A relativity question.

[Upchurch]

You have to ask the question in respect to a particular observer.

Man, I must be tired. I missed that completely. The concept of "simultaneity" is also a Newtonian concept. As has been said, there is no such thing as absolute time (which would be required for all observers to view an event happening "at one time".

The classic example of this is two observers at each end of a very long (light years long) rod. The observer at one end shoves the rod toward the second observer. Under Newtonian phsyics (and assuming that the rod is incompressible), the second observer feels the rod shove immediately. Under Relativity physics, however, such an event would violate the speed of light limitation. That is, information could be passed from one point to another at speeds faster than the speed of light.

I forget how this problem is resolved and I do not have time to look it up right now, but, in essence, it takes the rod time to propogate the "shove" from one end to the other, even though it is assume incompressible.

 

[Atlas]

RussDill and Upchurch, I really appreciate your posts here, especially the idea of simultaneity and the long rigid rod paradox. I don't get a chance to think about stuff like this.

I found a site perfect for my small brain - short answers to big questions . Here are it's answers to questions on simultaneity and the long rigid rod. I follow that one with one that I'm sure you'll like lifegazer, having to do with a photon's perspective.

... lifegazer, there is no such thing as simultaneously in relativity.

Question:
How can simultaneity be relative?

Answer:
We are used to thinking of time unfolding, and time being the same for all possible observers. This isn't the way it is in SR. Simultaneity is defined as "things happening at the same time." We might also define it in this way: Two events (two points in spacetime) are simultaneous in a given frame if a light signal emitted midway between the spatial locations of those two events arrives at those events. An example would be a lightbulb flashing at the center of a train car. In the train-car's frame the light hits the front and the back of the car at the same time, hence those two events are simultaneous. But to an observer watching the train car go by at some speed the two events cannot be simultaneous because the rear of the train comes forward to meet the light signal while the front of the train moves ahead of the light signal (put another way, the flashing of the light doesn't occur midway between the two events, according to the ground-based observer, because the train is in motion).
Because events that are simultaneous must be spacelike separated, they can have no causal relationship; hence, in a sense, simultaneity is a convention. When we say that a set of clocks is synchronized we are making a statement about how the clocks are set relative to one another in a certain frame. In another frame the clocks will show different times, although all the clocks in a given inertial frame will run at the same rate.

Man, I must be tired. I missed that completely. The concept of "simultaneity" is also a Newtonian concept. As has been said, there is no such thing as absolute time (which would be required for all observers to view an event happening "at one time".
The classic example of this is two observers at each end of a very long (light years long) rod. The observer at one end shoves the rod toward the second observer. Under Newtonian physics (and assuming that the rod is incompressible), the second observer feels the rod shove immediately. Under Relativity physics, however, such an event would violate the speed of light limitation. That is, information could be passed from one point to another at speeds faster than the speed of light.

Question:
OK, what if the two ends of the tunnel are blocked with the train inside, but the train is made out of an infinitely tough material that cannot be compressed. Something has to give, but what?

Answer:
What does it mean to be completely incompressible? It means that if you push on it, it doesn't give at all. If you had a completely incompressible rod, you could push at one end and have it move at the other end instantaneously. So is this a way to get around the finite speed of light? Make a big long rod one light year long and push on one end. If the rod cannot be compressed, then the whole thing will move at once, including the end a light year away. You have sent an instantaneous signal!
The problem is that the structural properties of something are determined by the intermolecular forces which are electromagnetic in nature. So when you push on one end of a rod, you apply forces to the molecules at that end, which in turn transmit forces on down the rod. How compressible a rod (or a train) is, is fundamentally limited by the need to transmit the force down its length, which must be limited by the speed of light. Real materials cannot evade this limit.
In the train case there is no way for the back end of the train to "know" about the front end being stopped by the blockage, except at the speed of light. If you suddenly stop the locomotive the rear of the train continues to come forward until it encounters the backward traveling signal (compression wave, shock wave, whatever). It can't avoid being crushed in this scenario.

Question:
If we go near the speed of light we have weird effects, time slowing, lengths contracting to zero. But light doesn't. Why isn't light subject to relativity?

Answer:
Light is certainly subject to relativity. Light has zero rest mass, and relativity says that anything with zero rest mass always has to go at the speed of light along lightlike trajectories in spacetime. If you want to be anthropomorphic about it, a photon doesn't experience the passage of time. To it, it is everywhere at once.

 

[uruk]

If you start by assuming some magical wheel that violates the laws of physics itself, then you might as well quit right there, because you have just thrown special and general relativity out the window.

Right! you can't violate one law of physics in order to explain another law or illustrate a concept otherwise the conclusion will not be valid. If you are going to make a claim about existance you have take all the rules that govern that existance into account.
You cannot arbitrarily ignore one rule and expect the result to give any meaningfull information.

Then, if yes, in what sense do we say that they have experienced time differently? If no, then we have a paradox that can only be explained in terms of the mind, imo.

It could also be explained in terms of time, distance and speed and referencial framework. this is the foundation of relativity.

I understand. But let's imagine a technology which enables us to impart an equal rotating force upon all parts of a wheel, simultaneously, so that all radials remain straight.

That could never happen in reality, to imagine that it could would violate certain laws of physics. any information you glean from this hypothetical situation would have no meaning in reference to this reality or this existance. you will be creating your own paradox.

Einstien engaged in these "thought experiments". His violation to the laws of physics is imagining that a "person" could "ride" along with a photon and "observe" the effects. other than this only violaton,(which does not affect the outcome) he used all the physical laws known to him to derive a conclusion. He then did the hard work to prove or confirm the conclusions (alot of really hairy math) Only now is our technology begining to approach ability to confirm some of those conclusions experimentaly. (i.e. particle accelerators, satellites, space telescopes...etc.)

 

[uruk]

The big philosophical point of all of this, is simply that our intuitive notions of how the World works, are not always very accurate.

which means that the workings of the universe is independent of our notions or perceptions.

 

[lifegazer]

The concept of "simultaneity" is also a Newtonian concept. As has been said, there is no such thing as absolute time (which would be required for all observers to view an event happening "at one time".

Okay. I understand. Others have made the same point too.

It appears that the scenario is impossible anyway, according to Stimpson and uruk.

I want to thank everyone who made a meaningful post here, especially stimpson, uruk and yourself.

I need to have a rethink and possibly approach this from an altogether different angle.

 

[lifegazer]

I found a site perfect for my small brain - short answers to big questions .

I great little site. I've saved it to favourites... thanks for sharing.

 

[Beleth]

I could have saved myself a lot o' trouble if I had just finished reading the thread before replying.

 

[MRC_Hans]

Okay. I understand. Others have made the same point too.

It appears that the scenario is impossible anyway, according to Stimpson and uruk.

I want to thank everyone who made a meaningful post here, especially stimpson, uruk and yourself.

I need to have a rethink and possibly approach this from an altogether different angle.

OK, you still have my respect.

Hans

 

[RussDill]

Okay. I understand. Others have made the same point too.

It appears that the scenario is impossible anyway, according to Stimpson and uruk.

I want to thank everyone who made a meaningful post here, especially stimpson, uruk and yourself.

I need to have a rethink and possibly approach this from an altogether different angle.

The rigid rotating disk is one of the most complex problems in GR. However, it can be worked out. Also, with the simultaneous thing, things can only be simultaneous in respect to a particular observer, which is why I was telling you to ask the question in respect to an observer.

 

[Elind]

Sorry; I don't get it. You choose to pose a question on understanding Relativity and Frames of Reference, while simultaneously defining a frame of reference that violates relativity, not to mention simpler physics principles. Next you will propose, perhaps, that we imagine that the universe was created 6000 years ago, while appearing like it happened 15 billion years or so ago, and ask how it is that creationists experience time differently?

scenario
Let's imagine a super-massive wheel as big as a galaxy (or even bigger if necessary), with fixed spokes/radials joining the circumference to the center.
Now, let's consider 3 points within that wheel upon a single radial. One point on the circumference(z), one point at center(x), and one point somewhere inbetween the other two, but much closer to the center(x) than the circumference(z).
Imagine the wheel is positioned so that gravitational considerations are approximately equal anywhere within the wheel. Hence, given that the wheel is stationary (at the start), we can say that an observer at each of our three points will be having an almost identical experience of time to one another. I.e., their clocks will all run the same and show the same time if they meet up.
... Now, the wheel begins to rotate so that the observer at position (z) on the circumference eventually accelerates to an extremely high velocity (c/2 for example). The other two observers, of course, are not moving very fast at all.

Question:-
All three observers share the same solid radial. The positions of other galaxies wrt that radial must be the same for all three observers, in reality. I.e., the galactic nightsky must appear to be in the same position for all three observers. If not, then howso? And if so, then in what sense do they all experience time differently?

I've posted this in philosophy because I might make philosophical claims which the science mob won't understand. Rather, they'll refuse to understand.

 

[epepke]

Man, I must be tired. I missed that completely. The concept of "simultaneity" is also a Newtonian concept. As has been said, there is no such thing as absolute time (which would be required for all observers to view an event happening "at one time".

It's a Galilean concept, actually, implicit in the Galilean transformations. This might seem picky, but I think that Newton gets short shrift sometimes. Newton's three laws of motion still work perfectly, given that Newton himself never expressed the law as equivalent to F=ma. It wasn't common back then to use this mathematical notation, but what Newton wrote is clearly better expressed as F=dp/dt. which still works perfectly.