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#53675 01/23/15 06:21 PM
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https://www.youtube.com/watch?v=MTY1Kje0yLg

It's just another look at the rubber sheet analogy, but I liked this one, especially as it introduced dark energy.

Downside? It's stirred up all those unanswered questions about gravity.

Is it a force, or not?

Does mass expend energy in warping spacetime, and maintaining the curvature?

Etc, etc......


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You can partly address some of your questions by realizing that in the video they had to clamp the fabric to a rigid frame to hold it taunt. Without that the whole thing would have collapsed in a bundle as would space collapse if there was not something holding space "stretched out" in their layman analogy.

That leads to the concept of a stress-energy tensor in relativity (Bill G and Rede didn't get that in a rather funny exchange on dark energy).

Background reading:
http://en.wikipedia.org/wiki/Stress-energy_tensor

So under Einstein relativity do you get what is proposed that stops the universe collapsing in on itself?

Hint: It's easy to see in EM and Radiation because they tend to radiate and spread out but not so obvious with matter because there are binding attractive forces to add in.

We could also tag two other pieces of information that will add in some useful things to consider in looking for a answer.

1.) Anything that moves gives off radiation. Your question to ponder is where does the energy come from?

2.) A not often understood fact from QM is that due to the uncertainty principle the bigger something is the tighter it can be packed. That is why the larger particles pack together in the nucleus of an atom. That is why even in a neutron star and beyond there are limits to the force gravity can exert as it tries to squeeze all the matter to a singularity and it must deal with QM. We refer to the process as degenerate matter (http://en.wikipedia.org/wiki/Degenerate_matter). If nothing else try to read and understand the concept section.


So there are things outside the simple picture and questions you asked to think about. Forces can be limited in really interesting ways that don't immediately come to mind in classic physics.

Last edited by Orac; 01/24/15 03:08 AM.

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Thanks, Orac; quite a lot to think about, there.

The thing that “jumps out” of your post is the question: “…where does the energy come from?” You probably recall that I have been asking that question about gravity, in one guise or another, for some considerable time. I think I even proposed an answer once. I would like to come back to that when I have had time to follow your links.

In the meantime, I have taken the next step in my own (layman) thought process, and as usual, present it as though I were explaining it to someone else.

Let’s start with a 2D image. On a sheet of paper, draw a circular spot to represent the gravitating body. According to GR, this body causes spacetime around it to bend. At some distance from the body draw a smaller spot to represent an object approaching the body. It is in uniform motion, so draw a straight line to show what the trajectory of the object would be if the body were not there. Now draw a line to represent the trajectory of the object, with the body present, such that the object is diverted by the curvature of spacetime, but not captured. Next draw a line to show the object going into orbit around the body.

All this fits the spacetime curvature scenario quite well. There are a few questions that arise, but at first glance they may not be obvious. Before you part with your sketch, draw a line representing the trajectory of the object if it is travelling straight towards the centre of the body. In this case, the curvature of spacetime will influence the object equally from both sides – we are still working in 2D – so the line will remain straight, there is no change in direction. There is, however, a change in the objects progress towards the body. As gravity takes hold, the object will accelerate. This is where an alarm bell rings. Somewhere from the depths of memory a Newtonian equation begins to emerge. F = ma. Force equals mass times acceleration. Whichever way you rearrange this equation, the force is still there. What is that force, if it is not gravity?


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Originally Posted By: Bill S.

Downside? It's stirred up all those unanswered questions about gravity.

Is it a force, or not?

Does mass expend energy in warping spacetime, and maintaining the curvature?

Whether gravity is a force depends, I think, on how you look at it. If you think of it as something that pulls you toward a massive object then it is a force. If you look at it as curved spacetime then it probably is just considered as a natural path, no real force required. Of course either way you look at it it acts like a force. That is it causes movement vectors to become curved so I will probably keep talking about the force of gravity.

In regard to mass expending energy to warp spacetime, My first thought is yes, but I'm not sure about how it works. The thing is that in order to have a mass expend energy in warping spacetime I have to go back to a thought experiment that never happens(1). If a mass suddenly appears in an "isolated" location spacetime will be warped. In that case energy will be required to create the warp. I assume that the energy comes from the mass creating the warp. That is a small amount of the energy required to create the mass, wherever that came from, will be expended in creating the warp. Of course for existing masses no further energy expenditure would be required to maintain the warp.

Then there is the energy of motion. Any kind of motion will of course cause changes in the warpage. Such changes will require energy and that energy comes from the motion. In other words anything that is moving is losing some of the energy of motion and slowing down. That is the source of gravity waves. Of course the amount of energy lost to gravity waves is normally very small. We can effectively ignore gravity waves when calculating the orbit of the Earth around the Sun.

(1) I am ignoring the question of where the mass of the universe came from prior to the big bang. Let's not go there.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Orac #53679 01/24/15 11:36 PM
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http://en.wikipedia.org/wiki/Degenerate_matter

“Imagine that a plasma is cooled and compressed repeatedly. Eventually, it will not be possible to compress the plasma any further, because the Pauli exclusion principle states that two fermions cannot share the same quantum state. When in this state, since there is no extra space for any particles, we can also say that a particle's location is extremely defined. Therefore, since (according to the Heisenberg uncertainty principle) ΔpΔx ≥ ħ/2 where Δp is the uncertainty in the particle's momentum and Δx is the uncertainty in position, then we must say that their momentum is extremely uncertain since the particles are located in a very confined space. Therefore, even though the plasma is cold, the particles must be moving very fast on average. This leads to the conclusion that in order to compress an object into a very small space, tremendous force is required to control its particles' momentum.”

I have a problem here. In the case of a particle that is free to move, its momentum can be measured, but, because it is moving, its position at any given time will be unknown. If its position at a given time is known precisely, then its momentum will not be known, but it is reasonable to assume that a second measurement of its position would reveal that its position had changed. It would not be possible to infer its momentum from this change in position because there is now way of knowing how it moved from position 1 to position 2. E.g. it could have taken all possible routes, or only one.

Is that reasoning correct?

The quote from Wiki seems to be saying that the plasma is compressed to a point where no further compression is possible, so the location of each particle “is extremely defined”. In spite of this maximum confinement the particles are “moving very fast on average”. How can they be moving if they have no space in which to move? Alternatively; how can they be still and have their positions “extremely well defined” without violating the HUP?


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Quote:
Whether gravity is a force depends, I think, on how you look at it. If you think of it as something that pulls you toward a massive object then it is a force. If you look at it as curved spacetime then it probably is just considered as a natural path, no real force required.


We seem to be thinking more or less along the same lines. The next step in my process went something like this:

When we move from 2D to 3D the usual explanatory image is that of the gravity well, which emerges directly from the rubber sheet analogy. The mass (usually represented by a bowling ball) depresses the sheet, and any passing object that comes close enough is diverted from its straight line course and may enter orbit around the mass.

Newtonian mechanics tells us that diverting the object from its straight line requires input of energy. A force must act on the object in order to divert it towards the mass. Does relativity change this? Relativity says that in following curved spacetime the object is following a geodesic. A geodesic is defined as the most direct route in curved spacetime. A geodesic seems to have a lot in common with a straight line. Could it be that a straight line in Euclidian geometry is equivalent to a curved line in the non-Euclidian geometry of curved spaces? Would it follow from that that, given an appropriate change in geometry, no force, and therefore no expenditure of energy, is required to alter the course of the object?

In a way, that would be a convenient line of reasoning, but, unsurprisingly, it leads to other problems.


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Originally Posted By: Bill S.
The quote from Wiki seems to be saying that the plasma is compressed to a point where no further compression is possible, so the location of each particle “is extremely defined”. In spite of this maximum confinement the particles are “moving very fast on average”. How can they be moving if they have no space in which to move? Alternatively; how can they be still and have their positions “extremely well defined” without violating the HUP?

You are applying a very classical concept remember the particle movement is more than just the kinetic classic physics movement it includes oscillations, spins and quantum vibrations. To try to give some classical relationship think of the typical ice skater spin they come skating in with speed and turn lateral movement into enormous rotational spin speed.

That is the classical version there is the same possibility in the quantum modes and it was explained thus
Quote:
At very high densities, where most of the particles are forced into quantum states with relativistic energies

So the particles are locked in your classical movement world but exhibit incredible vibration in the other degrees of freedom. If you like as in the ice skater example gravity is trying to lock the position to one physical location but the particle is then forced to shifting the uncertainty to the other degrees of freedom as the energy must go somewhere like spins and vibrations.

So in the classic world you would measure the plasma as cold as it's not moving in classic 3d space but there is still extreme uncertainty about which way it's spinning, vibrating etc. If you tried to touch the plasma as with trying to touch the spinning skater you would find out just how much energy it really contains.


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Originally Posted By: Bill S.
A geodesic seems to have a lot in common with a straight line. Could it be that a straight line in Euclidian geometry is equivalent to a curved line in the non-Euclidian geometry of curved spaces?

That is exactly what it is smile

http://en.wikipedia.org/wiki/Geodesic
=> In mathematics, particularly differential geometry, a geodesic is a generalization of the notion of a "straight line" to "curved spaces".

The equator is the maximal geodesic of the earth.

Originally Posted By: Bill S.
Would it follow from that that, given an appropriate change in geometry, no force, and therefore no expenditure of energy, is required to alter the course of the object?

So we could ask if the earth was all land does the amount of energy we require to walk or drive around it vary at any point if we are moving along a geodesic.

Originally Posted By: Bill S.
In a way, that would be a convenient line of reasoning, but, unsurprisingly, it leads to other problems.

I would be interested in what problems you think there are because this is all very classical. If we had the all land earth above and there was no friction and I rolled say a ball what would happen?

Last edited by Orac; 01/25/15 03:48 AM.

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If the Earth was all land and there was no friction the first thing that would happen is you would be propelled backward by an amount of energy equal to the energy you used to roll the ball. The ball would eventually roll back around to where you let go of it, and you would eventually slide back to where you were when you let it go. The ball, having less mass, will travel faster due to its greater momentum vs. mass. So you would pass the ball eventually, but at a point nearer to the origin than the ball.

When you specify no friction, all sorts of things become possible. :-)


If you don't care for reality, just wait a while; another will be along shortly. --A Rose

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The sillier among us have a large ball going forward and you going backward until the ball hits you behind the knees and knocks you flat. Of course this does not stop your motion, so next time it hits you on the head. Thank goodness for friction.


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I’ll be happy to return to the problems, but they might constitute a digression from the current “flow”.

Quote:
So we could ask if the earth was all land does the amount of energy we require to walk or drive around it vary at any point if we are moving along a geodesic.


Discounting surface irregularities, no, but we are permanently following the geodesic. Even when we feel as though we are travelling in a straight line we are still following the geodesic. Is that the same as travelling in a straight line in space, then transferring to a geodesic?


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Originally Posted By: Bill

Whether gravity is a force depends, I think, on how you look at it. If you think of it as something that pulls you toward a massive object then it is a force. If you look at it as curved spacetime then it probably is just considered as a natural path, no real force required.


That seems like a very reasonable view, but it links to the problems I hope to return to. Hold that thought.


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Correct x 2 smile

The key point is no energy was lost or gained while rolling around the geodesic and the balls energy is unchanging. So constant motion or stationary objects can both have a constant amount of energy. We could take the idea further and spin the ball and now roll it around the earth and now it has even more complex motions yet the energy remains constant.

So constant energy does not require simple straight line motion or no motion it can occur in the most complex movements.

Nothing changes in relativity with spacetime your ball is still rolling along a geodesic which has the same relative time and so no energy is lost or changed. However rather than need a force of gravity to hold the ball against earth like classic physics the ball is trying to run down the energy gradiant provided by the change in time towards the centre of earth and it is that energy gradient that holds it against the earth. The two situations are indistinguishable under classical physics tests you are simply replacing a force with an energy gradient.

You obviously need to do different tests like to see if time changes in the presence of a gravitational source.


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Originally Posted By: Bill S.
Discounting surface irregularities, no, but we are permanently following the geodesic. Even when we feel as though we are travelling in a straight line we are still following the geodesic. Is that the same as travelling in a straight line in space, then transferring to a geodesic?

You still aren't seeing the problem how would you ever know if you are traveling in a straight line in space smile

Even if I travelled directly at a close target say the moon. The milky way galaxy and our solar system are rotating and moving so I ask in what sense am I travelling to the moon in a straight line?

For the record:
The rotation speed of earth relative to the centre of the galaxy is 220 km per second, or 490,000 miles per hour.
The milky way is moving at around 600 km per second or 1,342,000 miles per hour.
The flight time for apollo 11 was around 3 days to the moon (259200 seconds) and its fastest speed was 11km per second or 25,000 Miles per hour.

Calculate the galaxy movement distances in that time smile

Bonus points: Can you work out what the true path looks like if you plotted it from some point outside the galaxy. Hint for every 11Km along the "straight line to the moon" it rotates 220km and moves laterally 600km.

This is the problem of trying to create a zero frame for the universe which is actually what you are trying to do so you can work your classic energy calcs from there .. fun isn't it smile

Straight lines don't really exist in space, they exist only in some relative sense to some reference point that has some general movement similar to you. So above we talk about a straight line to the moon because we are sort of moving in relatively similar way to the moon but the actual movement is anything but a straight line smile

Last edited by Orac; 01/25/15 04:33 PM.

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Originally Posted By: Orac
You can partly address some of your questions by realizing that in the video they had to clamp the fabric to a rigid frame to hold it taunt. Without that the whole thing would have collapsed in a bundle as would space collapse if there was not something holding space "stretched out" in their layman analogy.

That leads to the concept of a stress-energy tensor in relativity (Bill G and Rede didn't get that in a rather funny exchange on dark energy).

Background reading:
http://en.wikipedia.org/wiki/Stress-energy_tensor

So under Einstein relativity do you get what is proposed that stops the universe collapsing in on itself?

Hint: It's easy to see in EM and Radiation because they tend to radiate and spread out but not so obvious with matter because there are binding attractive forces to add in.

We could also tag two other pieces of information that will add in some useful things to consider in looking for a answer.

1.) Anything that moves gives off radiation. Your question to ponder is where does the energy come from?

2.) A not often understood fact from QM is that due to the uncertainty principle the bigger something is the tighter it can be packed. That is why the larger particles pack together in the nucleus of an atom. That is why even in a neutron star and beyond there are limits to the force gravity can exert as it tries to squeeze all the matter to a singularity and it must deal with QM. We refer to the process as degenerate matter (http://en.wikipedia.org/wiki/Degenerate_matter). If nothing else try to read and understand the concept section.


So there are things outside the simple picture and questions you asked to think about. Forces can be limited in really interesting ways that don't immediately come to mind in classic physics.



ABOUT EM RADIATION

http://en.wikipedia.org/wiki/Solar_sail

How body is generating CZERENKOW's Radiation


IN UNIVERSE EXIST N objects
N objects are sending N EM waves

Czerenkow ?


please try move in universe it is very similar situation to
swim

If You swiming under water You are pushing EM preasure
Your body is full of atoms ( atoms Joust like eats energy - absorbtion process )

Body that is moving joust makes TRAP

http://2.bp.blogspot.com/-iNHYy6u9bRg/VF9En99DNmI/AAAAAAAACFA/vOX3fZYchx4/s1600/em.jpg

Orac #53691 01/25/15 10:04 PM
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I am aware that one should not talk of a straight line without reference to something. However, if a moving object which, in our F of R, we measure as being in uniform motion, relative to something, changes course, relative to that same something, we deem it to have accelerated, relative to that something, and conclude that the object has been influenced by a force. Our conclusion may be erroneous if the motion, and change in motion, are considered relative to some other something in another F of R, but within ours, I think it does not conflict with the laws of relativistic physics.


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Returning to the gravity well, we have to ask how well the rubber sheet analogy illustrates the real situation. The gravity well of the rubber sheet analogy is really only a 2D illustration partially translated into 3D. The full 3D image would be practically impossible to represent on a sheet of paper. The mass becomes a sphere subtending a quasi-infinite number of gravity wells, impinging upon it from every direction. It might be tempting to visualise this as being like a sphere made up of closely packed gravity wells, like one of those paper Christmas decorations composed of closely packed hexagonal cones. Such would not be the case. In fact, every gravity well would have to overlap a boundless number of other gravity wells, such that if one imagines a sphere of any size centred on the mass, every gravity well would have to intersect the surface of that sphere so that its “mouth” formed a great circle. Any other configuration would not permit a stable orbit to be established by the object, irrespective of its direction of approach.


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That's a problem with all the analogies we use to help us visualize how the universe works. We start off looking at them and saying "OK, now I understand that". Then if we start looking more closely we find that things just aren't that simple. To see the way it really is I generally just look at the analogy, then remind myself that is not the whole story.

When you (Bill S.) ask some of your questions I wind up getting all tangled up trying to figure out what is really happening. I am not saying your questions are inappropriate, except that they make me have to thing about them. And I really don't much like to have to think.

Bill Gill


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C is the universal speed limit.
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How You want to recognize

force = gravitation or force = Em preasure

can You feel problem ?

http://1.bp.blogspot.com/-spFELf_xcJ0/U_CBPKLmCEI/AAAAAAAAB5M/FfYELmw3M6c/s1600/pytanie1.jpg


p1..p2..p3..me ---> motion

Bulb and rocket was in point p1
p2,p3 now is in point p4 will be in point p5

Please evaluate 45 degree problem ( two source )
how fast is rocket if EM preasure can not push rocket ?

ABOUT EM preasure
http://en.wikipedia.org/wiki/Solar_sail

Last edited by newton; 01/26/15 09:47 AM.
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Quote:
When you (Bill S.) ask some of your questions I wind up getting all tangled up trying to figure out what is really happening


Don't just wonder about it, Bill, ask, criticize, throw rocks! Most of my questions are asked because I'm trying to figure out what's going on, so any and all input is a bonus.


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