# Light Constant: Speed, Increase, Decrease?

• starstruck
In summary: So light is still special in that way.In summary, the speed of light is a constant value that has been observed to be the same for all unaccelerated observers, regardless of their relative speeds. This was a paradoxical concept until Einstein's theory of relativity, which rejects the absolute notions of time, distance, and simultaneity in order to account for the constant speed of light. This theory allows for adjustments in these concepts, making light a constant and unchanging factor. However, the unique property of light being unable to measure its one-way direction speed still remains.

#### starstruck

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I'm so new to this. So I would like to know what is meant when it is said that light is a constant. Is it the speed they are talking about.is it that light never increases speed or decreases speed? Does the speed never change? Thank you

No. The speed changes with the density of the medium, gives us rainbows. The speed is constant to all observers. Run, shine a flashlight ahead and the speed is the same. Run, shine a flashlight behind and the speed is the same.

starstruck said:
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I'm so new to this. So I would like to know what is meant when it is said that light is a constant. Is it the speed they are talking about.is it that light never increases speed or decreases speed? Does the speed never change? Thank you
The speed of light is assumed to be in vacuum and assumed to be with regard to equipment that is inertial (not accelerating). If you wanted to measure the value of the speed of light, you would have to set up a round-trip experiment with a clock or timer at one end and a mirror some measured distance away. People who have done this in the past have always gotten the same value so now scientists have defined that value to be an exact value and they use it to calibrate their distance device.

Einstein's second postulate is not about the constant value of the speed of light. Instead, it is with regard to the one-way speed of light which cannot be measured. It assumes that the light takes the same amount of time to reach the mirror as the reflection takes to get back to the source of light.

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starstruck
As a non-expert, I'll give it a try in simple language to provide something to work with, and gladly entertain any and all corrections...

All unaccelerated observers despite moving at various relative speeds among themselves always measure the speed of light* to be the same.. that is the starting observational thing to be accounted for. The first problem is that if you assume that time, distance, and simultaneity are absolute everywhere for all these observers then this does not make sense, seems paradoxical, and one must come up with some adjustments that would invent some very strange properties of light to account for it.

Relativity takes the observations as fundamental and the approach that the speed of light is assumed to be a constant as has always been observed... and then accounts for this by taking away the assumption of the absoluteness of time, distance, and simultaneity. This is revolutionary, but it ensures that all observations of light speed are constant and shifts the needed adjustments away from light and into the time, distance, and simultaneity calculations, with equations and concepts of time dilation, length contraction, and disagreement on simultaneity when one observer is accounting for what is happening to another.

Relativity adjusts these formerly absolute concepts in order for light to be assumed constant speed as observed. The nature of these adjustments conspire perfectly to make all light speed measures consistent for all observers, and amazingly the effects of these adjustments become transparent and unmeasurable to the one apparently subject to them when being measured and observed by another. This works so well that two observers will each observe and measure these adjustments in the other, but not in themselves.

This whole approach was based on a very careful step by step analysis of exactly what it means to measure time, distance, and simultaneity using light and the assumption that light speed is always constant.

* Light remains a peculiar thing in that one can't measure the one-way direction speed of light; the speed is always calculated from a two-way direction measurement (in which the speed in both directions is assumed to be the same).

for clarifying.

I can clarify that when we say light is a constant, we mean that the speed of light is always the same. This is known as the speed of light in a vacuum, and it is approximately 299,792,458 meters per second. This speed is a fundamental constant in physics and has been measured and confirmed through various experiments. It is not possible for light to increase or decrease its speed, as it is always traveling at this constant rate. However, the speed of light can change when it passes through different mediums, such as air or water, which can affect its path and wavelength. But in a vacuum, the speed of light remains constant. I hope this helps to clarify any confusion.

## 1. What is the speed of light?

The speed of light is a fundamental physical constant that is approximately 299,792,458 meters per second in a vacuum. It is denoted by the letter 'c' and is used as a universal constant in many equations in physics.

## 2. Can the speed of light increase or decrease?

No, the speed of light is considered to be constant in a vacuum and cannot be increased or decreased. However, it can be slowed down when passing through certain materials such as water or glass.

## 3. How was the speed of light determined?

The speed of light was first measured by the Danish astronomer Ole Rømer in the late 17th century using observations of the moons of Jupiter. Since then, it has been measured using various methods including the use of lasers and high-precision clocks.

## 4. Does the speed of light vary in different parts of the universe?

According to the theory of relativity, the speed of light is the same in all inertial reference frames. This means that it is constant throughout the universe, regardless of the observer's location or motion.

## 5. How does the speed of light affect our daily lives?

The speed of light plays a crucial role in our daily lives, as it is the basis for our understanding of time, distance, and the laws of physics. It also allows us to communicate through technology such as fiber optic cables and satellite communication. Additionally, it is used in many scientific and technological advancements, including medical imaging and space exploration.