My Zero Gravity Challenge

[AV1611VET]

Does zero gravity exist?

 

[Moral Orel]

No. At least some gravity exists.

 

[d taylor]

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No gravity does not exist. So yes it can be said there is zero gravity

 

[Bradskii]

Why is this a 'challenge'?

 

[SelfSim]

Does zero gravity exist?

Gravity is a fictitious force in the first place .. as you've been advised before. Go look it up for once.

 

[Nithavela]

If by "Zero Gravity" you mean "is there a place where no gravitic forces act on a body", then yes, kinda.

You can have points between two celestial bodies where the pull of the two bodies cancel each other out. Even then, other celestial bodies influence those points with their own gravity, it's just that those gravitiy forces are so weak that one can ignore them.

 

[AV1611VET]

If by "Zero Gravity" you mean "is there a place where no gravitic forces act on a body", then yes, kinda.

You can have points between two celestial bodies where the pull of the two bodies cancel each other out. Even then, other celestial bodies influence those points with their own gravity, it's just that those gravitiy forces are so weak that one can ignore them.

Yup.

As I understand it, scientists now prefer the term "microgravity" over "zero gravity."

 

[Nithavela]

Yup.

As I understand it, scientists now prefer the term "microgravity" over "zero gravity."

It certainly is a more precise word.

 

[Larniavc]

Does zero gravity exist?

I say yes. If two points are not gravitationally bound and there is nothing between them I think it’s far to say there is no gravity there.

 

[Tuur]

I voted "Don't know." Here's why:

Let's imagine a universe like ours, but with only one object. That one object has mass, and bends space time, which means it has gravity, even though there's nothing else to attract.

Now let's imagine a universe with only energy. Yes, I know E=mc^2, which implies energy has mass, but I'm confused. Aren't photons massless? I don't know, so I voted I don't know.

 

[sjastro]

Why is this a 'challenge'?

The challenge behind AV's challenges is to work out why they are challenges in the first place.

 

[AV1611VET]

Let's imagine a universe like ours, but with only one object. That one object has mass, and bends space time, which means it has gravity, even though there's nothing else to attract.

But the equation for gravity has two masses:

Gm₁m₂/r²

 

[AV1611VET]

The challenge behind AV's challenges is to work out why they are challenges in the first place.

Shouldn't be too hard.

Thirteen posts now, and not one vote yet.

 

[Tuur]

But the equation for gravity has two masses:

Gm₁m₂/r²

That's measuring the force of gravity between two objects, isn't it? G is the gravitational constant, m1 and m2 are the masses of the objects, and r2 the distance?

But can't we use the above equation by looking at gravitational acceleration? It's been so long ago that I cheated and did a web search, but since F = Gm1m2/r^2, and F = ma, we can say F/m2 = Gm1/r^2 = a = Gm1/r^2.

What I'm thinking is that we can use the gravitational acceleration of the option as a measure of how much an object bends space time. So, since, a = Gm1/r^2, that means a hypothetical single object in the universe would still distort space time, meaning there's still gravity, just there's nothing for it to affect.

 

[sjastro]

That's measuring the force of gravity between two objects, isn't it? G is the gravitational constant, m1 and m2 are the masses of the objects, and r2 the distance?

But can't we use the above equation by looking at gravitational acceleration? It's been so long ago that I cheated and did a web search, but since F = Gm1m2/r^2, and F = ma, we can say F/m2 = Gm1/r^2 = a = Gm1/r^2.

What I'm thinking is that we can use the gravitational acceleration of the option as a measure of how much an object bends space time. So, since, a = Gm1/r^2, that means a hypothetical single object in the universe would still distort space time, meaning there's still gravity, just there's nothing for it to affect.

As @SelfSim pointed out since gravity is a fictitious force in General Relativity this argument doesn’t work.

Einstein pointed out in the Equivalence Principle, a man in an elevator cannot differentiate if the elevator is stationary in a gravitational field or is being accelerated in empty space in which case gravity is the reaction or fictitious force defined by Newton’s third law.

In order to bring gravity into the picture it needs to be treated as a field rather than a force.
A gravitational field performs work on a particle of unit mass according to the equation;

ϕ is the Newtonian gravitational potential.

The Newtonian gravitational potential ϕ tells us nothing about how space-time curves this is defined by Einstein’s field equations for General relativity.

In a vacuum the field equations are;

In the presence of an external gravitational field the equations are;

Without going into specific details of how the field equations are formulated, the connection between these equations and ϕ is that for weak fields, the field equations for the vacuum and of an external field break down to their Newtonian field equation counterparts.

For the weak field case the gravitational field is so weak space-time can be modelled as being flat.

 

[Tuur]

As a fictitious force, meaning it's due to movement along a curved space time. As a curved space time, a hypothetical universe like ours but with a single object should still cause some curvature. I'm looking at it as the curvature being what we call gravity. Does that curvature exist? I think it does.

Back in the day I never dealt much with relativity (yes, I suppose it does show), but we did tinker with solenoids and equations based on a solenoid of infinite length in order not to deal with end effects. While in general we could disregard end effects, they still existed. I suspect that this also holds true for small objects mass in space time, just as we don't have to take into account relativity when dealing with objects traveling at slow speed. But those effects still exist.

Of course, I'm likely wrong, but that's my take on it.

 

[SelfSim]

As a fictitious force, meaning it's due to movement along a curved space time. As a curved space time, a hypothetical universe like ours but with a single object should still cause some curvature. I'm looking at it as the curvature being what we call gravity. Does that curvature exist? I think it does.

Its model dependent. Which is why the OP question, yet again, fails in framing an OP question in a specific context. It appears that @AV1611VET continually creates these ambiguities solely for his own entertainment purposes. This was highlighted by @sjastro earlier on in this thread with his comment about the real challenge is to discern why @AV1611VET sees any 'challenge' in them at all. His challenge is always about his (deliberately?) not recognising the importance of contexts in discussions. Its very tiresome .. and very childish behaviour, IMO.

Back in the day I never dealt much with relativity (yes, I suppose it does show), but we did tinker with solenoids and equations based on a solenoid of infinite length in order not to deal with end effects.

A classic example of framing a model specifically to highlight pertinent parameter relationships of practical importance when designing solenoids.
Existence models however, serve a different purpose.

 

[Lost4words]

I once experienced zero gravity!

It happened when my owners went around a corner super fast in their car! I floated from the back to the front like a feather! It was very scary which resulted in a big skid mark on the front windscreen!! Ewww

 

[dlamberth]

I voted "Don't Know" because I don't know.

 

[sjastro]

As a fictitious force, meaning it's due to movement along a curved space time.

Not necessarily.
When Einstein came up with the equivalence principle using the man in an elevator thought experiment, the elevator was accelerating in a straight line in flat space.
Gravity in this case is a fictitious force acting on the man in the opposite direction but of equal magnitude to the force exerted by accelerating elevator.

The field equations in the presence of external fields is;

The stress energy tensor T in the above equation if non zero can cause the motion of a particle or photon to curve due to external factors on space-time defined in the stress energy tensor T.

As a curved space time, a hypothetical universe like ours but with a single object should still cause some curvature. I'm looking at it as the curvature being what we call gravity. Does that curvature exist? I think it does.

Back in the day I never dealt much with relativity (yes, I suppose it does show), but we did tinker with solenoids and equations based on a solenoid of infinite length in order not to deal with end effects. While in general we could disregard end effects, they still existed. I suspect that this also holds true for small objects mass in space time, just as we don't have to take into account relativity when dealing with objects traveling at slow speed. But those effects still exist.

Of course, I'm likely wrong, but that's my take on it.

General relativity is called a field theory for good reason as it is the effect of fields on space-time.
The vacuum equation is an example which gives external solutions.

If we included the source of gravity in the equation such as a star or planet then we no longer have a vacuum equation!
This might sound like an odd way to make an argument but if the source was a black hole this causes serious problems with the theory due to the resultant physical and coordinate singularities.
For example if we include the space-time inside the event horizon of a black hole into the equations, the time coordinate behaves like a distance and distance coordinate acts like time.
Since the gravitational field of a black hole extends into the surrounding vacuum of space-time general relativity is modelled on how space-time behaves beyond the event horizon hence the term external solution.

A powerful example of general relativity which Newtonian gravity could not explain was the perihelion advance of Mercury's orbit.
The mathematics only examined the effect of the Sun's gravitational field on the planet not the source of the field itself.
Furthermore Mercury's small mass meant the space-time curvature effects from the planet was negligible and therefore could be ignored.