[Europan Ocean (OP)]

In space we can determine the rate at which the Earth travels, relative to the sun for example. And how fast we move determines how slowly we move through time. But how can we for example blast an object to perfect stillness, not relative to the sun... just perfect motionlessness?

Is the speed of light relative to the object it leaves or something else?

[lightarrow]

In space we can determine the rate at which the Earth travels, relative to the sun for example. And how fast we move determines how slowly we move through time. But how can we for example blast an object to perfect stillness, not relative to the sun... just perfect motionlessness?

Try to define the concept of velocity and you have the answer. Ok, since I don't know your knowledge, I will help you: when you want to find a car's velocity, you fix two points, A and B on the road and you measure the time it takes the car to go (straight) from A to B.
Now what happens if the road is on an aircraft carrier which is moving fast on the sea ? Which car's velocity are you measuring? The velocity with respect to the carrier or with respect to the sea, or to the Sun or to the absolute space or what?

Is the speed of light relative to the object it leaves or something else?

Light's velocity is relative to your frame of reference, because  velocity is a vector; light's speed results to be the same in every frame and respect to every source of light (moving or not).

[Pmb]

In space we can determine the rate at which the Earth travels, relative to the sun for example. And how fast we move determines how slowly we move through time. But how can we for example blast an object to perfect stillness, not relative to the sun... just perfect motionlessness?

Is the speed of light relative to the object it leaves or something else?

Before Einstein published his theory of relativity scientists used to think that there was an absolute rest frame of reference, i.e. a frame of reference which was at rest. Relativity changed all that. So whenever someone speaks of motion it's always with respect to something. Therefore speaking of absolute rest is meaningless, at least according to the theory of relativity.

The speed of light is always measured with respect to something which is considered at rest, at least in principle. However its measured value is independant of the speed of that which its motion is being measured against. If you think that's hard to understand then you'll understand why it took a genius like Einstein to figure it out.

[Ethos_]

Another interesting thought exercise is this:

Imagine your in a space craft somewhere in deep space. Now imagine there are absolutely no stars, planets or even the smallest particle of matter to establish relative motion against. Now, fire your rockets for a brief period and then shut them down.

While you are firing them, you will experience the effect of the acceleration as it presses you back in your navigators chair. But once you shut down the rockets, this acceleration will dissipate and you will no longer feel it.

We assume that your craft will continue to move in the opposite direction that your rockets were fired, however, with nothing to gage your movement by, how can you determine your velocity? The truth is, you can't.

Without another object to gage your motion against, there would be no evidence proving that you were moving at all. Even though the record of your past acceleration suggests that you are still moving, that is not sufficient evidence to prove it.

The only conclusion one can draw from this thought exercise is: Any and all degrees of motion can only be gaged relative to another observed object.

[Europan Ocean (OP)]

So if by chance as is possible, two stars pass each other, with great difference in speed, one going at 90% light speed and the other at 1% light speed, relative to a large body like Andromeda, light emitted in the same direction from the fast one is going faster than from the slower star?

[Bill S]

 E O,  this is how I sorted it out for myself when I was struggling with this sort of thing.  Hope it helps.  You may think of a clever way round the relative speed limit of a little under 300,000 kps.  Suppose you had access to an enormously long space craft that was capable of travelling at ninety percent of the speed of light.  Inside this, you have a small craft which is capable of twenty percent of light speed.  Surely, all you need to do is fly both of these craft, one within the other, at their maximum speeds and you will be exceeding the speed of light relative to the Earth.  It seems logical, but what we have to remember is that although the speed we are using for the big craft is its speed relative to the Earth, the speed of the small craft is its speed relative to the big craft.  Does this make any difference?  If I am on a train travelling at sixty miles per hour, and I run, in the same direction at ten miles per hour, then my speed relative to the track must be seventy miles per hour. So why would the same reasoning not work for space craft?  The truth is that the same reasoning does work for both; it is the earthbound example that is wrong, but because the speeds involved are so small in comparison to the speed of light, the straightforward addition we used is so close it makes no real difference.  We have no instruments capable of detecting so small a difference.  However, when we are dealing with speeds that are appreciable fractions of the speed of light, the difference becomes significant and we have to use the relativistic velocity addition formula.  The formula is expressed in the following equation:         
                                 v₁+ v₂ 
                      v' = ----------------------------------------
                                1 + (v₁+v₂)

In this equation the symbol v' (v prime) is the speed of the small craft relative to the Earth.
V₁ is the speed of the big craft relative to the Earth.  (90%c)
V₂ is the speed of the small craft relative to the big craft.  (20%c)

All speeds are expressed as fractions of the speed of light, so inserting the relevant values into the above equation, (which I'm not going to do because I had enough trouble with the other one)  gives us:   0.93 or 93% of the speed of light.

   

[dlorde]

So if by chance as is possible, two stars pass each other, with great difference in speed, one going at 90% light speed and the other at 1% light speed, relative to a large body like Andromeda, light emitted in the same direction from the fast one is going faster than from the slower star?

Turns out that wherever you measure it from, you get the same speed for light, whether it comes from the relatively fast star or the relatively slow star. But assuming they both are the same type of star and radiate the same spectrum, although the speed will measure the same, the frequencies of the light from each will look different - redder if the source is moving away from you, and bluer if it's coming towards you. The faster the speed of the source, the more pronounced the frequency shift.

Einstein's explanation for the constant speed of light regardless of source speed is that space and time themselves are distorted along the direction of travel.

[yor_on]

So if by chance as is possible, two stars pass each other, with great difference in speed, one going at 90% light speed and the other at 1% light speed, relative to a large body like Andromeda, light emitted in the same direction from the fast one is going faster than from the slower star?

No, all uniformly moving frames of reference can be considered 'at rest', macroscopically defined. You are at rest with Earth. Earth is in a way 'at rest' with its solar system, although orbiting the Sun. Easiest to think of it is defining 'at rest' as being still, and this property belongs to all uniform motion, no matter its speed. And light measured in a two way experiment will always give you 'c', no matter from what planet you measure, or its 'speed' as defined relative something. What happens though is that different uniform speeds should give you different definitions of a 'energy' if you measure both planets relative the rays from some, for both, fixed star in front of you. Also assuming that both are moving in this stars direction, loosely speaking.

So you can find the rays energy to change depending on your uniform motion, which then can be seen as a proof of there existing different uniform motions. But the speed will still be a constant, locally measured and globally, comparing.
=

Oops, sorry Dlorde. Should have looked.
Nice explanation.

(One way to think of it is asking yourself how you will prove a speed? Relative something else, isn't it? And there we have 'relative motion' to use, as defined relative mass, as you walking, relative your room. But light is treating all uniform motion as 'being still', when it comes to defining a speed, although it will admit to it existing by changing energy, but only as relative other objects of mass, as a sun you find to be in 'relative motion', relative your local definitions.)

[Europan Ocean (OP)]

So a light wave coming from a fast moving object, moving towards you, can have a wave that is slow relative to the source but fast as it hits your eyes, so it may become to you a gamma ray?

[Bill S]

So a light wave coming from a fast moving object, moving towards you, can have a wave that is slow relative to the source but fast as it hits your eyes

No. You, and an observer moving with the source, will always measure the same speed for the light. Only the wavelength will be observed as different.

Look again at the three posts before your last one.

[Europan Ocean (OP)]

I meant to ask if the wave is faster? So I will check above.

[yor_on]

if you hit a car that's coming at you from the opposite direction, a front to front collision, there will be more energy released, than if you hit it, going in the same direction as you, front to bumper. The same goes in space for light. And it doesn't matter who you define to be moving, neither on earth or up there. So light change energy with 'relative motion'. When it comes to why the speed of light, as defined from all inertially moving (uniform motion) objects, always are 'c' (locally measured)?

You can define it such as it always should be 'c', when you're not moving. Then define it such as all relative motion is being 'still'. Using a thought experiment in where you try to find a way to define a uniform motion in a, except your object, totally empty space. How can you prove a motion there?

You can prove a acceleration, but not a uniform motion.
And that is a proof.

[yor_on]

And well, it has been tested over the years too, experimentally.

[simplified]

Space has gravitational objects.Your motion relatively of the gravitational objects slows down your time.Your motion relatively of  tiny object considerably does not slow down your clock.Any exact experiment proves it and disproves Eistein's nonsense.