Mentor

## Redshift and Expansion of the Universe

 Quote by AlbertE Can you just give me a quick example which explains in laymans terms "frame of reference"?
See the post
 Quote by George Jones I think that some care is needed; it seems that you're describing the Doppler effect, although I could be wrong. In reality, the phrase "a moving clock runs slow" does not necessarily mean "a moving clock is seen visually to run slow." A clock moving directly away from an observer appears visually to run slow, but a clock moving directly towards an observer appears visually to run fast. In both cases, what is seen visually is given by the Doppler expression, which is always different than the time dilation expression. In both cases, the time dilation expression, used appropriately, does apply. Consider the following example. Assume that Alice is moving with constant speed directly towards Ted. When Ted uses his telescope to watch Alice's wristwatch, he sees her watch running at a faster rate than his watch. Ted sees Alice's moving watch running fast, not slow! Ted sees this because of the Doppler shift. Because Alice moves towards Ted, the light that Ted sees from her watch is Doppler-shifted to a higher frequency. But the rate at which a clock or watch runs is like frequency, i.e., a second-hand revolves at a certain frequency, and all frequencies are Doppler-Shifted., so ted see Alice's wristwatch running fast. To explain what "A moving clock runs slow." means, I first have to explain how Ted (with help from Bob) establishes his frame of reference. Starting from Ted, a series of metre sticks, all at rest with respect to Ted, are laid end-to-end by Bob along the straight line joining Alice and Ted. At each joint between two consecutive metre sticks, Bob places a small clock. The metre sticks and clocks all are at rest with respect to Ted. Initially, none of the clocks are running; before turning them on, the clocks have to be synchronized. To do this, Ted directs a laser pointer along the line joining Ted and Alice, and then sends a flash of light. Each clock is turned on when the flash of light reaches it. The speed of light is not infinite, so the time taken for the light to travel from Ted to each clock has to be taken into account. To do this, the clocks' hands are set initially as follows. The clock one metre away from Ted is set to the time taken for light to travel one metre; the clock two metres away from the tower is set to the time taken for light to travel two metres; ... . This whole setup of metre sticks and clocks establishes Ted's reference frame. Now, As Alice moves toward Ted, Ted uses his telescope to watch Alice's wristwatch, and to watch his clocks. First, he watches one of the distant clocks in his reference frame. The time he sees on the clock is the time at which the light he sees set out from the clock, so Ted sees an earlier time on the distant clock than he sees on his wristwatch. Because the clock is stationary in his frame, Ted does, however, see the distant clock running at the same rate as his watch. Similarly, Ted's sees all the clocks in his frame running at the same rate as his watch. As Alice approaches Ted, she whizzes by clock after clock of Ted's reference frame. Using his telescope, Ted sees that Alice is beside a particular clock, and he notes the time on her watch and the time on the clock adjacent to her. Some time later, Ted sees Alice beside a different clock, and he again notes the time on her watch and the time on the clock adjacent to her. Ted checks his notes, and he finds that the time that elapsed on Alice's watch as she moved between these two clocks of his frame is less than the difference of the readings of the two clocks. This what is meant by "A moving clock runs slow." Unfortunately, "time dilation" in general relativity and "time dilation" in special relativity often have different operational meanings. Suppose observer A hovers at a large distance from a Schwarzschild black hole, and that observer B hovers near the event horizon. If observer A uses a telescope to observe B's watch, A will see B's watch running more slowly than his own watch. In this context, gravitational time dilation is something that is seen visually.
 Sorry double post I didnt see that reply.

 Quote by AlbertE Can ANYTHING EVER be stationary? :)
No, both Newtonian/Galilean and Special Relativity quite clearly state that what you just asked means nothing. If you had asked if anything could be stationary with respect to a certain (inertial) reference frame, then the answer would be yes.

 Quote by AlbertE Can you just give me a quick example which explains in laymans terms "frame of reference"?
Read this. It's an excellence source for understanding Special relativity for those who don't wish indulge into the detailed mathematics , covers bit of a historical background.

One of the postulates of Einstein's theory is that no reference frame is superior to another , in other words we can't subject a particular reference frame as absolute.

EDIT: OH sorry about that : http://www.thebigview.com/spacetime/relativity.html
It's alright for the general readers , however, at the end the article gave me an impression of dealing with meta-physics..
 This is EXACTLY the kind of thing I hate so much about trying to learn something. From the link above...... ""Relativity teaches us the connection between the different descriptions of one and the same reality." This view of Relativity, that there are different realities," Look at the state of that drivel. E says "one and the same reality" - the author of the article then spews forth with "there are different realities" Its no wonder its hard to learn with authors like that. Why cant people learn HOW to teach before they attempt to start teaching. You see my problem now? Are there multiple realities? or is there one reality? Its a joke. What is actually happening when I read something like that is my brain is diving off in all directions making my brain cells form incorrect pathways and connections which results - I hate to discover - in my ultimate demise - a bottomless pit of meaningless irrelevant questions.

Recognitions:
Gold Member
 Quote by AlbertE This is EXACTLY the kind of thing I hate so much about trying to learn something. From the link above...... ""Relativity teaches us the connection between the different descriptions of one and the same reality." This view of Relativity, that there are different realities," Look at the state of that drivel. E says "one and the same reality" - the author of the article then spews forth with "there are different realities" Its no wonder its hard to learn with authors like that. Why cant people learn HOW to teach before they attempt to start teaching. You see my problem now? Are there multiple realities? or is there one reality? Its a joke. What is actually happening when I read something like that is my brain is diving off in all directions making my brain cells form incorrect pathways and connections which results - I hate to discover - in my ultimate demise - a bottomless pit of meaningless irrelevant questions.
This comes from using English, or any other every-day human language, to describe things that are best described with math.

I'd say his "multiple realities" is an English language conceit whereby what I see is reality for me and what you see is reality for you. As for the physical event, only one thing happened, so in that sense there is only one reality.
 I agree wholeheartedly. I would have written (infact E says it perfectly clearly) - but anyway - I would have said..... "This view of Relativity, that there are different perceptions of the same reality." Oh dear god - I just had a thought - what if General Relativity means different things for different observers. Oh - I just had another thought - it does!!!!! Eeeeek

 Quote by AlbertE Oh dear god - I just had a thought - what if General Relativity means different things for different observers. Oh - I just had another thought - it does!!!!!
Relational Quantum Mechanics (an interpretation of Quantum Mechanics) takes that to a whole new level. Just prepare yourself for different observers observing different things.

 Quote by AlbertE This is EXACTLY the kind of thing I hate so much about trying to learn something. From the link above...... ""Relativity teaches us the connection between the different descriptions of one and the same reality." This view of Relativity, that there are different realities," Look at the state of that drivel. E says "one and the same reality" - the author of the article then spews forth with "there are different realities" Its no wonder its hard to learn with authors like that. Why cant people learn HOW to teach before they attempt to start teaching. You see my problem now? Are there multiple realities? or is there one reality? Its a joke. What is actually happening when I read something like that is my brain is diving off in all directions making my brain cells form incorrect pathways and connections which results - I hate to discover - in my ultimate demise - a bottomless pit of meaningless irrelevant questions.
You seem to be arguing over semantics . I don't see any conflict with that particular statement.Author was highlighting towards the phenomena of SR, if we measure a string of wire in a moving refrence frame in comparison to a stationary one then its length would appear shorter to the observer in the moving R.F ... so you see both the stationary and moving are observers are right in concluding different lengths of the same wire , call it A.

The inclusion of gamma factor brings this phenomena of ' length contraction'.
Given by: L= $L_{0}/\sqrt{1-v^2/c^2}$

 Quote by phinds You are confusing doppler redshift with the redshift caused by expanding distances. They are not the same, although I found that a bit hard to get my head around when I first encountered it. Doppler shift happens when two objects are moving relative to each other in the same frame of reference. The expanding universe means they don't have the same frame of reference, so the effect is not a doppler shift. This is not just semantics, because saying that the red shift is due to a doppler effect is the same as saying that the universe it not expanding.
Is there a way to discern between the two redshift effects without preconceived knowledge of the radiation's state of motion? Or is most of the redshifting that we see simply assumed to be from the cosmic expansion?

Recognitions:
Gold Member
 Quote by TheBob Is there a way to discern between the two redshift effects without preconceived knowledge of the radiation's state of motion? Or is most of the redshifting that we see simply assumed to be from the cosmic expansion?
I am not aware of any way to tell the difference. As to relative contributions, I had always thought that it was almost entirely due to expansion, but I was told by someone on this forum that it is about 50/50. I have no idea if that is right but it seems to me to be absolutely wrong.