1. What is the Exact Speed of Light in MPH?

Definition: The speed of light in a vacuum is a fundamental physical constant exactly equal to 670,616,629.3844 miles per hour.

Importance: This constant is crucial in physics, astronomy, and many engineering applications.

2. About This Value

The exact speed of light is defined as:

Which converts to:

• 670,616,629.3844 miles per hour
• 1,079,252,848.8 kilometers per hour
• 186,282.397 miles per second

Note: This is the speed in a vacuum. Light travels slower in other media like water or glass.

3. Significance in Physics

Details: The speed of light is the maximum speed at which all conventional communication and matter can travel in the universe.

4. Historical Context

History: The speed of light was first measured with reasonable accuracy in 1676 by Ole Rømer. The exact value was established in 1983 when the meter was redefined in terms of the speed of light.

5. Frequently Asked Questions (FAQ)

Q1: Why is the speed of light considered exact?
A: Since 1983, the meter has been defined as the distance light travels in 1/299,792,458 seconds, making the speed of light exact by definition.

Q2: Can anything travel faster than light?
A: According to Einstein's theory of relativity, no information or matter can travel faster than light in a vacuum.

Q3: How was this exact value determined?
A: The value comes from the internationally agreed definition where the speed of light is set exactly at 299,792,458 m/s.

Q4: Does light always travel at this speed?
A: Only in a vacuum. Light slows down when passing through materials like water (about 140,000 mph slower) or glass.

Q5: Why is this value important for GPS systems?
A: GPS relies on precise timing of light-speed signals from satellites, requiring exact knowledge of light's speed for accurate positioning.