A thought experiment in special relativity

What if the speed of light was 5 km/h?

Hacker News — 278 points

rivendell.dmitrybrant.com/relativity

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The Thought Experiment

Understanding special relativity at human scale

Special Relativity: The Basics

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Speed of light is constant Light travels at ~300,000 km/s regardless of observer's motion
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Time dilation Moving clocks tick slower relative to stationary observers
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Length contraction Objects appear shorter in the direction of motion
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Mass increase Objects gain relativistic mass as they approach light speed

Key Relativistic Effects

What happens when you approach the speed of light

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Time Dilation

Time passes slower for fast-moving objects

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Length Contraction

Objects shrink in their direction of travel

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Mass Increase

Relativistic mass grows toward infinity

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Energy Equivalence

E=mc² binds mass and energy together

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Everyday Implications

What if light moved at walking speed?

Life at c = 5 km/h

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Walking is relativistic

At 5 km/h, you're moving at light speed—time nearly stops for you

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Colors shift dramatically

Objects ahead appear blue-shifted; behind you, red-shifted

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The world warps

Streets contract, buildings flatten as you move

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Light arrives late

You see events that happened moments ago, not now

Our Universe vs. Slow-Light Universe

Our Universe (c = 300,000 km/s)

Relativistic effects invisible in daily life
Light arrives instantly (for all practical purposes)
No perceptible time dilation at human speeds
Reality appears consistent and stable

Slow-Light Universe (c = 5 km/h) Thought Experiment
Relativistic effects obvious and dramatic
Light takes seconds to cross a room
Walking creates measurable time dilation
Reality warps with every movement

Thank You

Explore the interactive simulation at rivendell.dmitrybrant.com/relativity

Source: Hacker News Discussion