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HST and 'dark matter'
stevesliva
post Mar 11 2010, 05:08 PM
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I find it interesting that physicists have been considering inertia to be a "puzzling" concept. It has never even occurred to me to think, huh, it takes a force to impart velocity to an object? Why would that be? Of course I can't understand why it would be more "natural" to understand this as adding entropy rather than giving it a kick, but whatever! Yay physics.
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Mongo
post Mar 11 2010, 05:37 PM
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From Inertia Theory -- Paul Davies:

QUOTE
Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from.

Newton believed that inertia is an innate property of matter manifesting itself whenever matter accelerates (this includes rotation) relative to absolute space. You could think of the space in the vicinity of the accelerating body as somehow reacting to its motion to produce inertial forces. Newton never explained how, but took it to be a law of nature.

Newton's arch rival, Gottfried Leibniz, rejected this. Space, being empty, provides no reference against which a body can be said to accelerate. How can something move with respect to nothingness? We can judge motion, claimed Leibniz, only relative to other material bodies. Take the Earth's rotation. We observe the daily progression of the sun and stars across the sky. Our ancestors believed it was the heavens that turned, not the Earth. But suppose there were no sun and stars? Suppose Earth were alone in infinite space? Would it then make any sense to say it was rotating?

The debate raged on. Newton's hypothesis of absolute space predominated, but champions of the alternative "relative" view fought back, first in the guise of the Irish philosopher Bishop George Berkeley, then Austrian physicist-philosopher Ernst Mach - he of the Mach numbers. Mach, whose ideas greatly influenced Einstein when formulating his theory of relativity at the start of the century, insisted that acceleration can be defined only relative to the distant stars, a statement that came to be dignified with the name of "Mach's principle".

Mach's principle faced a thorny problem. Acceleration produces inertial forces. How can the distant stars be responsible for those? Could it really be that the child riding the roundabout is being tugged at by far-flung galaxies?

Einstein and others sought a mechanism to explain how a rotating body might experience a centrifugal force as a result of some sort of interaction with all the distant matter in the universe. A clue came from the theory of gravitation: after all, centrifugal force is sometimes even called artificial gravity.

Viewed from the roundabout, it is the rest of the universe that is rotating. We know that when electric charges circulate around a loop the resulting electric current produces a magnetic field. Could it be that the apparent rotation of the universe produces a gravitational version of a magnetic force that plucks at the clinging child? To test the idea, Einstein considered a small body at rest inside a rotating shell of material in otherwise empty space. Using his theory of relativity, he calculated what would happen. It turns out that the body should indeed feel a tiny gravito-magnetic force.

Further evidence in favour of Mach's principle comes from cosmology. If rotational motion is purely relative, then it is clearly nonsensical to talk about the rotation of the universe as a whole, for with respect to what would it rotate? In Newton's theory, it is entirely possible for the entire cosmos to spin about some axis. Given that almost all astronomical systems are observed to rotate to some extent, we might expect, if Newton is right, to observe a universal rotation too.

Astronomers find no evidence for a systematic rotation of the universe. Their observations imply that the universe cannot have turned by even one degree since the big bang. If rotation is absolute, the absence of a universal rotation seems to be a very special and contrived state of affairs, but if as Mach claimed it is relative, then the observations are explained.
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djellison
post Mar 11 2010, 05:47 PM
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Is it just me, or is this a catastrophically screwed up analogy?

"Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from"

Errr - F=MA and A=V^2 / R

Nothin mysterious about it.
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Mongo
post Mar 11 2010, 06:04 PM
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That was my first thought too, but how does the water 'know' that it is experiencing circular acceleration? The magnitude of the 'centrifugal' (or conversely, centripetal) force is easily calculated, but the reason it exists is apparently much harder to understand. Einstein never could. As the example stated, if the Earth were rotating in an otherwise empty universe, would it still be considered to be rotating? And would it still experience polar flattening as a result? If it does, how does it 'know' how quickly it is rotating (to generate that degree of flattening) with nothing to act as an external reference frame, and if it does not, why does the presence of some other random object(s) in the universe affect the shape of the earth, to a far larger degree than those caused by tidal effects? And is one other physical object in the universe enough to establish a reference frame, or is an entire universe equivalent to ours necessary? If so, why?

Inertia is very easy to handle mathematically, it's practically the first thing taught in high school physics, but its origin is apparently much more difficult to explain. It appears that this new theory of gravity by Verlinde has inertia fall out almost automatically, in terms of entropy gradients.
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djellison
post Mar 11 2010, 06:25 PM
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So it'snot a question about a bucket at all, it's just asking WHY F=MA
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centsworth_II
post Mar 11 2010, 06:46 PM
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QUOTE (djellison @ Mar 11 2010, 01:25 PM) *
...it's just asking WHY F=MA
And, why mass? Hence the search for the Higgs.
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Shaka
post Mar 11 2010, 06:47 PM
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If Earth were in an otherwise empty Universe, why would it have formed into a sphere? blink.gif

(Just when I had it figured out that a 'string' broke when the apple hit Newton.)


--------------------
My Grandpa goes to Mars every day and all I get are these lousy T-shirts!
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stevesliva
post Mar 11 2010, 07:13 PM
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QUOTE (Mongo @ Mar 11 2010, 01:04 PM) *
Inertia is very easy to handle mathematically, it's practically the first thing taught in high school physics, but its origin is apparently much more difficult to explain. It appears that this new theory of gravity by Verlinde has inertia fall out almost automatically, in terms of entropy gradients.


Yes, but I don't understand why an balanced entropy gradients are so much more gee-whiz than balanced forces. Someone will be unsatisfied with not knowing the origin for the 2nd law of thermodynamics, right?

I understand the elegance of describing inertia in terms of entropy, but in my mind, it's just saying that this thing we think is fundamental is just like this other thing that is fundamental. It's still fundamental.

I would think that if some young einstein asked his physics teacher "why is there inertia?" the answer would be "it's fundamental." Upon reaching university, a physics professor might say, "actually, it can be expressed as a manifestation of entropy." At which point the young einstein says, "why is there entropy?" Does the physics professor say, "it's fundamental?" tongue.gif

Is the reason it's so gee-whiz because entropy is a fundamental property of everything, not just matter and energy, but also information? We're better linking in the concepts of data and uncertainty with matter and energy?
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Mongo
post Mar 11 2010, 07:30 PM
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The following is just my own understanding of what this theory of gravity is saying.

It looks to me that it says that the only truly fundamental 'things' in our universe are the holographic screens that separate and define every point in (two-dimensional) spacetime, and the information / entropy associated with each such screen (plus whatever micro-level theory actually coordinates the individual points and screens, i.e. string theory or its big brother M-theory, loop quantum qravity, twistor theory, etc.), everything else emerging from that.

(Actually, the current word is that this theory is incompatible with string theory or M-theory, since it goes directly from 2D spacetime to our observed spacetime with no intervening extra dimensions. This is no surprise to me, since I have always been sceptical of string theory -- it must have the highest ratio of effort invested to usable results, of any physical theory ever.)

I have heard nothing about the strong and weak nuclear forces so far, but gravity, and very possibly electromagnetism, are already being described as derived forces, so I would not be surprised if the other two 'fundamental' forces are as well, in the full theory.
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Greg Hullender
post Mar 11 2010, 10:25 PM
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QUOTE (djellison @ Mar 11 2010, 10:25 AM) *
So it'snot a question about a bucket at all, it's just asking WHY F=MA

This has been a question for a long time. I remember my Physics professor at Caltech making a point of it when I was a freshman back in 1977.

When I was a kid, protons and neutrons were thought to be fundamenal particles, although there was a bewildering array of other, short-lived subatomic particles. It was exciting to witness the discoveries that replaced all that with the much more elegant system of quarks and leptons, and it's even exciting to think we may be witnessing another major refinement.

It'll be nice when someone has worked out an easier way to explain it to the educated layman. Meanwhile, I'm still waiting for someone to explain how they plan to test this hypothesis.

--Greg
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Marz
post Mar 11 2010, 10:56 PM
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QUOTE (Greg Hullender @ Mar 11 2010, 04:25 PM) *
This has been a question for a long time.


lol - who would've thought a spinning bucket is at the heart of modern physics?

I think the essence of the Bucket is to understand what aspects of gravity are mathematical abstractions or a natural phenomenon. From Greene's layman book "Fabric of the Cosmos" Chp 2, The Universe and the Bucket:

"If velocity is something that only makes sense by comparisons ... how is it that changes in velocity are somehow different, and don't also require comparisons to give them meaning? ... Could it be that there is some implicit or hidden comparison that is actually at work every time we refer to our experiences of accelerated motion? This is a central question .... it touches on the deepest issues surrounding the meaning of space and time."

Meanwhile... I'll have to decide what feels less silly:

Dark Matter (and perhaps Dark Energy) or holographic screens. Planetary geology is so lovingly concrete in comparison.
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Mongo
post Mar 12 2010, 12:27 AM
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QUOTE (Marz @ Mar 11 2010, 11:56 PM) *
Meanwhile... I'll have to decide what feels less silly:

Dark Matter (and perhaps Dark Energy) or holographic screens. Planetary geology is so lovingly concrete in comparison.


The term 'holographic screen' is perhaps a little unfortunate. We are not talking about some special movie screen on which to watch Avatar in glorious 3D! It is simply a way of describing the information associated with any volume of spacetime as being encoded on the surface boundary of that volume (when observed from outside that volume), hence the term 'screen', and since there is always more information contained in that screen than can be observed from outside, it is also 'holographic'. The term was originally applied to the event horizens of black holes, where it made perfect sense to refer to a 'screen' which screened the interior from outside observation, but it was found that the concept could be extended to any (continuous) 2D surface that fully enclosed some volume of spacetime.

In the case of this theory, they are taking the holographic screens to be the smallest possible, each one surrounding a single point of spacetime, which leads to gravity by means of thermodynamic arguments. At the other end of the scale, by taking a holographic screen that encompasses the entire 'universal horizon' of this universe, it is possible to generate a force that matches that of so-called 'dark energy', which is the cause for the cosmological acceleration at very large distance scales.
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monty python
post Mar 12 2010, 06:58 AM
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I'm really loving this thread, but for some reason I find comfort knowing that theories are just usefull tools we humans use to figure out how stuff works. They don't always need to be spot on correct.
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AndyG
post Mar 12 2010, 11:05 AM
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QUOTE (Marz @ Mar 11 2010, 10:56 PM) *
lol - who would've thought a spinning bucket is at the heart of modern physics?


smile.gif I prefer the late Douglas Adams' prescient insight into this. A nice cup of tea lies at the heart of Arthur Dent's well-being and, of course, stirring tea and then momentarily rotating the cup provides a demonstrably easier, tastier (and potentially less wet) appreciation of inertia and Mach's Principle.

Andy
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SteveM
post Apr 4 2010, 02:13 PM
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I've noticed that none of the arXiv papers are listed as having been accepted by (or even submitted to) a peer-reviewed journal. They may be interesting developments of a theoretical speculation, but caveat lector.

Steve M
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