Is the brightness of light invariant?

You mean like 'light travels at 299,792,458 m/s in a vacuum'?

 

Log in or Sign up to hide all adverts.

Back to basics!

A ship accelerates, changing its velocity by 20m/s.
What is its current velocity?

The answer, of course, depends of what the ship's velocity was to being with... and that velocity depends on your point of view.

If we take the point of view that the ship was at rest before accelerating, then the ship is now moving at 20m/s.

If we take the point of view that the ship is at rest after accelerating, then the ship was previously moving at 20m/s.

Which point of view is right? Both, or neither! There is no single right point of view, as far as we can tell.

Which point of view is better? Well, better for what? One point of view will be better for some problems, the other point of view will be better for others.

 

Log in or Sign up to hide all adverts.

Even that has no intrinsic meaning. To be complete, it should be "light travels at 299,792,458 m/s in a vacuum relative to all inertial reference frames."

It is not true in an accelerating reference frame, and it doesn't help us to find an absolute inertial frame, because it's true for all inertial frames.

 

Log in or Sign up to hide all adverts.

According to the captain's 'rest' frame, its current velocity is still '0' after accelerating.

What was the star's velocity to begin with? Why is it better for the captain to assume the star changed velocity after he feels the acceleration of his engines?

 

20m/s relative to a stationary star.

It depends on the only POV available and that is the ships captain.
There is no other POV he can rely upon, he is the only observer in this equation. He was stationary with the star and now after he has made a change he is no longer stationary with the star.

Correct according to the only observer we have. Certainly he cannot say he is at rest after he accelerated can he, a contradiction would be present.."Hey I accellerated to a position of rest...hmmmmm...."

Nope he would have to de-accellerate. And on top of that he would have to declare that he was actually at velocity to begin with other wise why would he change his velocity if he was already at rest. [Iknow that this is confusing the use of the words accellerate and de-accellerate etc.....]

The only time he can consider himself actually at rest is when he has zero relative velocity with the star.....hmmmm..now there's something new...rest is a relative thing.

Oh but we can because it was the ship that provided the changes and not the star.

The one that is true to what actually is happening to the observer by the observers actions.

 

Sorry for picking on you 2inq, this is really to all concerned:

Please stop using the word absolute, with or without quotation marks, when talking about motion. The concept of absolute rest is meaningless, and relativity certainly doesn't require any such notion.

Also, light doesn't just have a frequency! Light only has a frequency relative to an inertial frame , but unlike its speed, the color of a light pulse depends on how you look at it. Furthermore, one cannot move with respect to light since light doesn't have a reference frame.

 

So are you saying that we should not use relative coordinates?

If I am at rest on earth's equator (V_ned=0 m/s), and I wish to accelerate until I am moving West at a speed of the same magnatude as the equatorial velocity of earth (V_ned=465 m/s), are you saying that I should "de-accelerate" until I am at rest with the local star (Sol) even though I must fire my rocket engines in order to do so?

Would it not be easier for me, as captain, to accelerate from 0 to 465? Otherwise, how do I know whether I should hit the brakes or the gas pedal?

Please Register or Log in to view the hidden image!

 

Galilean Relativity and Special Relativity (not General Relativity) both require the observer to consider himself stationary in his 'rest' frame. Does this co-ordinate point move within the universe?

Correct, light has a wavelength, a physical property that doesn't change once the light is emitted from its source, barring gravitational effects or the accelerating expansion of the universe. Frequency is just a method for an observer to record the rate at which these wavelengths are recieved, whether his frame is an inertial frame OR a non-inertial frame. Yes, one CAN move relative to the wavelength of the emitted lightbeam. If one could not, he would never see a change in the frequency due to his motion.

 

Correct - assuming the star is stationary, of course, which it is in one particular point of view.

Any POV (reference frame) is available for consideration - it doesn't have to have an observer in it. Observers just make reference frame easier to think about.
You can use point of view we like. Use your imagination!
Imagine another star which is passing by our star at 1000m/s. In the point of view in which that star is stationary, what is the ship's velocity after accelerating?

Sure he can. Back to basics, QQ!

There is absolutely no physical difference between acceleration in one direction, and decelleration in the opposite direction.

Now you're getting it! What would be wrong with that declaration?

Very good - rest is relative. It depends on your point of view. You can always find another point of view in which you are not at rest.

But that's true in both points of view! All points of view, in fact.
In which point of view do you think that the star provided the changes?

 

Sorry Pete for my play on words.....probably not appropriate in the circumstances....

Of course there is no pont of view according to SRT that allows either the star or the ship to accellerate. Didn't someone say that SRT does not have any use when dealing with accellerations?

That it only deals with inertial non-accellerating frames or something to that effect.

 

I am not sure why our observer should worry to much about some other POV. I think he would busy enough with his own which is what?

When he acknowledges he accellerated and knows he has the velocity how does he interpret the speed of light? Does he have to use someone elses perspective?
What is wrong with his own perspective of being at relative velocity to the star and not at rest?

It is sort of like saying:
The speed of light will always be the same as long as the observer can record it as being the same.....a bit of fudge job...if you ask me.....hmmmmm

 

The ship accelerates in all points of view, the star doesn't accelerate in any point of view.

Maybe someone did... if so, they didn't know what they were talking about.

That's correct (close enough, anyway).
This doesn't stop us from dealing with the ship accelerating, it just stops us from attaching a reference frame to the accelerating ship.

Are you having fun yet?

 

or the memory of having accellerated to the "rest frame" label that is eventually applied to it.

Having fun.....hmmmmm.....maybe.....

 

Feel free to describe things however you wish in your own theories, but to claim as you did that it is the SR view is a gross misrepresentation. The First Postulate of SR is "The laws of physics (including electrodynamics) are the same in all inertial frames of reference." That no inertial frame is considered special or prefered or absolute in any way is the foundation of relativity. Any inertially moving object will have a frame in which it is at rest, known as that object's rest frame. The only thing absolute about that frame is that there is absolutely nothing special about it.

-Dale

PS If you are confused regarding this post and my posts to Kumar let me know.

 

Definitely. I am glad you understand this since it is a point that most anti-relativityists miss completely.

How can an inertially moving observer know he is at rest? What experiment can he perform to determine it? If there is such an experiment then you are completely correct and a moving observer knows he is moving and cannot assume he is at rest.

The foundation in reality is that there is no known test that can determine if an inertial observer is moving. If such a test is discovered then SR is done, but 100 years of trying has not found it. The problem with claiming that he has some absolute velocity is that you are making a choice between reference frames that nature does not appear to make. It is therefore the anti-relativityist position that lacks a foundation in reality.

I already answered this. May I ask why you are ignoring the answers you already received?

The only preposterous thing here is that you ignore all attempts at discussion and continue your ignorant monologue.

I'll give you another chance: in which inertial frame does the star accelerate when the captain uses his engines?

-Dale

 

The frame you are describing is not an inertial frame. In the frame in which the captain remains at the origin even during the acceleration, there is a frame force. This frame is experimentally distinguishable from any inertial frame by the simple fact that during the rocket burn a dropped ball will fall towards the back.

The only frames in which the star changes velocity are non-inertial frames and are therefore easily distinguished from inertial frames by the presence of their frame forces. The laws of physics are not the same in non-inertial frames!

-Dale

 

Have you never used brakes?

-Dale

 

Forgive me if I am mistaken but I was under the impression that an inertial frame is not able to accellerate. Possibly he may accelerate from an inertial position of frame but certainly an accellerating frame is no longer inertial yes?

The star can not accelerate ......physically impossible.... [ except if you wish to consider it's orbit]

How can a star accelerate? There is no observer in or on the star and no engines attached to the star.

You may think I am being deliberately awkward on this question. And in some ways I am. But you insist on talking about a physical impossibility, certainly impossible given our current understanding of the force needed to accelerate a star.

Maybe you would like to share with us how you intend to physically accelerate a star?

 

And now I know we are unable to communicate. How does using brakes imply acceleration?

A you may have guessed I am using plain english and common usage of these words, like acceleration etc...in common use braking would normally suggest de-acceleration would it not?

 

You obviously don't even understand the difference between an object accelerating in an inertial frame and an accelerating frame.

-Dale