I’m not a physicist, but I am an engineer interested in things like this. I’ve wondered this kind of question too.
After a bit of online research, I think I understand it well enough to explain it. Since DaFranker, EY, and shiminux all seem to know, I’d like to run this by you and get your edits.
Question: “Where the frak does gravity get its power source?”
Answer: “It’s not really a source like a battery or a motor. What you’re seeing is the changing of energy from one kind to another. The fact that masses curve space creates a way for the positions of potential energy to be changed into the motions of kinetic energy. Since it’s not a dissipative force like friction there’s no need to keep “pushing power into the system” like with a car’s motor or an airplane’s jet. Just like a spacecraft only needs to fire rockets at the beginning and the end to change direction and doesn’t need to keep engines going all the time to stay moving. Oh, wait, you weren’t talking about the equations of power, right? If that was it I’ll need to go read up some more.”
Upvotes!
I’ve had the space travel and the gravity pieces of the puzzle for a long time. A special thanks to shminux for mentioning potential and disipative forces. That’s how I was prompted to come to an answer.
Also, this really seems like a question that needs to be dissolved. I think it’s based on a misunderstanding and not a real problem. That may be why professors have a hard time explaining it—there isn’t a power source for gravity in the sense that was being asked.
Since DaFranker, EY, and shiminux all seem to know, I’d like to run this by you and get your edits.
I’d like to help you out, but I’m afraid I don’t understand gravity beyond rote application of the inverse-square law and a metaphor about rubber sheets.
I think it’s based on a misunderstanding and not a real problem. That may be why professors have a hard time explaining it
Someone who has a hard time answering questions based on incorrect understandings is not well qualified to be a teacher. Handling that sort of question is a major part of the job.
The other sort of question, those that are based on a clear and correct understanding, are the questions that teachers ask their students. Although as Eliezer suggests above, asking students the first sort could be awesome teaching material.
True, but college professors are often not expert teachers. I agree that ideally all teachers should be experts at understanding what the student is asking, but they often aren’t. Having a PhD means you have great depth of knowledge in your subject, but teaching skills only have to be acceptable, not stellar. And this question is an uncommon and challenging one. It doesn’t surprise me that he got answers that he personally felt didn’t answer the question. In one of the other splinter conversations that came out of this post someone told me that the answer to the question in relativity is an actual true unknown. Which means no average college professor would be expected to be able to answer.
As far as asking questions that deliberately lead the students the wrong way, I only think that’s acceptable if you VERY SOON tell them why, and what the real circumstance is. If you’re trying to teach people to challenge assumptions, yes, I agree, it’s a very valuable tool.
Thanks for the comment. I was fascinated by the question he had, and still am.
In one of the other splinter conversations that came out of this post someone told me that the answer to the question in relativity is an actual true unknown. Which means no average college professor would be expected to be able to answer.
In that case the right answer would consist of explaining conservative force fields and potential energy, and then to say something along the lines of “but nobody knows what this potential energy is.” Feynman tells a story of his father explaining inertia in the same way: things in motion tend to keep moving and things at rest tend to stay at rest, and we call this inertia, but no-one really knows why this happens.
One can always correctly answer a question. It’s just that sometimes, the correct answer is “I don’t know.”
I completely agree with you that an accurate answer to a student is “I don’t know”
But teaching in general, and PhD’s in particular are specifically trainined never to say that. I mean look at how much effort they have to put into proving that they DO know. Oral examinations are NOT a place to say “I don’t know.” Just in general smart people don’t like to say it, and authority figures don’t like to say it. But I’ve heard it said that the one thing a PhD will never say is “I don’t know”
A great story about that from the opposit direction is one about astronaut John Young. Apparently he would ask instructors question after question until he reached “I don’t know” and if he never got to it you would never gain his trust.
Is it important? Yes.
Should teachers say it? Absolutely.
Is it one of the hardest things for people to say? Oh yes. I mean, even my kids teachers never say it. I’ve met with my son’s teachers a lot over the years, and I ask tons of detailed questions. It’s really, really hard to get them, or any authority figure to say “I don’t know.”
I tell my kids lots of things. They ask me all kinds of questions and I give them all the info I’ve got to give. They’re like me and keep asking more and asking more. I did that so much growing up (and still do!) that I annoyed the heck out of people with my questions. So I’m generous when my kids do it and don’t get frustrated and keep giving the next answer I’ve got. Eventually I get to “I don’t know.” I’ve started saying things like “That’s one of the mysterious scientists are still trying to figure out” because I’ve said “I don’t know” so much that it’s gotten monotonous.
My point is that it’s not surprising to me that a questioning student gets frustrating answers from frustrated college professors. Even if the best answer in a perfect world should have been “I don’t know.”
I know a lot of PhDs and haven’t noticed any tendency for such people to be more reluctant than others to say “I don’t know”. By whom have you heard it said that that’s one thing a PhD will never say?
(Disclaimer: Some of those PhDs are friends are mine. One of them is me.)
That’s pretty good, though you could probably settle for the Newtonian version of gravity, relativity tends to complicate things. It is interesting to dig deeper, however:
Since it’s not a dissipative force like friction there’s no need to keep “pushing power into the system” like with a car’s motor or an airplane’s jet.
Why is friction dissipative? After all, no fundamental forces are.
I wouldn’t be able to answer using Newtonian gravity, I’ve never seen the theory explained (that I remember). I see more reading in my near future.
I obviously don’t understand the words “dissipative force” in the same way you do. I thought I had that part down too. I thought it means that the energy you are concerned about is getting changed into energy not useful to you, like “waste” heat. So then friction would be dissipative. Please point me in a direction to learn more.
Well, a more poetic description of Newtonian gravity (which may or may not also be reusable for relativity with some edits) goes that:
“The entire Universe longs to be in equilibrium, in balance. This equilibrium was disrupted some 13.75 billion years ago, and since then all the splinters have been trying to reunite into a single whole, the distance between them and various other forces hampering their quest to become one again. It is the natural equilibrium that all masses attract eachother in the same way that opposite electromagnetic charges must attract eachother. ”
it means that the energy you are concerned about is getting changed into energy not useful to you, like “waste” heat
What is waste heat and why is it less useful than any other form of energy? What’s the mechanism that changes useful energy into waste heat? These are some of the many of the questions you can still ask and find answers to, before you hit the limits of what is currently known.
Waste heat is an increase in entropy that doesn’t do something that we want it to. It differs from useful energy in that it has effects either irrelevant to or opposed to what we want.
Friction sometimes generates waste heat (as in the case where we want something to move) and sometimes generates useful work (as in the case where we want two things to stop moving relative to each other, and cause them to interact via friction to a common momentum.
The mechanism is electrical forces- given two crystal or ceramic matrices close to each other and moving, electrical forces near the interface cause electrons near the interface to enter different energy levels (heat). The electrons transfer the impulse to other particles in the same ceramic or crystal through other forces, slowing the relative movement of the macro objects.
Asking where gravity gets its power source is like asking where the electron gets its power source. After all, electrons exhibit a force on each of them, and so do baryons.
I think I do. I think I have a general, summarized, understangin of how gravity works. I would say I have a starting point of knowledge, and If I ever need to get more specific to solve specific problems, I know where to go research the details, and then run experiements to solve a specific problem. Or to challege the Fake Explanations.
I’m not set on Relativity, for example, and I don’t accept it as some kind of gospel. I love thories that try to poke holes in Reltivity. The day I posted this I read about several that tried and were demonstratably worse at predicting reality that Relitivity.
As far as I can tell my mental map of the universe works pretty well, but I’m ok to revise it if that turns out not to be true.
I’m putting this out there to clarify my understanding and get comments on it, so I accept your comment, but how would YOU phrase your answer to the question of how gravity works, in a better, non-Fake-Explanation way?
Or, alternatively, how would you rewrite my answer in a better, non-Fake-Explanation way?
Because if you mean that I need to send up my own Gravity Probe B to verify frame-dragging before I can help other students try to understand gravity, you’re out of luck. I’m planning on trusting teir results. (although I have to admit to being a bit disappointed when they confirmed Einstein instead of challenging him! )
As far as I can tell my mental map of the universe works pretty well,
Do you regularly encounter situations where your map of GR is tested?
I’m putting this out there to clarify my understanding and get comments on it, so I accept your comment, but how would YOU phrase your answer to the question of how gravity works, in a better, non-Fake-Explanation way?
Or, alternatively, how would you rewrite my answer in a better, non-Fake-Explanation way?
Sure:
Question: “Where the frak does gravity get its power source?”
Newtonian Answer: “It’s not really a source like a battery or a motor. What you’re seeing is the changing of energy from one kind to another. The fact that masses [creates a gravitational field] creates a way for the positions of potential energy to be changed into the motions of kinetic energy. …
GR Answer: “This question is related to a major unsolved problem in general relativity.
“I don’t know.” or “This seems impossible.” or even “My teacher mumbled something about gravity being canceled by the ground’s opposite upwards reaction once an object is at rest, but that still feels incomplete. I’m sure there’s a better explanation that I just don’t know about yet.”
(that third one is me verbatim five years ago, so it might not fit the golden standard of a SPARC model student)
A decent answer (exploring potential vs dissipative forces) probably requires more of a physics background than you want to presume.
Not giving an answer is also a valid answer.
I’m not a physicist, but I am an engineer interested in things like this. I’ve wondered this kind of question too.
After a bit of online research, I think I understand it well enough to explain it. Since DaFranker, EY, and shiminux all seem to know, I’d like to run this by you and get your edits.
Question: “Where the frak does gravity get its power source?” Answer: “It’s not really a source like a battery or a motor. What you’re seeing is the changing of energy from one kind to another. The fact that masses curve space creates a way for the positions of potential energy to be changed into the motions of kinetic energy. Since it’s not a dissipative force like friction there’s no need to keep “pushing power into the system” like with a car’s motor or an airplane’s jet. Just like a spacecraft only needs to fire rockets at the beginning and the end to change direction and doesn’t need to keep engines going all the time to stay moving. Oh, wait, you weren’t talking about the equations of power, right? If that was it I’ll need to go read up some more.”
Upvotes! I’ve had the space travel and the gravity pieces of the puzzle for a long time. A special thanks to shminux for mentioning potential and disipative forces. That’s how I was prompted to come to an answer.
Also, this really seems like a question that needs to be dissolved. I think it’s based on a misunderstanding and not a real problem. That may be why professors have a hard time explaining it—there isn’t a power source for gravity in the sense that was being asked.
I’d like to help you out, but I’m afraid I don’t understand gravity beyond rote application of the inverse-square law and a metaphor about rubber sheets.
Someone who has a hard time answering questions based on incorrect understandings is not well qualified to be a teacher. Handling that sort of question is a major part of the job.
The other sort of question, those that are based on a clear and correct understanding, are the questions that teachers ask their students. Although as Eliezer suggests above, asking students the first sort could be awesome teaching material.
True, but college professors are often not expert teachers. I agree that ideally all teachers should be experts at understanding what the student is asking, but they often aren’t. Having a PhD means you have great depth of knowledge in your subject, but teaching skills only have to be acceptable, not stellar. And this question is an uncommon and challenging one. It doesn’t surprise me that he got answers that he personally felt didn’t answer the question. In one of the other splinter conversations that came out of this post someone told me that the answer to the question in relativity is an actual true unknown. Which means no average college professor would be expected to be able to answer.
As far as asking questions that deliberately lead the students the wrong way, I only think that’s acceptable if you VERY SOON tell them why, and what the real circumstance is. If you’re trying to teach people to challenge assumptions, yes, I agree, it’s a very valuable tool.
Thanks for the comment. I was fascinated by the question he had, and still am.
In that case the right answer would consist of explaining conservative force fields and potential energy, and then to say something along the lines of “but nobody knows what this potential energy is.” Feynman tells a story of his father explaining inertia in the same way: things in motion tend to keep moving and things at rest tend to stay at rest, and we call this inertia, but no-one really knows why this happens.
One can always correctly answer a question. It’s just that sometimes, the correct answer is “I don’t know.”
I completely agree with you that an accurate answer to a student is “I don’t know”
But teaching in general, and PhD’s in particular are specifically trainined never to say that. I mean look at how much effort they have to put into proving that they DO know. Oral examinations are NOT a place to say “I don’t know.” Just in general smart people don’t like to say it, and authority figures don’t like to say it. But I’ve heard it said that the one thing a PhD will never say is “I don’t know”
A great story about that from the opposit direction is one about astronaut John Young. Apparently he would ask instructors question after question until he reached “I don’t know” and if he never got to it you would never gain his trust.
Is it important? Yes.
Should teachers say it? Absolutely.
Is it one of the hardest things for people to say? Oh yes. I mean, even my kids teachers never say it. I’ve met with my son’s teachers a lot over the years, and I ask tons of detailed questions. It’s really, really hard to get them, or any authority figure to say “I don’t know.”
I tell my kids lots of things. They ask me all kinds of questions and I give them all the info I’ve got to give. They’re like me and keep asking more and asking more. I did that so much growing up (and still do!) that I annoyed the heck out of people with my questions. So I’m generous when my kids do it and don’t get frustrated and keep giving the next answer I’ve got. Eventually I get to “I don’t know.” I’ve started saying things like “That’s one of the mysterious scientists are still trying to figure out” because I’ve said “I don’t know” so much that it’s gotten monotonous.
My point is that it’s not surprising to me that a questioning student gets frustrating answers from frustrated college professors. Even if the best answer in a perfect world should have been “I don’t know.”
I know a lot of PhDs and haven’t noticed any tendency for such people to be more reluctant than others to say “I don’t know”. By whom have you heard it said that that’s one thing a PhD will never say?
(Disclaimer: Some of those PhDs are friends are mine. One of them is me.)
Sorry, how did you form this impression?
That’s pretty good, though you could probably settle for the Newtonian version of gravity, relativity tends to complicate things. It is interesting to dig deeper, however:
Why is friction dissipative? After all, no fundamental forces are.
Thanks!
I wouldn’t be able to answer using Newtonian gravity, I’ve never seen the theory explained (that I remember). I see more reading in my near future.
I obviously don’t understand the words “dissipative force” in the same way you do. I thought I had that part down too. I thought it means that the energy you are concerned about is getting changed into energy not useful to you, like “waste” heat. So then friction would be dissipative. Please point me in a direction to learn more.
Well, a more poetic description of Newtonian gravity (which may or may not also be reusable for relativity with some edits) goes that:
“The entire Universe longs to be in equilibrium, in balance. This equilibrium was disrupted some 13.75 billion years ago, and since then all the splinters have been trying to reunite into a single whole, the distance between them and various other forces hampering their quest to become one again. It is the natural equilibrium that all masses attract eachother in the same way that opposite electromagnetic charges must attract eachother. ”
What is waste heat and why is it less useful than any other form of energy? What’s the mechanism that changes useful energy into waste heat? These are some of the many of the questions you can still ask and find answers to, before you hit the limits of what is currently known.
Waste heat is an increase in entropy that doesn’t do something that we want it to. It differs from useful energy in that it has effects either irrelevant to or opposed to what we want.
Friction sometimes generates waste heat (as in the case where we want something to move) and sometimes generates useful work (as in the case where we want two things to stop moving relative to each other, and cause them to interact via friction to a common momentum.
The mechanism is electrical forces- given two crystal or ceramic matrices close to each other and moving, electrical forces near the interface cause electrons near the interface to enter different energy levels (heat). The electrons transfer the impulse to other particles in the same ceramic or crystal through other forces, slowing the relative movement of the macro objects.
Asking where gravity gets its power source is like asking where the electron gets its power source. After all, electrons exhibit a force on each of them, and so do baryons.
Then what makes you think you know enough to use GR for anything besides a fake explanation?
Maybe I don’t.
I think I do. I think I have a general, summarized, understangin of how gravity works. I would say I have a starting point of knowledge, and If I ever need to get more specific to solve specific problems, I know where to go research the details, and then run experiements to solve a specific problem. Or to challege the Fake Explanations.
I’m not set on Relativity, for example, and I don’t accept it as some kind of gospel. I love thories that try to poke holes in Reltivity. The day I posted this I read about several that tried and were demonstratably worse at predicting reality that Relitivity.
As far as I can tell my mental map of the universe works pretty well, but I’m ok to revise it if that turns out not to be true.
I’m putting this out there to clarify my understanding and get comments on it, so I accept your comment, but how would YOU phrase your answer to the question of how gravity works, in a better, non-Fake-Explanation way?
Or, alternatively, how would you rewrite my answer in a better, non-Fake-Explanation way?
Because if you mean that I need to send up my own Gravity Probe B to verify frame-dragging before I can help other students try to understand gravity, you’re out of luck. I’m planning on trusting teir results. (although I have to admit to being a bit disappointed when they confirmed Einstein instead of challenging him! )
Do you regularly encounter situations where your map of GR is tested?
Sure:
Question: “Where the frak does gravity get its power source?”
Newtonian Answer: “It’s not really a source like a battery or a motor. What you’re seeing is the changing of energy from one kind to another. The fact that masses [creates a gravitational field] creates a way for the positions of potential energy to be changed into the motions of kinetic energy. …
GR Answer: “This question is related to a major unsolved problem in general relativity.
Indeed. Three relevant posts for how a good answer could be constructed.
Feel free to elaborate.
“I don’t know.” or “This seems impossible.” or even “My teacher mumbled something about gravity being canceled by the ground’s opposite upwards reaction once an object is at rest, but that still feels incomplete. I’m sure there’s a better explanation that I just don’t know about yet.”
(that third one is me verbatim five years ago, so it might not fit the golden standard of a SPARC model student)