The Strange Result of Light Speed being a Constant

by | Dec 8, 2017


The most famous equation in history is:  E=MC2   which defines the relationship between mass and energy. The “c” is a constant and is the speed of light (186,000 miles per second).*  This was the basis of Einstein’s relativity discovery – that the speed of light is a constant – and this gave rise to his theory of special relativity in 1905 (followed 10 years later with his theory of general relativity). What does it mean for speed of light to be constant? A mind boggling example below.

One aspect of special relativity answers the question of what would happen if you were traveling in a vehicle very fast – assume 100,000 miles per second – and turned on your headlights. How fast would the light of your headlights travel?

Let’s first consider something other than light emitted from your fast traveling vehicle.  Suppose you shoot a gun out your front windshield while traveling 100,000 miles per second and assume the bullet travels at 1 mile per second.** The observed speed of the bullet according to a stationary observer would be 100,001mps but to people inside your your car, the bullet will appear to be traveling away from you at the 1 mile per second (which it is).

If you think about it – that makes sense.

Now assume again that your vehicle is traveling at 100,000 miles per second and you put away your gun with bullets and instead shoot a laser gun (lasers are light and travel at 186,000 mps). How fast will the laser be traveling? Based on the bullet example above it would seem that a stationary person would observe the laser at 286,000 miles per second (100,000 mps + 186,000 mps) and that the passengers in the car would observe the laser shooting at 186,000 mps.

But that’s not the answer. What happens is that laser will travel at 186,000 mps relative to a stationary observer AND willshoot from the vehicle at 186,000 mps relative to the vehicle occupants. Wow. Mind blown.

This is of course is true even if you accelerate your vehicle to 99% of the speed of light.***  The laser will still be moving away from you at 186,000mps even though you are moving at 184,000mps. To a stationary observer the laser will appear to be moving 186,000mps and your vehicle will appear to be traveling 184,000 mps. That is what is meant by the fact that the speed of light is a constant – light does not travel at a different speeds even to observers traveling at relative different speeds. Further, whether a source of light is moving towards you or away from you, the light still travels at a steady 186,000mps completely contrary to common sense.

*A clarifying note – “c” is not actually the speed of light. Instead, think of it as the universe’s speed limit. Light and other electromagnetic energy can travel at this speed and no faster.  That is for travel in a vacuum.  It is possible for light to travel at less than less than “c” through mediums such as water and glass and a recent experiment at the University of Glasgow researchers were able to slow the movement of photons below “c” in free space.

**Note that bullets generally travel at around 2,500 feet per second, which is a bit less than 1/2 mile per second. The fastest high-speed bullet rounds can reach 4,000 feet per second. The 1 mile per second was used for clarity.

***What about travel at the speed of light? Or faster? Here’s a link to a previous IFOD on the speed of light and whether travel faster than the speed of light is possible:


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