A physicist explains gravity in terms we regular folks can understand.

[vorbis5]

It would seem logical that gravity would slow the expansion of the universe.

Nope. Gravity is way WAY to weak to slow down expansion. Look at the inverse square law.

 

[Blayz]

This doesn't explain why I don't fall off the planet when I jump.

I think it is wrong to emphatically state "gravity is this" or "gravity is that" We have a number of different models which work well in a particular "local" situation, but are not applicable in others.

 

[DontTreadOnMike]

This doesn't explain why I don't fall off the planet when I jump.

I think it is wrong to emphatically state "gravity is this" or "gravity is that" We have a number of different models which work well in a particular "local" situation, but are not applicable in others.

Examples?

 

[Doveaman]

Mmm nope gravity isn't a force. Everything moves in a straight line. When something gets pulled in and caught in earth orbit, it's still moving in a straight line. Draw a perfectly straight line on a piece of paper. Now bend the paper. That's what gravity is; bent space. So the planets are really traveling in a straight line through space that is curved by the sun

This line doesn't look straight to me:

 

[DontTreadOnMike]

This line doesn't look straight to me:

You sir have a good eye. That line is in fact, not straight.

 

[Doveaman]

Matter and energy bend space. It's really as simple as that.

If it's that simple, then demonstrate how matter and energy bend space.

 

[Doveaman]

You sir have a good eye. That line is in fact, not straight.

Neither are these:

 

[DontTreadOnMike]

If it's that simple, then demonstrate how matter and energy bend space.

Read the section on geometry and gravitation.
Introduction to general relativity - Wikipedia, the free encyclopedia

 

[DontTreadOnMike]

Neither are these:

You're right. Those are the 2 dimensional representations of the orbits of the planets on a euclidean plane and not ACTUALLY the orbits of the planets, they are limited representations. Space is not euclidean and this is not a pipe:

Is this a straight line?

Now zoom out.

 

[vorbis5]

You're right. Those are the 2 dimensional representations of the orbits of the planets on a euclidean plane. Space is not euclidean.




Is this a straight line?



Now zoom out.

Exactly.

 

[Blayz]

Examples?

I gave one. The relativistic model of gravity does not explain why I do not fall off the Earth when I jump, nor does it take into account quantum based approaches, such as gravitons.

More generally, once you start saying that some aspect of science is this and only this, you might as well put on a mitre and become a bishop.

 

[DontTreadOnMike]

I gave one. The relativistic model of gravity does not explain why I do not fall off the Earth when I jump, nor does it take into account quantum based approaches, such as gravitons.

More generally, once you start saying that some aspect of science is this and only this, you might as well put on a mitre and become a bishop.

I'm not a scientist so all I can say is that the scientists I talk to seem to be confident about the relativistic model of gravity. You have me intrigued now. I think I'll submit this question because if I'm wrong I want to know it, and if I'm right I want to know why. Knowing the right answer doesn't matter if you don't actually understand it.

EDIT: Oh wow that was fast. A scientist specializing in "Experimental strong force physics and quark-gloun plasma" says in response to the idea that The relativistic model can't explain why I don't fall off the earth when I jump:

"Yes it does. Work out a Lagrangian using the derivatives from General relativity and you'll get back Newtonian gravity as a limiting case. Ie, it behaves almost exactly like normal gravity, with some extreme cases as exceptions. (the orbit of mercury, the deflection of light, etc.) If you're not familiar with "Lagrangian" mechanics, it's just a more general way of stating Newton's laws. It tends to be a more useful way of doing problems that are more complicated than first year physics problems."


I have some studying to do before I know what he's saying though haha

 

[DontTreadOnMike]

The same person comments of gravitons:

"Also, I'd say, as for gravitons, there's no need to invent a new unobserved particle just to resolve a problem that doesn't really seem to exist to my knowledge. Gravity isn't a force. It isn't an exchange of momentum. It's a curvature of space. Particles don't need to be introduced to the situation."

 

[Blayz]

Thanks for the previous response (not quoted) here, any chance we could get one of those easy-to-understand explanations of how it works that doesn't involve words like Langrangian?

The same person comments of gravitons:

"Also, I'd say, as for gravitons, there's no need to invent a new unobserved particle just to resolve a problem that doesn't really seem to exist to my knowledge. Gravity isn't a force. It isn't an exchange of momentum. It's a curvature of space. Particles don't need to be introduced to the situation."

Would be interesting to see a conversation between this guy and someone of equal skill who works with quantum fields.

More generally, the issue of having multiple overlapping theories rather than one is covered in Hawkings' Grand Design...though I hesitate suggesting buying it since Personally speaking I found it a bit thin and disappointing.

 

[Wiccan_Child]

The same person comments of gravitons:

"Also, I'd say, as for gravitons, there's no need to invent a new unobserved particle just to resolve a problem that doesn't really seem to exist to my knowledge. Gravity isn't a force. It isn't an exchange of momentum. It's a curvature of space. Particles don't need to be introduced to the situation."

I would disagree with this person. If gravity isn't a change of momentum, I invite him to jump of a bridge and witness his momentum change quite rapidly indeed.

More generally, his opinion based on supposition: he interprets the mathematical model of GR to mean that space is warped by massive objects without the use of particles, which is pure conjecture.
It boils down to whether you put precedence in GR or QM, and most physicists err on the side of QM.

 

[DontTreadOnMike]

I would disagree with this person. If gravity isn't a change of momentum, I invite him to jump of a bridge and witness his momentum change quite rapidly indeed.

More generally, his opinion based on supposition: he interprets the mathematical model of GR to mean that space is warped by massive objects without the use of particles, which is pure conjecture.
It boils down to whether you put precedence in GR or QM, and most physicists err on the side of QM.

I'm out of my league here so I'll defer to the experts. Here's how RobotRollCall (the guy who wrote the OP) responded to my question above about gravitons. Afterward I'll find some copypasta from him about how gravity isn't a change in momentum.

"Graviton," the word, has two different meanings: an old one, and a contemporary one.
The old meaning was (jargon incoming) a massless spin-2 tensor boson that was the quantum of the gravitational field. This particle was the result of naively applying the methodology of quantum electrodynamics to gravitation. This method is now known to be a dead-end, for technical reasons relating to a process called renormalization. In short, if you can't renormalize a theory, you can't actually use that theory for anything, and this first attempt at a quantum theory of gravity cannot be renormalized. So back to the drawing board.
The contemporary meaning of "graviton" is simply any boson that arises from a quantum-field-theory formulation of gravitation. To date, there is no such formulation, but if one is ever devised that includes a single boson, it will be called a "graviton."
Except a quantum field theory of gravitation that includes a single field and a single boson may well be impossible. That's where ideas like gravitinos, graviscalars and graviphotons come in. But these are all products of speculative theories right now, and are not known to be valid descriptions of reality. You might hear the terms mentioned in popular science articles or what have you; if you do, understand that you're reading about something that may be true, not something that is definitely, or even widely believed, to be true.
Of all the physicists who have an opinion on the subject, I'd say — and this is really just based on personal experience, not any rigorous poll-taking or anything — that most agree that there probably is a way to reformulate the theory of general relativity in quantum-field-theory terms, but that such a theory would just be a rewriting of what's already known, and not the key to some fundamental new insights into gravitation. It'd just be a different-but-equivalent — and not necessarily more useful! — way of solving problems, in other words, and not a better theory of gravity than we already have. This is not the universal consensus or anything, but it seems to me that it's the most popularly held opinion.

In regards to gravity and acceleration/momentum, here's a conversation I had with RobotRollCall a week ago:

Me:I'm trying to understand how objects can seem to be accelerating when they fall towards earth when they really aren't. Without knowing the right answer, this is what my brain came up with.

To simplify it, we'll just look at a 2 dimensional plane with a grid of one meter by one meter squares. (totally arbitrary). Don't worry, this isn't the bowling ball and rubber sheet analogy. The grid isn't something, it's just to help us measure space.
Anyway we have two circles, one representing a planet and one representing, say, an astronaut. The two dimensional person is moving directly towards the two dimensional planet at a constant speed of one meter per second, he's not yet close enough to be falling. As he gets closer, he would seem to outside observers to accelerate towards the planet but if I understand this right, he's not actually experiencing acceleration in his own frame of reference. Is this because space is stretched towards the gravitating body so that the arbitrary grid units are longer? So he's still traveling one grid unit per second, but to an outside observer, he would seem to accelerate because the the grid units are bent or stretched.
How wrong am I?




RRC:

The thing most new students of general relativity struggle with is the fact that it's not space that curves, but spacetime. If you visualize just space as being the thing that's curved — as you have here, with your imaginary grid that represents a three-dimensional orthonormal coordinate basis in space — then you'll be confused for exactly the reasons you described.
If you want to try visualizing the effect of curvature on inertial motion, you're going to have to think about vectors, because visualizing spacetime — even a two-dimensional slice of spacetime — is not really something people can do very well. Instead, think of the four-velocity vector of the falling object as seen by both the falling observer and the stationary observer.
The falling observer will see her own four-velocity vector point straight toward the future, just as it always does. You're always at rest relative to yourself, obviously, so you will never observe your four-velocity vector to be pointed in any other direction than straight futureward.
But the stationary observer, at any given instant, sees the falling observer's four-velocity vector tilt toward the source of gravitation. It still points mostly toward the future, of course, but it also gains a spatial component due to the curvature of spacetime. This looks, to the stationary observer, like motion. The falling observer does not notice any acceleration, but the stationary observer sees the falling observer move along a curved worldline through spacetime.
What's more, as the falling observer gets closer to the source of gravitation, she moves continually from regions of less spacetime curvature to areas of more spacetime curvature. Which means that, again, she experiences no acceleration, but the stationary observer sees the falling observer's four-velocity vector tilt more and more toward the source of gravitation. The spatial component of four-velocity of the falling observer as measured in the stationary observer's reference frame therefore increases with time, which is equivalent to three-acceleration.
So the falling observer never experiences any acceleration at all, but the stationary observer sees the falling observer move with ever-increasing three-velocity through the stationary coordinate frame. Thus, the illusion of acceleration.




Me:Then what is the feeling of falling?


RRC: The "feeling of falling" is actually the absence of the ubiquitous acceleration you feel every moment of every day of your life (unless you're a skydiver or an astronaut or such like).

Third-party jumps in:
I would imagine the "feeling of falling" as the absence of a normal/support force on my body. As I stand at rest on the floor I am not accelerating, but I do feel the force of the floor on my feet which tells my senses that I am properly supported.
If I'm in a car that crests a hill, I feel lighter (almost like falling) because my normal force is smaller than normal for half a second.



RRC (Responding to "As I stand at rest on the floor I am not accelerating"): Actually you are. What you characterize here as a force is in fact an acceleration.

Unless you're doing a statics problem, it's best not to think in terms of forces. They're poorly defined in any situation where there's no measurable change in momentum, whereas acceleration is always directly measurable with nothing necessarily more sophisticated than a mass on a spring.

 

[DontTreadOnMike]

Thanks for the previous response (not quoted) here, any chance we could get one of those easy-to-understand explanations of how it works that doesn't involve words like Langrangian?

I know right? haha

The the quote in my op really helped me and so did the conversation I posted directly above this post in response to wiccan child. Check it out.

Would be interesting to see a conversation between this guy and someone of equal skill who works with quantum fields.

I agree but so far everyone who has responded to all of my questions about gravitation have had the same opinion. A quantum view of gravitation MAY be possible but it's not very useful, redundant, and obsolete because the relativistic view already answers our questions and no particle like a graviton has ever been observed, so why go with that view?

 

[Wiccan_Child]

I'm out of my league here so I'll defer to the experts. Here's how RobotRollCall (the guy who wrote the OP) responded to my question above about gravitons. Afterward I'll find some copypasta from him about how gravity isn't a change in momentum.




In regards to gravity and acceleration/momentum, here's a conversation I had with RobotRollCall a week ago:

An interesting discussion. But anything I want to say in response would be directed at the original poster, not yourself; a conservation by proxy isn't really a conversation at all

 

[DontTreadOnMike]

An interesting discussion. But anything I want to say in response would be directed at the original poster, not yourself; a conservation by proxy isn't really a conversation at all

Here's the thread I submitted because of this conversation:
Friend had a question about a relativistic model of gravity vs. gravitons. : askscience


And here's where I ask about accelleration due to gravity:
Critique my over simplified view of acceleration due to gravity. : askscience

The users you want to talk to are RobotRollCall and Shavera. making an account at Reddit is really easy, you don't even need to provide an email address. reddit.com/r/askscience is the subforum I frequent the most.

A word of warning however, Reddit is addictive. You may never be productive again haha

 

[BobAliceEve]

What we have so far is not an explanation but only a list. An explanation requires a description of interaction of things in the list. I will offer an example of an explanation. The explanation I give may not be true but the example it offers will be.

Let's start with something we understand: a laser hologram. We see the hologram because laser light flows into a region. The picture stops when the flow of laser light into the region stops. Similarly, a solid exists because energy flows into a region. If the energy were to stop flowing into the region then the solid would cease to exist. What we see as gravity is really our self being pushed toward the planet by the wind created by the energy moving into the region to keep the solid instantiated.