What exactly is gravity?

[CycloWizard]

Originally posted by: VinDSL
Gravity is a 'push', not a 'pull'...

Here's a little experiment you can do... even sitting at your computer.

Hold your arms straight out to your side -- palms down. Then, turn your palms up -- then back down again -- back n' forth -- yada, yada, yada...

Do you feel a 'pull' toward the earth, or a 'push' from above?

It's a vector body force. Whether it's a 'push' or 'pull' simply depends upon your reference frame relative to the direction of the vector.

 

[VinDSL]

Originally posted by: CycloWizard
It's a vector body force. Whether it's a 'push' or 'pull' simply depends...

Good enough!

I just want ppl to get used to the idea of gravity being a 'push', not a 'pull'.

Once you start thinking in this (ahem) direction, gravity makes more sense!

So, to that end, your declaration is a suitable compromise!

 

[CycloWizard]

Originally posted by: VinDSL
Good enough!

I just want ppl to get used to the idea of gravity being a 'push', not a 'pull'.

Once you start thinking in this (ahem) direction, gravity makes more sense!

So, to that end, your declaration is a suitable compromise!

No, my declaration isn't a compromise. You can't call it a 'push' or a 'pull', as both are completely arbitrary. It's simply an attractive force between two bodies with non-zero masses. Or are you trying to argue that it's a repulsive force?

 

[aj654987]

Originally posted by: f95toli

Originally posted by: gerwen

Quantum physics describes the really small, and General relativity describes the really big.

That is not really correct. There is no "scale" in either theories. GR is perfectly valid even for "small" things, it is just that the effects are usually (but not always) so small that we might as well just use Newtons theory of gravity instead.

Yes, a better way to put it would be that on the smaller scales, quatum forces control where as on a larger scale gravitational forces control.

For example, on an atomic level, gravity is still there and general relativity is still valid. However, with a very small mass such as an electron and a proton, there force of gravitational attraction is many many orders of magnitude smaller than the electromagnetic force between the two.

Ordinarily this isnt a problem as the gravitational force can be neglected and only the other three forces are considered on atomic levels using quantum mechanics.

They have the most problems when both gravity and the other forces both become important in situations like a black hole where all the atomic forces and gravitational forces are both very strong. With no current way to combine all four forces, they have many unanswered questions regarding black holes and the big bang.

 

[chcarnage]

If accidentally somebody here is an advanced German speaker, I highly recommend the astronomy broadcast Alpha Centauri. Recently their subject was: Are the natural laws a coincidence?

Natural laws and natural constants describe not more than four natural powers. The four natural powers can't be explained any further (for now?) but vary greatly in their force. They are (translated by myself, maybe there are more appropriate terms):

- The strong nuclear power (responsible for the existence of atomic nuclei)
- The electrodynamic force (responsible for the attraction of differently charged particles, 100 times weaker than the strong nuclear power)
- The weak nuclear power (responsible for nuclear disintegration/radioactivity, 10^11 times weaker than the strong nuclear power)
- The gravitational power (a whopping 10^40 times weaker than the strong nuclear power)

The professor then countinued to explain what would happen to the universe if the ratio between this powers was changed by increasing one of the powers, the results would be catastrophic. The conclusion was that we couldn't exist with another ratio.

 

[Nathelion]

As to gravity being a push and not a pull... there was (before einstein) a corpuscular theory of gravity that posited that gravity was caused by little particles flying around in great densities in random directions everywhere in space. When two bodies are introduced into the space, they "block", or absorb, some of the corpuscles that fly through them. This would happen in all directions, but when two bodies are considered they would essentially cast a "shadow" on each other. This would account for the mutual attraction of large bodies of mass. This theory has been decisively disproved since then, but interestingly it turned out that the same concepts could be applied to bodies suspended in hot gases. The theoretical framework of corpusclular gravity played a role in forming the molecular theory of gases.

Anyway, as far as Theories of Everything go, the two most "popular" candidates for a succesful theory seem to be String Theory (or, more specifically, m-theory), and Loop Quantum Gravity. String theory is basically plagued by the problem that it claims a great deal, but none of these claims have turned out to be testable (yet). Loop Quantum Gravity makes some testable predictions (such as different wavelengths of light traveling at different speeds), but much work remains to be done before it can be considered a mature theory, and the measurements that would support it are difficult and have not been carried out (yet). I personally think it'll be quite some time before Quantum Theory and General Relativity are reconciled. And to think that physicists in the early twentieth century were discouraging new students from entering the field - because "everything is already explained". LoL!

 

[PooBeetle]

What exactly is gravity?

Depends on your desired microscope power.
In Newtonian physics, gravity sucks, and thats it.
In relativistic physics, gravity is instantaneous because it is in tensor form. (nothing exists, suddenly you have one partical in your universe, gravity exists (for all other forces known, there would need to be two particles).
In string theory, there is only resonance ( two particles exist and one says to the other, semi hypnotically, you are travelling towards me, the second particle, IF inclined finds resonance with this idea, absorbs this information and incorporates this new order into himself). thus gravity depends on sharing the information only.
In many worlds theory, a body ( a single, life entity) has their own multi-verse, and only while gravity is life oriented (increases order) is it permitted, and defined.

hope this helps ;p

 

[CycloWizard]

Originally posted by: PooBeetle
What exactly is gravity?

Depends on your desired microscope power.
In Newtonian physics, gravity sucks, and thats it.
In relativistic physics, gravity is instantaneous because it is in tensor form. (nothing exists, suddenly you have one partical in your universe, gravity exists (for all other forces known, there would need to be two particles).
In string theory, there is only resonance ( two particles exist and one says to the other, semi hypnotically, you are travelling towards me, the second particle, IF inclined finds resonance with this idea, absorbs this information and incorporates this new order into himself). thus gravity depends on sharing the information only.
In many worlds theory, a body ( a single, life entity) has their own multi-verse, and only while gravity is life oriented (increases order) is it permitted, and defined.

hope this helps ;p

 

[PooBeetle]

Why is this no longer accepted as the 'cause' of gravity?

Because . . . it Depends on your desired microscope power, how many decimal points you are interested in.
(add ten to twenty decimal places of accuracy for each newer theory, and lots more aspects, behaviours etc)


Newtonian physics describes one, and only one aspect (feature/ability) of gravity, that it sucks. That the apple drops at all is all the accuracy you were interested in. (but you get another 20 anyway, for free)

In Relativistic physics, the space time continuum was created by the big bang (and gravity/time/space are so intertwined as to be inseperable. gravity is not a distortion of the fabric of space/time, but defines it. (remember the sheet of stretched rubber with one cluster of heavy ball bearings in the middle? big bang = moving all the ball bearings apart, you can now see space time in a 2D representation. gravity would = the topology of the rubber sheet).(don't be confused by einstein himself calling inertial gravity indistinguishable from mass gravity (that was just for a single starting point in an argument, nothing more) - they are obviousy different when viewed on the rubber sheet)

In string theory, or derivatives, an entity called Mr. Gravity (or graviton of you prefer) says listen up, and you will believe the information that I am constantly spouting. (resonance occured. think of a cloud made up not of water, but of tiny shimmering pieces of string. Mr. Gravity (Esq.), was able to get all the other pieces of string to believe him, AND start transmiting (shimmering, vibrating at a certain frequency, whatever) the same information themselves, for reasons unknown.)

In a multi-verse (a path through an infinity of infinities (now routinely used by universities and their quantum computers) you would ask, can a NEW! invention/idea called "gravity" increase entropy (order) WHILE at the SAME TIME either increasing energy, or at the bare minimum, not destroying any? (totally outside of all standard entropy definitions except the most modern)(remember "matter can neither be created nor destroyed" is newtonian. look at your finger. matter has obviously been created, and is now routinely created (admitedly as self anihilating twined particle pairs) in the particle accelerator, protons do have a measurable decay (they simply collapse into nothingness), and are also created spontaneously out of vacuum at a very slow rate.)

well lets call it permitted in the narrow path then (an entropy/energy equivalence equation that is within the bounds of foreverness, (an infinity of universal ages (from big bang to big ?, say 100 billion years)(most classical entropy definitions are newtonian, to go multiversial you have to start thinking about principles and physics laws that are sustainable over essentially infinite repeated universal ages. un-surprisingly, you come out with constants that match exactly what we've got now.)



(
mapping problems and incompleteness theory.
1+1 = 2 has never been true in the real world.
if i sold you one large delicious orange and one small, green, bitter, inedible orange, would you come back to my shop? despite my being the worlds foremost mathematical shop keeper? and being able to "prove" that I sold you two oranges?

ALL maths assumptions break down near boundaries. ie newtonian physics is good for everyday life, but when you need more power, more accuracy, you have to go relativistic, which is only good up till the next boundary, (relativistic quantum field theory only works when you pretend gravity doesn't exist), then you need to go string, then multiversial. each is nothing more than a telescope that gives you more decimal places. (and each has astounding philosophical implications.)

all logics are incomplete, and worse, inconsistent (any system of logic can be used to create a paradox, ie to prove that it has mathematical inconsistencies.)
)



personaly,
I'm "Dropping science, like Gallileo dropped the orange" - "Beastie Boys"

 

[aCynic2]

Originally posted by: GimpyFuzznut

Originally posted by: silverpig
It's called General Relativity.

IE space is a "fabric" of sorts that bends. I think the commonly used analogy is imagine space as a blanket and then drop a bowling ball on the blanket. That bowling ball will cause a warp in the fabric and so other things around will also conform to this warp.

I'm coming in late and I'm not an astrophysicist, so bear with me:

Isn't that sort of a...a...2D or 2.5D representation? Wouldn't a true representation in 3D/4D space look more like, for lack of a better wording (remember, I'm not a physicist), sheesh, I'm having trouble describing in words...

Well, since gravity works in 4D spacetime, the example needs to extend in all directions, so that no matter who you cut through spacetime, you get exactly the same stretch in spacetime. So, if you 3D grid the representation, it would show the grid marks closer to the gravitational body elongated and stretched. I hope this makes sense, because I have no other way to explain then to draw something out.

 

[Biftheunderstudy]

Thats pretty much the gist of it. The 2D spacetime sheet is a bad analogy, a more accurate one needs 3D Space + a time dimension. This space time gets warped and curved by an energy density. According to the theory, light always travels the shortest distance between 2 points. In a curved space time this is a geodesic which can have bends and such. In general relativity this curving of space time is gravity, there is no force.

A good way to think of this is as follows. Pretend you are living on the surface of a sphere and you are only 2 dimensional. As a 2 dimensional being you don't know you are living on a sphere, and your local place on the sphere is flat. Now you do an experiment, send two machines to walk in a straight line parallel to eachother(for ease pretend that these machines are perfect and do actually move in straight lines). When the distance between them is measured at the beginning and again at the end one finds that the distance has gotten shorter. The 2D scientist then concludes that there is an attractive force between the 2 machines. We know this is absurd, there is no force its just because they are moving on a sphere, parallel lines converge. But we can extend this to 4 dimensional space time and we have GR.

Gravity is the result of objects following geodesics in a curved space time.


P.S.
I don't know much about the many worlds interpretation but I do know its an interpretation not a separate theory. As such it does not solve any new mysteries, does not provide us with a quantum theory of gravity, or provide more accuracy than the standard model. It does provide us with an alternate explanation for the wave function collapse.

Also, poobeetle if you could provide some proof that many worlds is more mathematically consistent than the Copenhagen interpretation I would like to see them. I'm curious where the 2 interpretations diverge in the math.

 

[gerwen]

Originally posted by: f95toli

Originally posted by: gerwen

Quantum physics describes the really small, and General relativity describes the really big.

That is not really correct. There is no "scale" in either theories. GR is perfectly valid even for "small" things, it is just that the effects are usually (but not always) so small that we might as well just use Newtons theory of gravity instead.

<snip>
The systems I work can be pretty big, something like 10x10 micrometer, meaning you can easily see them in an ordinary optical microscope.

<friendly sarcasm>Seen with a microscope? Wow that IS big.</friendly sarcasm> ( I understand that big here is relative.)

I stand corrected. I'm just a layman with an interest in science, and not afraid to admit my ignorance. I honestly didn't realize quantum physics still applied at large scales however. I thought it fell apart above the microscopic level, and that was the basis for looking for a GUT.

However, isn't my statement functionally true? Nobody describes the motion of the planets or galaxies using quantum physics, and the same goes for atomic processes and gravity. While the physics may apply, they are insignificant at those scales. I suppose my statement could be modified to say; 'Quantum physics is used to describe the really small, and General relativity is used to describe the really big.'

*edit* missed aj654987's post when i wrote this. I guess there are places where both are needed to fully (or even partially) describe something. That's pretty inconvenient for researchers. <--Understatement of the day!

 

[f95toli]

While it is true that you do not NEED to use QM to describe the motion of a planet, the point is that there is no reason why you couldn't (unless, of course, general relativity is imporant for the motion, if so you would need a GUT)

Also, while "microscopic" might seem small it really isn't, you need to remember that even a micrometer is a very long distance compared to e.g. the diameter of an atom and something so big that you can see it in optical microsope (which typically magnifies about 100x) will probably contain about a trillion atoms.

If you want something bigger a good example would be high quality resonators, these can be centimetres in size but if you cool them down (to get rid of thermal noise) their electromagnetic properties are still governed by quantum electrodynamics. Or, even bigger still; if you have a few litres of superfluid helium in a glas cryostat you will see that it behaves in a really weird way ("climbing" over walls etc).

Moreover, quantum mechanical tunnelling is a real problem in modern CPU design since it is a major source of leakage currents: If the barrier is too thin (in an electrical sense) electrons will simly tunnell thorugh the barrier. This is one reason why e.g. Intel recently started using high-kappa materials. Hence, quantum mechanics does really affect the behaviour of things we use in our daily life.

 

[gerwen]

Interesting stuff, thanks for moving an esoteric concept into the real world for me.

 

[GPett]

Originally posted by: piasabird


I have no clue how to measure gravity.

Gravity can be measured in a magnetically shielded vacuum environment. There is equiment that exists that can record gravity.

What intreagues me is when enough of hese machines are made that we can generate a central observation of the data from hese machines to build a model of the earths gravity. Then compare that data to any coroborating flucutiations to see if anything such as cosmic, solar, or envorionmental changes falls in line with that data collected.