Relativity - Oh dear, here we go again!

[BillyJoe]

The rock only exists in its own rest frame and falls in a straight line toward the force of gravity when dropped. That it is perceived to be moving on an angle when observed from another rest frame is an illusion.

Okay, from thew point of view of the centre of the Earth, the stone is moving in a straight line towards the centre of the Earth. From the POV of an observer on the ground, it is moving away at an angle. From the point of view of the car, the stone falls straight down. From the point of view of the stone, it remains still.

There are no preferred reference frames.

To use the movie analogy used earlier, the single event is being separated in to a strip of many events and this creates the illusion.

I would drop the movie analogy. It doesn't work.

 

[ynot]

Well, the point is, there's no absolute "is" - no preferred frame of reference.

If we’re trying to discover a most accurate level of what is true, fact, actual, etc (I agree it can’t ever be absolute) then I would think that the closest, most local frame of observation, would be by far the most preferred.

There's just what you observe. You observe the clock running slower, so for you, the clock is running slower. It's the same clock, as viewed by two different observers.

So observation creates the reality? I thought it was the reality that made the observation possible. The clock is perceived to be running slower as you observe it, but it’s not actually running slower as a clock

 

[BillyJoe]

You say the clock “is” running slower when observed from the other frame.

Yes it IS running slower form the other reference frame.

In other words the clock is actually simultaneously running both correctly and slower.

Correctly?
There is no correct time, just different times from the point of view of different reference frames.

Surely this is claiming two realities of the same clock.

The reality is that the clock runs different speeds from the point of view of different reference frames.

We're not “just talking about two different points of view”. You’re saying that, viewed from the other frame, the clock has a different reality. If you were saying that it was perceived to be running slower, I would quickly agree.

It's not just perception. It really does run at different speeds from the point of view of different reference frames.
That's why the second twin returns to Earth having aged less.
It's not an illusion. It's not a misperception. It is real. And it's been experimentally confirmed

 

[bjb]

Try this:

http://metaresearch.org/cosmology/gps-relativity.asp

The bottom line is that the clock does run slower (or faster) depending upon its speed and altitude.

 

[BillyJoe]

I think that should be how it appears or is percieved to act. How it actually acts at its most local level is the reality. Not how it appears from the frame of reference you are observing it from.

There is no illusion.
There is no misperception.
There is no correct frame of reference.

The observation is essentially the same but the scenario is different. The camera is observing from the frame of the person, not from the frame of the clock. The camera is making the same observation that the observer would, but it is able to filter out factors that cause the illusion.

The observer, B, cannot do what the camera does.
B cannot zoom in and out to keep the clock size constant.
The camera can zoom in and out and, in doing so, creates the illusion of being a camera in the same frame of reference as A. The video produced is the same as one produced by a camera in the frame of reference of A.

 

[ynot]

Okay, from thew point of view of the centre of the Earth, the stone is moving in a straight line towards the centre of the Earth. From the POV of an observer on the ground, it is moving away at an angle. From the point of view of the car, the stone falls straight down. From the point of view of the stone, it remains still.

There are no preferred reference frames.

What I am questioning is whether a clock actually slows down when observed from an external frame, or whether is just appears to slow down. The two things are completely different, and the difference is important to me in my attempt to understand and accept (or the highly unlikely disprove) Relativity.

I would drop the movie analogy. It doesn't work.

Doesn’t work by your decree, or have you consulted the rest of humankind?
I agree that it’s not a particularly good analogy so I will stop using it. It’s not always easy to come up with good analogies and perhaps we shouldn’t do it at all. Personally I don’t particularly like 2D flat earth, balloon curved space, triangle on spheres analogies either.

 

[BillyJoe]

I didn’t say that he rock moves relative to its own rest frame. I said it moves within it’s own rest frame.

It doesn't move within it's own frame of reference.
An object is always at rest within its own frame of reference.

Actually it doesn’t, it’s accelerating to another rest frame.

Okay, we overlooked the acceleration bit.
That is GR not SR, so let's let that go.

Given that everything is constantly moving relative to everything else at an atomic level, a true rest frame is impossible.

No.
Everything is at rest within its own frame of reference.

Or perhaps only possible for the smallest possible particle.

I've got no idea what this means.

Perhaps illusion is not a good word to use.

I think you should drop the words "illusion", "misperception" in the bin with your movie analogy.
Now....just try to think about the word "relative"

A perfectly accurate and correct observation from another frame of reference I agree, but not a perfectly accurate and correct observation of the actual event as it actually occured at it’s most local level.

There are no correct reference frames.
There are just different reference frames.
They are all equally valid.

The twin arrives back on Earth younger than the twin who stayed behind.

 

[BillyJoe]

What I am questioning is whether a clock actually slows down when observed from an external frame, or whether is just appears to slow down.

It doesn't slow down. It just runs slower.
And, yes, it actually does run slower.

The two things are completely different, and the difference is important to me in my attempt to understand and accept (or the highly unlikely disprove) Relativity.

Well okay, the clock does actually run slower and this has been experimentally confirmed.

Doesn’t work by your decree, or have you consulted the rest of humankind?

No, there is no analogy there at all.
One has to do with the persistence of image to create the illusion of smooth movement.
The other has to do with different clock speeds in different reference frames and how all are equally ligitimate.

I agree that it’s not a particularly good analogy so I will stop using it. It’s not always easy to come up with good analogies and perhaps we shouldn’t do it at all. Personally I don’t particularly like 2D flat earth, balloon curved space, triangle on spheres analogies either.

Analogies are useful if they help to illustrate a point you are trying to make.

 

[bjb]

Here's another example of relativity (space-time compression) in action:

http://zebu.uoregon.edu/~rayfrey/QNet/QNet05/muon_lifetime.pdf

Take a look at page 11 in the document. The muon lives longer when it is travelling at high speed than when it is at rest. The way I understand it, we 'observe' the muon as living longer because of time dilation. The muon is moving fast so we can compute its lifespan based upon time dilation. However, the muon 'thinks' it is living a normal lifespan and it reaches the ground because from its own point of view, it has traveled a shorter distance. The distance is shorter because of *space* compression. Put the two observations together and you get space-time compression, which is what we are trying to understand here.

 

[Fredrik]

You say the clock “is” running slower when observed from the other frame.

I wouldn't put it like that, because you obviously can't change the properties of a clock just by observing it. I would just say that the clock is running at different rates in different frames.

In other words the clock is actually simultaneously running both correctly and slower.

It is definitely not the case that what's observed in one frame is more "correct" than what's observed in another. But yes, the clock is observed to be running at different rates in different frames (because it is running at different rates in different frames).

Surely this is claiming two realities of the same clock.

No, it's not. This part is very difficult to explain. There's only one space-time, and each microscopic part of the clock traces out exactly one path through it, not a different path for each frame. Different observers will disagree about which three-dimensional slices of space-time you should call "space". This will make them disagree about other things, like the physical shape of the clock in your example. But they only disagree about things that depend on how the "slicing" is done. The ticking rate is another example. How big the disagreement is depends on the angle between observer A's slices and observer B's slices.

I don’t think that the cylinder example is valid comparison. The shape of the silhouette of the cylinder changes when viewed from different an angles, but the time shown face of the clock doesn’t.

It was an analogy, and not meant to be exact. But it is pretty good. It's very difficult to explain it though. I'm pretty sure you won't really understand this until you've learned to draw space-time diagrams. If you're serious about understanding these things, then learning about space-time diagrams should be your highest priority. It's totally worth it, because they make everything else much easier.

 

[ynot]

Yes it IS running slower form the other reference frame.
Correctly?
There is no correct time, just different times from the point of view of different reference frames.
The reality is that the clock runs different speeds from the point of view of different reference frames.
It's not just perception. It really does run at different speeds from the point of view of different reference frames.
That's why the second twin returns to Earth having aged less.
It's not an illusion. It's not a misperception. It is real. And it's been experimentally confirmed

There is the clock as it exists and there is the clock as it is observed. The existence is the reality, the observation is not. A thing doesn’t have to be observed to exist. A thing that exists is potentially observable. If we’re talking about human created clock time, then the “correct time” is the “correct speed” that the clock runs at according to it’s design and purpose. In other words, a correctly functioning clock runs at a predictable, constant rate when observed at a local level (in it’s frame). That the clock can be observed to be different than it’s actual existence, from the distance and movement of another frame, doesn’t mean that that it is different.

 

[ynot]

I wouldn't put it like that, because you obviously can't change the properties of a clock just by observing it. I would just say that the clock is running at different rates in different frames.


It is definitely not the case that what's observed in one frame is more "correct" than what's observed in another. But yes, the clock is observed to be running at different rates in different frames (because it is running at different rates in different frames).


No, it's not. This part is very difficult to explain. There's only one space-time, and each microscopic part of the clock traces out exactly one path through it, not a different path for each frame. Different observers will disagree about which three-dimensional slices of space-time you should call "space". This will make them disagree about other things, like the physical shape of the clock in your example. But they only disagree about things that depend on how the "slicing" is done. The ticking rate is another example. How big the disagreement is depends on the angle between observer A's slices and observer B's slices.


It was an analogy, and not meant to be exact. But it is pretty good. It's very difficult to explain it though. I'm pretty sure you won't really understand this until you've learned to draw space-time diagrams. If you're serious about understanding these things, then learning about space-time diagrams should be your highest priority. It's totally worth it, because they make everything else much easier.

I disagree. The clock is only running in it’s own frame. It is only being observed to be running from another frame. It is never actually running in another frame. It can’t be in two frames at once.

Until I find a good reason to accept that any observation is always an valid, accurate and correct representation of reality, writings on space-time diagrams, math equations, gods, etc isn’t going to help me understand and accept Relativity. Having said that, I can, have and do read quite a lot about the subject.

 

[ynot]

Here's another example of relativity (space-time compression) in action:

http://zebu.uoregon.edu/~rayfrey/QNet/QNet05/muon_lifetime.pdf

Take a look at page 11 in the document. The muon lives longer when it is travelling at high speed than when it is at rest. The way I understand it, we 'observe' the muon as living longer because of time dilation. The muon is moving fast so we can compute its lifespan based upon time dilation. However, the muon 'thinks' it is living a normal lifespan and it reaches the ground because from its own point of view, it has traveled a shorter distance. The distance is shorter because of *space* compression. Put the two observations together and you get space-time compression, which is what we are trying to understand here.

At this minute level of observation and interpretation I would have to have faith that the experts have it right. I have to have faith in their claim that a muon actually exists. I don’t think they are deliberately lying, but they could be wrong. Just because a guy wears a white coat and has letters after his (or her) name, it doesn’t mean they are infallible. Is time dilation the reason the muon lives longer, or is it another reason? I certainly don't know and don't pretend to..

 

[ynot]

At this minute level of observation and interpretation I would have to have faith that the experts have it right. I have to have faith in their claim that a muon actually exists. I don’t think they are deliberately lying, but they could be wrong. Just because a guy wears a white coat and has letters after his (or her) name, it doesn’t mean they are infallible. Is time dilation the reason the muon lives longer, or is it another reason? I certainly don't know and don't pretend to.

Ooops, I pushed quote instead of edit.

 

[PixyMisa]

At this minute level of observation and interpretation I would have to have faith that the experts have it right. I have to have faith in their claim that a muon actually exists. I don’t think they are deliberately lying, but they could be wrong. Just because a guy wears a white coat and has letters after his (or her) name, it doesn’t mean they are infallible. I guess I’m a sceptic.

These experiments have been independently verified many many times. And they confirm the same Theory of Relativity that is built into GPS (which wouldn't work if the mathematics was incorrect). There isn't a whole lot of room for skepticism here. Millions of people are relying on relativity every single day.

Another thing, you are way too hung up on the local frame of reference. As I said earlier, frames of reference aren't real. They're mathematical constructs. A given local frame has no greater significance than any other. And the local frame is arbitrary. An object at rest is at rest with respect to itself, which means absolutely nothing to anyone else.

 

[ynot]

These experiments have been independently verified many many times. And they confirm the same Theory of Relativity that is built into GPS (which wouldn't work if the mathematics was incorrect). There isn't a whole lot of room for skepticism here. Millions of people are relying on relativity every single day.

Another thing, you are way too hung up on the local frame of reference. As I said earlier, frames of reference aren't real. They're mathematical constructs. A given local frame has no greater significance than any other. And the local frame is arbitrary. An object at rest is at rest with respect to itself, which means absolutely nothing to anyone else.

I thought that the GPS thing was to do with time delay not time dilation. Perhaps I just read something written by a nutter.

Regardless of the terminology used, I think that it’s obvious that the closer and more immediate that a thing is observed the more accurately the observation is likely to represent the reality. I believe that every thing only has one reality. I'm not making a claim, I'm asking a question. Why is is an observation that has been distorted by distance and motion claimed to be an accurate representation of the reality of the thing being observed?

 

[PixyMisa]

I thought that the GPS thing was to do with time delay not time dilation. Perhaps I just read something written by a nutter.

Here's an article on the relativistic effects that have to be accounted for to keep the GPS accurate.

Regardless of the terminology used, I think that it’s obvious that the closer and more immediate that a thing is observed the more accurately the observation is likely to represent the reality. I believe that every thing only has one reality. I'm not making a claim, I'm asking a question. Why is is an observation that has been distorted by distance and motion claimed to be an accurate representation of the reality of the thing being observed?

Because that "distortion" always happens everywhere and is precisely predictable. It doesn't matter what you think is obvious, or what you believe about reality; relativity is an accurate mathematical description of what really happens.

 

[Ziggurat]

There is the clock as it exists and there is the clock as it is observed. The existence is the reality, the observation is not. A thing doesn’t have to be observed to exist.

Your last sentence is correct, but it obscures some rather important distinctions. In the context of special relativity, the word "observe" has a rather specific meaning, and it is a different meaning from what you merely see. An observation in special relativity is an idealized measurement. It does not create reality, certainly, but if it is an observation (again, using the term in a rather specific sense) and not just any old measurement, then it does indicate reality. That's the whole point of using the term. There are all sorts of optical illusions which result from the finite speed of light which apply to what you see or even measure, but when talking about what you observe none of those apply: observations mean what's really going on.

The example I like to use is airplanes flying overhead. Because of the finite speed of sound, you hear noise coming from some point noticeably behind where the plane overhead is. But if you know the speed of sound in air and you know how fast the plane is going, you can calculate when the sound was emitted and where the plane was when it was emitted, and you can observe that the sound is indeed coming from the plane and not from behind it.

A thing that exists is potentially observable. If we’re talking about human created clock time, then the “correct time” is the “correct speed” that the clock runs at according to it’s design and purpose. In other words, a correctly functioning clock runs at a predictable, constant rate when observed at a local level (in it’s frame). That the clock can be observed to be different than it’s actual existence, from the distance and movement of another frame, doesn’t mean that that it is different.

The clock is not different: time itself is.

Let me give you an example. Consider just Newtonian physics (with Galilean relativity). You're standing on the ground, and your friend is riding on a train car moving at a uniform speed of 2 m/s. In your reference frame, two events occur, one at x=0, t=0, and another one at x=10 m, t=3 s. Now, what's the distance between these two events? Well, in your reference, it's 10 meters. What about in your friend's reference frame? Well, if we set it up so his origin (x'=0, t'=0) is the same as yours, then the first event happens at his origin, but the second event happens at x'=4 m, t'=3 s, and the distance between the events is only 4 meters. Why the disagreement? Is his ruler broken? Is it changed? No, it isn't. All that happened is that we had to transform our coordinates: x' = f(x). But when I ask the question, what's the distance between the two events in the two different coordinate systems, is the fact that the answers are different somehow artificial? No, the difference is quite real.

So what's different in special relativity? Well, in special relativity, you can't simply say x' = f(x): it's really (x',t') = f(x,t). You need to transform both time and space coordinates. And one of the effects is that the time between two events (say, two ticks on a clock) isn't the same in different reference frames. Nothing is broken or even changed about the moving clock, other than the fact that it's in a moving reference frame. It's easy to see why this happens with rulers, it only seems strange because from our everyday experience, we don't ever notice that time has to change for moving frames because the change is so small at non-relativistic frames. But the change is quite real, it is not an artifact, and it's been experimentally confirmed plenty of times.

 

[BillyJoe]

If we’re talking about human created clock time, then the “correct time” is the “correct speed” that the clock runs at according to it’s design and purpose. In other words, a correctly functioning clock runs at a predictable, constant rate when observed at a local level (in it’s frame). That the clock can be observed to be different than it’s actual existence, from the distance and movement of another frame, doesn’t mean that that it is different.

Have you studied "The twin paradox"?

The twin who remains on Earth has a clock and he observes his clock to run at the normal rate.
The twin who travels to alpha centauri and back also has a clock and she observes her clock to run at the normal rate.
Yet, when they meet up again, less time has passed for the twin that travelled to Alpha centauri than for the twin who stayed at home.
Which they can both confirm by comparing their clocks.

As I said, this has been proven experimentally to be true on a smaller scale. There is the muon example cited earlier. Also, extremely accurate atomic clocks have been carried on aircraft as confirmation.

These are the facts.

Someone who knew what they were talking about once said:
"You are entitled to your opinion, but not your facts"

 

[ynot]

It seems to me that the clock experiment only “works” when the observer and clock are passing each other.

An observer is orbiting the ball clock described earlier in a circular motion. The observer is moving in relation to the clock but the clock doesn’t appear to the observer to be moving. The clock is not seen to be moving sideways and the ball is not seen to be moving in a zigzag manner. The clock is observed to be function normally.

Move the clock out from the centre, half way along the same diagonal that the observer is on, and have them both move in a concentric orbit. The observer and clock always remaining on the same diagonal. The observer is moving at a different speed to the clock but the clock doesn’t appear to the observer to be moving. The clock is not seen to be moving sideways and the ball is not seen to be moving in a zigzag manner. The clock is observed to be function normally.

Reverse the rotational direction of the clock and have it orbit at it’s previous speed. The observer and clock now pass each other and the clock is seen to be moving sideways and the ball is seen to be moving in a zigzag manner. According to Relativity, the clock now appears to (and actually would) slow down (I can only agree with the appears bit).

The observer and clock are the only things that exist so no third object can be used as a reference.