## Is Gravity really a force, like e.g. magnetism?

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simonh
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### Is Gravity really a force, like e.g. magnetism?

I've split this from the giant battle dress thread as being OT.
Reynard wrote:"Except that gravity is not really a force. Free floating objects in a gravitational field, such as falling objects or objects in orbit, expereince no acceleration and no forces."

Where in the world did you find that definition. Every source I found does call it 'gravitational force'. It causes attraction between objects, even light and imparts acceleration as part of attraction. Gravity exists! ...
Of course it exists, but it doesn't work the same way forces like electromagnetism do. Specifically, the idea that it exerts some force proportional to that other object's mass is a missunderstanding of how it works. It's an easy one to fall into though, after all it's what Newton thought and he was a true genius.

For gravity to cause an attractive force between objects, that attraction must be due to some property those objects share, and those properties must interact with each other. You might think that is mass, but that's not the case.

Massive objects warp the space around them and in turm that curvature of space affects objects moving through that space, but that influence on other objects has nothing to do with any properties of those objects. Specifically, it's independent of the mass of those objects. That's why gravity affects light which has no mass. If gravity actualy exerted an attractive force on objects the way magnetism does, the force of that attraction would be proportional to their mass. But as light has no mass, that attractive force would be zero. Yet light is affected by gravity. Therefore gravity can't be considered to be two masses acting on each other. QED.

Magnetism is an interation between two magnetic particles. It has no effect on particles that are not themselves magnetic, and the strength of the interaction is proportional to the magnetic charge of the particles involved. Gravity does not work like that at all. It is not an interaction between two particles with mass. Two massive particles do influence each other due to the fact that they both curve space, but the influence of one object on the other has nothing whatever to do with the other object's own mass, or even dependent on it having any mass whatsoever.

That's why saying something like "gravity affects the other object proportional to it's mass" is nonsense, whereas saying equivalent things about magnetism and electrical fields (or even the weak and strong nuclear forces) would make perfect sense. Gravity is fundamentally a different sort of thing.
Go read a book!

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### Re: Is Gravity really a force, like e.g. magnetism?

A good place to start regarding the nature of gravity as it's known today is http://en.wikipedia.org/wiki/Gravitation

Reading books to learn greater detail is definitely recommended.

On a personal note, thanks to my playing Traveller at the time, asking my astronomy professor 'way back in college if, given its properties, gravity was a form of EM energy and explaining why I was asking using classroom proofs ended up getting me a job with the university physics department while I was working on my undergrad business degree. Let's hear it for Traveller!
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### Re: Is Gravity really a force, like e.g. magnetism?

Beyond lensing, gravity also effects time.
ShawnDriscoll
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### Re: Is Gravity really a force, like e.g. magnetism?

No one understands how gravity works yet. It'll probably be cool once someone does.
CosmicGamer
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### Re: Is Gravity really a force, like e.g. magnetism?

I'm just a simpleton and no science whiz but I can read. So from this definition
a force is any interaction which tends to change the motion of an object
I am a force to be reckoned with!

Again, I am no science whiz but is not light photons? Photons are particles? And a particle is a minute fragment or quantity of matter?

Now as to how similar the force of gravity is to magnetic force, I'm not sure why that has any more relevance than comparing the force of my bicep with the force of gravity, but I was not reading the thread that spawned this one so I can't provide any layman's insight.
simonh
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### Re: Is Gravity really a force, like e.g. magnetism?

CosmicGamer wrote:Again, I am no science whiz but is not light photons? Photons are particles? And a particle is a minute fragment or quantity of matter?
Light is a wave in an electromagnetic field. Imagine holding one end of a 10m piece of string, the other end is tied to a poll and it's hanging slightly loose. If you wave your hand from side to side, you will see waves or ripples in the string. The mass of the string doesn't increase just because you are shaking it and creating waves in it. However those waves do carry energy - if someone else was holding the other end of the string their hand would get shaken by the string.

Now imagine holding a powerful magnet in your hand. The magnet projects out a magnetic field in the space around you. If you wave the magnet around in your hand, the magnetic field will wave and ripple in the space around you. Those are very low frequency electromagnetic waves. They're the same thing as light, radio waves, x-rays, etc. Higher frequency electromagnetic waves are caused by atoms or electrons jiggling about and creating waves.

It turns out the energy in those waves is generated in fixed quantities of energy. You can think in terms that you can only create whole waves. You cant generate 1/10th of a wave or 11/10ths of a wave (that's not quite right, but it's not far off). Those fixed quantities of energy are photons. They do carry energy, but they have no mass. Just as with the string, creating waves in the electromagnetic field doesn't increase it's mass. Unlike with the string though, creating waves in an electromagnetic field projects out quanta of energy that carry some of the energy of the field with them over long distances. That's how we can transmit signals, light beams, etc. But they don't have mass either, they're still just waves in a very localised magnetic field.
Now as to how similar the force of gravity is to magnetic force, I'm not sure why that has any more relevance than comparing the force of my bicep with the force of gravity, but I was not reading the thread that spawned this one so I can't provide any layman's insight.
The point is that gravity is not two masses acting on each other, because it affects things that have no mass. We can't say that because electromagnetic fields work a certain way that therefore gravity must as well. The rest of it was realy just supporting that point.

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CosmicGamer
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### Re: Is Gravity really a force, like e.g. magnetism?

There are several different things getting jumbled together and I get confused by all the science.

The topic of this thread is titled "Is Gravity really a force"

As I quoted before, a force
a force is any interaction which tends to change the motion of an object
The object may be a planet, a moon, your body, and so on. Whether or not gravity effects something that is not an object, or does not have mass, as light is mentioned, I don't think negates it's being a force. (and besides, "object" has multiple definitions and that includes things with no mass)
simonh wrote:The point is that gravity is not two masses acting on each other, because it affects things that have no mass.
From what others smarter than me have written and I have read, I'm confused by the whole light/mass thing. Here is one quote
If light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in both directions symmetrically in the box, then the total momentum is zero in the box's frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured--in principle at least--either by the greater force required to accelerate the box, or by an increase in its gravitational pull. You might say that the light in the box has mass, but it would be more correct to say that the light contributes to the total mass of the box of light. You should not use this to justify the statement that light has mass in general.
Huh? Thought it might be of interest to this discussion since it mentions light and gravity - not that I understand it.

The concept some have given is of photons having "relativistic mass". Again, beyond my understanding.

A glass that is empty is still a glass. Perhaps there is a distinction between no mass and a mass of zero?

I'm not sure if any of this even matters. Why can't gravity have more than one property? One that effects mass and one that effects light or other items of "no mass"? Does gravity effect the other types of phonic energy waves too?
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### Re: Is Gravity really a force, like e.g. magnetism?

It’s considered one of the four fundamental forces, but it’s much weaker than the others and doesn’t appear to comply with their physical laws and behaviours.

The advent of time-space as a concept, suggests that gravity is more akin to objects ‘falling’ into a space brought about by the curvature of timespace in accordance to bodies of mass. That is, the force that holds the planets in the Solar system is the manner in which spacetime is curved by the mass of the Sun.

As such, it’s arguably not a force - merely a natural place for bodies of mass to be in while spacetime curves around it.
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simonh
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### Re: Is Gravity really a force, like e.g. magnetism?

CosmicGamer wrote:There are several different things getting jumbled together and I get confused by all the science.
That's quite natural, this is complicated stuff. Even experts sometimes get muddled about this, partly because the terminology we commonly use is ambiguous.

[quoteThe topic of this thread is titled "Is Gravity really a force"[/quote]

Not quite, the 'like e.g. magnetism' is a very important qualifier because it specifies the kinds of force we are talking about. It is arguable that Gravity is a force, but it not arguable that it is like magnetism except in some extremely general and vague ways that will very quickly lead you astray if you take them too far.
As I quoted before, a force
a force is any interaction which tends to change the motion of an object
The object may be a planet, a moon, your body, and so on. Whether or not gravity effects something that is not an object, or does not have mass, as light is mentioned, I don't think negates it's being a force. (and besides, "object" has multiple definitions and that includes things with no mass)
Fair enough. In that sense, specifically within the terms of that definition, we can say gravity is a force.

The key difference between the force of gravity and the force of magnetism (and others) is that magnetism does exert actual forces on things. A magnet physically pulls iron filings towards it. Those iron filings experience an acceleration (force = mass x acceleration) due to the action of the magnet. If you could fit an accelerometer on the iron filing, it could measure the acceleration due to the magnetic field.

While objects in gravity fields appear to act as though they are experiencing a force due to gravity in the same way, they aren't. An accelerometer will measure zero, and in fact there's no experiment you can conduct at a point in a gravity field that will detect the presence of the field. That's because it's just a curvature in spacetime. That's why callign gravity a force is misleading - according to relativity it doesn't actualy exert forces.

Of course we still pretend it does for everyday purposes. When you're taught Newtonian mechanics we talk about the froce of gravity, the force applied to two objects due to each other's mass, etc. Physics texts are full of this stuff. That's why I have been experemely careful to be very specific at every step exactly what I mean by force, and what different kinds of force there are.
Huh? Thought it might be of interest to this discussion since it mentions light and gravity - not that I understand it.

The concept some have given is of photons having "relativistic mass". Again, beyond my understanding.
It depends on how deep down the rabbit hole you want to go. We're actually talking about several different properties that are often conflated. They are rest mass, momentum, inertia, relativistic mass and energy. When I've been talking about mass previously I've really been talking about rest mass - if you dropped an object at a point in a gravity or magnetic field, how would it move and what causes that motion.

Light has zero rest mass. You can consider it to have relativistic mass, but that is completely dependent on relative motion. It's not generalisable. For example, two light rays traveling parallel to each other will not gravitationally attract each other at all.

The situation with the box of light is very complex. Are the photons in the box free-moving objects independent of the box, or are they trapped energy that we can consider to be part of the box? Those are two fundamentally different ways of looking at the situation, and it doesn't make sense to try to look at it both ways at the same time.
I'm not sure if any of this even matters. Why can't gravity have more than one property? One that effects mass and one that effects light or other items of "no mass"? Does gravity effect the other types of phonic energy waves too?
If we accept relativity, then we know the answer to that. Gravity is a curvature in the geometry of the universe. Objects traveling through that distortion are all affected by it in the same way because they are all in the universe. Any change in the shape of the universe around them affects their positional relationship to the rest of the universe equally, regardless of their individual properties.

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CosmicGamer
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### Re: Is Gravity really a force, like e.g. magnetism?

simonh wrote:The key difference between the force of gravity and the force of magnetism (and others) is that magnetism does exert actual forces on things. A magnet physically pulls iron filings towards it. Those iron filings experience an acceleration (force = mass x acceleration) due to the action of the magnet. If you could fit an accelerometer on the iron filing, it could measure the acceleration due to the magnetic field.

While objects in gravity fields appear to act as though they are experiencing a force due to gravity in the same way, they aren't. An accelerometer will measure zero
I find this hard to comprehend. Again, no science whiz, but I thought an objects speed can be altered by gravity. If you let go of a object from the top of a cliff it falls, accelerating, due to gravity, no? Space craft use planets and moons to slingshot and increase their speed, no?

It makes no sense to me. An object falling does not measure as accelerating on an accelerometer? So while no science whiz, I can read and I like to think I'm bright, and this is something that I want to try and understand as it is contrary to my current understanding.

What you say matches what I read. Somewhat.
Accelerometers in free fall orbiting and accelerating due to the gravity of Earth will measure zero.
The object is accelerating but the accelerometer reads zero? So what exactly is an accelerometer measuring? I read on.

Continuing to look things up I find terms like "proper acceleration" and "coordinate acceleration". So I start getting more confused. But one thing keeps popping up. The accelerometer is measuring G force. I also see that
an accelerometer at rest on the surface of the Earth will measure an acceleration g= 9.81 m/s2
There is a change from an accelerometer at rest measuring 1 G and an accelerometer in free fall measuring zero G. There is a change in the devices readings. Not exactly sure what this implies. What "force" is causing this difference?

I believe the earths gravity is a constant and it's force is not varying. Nor is the gravitational force of the object changing as far as I understand it. Maybe this is why there is no reading on the accelerometer when it is in free fall, because there is no change in the g force? Shrug.

I come back to the fact that your example with magnetism and iron filings and an accelerometer. It is using a device that measure G force!

So a magnetic field pulls iron which registers on an accelerometer which measure g force... is a magnetic field an artificial gravity field?

I am certainly not saying Gravity = Magnetism, but I am trying to figure out what your definition of "like" means as
simonh wrote:Not quite, the 'like e.g. magnetism' is a very important qualifier because it specifies the kinds of force we are talking about. It is arguable that Gravity is a force, but it not arguable that it is like magnetism except in some extremely general and vague ways that will very quickly lead you astray if you take them too far.
from what you yourself give as an example, it appears to me, a layperson, that the force of gravity is "like" magnetism as it is the very thing that you are using to measure the force of magnetism.
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### Re: Is Gravity really a force, like e.g. magnetism?

Go check this out. It is a nice, easy to understand video talking about the four fundamental forces of nature as we know them today. Two of them, gravitation and and electromagnetism, have been known for a rather long time. The other two, the Strong Force and the Weak Force, were discovered in the 20th century. The are sometimes called the Nuclear Forces. All of this is explained by Dr. Michio Kaku and Bill Nye. https://www.youtube.com/watch?v=R9FVFh3HYaY
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simonh
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### Re: Is Gravity really a force, like e.g. magnetism?

CosmicGamer wrote: I also see that
an accelerometer at rest on the surface of the Earth will measure an acceleration g= 9.81 m/s2
There is a change from an accelerometer at rest measuring 1 G and an accelerometer in free fall measuring zero G. There is a change in the devices readings. Not exactly sure what this implies. What "force" is causing this difference?
For the accelerometer resting on the surface of the earth, it is being 'pushed' upwards by the surface of the Earth, preventing it from falling freely. That is the force it's really measuring.
I believe the earths gravity is a constant and it's force is not varying. Nor is the gravitational force of the object changing as far as I understand it. Maybe this is why there is no reading on the accelerometer when it is in free fall, because there is no change in the g force? Shrug.
An accelerometer in free-fall, whether it's in a gravity field or not, experiences weightlessness. An astronaut holding an accelerometer in orbit would see a reading of zero on the device, and would feel no forces acting on themselves. They're not being shoved around by any direct force pushing or pulling on them. I know this way of looking at it is conter-intuitive, because our everyday experience on the ground is to feel that we are being pulled down by gravity. It doesn't seem ot make any intuitive sense to talk about the surface of the earth pushing us upwards.

It's a bit like when you look up into the sky and imagine that the earth is a ball spinning round it's axis and hurtling round the sun. To our naked senses it feels like the earth is stationary and the sun is travelling through the sky. Intuitions can be desceptive.

I've not answered your other points directly, because it's this distinction that seems at the root of it.

This is the mind-blowing bit, I've saved it up because I wasn't sure if it's actualy useful to mention it here, but let's go for it:

In relativity all intertial frames of reference are equally valid. You can tell your intertial frame of reference is changing because you experience acceleraton. That's what acceleration is, it's a transition from one inertial frame of reference to another. You are pushed by a rocket, attracted by a magnet, suspended by the ground under your feet.

If you experience no acceleration, your inertial frame of reference isn't changing. Within that frame of reference you are stationary. So in it's inertial frame of reference, a satelite in orbit around the earth is not moving. This is where relativity gets really surreal.

Of course in our inertial frame of reference(s) down here on earth, the satelite is whizzing around us. But relative to it's inertial frame of reference, it never moves at all. In relativity, that's a perfectly valid way to view the satelite's situation - as valid as any other.

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CosmicGamer
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### Re: Is Gravity really a force, like e.g. magnetism?

You are getting way beyond my ability to comprehend when you discuss relativity in this manner.

I do understand that everything is relative to other things. Is a space ship moving at x speed? x speed compared to the moon, the earth, another ship, the sun...

If you are going to give relativity as an explanation, give all the relative data. (pun intended) I can comprehend that objects in space have their course in the cosmos and the earth has it's and it may be the earth smacking into the meteoroids and not the meteoroids falling to earth.

The fact that it was the objects relative motion and not the iron core of an object being attracted by the earths magnetic field does not disprove magnetism to me. Nor does it disprove gravity to me.

I'll need more detail in how relativity disproves gravity.
simonh wrote:An accelerometer in free-fall, whether it's in a gravity field or not, experiences weightlessness.
I understand that weightlessness is a relative thing too. You can experience it on an amusement park ride. A ship can experience varying degrees of weight under thrust.
simonh wrote:An astronaut holding an accelerometer in orbit would see a reading of zero on the device, and would feel no forces acting on themselves.
First off, I thought the force of a magnetic field vary in strength based on distance and other factors? Isn't it similar for gravity?

Second, as it seams you say yourself, the accelerometer readings are relative
NASA Website wrote:This apparent weightlessness is created as the Space Station circles and falls around the Earth, and the continuous free fall simulates the absence of gravity.
so I'm not sure what point you are trying to make with these readings - which brings up: Third
NASA website wrote:Two accelerometer systems developed by NASA's Glenn Research Center in Cleveland, Ohio, are being used aboard the Station.
Without going into the paragraphs of information which I only partly understand, the accelerometers are making measurements. Again, it is relative as the purpose of these instruments is not in measuring earths gravity but the relative "forces" of objects in space.

You are making it sound like gravity is a non existent and it is something else that causes one thing to gravitate toward another. I'm ok with that if you can somehow describe what this other thing is.

Lets say there are thousands of cargo airplanes that are in the sky all around the earth. All of these planes open their bottom cargo bay doors and the contents comes out decreasing in distance to the earth until they impact the surface.
Is the earth moving in every direction toward these objects or are the objects all moving toward the earth?
What is causing all these objects, in every direction, to impact with the earth?
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### Re: Is Gravity really a force, like e.g. magnetism?

You saythat gravity is not a force because it affects light, which is energy, not mass. However, is not energy simply mass converted? As gravity is so different from the other three known forces, is it not posdible that there is some property of gravity that causes it to act irregardless of that conversion?
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### Re: Is Gravity really a force, like e.g. magnetism?

Other forces affect light too, such as the forces that make up a glass lens which bends light.
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### Re: Is Gravity really a force, like e.g. magnetism?

Which of the four known forces make up glass?
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### Re: Is Gravity really a force, like e.g. magnetism?

Reynard wrote:Which of the four known forces make up glass?
electromanetism: The electric field holds the electrons near atomic nuclei to make atoms. The Strong and Weak force hold the nuclei together. Gravity plays little role unless you have an awful lot of glass. My feeling is electromagnetism affects photons the most because photons are wave/particles of electromagnetic fields, so atoms and molecules either bend light paths, reflect light or absorb light.
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### Re: Is Gravity really a force, like e.g. magnetism?

Approximately 1/3 of the Earth is silicon - glass. That's a lot of matter and a lot of gravitational attraction. To better understand the relation of gravity to the other three forces is the statement:

"Gravity is the weakest of the four fundamental forces of nature. The gravitational force is approximately 10−38 times the strength of the strong force (i.e. gravity is 38 orders of magnitude weaker), 10−36 times the strength of the electromagnetic force, and 10−29 times the strength of the weak force. As a consequence, gravity has a negligible influence on the behavior of sub-atomic particles, and plays no role in determining the internal properties of everyday matter. On the other hand, gravity is the dominant force at the macroscopic scale, that is the cause of the formation, shape, and trajectory (orbit) of astronomical bodies, including those of asteroids, comets, planets, stars, and galaxies. It is responsible for causing the Earth and the other planets to orbit the Sun; for causing the Moon to orbit the Earth; for the formation of tides; for natural convection, by which fluid flow occurs under the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to very high temperatures; for solar system, galaxy, stellar formation and evolution; and for various other phenomena observed on Earth and throughout the universe. This is the case for several reasons: gravity is the only force acting on all particles; it has an infinite range; it is always attractive and never repulsive; and it cannot be absorbed, transformed, or shielded against. Even though electromagnetism is far stronger than gravity, electromagnetism is not relevant to astronomical objects, since such bodies have an equal number of protons and electrons that cancel out (i.e., a net electric charge of zero)."

On a nano scale, gravity is the runt while on the macro it is all!
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### Re: Is Gravity really a force, like e.g. magnetism?

I'm not saying gravity doesn't exit.

I tried to explain how Gravity works in my first post above, but it was a bit brief. I'll try to be a bit more expressive.

In relativity, Gravity is a distortion in the geometry of spacetime. Masses curve the space and time around them, so that anything travelling ballistically (that is, without any motive power) through that space will follow a curved path through the space.

A magnet creates a curved magnetic field around itself, which is an energy field that exerts forces on magnetic particles that enter it. Gravity doesn't have an special energy field it needs or creates in order to exert force on things. Instead, it curves space itself. In order to be affected by that curvature you don't need to have any special property like charge or magnetic poles, or even mass.

To be affected by the magnetic field, a particle needs to be magnetic itself, or to have an an electrical charge. To be affected by the curvature of spacetime created by Gravity, a particle just has to be in spacetime. No special properties have to be present, other than the property of existing.

The reason they have two accelerometers on the ISS is to measure the forces on the space station. I was very careful and very specific when talking about accelerometers in a gravity field. I already pointed out that I was only talking about measurements at a point. For a large structure like the space station, some parts of it are actually at a slightly different altitude than others. That means their natural orbital period would be different, but the structure of the station holds them together and that causes stresses on the space station. You also get similar effects laterally across the station as well. The forces those accelerometers measure are the forces the structure of the space station exert on them to prevent them following their natural ballistic path in orbit, forcing them to stay inside the space station. If the station want there, the accelerometers would follow different orbital paths an gradualy drift many (hundreds, maybe thousands of) kilometers away from each other.

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