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Molecular Weight Calc

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The Universal Speed Limit

According to our current understanding of physics (Einstein's Theory of Special Relativity), nothing in the universe can travel faster than light in a vacuum. It is the ultimate cosmic speed limit.

This constant is denoted by the letter c (from the Latin word celeritas, meaning "swiftness"). It connects energy and mass in the famous equation E = mc².

The Value of c:
299,792,458 meters / second
For quick mental math, we often approximate this as:
3 × 108 m/s (or 300,000 km/s).

Light Slows Down? (Refraction)

The speed limit c applies only in a perfect Vacuum (empty space). When light travels through a material (a medium) like water or glass, it interacts with the atoms in that material, which effectively slows it down.

The measure of how much a material slows down light is called the Refractive Index (n).

The formula for the speed of light in a medium (v) is:

v = c / n
Material Refractive Index (n) Speed (% of c)
Vacuum 1.0000 100%
Air 1.0003 99.97%
Water 1.333 75%
Glass ~1.50 66%
Diamond 2.42 41%

Looking into the Past

Because light takes time to travel, everything you see is actually an image from the past.

  • The Moon: 1.3 seconds ago.
  • The Sun: 8 minutes and 20 seconds ago. If the Sun exploded right now, we wouldn't know for 8 minutes.
  • Alpha Centauri (Nearest Star): 4.3 years ago.
  • Andromeda Galaxy: 2.5 million years ago.

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