Speed of light

[Oldboy913]

How can the speed of light be constant when time changes dependent on speed?

[vascop]

That's true for things moving at LESS than the speed of light - in other words everything except light.

[jake-the-peg]

One of the things often not well understood is that the fact that the speed of light is constand is not the result of the Special theory of relativity.

The observation that the speed of light is constant is what lead to the theory of relativity.

It is as the name suggests relative.

If I measure the distance to the nearest star as 4 light years then hop in a spacecraft and travel there at close to the speed of light time slows for me and I get there in what is for me a few days.

However because of time dilation the "stationary" observer back on Earth sees me getting there in 4 years.

You can't calculate speed by measuring the distance for one observer in one frame of reference and dividing by the time observed by a different observer in a different frame of reference

[vascop]

Where are you old boy?

[Berkshirelad]

The speed of light is not constant - it entirely depends on the medium through which it is passing.

In a vacuum, the speed of light is a constant.

[Davethedog]

Time doesn't exist as minutes and hours. Time as we use it and understand it is a man made conceit, isn't it? I'm not stating this as a fact and am prepared to be shot down and/or educated.

We base it on the time it takes our planet to revolve on its axis, it is of course a needed concept for many variables.

[beso]

The speed of light is constant. When time changes due to relative motion, so does the perception of distance, mass and kinetic energy.

Only the momentum of a moving object is independent of perception.

[mibn2cweus]

The speed of light (in a vacuum) is so constant you can calibrate the rate at which your clock measures the passage of time by it. Although the time indicated by your clock might not agree with that shown on the clock of a stationary observer, both clocks so calibrated in their respective laboratories would be found upon comparison to tick at precisely the same rate.

[Oldboy913]

VASCOP I'm sorry that I have taken so long to answer but I am 79 and I am putting my life story up on the Internet, and as you can imagine there is quite a lot of it.
I actually live in Weston super Mare in Somerset UK
Try www.Story of my life.com my initials are J. E.

[Oldboy913]

VASCOP sorry Try www.Storyofmylife.com my initials are J. E.

[vascop]

When it is stated that the speed of light is constant, this is a very loose way of making the full more precise statement, which is:
"The velocity of light in empty space is the same for all observers in uniform relative motion, its value being independent of the velocity of its source."
Relativity Physics by W. H. McCrea 1960