Is Light's Speed Truly Constant in All Frames of Reference?

[madhumadanan]

Dear PF i am new to this forum .

Just wanted to clear a doubt about relativity.

consider a inertial system on a light ray moving at c . If the light ray has to travel betwwen two points in space won't the observer on the light ray feel that the light has traveled almost instantly form the source to destination because of the length contraction observed . So how can light's speed be finite when it is almost instantaneous!

just tried to substitue the muon example with a light ray and got confused can anybody clarify that for me

 

[Physics Monkey]

You can't have an observer "on the light ray", this isn't allowed in relativity because light always moves (at the speed of light) in every inertial frame. A light ray doesn't have a rest frame, which is the frame an observer "on the light ray" would have to be in. Hence, no such observer can exist.

So how can light's speed be finite when it is almost instantaneous!

You answered your own question! Light is only "almost" instaneous because it moves very fast compared to our everyday experience.

Also, this is the quantum physics forum. If you have more questions about relativity, you should post in the relativity forum https://www.physicsforums.com/forumdisplay.php?f=70 . You will get a better response there.

 

[Entropia]

.

Hello and welcome to the forum! Your question is a common one when it comes to understanding the constancy of the speed of light in relativity. It may seem counterintuitive, but the speed of light being constant is one of the fundamental principles of relativity. Let me try to explain it in a simple way.

In relativity, there are two main principles: the principle of special relativity and the principle of general relativity. The principle of special relativity states that the laws of physics are the same for all observers in uniform motion, regardless of their relative velocities. This means that no matter how fast you are moving, the laws of physics will remain the same.

Now, let's apply this principle to the speed of light. Say you are on a train moving at a constant speed and you shine a flashlight in front of you. The speed of light from that flashlight will be measured as c, the speed of light in a vacuum, by both you and someone standing outside of the train. This is because the speed of light is constant for all observers, regardless of their relative velocities.

So, in your example of the light ray moving at c, the observer on the light ray will still measure the speed of light as c, even though they are also moving at that speed. This is because the laws of physics, including the speed of light, are the same for all observers in uniform motion.

As for your question about the length contraction, it is true that the observer on the light ray will see the distance between the source and destination as contracted due to their high speed. However, this does not affect the speed of light itself. The light ray will still travel at c, but the distance it has to cover will appear shorter to the observer due to the length contraction.

I hope this helps clarify the concept of the constancy of the speed of light in relativity. It may take some time to wrap your head around, but it is a fundamental principle that has been proven through countless experiments. Keep asking questions and exploring the fascinating world of relativity!