The discussion centers on the measurement of the speed of light in relation to moving observers, specifically in the context of Einstein's theory of special relativity. Participants clarify that the speed of light remains constant at approximately 299,792 kilometers per second (c) regardless of the observer's motion. They emphasize the importance of understanding the second postulate of relativity, which states that light's speed is invariant across all inertial frames. Additionally, the relativity of simultaneity is highlighted as a crucial concept for comprehending these measurements.
PREREQUISITESStudents of physics, educators teaching relativity, and anyone interested in the foundational principles of modern physics will benefit from this discussion.
The answer to your question is the second postulate if relativity. Can you quote it?CHAIM123 said:is the speed measured constant even then?
This is half of the story, which may be confusing you. You also need to take into account the relativity of simultaneity.CHAIM123 said:if I move at half the speed of light then time is slower, so I can't measure the speed of light as higher.
Thank you !!!Ibix said:This is half of the story, which may be confusing you. You also need to take into account the relativity of simultaneity.
There is no such thing as "moving at half the speed of light". All motion is relative. You must be moving at half the speed of light relative to something. There is also no such thing as "time running slower".CHAIM123 said:If I'm not understood it's because I have to translate into English. I'm sorry.
I learned that the speed of light is always constant, and if I move at half the speed of light then time is slower, so I can't measure the speed of light as higher. The question is in relation to the light coming from a light source from which I move away, is the speed measured constant even then?
I apologize for my ignorance... I'm barely a beginner...
Isn't time slower in a body that is in fast motion or when the density of matter in its environment is high?PeroK said:There is no such thing as "moving at half the speed of light". All motion is relative. You must be moving at half the speed of light relative to something. There is also no such thing as "time running slower".
Consider these two scenarios, which are physically equivalent:
You are on Earth and a spacecraft is moving towards the Earth at half the speed of light. Note that this means the Earth is moving towards the spacecraft at half the speed of light.
You emit a light signal towards the spacecraft. Both you and the spacecraft measure the speed of that light signal as ##c##.
The spacecraft emits a light signal towards Earth. Both you and the spacecraft measure the speed of that light signal as ##c##.
In relativity, there is no physical difference between the Earth and spacecraft in this scenario. There is no sense in which the spacecraft is "really" moving or the Earth is "really" moving. The motion is relative.
No. If you are in inertial motion moving fast relative to me then in my rest frame I will measure your clocks to be ticking slowly. But you will measure my clocks to be ticking slowly. So "time runs slowly" is an inadequate description.CHAIM123 said:Isn't time slower in a body that is in fast motion
"Time runs slowly" is always a terrible description. But with some very large caveats, it's less terrible in this context. Clocks at lower gravitational potential will tick slower than those at high potential.CHAIM123 said:or when the density of matter in its environment is high?
There is no such thing as "a body in fast motion".CHAIM123 said:Isn't time slower in a body that is in fast motion
It's best to leave gravitation (general relativity) until you understand special relativity (no gravity).CHAIM123 said:or when the density of matter in its environment is high?
Yes. In your rest-frame, the source is moving away from you.CHAIM123 said:The question is in relation to the light coming from a light source from which I move away, is the speed measured constant even then?
Source:Wikipedia said:1. First postulate (principle of relativity)
The laws of physics take the same form in all inertial frames of reference.
2. Second postulate (invariance of c)
As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body. Or: the speed of light in free space has the same value c in all inertial frames of reference.
It's dependent on the speed of the medium, though, so it isn't frame invariant and doesn't lead to relativity.cianfa72 said:Sorry, what if we take the constancy of the speed of the sound as second postulate of special relativity ? As far I know the speed of the sound is independent from the motion of the source as well.
So consider the following scenario: let's take an inertial frame in which the medium is at rest and source in moving with a given velocity. The measured speed of sound in this inertial frame will be let me say ##s## regardless of the motion of the source.Ibix said:It's dependent on the speed of the medium, though, so it isn't frame invariant and doesn't lead to relativity.