What exactly is gravity?
- Thread starter TheNiceGuy
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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.
http://www.edwardtbabinski.us/geocentrism/fabric_of_space.jpg
That doesn't exactly give an explanation of what gravity is however. The original link to Gravitons might be a more accurate explanation.
One book I read (I can't recall which off the top of my head) suggested that gravity is simply the manifestation of the curvature of space-time. Thus, it is simply a direct result of the four-dimensional universe.
Ah, right. It's been a while since I read anything about pure physics.
Why is this no longer accepted as the 'cause' of gravity?Originally posted by: CycloWizard
Ah, right. It's been a while since I read anything about pure physics.
I'm not an expert in the area, but IIRC general relativity has problems explaining some observed phenomenon. While it is internally consistent, it doesn't seem to explain everything, hence why things like quantum theory and string theory have come about. Not that anyone agrees on those either.
The more-or-less 'standard' quantum physics explanations require some kind of particle to transmit the force we see as 'gravity'. Otherwise it would seem that "empty space" is transmitting information or force.
silverpig
Lifer
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Originally posted by: Matthias99
Originally posted by: CycloWizard
Ah, right. It's been a while since I read anything about pure physics.
I'm not an expert in the area, but IIRC general relativity has problems explaining some observed phenomenon. While it is internally consistent, it doesn't seem to explain everything, hence why things like quantum theory and string theory have come about. Not that anyone agrees on those either.
The more-or-less 'standard' quantum physics explanations require some kind of particle to transmit the force we see as 'gravity'. Otherwise it would seem that "empty space" is transmitting information or force.
GR works perfectly in every experiment we have run. So does quantum field theory. However, when you try to apply QFT to gravity you get infinities in your equations. That's bad.
Think of it like the blind men and the elephant. One blind man touches the elephant's leg and says it's a tree, the next one touches the elephant's side and says it's a wall. Those are fairly accurate descriptions of the parts they both touch, but now you try to combine them. Maybe the object is a log cabin? That means there should be a door somewhere to get in. So you send another blind man to go around back to find a door. He grabs the tail and comes back saying that there's no door, only a rope.
So now, you know that the idea of wall + tree = cabin doesn't work, you're also pretty sure it's neither a wall nor a tree, and you have no idea what to do about this rope either.
But in the mean time you can still scratch your back against the tree, and sit behind the wall for shade.
I think it's something more simple. I think it's just when you get a mass of molecules together that they all combine to cause a pull. So each one has a tiny almost inmeaserable pull to it by it self (maybe based on what keeps them together at all). And when you start combining them that pull increases. The bigger the planet the larger the pull for example. That just seems to make the most sense to me.
Quantum physics describes the really small, and General relativity describes the really big. What you're talking about is (or was) called the Grand Unifying Theory, which describes both. And doesn't exist yet, although string theory is a candidate.
Gravity
Definition: - It is the force originating from the non-equilibrium of charge on the left and right of the particle and the resulting flow out of it.
Does this confuse you enough?
I have no clue how to measure gravity.
There was some article on a space probe, I think from the EU, which stated that as the Earth goes through space the field of gravity or magnetic field is causing a disruption in time. It is a little over my head how you prove such a claim. They used some kind of super-sensitive gyro-scopes.
Definition: - It is the force originating from the non-equilibrium of charge on the left and right of the particle and the resulting flow out of it.
Does this confuse you enough?
I have no clue how to measure gravity.
There was some article on a space probe, I think from the EU, which stated that as the Earth goes through space the field of gravity or magnetic field is causing a disruption in time. It is a little over my head how you prove such a claim. They used some kind of super-sensitive gyro-scopes.
I am no where near an expert on this subject, but I find Van der Waals forces to be increasingly interesting. Especially the fact that they can not even get to the amount of perfection already displayed in nature by creatures such as the gecko.
Obviously, this does not really relate exactly to gravity, but this force seems to defy gravity to me. The fact that there can be so much adhesion from such a small surface area is fascinating.
Obviously, this does not really relate exactly to gravity, but this force seems to defy gravity to me. The fact that there can be so much adhesion from such a small surface area is fascinating.
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.
Quantum mechanics is not limited to "small" things. It is a general theory for everything; is jist that, for reasons we don't understand, it doesn't work when gravity is inolved (unless you can make a semi-classical approximation and use Newtons theory instead) REGARDLESS of scale; it is not "compatible" with GR.
The reason why QM has traditionally been limited to small things like atoms is that when you try to apply the theory to big things you will find that they tend to interact with the enironment to much larger extent than e.g. an atom. This interaction "smears out" all quantum effects and you get results which might as well be described by classical physics. It is only during the past 20 years that we have learned how to "isolate" macroscopic objects well enough to see true quantum effects such as entanglement. The systems I work can be pretty big, something like 10x10 micrometer, meaning you can easily see them in an ordinary optical microscope.
PhatoseAlpha
Platinum Member
- Apr 10, 2005
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Because there's more to it then that. Distance is also involved, and when you get right down to it, gravity increases linearly with mass, but decreases quadratically with distance.
You figure when the big bang went bang, it sent all the matter in the universe outward with vast velocities. Gravity would work to pull them together, slowing them down, but since gravity gets weaker across distances, if they're moving apart too fast, gravity will never be able to actually stop them - it gets weaker and weaker as the distance grows, so it just can't rustle up the force, even over all of time, to cause it to come back together.
Mathematically, if I start with a velocity of 1, and a force takes away 1/2, then 1/4, then 1/8, etc, even if that force works on my velocity for all of time, it's effect is never going to be enough to actually stop me, much less turn me around.
There are actually 3 scenarios for the endstate of the universe. They all depend crucially on the density of matter in the universe. If this density is below a certain critical density then the universe will continue to expand forever and end in a deep freeze. If the density is above the critical density it will halt its expansion and begin to collapse on itself and possibly begin anew(this part is theoretical since we don't know if it would actually big bang again). If the density is exactly the critical density then the universe will come to a halt after an infinite amount of time. The current observational evidence points to a density that is very close to the critical density, indeed the theory predicts that the universe should be at the critical density. Dark matter plays a large role in in this theory as most of the matter in the universe is Dark Matter.
There are a couple of complications to this picture, like dark energy which is accelerating the expansion.
As a side note, GR doesn't get along with QM. This usually isn't a problem in most cases except for black holes and big bang cosmology. If you can name more cases where GR+QM is important I'm curious. My understanding is that GR is perfectly valid until you reach planck length scales and then one of the assumptions of GR doesn't make physical sense anymore.
There are a couple of complications to this picture, like dark energy which is accelerating the expansion.
As a side note, GR doesn't get along with QM. This usually isn't a problem in most cases except for black holes and big bang cosmology. If you can name more cases where GR+QM is important I'm curious. My understanding is that GR is perfectly valid until you reach planck length scales and then one of the assumptions of GR doesn't make physical sense anymore.
DrPizza
Administrator Elite Member Goat Whisperer
It's my understanding that those three states were postulated quite a few years ago. However, in the last few years, the results are "Holy Cow! The expansion of the universe is actually accelerating!" Meaning it's not going to end in a big crunch (nor slow down, approaching zero expansion.) Dark energy, as you mentioned, is one of several competing explanations for the expansion. But, accelerated expansion it is.
There are a lot more than that, again, let's try to point out which comments are theories vs facts.
Links please, this sounds like research from the 70s-80s which has mostly been disfavored over current theories. The current consensus view is that the universe is expanding infinitely and the expansion is actually speeding up (everything from dark energy to gravitational effects lessing are considered for this)
Unless of course you believe in the multi-brane model, in which case the universe gets refreshed when the brains collide again
I don't have any links as these theories are out of textbooks. Introduction to Modern Astrophysics-Corroll and Ostlie(1996), Gravitation and Cosmolgy-Tei Pei Cheng(I don't know how to spell the name, textbook is in another room). The first is fairly good, and the second is ok.
These 'theories' are derived straight from the Friedman equations and depend on the curvature of the universe often called k, it can be -1,0,1 for open, flat, closed respectively. (Note that there is no cosmological constant in these equations)
As DrPizza stated these are relatively old theories, I believe they were originally derived in the 20's but still valid since there haven't been any changes to GR over the years. When the Friedman equations are rederived with a cosmological constant one finds that unless the density is high enough, the universe will expand forever even for a closed universe. The cosmological constant is a term which is a constant density of vacuum energy responsible for a repulsive force or negative pressure. However, the universe is still thought to be very close to this originally proposed critical density--and accelerating in it expansion because of a non-zero Constant.
These results depend heavily on dark energy, and no one wants to say anything too specific about that beast so thats why I didn't get into it.
Keep in mind that no scientific theory can be proven to be correct, the best we can hope for is that it matches our observations and can predict further phenomenon.
These 'theories' are derived straight from the Friedman equations and depend on the curvature of the universe often called k, it can be -1,0,1 for open, flat, closed respectively. (Note that there is no cosmological constant in these equations)
As DrPizza stated these are relatively old theories, I believe they were originally derived in the 20's but still valid since there haven't been any changes to GR over the years. When the Friedman equations are rederived with a cosmological constant one finds that unless the density is high enough, the universe will expand forever even for a closed universe. The cosmological constant is a term which is a constant density of vacuum energy responsible for a repulsive force or negative pressure. However, the universe is still thought to be very close to this originally proposed critical density--and accelerating in it expansion because of a non-zero Constant.
These results depend heavily on dark energy, and no one wants to say anything too specific about that beast so thats why I didn't get into it.
Keep in mind that no scientific theory can be proven to be correct, the best we can hope for is that it matches our observations and can predict further phenomenon.
silverpig
Lifer
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WMAP determination of cosmological parameters.
This is basically just a list. There are longer papers on similar pages dealing with the science.
VinDSL
Diamond Member
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
Do you feel a 'pull' toward the earth, or a 'push' from above?
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?
Still trying to figure out if your really that dense, or just trolling (perhaps my sarcasm meter is off today)
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