# Why should the speed of light be the same in every frame of reference.

• Lizwi
In summary, the speed of light is the same to all observers, but it depends on the relative motion of the observer and the object being observed.
Lizwi
If we have light, any particle, and maybe car, the particles travel at 0.98c and car let say 50m/s. Let them start moving at the same time to the given point let say 300 metres away. The difference between times of arrival of light and particle will be small compared to that between light and car. This means that light and particles spend more time together than the time light spend with car, hence the speed of light relative to particle is not the same as speed of light relative to car.

I have no idea what you could mean by "spend more time together".

I suspect your error is that you are not taking into account the Lorentz contraction of time for the particles as seen from the car's frame of reference.

Lizwi said:
If we have light, any particle, and maybe car, the particles travel at 0.98c and car let say 50m/s. Let them start moving at the same time to the given point let say 300 metres away. The difference between times of arrival of light and particle will be small compared to that between light and car. This means that light and particles spend more time together than the time light spend with car, hence the speed of light relative to particle is not the same as speed of light relative to car.
You are implicitly assuming a Newtonian universe here, where length and time are absolute.

That the speed of light is experimentally the same to all observers was a big clue that this Newtonian view is not how the universe works, at least not at very high speeds. And indeed, length and time are not absolute.

Hi HallsofIvy: By saying they spend more time together I mean it take some time for light to leave these particles back if they are traveling at may 0.9c.

Lizwi said:
If we have light, any particle, and maybe car, the particles travel at 0.98c and car let say 50m/s. Let them start moving at the same time to the given point let say 300 metres away. The difference between times of arrival of light and particle will be small compared to that between light and car. This means that light and particles spend more time together than the time light spend with car, hence the speed of light relative to particle is not the same as speed of light relative to car.

If you're sitting in the moving car, and you measure the speed of light, and if your sitting on the particle and you measure the speed of light, you will get the same speed.

This is because time dilates and length contracts in the moving frame of reference WHEN MEASURED BY A FRAME AT REST. When measured from inside the moving frame, time and length are normal. These factors mean that the speed of light is always measured to be the same in all frames, regardless of their relative motion.

BTW, the light and the particle are only together at the instant they start. The light and the particle spend no time together because the light still moves at c, as measured from the particle.

## 1. Why is the speed of light considered a constant in every frame of reference?

The speed of light is considered a constant in every frame of reference because it is one of the fundamental constants of the universe. It was first measured by the famous experiments conducted by Albert Michelson and Edward Morley in the late 19th century. Subsequent experiments have confirmed that the speed of light is indeed constant, regardless of the observer's frame of reference.

## 2. How does the theory of relativity explain the constancy of the speed of light?

The theory of relativity, proposed by Albert Einstein, explains the constancy of the speed of light through the concept of space-time. According to this theory, space and time are not absolute, but are relative to the observer's frame of reference. The speed of light is the only constant in this relative space-time, and it remains the same regardless of the observer's perspective.

## 3. What is the significance of the constancy of the speed of light in modern physics?

The constancy of the speed of light has significant implications in modern physics. It is a crucial component of Einstein's theory of relativity, which forms the basis of our understanding of the universe. It also plays a fundamental role in quantum mechanics and other branches of physics, and is used in many important equations and theories, such as the famous E=mc^2 equation.

## 4. Can the speed of light ever be exceeded?

Based on our current understanding of physics, the speed of light cannot be exceeded. This is because as an object approaches the speed of light, its mass increases, and it requires an infinite amount of energy to reach the speed of light. Additionally, as an object approaches the speed of light, time dilation occurs, making it impossible to exceed the speed of light.

## 5. How does the constancy of the speed of light impact our perception of time and space?

The constancy of the speed of light has significant implications for our perception of time and space. It means that time and space are not absolute, but are relative to an observer's frame of reference. This concept is known as time dilation and length contraction, where time and space appear to be different for observers in different frames of reference. This has been confirmed through various experiments and is a crucial aspect of Einstein's theory of relativity.

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