# Measuring Light Speed - Is Light Massless?

• lneilson
In summary, light has mass, but it doesn't interact with matter as much as we think it does. The speed of light in a vacuum is 299,792.458 km/s.
lneilson
Hi

I was reading about the measurement of light years and was suprised to see it is the speed at which light goes through a vaacum. Is that because of the vaacum of space or does light have mass?

thanks

lneilson

lneilson said:
Hi

I was reading about the measurement of light years and was suprised to see it is the speed at which light goes through a vaacum. Is that because of the vaacum of space or does light have mass?

thanks

lneilson

Light is massless, yet has momentum.
The velocity of light was actually determined on the chalkboard. What do I mean by that? Well, lightspeed was determined by James Maxwell in 1856 after he divided his equation for magnetism by his equation for electricity. After Maxwell divided these two equations, he obtained a constant: 300,000/sec, which is precisely the velocity of light in vacuo.

Experiments to verify this constant have been performed to a very high degree of accuracy.

Neo_Anderson said:
300,000/sec, which is precisely the velocity of light in vacuo
Well, actually 299,792.458 km/s is precisely the value.

lneilson said:
Hi

I was reading about the measurement of light years and was suprised to see it is the speed at which light goes through a vaacum. Is that because of the vaacum of space or does light have mass?

thanks

lneilson

As Neo mentioned, light does not have mass (but it can be bent by a gravitational field!). The reason we talk about a vacuum speed of light and a speed of light in a medium is because light interacts with the medium that it passes through, and this retards its speed. Light rays are actually oscillating electric and magnetic fields. When an electric field passes through matter, it can polarize the material, which will have an effect on the field. Likewise, magnetic fields can magnetize materials, which will also have an effect on the magnetic field. Polarization and magnetization affect light rays, and change its wave speed.

Another way to explain this effect is to think about light as a particle. When photons (i.e. the fundamental quanta of light) pass through matter, they can be absorbed by atomic electrons and then reradiated. This is a quantum effect caused by the fact that atomic electrons have discretized energy levels. The process of absorption and reradiation introduces a delay, which retards the speed of light in matter.

The speed of light in matter is a property of the material through which the light is passing. The vacuum speed of light, however, is a fundamental property of the universe. In the nineteenth century, physicists did experiments on electric and magnetic fields. They discovered how electric charges interact, as well as how magnetic dipoles interact. Because light is an electromagnetic wave, the information about electric charges and magnetic dipoles can be used to deduce the speed of light, without ever calculating the speed of a light ray. The speed of light was also directly measured by using an apparatus called a Fizeau Wheel (you can Google to get the details of this experiment). Remarkably, the two methods of calculating the speed of light agree, which demonstrates that light is indeed an electromagnetic wave.

they really gave me something to think about

lneilson

## 1. What is light speed?

Light speed, also known as the speed of light, refers to the rate at which light travels through space. It is considered to be the fastest possible speed in the universe, and it is denoted by the letter "c". In a vacuum, light travels at a speed of approximately 299,792,458 meters per second.

## 2. How is light speed measured?

Light speed can be measured using various techniques and instruments, such as lasers, interferometers, and astronomical observations. One of the most accurate methods is the use of a Michelson interferometer, which measures the time it takes for light to travel a known distance and then calculates the speed based on this time and distance.

## 3. Is light massless?

According to the theory of relativity, light is considered to be massless. This means that it does not have any rest mass and only has energy and momentum. While light does have energy and properties that can be affected by gravity, it does not have any physical mass like other particles do.

## 4. How was it determined that light is massless?

The concept of light being massless was first proposed by theoretical physicist James Clerk Maxwell in the 19th century, based on his equations that described the behavior of electromagnetic radiation. This was further supported by experiments such as the Michelson-Morley experiment and Einstein's theory of relativity, which showed that light does not have any physical mass.

## 5. Can light speed be exceeded?

According to our current understanding of physics, it is not possible for any object or particle, including light, to exceed the speed of light. This is because as an object approaches the speed of light, its mass increases and it requires an infinite amount of energy to accelerate any further. This is known as the theory of special relativity.

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