Drawing on Albert Einstein's Special Relativity and Hermann Minkowski's four-dimensional space-time, Sabine Hossenfelder argues that the relativity of simultaneity implies a static "block universe" where the past exists in the exact same manner as the present. Four-Dimensional Space-Time and Coordinate Invariance (0:00) Human intuition views time as an absolute, globally shared clock marking a universal present. Modern physics shifted away from this paradigm when Hermann Minkowski demonstrated that James Clerk Maxwell’s equations of electrodynamics exhibit rotational symmetry if time is treated as a fourth dimension rather than an external parameter. Electric and magnetic field dynamics thereby emerge as geometric features of four-dimensional space-time. Operationalizing Simultaneity Through Einstein's Postulates (2:09) Einstein founded Special Relativity on four core premises: the speed of light in a vacuum is finite, invariant for all observers, and serves as the universal speed limit, while all coordinate systems (observers) are equally valid. Because finite light speed prevents immediate verification of distant events, defining "now" requires an operational procedure. Einstein modeled this by placing an observer equidistant between two mirrors; emitting simultaneous photons that reflect back at the identical instant establishes that the reflections occurred simultaneously at half the round-trip travel time. Observer Dependence and the Relativity of Simultaneity (8:06) When an observer in motion relative to a stationary frame executes this reflection procedure, the two viewpoints diverge. A stationary observer sees the moving system's forward mirror receding from the light pulse while the rear mirror rushes toward it, causing the photon impacts to occur at different times. Because Einstein's postulates dictate that both inertial frames are equally valid, neither observer is incorrect. Consequently, simultaneity is observer-dependent: any two causally disconnected events (separated by a space-like interval) can be rendered simultaneous under a suitable reference frame. The Logic of the Block Universe (11:43) This relativity of simultaneity leads directly to the concept of the block universe. For instance, an event in an individual's personal past (such as a previous evening's dinner) and an event in their future (such as the following morning) can each be simultaneous with the same distant cosmic event, such as a supernova, depending on the observer's relative velocity. Chaining these valid reference frames together demonstrates that past, present, and future events are interconnected across space-time, suggesting that the entire four-dimensional manifold exists statically without an objective, progressing "now." Counterarguments and Quantum Indeterminism (12:46) Escaping the block universe requires alternatives that introduce severe conceptual hurdles. Strict presentism avoids the conclusion by claiming only the immediate spatial "here and now" exists, which collapses into extreme skepticism. Introducing a mathematically preferred space-time "slice" to establish universal time either contradicts Special Relativity or relies on unobservable mechanics. Furthermore, while quantum mechanics involves instantaneous wave-function collapses and potential indeterminism—leading some to propose a "growing block" where the future remains open—quantum collapse cannot transmit signals faster than light. Consequently, quantum mechanics fails to establish an observable universal simultaneity slice, leaving the physical reality of the past intact. Hossenfelder concludes that, within the current empirical framework of physics, there is no viable mechanism to dispute the physical reality of the past; all past moments remain as real as the present.

精炼阅读摘要

物理学者萨宾·霍森菲尔德（Sabine Hossenfelder）指出，基于狭义相对论对光速不变与参考系平权的设定，不同观察者对“此时此刻”的判定存在相对性；由此推导出的“块状宇宙”模型表明，过去、现在与未来在物理层面上具有同等真实的存在性，时间本身并未发生宏观维度的流动。从经典绝对时间到四维时空连续体 (0:00) 在人类直觉与经典物理学中，时间被视为一个普遍共享的宇宙时钟参数，定义了所有人共享的绝对“现在”。然而，赫尔曼·闵可夫斯基（Hermann Minkowski）通过麦克斯韦电动力学方程发现，空间方向在数学上可以旋转进入时间，这一内在对称性促使时间被重新定义为一个物理维度，与三维空间融合成统一的四维时空。在爱因斯坦的狭义相对论中，这一结构基于四个核心公设建立：真空光速有限、光速对所有观察者恒定不变、任何物体运动无法超光速，以及所有观察者的视角（即理论坐标系）在描述物理现实时地位均等。在二维时空图中，静止物体沿时间轴垂直上升，匀速运动则表现为轨迹的倾斜旋转，光速对应45度角。由于时间维度的引入，时间坐标轴的选择纯属人为标定，并不存在一个客观优选的宇宙基准。 “现在”的操作性定义与同时性的相对性 (5:05) 由于光速有限且生物神经传导存在延迟，物理学无法直接感知远处正在发生的事件，必须借助具体的操作性测量来定义“现在”。爱因斯坦的思想实验展示了这一过程：观察者位于两面等距镜子的正中央，向两侧发射光子；若两束光子同时返回，往返时间的中点便被操作性地定义为光线击中两侧镜面的“同时”时刻。然而，一旦引入相对运动，这种同时性便因观察者而异。假设观察者爱丽丝乘坐高速飞船掠过地面，在飞船内部她同样测得两束光同时撞击两侧镜子；但地面静止观察者会看到，前方镜子正迎向光子使其更早碰撞，后方镜子则远离光子导致碰撞延后。根据相对论对所有惯性参考系地位平等的假设，双方的测量结论均属正确，这表明“现在”并非宇宙共有的客观事实，而是完全依赖于观察者的参考系，即同时性的相对性。块状宇宙逻辑与时间链条的客观存在 (11:43) 霍森菲尔德阐明，对于任意两个在时空图上斜率小于45度且因果解耦（空间类间隔）的事件，总能找到一个特定的惯性参考系，使得这两个事件在其中同时发生。基于这一几何特征，时空中的所有事件将通过不同观察者的同时性平面相互连接。例如，某人在地球上吃晚餐（过去的事件）与遥远星系的超新星爆发在某一参考系中同时发生；而对于以另一速度运动的参考系，该超新星爆发可能恰好与该人次日清晨醒来（未来的事件）同时发生。由于所有观察者的物理现实描述均等且有效，承认远方事件客观存在便意味着过去、现在与未来以完全相同的方式存在。这一四维静态结构被称为“块状宇宙”，从整体时空几何的视角来看，它静止存在，不生不灭，也不发生实质演变。对块状宇宙的反驳尝试与量子力学检验 (12:46) 面对块状宇宙反直觉的结论，物理学与哲学常提出替代设想，但均面临严格限制。极端唯现论（Strict Presentism）主张仅有观察者立足的“此时此地”真实存在，但霍森菲尔德指出这会导致极端唯我论，违背基本物理经验。另一种方案是引入数学上的“时空切片”（Slicing）人为确立优选时间坐标，然而这一假想切片若能产生物理效应，将直接与已验证的狭义相对论冲突；若无任何可观测效应，则纯属无物理意义的虚构。即便转向以非决定论著称的量子力学，波函数坍缩虽然表现为瞬时更新，但无法用于传递超光速信号，因而在观测上无法构建绝对同时性标准。某些试图保留开放未来的“增长块模型”（Growing Block）即便成立，霍森菲尔德强调，只要相对论的同时性链条保持有效，过去事件便已被彻底锁定于客观现实之中，过去依然与现在一样真实存在。霍森菲尔德总结指出，就现代物理学目前的认知而言，过去与现在具有同等的实在性。无论是引入量子力学的非决定论，还是尝试设立特权参考系切片，都无法否定狭义相对论下过去已固化于四维时空的事实；时间的单向流逝感属于局限于生命感知的体验，而非客观宇宙的基础结构。

Detailed notes — zh

## Background 该视频由物理学者 Sabine Hossenfelder 于 2022 年 7 月 23 日发布在 YouTube 平台。视频简介指出，爱因斯坦揭示了空间与时间共同构成了“时空”，这让时间成为一个类似于空间的维度，并由此推导出了“块状宇宙”（Block Universe）的概念——在其中，过去、现在与未来以完全相同的方式共同存在，时间的流逝不过是人类感知造成的错觉。 ## TL;DR 视频探讨了物理学如何彻底颠覆日常中对时间“流逝”与“当下”的直觉认知。基于闵可夫斯基时空理论与狭义相对论，由于真空光速有限且对所有观测者等价，“现在”的定义完全依赖于观测者的参考系，即“同时性的相对性”。通过将无因果关联的事件相串联，逻辑上必然推导出过去、现在与未来同时存在的“块状宇宙”模型。Sabine 进一步分析了极度局域论、时空切片以及量子力学波函数塌缩等反驳观点，论证了就现代物理学所知而言，过去依然真实而客观地存在着。 ## Atmosphere & style 整期内容兼具严密的科学理性与标志性的冷幽默反讽，Sabine 用平实接地气的比喻（如吃奶酪与宿醉）拆解深奥抽象的相对论佯谬，语言干脆利落且不失锋芒。叙事从打破日常直觉的怀疑论起步，层层递进地铺展思想实验与几何图景，带给观众一种步步紧逼、最终直面现实本质被连根拔起的智力震撼感。尽管论证涉及宏大的时空存在论，其讲述风格始终保持着清醒、克制且充满哲理思辨的沉浸氛围。 ## Bulletin - [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) 狭义相对论证明了同时性的相对性：“现在”的概念完全依赖于观察者，不存在全宇宙通用的绝对时间基准。 - [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) 基于相对论对无因果关联事件的逻辑推导表明，过去、现在和未来同等客观存在，构成了静止不变的“块状宇宙”。 - [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) 即使量子力学存在波函数塌缩与本质不确定性，其超光速塌缩仍不可直接观测，无法推翻过去与现在同等存在的物理结论。 ## Key takeaways - [00:00](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=0s) 时间的日常主观流动感与物理学现实截然相反，爱因斯坦的相对论彻底颠覆了人类对时间的传统直觉。 - [00:58](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=58s) 闵可夫斯基将时间从通用参数重塑为第四个几何维度，使麦克斯韦电动力学方程组获得了天然的时空旋转对称性。 - [02:09](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=129s) 在时空图中，速度变化本质上是坐标系在时空中的几何旋转，时间轴退化为一种可任意选取的坐标标签。 - [03:40](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=220s) 爱因斯坦狭义相对论将“观测者”数学化定义为坐标系，并确立了所有惯性参考系在描述物理规律时完全平权的假设。 - [05:05](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=305s) 受限于有限且不可超越的光速，操作上必须通过双向发射光子及反弹测量等程序来构建远方的“现在”。 - [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) 相对运动会导致各方对远端事件发生次序的判定产生分歧，从而确立了“同时性的相对性”。 - [10:00](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=600s) 对于任意两个无因果关联的时空事件，宇宙中必然存在某一观察者视角，认定这两个事件是绝对同时发生的。 - [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) 通过引入空间极远且无因果关系的第三方事件（如超新星爆发）作为枢纽，不同时刻的事件被逻辑绑定为同等存在。 - [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) “块状宇宙”（Block Universe）理论指出，所有时间点从一开始就静止地坐落于时空中，时间的流动纯属感知错觉。 - [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) 否定块状宇宙需要退回到极端的“此时此地主义”（唯我论），或者强行引入无任何物理观测效应的时空切片。 - [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) 量子力学的不可观测波函数塌缩无法构建绝对同时性基准，就目前所知，过去依然以与现在完全相同的方式真实存在。 ## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=0s) 时间的直觉感知与爱因斯坦的颠覆人类存在的核心谜团之一，恰好也是物理学中最深奥的谜题：时间。在日常感知中，时间像水流一样流逝，伴随着一个被称为“现在”（now）的特殊时刻。此时此刻你在看视频，半小时前你或许在做别的事；无论过去做了什么，都已经无法更改，而半小时后你要做什么则完全由你决定。然而，物理学所揭示的时间本质与这种主观感知大相径庭。揭开这一谜底的正是阿尔伯特·爱因斯坦（Albert Einstein）。Sabine Hossenfelder 幽默地感叹道：“我知道，又是那家伙。事实证明他好像无所不知。”（“I know. That guy again. Turns out he kind of knew it all.”）关于过去、现在与未来的物理真相，也在她的著作《Existential Physics》（《存在物理学》）中进行了更深入的探讨。 ### [00:58](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=58s) 闵可夫斯基时空：将时间化为维度在常识中，时间被视为对所有人与物体都普遍适用的通用参数（universal parameter）。如果 Sabine 度过了一秒，你度过了一秒，天上的云朵也同样度过了一秒。这一参数标记了流逝的时光，也标定了全人类共同认可的“现在”。赫尔曼·闵可夫斯基（Hermann Minkowski）率先察觉到这种预设有误。他发现，如果将时间视作一个“维度”而非单纯的参数，麦克斯韦电动力学方程组（Maxwell's equations）就会变得极其合理。正如将空间中的一个方向旋转到另一个方向时小球的性质不变一样，如果将空间方向与时间方向相互旋转，麦克斯韦方程组的形式同样保持不变。 - 闵可夫斯基提议将三维空间与一维时间合并为四维时空（4-dimensional space-time）。 - 麦克斯韦方程组的固有对称性并不来源于电场和磁场本身，而是直接源于空间与时间自身的几何性质。 ### [02:09](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=129s) 时空图与坐标标签的破灭为了在二维纸面上直观表达时空，物理学家绘制了“时空图”（space-time diagram）：一条坐标轴代表时间，另一条代表至少一个维度的空间。静止不动的物体在时空图中表现为垂直向上的直线；以恒定速度运动的物体表现为倾斜的直线。因此，速度的改变在几何本质上就是在时空中的旋转。在常规画法中，物体运动的极限速度——光速（speed of light）通常被规定画成 45 度角。在空间维度上，我们可以前后、左右、上下自由移动；而在时间维度上，我们只能勇往直前，既无法掉头（U-turn），也没有供人别扭倒车的三分掉头支路（three-point turns）。尽管时间在这一点上依然特殊，但一旦时间变成了维度，它就彻底降级为一种坐标标签。正如二维平面上的坐标系可以任意旋转选取一样，四维时空中的时间轴标签也没有任何绝对的普适意义。 ### [03:40](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=220s) 狭义相对论的四大假设与“观测者”的定义为了弄懂时空维度的物理含义，需要确立爱因斯坦狭义相对论（Special Relativity）的四项基石假设： 1. 真空中的光速是有限的（The speed of light in vacuum is finite）。 2. 真空中的光速恒定不变（It's always the same）。 3. 任何物体的运动速度都不能超越光速（Nothing can go faster than the speed of light）。 4. 所有观测者的视角都是同等有效的（All observers' viewpoints are equally valid）。在理论物理中，所谓的“观测者”并不必须是真实存在的生命体。面对学术界关于蚂蚁、树木或海豚算不算观测者的冗长讨论，爱因斯坦直接给出了极其纯粹的数学定义：观测者本质上就是一个坐标系（coordinate system）。因此，爱因斯坦的第四项假设实质上表明：“任何坐标系在描述物理现实时都应该与其他坐标系一样好。”（“So Einstein's assumption basically means any coordinate system should be as good as any other for describing physical reality.”） ### [05:05](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=305s) “现在”的测量困境与爱因斯坦的思想实验将“光速有限且不可超越”结合起来，会对现实感知产生强烈冲击。例如，你此刻正在屏幕前看视频，这个屏幕“现在”真的存在吗？严格从物理学讲你并不知道，因为屏幕在“这一刻”发出的光根本还没抵达你的眼睛；即使手中握着手机，神经电信号的传导速度相比光速也奇慢无比，若是有人抽走手机，信号需要数微秒才能传到大脑。正如笛卡尔的名言“我思故我在”（I think, therefore I am）所总结的，严格来说除了自己当下的思绪，你无法确认任何外界事物在“现在”存在。日常生活中人们不可能采用这种极端怀疑论，物理学必须提供一种能够测量“现在”的操作性程序（operational procedure）。为此，爱因斯坦设计了一个巧妙的“思想实验”（Gedankenexperiment）： ```mermaid flowchart TD accTitle: Einstein's Simultaneity Thought Experiment accDescr: An operational method to define 'now' by emitting photons to equidistant mirrors and detecting their return. subgraph Emit["发射阶段"] A["观测者居中站立"] --> B["同时向左右两面镜子发射光子"] end subgraph Reflect["反射与校准阶段"] B --> C["光子飞行并在两侧镜面反弹"] C --> D{"两侧光子是否同时返回？"} D -- "否" --> E["调整自身位置重新测试"] E --> B D -- "是" --> F["确认自己处于两镜正中央"] end subgraph DefineNow["确定『现在』"] F --> G["取往返总时间的一半时刻 (t/2)"] G --> H["定义该时刻即为光子击中镜面的『现在』"] end ``` 依靠光速恒定假设，只要两侧返回的光子同时到达，观测者就能断定自己在两面镜子的正中间；进而可以确信，在光子往返总时间恰好一半的瞬间，正是光子在远端击中镜子的时刻。通过这种测量程序，观测者成功构建出了远方的“现在”。 ### [08:06](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=486s) 同时性的相对性与因果未关联事件当引入处于相对运动的观测者时，这一“现在”的定义便轰然瓦解。假设你的朋友爱丽丝（Alice）驾驶飞船以恒定速度从左往右掠过。在你的时空图中，爱丽丝沿着倾斜直线疾驰。如果爱丽丝在飞船上也架设两面间距相等的镜子，并执行完全相同的光子对撞测验： - 从你的角度看：爱丽丝右侧的镜子正顺着飞行方向逃逸，左侧的镜子则迎着光子运动。因此，向左飞行的光子显然会先击中左侧镜子。虽然反弹光子在返回爱丽丝处时行程差会抵消，使她误以为两端对称，但你确信两次撞击绝非同时发生。 - 从爱丽丝的角度看：由于运动是相对的，她认为自己静止而你在向后飞退；她认定自己的“现在”正确无误，反而是你的判断出了偏差。依据爱因斯坦第四假设，所有观测者的视角完全等价。爱因斯坦对此给出的裁决是：“所以你们两个都是对的。或者换句话说，‘现在’的概念取决于观测者，正如物理学家所说，它是‘依赖于观测者的’。”（“So you’re both right. Or, to put it differently, the notion of “now” depends on the observer, it is “observer-dependent” as physicists say.”）这就是狭义相对论的核心结论之一——同时性的相对性（relativity of simultaneity）。在日常低速世界中，这种对“现在”理解的微小错位完全可以忽略不计；但若以接近光速移动，“现在”的定义将彻底陷入混乱。这种几何特性进一步衍生出一个惊人事实：时空图中任意两个**无因果关联的事件**（causally disconnected events，即连接两事件的直线倾角小于 45 度，光信号都无法在两者间传递），必然存在某一个以特定速度飞行的观测者，在其参考系中，这两个事件是**绝对同时发生的**。物理学家只需让两面镜子刚好掠过这两个事件，并调整飞船速度让观测者居于中央，就能在该参考系内将它们同时捕获。 ### [11:43](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=703s) 块状宇宙：过去、现在与未来同等存在在讨论时间的本质时，Sabine Hossenfelder 举了一个接地气的比喻：假设你在前一天晚上吃了一顿奶酪大餐，而第二天早上醒来时感到身体极不舒服。现在，在浩瀚宇宙中寻找一个与这两起事件完全没有因果关联的事件——比如遥远星系中发生的一场超新星爆发。基于狭义相对论的时空特性，宇宙中必定存在某些观察者，对他们而言，这场超新星爆发与你吃奶酪大餐是同时发生的；同时，也必然存在另一些观察者，对他们而言，超新星爆发与你次日早晨难受醒来的时刻是同时发生的。如果把这些逻辑串联起来，其推导结果将颠覆我们对日常现实的认知。只要你承认某个此时此刻并不在“这里”（异地）的事物依然存在于“现在”，那么根据爱因斯坦的第四个假设，这种存在性对所有观察者都必须成立。如果所有你认为“现在”发生的事件都存在，而其他观察者所认定的“与那些事件同时发生”的事件也同样存在，那么推导到极致，所有的事件就都存在于“现在”。换言之，所有的时间点都是以完全相同的方式客观存在的。这就是物理学中著名的“块状宇宙”（Block Universe）。正如 Sabine Hossenfelder 所总结的：“This is called the ‘block universe’. It’s just there. It doesn’t come into being, it doesn’t change. It just sits there.”（这就是所谓的‘块状宇宙’。它就在那里。它并非生成出来的，它也不会改变。它只是静静地待在那里。） ```mermaid flowchart TD accTitle: 块状宇宙逻辑推导 accDescr: 展示相对论中同时性的相对性如何推导出所有时间以相同方式存在的块状宇宙模型。 subgraph S1["事件关联"] A["事件 A: 吃奶酪晚餐"] B["非因果事件: 遥远超新星爆发"] C["事件 B: 宿醉/次日清晨难受"] end subgraph S2["不同观察者视角"] O1["观察者 1 判定:<br/>超新星与奶酪晚餐同时"] O2["观察者 2 判定:<br/>超新星与次日清晨同时"] end subgraph S3["推论"] D["所有时间点以同等方式客观存在<br/>(块状宇宙)"] end A --- O1 B --- O1 B --- O2 C --- O2 O1 --> D O2 --> D ``` - 块状宇宙的核心结论在于：过去、现在和未来之间并不存在客观的物理流动，时间的流逝感纯粹源于人类的主观感知。 - 在逻辑链条中，超新星爆发作为一个空间距离极远且无因果联系的事件，成为了连接不同观察者“同时性平面”的枢纽。 ### [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) 质疑与反思：极度局域论、切片假设与量子力学对于许多人而言，接受块状宇宙是一个巨大的认知挑战，但要在狭义相对论的框架下否定它，所要付出的代价可能更加极端。Sabine 指出，推导出块状宇宙的前提是：你愿意承认某些你看不到、也无法直接体验到的事物在“当下”仍然存在。如果你退回到最极端的立场，主张只有“此时此地”（Now and Here）的事物才算存在，确实可以避开块状宇宙的结论，但这显然更加令人难以接受。另一种方案是人为发明一种“存在”的概念，在时空中为每一个时间点人为划定一个特定的截面，这在物理上被称为“切片”（slicing）。Sabine 风趣地打趣道，这种切片可惜和披萨切片没有任何关系。但问题在于，如果这种时空切片带有任何可观测的物理后果，它就会直接违背狭义相对论的基础假设；而狭义相对论早已获得无数高精度实验的极力证实。如果非要定义一个没有任何物理可观测效应的特权“现在”，在数学上固然可行，但毫无科学意义，正如 Sabine 所吐槽的那样，完全不知道为什么有人会想这么做。此外，量子力学的介入也无法推翻块状宇宙。尽管量子力学中波函数的测量塌缩发生得比光速还要快，但由于这种超光速塌缩在物理上是不可直接观测的，因此根本无法用来定义一个绝对的“同时性”基准，它与狭义相对论并不冲突。有人可能会反驳说，量子力学具有本质的不确定性，因此未来不可能事先存在于块状宇宙中，必须存在一个特殊的“现在”将过去与未来割裂开来。对此，Sabine 坦言即便未来尚不确定，但这套逻辑依然完全适用于过去。Sabine Hossenfelder 郑重指出：“So, yeah, it’s true. For all we currently know, the past exists the same way as the present.”（所以，是的，这是真的。就我们目前所知的一切而言，过去与现在是以完全相同的方式存在的。） - **此时此地主义（Here-and-Now）**：彻底拒绝块状宇宙的代价是承认只有自己当下感官所及的狭小奇点才存在，这会导致极端的唯我论倾向。 - **时空切片（Slicing）**：人为定义特权参照系会破坏洛伦兹不变性，除非该切片没有任何可观测效应，否则与已被证实的相对论相悖。 - **波函数塌缩与因果**：波函数更新的瞬时性无法传递信息或作为物理观测基准，因而不构成对相对论同时性相对性的反驳。 ### [14:50](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=890s) 赞助推荐：深入量子力学的互动学习在厘清了狭义相对论与量子力学对时间本质的探讨后，Sabine 话锋一转，打趣道大家本来以为这是一期纯粹关于狭义相对论的视频，结果自己又不可避免地聊到了量子力学。她顺势介绍了赞助商 Brilliant 平台，并宣布自己在 Brilliant 上线了一门与她的视频内容相配套的全新量子力学系统课程。 Sabine 介绍，Brilliant 是一个优秀的科学与数学学习工具，拥有涵盖众多领域的丰富课程体系。她亲自参与制作的这门全新量子力学课程涵盖了量子叠加态（superpositions）、量子纠缠（entanglement）、不确定性原理（uncertainty principle）、非对易性（non-commutativity）以及贝尔定理（Bell's theorem）等核心概念。学生在学完这门入门课后，还可以顺畅衔接 Brilliant 提供的量子客体、量子计算或线性代数等进阶内容。课程延续了 Brilliant 一贯的互动式教学风格，通过即时提问让学习者验证自己的理解，Sabine 提到与 Brilliant 团队的合作过程非常愉快。 - **课程涵盖主题**：量子叠加态、量子纠缠、不确定性原理、非对易性以及贝尔定理。 - **后续进阶方向**：涵盖量子计算（Quantum Computing）、量子客体（Quantum Objects）和线性代数（Linear Algebra）等模块。 - **推广与优惠信息**：观众可通过访问专属链接 `brilliant.org/sabine` 免费注册，前 200 名订阅者可享受年度高级会员（Annual Premium Subscription）20% 的折扣。 ## Notable quotes > [12:46](https://www.youtube.com/watch?v=GwzN5YwMzv0&t=766s) For all we currently know, the past exists the same way as the present. — Sabine Hossenfelder ## Mentioned Sabine Hossenfelder, 阿尔伯特·爱因斯坦, 赫尔曼·闵可夫斯基, 勒内·笛卡尔, Brilliant, Existential Physics, 麦克斯韦电动力学方程组, 狭义相对论, 块状宇宙 ## Video description To try out our new course (and many others on math and science), go to https://brilliant.org/sabine. You can get started for free, and the first 200 will get 20% off the annual premium subscription. Albert Einstein taught us that space and time belong together to a common entity: space-time. This means that time becomes a dimension, similar to space, and has profound consequences for the nature of time. Most importantly it leads to what has been called the block universe, a universe in which all moments of time exist the same way together. The future, the present, and the past are the same, it is just our perception that suggests otherwise. 💌 Sign up for my weekly science newsletter. It's free! ➜ http://sabinehossenfelder.com/ 👉 Support me on Patreon ➜ https://www.patreon.com/Sabine 📚 Buy my book ➜ https://amzn.to/3HSAWJW 0:00 Intro 00:58 Space-time 02:09 Space-time diagrams 03:40 Special Relativity 05:05 The Relativity of Simultaneity 10:36 The Block Universe 12:46 The if's and but's 14:50 Sponsor Message [Source](https://www.youtube.com/watch?v=GwzN5YwMzv0)

Source quotes

[12:46] Sabine Hossenfelder: For all we currently know, the past exists the same way as the present.

Source quotes

[12:46] Sabine Hossenfelder: For all we currently know, the past exists the same way as the present. [8:06] Sabine Hossenfelder: The notion of ‘now’ depends on the observer, it is ‘observer-dependent’ as physicists say. Your ‘now’ is not the same as my ‘now’.

Original transcript

[0:00] One of the biggest mysteries of our existence is also one of the biggest mysteries of physics: [0:05] time. We experience time as passing, with a special moment that we call “now”. Now you’re [0:12] watching this video, half an hour ago you were doing something else. Whatever you did, [0:16] there’s no way to change it. And what you will do in half an hour is up to you. At least that’s [0:22] how we perceive time. But what physics tells us about time is very different from our perception. [0:28] The person who figured this out was none other than Albert Einstein. [0:32] I know. That guy again. Turns out he kind of knew it all. What did Einstein [0:38] teach us about the past, the present, and the future? That’s what we’ll talk about today. [0:47] The topic we’re talking about today is covered in more detail in my new book [0:51] “existential physics” which will be published in August. You find more info [0:55] about the book at existentialphysics dot com We think about time as something that works [1:00] the same for everyone and every object. If one second passes for me, one second passes for you, [1:06] and one second passes for the clouds above. This makes time a universal parameter. This [1:13] parameter labels how much time passes and also what we all mean by “now”. [1:18] Hermann Minkowski was the first to notice that this may not be quite right. He noticed that [1:25] Maxwell’s equations of electrodynamics make much more sense if one treats time as a dimension, [1:31] not as a parameter. Just like a ball doesn’t change if you rotate one direction of space [1:37] into another, Maxwell’s equations don’t change if you rotate one direction of space into time. [1:43] So, Minkowski said, we just combine space with time to a 4 dimensional space-time, and then [1:50] we can rotate space into time just like we can rotate two directions of space into each other. [1:56] And that naturally explains why Maxwell’s equations have the symmetry they do have. [2:02] It doesn’t have anything to do with electric and magnetic fields, [2:06] it comes from the properties of space and time themselves. [2:10] I can’t draw a flower, let alone four dimensions, but I can just about manage two straight lines, [2:16] one for time and the other for at least one dimension of space. This is called a space-time [2:22] diagram. If you just stand still, then your motion in such a diagram is a straight vertical line. [2:28] If you move at a constant velocity, your motion is a straight line tilted at some angle. [2:34] So if you change velocity, you rotate in space-time. The maximal velocity at which [2:39] you can move is the speed of light, which by convention is usually drawn at a 45-degree angle. [2:45] In space we can go forward-backward, left right, or up down. In time we can only go forward, [2:52] we can’t make a u-turn, and there aren’t any driveways for awkward three-point turns either. [2:57] So time is still different from space in some respect. But now that time is also a dimension, [3:03] it’s clear that it’s just a label for coordinates, there’s nothing universal about it. There are many [3:09] ways to put labels on a two-dimensional space because you can choose your axes as you want. [3:14] The same is the case now in space-time. Once you have made time into a dimension, the labels on it [3:21] don’t mean much. So what then is the time that we talk about? What does it even mean that time is a [3:28] dimension? Do other dimensions exist? Supernatural ones? That could explain the strange sounds you’ve [3:34] been hearing at night? No. That's a separate problem I'm afraid I can't help you with. [3:40] It was Albert Einstein who understood what this means. If we also want to understand it, [3:45] we need four assumptions. The speed of light in vacuum is finite, it’s always the same, [3:51] nothing can go faster than the speed of light, and all observers’ viewpoints are equally valid. [3:57] This formed the basis of Einstein’s theory of Special Relativity. [4:01] Oh, and also, the observers don’t have to exist. I mean, this is theoretical physics, [4:06] so we’re talking about theoretical observers, basically. So, if there could be an observer [4:12] with a certain viewpoint then then that viewpoint is equally valid as yours. [4:17] Who or what is an observer? Is an ant an observer? A tree? How about a dolphin? What do you need to [4:24] observe to deserve being called an observer and what do you have to observe with? Believe it or [4:30] not, there’s actually quite some discussion about this in the scientific literature. We’ll side-step [4:36] this, erm, interesting discussion and use the word “observer” the same way that Einstein did, [4:41] which is a coordinate system. You see, it’s a coordinate system that a theoretical observer [4:46] might use, dolphin or otherwise. Yeah, maybe not exactly what the FBI thinks an observer is, [4:53] but then if it was good enough for Einstein, it’ll be good enough for us. So Einstein’s assumption [4:59] basically means any coordinate system should be as good as any other for describing physical reality. [5:05] These four assumptions sound rather innocent at first but they have profound consequences. Let’s [5:11] start with the first and third: The speed of light is finite and nothing goes faster than light. You [5:18] are probably watching this video on a screen, a phone or laptop. Is the screen there now? [5:24] Unless you are from the future watching this video as a hologram in your space house, I'm going to [5:29] assume the answer is yes. But a physicist might point out that actually you don’t know. Because [5:36] the light that’s emitted from the screen now hasn’t reached you yet. Also if you are from [5:42] the future watching this as a hologram, make sure to look at me from the right. It’s my good side. [5:47] Maybe you hold the phone in your hand, but nerve signals are ridiculously slow compared to light. [5:54] If you couldn’t see your hand and someone snatched your phone, [5:57] it’d take several microseconds for the information that the phone is gone [6:01] to even arrive in your brain. So how do you know your phone is there now? [6:07] One way to answer this question is to say, well, you don’t know, [6:11] and really you don’t know that anything exists now, other than your own thoughts. [6:17] I think, therefore I am, as Descartes summed it up. [6:21] This isn’t wrong – I’ll come back to this later – but it’s not how normal people use the word “now”. [6:28] We talk about things that happen “now” all the time, and we never worry about how long it takes [6:33] for light to travel. Why can’t we just agree on some “now” and get on with it? I mean, think back [6:40] to that space-time diagram. Clearly this flat line is “now”, so let’s just agree on this and move on. [6:47] Okay, but if this is to be physics rather than just a diagram you have to come up with [6:53] an operational procedure to determine what we mean by “now”. You have to find a way to measure [6:58] it. Einstein did just that in what he called Gedankenexperiment, a “thought experiment”. [7:05] He said, suppose you place a mirror to your right and one to your left. You and the mirrors [7:11] are at fixed distance to each other, so in the space time diagram it looks like this. [7:16] You send one photon left and one right, and make sure that both photons leave you at the same time. [7:22] Then you wait to see whether the photons come back at the same time. If they don’t, [7:27] you adjust your position until they do. Now remember Einstein’s second assumption, [7:34] the speed of light is always the same. This means if you can send photons to both mirrors and they [7:41] come back at the same time, then you must be exactly in the middle between the mirrors. [7:46] The final step is then to say that at exactly half the time it takes for the photons to return, [7:53] you know they must be bouncing off the mirror. You could say “now” at the right moment even though [8:00] the light from there hasn’t reached you yet. It looks like you’ve found a way to construct “now”. [8:06] But here’s the problem. Suppose you have a friend who flies by at some constant velocity, maybe in a [8:11] space-ship. Her name is Alice, she is much cooler than you, and you have no idea why she's agreed to [8:17] be friends with you. But here she is, speeding by in her space-ship left to right. As we saw [8:22] earlier, in your space-time diagram, Alice moves on a tilted straight line. She does the exact [8:29] same thing as you, places mirrors to both sides, sends photons and waits for them to come back, [8:35] and then says when half the time has passed that’s the moment the photons hit the mirrors. [8:42] Except that this clearly isn’t right from your point of view. [8:46] Because the mirrors to her right are in the direction of her flight, so the light takes longer [8:51] to get there than it does to the mirrors on the left, which move towards the light. You would say [8:57] that the photon which goes left clearly hits the mirror first because the mirror’s coming at it. [9:03] From your perspective, she just doesn’t notice because when the photons go back to Alice, [9:09] the exact opposite happens. The photon coming from left takes longer to get back, so the net effect [9:15] cancels out. What Alice says happens “now” is clearly not what you think happens “now”. [9:23] For Alice on the other hand, you are the one moving relative to her. And she thinks that [9:29] her notion of “now” is right and yours is wrong. [9:32] So who is right? Probably Alice, you might say. Because she’s much cooler [9:37] than you. She owns a spaceship, after all. Maybe. But let’s ask Einstein. [9:42] Here is where Einstein’s forth assumption comes in. The viewpoints of all observers are equally [9:48] valid. So you’re both right. Or, to put it differently, the notion of “now” depends on the [9:56] observer, it is “observer-dependent” as physicists say. Your “now” is not the same as my “now”. [10:04] If you like technical terms, this is also called the relativity of simultaneity. [10:10] These mismatches in what different observers think happens “now” are extremely tiny in every-day [10:16] life. They only become noticeable when relative velocities are close by the speed of light, [10:22] so we don’t normally notice them. If you and I talk about who knocked at the door right now, [10:28] we won’t misunderstand each other. If we’d zipped around with nearly the speed of light, however, [10:33] referring to “now” would get very confusing. [10:36] This is pretty mind-bending already, but wait, it gets wilder. Let us have a look at the space-time [10:43] diagrams again. Now let us take *any* two events that are not causally connected. This just means [10:49] that if you wanted to send a signal from one to the other, the signal would have to go faster [10:54] than light, so signaling from one to the other isn’t possible. Diagrammatically this means [11:01] if you connect the two events the line has an angle less than 45 degrees to the horizontal. [11:07] The previous construction with the mirrors shows that for any two such events there is always some [11:14] observer for whom those two events happen at the same time. You just have to imagine [11:19] the mirrors fly through the events and the observer flies through directly in the middle. [11:25] And then you adjust the velocity until the photons hit both events at the same time. [11:31] Okay, so any two causally disconnected events happen simultaneously for some observer. Now [11:38] take any two events that are causally connected. Like eating too much cheese for dinner and then [11:43] feeling terrible the morning after. Find some event that isn’t causally connected to either. [11:49] Let’s say this event is a supernova going off in a distant galaxy. [11:54] There are then always observers for whom the supernova and your cheese dinner are simultaneous, [11:59] and there are observers for whom the supernova and your morning after are simultaneous. [12:05] Let’s then put all those together. If you are comfortable with saying that something, [12:10] anything, exists “now” which isn’t here, then, according to Einstein’s fourth assumption, [12:17] this must be the case for all observers. But if all the events that you think happen “now” exist [12:23] and all other observers say the events that happen at the same time as those events, [12:29] then all events exist “now”. Another way to put it is that all times exist in the same way. [12:37] This is called the “block universe”. It’s just there. It doesn’t come into being, [12:43] it doesn’t change. It just sits there. [12:45] If you find that somewhat hard to accept, there is another possibility to consistently combine [12:51] a notion of existence with Einstein’s Special Relativity. All that I just said [12:56] came from assuming that you are willing to say something exists now even though you can’t see [13:03] or experience it in any way. If you are willing to say that only things exist which are now [13:09] and here, then you don’t get a block universe. But maybe that’s even more difficult to accept. [13:16] Another option is to simply invent a notion of “existence” and define it to be a particular [13:21] slice in space-time for each moment in time. This is called a “slicing” but unfortunately [13:28] it has nothing to do with pizza. If it had any observable consequences, that [13:33] would contradict the fourth assumption Einstein made. So it’s in conflict with Special Relativity [13:39] and since this theory is experimentally extremely well confirmed, this would almost certainly mean [13:45] the idea is in conflict with observation. But if you just want to define a “now” that [13:50] doesn’t have observable consequences, you can do that. Though I’m not sure why you would want to. [13:57] Quantum mechanics doesn’t change anything about the block universe because it’s still compatible [14:02] with Special Relativity. The measurement update of the wave-function, which I talked about in [14:07] this earlier video, happens faster than the speed of light. If it could be observed, you could use [14:14] it to define a notion of simultaneity. But it can’t be observed, so there’s no contradiction. [14:20] Some people have argued that since quantum mechanics is indeterministic, [14:25] the future can’t already exist in the block universe, and that therefore there must also [14:30] be a special moment of “now” that divides the past from the future. And maybe that [14:37] is so. But even if that was the case, the previous argument still applies to the past. [14:43] So, yeah, it’s true. For all we currently know, the past exists the same way as the present. [14:50] So you thought this is a video about Special Relativity, [14:53] but then I’ve been talking about quantum mechanics again. [14:57] Indeed, I now have an entire course about quantum mechanics up at Brilliant [15:01] which accompanies my videos on the topic. [15:03] Brilliant is an amazing tool for learning with courses on a large variety of topics in science [15:08] and mathematics. Our new course will give you an introduction to superpositions and entanglement, [15:14] the uncertainty principle, non-commutativity, and Bell’s theorem. And you can then build up [15:20] your knowledge with Brilliant’s courses on quantum objects and quantum computing or linear algebra, [15:26] or wherever you want to go after this. Like all their courses, our new course is interactive and [15:31] will challenge you with questions so you can check your understanding right away. [15:36] I had a lot of fun working with Brilliant’s team on this, [15:38] and I hope you’ll enjoy it too. To support this channel and learn more about Brilliant, [15:44] go to Brilliant dot org slash Sabine and sign up for free. [15:48] The first 200 subscribers using this link will get 20 percent off the annual premium subscription. [15:54] Thanks for watching, see you next week.