# Exploring the Constant Speed of Light in a Moving Spacecraft

• JKaufinger
In summary, when people say that light will always be measured at a constant speed, no matter where you are, they mean that it will always have the same speed regardless of the observer's speed. This principle is known as the speed of light being constant in all inertial frames, as explained by Einstein's theory of relativity. This means that the speed of light cannot be used to measure the speed of an observer, and as an object approaches the speed of light, it experiences time-dilation and length-contraction. The Doppler effect also plays a role in high-speed situations, shifting the light spectrum but not affecting the speed of light itself.
JKaufinger
I know there is an answer to this somewhere, so please forgive me for not being able to see it :)

When people say that light will always be measured at a constant speed, no matter where you are, what do they mean?

Say we are in a spaceship which is standing still. Inside the spaceship, we throw a ball at 5 m/s. Now the spaceship goes to a fast speed. We throw the ball, and inside the ship, we still see it as 5 m/s.

So are they saying that if you are in the fast-moving spaceship, you will measure light the same as in the standing still space ship, just as you would the ball? That doesn't make any sense, because that would imply that light, like the ball, is affected by the inertia of the ship. It also implies that if the ship is moving at 100 m/s and the ball is 5 m/s from inside the ship, then to the outside, the ball is 105 m/s. Thus, if the light was also affected, then that means that the light would be going at c+5 m/s to the outside observer.

You see where I have this confusion?

Thanks.

JKaufinger said:
When people say that light will always be measured at a constant speed, no matter where you are, what do they mean?
You can never know what speed you are moving at if you can't see some background object to measure against. So by this principle you can't use a measurement of the speed of light to tell your speed - whatever speed you are doing you will measure the same speed for light

It also implies that if the ship is moving at 100 m/s and the ball is 5 m/s from inside the ship, then to the outside, the ball is 105 m/s.
No it doesn't - speeds don't actually add like that. At low speeds they almost do so the speed would be something like 104.99999999 m/s. As things get closer to the speed of light this effect is bigger.
At the speed of light the speed is the same measured by any observer.

In layman's terms, you can't apply conventional physics (Newtonian) to speed of light or anything that approaches such high speed. Einstein showed that speed of light (c) is constant and as you approach this limit, time-dilation and length-contraction get introduced. If an object were to reach the speed of light, which is impossible by the way (anything with mass can't), then in theory, it would have zero-length in the direction it's heading, gain infinite amount of mass, and literally be turned into pure infinite energy.

We are so used to relatively slow speeds in our everyday-experiences that we tend to automatically think in terms of additive velocities. What happens instead in high-speed situations is that "Doppler-Effect" shifts the light spectrum to lower frequency when the object is moving away from the observer (and vice-versa) while the speed of light remains constant. That is as simple as I can explain it.

## 1. What is the constant speed of light?

The constant speed of light, denoted by the symbol c, is the speed at which electromagnetic radiation (such as light) travels in a vacuum. This speed is approximately 299,792,458 meters per second.

## 2. How does the speed of light relate to the theory of relativity?

The theory of relativity, developed by Albert Einstein, states that the laws of physics are the same for all observers, regardless of their relative motion. This includes the speed of light, which remains constant for all observers and is the maximum speed at which anything can travel.

## 3. Can the speed of light change in different environments?

No, the speed of light remains constant regardless of the environment it is in. This is one of the fundamental principles of physics and has been extensively tested and confirmed by experiments.

## 4. How does the constant speed of light affect space travel?

The constant speed of light presents a challenge for space travel because as an object approaches the speed of light, it requires an infinite amount of energy to accelerate further. This makes it difficult for spacecraft to travel at high speeds, and is a key factor in the design of space missions.

## 5. How do scientists measure the speed of light in a moving spacecraft?

Scientists use a variety of methods to measure the speed of light in a moving spacecraft, including using lasers and mirrors to measure the time it takes for light to travel a known distance. These experiments have confirmed that the speed of light remains constant, regardless of the motion of the observer.

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