Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 07/03/2011 12:04:18
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Imagine a universe devoid of matter in which we introduce a spacecraft. We accelerate it.
What is its 'potential energy' or momentum, or 'relative mass'
How will you measure it?
And if you can't, does it exist?
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Now try to define a 'speed'.
Can you?
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When you've done that, now tell me if there will be an inertia experienced at the acceleration?
Relative what?
Lastly, will there be a 'gravity' experienced in that acceleration?
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You can only accelerate your spaceship in the otherwise empty universe by ejecting either matter or radiation then it would not be the only thing in the universe and the normal laws would apply
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I notice your 'empty' universe is only devoid of matter, does radiation such as the CMBR exist there.
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Your question boils down to 'is there an æther' the approved answer is no!
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Nope, if you accept 'space', then that is sufficient for my questions. Why any ejecta would make it different? Explain why please. Do you expect it possible to use radiation from the acceleration as a 'measure'?
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Hmm, are you suggesting that the radiation would make inertia and gravity possible?
So a radiative universe must contain inertia and gravity?
That's interesting, but can you prove it.
Any light experiments showing that all light contain gravity?
As well as inertia?
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By the way, you didn't answer the questions?
Seems like you attacked the concept instead.
Why not attack Mach principle too then?
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I'm not saying that such a universe can exist by the way. Although if we assume that even radiation 'clings out' in interactions, as when colliding with some other spontaneously created pair production which in infinite 'times arrow' should happen, then you need to ask yourself where that 'energy' goes. Also consider the fact that any light existing contradict the entropic statement of all 'work done' thermodynamically. One possible idea might be to consider it becoming part of that mysterious 'stress energy tensor' as the conservation laws demands all 'parts' of a universe to stay inside it, only transform from usable to unusable 'work'. Or we rewrite it slightly defining 'unusable' work as any energy you can't get to physically, which then seem to include a whole universe, here and now.
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It's a tricky one. We already know that we have a lot of 'states' more than what we observe macroscopically. So 'ordinary light' is not a ultimate answer. Maybe 'time' is though?
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You can only accelerate your spaceship in the otherwise empty universe by ejecting either matter or radiation then it would not be the only thing in the universe and the normal laws would apply
This is very smart, compliments!
(I say seriously).
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Now I have two views without explanations here?
Any of you wanting to define exactly what you mean?
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If I place a spacecraft there it will by definition not be 'empty' anymore.
I defined it as empty relative the spacecraft. Assuming that I throw out a apple from it :) you will now have two objects in that 'space'. Looking inside the ship I can find more objects to throw out. The question wasn't how many objects I can free, unless you expect gravity to need those objects to exist. If so you seem to be defining gravity as something only existing through matter. In which case my spacecraft in itself should be enough as I assume it to be made of just that 'substance'?
So you say that gravity comes out of matter then?
Or 'radiation' possibly?
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Or possibly the idea is that 'gravity' is a relation between?
Radiations?
But that one needs to be proved.
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Let us look at how I understands it from Einsteins General relativity. Then Einstein have defined 'gravitational waves' which from my view is 'SpaceTime' itself 'moving'. You might want to use that as a proof for 'gravitons', but I don't think Einstein did so? Take that lovely cube of Jello and hit it, see it flutter. Did shock waves pass through it? Yes. Did we need to invent 'shockitons' for it? No. What we had was 'kinetic energy'. But then, 'energy'. That exist, doesn't it? Sure, in any interaction it exist but there is no substance or particle existing that you can observe as 'energy' solely. The best approximation I've seen seems to be the stress energy tensor as a expression of 'energy'.
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You could want to argue that photons is 'pure energy' though?
But, they only exist in their interaction, don't they?
Even as waves this is true. You can define a wave as infinite if you like, but to see it you still need that localized interaction.
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If you take inertia as it is in SpaceTime.
It exists everywhere, right?
What would you define it against?
Assume that you are uniformly following a geodesic. Would that be the same as being still in a place where no gravity is measurable? If you think it is? Then we have defined a experiment similar to what I described before with the spacecraft. Make it a 'black box experiment'. Would you be able to differ this from my first question about that Spacecraft before we accelerate it? If you can't find a way, are they then equivalent?
And if we follow the logic, in this case that 'gravity' and 'inertia' need something to react against as our acceleration, and its ejecta. Assuming so, it seems to become a Newtonian description of action and reaction? Using that argument I can now state that any acceleration, creating a gravity, and inertia is solely local. That should then mean that we have no 'gravity' permeating space, right? As they need an 'action' to 'react'?
But there is a gravitational potential in space too, isn't there? You might argue that those parts of space are accelerating too of course? But, exactly how do space 'accelerate' relative space?
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Let's look at compression. How does that create a black hole? Does the distribution of that black hole change the SpaceTime it exists in, the 'field' if you like? Do motion produce a black hole?
First motion. Not as I understands it. Why? The best idea is to consider if you could be still relative that motion. Could you? Sure. and when you are still relative it, 'uniformly moving' with it for example, will you be able to see it? If you're close enough you should, and for this question we assume you are, and if you can see it, can it be a black hole for someone else? I don't think so myself, although there is the question about what a infinite Lorentz contraction can do to a moving object of course? There I don't know if that is enough, assuming that you really 'contract', as observed by someone being being still relative your origin (Earth). I know it's only in the direction of its motion it contracts, but assume something really thin and long for this :) But, no black hole made by motion, as I think of it now that is.
Does the distribution of that black hole change the SpaceTime it exists in, the 'field' if you like? Not as I understands it, it will have the same gravitational 'pull' as before, the only thing changing that it actually have become what Newton described 'gravity' as, emanating from the 'center' of any sphere. That center now being a indefinite 'point' containing a 'infinite gravity', same as I understands it for any invariant mass becoming a black hole, and that's real strange, but as I understands it also true. for those of you arguing that nothing can pass the EV (event horizon) I don't agree. You're mixing the frames there, if you mean that we won't notice any motion near the EV, observing from afar, you might be correct though. doesn't change its center though, being 'infinite'.
How does compression create a black hole? Yeah, that one puzzles me too, what happens when it gets compressed, and why does it get compressed. "As a star runs out of fuel it can expand and will begin to form heavier elements such as carbon and iron (most of the matter in our solar system comes from extra-solar sources). Once it finally exhausts all of its fuel it will begin to collapse. It is here that the stars begin to undergo different fates.
Our own sun will collapse until it becomes a white dwarf, at this point the Pauli exclusion principle keeps the electrons in the star far enough apart to resist further collapse - this energy is called 'electron degeneracy'. Stars greater than 1.4 times the mass of the Sun (called the Chandrasekhar limit after the Indian physicist who discovered it on his way to England) will tend to explode in a supernova casting off much of their mass. A small central core will remain and like smaller stars this will collapse only this time electron degeneracy will not be enough to support the star's mass against its gravitational collapse and it will continue to shrink until it becomes a tiny, but hugely massive, neutron star held together by neutron degeneracy. If neutron degeneracy is not enough to resist the star's collapse it will continue to shrink until the matter is all compressed into an infinitely small, infinitely dense point called a singularity. This is the centre of a black hole." Take a look here for a definition of a 'simple' non-rotating black hole Schwarzschild solution. (http://en.wikipedia.org/wiki/Schwarzschild_metric#Singularities_and_black_holes)
"Degenerate matter is matter which has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle prevents the constituent particles from occupying identical quantum states. Any attempt to force them close enough together that they are not clearly separated by position must place them in different energy levels. Therefore, reducing the volume requires forcing many of the particles into higher-energy quantum states. This requires additional compression force, and is made manifest as a resisting pressure." "Although the precise figure is uncertain, mainly because the behavior of matter at very high densities is not well understood, most researchers concur that the mass of a neutron star cannot exceed about three times the mass of the Sun. This is the neutron-star equivalent of the white dwarf mass limit discussed in the previous chapter. Above this mass, not even tightly packed neutrons can withstand the star’s gravitational pull. In fact, we know of no force that can counteract gravity beyond this point."
What we might want to define 'infinite gravity' from, intuitively is the black holes Event horizon. And what makes that so remarkable is that this is the place where light, and matter, disappear. If you turn it around the EV defines the place where the Black holes existence 'ends'. Inside it the light have only one direction as I understands it, towards its center, there can be no 'reflections' back towards where it came. That you can assume a more or less 'infinite space' inside it, as observed from the inside, have nothing to do with this statement. As seen from outside light have only one direction, well past the event horizon. And notice that the possible size of this 'inside' is related to the black holes 'invariant mass', but the 'infinite gravity' at any black holes center will be the same, no matter its mass.
So you might want to define this compression as being decided by the creation of a event horizon. And as I like to look at it, as that light only have one way as observed by the far observer.
Now, doesn't this state that gravity is a matter of eh, matter?
Maybe.
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Did you notice the definition of degenerate matter? As matter that has been stripped of its orbital electrons with only the nuclei left, incredibly close to each other. As a star 'shrinks' and becomes more massive its neutrons become relativistic, prevented from slowing down, cooling and losing energy by the Pauli exclusion principle, for a neutron star ending up as a 'relativistic degenerate gas' of neutrons. In a white dwarf, as its low energy states fills up with energy, the remaining electrons will be forced to keep their speeds and so also create a higher temperature, and pressure. That creates a 'outward' pressure that keeps gravity from collapsing them further.
But only to a certain point, the more massive the white dwarfs becomes the higher the density, and so energy, they contain. And at some point their electrons will be approaching the speed of light, meaning that they, in the end, would need to 'move' faster than light to support that 'outward' pressure. The limit where they fail is called the Chandraskhar Limit and is the mass beyond which a white dwarf must collapse into a neutron star, about 1.4 solar masses. And in a neutron star we only had the nucleus left, right? No electrons anymore. Only a incredibly hot star emitting short wave radiation x-rays, the next most energetic radiation we know to supernova explosions (gamma rays). You might also notice here that the 'spinning around' the electrons do here actually have a real effect, as compared to the 'spin' we speak of measuring in particles, those already being faster that lights speed in a vacuum (also called polarization in photons)
"As the wavelengths of light decrease, they increase in energy. X-rays have smaller wavelengths and therefore higher energy than ultraviolet waves. We usually talk about X-rays in terms of their energy rather than wavelength. This is partially because X-rays have very small wavelengths. It is also because X-ray light tends to act more like a particle than a wave. X-ray detectors collect actual photons of X-ray light - which is very different from the radio telescopes that have large dishes designed to focus radio waves."
So what is a nuclei?
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A nuclei consist of what we call protons and neutrons. Neutrons are neutral, meaning that they have no charge. Protons has a positive charge. And those electrons, disappearing from our Neutron star, had a negative charge. According to the standard theory those protons and neutrons are, in 'reality', made out of quarks. The proton has two 'up quarks' and one 'down quark' and the neutron two 'down quarks' and one 'up quark'. As the proton carried an electrical charge it mean that at least some of the quarks should be 'electrically charged'. And as the neutron was 'neutral', having no charge, while built by the same quarks as the proton there has be something making them differ, and that is how you combine those quarks.
Before the discovery of quarks all charges was thought to be multiples of the proton charge but finding that it was made of quarks the protons charge had to be split up. The standard model describe three basic amounts for a charge. + 2/3, −1/3, and −1. When it comes to electrons they are similar to the muon and the tau, having the same electrical charge and acting similarly, although the electron having a different mass and that the muon and tau could decay into other particles, whereas the electron was stable and unchanging.
So .. "Everything around us is made of matter particles. These occur in two basic types called quarks and leptons. ('Leptons can either carry one unit of electric 'negative' charge, or be neutral. Moreover, all leptons have antiparticles called antileptons.')
Each group consists of six particles, which are related in pairs, or ‘generations’. The lightest and most stable particles make up the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter in the Universe is made from particles that belong to the first generation; any heavier particles quickly decay to the next most stable level.
The six quarks are paired in the three generations – the 'up quark' and the 'down quark' form the first generation, followed by the 'charm quark' and 'strange quark', then the 'top quark' and 'bottom quark'. The six leptons are similarly arranged in three generations – the 'electron' and the 'electron-neutrino', the 'muon' and the 'muon-neutrino', and the 'tau' and the 'tau-neutrino'. The electron, the muon and the tau all have an electric charge and a mass, whereas the neutrinos are electrically neutral with very little mass."
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A lot to stomach right. But I believe me to have a point somewhere :)
And here it comes. "In the modern theory, known as the Standard Model there are 12 fundamental matter particle types and their corresponding antiparticles. The matter particles divide into two classes: quarks and leptons. There are six particles of each class and six corresponding antiparticles. In addition, there are gluons, photons, and W and Z bosons, the force carrier particles that are responsible for strong, electromagnetic, and weak interactions respectively.
These force carriers are also fundamental particles. All we know is that quarks and leptons are smaller than 10^-19 meters in radius. As far as we can tell, they have no internal structure or even any size. It is possible that future evidence will, once again, show this understanding to be an illusion and demonstrate that there is substructure within the particles that we now view as fundamental."
So, what is matter?
Or in this case, the nucleus.
And, how does any/one/thing 'compress' it?
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And, what do you expect to be left as it becomes a 'black hole'?
Energy maybe? Concentrated to a infinitesimally small point then?
So, what is gravity?
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Imagine a universe devoid of matter in which we introduce a spacecraft. We accelerate it.
What is its 'potential energy' or momentum, or 'relative mass'
How will you measure it?
And if you can't, does it exist?
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Now try to define a 'speed'.
Can you?
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Well, all you need is an accelerometer and a clock on your craft. You can determine everything you need to know if you have a record of acceleration against time.
(This is too easy - there must be some sort of catch here [???])
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It seems a universe devoid of matter would not have any frame of reference of its own. The spacecraft would be the biggest and smallest entity in it. Its acceleration would be both zero and infinite. Sounds like a potential big bang.
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Yes, you could do that Geezer. You would need to have a reference frame for what the energy you expended was, in form of speed, then you would need a referenceframe for what time. But they won't tell you what your speed is. They will only tell you what you already know, that you are accelerating. Any uniform motion is by definition a inertial frame and in that eternal blackness you have no reference frames to use for defining a speed, no 'fixed stars'.
And I think I should have put those questions as number two really. The first one to answer should really have been the second post about if there would be any inertia and gravity to be found. And in that case, related from 'what' to 'what'. We use matter as our easy definition of a reference frame for gravity, physicists may use the stress energy tensor, QM may use gravitons and higg's 'fields/bosons'.
So, the really interesting thing to me is if there will be a gravity? Will there be a inertia? and where/how should it come to be? I assume inertia as my proof for 'gravity' existing everywhere. But maybe I'm wrong, maybe they are two different effects? You can always experience inertia, even when there is no 'gravity' to be measured. And looking at it from the perspective of a 'force' when you do not have any electricity in your cable, is it still there? When you have no gravity to measure, is it still there? I build a lot on the expectation that gravity always is 'there'.
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A 'inertial frame', as I see it, is defined by (any) uniform motion/'speed', inseparable from being still inside a 'black box' scenario. Inertia is defined by it proposing 'I will not be changed' (resistance to change), and that goes from both uniform motion/being still and acceleration. In a acceleration you will have inertia working on you constantly, as I see it.
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Interesting ideas Grizelda, but let's take my posts in the opposite order first, same as I suggested to Geezer.
My fault that, but I didn't think it through before writing.
So what's your call on the question of Gravity and Inertia in that 'empty space'? That one will become the reference frame from where you build the rest I think.
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Yoron and Geezer - I must go along with Geezer. If I have a nice sensitive accelerometer and a nice clock that I trust. I can fire my retro rockets and measure the acceleration over a certain time - how can I not be said to have a speed? I can arbitrarily set my initial velocity to zero (let's face it we do this all the time) and I can find out my final velocity based on my assumed initial velocity, my measured acceleration, and my measured time. That I cannot find any alternative frame of reference is neither here nor there [;D] As long as my space ship is large enough and my instruments accurate enough to rule out gravitation (ie rule out through tidal measurement) I know that acceleration will make my velocity change. As all units and measurements are based on arbitrary scales and origins why is this any different.
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Yes, I agree. Geezer had the practical solution to the problem. As we can't define a 'zero speed' why not use what we call 'inertial frames' and EM 'vibrations', to combine into what we then define as our 'speed'. That's also why that question should have been number two, as it when being number one make you forget about the universe you now are in. The difference more than it just being bereft of stars (all invariant mass that is, excepting your spacecraft).
So if you like :) define my second question to your pleasure first, and then see if/what you think it can build. It's a tricky one to me.
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There is one point to be made though, you're importing reference frames from this universe into the 'empty one' when you use those definitions, and I'm not sure they are the same? But that's my fault not thinking it through before posting..
(Damn*n, I'm sure there will be answers to that first Q :) years to come now, without reference to the second Q which really needs to be the first one to define, Awhh :)
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There is one point to be made though, you're importing reference frames from this universe into the 'empty one' when you use those definitions, and I'm not sure they are the same? But that's my fault not thinking it through before posting..
I am not sure you have to move in any reference frame or preconceptions apart from the equations, the constancy of light speed. There are no fundamental units (I know about planck scale - but is that fundamental or just very useful) so every set of units relies on arbitrary origins. All we know is that our lightbeam accelerometer (for instance) shows a horizontal beam across ship when under uniform velocity or at a standstill, and that it is deflected when under gravity or acceleration. In our perfectly equipped and very large space ship we can used differences in tidal forces to determine whether this is gravitional or accelerational. Its maths and laws with no further physics assumptions from then on.
What are we importing from our reference frame apart from the knowledge and understanding we gained through observation of our frame and the great minds of newton, einstein et al?
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Yep, that how it looks here. But over there, as they say, you have nothing left, except yourself and that ship. Now try to define the gravity there, then ask yourself how you would define inertia? After you defined it for that space, you might have our universe, or another, depending on definition.
You might look at what a black hole is expected to be, and then ask yourself why its gravity would be 'infinite', as I define it that is. Also take a look at the stress energy tensor. You can use that to define how a SpaceTime may look 'geometrically'. The left side of it is described as representing the geometry of SpaceTime, and the right side of it represents things like pointlike density (matter), momentum, pressure and stress, all related to Einstein's formula E=mc2, stating that energy has mass.
So my second question will define the first I think.
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What I mean about 'have our universe, or another' is just that we might see it differently, although both of us expecting us to refer to the same universe :) It happens in all interesting discussions ::))
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I'm not even sure if you can use the stress energy tensor there. It depends on how you will define it I think. You might look at it as having a undefined state, before your acceleration. I'm not sure, maybe JP know? Grizelda's idea is interesting too even if I'm not sure how he/she got to it.
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Looking at it, 'density' is very much a question about from where you define it as being 'nul'. A fish may define that as being the specific depth, salinity etc, of the water it is accustomed too live in. The same could be said for pressure. Momentum is a relation as I think of it, as you will have it in any uniform motion, but are unable to define is as anything other than as defined relative another 'frame of reference', (as Earth). That means, no 'jiggling', as I know of, relative the spacecrafts 'atoms'? And stress? that one I don't know how to define at all here?
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You might ask yourself what define 'nul' for us there?
And not 'forces' this ah, time :) At least I think it has a importance, it's a unrelenting part of SpaceTime after all.
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But there is also time dilation, as well as Lorentz contraction to consider. In a 'empty' universe I don't know how though, you might assume that a Lorentz contraction should be existing in the acceleration though? Any SpaceTime existing should have the ability to represent 'energy' and so also allow you to become a Rindler observer observing a Unruh effect (radiation)?
Maybe :)