Question

Imagine you are riding on a yacht in the ocean and traveling at 20 mph. You then hit a golf ball at 100 mph from the deck of the yacht. You see the ball move away from you at 100mph, while a person standing on a nearby beach would observe your golf ball traveling at 120 mph (20 mph + 100 mph).

Now imagine you are aboard the Hermes spacecraft traveling at 0.1c (1/10 the speed of light) past Mars and shine a laser from the front of the ship. You would see the light traveling at c (the speed of light) away from your ship.
According to Einstein’s special relativity, how fast will a person on Mars observe the light to be traveling?


0.1c (1/10 the speed of light)

c (the speed of light)

1.1c (c+0.1c)

Answer 1

According to Einstein's special relativity, the speed of light in a vacuum is always observed to be \(c\) (the speed of light) by all observers, regardless of their relative motion. This is one of the fundamental postulates of special relativity.

In your scenario, while you are aboard the Hermes spacecraft traveling at \(0.1c\) and shine a laser from the front of the ship, you would indeed see the light traveling at speed \(c\) relative to your spacecraft.

A person on Mars, observing the event, would also measure the light traveling at speed \(c\). The relativistic addition of velocities means that you can't simply add the speeds together like in classical physics.

Hence, the correct answer is:

**c (the speed of light)**

No observer, regardless of their relative velocity, can measure the speed of light to be anything other than \(c\).