Weightless in a Shielded Room: Does Gravity Still Apply?

[satelliteguy]

New to trying to understand the universe, so may be off on some things, but please let me know if I am.

Part of Einstein's general theory of relativity is that "gravity" doesn't pull on us, but rather "space" pushes on us and that's what keeps us on earth. But if that's the case, wouldn't you be weightless in a shielded underground room?

 

[CompuChip]

You would still be in a gravitational field. But if you could somehow shield even that, then I suppose: yes?

 

[A.T.]

Part of Einstein's general theory of relativity is that "gravity" doesn't pull on us, but rather "space" pushes on us and that's what keeps us on earth.

No, its the ground that pushes us up. When nothing pushes you, you are in free fall.

But if that's the case, wouldn't you be weightless in a shielded underground room?

Gravity cannot be shielded. But if the Earth was a hollow shell, you would be weightless inside, even in Newtonian gravity.

 

[neopolitan]

Gravity cannot be shielded. But if the Earth was a hollow shell, you would be weightless inside, even in Newtonian gravity.

I do hope that the latest movie adaptation of Jules Verne's novel takes this into account to have have Brendan Fraser and the gigantosaur floating around in the centre of the Earth. I'll be terribly disappointed if they make a scientific error by having Earth normal gravity in the centre.

It'd be like having later Cretaceous dinosaurs in Jurassic Park (only out by about 80 million years) or ignoring the molten core of the Earth, which I am quite certain that film makers won't ignore ... will they?

 

[satelliteguy]

Alright, so either a)Gravity pulls us towards the center of the Earth (but nothing can be "pulled") b)The Earth pushes us up (free fall toward?) c)space around us is pushing us back down toward Earth because the Earth is moving through space, and space offers some kind of resistance on earth, and hence, on us.

 

[atyy]

You are weightless if gravity, and gravity alone acts on you, ie. if you are falling freely.

If, in addition to gravity, other forces act on you, then you are not falling freely. For example, when you are standing on the floor, the non-gravitational forces between the atoms of the floor and the atoms of your feet are preventing you from falling freely through the floor.

When you are high above the floor, the only force between your atoms and the atoms of the Earth is the gravitational force. You don't feel the non-gravitational forces either because they are short-ranged; or in the case of the electric force, positive and negative electric charges cancel each other out to very high precision. Thus high above the floor you are freely falling.

Of course, this is just an approximation, because even high above the floor, the atoms in your fingers are connected to the atoms in your arm by a non-gravitational force, so the atoms in your fingers cannot be exactly freely falling. But this is nitpicking.

 

[satelliteguy]

Thought "weight" was the effecÞ of "gravity" on us, and w/out gravity (space) we would be weightless? We weigh less when we free-fall because we have no resistance besides air for "gravity" to force on us?

Didn't Newton give-up on understanding what "gravity" is, and simply said, "I'll call it gravity". (More or less) ?

 

[A.T.]

We weigh less when we free-fall because we have no resistance besides air for "gravity" to force on us?

And therefore free fall in gravity is equivalent to being in space with no forces acting. So the idea in GR is to assume that free fall in gravity means also no forces acting (gravity is not modeled as a force).

Didn't Newton give-up on understanding what "gravity" is, and simply said, "I'll call it gravity". (More or less)

General relativity also doesn't say what it is, just how it affects objects. The mathematical models to quantify its effects are just different.

 

[atyy]

Thought "weight" was the effecÞ of "gravity" on us, and w/out gravity (space) we would be weightless? We weigh less when we free-fall because we have no resistance besides air for "gravity" to force on us?

Didn't Newton give-up on understanding what "gravity" is, and simply said, "I'll call it gravity". (More or less) ?

When you are standing on a scale on the floor, the scale reads your weight. The forces on you are the downward force of gravity caused by the entire earth, and the upward force of the scale that balances gravity and prevents you from falling through it. By Newton's third law, you exert a downward force on the scale which is what the scale reads which is your weight.

If you are free falling, and the scale you are trying to stand on is also free falling, then because everything falls at the same rate (the legendary Galileo at Pisa demo), how are you going to exert a force on the scale? You can't. So you are weightless.

 

[DrGreg]

When you are standing on a scale on the floor, the scale reads your weight. The forces on you are the downward force of gravity caused by the entire earth, and the upward force of the scale that balances gravity and prevents you from falling through it. By Newton's third law, you exert a downward force on the scale which is what the scale reads which is your weight.

That's the Newtonian explanation. The relativistic explanation is that no force acts downward, the Earth pushes you upwards, through the scale, and you accelerate upwards according to Newton's second law. (Accelerate upwards relative to any free-falling objects, that is.)

In both the Newtonian and relativistic explanations, the scale measures the upwards force exerted by the Earth.

 

[satelliteguy]

Oh wow, think I just got it. We are where we are, and as the Earth moves, it moves us, and exerts force on us. Is that basically it? If that's right, then...wow, exhilirating.

 

[Englishman]

Well, actually, to make things clear, the Earth does not pull upon anything.

Picture this: A three dimensional co-ordinate plane that is somewhat elastic.
Let's say you place a large ball in the middle of the plane. What happens? The plane becomes stretched, because of the "weight" of the ball.
Next, imagine a much, much smaller ball being thrown (of course with the exact speed necessary) along the stretched portion of the plane. What happens? The smaller ball travels around the much bigger ball. Imagine that it never stops.

That is the gravitational relationship between the Sun and the Earth. The perpetual motion of the smaller "ball" is the Earth orbiting around the Sun. The "weight" is the affect of gravity, which is the warping of spacetime. As you know, everything with mass exerts a force upon anything else with mass. The force is the reason why the Earth is in orbit around the Sun.
It might difficult for one to understand because it is difficult to explain. Just picture those coin donation things at the mall. When you throw a coin there, it takes a while to finally go in the black hole. Picture that, but with the Earth replacing the coin, and the Sun replacing the hole in the middle (and remember that instead of the Earth going towards the center like the coin, it would be going in an elliptical orbit, permanently).

 

[Englishman]

You are weightless if gravity, and gravity alone acts on you, ie. if you are falling freely.

If, in addition to gravity, other forces act on you, then you are not falling freely. For example, when you are standing on the floor, the non-gravitational forces between the atoms of the floor and the atoms of your feet are preventing you from falling freely through the floor.

When you are high above the floor, the only force between your atoms and the atoms of the Earth is the gravitational force. You don't feel the non-gravitational forces either because they are short-ranged; or in the case of the electric force, positive and negative electric charges cancel each other out to very high precision. Thus high above the floor you are freely falling.

Of course, this is just an approximation, because even high above the floor, the atoms in your fingers are connected to the atoms in your arm by a non-gravitational force, so the atoms in your fingers cannot be exactly freely falling. But this is nitpicking.

But it's true; another force is the Earth's electromagnetism.

 

[Englishman]

Alright, so either a)Gravity pulls us towards the center of the Earth (but nothing can be "pulled") b)The Earth pushes us up (free fall toward?) c)space around us is pushing us back down toward Earth because the Earth is moving through space, and space offers some kind of resistance on earth, and hence, on us.

He's right; this is what I was talking about, that there is always an attraction between objects (just don't follow Newtonian thinking; gravity is not an invisible tether between us and Earth).

 

[satelliteguy]

Alright, I think I'm getting it. If I go skydiving and jump out of a plane, the Earth is moving toward me, and not deploying my 'chute to use wind resistance, the Earth would eventually *hit me* at ?166k? mph w/ several kg's of mass. Is that mostly it?
Also, space itself is pushing on me, also pushing me toward earth.
But the Earth's electromagnetic waves/energy wouldn't effect me because I'm made up of a neutral charge?

Also had a deep passion for this and questions like it, have just ignored 'em for too long. TY for your help.

 

[Englishman]

Alright, I think I'm getting it. If I go skydiving and jump out of a plane, the Earth is moving toward me, and not deploying my 'chute to use wind resistance, the Earth would eventually *hit me* at ?166k? mph w/ several kg's of mass. Is that mostly it?
Also, space itself is pushing on me, also pushing me toward earth.
But the Earth's electromagnetic waves/energy wouldn't effect me because I'm made up of a neutral charge?

Also had a deep passion for this and questions like it, have just ignored 'em for too long. TY for your help.

The Earth will hit you, but empty space itself does not push on you. Remember, only objects with mass exert force upon other objects with mass, not empty space. The electromagnetic force, not waves/energy, is part of the reason why you remain on the earth. We are talking about the electromagnetic force, not electromagnetic waves (I'm not trying to be condescending).

Yeah, I understand; Einstein's relativity is very captivating.

 

[A.T.]

Alright, I think I'm getting it. If I go skydiving and jump out of a plane, the Earth is moving toward me,

Movement is relative. What matters is acceleration caused by forces:

Newtons theory (gravity is a force):
- You are accelerated towards the Earth's center by the force of gravity
- The Earth's surface is not accelerated, because the force of gravity is canceled out by the electromagnetic repelling force from the matter below.

General relativity (force of gravity is gone):
- You are not accelerated as no forces act on you
- The Earth's surface is accelerated away from the center, by the electromagnetic repelling force of the matter below.

Important note about a common misconception: Acceleration away from the Earth's center, does not imply movement away from the Earth's center. The Earth is not inflating and has a constant radius.

 

[jtbell]

General relativity (force of gravity is gone):
- You are not accelerated as no forces act on you
- The Earth's surface is accelerated away from the center, by the electromagnetic repelling force of the matter below.

No, both you and the Earth are in free fall (zero proper acceleration), moving along space-time geodesics which intersect. (ignoring the fact that the Earth is an extended object whose parts exert forces on each other to keep them from collapsing towards the center)

 

[A.T.]

General relativity (force of gravity is gone):
- You are not accelerated as no forces act on you
- The earth's surface is accelerated away from the center, by the electromagnetic repelling force of the matter below.

No, both you and the Earth are in free fall (zero proper acceleration), moving along space-time geodesics which intersect.(ignoring the fact that the Earth is an extended object whose parts exert forces on each other to keep them from collapsing towards the center)

I was specifically talking about the earth's surface, meaning the rocks you hit if your parachute doesn't open.

 

[Naty1]

Gravity cannot be shielded

I'd prefer to say we don't currently know how to shield against gravity...we do know how to shield against other forces (strong,weak, electromagnetic) so there would seem to be a potential opportunity to do so...but gravity IS different...

But if that's the case, wouldn't you be weightless in a shielded underground room? (QUOTE]

I agree, BUT If you were in a shielded room, you'd be weightless regardless of the mechanism...maybe a hollow sphere of dark energy could "shield" you...

 

[satelliteguy]

Alright, when I first said shielded room, was thinking we were held here by space pushing against us. Know that's not right now.

Still feels like somethings missing in the current knowledge.

 

[Dale]

Still feels like somethings missing in the current knowledge.

Yes. That's why we haven't fired all of the research physicists.

 

[Englishman]

Alright, when I first said shielded room, was thinking we were held here by space pushing against us. Know that's not right now.

Still feels like somethings missing in the current knowledge.

What do you want to understand?

 

[satelliteguy]

Everything in existence. :). Kind of a tall order, but if I had my drathers, it'd be everything about how the universe works.

 

[Naty1]

Still feels like somethings missing in the current knowledge.

Yes, of course there is something missing as Dalespam posted following this comment...for example, the massive effects of repulsive gravity were only discovered quite recently...who ordered THAT? (to paraphrase a famous physicst)...and nobody understands dark matter and dark energy...

What people in the above posts have been providing are classical Newtonian and relativistic interpretations of gravity...what we understand so far...which is appropriate...
we KNOW neither is correct and so far neither is quantum gravity complete...Had you posted your question(s) in the quantum mechanics section you would have received some different interpretations about gravity...via quantum gravity, loop quantum gravity,gravitons,etc...

How could anyone REALLY understand gravity when neither space nor mass is understood...

 

[bhornbuckle75]

Ok so I am trying to understand the concepts talked about here about relatavistic gravity...It seems to have been described a few different ways by different posters and I seem to be getting more and more confused. Ok so I will just try to (without qouting) reflect on what the various posters have said and perhaps someone can help me with my confusion. It was suggested that it is the Earth's acceleration away from its center which causes a person to stick to it, rather than a force pushing or pulling from without, and also that both the Earth and a person on it is in a state of free fall. Ok I can imagine that if I was falling down a very big hole and there was a massive rock beside me, that if I were to push against the rock (its inertia being much greater than mine, and therefore I could push off of it as if it were a stationary object). Is this concept in any way similar to the whole free falling Earth idea? If I am free falling with the Earth (and for this example I'll shrink it down to the size of a large truck, but assume that it is still just as massive) down a hole - and at the end of the hole would obviously be something super massive so that the Earth would fall towards it, such as the sun or something equivalent. Anyway Me and the Earth are free falling down a hole, and let's say that I am 20 feet from the earth. The Earth is going to draw me towards it. How can the Earth pushing away from itself cause such a thing? If Relativity says that the Earth warps space around it, what difference would that make if the force of Gravity is not coming from the warped space? If space itself can not be a force that acts upon me then where is this force coming from which can affect me at a distance from the earth. I can understand that the Earth is accelerating away from itself (sort of...-still kind of iffy on the idea of acceleration away from something without movement from the same) But why would I move toward something unless I am being acted upon by some kind of force. Someone on the other side of this small truck sized yet massive Earth would experience the same thing as me. We would both be moving toward something in between us-yet if I am understanding the other posts there is no force acting upon us-rather it is the force of the Earth which is accelerating away from itself and towards us-yet without moving. WHAT?! I thought the whole quantum double slit thing and Cats being both dead and alive at the same time until their probability fields collapsed were wierd, but at least those ideas make sense in a sort of sci-fi TV show kinda way. I've got to be misunderstanding the posts here somewhat because the reality of the universe just went from being a kind of cool zen-like sci-fi Show to being a Monty Python sketch...and not one of the funny classic ones either, nope its one of the really lame surrealistic 'weird for the sake of being weird' ones!

 

[satelliteguy]

Like the last few posts said, it is not understood. I don't think anyone here really knows, these are just repeated ideas from Newton, einstein, etc.

Even those 2 guys knew they didn't have the whole answer. I'm disbelieving einsteins theory more and more

 

[A.T.]

Even those 2 guys knew they didn't have the whole answer. I'm disbelieving einsteins theory more and more

Physical theories are not about believing and having the whole answer, but about understanding and using them to make predictions. If you need something to believe in, try religion. It gives you the whole answer for sure.

 

[satelliteguy]

Haha, yeah right. After seeing pictures of the known universe, it's hard to believe that our one little speck means anything to any higher power.

I know that the theories were a huge step forward in even thinking about what's going on with the universe, but so many people think that these things are already understood, and etc, but nothing's really known. I mean, when I was in school, we were taught Newtonian gravity, as if it were fact.

Wish more people wondered about it all.

 

[Englishman]

Haha, yeah right. After seeing pictures of the known universe, it's hard to believe that our one little speck means anything to any higher power.
I mean, when I was in school, we were taught Newtonian gravity, as if it were fact.

Yeah well, Newtonian gravity is highly inaccurate compared to Einsteinian relativity, just like Euclidean geometry is highly inaccurate compared to Riemannian geometry. Euclidean geometry deals with the geometry of planes, which are flat. Absolutely nothing is flat in this world; some things may look flat to the naked eye. But if you look at the clusters of atoms that make up, let's say, a table, you will see that it is very rough, and in turn, very curved at the molecular level. Riemannian geometry is has been the "geometry of spacetime." Riemannian geometry is for curvature (spacetime is curved). Einstein used Riemmanian geometry as his mathematical tool for relativity.

 

[whybother]

Yeah well, Newtonian gravity is highly inaccurate compared to Einsteinian relativity, just like Euclidean geometry is highly inaccurate compared to Riemannian geometry. Euclidean geometry deals with the geometry of planes, which are flat. Absolutely nothing is flat in this world; some things may look flat to the naked eye. But if you look at the clusters of atoms that make up, let's say, a table, you will see that it is very rough, and in turn, very curved at the molecular level. Riemannian geometry is has been the "geometry of spacetime." Riemannian geometry is for curvature (spacetime is curved). Einstein used Riemmanian geometry as his mathematical tool for relativity.

I'm sorry, but are you kidding? "Euclidean geometry is highly inaccurate compared to Riemannian geometry"? No, not inaccurate... just used for different purposes. Things being 'curved' at the molecular level doesn't mean Euclidean geometry is invalid or doesn't have it's purpose - especially when most of the time you are taking about 2-curvature embedded in a flat 3D Euclidean space...

 

[dendros]

Hello, I am passionate about physics and I'd like some clarification: is it true that in the General Theory of Relativity the speed of light is no longer a constant (in magnitude)?

 

[A.T.]

Hello, I am passionate about physics and I'd like some clarification: is it true that in the General Theory of Relativity the speed of light is no longer a constant (in magnitude)?

The speed of light in vacuum is locally still constant.

Actually it is only if you assume a flat space-time, you will come to the conclusion that light is slowed down by masses. But the curvature of space-time stated by GR prevents exactly that. It makes sure that locally it is still constant.

 

[dendros]

The speed of light in vacuum is locally still constant.

Actually it is only if you assume a flat space-time, you will come to the conclusion that light is slowed down by masses. But the curvature of space-time stated by GR prevents exactly that. It makes sure that locally it is still constant.

Hello. Thank you for your answer. If the speed of light is locally constant, how about globally: also constant (same magnitude as locally), or variable? That's what I'd like to know, because from Einstein's book "On the Special and General Theory of Relativity (A Popular Account)" I understood that the speed of light was variable (different from the local magnitude) globally.

 

[Dale]

The coordinate speed of light is not constant, even in flat spacetime in non-inertial reference frames. Coordinate speeds are relative to a specific coordinate system.

However, what is coordinate independent is the light cone. In other words, if you have a flash of light at some event, all coordinate systems will agree on the set of events that receive that flash. Geometrically that is the set of all events with a spacetime interval of 0 from the flash.

In local inertial reference frames the two statements "the speed of light is constant" and "all frames agree on the light cone" are equivalent. In other frames the latter is still true although the former is not. So the frame invariance of the light cone is more fundamental in that sense.

 

[DrGreg]

Hello. Thank you for your answer. If the speed of light is locally constant, how about globally: also constant (same magnitude as locally), or variable? That's what I'd like to know, because from Einstein's book "On the Special and General Theory of Relativity (A Popular Account)" I understood that the speed of light was variable (different from the local magnitude) globally.

To expand on what DaleSpam said, speed is measured relative to a frame of reference. The speed of light is constant when measured in an inertial reference frame, but not necessarily in other frames. An inertial frame is one relative to which free-falling objects move at constant velocity.

In special relativity, inertial frames fill the whole of spacetime, so the speed of light, measured in any inertial frame, is constant everywhere. (But not so in non-inertial frames.)

In general relativity, there are no such things as global inertial frames, but there are "local inertial frames", i.e. frames that are good approximations to inertial frames within a small enough region around the free-falling observer. In these frames the speed of light always takes the same value at the location of the observer. But when you try to extend such a frame over a larger area, it becomes non-inertial; gravitational tidal effects cause free-falling objects (that are not near the observer) to accelerate relative to the frame, and the speed of light may vary.

So a free-falling observer always measures the same value locally for the speed of light, but not at a distance from the observer.

 

[dendros]

Thanks for the explanation. I want to know if the speed of light in GR may be outdated, if it is constant?

 

[DrGreg]

Thanks for the explanation. I want to know if the speed of light in GR may be outdated, if it is constant?

Sorry, I've no idea what you mean by "outdated" in this context. Can you ask your question with different words?

 

[George Jones]

In general relativity, there are no such things as global inertial frames, but there are "local inertial frames", i.e. frames that are good approximations to inertial frames within a small enough region around the free-falling observer. In these frames the speed of light always takes the same value at the location of the observer.

And in local non-inertial frames, too!

So a free-falling observer always measures the same value locally for the speed of light, but not at a distance from the observer.

All observer's, relative to their own local frame measure the same value for the speed of light.

For example, non-inertial observers that hover above a Schwarzschild black hole all measure c to be the local speed of light relative to their frames.

 

[dendros]

Sorry, I've no idea what you mean by "outdated" in this context. Can you ask your question with different words?

Sorry. Not speaking English, are from Romania, I used Google translator to post. I wanted to know if is possible in GR speeds greater than light speed.

 

[Dale]

I wanted to know if is possible in GR speeds greater than light speed.

No, it is not possible to travel faster than light (spacelike worldline) at any point in GR. However, it is theoretically possible to take a shorter path through spacetime and arrive before light that took the longer path.

 

[dendros]

Oh, I understand. From a theoretical point of view, you can create a shortcut through space-time? I understand that in GR space is equivalent to the gravitational field.

 

[D H]

Yeah well, Newtonian gravity is highly inaccurate compared to Einsteinian relativity

Where? In the vicinity of a black hole? General relativity predicts a paltry 43 arcseconds/century anomalistic precession that Newtonian gravity doesn't explain. This is fairly tiny compared to Mercury's mean motion of 538,091,702 arcseconds/century. The impact of relativity on planets other than Mercury is smaller yet. Other than GPS (relativity is very important when it comes to GPS), spacefaring nations use good old Newtonian gravity to model the motions of their spacecraft for the simple reason that Newtonian gravity is just as accurate as a full-blown relativistic model. The errors from using Newtonian gravity is orders of magnitude smaller than the errors induced by imperfect sensors and effectors.

 

[dendros]

On the forum of Romania have submitted an idea that I do not know if original or not.
In this idea, I imagine the space as equivalent to all the many fields in the Universe (not just with the gravity field, as in GR).
This equivalence understand the meaning that each field is a component, an aspect of space, and many these issues is what we call space, in the most general sense.
And so I said it could be possible speeds grater than c, if there are fields in which the energy (and implied mass) could transmit (move) with higher speeds than the speed of light in vacuum.
Maybe the idea is just silly, but present here, to clarify definitively whether so or not.

 

[George Jones]

On the forum of Romania have submitted an idea that I do not know if original or not.
In this idea, I imagine the space as equivalent to all the many fields in the Universe (not just with the gravity field, as in GR).
This equivalence understand the meaning that each field is a component, an aspect of space, and many these issues is what we call space, in the most general sense.
And so I said it could be possible speeds grater than c, if there are fields in which the energy (and implied mass) could transmit (move) with higher speeds than the speed of light in vacuum.
Maybe the idea is just silly, but present here, to clarify definitively whether so or not.

As they stand, your ideas look to fuzzy to be meaningful. Also, Physics Forums is meant for the discussion of mainstream physics ideas. From the Physics Forums rules,

Overly Speculative Posts:
One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion.

 

[dendros]

As they stand, your ideas look to fuzzy to be meaningful. Also, Physics Forums is meant for the discussion of mainstream physics ideas. From the Physics Forums rules,

Sorry, I read your forum rules. Now understand that this is a forum for study, not ideas. Sorry to bother you, I think I will quit the account here.

 

[dendros]

Hello. I hope not bother you if you post again. I apologize again because I violated the rules of the forum. From now on, I keep ideas for me and I will refer only to GR. I read his book again Einstein, cited above, and we reached some conclusions about which I'm not sure it is correct. From what I read, that in a local reference system placed in a variable gravity field are non-inertial effects. Ie the reference system placed in the gravitational field continuously-variable physical sizes, such as the mass (energy by default), geometric dimensions, speeds (including speed of light) are also continuous-variable. I understood correctly? If so, why they say that the speed of light is a limit, since it is continuously-variable?