Gravity?

[Ecowiz Returns]

Originally posted by Lung
Another way to look at it is this:

If you walk along the equator and see a longitudinal line perpendicular to the equator. Just to be sure, you measure the angle, and sure enough, it's 90 degrees. You walk another couple of hundred miles, and you see another. Again, you find it's 90 degrees. You turn and follow the longitudinal all the way to the north pole, only to find the other longitudinal line you passed earlier! However, if they were the same angle, they must be parrallel, right? Wrong. Why? Because you've applied 2 dimnesional geometry to a 3-dimensional world. Applying 3-dimensional geometry to 4-dimensional spacetime will also fail. in other words, your straight line is another dimension's curve.

At least that's my understanding of the geometry of multidimensional spacetime

Lung:

Although your example gives a good explanation on the efects of aplying 2 dimensional geometry to a 3 dimensional world the problem I see is this:
In your example, the two 2 dimensional paralel straight lines are actually two 3 dimensional intersecting curves. Ultimatelly, if I were to walk in a 2 dimensions straight line, forever, I would ultimatelly drawing a circular form.
In time-space we are called to believe that a 3 dimensional elipse is actually a 4 dimensional line which is, exactly, the oposite result.

How can this be?

[Lung]

Originally posted by Ecowiz Returns

In time-space we are called to believe that a 3 dimensional elipse is actually a 4 dimensional line which is, exactly, the oposite result.

How can this be?

Buggered if i know!

Sorry, i think it was just my way of grasping something i read from Stephen Hawking or Timothy Ferris (not the INXS variety!). I guess the point i was making was one of perception. When you walk around the world you are of course walking in an enourmous circle, even though you appear to be walking straight. It is a perception that is misleading. We do apply our 3-dimensional geometry to space, and end up with the confusion over how gravity works.

Another way, i found, of viewing space in a gravitation field is having a 3-dimensional grid where an object, such as a planet, draws the gridlines into it by a degree relative to it's mass. At a certain speed, a small object passing the planet will follow the gridlines, so in a way, it is moving straight. The point is that gravity distorts space, and i found this to be a good way of conceptualising it.

I may be wrong, but i feel i understand gravity. Am i deluding myself?

[Ari Rahikkala]

Lancer: If you're posting pseudo-intoxicated by a lack of sleep, it's only a matter of time when you seriously start mixing stuff up .

Originally posted by Ramo
The Heisenburg Uncertainty Principle states that one cannot know the precise position and momentum of a particle at the same time (as well as other coupled quantites such as energy and time, which, BTW, is essential for the operation of gauge bosons). I don't know what you're referring to...

I mean... does it take 14 billion years or zero seconds for two electrons on different edges of the universe to interact with each other?

[MrWhereItsAt]

In the current theory, no particle can travel FTL, thus limiting the EM force to this speed. Thus, if you have two particles that can interact electromagnetically at opposite ends of the universe, the "information" of interaction from one to the other would take that 14 billion years or whatever to reach the other, and vice versa. BUT, unless the particles were created from energy (can energy in any form NOT interact with anything else?), the particles were always there - including at the beginning of the universe, when they were in the same place.

But at this point the laws of Physics are open season. After some expansion of the universe, the forces we know of, Gravity, EM, Strong and Weak, "crystallize" out and become distinct. At this point, the two particles could start interacting. BUT, since their interaction can only travel at the speed of light, if the Universe itself was expanding at FTL (and how would you define or measure this, since your measuring device is expanding at the same velocity!), then these two particles, unless they moved closer together by some other means, would NEVER be able to interact! That is, unless the Universe DOES eventually slow down and start to contract.

And all this is conditional on the speed of light being absolute, not just in the Universe as we measure it, but absolute when measured by some hypothetical way OUTSIDE our Universe, looking at the expansion of the Universe as well!

As if I haven't confused everyone (and myself) enough, I just had to say something in response to someone's post on the first page that sent me into paroxysms of Physics-class induced horror - there is no "universally accepted" quantum theory of gravity at present. String theory is not convincing enough to be taught as part of any GUTS or TOES.

Though one can hope.

[MrWhereItsAt]

And it is a consequence of the HUP that black holes can emit radiation - the uncertainty in energy/time at space near the event horizon means particles and anti-particles (or is that virtual particles?) are constantly being pair-produced and then mutually annihilated. If one is lost in the black hole before annihilation, this creates a kind of"energy from nothing". Of course, this isn't REALLY energy from nothing, and the entropy of the black hole balances everything out in the long LOOOONG run.

[Dauphin]

Originally posted by LightEning
I mean... does it take 14 billion years or zero seconds for two electrons on different edges of the universe to interact with each other?

Metaphorically 14 billion, unless you believe in the whole "spookie action at a distnace" stuff. However the electrons don't directly interact; they emit (virtual) photons that then interact with the opposing electron 14 billion years later giving the appearance of a direct interaction.

[Ari Rahikkala]

Umm... allright. I'll take a new approach to the whole modern physics thing.

* goes into a cryogenic tank *

Wake me up in a few million years... Or when you've got this all sorted out. Whichever comes first.

* ahem *

But back to seriousness. I still have a question about virtual particles (this obviously means that I've been thinking of them too much, but in any case, there is no funner way of masturbation than modern physics).

A virtual photon is "sent" (forgive me for not knowing the proper vocabulary) from electron A to electron B (A and B being, of course, as far away from each other as necessary). However, before the virtual photon gets there, electron B bumps into a random positron C that got into its way, and annihilates. What happens to the photon? (since I have never seen the answer to this question, I figure it's probably irrelevant, but I'd like to know it anyway)

[Rogan Josh]

I just wrote a lenghty response to Lung and Mrwhereitsat and then the bloody browser crashed!!!

Briefly....

Lung: it really depends what you mean by 'interaction' . If you mean 'exchange a photon' (which is how I would define 'interact') then yes it would take them 14 billion years. If you include the collapse of a mutual wavefunction in your definition then it is instantaneous. Your objection then becomes the EPR paradox.....

I wanted to point out to Mrwheritsat that the expansion of the universe is a large scale expansion. The metric locally is not expanding in this way, otherwise you would not be able to observe the expansion because your 'ruler' (and you) would also expand making everything look the same.

Originally posted by LightEning
A virtual photon is "sent" (forgive me for not knowing the proper vocabulary) from electron A to electron B (A and B being, of course, as far away from each other as necessary). However, before the virtual photon gets there, electron B bumps into a random positron C that got into its way, and annihilates. What happens to the photon? (since I have never seen the answer to this question, I figure it's probably irrelevant, but I'd like to know it anyway)

this was what I was meaning earlier about the idea of virtual particles being misleading. In some sense, the virtual particle is guaranteed to meet the other particle before it is emitted, or put another way, it would not be emitted if it wasn't going to meet the other particle. This doesn't really make sense in terms of cause and effect but the quantum theory which is underlying the analogy is still sound. It is just the analogy which is breaking down. Indeed, in what sense can the virtual particle be said to exist at all since it is by definition, unobservable.

Feynman's explanation was that there is a sort of sum over all possible histories. The virtual particle takes all possible routes out from its parent, and the physical reality is a superposition of all of these possible realities weighted by the exponential of (i times) the action (I am not going to go into the definition of the action, but think of it as how 'difficult' the journey of the particle is). The paths with high action interfere with one another and cancel out, leaving the actual reality as (approximately) the path with least action.

Hmm... this is hard to explain without the maths....

[Adalbertus]

this was what I was meaning earlier about the idea of virtual particles being misleading. ...

I'm not a particle physicist, so forgive me any incorrectness, but I think this is just the reason why it is called virtual. One tends (and perhaps is forced by the structure of our brain) that there are real photons bouncing around an electron which kick another electron off. This isn't the case. The virtual photons become real photons when absorbed by another charged particle. We cannot make any experiments on those virtual photons, even not confirm their existence, because to detect them they have to interact and thus are real photons.

Originally posted by Sagacious Dolphin:
If the photon is propogated by Electric and Magnetic fields, the graviton will be propogated by .......??

Spacetime.

The distribution of masses is tied to the curvature of spacetime in a similar way as the Maxwell equations tie the distribution of electrical charges to the electromagnetic field. (Equations look very different, however).
So: Masses tell spacetime how to curve, spacetime tells masses how to move.
Btw: Is there someone who has studied the subject who can tell if gravitational waves can be deduced from Einsteins Field Equations? If gravitational waves are more than speculation, gravitons can be "simply" explained as the quanta of gravitational waves in the same way as photons are quanta of electromagnetic waves.

[Dauphin]

Originally posted by Rogan Josh
I just wrote a lenghty response to Lung and Mrwhereitsat and then the bloody browser crashed!!!

My sob story wins - I had a lengthy response, the system crashed and I had to re-install all software back onto my computer after severe viral infection.

In some sense, the virtual particle is guaranteed to meet the other particle before it is emitted, or put another way, it would not be emitted if it wasn't going to meet the other particle.

I take the view that a virtual photon is no different to the real photon. When it is emitted it didn't know it would be "met", but when it is received the interaction is complete. Besides the sum of histories applies as much to real particles as to virtual ones, no?

So: Masses tell spacetime how to curve, spacetime tells masses how to move.

I was thinking more about the effect of physical manifestation. When an object moves close to the speed of light it gains mass/energy in our frame. If two objects (planets say) move at great speed (comparable to c) they will have greatly increased mass/energy. Gravitational attraction towards each other would have increased but so would "inertial" mass. What would the net effect be - increased or decreased acceleration towards each other?

I hope you see why I ask this...

[Ecowiz Returns]

Originally posted by Lung
Buggered if i know!

Count me in!

I'm reading Stephen Hawking's "Brief History of Time" and I'm really having problems in understanding that "In 4 dimensions the planet is walking in a straight line but in three dimensions it is making a curve" thing

I mean. Isn't a fixed point in space also "drawing" a straight line in the time dimension? Shouldn't we notice the diference?

Or am I seeing things all wrong?

[Adalbertus]

I was thinking more about the effect of physical manifestation. When an object moves close to the speed of light it gains mass/energy in our frame. If two objects (planets say) move at great speed (comparable to c) they will have greatly increased mass/energy. Gravitational attraction towards each other would have increased but so would "inertial" mass. What would the net effect be - increased or decreased acceleration towards each other?

What I think you mean is that the two bodies should fall to each other at different speeds depending on the frame of reference.
In fact this is not a problem because time is relative to the inertial frame. In general there are two effects to be taken into account when you want to undergo the pain to calculate everything in the "solar system" frame of reference. The one is the mass increasing with velocity. The other is the time dilatation (moving clocks go slower, which has been experimentally verified with high precision clocks in a plane). Both effects counteract (higher mass accelerates, slower time decelerates). One problem here: At the moment we are discussing mostly in terms of special relativity which doesn't apply to gravity. I've not sufficiently studied general relativity and before I post nonsense, I'll better shut up.

I'm reading Stephen Hawking's "Brief History of Time" and I'm really having problems in understanding that "In 4 dimensions the planet is walking in a straight line but in three dimensions it is making a curve" thing

I mean. Isn't a fixed point in space also "drawing" a straight line in the time dimension? Shouldn't we notice the diference?

A fixed point in space isn't necessarily at rest in any inertial frame (in the view of general relativity). Someone falling out of a plane is at rest as long as you can neglect air resistance. When you are sitting on your chair, you are not.

[Rogan Josh]

Originally posted by Adalbertus
Btw: Is there someone who has studied the subject who can tell if gravitational waves can be deduced from Einsteins Field Equations? If gravitational waves are more than speculation, gravitons can be "simply" explained as the quanta of gravitational waves in the same way as photons are quanta of electromagnetic waves.

Yes. Since Einstein's Field equations are exactly that - field equations - they have solutions which are fields, and therefore gravitational waves. Therefore, a graviton can be thought of a quantum of these fields. The theory actually looka a lot like electromagnetism.

The problem is that when you make the theory quantised you find that every observable depends on the physics applicable at the Planck scale (or some other cut-off scale). In other words there is no factorisation between the physics of gravity at our energy scale and of very high energy scales, meaning that we have to understand the physics at these high energy scales to explain the physics in our realm of experience. This is not nice (technically we say the theory is non-renormalizable).

There is an even better problem with gravity though (which I mentioned earlier but was ignored). Why is it so weak? If gravity is so much like electromagnetism, shouldn't it have a similar strength? But look at the molecules in your body - they are held together by electromagnetic effects, but these electromagnetic effects are strong enough to counter the gravitational pull on the atoms from the entire Earth! If electromagnetism and gravity were of similar strangths we would all be crushed to nothingness - ripped apart by the Earths gravity!

In fact gravity is roughly 10^17 times weaker than elecromagnetism. This is a huge number and is thus far, completely unexplained.

[ZoboZeWarrior]

Forces are created by the existence of a gap between two potential of same kind of energy. Forces tend to annihilate such gap.

There is a theory which considere that each forces needed a substart to exist, a "messenger". This messenger links the the potential.

Such "messenger" could have mater properties or wave properties or both ...

Actually we are seeking the substrat of gravity : the gravitron.
Wich is theorical.

But all this don't explain me why I have headach after drinking too much ...

[Ecowiz Returns]

Originally posted by Adalbertus
A fixed point in space isn't necessarily at rest in any inertial frame (in the view of general relativity). Someone falling out of a plane is at rest as long as you can neglect air resistance. When you are sitting on your chair, you are not.

That much I can understand.

What i have some trouble in visualising is how something that is (more or less) "drawing" a closed curve in a 3 dimensions representation is said to "draw" a straight line in a 4 dimensions one.

For the sake of the argument lets assume that the Earth indeed draws an exact elipse around the sun, in our intuitive 3 dimensional scope.

If I disregard time, I could state that the Earth would eventually return to some point in space where it already had been.

When I add the time dimension I have to say that, in fact, the Earth never returns to the same space-time point, for time as elapsed. We would have a kind of cilindrical elipse, but hardly a straight line, wouldn't we?

The way I see it, it is not intuitive at all that, by adding another dimension to someting I end up with a simpler, rather than more complex geometrical shape.

I really can't see where I'm wrong, although I'm sure I must be (otherwize, someone else would have stated this same argument, I supose).

I also don't see the use of that "curve is actually a straigh line" rational in explaining the "small hole" is space-time caused by mass that seems to ilustrate the gravitational effects.

[Dauphin]

Originally posted by Adalbertus


What I think you mean is that the two bodies should fall to each other at different speeds depending on the frame of reference.

Errr. not quite.

When two electrons are moving parallel to each other they are repelled from each other by the electric force, and attracted by the magnetic force. A massless electron pair moving and approaching c would have these forces cancel each other out.

This scenario is reconciled to the pure electric treatment (electrons frame of reference) by saying they are experiencing time dialation. Hence magnetism is often referred to as a relativistic correction to the electric force.

Presumably gravity should be analagous. As two test masses approach c, they should accelerate towards each other at a decreasing rate, no? If they don't then what is the reconciling feature.

[Adalbertus]

What i have some trouble in visualising is how something that is (more or less) "drawing" a closed curve in a 3 dimensions representation is said to "draw" a straight line in a 4 dimensions one.

You've answered that part already in your post, in four dimensions the path of the moon is a sort of spiral.

When I add the time dimension I have to say that, in fact, the Earth never returns to the same space-time point, for time as elapsed. We would have a kind of cilindrical elipse, but hardly a straight line, wouldn't we?

In some sense, it is. The problem is that we normally think to know what is a straight line. Then you have to define it in some way. The most practical way is to take the paths of the light in vacuum as straight lines, also in four dimensions.
On earth, this fully agrees with our notion of straight lines.
One of the first tests of General Relativity was to take a photograph of the sun and surrounding starts during a solar eclipse, and of the same stars later when the sun was far away. One has found that the stars that appeared close to the sun apparently moved apart, and that even stars were visible that should have been hidden by the sun. The reason is that the gravitational field was such that the sun attracted the light (from a flat geometry point of view), or that spacetime around the sun was curved.
Note that we cannot know the "truth" about the geometry. We can measure the universe as we like to. You can make up a theory with a flat spacetime, and an attractive force of the sun to light. The only problem is that this theory would become way more complicated than the theory with a curved spacetime is.
(To get a better notion of the problems with a curved geometry you might find something in a good atlas, when it comes to the problems of drawing a world map. It is impossible to draw a flat world map in which all shortest paths on earth show up as straight lines on the map. Things are similar in a curved spacetime).

So the next thing you'll probably ask is the following: The deflection of light by earths gravity is not noticeable, one has a hard time to see this from the sun, why is the path of the moon so much more curved than that of light?

The answer: Earth curves spacetime but the moon does, too. This gives a far stronger effect than of the earth alone. And the path of the moon is not very strongly curved. You can see this by a short estimate: The velocity of light is the conversion factor time - space. The moon is a bit more than a light second away from us. It takes one month for a turn around the earth. Which is that said spiral has a ratio of radius :thread height of about 1:2419200. To me, this is quite straight.

In fact gravity is roughly 10^17 times weaker than elecromagnetism. This is a huge number and is thus far, completely unexplained.

The antropic principle would simply state that if gravity were as strong as electromagnetism, everything had ended up in a black hole or complete void, as gravity is (at least mostly) attractive; thus there were nobody to notice that situation. Of course it doesn't help here.
I just had a look in a handbook of physics, and found a table of the strengths of the fundamental forces
strong interaction 1
electromagnetism 1/137
weak interaction 10^-14
If the numbers are comparable to what you state, gravity should go in as something like 10^-19, which I would not see as too far off. Electromagnetism and weak interaction were the first to be unified.

[Dauphin]

If you want to talk coincidental numbers -

Why is the ratio of the electic force between an electron and proton to the gravitational force between an electron and proton the same as the ratio of the size of the universe to the size of the electron, which is also the square root of the number of particles in the universe?

e²/ G m_p m_e = 10⁴⁰ = ct * m_e c² / e²

Which incidentally is the square root of the number of particles in the universe

[Lancer]

Damn that's a good question.

This thread isn't teaching me about gravity sadly, though that's entirely my fault I'm sure. No, what this thread is teaching me is how deeply stupid I am compared to those who can debate such stuff. I am unable to wrap my brain around any of this very interesting but perplexing read. Ahh well, to each their own, but I tell you what, my hat is off to you all.

Now, I'm very much looking forward to being perplexed by the answer to SD's question...

Who can answer it?

[Adalbertus]

If you want to talk coincidental numbers -

Given the error some of your data have, such as "number of particles in the universe" or "size of universe", it's rather coincidental orders of magnitude.

Let's discuss real numbers, small ones:
We perceive four dimensions, one of which (time) is somewhat weird.
We perceive four fundamental forces, one of which (gravity) is somewhat weird.
Why?

This thread isn't teaching me about gravity sadly, though that's entirely my fault I'm sure

Maybe it's not only your fault, there are many intelligent people who don't have a clue on general relativity. And there are two inherent difficulties: Most people who are untrained in university maths have difficulties in understanding geometries other than the usual Euclidean, or a spacetime other than our normal Galilean. And even if they did there are huge problems in communication inherent to the subject: To get some decent and mathematically correct understanding of GR, you have to take a one year's course at the university (I didn't get through all of it because of other interests...), after having done physics for some time. It is close to impossible to fit the essentials of what you've learned in this time into a post which roughly stays in limits (Imagine writing a data compression software such as zip which reduces the size of any given file by a ratio of 1:10000). And scientists aren't really famous for their communication skills.

[Dauphin]

Originally posted by Adalbertus

Given the error some of your data have, such as "number of particles in the universe" or "size of universe", it's rather coincidental orders of magnitude.

The size of the "causal" universe is known to with an order of magnitude, as is the size of the electron.

The number of particles in the universe is correct to within an order of magnitude if space is flat (which it appears to be).

[Adalbertus]

Order of a magnitude. Exactly what I mean. I wouldn't try to find any meaning in it. OTOH, astrophysicists seem to be quite happy if reality fits to theory by an order of magnitude...

[Dauphin]

Did you know I was going to take a PhD in astrophysics/cosmology, but dropped the idea at the last moment?

[KrazyHorse]

This thread isn't teaching me about gravity sadly, though that's entirely my fault I'm sure. No, what this thread is teaching me is how deeply stupid I am compared to those who can debate such stuff. I am unable to wrap my brain around any of this very interesting but perplexing read

Gravity's simple. Tensor algebra isn't. Nothing in this world should have four indices tagged on it.

[Adalbertus]

Gravity's simple. Tensor algebra isn't. Nothing in this world should have four indices tagged on it.

Gravity isn't quite thqt simple, because its geometry collides with normal imagination.
Tensor algebra. Tsts. Differential geometry and exterior calculus rocks. Gets you rid of most nasty indices unless you want your result in coordinates

edit: thqt??? s*** french kezboqrds!