Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: james Muirhead on 20/12/2016 09:49:45
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james Muirhead asked the Naked Scientists:
What is the source of the force of gravity or what is it that reaches up from the Earth to pull the apple downward?
What do you think?
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hi James. This is just a thought from a non-expert.
If two, or more, massive objects are moved apart they are given gravitational potential energy. If you pick up a rock, you are the source of that GPE, which causes the rock to return to the surface when you let go.
Extrapolate to the Big Bang, when all matter/energy was in intimate contact. The expansion of the Universe moved all that apart, presumably, imparting GPE, so all the matter of the Universe has the energy to return to its original compact state if nothing prevents it from doing so.
This would mean that the Big Bang was the source of gravitational energy. Naturally, that leaves the question of the source of the force that drives expansion, but that's another problem.
Hopefully, someone with more knowledge than I have, will come in on this.
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The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works.
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I thought it had been recently established, that it's all due to the Higg's boson. This boson creates a "field" which has a "dragging" effect on particles, which we perceive as "mass".
And this "mass" produces a "heaviness", which we used to attribute to a "gravitational force", but now we know it isn't, it's just the result of the Higg's boson's "field".
I mean how much clearer can you get?
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I thought it had been recently established, that it's all due to the Higg's boson. This boson creates a "field" which has a "dragging" effect on particles, which we perceive as "mass".
And this "mass" produces a "heaviness", which we used to attribute to a "gravitational force", but now we know it isn't, it's just the result of the Higg's boson's "field".
I mean how much clearer can you get?
You appear to be confusing inertial mass with active gravitational mass. The Higgs boson is part of particle physics which is a quantum theory of particles. As of yet there is no quantum theory of gravity. The reality will certainly be clearer than your belief as stated above.
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With the electromagnetic field the force carrier is the photon. For the gravitational field the force carrier is the hypothetical graviton. This has been described as a two copy gluon. However, the graviton cannot be made of gluons since the force is inverse square. The gluon is the reverse. It gets stronger with distance. Gluons can absorb and emit other gluons. This could be the same for gravitons. None of this speculation about the graviton should be taken as the truth.
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I thought the current theory of gravity was geometrically based (GR) and that the mass of objects did not cause an "attraction" effect as all objects appear to be attracted whether or not they were massive.
Now I have heard that massive objects are indeed responsible for the way the co ordinates used to model space time become curved locally.
Is it the mechanics of this action which are not understood?
All we have is the description that (loose quote) "mass curves space and curved space tell mass how to move" ,it seems to me.
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I thought it had been recently established, that it's all due to the Higg's boson. This boson creates a "field" which has a "dragging" effect on particles, which we perceive as "mass".
I don’t think the boson creates the field. Doesn’t the particle arise as a disturbance in the field?
The idea that the Higgs field gives mass via a drag effect is misleading. This arose from an analogy thought up by David Miller when William Waldgrave, (science minister in John Major's Government, needed a layman-level explanation of the Higgs mechanism in order to make a case for continuing the British share of funding for the CERN project). It’s the one about a famous person crossing a room full of people. This is not how the Higgs field works.
BTW. It won Miller a bottle of Champagne, but his family beat him to it – he got none.
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Frankly, I'm disappointed. I thought the Higg's Boson would explain the cause of mass. That's to say, mass is caused by the "dragging-effect" of a field caused by the Higgs thing.
But apparently it's not so simple, and nobody knows what they're talking about.
Is 21st century physics in a crisis?
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Is 21st century physics in a crisis?
No, but the popular press like to oversimplify it.
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The crisis of 21st century physics is this:
It has produced two different, and entirely irreconcilable, descriptions of what happens in the Universe:
1. Relativity Theory
2. Quantum Mechanics
They don't agree with each other. Therefore there is something wrong about both of them.
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Relativity and Quantum Mechanics... don't agree with each other. Therefore there is something wrong about both of them.
General Relativity describes the universe on a large scale, which is dominated by the force of gravity.
Quantum Mechanics describes the universe on a very small scale, which is dominated by forces much stronger than gravity.
They both work extremely well in the universe we see around us:
- in the LHC, which studies the world of the very small (and uses the principles of Relativity to do it),
- and in Astronomy, which studies the world of the very large (and uses the principles of Quantum Mechanics to do it)
However, at the event horizon of a black hole, the effects of gravity rivals the forces normally studied in Quantum Mechanics.
- It is in this (to date) unseen domain that General Relativity and Quantum Mechanics contradict
- Theoreticians are working on various approaches to a unified theory
- Rather than consider this conflict a disaster, researchers consider it an opportunity!
- Fortunately for us, applied physicists don't currently have any nearby black holes to study in microscopic detail, to resolve these open questions!
But for the rest of the universe outside a black hole, we think that both theories are happily compatible, and explain events in our solar system and beyond with great precision.
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Want to hear something weird :)
What if 'space' is 'gravity'?
I've started to wonder, I find 'vacuums' very bothersome myself.
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General Relativity describes the universe on a large scale, which is dominated by the force of gravity.
What is the reasoning behind describing Gravity as a force?I thought GR viewed it through the prism of the geometry of spacetime.
Is there a reason it can be called a "force"? It is not the same as Newtonian force ,is it?
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General Relativity describes the universe on a large scale, which is dominated by the force of gravity.
What is the reasoning behind describing Gravity as a force?I thought GR viewed it through the prism of the geometry of spacetime.
Is there a reason it can be called a "force"? It is not the same as Newtonian force ,is it?
F = ma
g = Gm/r2
g = a
F = force
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In a gravitational field that is uniform everywhere you get a linear acceleration.
If the potential of the field varies from place to place this is not the case.
When the potential varies a graph of acceleration is curved and not a straight line.
Remembering that acceleration is the derivative of the change in velocity with respect to time.
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F = ma
g = Gm/r2
g = a
F = force
Those are Newtonian equations. Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity does not apply?
EDIT: we cross-posted..
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Incidentally the derivative of acceleration is jerk. The derivative of jerk is snap.
This adequately sums up tidal forces.
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F = ma
g = Gm/r2
g = a
F = force
Those are Newtonian equations? Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity does not apply?
EDIT: we cross-posted..
They are approximations. You have to understand what is meant by curvature.
Since time dilates and length 'contracts' neither time nor space are linear.
This means that things don't happen simultaneously everywhere.
Add in the speed limit on information transfer (the speed of light) and viola!
Curved
spacetime.
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Add in the speed limit on information transfer (the speed of light) and viola!
Curved
spacetime.
What ,not the whole orchestra (or even the string section) ? ;)
PS still no formatting on the site?
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Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity does not apply?
Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.
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Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity does not apply?
Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.
That is not gravitational force though,is it?.Is it not simple acceleration?
I thought the main idea was that you do not feel such a force from gravity.(in freefall). and that accelerometers read zero.
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Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity does not apply?
Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.
That is not gravitational force though,is it?.Is it not simple acceleration?
I thought the main idea was that you do not feel such a force from gravity.(in freefall). and that accelerometers read zero.
The reason behind this is that gravity acts upon every individual particle 'at once'.
When we push something we apply a force to a particular point on a surface.
Gravity applies its force throughout. It is like having a cord tied to every particle.
If they all move in unison then the whole frame does.
It can be thought of as an inertial frame being accelerated.
This appears to remove the force of gravity since the frame is keeping pace with the force.
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That is not gravitational force though,is it?.Is it not simple acceleration?
No, it was only intended as an illustration of how a force can occur because of a curved path. However, remember that
Einstein used the illustration of gravity being the equivalent of an acceleration.
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No, it was only intended as an illustration of how a force can occur because of a curved path. However, remember that
Einstein used the illustration of gravity being the equivalent of an acceleration.
Is not the acceleration in the instance adduced by Einstein actually caused by the electro magnetic forces rather than gravity?
I think you are referring to the closed room where the occupant is unable to decide if the gravitational effects are due to gravity or an artificial acceleration.
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Is there a reason (gravity) can be called a "force"? It is not the same as Newtonian force, is it?
I was using gravity as a force the way Newton used it. But I could have used gravity as a curvature in spacetime as Einstein used it.
Gravity the familiar force we feel on our pants as we sit on a seat. And Newton's equations are not too bad an approximation (provided you are conscious of its limitations); General Relativity is a better approximation (provided you are aware of its fewer limitations).
However, when we are talking about quantum effects at the event horizon of a black hole, we are in a zone where neither Newton, Einstein or Heisenberg generate accurate predictions. So I used the more familiar model from Newton to describe why all current theories fail in this zone.
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Is not the acceleration in the instance adduced by Einstein actually caused by the electro magnetic forces rather than gravity?
Can you explain why you think that? Perhaps we are thinking of different scenarios here because I'm not aware of this example.
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Is not the acceleration in the instance adduced by Einstein actually caused by the electro magnetic forces rather than gravity?
Can you explain why you think that? Perhaps we are thinking of different scenarios here because I'm not aware of this example.
I am thinking of the scenario ** when an observer is in an enclosed chamber. He or she will notice objects falling in the direction of the "floor"
The question is posed "Is the chamber artificially accelerating or is it in a gravitational field?"
He cannot give an answer as either scenario fits the observation
I understand the only "force" involved to be that of the electro magnetism in the atoms of the floor preventing objects escaping the chamber.
In my recent post I think I may have garbled the ideas of force and acceleration in my mind:( Apologies if so.
**is this not a very well known example used by Einstein?
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The question is posed "Is the chamber artificially accelerating or is it in a gravitational field?"
He cannot give an answer as either scenario fits the observation
Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.
Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.
Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration. Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase.
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What defines a planet is its spin. That will show you if you're at one or accelerating inside a rocket in 'deep space'- Reading "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase. " lose me slightly Bill, what do you mean there?
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The question is posed "Is the chamber artificially accelerating or is it in a gravitational field?"
He cannot give an answer as either scenario fits the observation
Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.
Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.
Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration. Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase.
True , I had not heard that explanation/resolution before but it is valid. Nevertheless in a uniform (ie very large) gravitational field it would not be possible other than in principle.
By the way,Is my scenario not the one that Einstein used for his Equivalence Theory? (have I got scenarios mixed up?)
Also this link may be relevant or helpful-or just nice:
http://www.feynmanlectures.caltech.edu/II_42.html -go down to 42–5Gravity and the principle of equivalence
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I am thinking of the scenario ** when an observer is in an enclosed chamber.
**is this not a very well known example used by Einstein?
Yes, this is the famous example, but it doesn't equate gravity to electromagnetic forces. If the astronaut is at the front of the spaceship when it begins to accelerate s/he will experience freefall relative to the ship.
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I am thinking of the scenario ** when an observer is in an enclosed chamber.
**is this not a very well known example used by Einstein?
Yes, this is the famous example, but it doesn't equate gravity to electromagnetic forces. If the astronaut is at the front of the spaceship when it begins to accelerate s/he will experience freefall relative to the ship.
I don’t think I was attempting to equate gravity to em forces.
My suggestion was that it was wrong to talk about gravity as a force –and that, in the example we are looking at , the only forces at play are the forces stopping objects from falling through the floor.
I realize that force does play a role in Newtonian gravity but I thought that conceptually it was wrong as an idea although it may perhaps be correct within its limits of application(non relativistic speeds?)
I also realize that I may be being pedantic and merely stating the obvious.
Perhaps also I have confused in my own mind the prohibition of “an instantaneous force at a distance” with a prohibition of a force otherwise defined (f=ma)
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I don’t think I was attempting to equate gravity to em forces.
OK, I misunderstood what you were implying here:
Is not the acceleration in the instance adduced by Einstein actually caused by the electro magnetic forces rather than gravity?
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The question is posed "Is the chamber artificially accelerating or is it in a gravitational field?"
He cannot give an answer as either scenario fits the observation
Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.
Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.
Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration. Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase.
Oh that's clever. I do like that argument.
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Oh that's clever. I do like that argument.
It is neat, question is would it be possible to measure it in such a small volume?
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Oh that's clever. I do like that argument.
It is neat, question is would it be possible to measure it in such a small volume?
Feynman went on to say that the equivalence was just applicable to a point in the chamber/spacecraft
quote "To be strictly correct, that is true only for one point inside the ship"
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So equivalence is singular. Now that is interesting and not trivial.
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Well, I like it too, independent thinking I would call it :)
I'm not sure what Bill refers to with his last argument though? "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase."
1. The higher one climb the slower ones time, as described from a ground-sitter, might be one variable
2. Terminal velocity at higher altitudes is greater due to the thinner atmosphere, another
but the speed (acceleration) is the same, or is it :)
It may have to do with what elevation the marbles are at?
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Well, I like it too, independent thinking I would call it :)
I'm not sure what Bill refers to with his last argument though? "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase."
1. The higher one climb the slower ones time, as described from a ground-sitter, might be one variable
2. Terminal velocity at higher altitudes is greater due to the thinner atmosphere, another
but the speed (acceleration) is the same, or is it :)
It may have to do with what elevation the marbles are at?
I think it is because ,if you release two objects at a two different distances from the Sun (as an example of a body with gravitational attraction) and both initially stationary wrt the Sun, then the object which is nearer to the Sun will accelerate more quickly than the one that is further away -and so the distance between the two objects will increase over time.
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So equivalence is singular. Now that is interesting and not trivial.
Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?
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I think it is because ,if you release two objects at a two different distances from the Sun (as an example of a body with gravitational attraction) and both initially stationary wrt the Sun, then the object which is nearer to the Sun will accelerate more quickly than the one that is further away -and so the distance between the two objects will increase over time.
Yes; that was my thinking. Gravity decreases as a square of distance; so the nearer one is more strongly attracted.
It is neat, question is would it be possible to measure it in such a small volume?
That's why I included the " sufficiently sensitive measuring device". [:)]
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Gravity decreases as a square of distance; so the nearer one is more strongly attracted
This is the origin of "spaghettification (https://en.wikipedia.org/wiki/Spaghettification)" - in an extreme gravitational gradient (eg near a black hole), your feet would be attracted much more strongly than your head, so you get turned into spaghetti.
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I thought the source of Gravity was supposed to be a single, unique, "God-Particle" - the Higg's Boson?
But the Higg's is already postulated as coming in different varieties - "colours" or "flavours" or "spin", or whatever.
Physicists achieved their best work by discovering the Electron, the Proton, and the Neutron.
These are particles which seem firmly grounded in physical reality.
But later phantasms like the so-called "Higg's Boson", I don't believe in at all. It's just a fairy-tale. Doesn't everyone know it really?
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But later phantasms like the so-called "Higg's Boson", I don't believe in at all. It's just a fairy-tale. Doesn't everyone know it really?
May I suggest that you try to make three small adjustments to your thinking; then start the understanding process again. I suggest this, not as any sort of expert, but as one who has had to go through that process, more than once.
1. Get rid of the “God particle” idea. It seems to lead to all kinds of off-track thinking. It was such a bad idea, from the start, that its originator tried to blame it on his publisher.
2. Give some serious thought to what you mean by physical reality.
3. Ask yourself why you “don't believe in [that] at all”; just to be sure your belief is based on science, rather than prejudice.
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So equivalence is singular. Now that is interesting and not trivial.
Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?
Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.
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So equivalence is singular. Now that is interesting and not trivial.
Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?
Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.
Which point within the ship did Feynman mean? Any infinitesimal point or one both strategically positioned and infinitesimal ?
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So equivalence is singular. Now that is interesting and not trivial.
Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?
Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.
Which point within the ship did Feynman mean? Any infinitesimal point or one both strategically positioned and infinitesimal ?
The exact centre of gravity seems to be a good bet to me. Since objects are treated as point sources.
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But later phantasms like the so-called "Higg's Boson", I don't believe in at all. It's just a fairy-tale. Doesn't everyone know it really?
May I suggest that you try to make three small adjustments to your thinking; then start the understanding process again. I suggest this, not as any sort of expert, but as one who has had to go through that process, more than once.
1. Get rid of the “God particle” idea. It seems to lead to all kinds of off-track thinking. It was such a bad idea, from the start, that its originator tried to blame it on his publisher.
2. Give some serious thought to what you mean by physical reality.
3. Ask yourself why you “don't believe in [that] at all”; just to be sure your belief is based on science, rather than prejudice.
Gravity seems to be causing so much trouble to physics. Could this be, because "Gravity" isn't really a "force" at all. It's just an in-built tendency of matter to gather together.
Without any force involved. Matter just intrinsically "wants" to be with matter. As a kind of a natural Universal "homing" instinct. Like the proverbial "birds of a feather, flock together"
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All matter, however widely scattered it is, and whether it's composed of grains of dust, or planets, or stars, or entire galaxies, will naturally move together, until it's reached "home".
This was old Aristotle's teleological explanation. Was he right after all?
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Could this be, because "Gravity" isn't really a "force" at all. It's just an in-built tendency of matter to gather together.
Without any force involved.
You then have to ask "so how strong is this tendency?".
And you discover that this "tendency" increases proportional to the mass of either object, and decreases in proportion to the square of the distance between their centres.
And this "tendency" can be measured in units of Newtons - which is a unit of Force.
So gravity can be considered qualitatively as a "Tendency", and quantitatively as a "Force".
And if you follow Einsteins' General Relativity, you may consider it causatively as a "Curvature in Spacetime".
Nobody yet has a consistent quantum theory of gravity, but probably someone will eventually describe it in terms of an "Exchange of Gravitons".
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Could this be, because "Gravity" isn't really a "force" at all. It's just an in-built tendency of matter to gather together.
Without any force involved.
You then have to ask "so how strong is this tendency?".
And you discover that this "tendency" increases proportional to the mass of either object, and decreases in proportion to the square of the distance between their centres.
And this "tendency" can be measured in units of Newtons - which is a unit of Force.
So gravity can be considered qualitatively as a "Tendency", and quantitatively as a "Force".
And if you follow Einsteins' General Relativity, you may consider it causatively as a "Curvature in Spacetime".
Nobody yet has a consistent quantum theory of gravity, but probably someone will eventually describe it in terms of an "Exchange of Gravitons".
Are there any theories as to the mechanism that might be involved if "gravitons" were involved?
What might be their range ,for example?
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Are there any theories as to the mechanism that might be involved if "gravitons" were involved?
What might be their range ,for example?
Gravitons are a hypothetical particle that carries the gravitational force; the individual particles have very low energy, which makes confirming their existence quite a challenge.
They are most commonly thought to be massless (zero "rest mass"), travel at the speed of light, and have an infinite range (provided the general expansion of the universe does not carry distant objects away faster than c).
However, there are a number of alternative theories about the graviton, and at present no real way to distinguish between them.
See: https://en.wikipedia.org/wiki/Graviton (https://en.wikipedia.org/wiki/Graviton)
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Einstein said " everything should be as simple as it can be, but not simpler " it is just as simple as this : there is equivalence between the energy contained inside matter ( mass/energy ) and the sum of all possible potential energy it can exert outside this mass until infinity.
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Einstein said " everything should be as simple as it can be, but not simpler " it is just as simple as this : there is equivalence between the energy contained inside matter ( mass/energy ) and the sum of all possible potential energy it can exert outside this mass until infinity.
I wouldn't put too much trust in what Einstein said. He was just an early 20th-century theorist. Isn't it time to move on?
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yes , Einstein intelligence is a constant, but the equation also inversely proportion to years taken! but his wisdom never vanish.
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james Muirhead asked the Naked Scientists:What is the source of the force of gravity or what is it that reaches up from the Earth to pull the apple downward? What do you think?
That is the very question that Isaac Newton spent a life time trying to solve, unfortunately although he was able to come up with possible hypotheses for the phenomena of gravity there were never any supporting empirical proofs that could be proved by experiment. Hence his famous quote:' Hypotheses non finga ' (I make no hypotheses.). The hypotheses that Newton had in mind was one involving the existence of an aether because he could not comprehend the fact that gravity apparently acted at a distance with no intervening medium in between:
“It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter without mutual contact, as it must be, if gravitation in the sense of Epicurus, be essential and inherent in it. And this is one reason why I desired you would not ascribe innate gravity to me. That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left open to the consideration of my readers."
However, he considered that even action at a distance was preferable to the vortice theory put forward by Leibniz and Huygens:
“For since celestial motions are more regular than if they arose from vortices and observe other laws, so much so that vortices contribute not to the regulation but the disturbance of the motions of planets and comets; and since all phenomena of the heavens and of the sea follow precisely, so far as I am aware, from nothing but gravity acting in accordance with the laws described by me; and since nature is very simple, I have myself concluded that all other causes are to be rejected and that the heavens are to be stripped as far as may be of all matter, lest the motions of planets and comets be hindered or rendered irregular. But if, meanwhile, someone explains gravity along with all its laws by the action of some subtle matter, and shows that the motion of planets and comets will not be disturbed by this matter, I shall be far from objecting.”
Today it is Einstein's version of gravity as illustrated by General Relativity that captures the imagination of physicists. Yet Einstein's theory of General Relativity is very far from being perfect and it is hard to imagination future generations of astronauts on a Mars mission relying solely on General Relativity to navigate to Mars or anywhere else in the Solar system or for that matter the Universe, which in effect speaks for itself. As someone once said of the General Theory of Relativity:
“Gravity is the most familiar force. We are subject to it every day of our lives. Newton gave us his ‘law of gravity,’ which describes its effect but doesn’t explain it. “I frame no hypotheses,” he wrote. Einstein wasn’t so prudent when he introduced his “postulates.” Unfortunately, his unreal geometry doesn’t explain gravity either. The usual demonstration using heavy steel balls on a rubber sheet to represent ‘gravity wells’ relies on gravity as its own explanation!”New Scientist, 5th. July 1973.
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it is hard to imagine future generations of astronauts on a Mars mission relying solely on General Relativity to navigate
General Relativity is already used for multiple gravitational slingshots, so it will be fine for a "simple" Earth-Mars transfer orbit.
Probably the main difference with a manned mission is that a manned mission occasionally vents gases or liquids into space, an orbital disturbance that must be corrected.
Getting safely through the atmosphere of Mars has always been a challenge, but this requires a better understanding of hypersonic airflow, not an improvement on General Relativity.
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#56 evan_au said:
"General Relativity is already used for multiple gravitational slingshots, so it will be fine for a "simple" Earth-Mars transfer orbit".
[/size]Could you please why G. R. is necessary to gravitational slingshots?[/color]
[/size]I consider that, similarly to original "slingshots" (arm-wrist-hand powered sling), they can be explained within the limits of Newton´s Mechanics. [/color]
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it is hard to imagine future generations of astronauts on a Mars mission relying solely on General Relativity to navigate...
...I consider that, similarly to original "slingshots" (arm-wrist-hand powered sling), they can be explained within the limits of Newton´s Mechanics.
I agree with this. Provided you stay well outside the orbit of Mercury, the difference between Newton & Einstein's model of gravity is so small that it will not affect orbital calculations.
Functionally, Newton's model of gravity is a subset of Einstein's model of gravity, in weak gravitational fields (such as most of the Solar System).
- So orbital planning could rely solely on general relativity
- But they may choose to use Newton's gravity because the calculations are a tiny bit faster on a computer
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"Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase."
There are two variables to the pebbles falling Bill, well, as I see it. One is about the ships density, the 'material' as it might be, I would prefer 'unobtanium' of a 'infinite rigidity' for this one. Then it is the source of the ships gravity (the bottom of the ship) creating the ever increasing displacements in time. The origin of those displacements are its 'center of mass' as described from a inside so I would expect those pebbles to increasingly separate just as they should, if let go from some separation of elevation, on Earth. The first argument you use is one where you assume that the rockets center of mass (engine) is evenly distributed over the whole 'floor' of the rocket, right? I actually wonder there whether the rockets 'center of mass' won't be situated in a 'middle' of that floor too?
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The crisis of 21st century physics is this:
It has produced two different, and entirely irreconcilable, descriptions of what happens in the Universe:
1. Relativity Theory
2. Quantum Mechanics
They don't agree with each other. Therefore there is something wrong about both of them.
Not necessarily. The various formulations of quantum physics sometimes appeared to disagree with one another but were found to be just different ways of expressing the same truth. Relativity is a theory of a continuum whereas quantum physics is a theory of discrete particles. i.e analogue versus digital. Think of a TV set where the same picture is produced by analogue and digital transmission types. Quantum theory may in fact be a simulation of relativity. Gravity in quantum theory is defined via the graviton particle whereas gravity in relativity is intrinsic in the geometry of the continuum.
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james Muirhead asked the Naked Scientists:
What is the source of the force of gravity or what is it that reaches up from the Earth to pull the apple downward?
What do you think?
Mass attracts other mass, via gravity, and by doing so clumps in such a way to cause the shared space-time to contract. If we keep on adding mass to the clump, common space-time will continue to contract until it reaches a limit; singularity, which has a reference very similar to a speed of light reference; point-instant. This reference is where the gravitational potential of all the mass units are all at a minimum. If you add this all together, gravity simply reflects a movement of mass toward a common ground state, that is analogous to the speed of light reference.
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Release your marbles simultaneously from the top of the box. They will fall to the bottom. If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.
The first argument you use is one where you assume that the rockets center of mass (engine) is evenly distributed over the whole 'floor' of the rocket, right? I actually wonder there whether the rockets 'center of mass' won't be situated in a 'middle' of that floor too?
I was making no assumption about the centre of mass of the rocket. To do so would be to introduce gravity to the part of the scenario that should relate to the acceleration, surely this would negate the equivalence argument because you would be comparing gravity to acceleration + gravity.
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That's okay. I'm doing it. I started thinking about it in terms of where the 'center of mass' should be. Earth can be treated as if the center of its mass is inside its 'middle' approximately, that's also a reason to why the pebbles diverge as well as they 'accelerate', and it was those pebbles that made me start to wonder :) Because, assuming it to be correct then what it would say is that the 'center of mass' is 'everywhere' inside that rocket.
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don't read me wrong Bill, it was one of the more interesting ideas I've seen, no matter its outcome, it made me wonder. Better add that even if it is this way the equivalence principle should stand, as it uses very small patches to describe the equivalence from, but generally speaking, I would still wonder a little :)
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The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works.
Anybody like to speculate why it works?
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The following quote from Christopher Baird may help:
“The bottom line is that a uniform gravitational field is NOT a flat spacetime in the strict scientific sense since clocks tick at different rates at different locations in the uniform gravitational field. A uniform gravitational field may not look like a bar that is bent or a rubber sheet that is warped, but this just means that our analogies have limitations. It does not mean that there is no spacetime curvature.”
“If you stand motionless in the field (not in free-fall), and let go of a ball, it will fall in a special direction - in the direction in which the gravitational field strength is changing (i.e. down). Therefore, the existence of a special direction indicates that a uniform gravitational field is indeed a case of a curved spacetime. Again, it's not curved in the sense of a bar being bent to one side. It's curved in the scientific sense of spacetime behaving differently from one point to the next.
Yes, gravity is a force. But it is a force that is more completely described by spacetime curvature and not Newton's law.”
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It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.
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Anybody like to speculate why it works?
I wondered if thoughts along those lines might arise from #1, but no one has picked up on it, so perhaps it’s a non-starter.
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Quote from: PmbPhy on 27/12/2016 23:23:27The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works. Anybody like to speculate why it works?
So many things in this weird world/universe that we know how it works but not why it works.
This last hundred years or so has been a most exceptional period of advancement, even rivaling the introduction of the printing press in the 15th century. We have achieved so much and we are on the brink of so much more.
I would dearly like to hear more of the why rather than the how.
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It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.
This should be possible - the surface would be a straight line (at least in regions small enough where you could consider gravity to be constant).
You need to calculate the frictional/aerodynamic/magnetic induction energy losses of the ball rolling at the intended speed.
- Then place the surface at an angle that just compensates for all of these losses.
- If you start the ball rolling at the correct speed, it will keep rolling at this speed.
- It would be less complicated if the experiment were done in a vacuum (air pressure increases much more rapidly than Earth's gravity), and if the ball were non-conductive and non-magnetic (magnetic field intensity varies dramatically at different points on Earth's surface).
How would this help the study of gravity?
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The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works.
Anybody like to speculate why it works?
I think that what one should really be asking about is why it is that open space, which doesn't even comprise of thin air, is actually curved...
Q: What is curving?
A: There is nothing there to be curved.
Then one can look to gravity and ask if there is something about gravity that could cause a body of mass, or light to move in a curve.
Different values of M cause differing curves...
But any value of m in relation to M follows the same curve, therefore, although M is the cause of gravity, it can only be the g-field that is causing the curve.
When describing an acceleration or deceleration - acceleration is simply a phenomenon where it takes a lesser amount of time to cover the same distance. And a deceleration is a phenomenon where it takes a longer amount of time to cover same distance.
If you draw a line describing a linear deceleration followed by a linear acceleration between 2 bodies of M, the line will be a curve.
Therefore there lies the possibility that space itself is flat, and that gravitation is a time dilation phenomenon within the g-field itself that is causing the geodesics that m and light must travel, and as a result space is only appearing to be curved.
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When describing an acceleration or deceleration - acceleration is simply a phenomenon where it takes a lesser amount of time to cover the same distance. And a deceleration is a phenomenon where it takes a longer amount of time to cover same distance.
If you draw a line describing a linear deceleration followed by a linear acceleration between 2 bodies of M, the line will be a curve.
Therefore there lies the possibility that space itself is flat, and that gravitation is a time dilation phenomenon within the g-field itself that is causing the geodesics that m and light must travel, and as a result space is only appearing to be curved.
If this hypothetical** time dilation effect is increased without limit I take it that we approach a (relative) velocity of c for any massive object.
Does the value of c act as an impediment to the otherwise seeming possibility that succeeding events could happen simultaneously (in an absolute way)?
And does this value of c act as a practical impediment to the ideas that seem to go around of "tears in the fabric of space time" ,wormholes etc ?
I may not have made myself clear(because my reasoning is faulty no doubt) but perhaps you understand the (fuzzy?) point I am trying to make....
** I am not making out time dilation itself to be hypothetical but I think you yourself introduced it as a relevant factor in a hypothetical schema.(I have bolded and underlined where I think you are introducing this as speculation )
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Well that is a bit on the fuzzy side, but I think I understand what you are getting at.
However, this hypothetical time dilation of the g-field is decelerated at h from M, and can be considered as an aether type scenario.
The speed of light in open space will be being held relative to longer seconds, but in the reference frame of the longer seconds, the speed of light will be 299 792 458 metres per second of that reference frame.
Similarly, when approaching a black hole, the speed of light will be held relative to shorter seconds, but in the reference frame of the shorter seconds the speed of light will be 299 792 458 metres per second.
Now all that remains is to add GR time dilation and SR time dilation for mass, but not the length contraction/dilation of space associated with SR, for the reason that this effect is already taken care of via the g-field as a temporal measurement rather than a spatial measurement.
Now because it is the g-field that is inherent with the 'hypothetical' time dilation of the g-field, we can say that GR time dilation for m in relation to the g-field is gravity potential energy related, and is the exact opposite to the hypothetical g-field time dilation with respect to M, whereas GR time dilation is positive value, and the g-field time dilation is negative value.
When adding SR time dilation for mass to this picture, we can observe that because the speed of light is held relative to the negative value, that the speed that m is travelling at will be a higher percentage of the speed of light in that reference frame and the SR time dilation effects that m will experience will escalate.
In this manner, the m cannot exceed the speed of light of the reference frame it is travelling through.
Within this set up there lies the possibility to warp the slower time of the g-field reference frames in order that m will experience its travel path through space at a faster rate by regulating SR time dilation effects, and get 'there', wherever that may be quicker, but that is a far more advanced conversation
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Q: What is curving?
One way to look at the curvature of space is to add up the angles inside a triangle.
- In high-school geometry (as developed by Euclid), the sum of the internal angles of a triangle is exactly 180 degrees.
- On the surface of the Earth (or in a closed universe, or near a black hole), the sum of these angles is slightly larger than 180 degrees
- In an open universe (or on the surface of a horse-saddle), the sum of these angles is slightly less than 180 degrees.
So grab your protractor, and head off to the nearest black hole...
See: https://en.wikipedia.org/wiki/Non-Euclidean_geometry#Uncommon_properties
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Head off to the nearest black hole?
"Why whatever do you mean?"
(chuckle)
Should my feelings now be hurt I wonder...
But I believe you are missing the fact that if it takes a constant velocity a longer amount of time to cover a distance, this will cause the appearance of a greater distance. ie: a curve.
But this curve will only be an appearance because the distance remains a constant straight line of 180 degrees in the face of the longer time value...
...and without a horse saddle, dependent on the gait, less than 180 degrees can be quite lumpy, unless you're riding a nice wide cob!
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It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.
This should be possible - the surface would be a straight line (at least in regions small enough where you could consider gravity to be constant).
You need to calculate the frictional/aerodynamic/magnetic induction energy losses of the ball rolling at the intended speed.
- Then place the surface at an angle that just compensates for all of these losses.
- If you start the ball rolling at the correct speed, it will keep rolling at this speed.
- It would be less complicated if the experiment were done in a vacuum (air pressure increases much more rapidly than Earth's gravity), and if the ball were non-conductive and non-magnetic (magnetic field intensity varies dramatically at different points on Earth's surface).
How would this help the study of gravity?
I wrote that on 9th Feb and you expect me to remember my train of thought? I have a kitchen nightmare going on here. I will get back to you on this point.
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One way to look at the curvature of space is to add up the angles inside a triangle.....
I agree, but would also add the quote from Bill:
….....it's not curved in the sense of a bar being bent to one side. It's curved in the scientific sense of spacetime behaving differently from one point to the next.
So if we measure distance and time, near a mass, and find that the intervals (spacings) change in such a way that we can describe them as being on a curve, then it's reasonable to say that spacetime curves. It's only a description of what we observe.
We can take Evan's example of curvature even further Bill. If you look at a globe you see parallel lines of latitude. Someone might mistakenly think that if they stood on the intersection of a latitude and longitude and set off in a straight line along the surface, that they would follow the line of latitude. In fact they would follow a great circle which curves down to meet the equator.
If we were to imagine the latitude direction to be time – and we are always travelling through time like it or not – then the curvature would pull us towards the equator. Of course, we don't think of ourselves as travelling through time, so it seems to us that we are falling, or being pulled down, a line of longitude directly towards the equator.
Of course the real problem is that most physics is descriptive of what we observe, rather than trying to assign a cause. Every time we dig deeper to find a root cause we have to find words to describe that deeper layer and that description can only be in terms of a yet deeper layer. So even if someone finds a graviton say, the next question will be “how does it cause the attraction?”.
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It's turtles all the way down.
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At the moment I am looking at the inertia tensor when I get the rare half hour free. Since inertia and gravity appear to be related then I may have something useful soon.
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Gravitons or space time warping I think that we must use double think like waves and particles and agree that both are correct.
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I agree Syhprum - however it would also be nice to know why both are correct.
In examining the scenario where both descriptions are correct, it becomes necessary to consider that a vital peice of information that links the waves to the particles, and the particles to the waves must indeed exist.
And in accepting that a peice of information is missing - and this is the most important bit - it is paramount that one be open and willing to consider ideas!
An 'open' state of mind on the whole being a quality that appears to be somewhat lacking...
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It is not even a case of open mindedness. Theory has to be driven by observation and be able to make verifiable predictions. That is just the way things have to be done. Established theories work because they fit the above criteria. Rewriting them for the sake of it is unproductive.
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In addition to post above:
It is not even a case of open mindedness. Theory has to be driven by observation and be able to make verifiable predictions. That is just the way things have to be done. Established theories work because they fit the above criteria. Rewriting them for the sake of it is unproductive.
There is nothing unproductive about re-writing the laws of physics, if doing so unites the standard model with gravity.
That has been the goal of physicists for the best part of a century, or are the books I'm reading lying?
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I am inclined to a push theory in a field that pushes on matter, I have proof for that, other than this idea seems to continually crop up.No smoke without fire. Such theorys also give a very good reason for for dark matter.