Currently I'm 17 years old, going to secondary school. So, my ideas might be totally wrong…
I know that everything is relative. In the example of speed, the earth moves, and the galaxy moves, etc.
My physics teacher told me that the speed of light is absolute, which means that the speed of the light source doesn't influence the speed of light in space. So, I was thinking that that fact could helps us to measure the absolute speed of our planet in space. Not relative to the sun, or the galaxy.
The way of measuring it was following:
A is the light emitter.
B is the light sensor, in combination with a very very precise timer.
D is the signal broadcasting point.
~~> is light, going from A to B.
A ~~~~~>~~~~~~~>~~~~>~~~~~>~~~~~~~~>~~~~~~>~~~~~~~~~> B
\__________________________D_________________________/
So, how it works — in my head — is that you send a signal from D to both A and B. The distances from A to D and from B to D are equal, so this should get the signal to both A and B in the same time. The distance between A and B is constant, say K.
As soon as the signal reaches B and A at the same moment, B starts the very precise timer and waits for the light coming from A, at the same time, A starts emitting light.
According to my knowledge, you should be able to calculate the speed of the whole situation along the axis A,B. Why? Because if our absolute speed is along with the light direction, it should take longer for the light to reach B, because the distance is bigger. Otherwise, we are moving in the opposite direction of the emitted light, so we are going towards the light, so, we make the distance for the light to travel shorter, because we are concede towards the light.
Compare it with a car (C) driving on the highway, next to a high speed train (T). The train goes faster than the car.
Compare both situations:
1) Train and car moving in the same direction, train starts behind the car.
T ---------------->
C ------>
2) Train and car moving in opposite direction towards each other.
T ---------------->
<------- C
In Situation 1, it will take longer for the car and train to meet.
In Situation 2, it will be pretty quick that they meet, because they going towards each other.
It is that difference in time that can be used to calculate our absolute speed, I think.
To define our absolute velocity vector, we can do this measurements three times, each test perpendicular to the two others, so we can apply Pythagoras to get our absolute speed as a scalar.
My teacher could hardly believe that it would work, so he thought that something should be wrong to my theory. What do you think, assuming that we have very precise measuring tools?
Best Answer
Your experiment will measure the speed of light to be the same no matter what direction you do you it in. When we say the speed of light is constant we mean that every local experiment to measure the speed of light will find the same value. Even if we put our apparatus into a rocket and fire it off at 0.999c the experiment will still measure the same speed of light as it did when it was sitting on the Earth.
From a common sense perspective this seems silly. How can the measured speed of light be unaffected by the motion of the experiment? I don't know of any intuitive way to explain this, but it's a premise of special relativity, and all the weird effects like time dilation can be explained from the constancy of the speed of light.
I think the best way to explain the constancy of the speed of light is to explain it as a geometirical property of the universe. I went into some detail about this in the answer to Special Relativity Second Postulate. You might be interested to have a look at this.