Thread: Gravity is awesome

That's tautological, and basically wrong. This is something that they do teach to school children, but then when they get older, they ask, "what force other than gravity causes the ball to go down into the pit"? It's a good question.

As it turns out, I'm actually taking an advanced course on it right now, so I can explain. The actual force of gravity isn't a force at all. It's a fictitious force that only appears when you're in a 'weird' reference frame. Standing on the surface of the Earth is one such 'weird' frame. The proper frames for gravity are actually spacetime geodesics, and in these frames, ignoring tidal forces, you don't feel gravity at all. Now, because these are paths in space AND time, this involves movement, and hence this is why things fall. Or, in other words, in curved spacetime, being "at rest" is falling.

I've been thinking about that too, thank you for your reply

I still don't get it. How is your explanation different from mine, except in some tough words?

But then gravity is also hyptothesised to be due to gravitons, which would make them one of four basic force-causing particles.

I don't personally remotely understand how the ideas of particles and quanta can lead to General Relativity though. It seems quite contradictory to me.

That's because the ideas about gravity don't lead to General Relativity.

The relativity principle and constant speed of light lead to General Relativity

So why are gravitons needed?

Originally Posted by Xei

So why are gravitons needed?

They're not. Gravitons were invented long after GR as a sort of apologetics for the up-and-coming field of QCD. With the finalizing of the Standard Model, it became clear that there probably would, some day, be a gravity particle, so they just fudged it in. Gravitons are not required for or are a result of GR.

The reason GR doesn't mesh with QCD is that the Heisenberg Uncertainty Principle allows particles to occupy large volumes of space over short time intervals, which violates the speed of light axiom. Now obviously, we're just talking about probability clouds, not actual particles, and this fact is why some physicists are hopeful that there will be a resolution.

I don't understand; are gravitons real things? Or are they just an abstraction which arises as a result of GR? Which is it which causes the other?

Originally Posted by Xei

I don't understand; are gravitons real things? Or are they just an abstraction which arises as a result of GR? Which is it which causes the other?

Gravitons have NOTHING to do with GR. They were made up much later by folks trying to reconcile GR with the QCD.

Okay. So they don't actually exist.

Although I thought they were looking for them...

Originally Posted by Xei

Okay. So they don't actually exist.

Although I thought they were looking for them...

Only the people that think they exist. I'm not saying they don't, but GR does not require them.

In fact, I don't even think photons really exist, but that's another story.

Originally Posted by Xei

Okay. So they don't actually exist.

Although I thought they were looking for them...

Only the people that think they exist. I'm not necessarily saying they don't, but GR does not require them.

In fact, I don't even think photons really exist, but that's another story.

That's part of what I'm interested in... how would you explain the photoelectric effect?

Originally Posted by Xei

That's part of what I'm interested in... how would you explain the photoelectric effect?

The photoelectric effect was discovered long before the study of more complicated types of waves, such as solitons. I don't know the math here, because it's incredibly complicated, but I do know that light can't be both a particle and a wave. It can appear to be both, but those two concepts are, in fact, mutually exclusive. Unless you redefine the terms themselves, of course, but that's wordplay. My point is, I wouldn't be surprised if the apparent particle nature of photons was due to some sort of artifact, sort of like how a phonon isn't really a physical particle.

For example, if you look at the double slit, we see individual packets of light coming through, but that only happens when they go through the slits. If QM has taught us anything, it's that we always must consider then entire apparatus as part of the system. What if the molecules in the slits themselves modulated the light in such a way as to come out in packets rather than a beam? You can't give any proof that this doesn't happen, because the slits are required to see the packets in the first place. I'm really starting to think that wave/particle duality is just an overlooked pre-QM misconception that somehow avoided the scrutiny of modern physics for historical, or other, reasons. Maybe people don't want to contradict the great and all-knowing Einstein, who knows.

Originally Posted by drewmandan

Or, in other words, in curved spacetime, being "at rest" is falling.

I don't understand. Do you mean being at rest "on Earth", for example, or being generally at rest somewhere in the middle of nowhere, where no other mass is attracting you. If the latter then I don't understand even further

Originally Posted by Bonsay

I don't understand. Do you mean being at rest "on Earth", for example, or being generally at rest somewhere in the middle of nowhere, where no other mass is attracting you. If the latter then I don't understand even further

At rest means that no real force is acting upon you. Near a gravitating body, the only way to be at rest is to orbit the the body in some way. Near the surface of the Earth, these orbits are almost perfect parabolas, which is why thrown objects follow parabolic trajectories. Note that during the entire flight, a thrown object never "feels" gravity, whereas a person standing on the ground does. In fact, the person not moving is in a sort of non-inertial reference frame, being accelerated by the normal force of the ground.

Gravity is very not awesome.

It's a pussy compared to electromagnetism.

Originally Posted by Elis D.

Gravity is very not awesome.

It's a pussy compared to electromagnetism.

+1

Gravity is very not awesome.

It's a pussy compared to electromagnetism.

Strong force would fuck both of them up. >:V

Strange how it's the least understood of all the forces.

Quick, without thinking .. if the sun were to disappear, how long would it take for the earth to notice that it was no longer under the influence of the sun's gravity?
(We wouldn't notice the light gone for over 8 minutes)

It's too bad the startup of the Hadyron accelerator's been pushed back until September. Hopefully that will shed a little more light. Speaking of gravity, am I the only one in a lucid dream where the earth's gravity is like that of the moon's? I never walk I always bounce like the astronauts did on the moon. I bounce everywhere, hover at will but flying is still difficult because once I get airborne gravity slowly pulls me down. I can leap to the top of a skyscraper but I've never really flown significant distances and it's getting frustrating having to bounce everywhere or drive.

Note: For all you GPS buffs out there, the U.S. (which runs the GPS satellites) has to adjust their atomic clocks periodically to account for the difference in the passage of time onboard them and the time on earth because of .. velocity and gravity. Fun stuff.

Originally Posted by Lucid Lobster

Strange how it's the least understood of all the forces.

Quick, without thinking .. if the sun were to disappear, how long would it take for the earth to notice that it was no longer under the influence of the sun's gravity?
(We wouldn't notice the light gone for over 8 minutes)

good point, because we would be like floating away from something which we see, but its not actually there anymore.

To be honest there is no 'lack of understanding' about gravity. It's very well understood what causes it.

And the changes in spacetime travel at the same speed as the speed of light; as soon as we see the sun disappear, the gravitational force would also dissapear; Earth would cease circular motion and would just carry on in a straight line into space.

Originally Posted by Xei

To be honest there is no 'lack of understanding' about gravity. It's very well understood what causes it.

And the changes in spacetime travel at the same speed as the speed of light; as soon as we see the sun disappear, the gravitational force would also dissapear; Earth would cease circular motion and would just carry on in a straight line into space.

That’s a far reach, knowing what causes something doesn’t necessarily mean that you know the minute nature of it, and we obviously do not have enough understanding of gravity to make such a postulation. Yes we can identify the basics, we can use gravity and make sound predictions regarding the constellation, send spacecraft to planets, predict the motion of celestial bodies, etc… We have qualitative AND quantitative predictions that agree with observations. However we do not have the fundamental understanding to manipulate it for propulsion. Or make modifications for that matter. We simply do not have enough understanding of gravity to repel a force large enough to have flight without rockets. This should be quite possible considering how incredibly weak gravity is. In short there is a lack per-se’ of understanding gravity, our understanding of it is incomplete just as all scientific theories are incomplete, because they do not – and can not - exist in isolation. They relate in some way to every other scientific theory. No scientific theory is completely understood or “finished” sort-of-speak.

Originally Posted by Xei

To be honest there is no 'lack of understanding' about gravity. It's very well understood what causes it.

its not really, its the weakest force in the universe, and they are trying to figure out why. they have their little theories, but they still dont understand why yet (unless ive missed something)

True. Gravity is the "least" understood force even if we do know the 32 feet per second per second rhyme. The only practical use that we can even find for it so far is to power primitive waterwheels, ensure that bombs hit the ground when dropped and falling bread lands buttered side down. When/if we "figure out" gravity we can negate it and get to Alpha Centauri without it taking 100,000 years and an infinite amount of fuel. And once we conquer gravity inertia can't far behind. Our 767's will then be able to make those amazing 20,000 mph right angle turns in mid air just like the UFO's that everybody but me sees apparently on a daily basis. As far as the speed of gravity, there are still opposing theories as to whether gravity propogates at the speed of light or causes interactions instantaneously like quantum entanglement does.