Constancy of SOL from all objects

[Markus Hanke]

It is taking something simple and making it unnecessarily complicated.

 

 

It's actually the exact opposite - we put things into the context of spacetime precisely because the result no longer depends on the observer. It is the simplest possible description of the universe, because everyone agrees on it. I think that is as close as you can get to a description of reality. The complications only arise once we abandon the global point of view, and restrict ourselves to the "projections" of particular observers. To me, it really doesn't get any simpler - but we may need to just agree to disagree on this. All I can really say is that - had you adopted the "spacetime" view right from the beginning - the 150+ posts on this thread would have been unnecessary, because there are no longer any disagreements between observers.

 

 

 

Okay, but one can ask about the wavelength of the light reflected from the cow... and then like just about everything we have to ask 'as measured by who?'

 

Exactly.

On a somewhat unusual note, I am a synesthetic. Quite separate from what I physically see on a cow in a field, the very concept of cow itself ( regardless of whether I actually see it or not ) has sensory properties for me too - "cow" is a deep dark brown ( in English ), it feels velvet-y in an unpleasant way, it sounds sharp like broken glass, and it is located centre-right. Plus some others which are less easy to describe in words. Are these things "real" ? They are to me, and I don't know what "cow" would be like for someone who isn't a synesthetic

 

 

You just became accustomed to the concepts that were put in your brain with the help of academic hammer.

 

It's funny that you should say this, because I left school after 2nd level and never went to university. I work in a supermarket, and have never been - nor will I ever be - part of professional academia. I have acquired my own understanding through self-study in my free time from a large variety of sources, both academic and otherwise, and hence from a variety of different angles and perspectives. I do this because I have a deep passion for understanding how the universe works at its core - and even without any "academic hammers" it appears intuitively obvious to me that Lorentz invariance, and hence also CPT invariance in quantum field theory, are fundamental parts of how the world works. It is also by far the simplest and most natural description of the world. It is your own choice if you wish to disagree; I have attempted to explain, but I cannot convince. Only you yourself can do that. And just so that you know - the "unthinking academic zombie" fallacy is just really ridiculous in general.

 

 

 

You didn't answer again.

 

The answer I gave is the best anyone can give, really. If I stand still, the cow is black and white. If I move towards it a high speed, the cow will be bluish-black. If I move away from it at high speed, it is red-shifted. If I close my left eye, it becomes monochrome since I am almost colour blind on one side. If I was to ask a bat, then it would have no idea what I am even talking about, because it uses other ways to "see" the cow.

 

Who's right ? I depends who asks the question. I would have thought this is quite obvious ( pun intended ).

Edited July 30, 2016 by Markus Hanke

[geordief]

 

 

 

On a somewhat unusual note, I am a synesthetic. Quite separate from what I physically see on a cow in a field, the very concept of cow itself ( regardless of whether I actually see it or not ) has sensory properties for me too - "cow" is a deep dark brown ( in English ), it feels velvet-y in an unpleasant way, it sounds sharp like broken glass, and it is located centre-right. Plus some others which are less easy to describe in words. Are these things "real" ? They are to me, and I don't know what "cow" would be like for someone who isn't a synesthetic

 

It's funny that you should say this, because I left school after 2nd level and never went to university. I work in a supermarket, and have never been - nor will I ever be - part of professional academia. I have acquired my own understanding through self-study in my free time from a large variety of sources, both academic and otherwise, and hence from a variety of different angles and perspectives.

 

 

What? Well I suppose that is better than Macdonalds.

 

http://scienceblogs.com/cognitivedaily/2007/09/20/more-evidence-that-everyone-ha/

 

I can relate to that ,even though I would be on the "minimallly significant" end of the scale.

Edited July 30, 2016 by geordief

[MigL]

Is the cow 'really' blue ?

Don't be silly. Cows don't have a color !

 

What you're seeing is the frequency/wavelength of the light reaching you from the cow.

And yes in one case the light is REALLY blue, in the other the light is REALLY green.

The cow itself doesn't change, but the 'signal' from the cow ( the light ) does change in accordance with relativity.

 

Relativity hasn't made the notion of reality redundant. As a matter of fact. it make it a valid notion again.

The differing observations dependent, on the observer's frame, only make sense if you consider the space-time and the interval ( as Marcus has previously explained ). If you take the non-relativistic approach and consider just space or just time independently, you end up extremely confused, Michel, as there appears to be no 'reality'.

[swansont]

You didn't answer again.

 

Stop asking nonsensical questions, or at least complaining when nobody can answer them.

 

Or perhaps I should ask "What's the difference between a duck?" and whine when you don't answer it.

[Mordred]

 

 

The answer I gave is the best anyone can give, really. If I stand still, the cow is black and white. If I move towards it a high speed, the cow will be bluish-black. If I move away from it at high speed, it is red-shifted. If I close my left eye, it becomes monochrome since I am almost colour blind on one side. If I was to ask a bat, then it would have no idea what I am even talking about, because it uses other ways to "see" the cow.

 

Who's right ? I depends who asks the question. I would have thought this is quite obvious ( pun intended ).

I certainly wasn't expecting the cow example to cause extra complications.

 

I would have thought it would have been as obvious as observers comparing size.

 

ie To a distant observer an object appearing smaller. It certainly doesnt mean the object is smaller.

 

I suppose whats obvious to one person isn't to another.

Edited July 30, 2016 by Mordred

[ajb]

Things can look different depending on what frame you view them from... Edited July 30, 2016 by ajb

[Mordred]

roflmao I often feel like the instructor on that video 😞

 

Just a side note there is a humorous story behind "cows are blue". I used to work as an automation tech at a Pork slaughterhouse years ago.

 

My knowledge of automation far exceeeded anyone else I worked with.

 

The "cows are blue" expression used to be my favourite to describe how reliant that plant was on my descriptions of a breakdown. I could literally tell the managers anything and they had no choice but to accept it. "cows are blue" described that.

 

At university in my career change I decided to mathematically show cows are blue via redshift. It was one of my report assignments (got an A on the report lol)

Edited July 30, 2016 by Mordred

[imatfaal]

roflmao I often feel like the instructor on that video

 

...

 

If you don't know that Irish comedy already then do yourself a huge favour and buy a box set of Father Ted - one of the most innovative and funny series ever; up there with Faulty Towers, the Office, and Cheers

[Mordred]

 

If you don't know that Irish comedy already then do yourself a huge favour and buy a box set of Father Ted - one of the most innovative and funny series ever; up there with Faulty Towers, the Office, and Cheers

Ive never seen it before in Canada. I will definetely look into it. My wife will probably enjoy it as well.

[Delta1212]

I have no problem with anything you wrote above.

BUT tell me, when observer B that sees no hole in the donut will extend his arm and take the donut at hand, will he get a better information or not? Will he see the hole in the donut or not? Will the donut "truly" lack a hole?

That is the question.

But the rest frame is not taking the donut in hand and looking at it from all perspectives. It is looking at the top down perspective, where it looks like a flat circle with a hole in the middle. But it also looks like it's completely covered in frosting.

 

There is no one single perspective that gives you a total view of the donut, and there is no single frame of reference that gives you the total "true" measurement of an object.

[Mordred]

This seems to be a common problem for learning the complexities of physics.

 

People tend to think "what is real" so they look one observer view that is more "real" than another observer viewpoint.

 

Relativity teaches us that all viewpoints are equally accurate to the observer viewpoint.

 

You get the same problem with "artifacts of coordinate systems". People tend to think a scenario in say the Schwartzchild metric is accurate though in other metrics such as turtle coordinates that scenario has a different result.

 

One might believe "real is what you see" but therein lies the problem not everyone sees the same thing. To each observer "real is what they see and measure". This is certainly accurate from the viewpoint of each observer, provided one recognizes that their descriptions of reality is an observer viewpoint.

 

For example "describe a table". We often view this table as being solid. To us the table certainly feels solid. Yet if you are an observer who is tiny enough to see the atoms of the table.

 

That table may very well look extremely full of empty space and probably wouldn't even be recognizable as a table.

 

You have two completely different viewpoints of reality. Which one is correct.

 

the answer is both of them.

 

On a side note on the last bit. Another common misunderstanding is the ideal gas laws. One might think these laws are only usable on a gas. Yet if you consider the main difference between a gas and solid is mainly density. Then one realizes that the ideal gas laws can be applied to any solid. Even the pebble in your shoe.

 

This last bit if you follow it will help better understand relativity.

Edited July 30, 2016 by Mordred

[Strange]

 

If you don't know that Irish comedy already then do yourself a huge favour and buy a box set of Father Ted - one of the most innovative and funny series ever; up there with Faulty Towers, the Office, and Cheers

 

 

There is another great episode where Father Ted draws a circle to explain the idea of "inside your head" (i.e. things you imagined) and "outside" (the real world).

 

And now back to your scheduled argument discussion.

[Markus Hanke]

I'm glad to see that others outside Ireland can appreciate the subtleties of Father Ted too He's a true legend here !

 

On a more serious note, perhaps a practical example of what these two different viewpoints imply is in order. Let's take something really simple, like a magnetic field; the "3D + time" ( i.e. Newtonian ) viewpoint for an observer who is at rest with respect to the field is

 

[latex]\displaystyle{div\: \mathbf{B}=0}[/latex]

[latex]\displaystyle{rot \: \mathbf{E}=-\frac{\partial \mathbf{B}}{\partial t}}[/latex]

 

which is fairly straightforward, so long as I don't ask what another observer, who is perhaps in relative motion with velocity v, might say about the field. If I do that, my field will start to transform as

 

[latex]\displaystyle{\mathbf{B'}=\gamma \left ( \mathbf{B}-\frac{\mathbf{v}\times \mathbf{E}}{c^2} \right )-\left ( \gamma -1 \right )\left ( \mathbf{B}\cdot \mathbf{\hat{v}} \right )\mathbf{\hat{v}}}[/latex]

 

and something similar for the E field too. Not only that, but suddenly my div and rot operators take on a different form too ( they are coordinate-dependent ), and the initially simple equations become a complicated mess. The two observers will disagree on the description of the magnetic field, because they both argue from a space+time point of view. Not only that, but they will even disagree on the "mix" between E and B fields which they see !

 

I on the other hand come along and base everything on 4D spacetime. I do away with the E and B 3-vector fields, and instead define my magnetic field as a differential 2-form F in four dimensions, which makes my description of it

 

[latex]\displaystyle{dF=0}[/latex]

 

where "d" is the exterior derivative operator. The above quite simply and intuitively means that my magnetic field has no boundary, so flux tubes of the field do not end anywhere, hence they must form closed loops instead ( also meaning there are no "magnetic charges" ). This description of the field is valid no matter which observer looks at it; that is born out by the fact that differential forms are in fact antisymmetric tensors, so the description dF=0 is valid in this exact same form for all observers, regardless of where they are and how they move. This is possible because I am describing the global form of the field in spacetime, and not just a particular projection of it in space+time, which would be intrinsically observer-dependent. This is the simplest and most natural description of a magnetic field, and its meaning is easy to understand and quite clear ( unlike the original 3-vector equations ).

 

Similar arguments can be made for other scenarios too - for example, it was mentioned before that observers will disagree on the frequency of light, depending on how they move. That is because frequency is a concept from the "space+time" point of view, and hence observer-dependent. In 4D spacetime, one can instead describe the radiation field ( =the light ) in terms of its associated stress-energy-momentum tensor, and one will find that not only will all observers once again agree on this description, but it also allows us to naturally deduce certain properties such as conservation of energy-momentum ( the divergence of this tensor identically vanishes ). And so on.

 

I think this should provide some clarity on why spacetime is the simplest and most natural description of physics, as opposed to the old "space+time". It is no accident that modern physics is formulated in terms of the former.

Edited July 30, 2016 by Markus Hanke

[MigL]

Great explanation as always Markus.

 

I hope we all learned a lesson...

Never bring a cow to an argument !

[Mordred]

 

I hope we all learned a lesson...

Never bring a cow to an argument !

But the cow doesn't have any misconceptions... Might be easier to teach relativity 👌

 

I agree great explanation Marcus

Edited July 30, 2016 by Mordred

[Markus Hanke]

Learning relativity is a funny thing, though - michel12345 said I got "indoctrinated" by academic ideas, but in actual fact the exact opposite is the case. Understanding relativity means we need to unlearn our innate tendency to view the world in terms of "space+time"; this conditioning is a natural consequence of the fact that humans live in a low-energy, low-velocity environment, i.e. within a purely Newtonian domain. This conditions the way we think about the world. To understand why relativity is necessary, one must step outside those boundaries, and adopt a more general point of view, just like the 2D cartoon character needs to step outside his flat plane into the 3D world to understand why his environment is the way it is. And unlearning is always very much harder than learning, because we tend to get attached to what we consider to be basic truths. I think this is one of the reasons why so many people have difficulties accepting the ideas of relativity, even though it is by far the most simple and natural way to view the world. It's an issue of psychology, moreso than physics - acquiring knowledge is easy, but letting go of it is exceedingly hard.

[StringJunky]

Learning relativity is a funny thing, though - michel12345 said I got "indoctrinated" by academic ideas, but in actual fact the exact opposite is the case. Understanding relativity means we need to unlearn our innate tendency to view the world in terms of "space+time"; this conditioning is a natural consequence of the fact that humans live in a low-energy, low-velocity environment, i.e. within a purely Newtonian domain. This conditions the way we think about the world. To understand why relativity is necessary, one must step outside those boundaries, and adopt a more general point of view, just like the 2D cartoon character needs to step outside his flat plane into the 3D world to understand why his environment is the way it is. And unlearning is always very much harder than learning, because we tend to get attached to what we consider to be basic truths. I think this is one of the reasons why so many people have difficulties accepting the ideas of relativity, even though it is by far the most simple and natural way to view the world. It's an issue of psychology, moreso than physics - acquiring knowledge is easy, but letting go of it is exceedingly hard.

I agree, Markus, it's been hard work for me accepting what SR and GR says...what little I understand of it. Once I realised that nothing exists in the way we think it does macroscopically at the atomic level, ie not tangible, it was a lot easier. I am now a fully "indoctrinated", diehard Relativist. It's awesome.

[michel123456]

i disagree completely.

 

Let me try to explain:

 

300 observers are looking at point C (the Cow, the center)

 

One observer says the cow is white, the other blue, the other red, etc.

They all get different informations about point C, BUT they have a great Theory that make them able to compare their measurements. So in the end they all agree that point C is a 4D object that IS at the same time red, blue, yellow, white, etc.

 

But at the end of the conversation, one should realize that there is only one point C. There is a single intersection to the 300 lines of sight. There are not 300 different points C. This point C doesn't change color because it is observed 300 times. It may be that point C has no intrinsic color at all but there is only one reality which is that point C somehow exists. There are no 300 overlapping realities*, and putting measurements above that is madness.

 

AND to me, there is a "best frame" between the 300 which is the frame that is exactly at the intersection. This frame measures proper length (1), proper time(2), proper mass (3 rest mass) in as much as possible present time(4). These are 4 reasons for putting this frame a better one.

 

*That must be mathematically provable.

Edited July 31, 2016 by michel123456

[swansont]

 

These are 4 reasons for putting this frame a better one.

 

 

Not from a physics standpoint. And this is supposedly a physics discussion.

[michel123456]

 

 

Not from a physics standpoint. And this is supposedly a physics discussion.

Is that the only point that bothers you in my post?

[Endy0816]

Infinite number of frames...

[MigL]

And again you're missing the big picture...

 

The 'cow' is the cow , locally; that doesn't change.

But that information has to get to the 300 different observers in their various frames.

That information transfer is subject to the various constraints of relativity, such as finite SoL, invariant SoL, etc.

 

So whereas previously you would have had 300 different observations/measurements of the cow's color, and the world would have made no sense, with relativity we know how and why we have the 300 different observations.

 

And the world makes sense again.

[michel123456]

And again you're missing the big picture...

 

The 'cow' is the cow , locally; that doesn't change.

But that information has to get to the 300 different observers in their various frames.

That information transfer is subject to the various constraints of relativity, such as finite SoL, invariant SoL, etc.

 

So whereas previously you would have had 300 different observations/measurements of the cow's color, and the world would have made no sense, with relativity we know how and why we have the 300 different observations.

 

And the world makes sense again.

OK so far.

 

But if observer 6 takes the time to travel till point C, he will observe the cow slightly changing from a reddish ccclrrrrrrrstowish thing into a cow. At some point he will not be able to go further and that's it.

I find it so wrong to believe that observation from 6 is the same valuable as observation from C.

 

And reversely if observer 6 extends his hands and takes the cow inside his own FOR, I seriously doubt he will obtain that reddish ccclrrrrrrrstowish thing. If he believes he will, then he made a mistake calculating the transform from one frame to another.

[Mordred]

Why not? You have two or more observers of the same event. Both observers accurately measure that event according to his reference frame.

 

GR explains why both observers get slightly different measurements.

 

It is no different than having different observers seeing an object as visually appearing as different size according to his distance.

 

You have no problem understanding why different observers will see a different size of an object. Ie someone on a plane.

 

Relativity is essentially the same.

[MigL]

Sooooo...

 

All observations/measurements are valid/real in the frame they're made in.

 

All information, i.e. anything from the cow that can affect you ( the observer in your own particular frame ), is what is changing.

The cow itself, in its local frame is not. But you will never experience THAT cow unless you join it in its local frame.