What is Time?

“Time is the universe’s greatest mystery. We all live in time, but few truly understand it.”

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Starting from Clocks

Imagine the watch on your wrist. The second hand ticks, one second, two seconds, three seconds…

What is this “tick”? What is it measuring?

🕰️ Principle of Mechanical Clocks

Open an old mechanical clock, and you’ll see:

graph LR
    A["Mainspring<br/>Stores Energy"] --> B["Gear System<br/>Transmits Motion"]
    B --> C["Balance Wheel<br/>Periodic Oscillation"]
    C --> D["Escapement<br/>Controls Release"]
    D --> E["Hands<br/>Display Time"]

    style C fill:#ff6b6b,color:#fff

The core component is the balance wheel: A small wheel oscillating back and forth with very stable period.

• Each oscillation → Gear turns one notch
• Gear turns → Hand moves
• Hand moves → We say “time has passed”

💡 Key Insight: The “passing” of time is essentially counting periodic oscillations!

⚛️ Atomic Clock: More Precise “Tick”

The most precise modern clocks are atomic clocks. Their principle:

1. Cesium atoms transition between two energy levels
2. Absorb or emit electromagnetic waves of specific frequency
3. Frequency is extremely stable: $\nu =9,192,631,770$ Hz (per second)
4. Count this oscillation to define the “second”

📌 Analogy: If the mechanical clock’s balance wheel is a pendulum, then atomic clocks use atoms as “super-precise pendulums” to tell time.

But the question arises: Why is counting oscillations “time”?

Two Views of Time: Flow vs Geometry

View 1: Time is a Flowing River (Everyday Intuition)

Our intuition tells us:

• Time “flows” like a river
• The past has flowed away, cannot return
• The future hasn’t arrived yet
• Only “now” is real

graph LR
    Past["Past<br/>Has Happened"] -->|flow| Present["Present<br/>Happening Now"]
    Present -->|flow| Future["Future<br/>Will Happen"]

    style Present fill:#ffe66d,stroke:#f59f00,stroke-width:2px

This is A-theory: Time has an absolute “now,” time really flows.

View 2: Time is a Geometric Dimension (Physics)

Einstein’s relativity tells us another picture:

• Time is a dimension of four-dimensional spacetime
• Past, present, and future all equally “exist”
• “Flow” is an illusion, a feeling of our consciousness
• Time is more like a direction in space

graph TB
    subgraph "Four-Dimensional Spacetime (Like a Map)"
        T1["Past<br/>t=-1"]
        T2["Present<br/>t=0"]
        T3["Future<br/>t=+1"]
    end

    T1 -.your memory.-> T2
    T2 -.your expectation.-> T3

    style T2 fill:#ffe66d,stroke:#f59f00,stroke-width:2px

This is B-theory: Time doesn’t flow, all moments equally exist, just like all locations on a map exist.

⚠️ Common Misconception: Many think relativity says “time can flow backward.” Wrong! Relativity says time is relative (fast-moving people experience slower time), but time still has a direction (entropy increase).

GLS Theory’s View of Time: Three in One

GLS unified theory proposes a third perspective that attempts to merge the above two views.

Core Insight: Time = Change of Phase

In quantum mechanics, any object has a “phase” $\phi$, like a constantly rotating pointer:

graph TD
    A["Quantum State<br/>Like a Rotating Clock"] --> B["Phase φ<br/>How Many Degrees Rotated?"]
    B --> C["Phase Change Rate dφ/dt<br/>How Fast is it Rotating?"]
    C --> D["This is Energy E = ℏω<br/>ω is Angular Frequency"]

    style B fill:#ff6b6b,color:#fff
    style D fill:#4ecdc4,color:#fff

Key Formula (don’t be afraid, we’ll explain):

$$\phi =\frac{m{c}^2}{\hslash }\int d\tau$$

Translation:

• $\tau$ = Proper time (time elapsed on the object’s own “watch”)
• $\phi$ = Quantum phase (“rotation angle” of wave function)
• $m{c}^2/\hslash$ = A constant (tells you how fast it rotates)

Meaning: Object’s proper time = Its quantum phase change (divided by a constant)

Three Times, One Essence

A core theoretical inference of GLS theory is: Three seemingly different “times” might be mathematically equivalent!

graph TD
    subgraph "Unified Time Scale"
        Unity["Same Time<br/>Just Different Ways to Measure"]
    end

    A["Scattering Time<br/>How Long Particle Delayed After Collision"] --> Unity
    B["Modular Time<br/>Intrinsic 'Clock' of Thermal Equilibrium"] --> Unity
    C["Geometric Time<br/>Time Coordinate t of Spacetime"] --> Unity

    Unity --> D["Unified Scale Identity<br/>κ = φ'/π = ρ_rel = tr Q/2π"]

    style Unity fill:#ff6b6b,stroke:#c92a2a,stroke-width:3px,color:#fff
    style D fill:#4ecdc4,color:#fff

Three Times:

1. Scattering Time: How long a particle is delayed after scattering
2. Modular Time: Intrinsic “thermal time” after a system reaches thermal equilibrium
3. Geometric Time: Time component of spacetime coordinates in general relativity

Unified Scale Identity:

$$\kappa (\omega )=\frac{{\phi }^{\prime }(\omega )}{\pi }={\rho }_{\text{rel}}(\omega )=\frac{1}{2\pi }\text{tr}Q(\omega )$$

This formula says: These four quantities are completely equal!

• $\kappa$ = Scattering delay
• ${\phi }^{\prime }/\pi$ = Phase change rate
• ${\rho }_{\text{rel}}$ = Density of energy levels
• $\text{tr}Q/2\pi$ = Wigner-Smith group delay

💡 Key Insight: You measure “time” using different methods, and you get the same answer! This isn’t coincidence—they’re essentially the same thing.

Three Faces of Time

Let’s understand with an analogy:

📦 Analogy: Projections of a Cube

Imagine a cube, viewed from three different angles:

graph TD
    Cube["Cube<br/>(Real Time)"] --> Front["Viewed from Front<br/>(Scattering Time)<br/>See Square"]
    Cube --> Side["Viewed from Side<br/>(Modular Time)<br/>Also See Square"]
    Cube --> Top["Viewed from Top<br/>(Geometric Time)<br/>Still Square"]

    Front -.same cube.-> Unity["They Are All<br/>Different Projections<br/>of the Same Cube"]
    Side -.same cube.-> Unity
    Top -.same cube.-> Unity

    style Cube fill:#ff6b6b,color:#fff
    style Unity fill:#4ecdc4,color:#fff

• Cube = True “time” (unified time scale)
• Front Projection = Scattering time (physicists measure with scattering experiments)
• Side Projection = Modular time (measured with intrinsic evolution of thermal equilibrium states)
• Top Projection = Geometric time (coordinate time in general relativity)

The three projections look different, but they describe the same cube!

This constitutes GLS theory’s core hypothesis on time: Time might not be three independent things, but three ways to measure the same reality.

Direction of Time: Why Can’t It Flow Backward?

Since time is a geometric dimension (like space), why can’t we “walk back and forth” in time, like walking forward and backward in space?

🔑 Answer: Entropy

Second Law of Thermodynamics: In a closed system, entropy (disorder) always increases or stays constant, never decreases.

graph LR
    A["Ordered State<br/>Low Entropy"] -->|time passes| B["Disordered State<br/>High Entropy"]
    B -.impossible.-> A

    style A fill:#a8e6cf
    style B fill:#ffaaa5

Examples:

• Breaking an egg: Egg → Fragments (entropy increase) ✓ Possible
• Restoring an egg: Fragments → Whole egg (entropy decrease) ✗ Almost impossible

📌 Analogy: A room only gets messier (if you don’t clean it), doesn’t automatically become tidy. The “arrow” of time is the direction of increasing disorder.

Three Formulations of Time Arrow

GLS theory points out that the time arrow has three equivalent formulations:

graph TD
    Arrow["Time Arrow"] --> E["Thermodynamic Arrow<br/>Entropy Always Increases"]
    Arrow --> C["Causal Arrow<br/>Cause Before Effect"]
    Arrow --> P["Psychological Arrow<br/>We Remember Past, Not Future"]

    E -.equivalent.-> C
    C -.equivalent.-> P
    P -.equivalent.-> E

    style Arrow fill:#ff6b6b,color:#fff

They are essentially the same thing:

• Entropy increase → Defines causal order
• Causal order → Determines memory direction
• Memory direction → Feels time “flowing”

Relativity of Time: Your Clock and My Clock Run Differently

Special Relativity: Motion Slows Time

Twin Paradox:

• Older brother stays on Earth
• Younger brother travels in rocket at near light speed
• When younger brother returns, finds older brother much older, while he’s still young

graph TD
    Start["Departure<br/>Same Age"] --> Earth["Brother on Earth<br/>Normal Time Flow<br/>Aged 20 Years"]
    Start --> Rocket["Brother in Rocket<br/>High Speed<br/>Aged Only 2 Years"]

    Earth --> Meet["Reunion<br/>18-Year Age Difference"]
    Rocket --> Meet

    style Meet fill:#ffaaa5

Reason: Moving clocks run slow!

$$\Delta \tau =\Delta t\sqrt{1-\frac{v^2}{c^2}}$$

• $\Delta t$ = Time on Earth
• $\Delta \tau$ = Time on rocket (proper time)
• $v$ = Rocket speed
• $c$ = Speed of light

When $v$ approaches $c$, $\Delta \tau \ll \Delta t$ (time on rocket is much slower)

General Relativity: Gravity Slows Time

Near a gravitational source (like a black hole), time also slows!

Example:

• Clock on Earth’s surface
• Clock on GPS satellite (farther from Earth, weaker gravity)

Time on GPS satellite runs faster, about 38 microseconds per day. Without correction, GPS positioning would drift 10 km per day!

graph TB
    BH["Black Hole<br/>Extremely Strong Gravity"] --> Near["Near Black Hole<br/>Time Very Slow"]
    BH --> Far["Far from Black Hole<br/>Time Normal"]

    Near -.time dilation.-> Far

    style BH fill:#000,color:#fff
    style Near fill:#ff6b6b,color:#fff

Tolman-Ehrenfest Redshift: Deeper gravitational potential → Slower time → Lower frequency (redshift)

Summary: Multiple Faces of Time

Let’s summarize different understandings of “time”:

🎯 Key Points

1. Time is not absolute: Speed and gravity change the rate of time flow
2. Time has direction: Entropy increase defines the time arrow
3. Time is geometry: Time is a dimension of spacetime, not an independent parameter
4. Time is phase: At quantum level, time equals change of phase
5. Time is unified: Three seemingly different times (scattering, modular, geometric) are essentially different ways to measure the same time

💡 Most Profound Insight

GLS theory proposes: Time might not be an externally imposed “parameter,” but an intrinsic emergent property of physical systems.

Just as temperature is not fundamental (it’s average kinetic energy of molecules), time is not fundamental—it’s an emergent property of deeper structures.

What’s Next

Now you have a deeper understanding of “time.” Next we ask:

• What is causality? Why does A cause B?
• If time isn’t absolute, does causality still hold?
• What’s the relationship between causality and time?

Answers to these questions are in the next article:

Next: What is Causality? →

Remember: Understanding time is the first step to understanding the universe. Time isn’t as simple as we imagine, but its true nature is even more beautiful.

← Back to Home | Next: What is Causality →