Existence

But if I travel towards you at 30mph I will measure your car moving at 90mph relative to me (if you started 90 miles away, it would only take you an hour to reach me). This cannot be said of light. If I travel towards light at 300 m/s I measure light traveling the same speed relative to me as when I travel away from it at the same speed.The speed of your car is relative, the speed of light isn't, it's constant. That doesn't mean it won't seem to slow down in water or air, it means it doesn't vary between observers.

Yes. If you like. If you travel towards light at 300m/s you won't measure the distance between you decreasing at a rate of 299 792 758 m/s but the distance will decrease at 299 792 458 metres a second. The same applies if you were moving away from the light - it would still be gaining ground on you at the same rate. Relative to the ground, but not relative to me. Relative to me you are approaching at 90mph. It seems to be a fact about light, I don't know why. It would be gaining on you the same amount as when you were traveling towards it at 300m/s which in turn is the same amount as when you aren't traveling away or towards it. It seems to be constant no matter what your speed. This has been observed. Then we have directly observed magic. If 'It will take the light 300 seconds to travel the 55,800,000 miles' then it stands to reason it will take a little longer than 300 seconds to travel a little over 55,890,000 miles. What were you trying to illustrate with this maths?

Not according to actual observation. What do we open minded people do do when reality is different in observation than it is in our intuition? I already commented on the maths. What did you think it proves?Show me where the observations are wrong.Show me where the observations are wrong.Edited by Modulous, : No reason given.

You are doing 60mph relative to the ground, which is the standard reference point, I agree. But you also agree that this would be 90mph relative to me. That's all that matters. When someone says the speed of light is constant they are suggesting that the same thing does not go for the beam of light. This is consistent with our observations. Your version is not. I'll leave it for you to decide how to deal with that.As long as you understand that the constancy of the speed of light is not about its transmission through various media such as interstellar matter, but rather it is meant in the sense that it is not like the car example. Remember that speed is just an expression of distance traveled over some time. One possibility is that the speed of light varies like that speed of cars varies depending on what reference point you are measuring distance and time from. Another possibility is that the speed of light is constant but time or distances vary. The one where the speed of light is constant while the other components of that speed vary has evidential support, and even makes specific quantitative predictions which have been observed to be accurate within the measuring capacities of our equipment.I won't even attempt to persuade you which version you should pick.

You perform the experiment. You choose a distance, d, and measure how long it takes light approaching from behind you to travel that distance, t. You divide the distance by the time and it comes out as c. We get the same measurement for the speed of light relative to us when we travel towards it. This is unlike with cars. You are perfectly justified in having difficulty with this, but that's what we observe and what is meant by the constancy of the speed of light.There is a solution, and it is not intuitive.

So your refutation of the constancy of the speed of light is to measure the speed of light at several points during the experiment and calculating that the speed of light is the same at each point?

Yes, that's what you were doing, it doesn't bode well that you didn't notice. I suggest that if you move towards light at 0.5c you will measure oncoming light as moving at c relative to you and not 1.5c as we might expect as in the example of cars. If you are now conceding that we will observe the light passing us at c then, I challenge you to make the maths work so that c is the same for all observors without changing the rate of time passage for someone. I agree that if you measure the time it takes for light to travel any given distance you will always get c, regardless of your direction and magnitude of your speed, unlike with any other thing in the universe.What happens if I set up a bi-directional laser on a one light year long train travelling at 0.5c, and put light sensors at either end of the train and then turn the laser on?If I am on the train, both sensors come on at the same time, since I measure the laser travels 0.5ly forwards and backwards taking half a year to do so: both light sensors will come on in half a year and I will notice it in one year. The speed I calculate light to be travelling relative to me is c.If I am standing still relative to the train, I will see the rear sensor light come on first since the distance between the sensor and the light decreases as the rear of the train 'catches up' to the light. Likewise the front sensors are moving away from the place where the light was emitted and so it takes longer than half a year for the light to get there. The speed I calculate light to be travelling relative to me is is c.This is the constancy of the speed of light and the consequences thereof. If you think otherwise, you are indeed 'trying to refute the constancy of the speed of light '

When we say the speed of light is constant we mean that if you are on the train both light sensors come on at the same time. We measure the distance the light has travelled relative to us (half a light year) the time it takes (half a year) and determine relative speed to be c.The person on the platform also determines the distance the light has travelled relative to them and for all the reasons you just they get different relative distances but the same relative speed, cIf we did the experiment with bullets, the person on the platform would get different relative speeds for the bullets than the person on the train. The speed of bullets (even bullets that don't slow down (fired in a vacuum)) is not constant in the same way the speed of light is.Edited by Modulous, : No reason given.Edited by Modulous, : No reason given.