Any physics majors here? Discussion on general relativity.

[ChapelHillSooner]

First, I am having deju vu a bit so if I already discussed this recently then, well, I am getting older so forgive me. (In seriousness I asked this on reddit recently so not sure if I also asked it here.)

My question isn't about general relativity itself but about the motivation to come up with it. My understanding is that one motivation is that Einstein wanted to bring gravity in line with special relativity. I'm getting way over my skis but one problem is that changes in gravitational fields would be immediately apparent everywhere. Ok, so, I get that.

Where I have a problem is when people claim that a motivating factor is that we always feel an acceleration (or instruments can detect it) but we don't feel acceleration caused by gravity when in orbit or free fall.

But looking back at Newtonian physics, we understand that every single particle is accelerated at the same rate. If every particle in my body is experiencing the exact same acceleration, then it seems obvious that we could not feel that acceleration. It is clearly a different situation than being accelerated in a car where the seat is pushing only on the surface of your back and those forces/accelerations have to work their way through your body.

The analogy would be if I were using a spring to detect acceleration, if the force is applied to one end of the spring then that would be noticeable but if it were applied to every single particle within the spring (and the acceleration is identical for every particle) then the spring would not be compressed.

So my argument is that from a physicist in the 1800's, I see no contradiction in saying that I can't feel/detect gravity.

Now, the question is why does gravity accelerate every particle at the same rate. That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity. Put those together and mass/gravity is somewhat unique.

One could ask, why does mass play this dual role and why does it have this dual role in just the perfect way to make it impossible to detect whether or not you are being accelerated by gravity? And that is a valid question. But to me, that question is the starting point, not that we must always "feel" acceleration.

I admit that that may be a distinction without a difference but it actually made it harder for me (as someone who never studied general relativity) to understand it.

 

[Bobfripp]

But looking back at Newtonian physics, we understand that every single particle is accelerated at the same rate.

I have a BS in Physics, and still keep up with it, although I work in neuroimaging these days. Just a quick comment. Particles on or near the Earth only accelerate at the same rate (9.81 meters per second squared at sea level) if they are at the same distance from Earth's center of mass. A person on the top of Mt. Everest actually experiences a tiny bit less acceleration than someone at sea level.

As far as 'feeling' gravity, if you jump out of plane when skydiving you feel the force of air/wind hitting you, which is usually hurricane force (120 mph). If the Earth had no atmosphere, but was otherwise the same size and mass, you'd feel absolutely nothing, until you hit the surface .

 

[nycfan]

I’m always interested to read about it in terms that are accessible to people who don’t speak the math.

I’ll just follow along quietly in the back.

 

[superrific]

First, I am having deju vu a bit so if I already discussed this recently then, well, I am getting older so forgive me. (In seriousness I asked this on reddit recently so not sure if I also asked it here.)

My question isn't about general relativity itself but about the motivation to come up with it. My understanding is that one motivation is that Einstein wanted to bring gravity in line with special relativity. I'm getting way over my skis but one problem is that changes in gravitational fields would be immediately apparent everywhere. Ok, so, I get that.

Where I have a problem is when people claim that a motivating factor is that we always feel an acceleration (or instruments can detect it) but we don't feel acceleration caused by gravity when in orbit or free fall.

But looking back at Newtonian physics, we understand that every single particle is accelerated at the same rate. If every particle in my body is experiencing the exact same acceleration, then it seems obvious that we could not feel that acceleration. It is clearly a different situation than being accelerated in a car where the seat is pushing only on the surface of your back and those forces/accelerations have to work their way through your body.

The analogy would be if I were using a spring to detect acceleration, if the force is applied to one end of the spring then that would be noticeable but if it were applied to every single particle within the spring (and the acceleration is identical for every particle) then the spring would not be compressed.

So my argument is that from a physicist in the 1800's, I see no contradiction in saying that I can't feel/detect gravity.

Now, the question is why does gravity accelerate every particle at the same rate. That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity. Put those together and mass/gravity is somewhat unique.

One could ask, why does mass play this dual role and why does it have this dual role in just the perfect way to make it impossible to detect whether or not you are being accelerated by gravity? And that is a valid question. But to me, that question is the starting point, not that we must always "feel" acceleration.

I admit that that may be a distinction without a difference but it actually made it harder for me (as someone who never studied general relativity) to understand it.

Whether you feel gravity isn't a question about human biology. A particle in free fall and a particle in a gravitational field cannot be distinguished. It's provable and indeed proven (well, it's the basis for the theory, which is extremely well established).

I've got some bad news for you about mass. It doesn't even exist in the way you think. "Mass" is the magnitude of a particle's interaction with the Higgs field. More specifically, the electroweak force spontaneously breaks symmetry, which roughly means that nucleii have a natural direction, so to speak -- i.e. if you spin the particle around an axis, its physical properties are inconstant. This gives the electroweak force four degrees of freedom, and thus four different solutions: three of them are the W and Z bosons. The fourth is the Higgs Boson, which is the mediating particle of the Higgs field. Photon: EM field -> Higgs Boson: Higgs field. Other particles interactions' are their masses.

Gravity, in general relativity, does not exist as a force. It is actually a way of describing the curvature of the spacetime continuum. When you start accelerating after jumping out of the plane, you can call it a "force" but really what's happening is that spacetime is curved and your "arc" so to speak has you accelerate.

 

[Bobfripp]

Whether you feel gravity isn't a question about human biology.

I disagree. 'Feel' is entirely human biology and human CNS. You 'feel' the air hitting your body as you fall (e.g., skydiving). Take away the air, you don't feel anything.

 

[Bobfripp]

I've got some bad news for you about mass. It doesn't even exist in the way you think. "Mass" is the magnitude of a particle's interaction with the Higgs field. More specifically, the electroweak force spontaneously breaks symmetry, which roughly means that nucleii have a natural direction, so to speak -- i.e. if you spin the particle around an axis, its physical properties are inconstant. This gives the electroweak force four degrees of freedom, and thus four different solutions: three of them are the W and Z bosons. The fourth is the Higgs Boson, which is the mediating particle of the Higgs field. Photon: EM field -> Higgs Boson: Higgs field. Other particles interactions' are their masses.

True, but this is deep dive that doesn't really answer the OP's questions.

 

[stankeylegjones]

My question isn't about general relativity itself but about the motivation to come up with it. My understanding is that one motivation is that Einstein wanted to bring gravity in line with special relativity. I'm getting way over my skis but one problem is that changes in gravitational fields would be immediately apparent everywhere. Ok, so, I get that.

Yes, “action-at-distance” violated the finite speed limit set forth by light traveling, well, at the speed of light.

Where I have a problem is when people claim that a motivating factor is that we always feel an acceleration (or instruments can detect it) but we don't feel acceleration caused by gravity when in orbit or free fall.

But looking back at Newtonian physics, we understand that every single particle is accelerated at the same rate. If every particle in my body is experiencing the exact same acceleration, then it seems obvious that we could not feel that acceleration. It is clearly a different situation than being accelerated in a car where the seat is pushing only on the surface of your back and those forces/accelerations have to work their way through your body.

The analogy would be if I were using a spring to detect acceleration, if the force is applied to one end of the spring then that would be noticeable but if it were applied to every single particle within the spring (and the acceleration is identical for every particle) then the spring would not be compressed.

So my argument is that from a physicist in the 1800's, I see no contradiction in saying that I can't feel/detect gravity.

There is no contradiction with Newtonian mechanics, and a physicist from the 1800’s, or even prior to then, would not gripe. They would say, of course you don’t feel gravity in free-fall.

Now, the question is why does gravity accelerate every particle at the same rate. That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity. Put those together and mass/gravity is somewhat unique.

The reason is that energy (stress, mass, etc.) curves the spacetime around it. Particles follow along geodesics, curved by this curvature of the spacetime. To add on to what you’ve said and to answer a few of your questions, in Newtonian physics you assume that the inertial mass (the m appearing on the RHS of Newton’s second law) is equivalent to the gravitational mass (the m appearing in the formula for the gravitational force) are equivalent. Many folks have measured the two and found them to be identical up to many orders of significant digits, but it is still an assumption. Einstein made it a principle, m_in = m_gr, a starting point per se. What lead him to this was thinking of a worker falling off of a roof during his morning coffee (it was apparently a story in the newspaper he was reading). As the worker falls, he looks over and sees his hammer falling with him. Einstein realized that the worker, being accelerated, would still serve as an inertial frame (defined via Newton’s first law) w.r.t. to the hammer. So, to the worker, it would appear that the hammer is not experiencing any gravitational field. That is, locally, free fall is indistinguishable from inertial motion, while uniform acceleration is locally indistinguishable from being in a constant gravitational field. Now, if you’re not “local” then you would be able to tell because of tidal forces. So, locally, our spacetime is flat (this is special relativity) and we can cover it with a single chart. However, this cannot happen globally due to tidal forces (if it could then there would be no way to distinguish gravity from acceleration). This leads directly to proposing that spacetime is a curved manifold, and particles follow geodesics on this manifold - the force of gravity is their inertial mass as they’re moving through spacetime.

What are your questions? (This is something that I really enjoy talking about, so please ask your questions.)

 

[Turd Ferguson]

 

[stankeylegjones]

Whether you feel gravity isn't a question about human biology. A particle in free fall and a particle in a gravitational field cannot be distinguished. It's provable and indeed proven (well, it's the basis for the theory, which is extremely well established).

I've got some bad news for you about mass. It doesn't even exist in the way you think. "Mass" is the magnitude of a particle's interaction with the Higgs field. More specifically, the electroweak force spontaneously breaks symmetry, which roughly means that nucleii have a natural direction, so to speak -- i.e. if you spin the particle around an axis, its physical properties are inconstant. This gives the electroweak force four degrees of freedom, and thus four different solutions: three of them are the W and Z bosons. The fourth is the Higgs Boson, which is the mediating particle of the Higgs field. Photon: EM field -> Higgs Boson: Higgs field. Other particles interactions' are their masses.

Gravity, in general relativity, does not exist as a force. It is actually a way of describing the curvature of the spacetime continuum. When you start accelerating after jumping out of the plane, you can call it a "force" but really what's happening is that spacetime is curved and your "arc" so to speak has you accelerate.

Your explanation of "mass" is not entirely correct (and includes some errors), but I don't think this thread is the right place for discussing it. Maybe we can create another thread for that...the more threads on theoretical physics the better, imo!

 

[superrific]

Your explanation of "mass" is not entirely correct (and includes some errors), but I don't think this thread is the right place for discussing it. Maybe we can create another thread for that...the more threads on theoretical physics the better, imo!

I was answering the question of "That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity" and why is it like that.

I was also trying to be funny by making it more complicated than necessary, and if there are errors, it's probably because I included a derivation I never use. But anyway.

 

[superrific]

I disagree. 'Feel' is entirely human biology and human CNS. You 'feel' the air hitting your body as you fall (e.g., skydiving). Take away the air, you don't feel anything.

That just depends on what we mean by feel. I mean, particles can't "feel" anything in that sense. I meant that a particle's state would be invariant in free fall or gravity.

 

[Bobfripp]

From google AI

Einstein's idea for general relativity stemmed from his 1907 "happiest thought"—the equivalence principle, which stated that gravity and acceleration are locally indistinguishable. This led him to think gravity isn't a force but a curvature of spacetime, a concept he developed over several years with the help of mathematicians like Marcel Grossmann and using the mathematics of Riemannian geometry, eventually publishing the final theory in 1915.

 

[stankeylegjones]

I was answering the question of "That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity" and why is it like that.

I was also trying to be funny by making it more complicated than necessary, and if there are errors, it's probably because I included a derivation I never use. But anyway.

What derivation are you referring to? I didn’t see any in your post. Did you modify it?

 

[stankeylegjones]

From google AI

Einstein's idea for general relativity stemmed from his 1907 "happiest thought"—the equivalence principle, which stated that gravity and acceleration are locally indistinguishable. This led him to think gravity isn't a force but a curvature of spacetime, a concept he developed over several years with the help of mathematicians like Marcel Grossmann and using the mathematics of Riemannian geometry, eventually publishing the final theory in 1915.

The history of Einstein’s work on general relativity is very fascinating. I encourage everyone to dive into it.

 

[ChileG]

I’m always interested to read about it in terms that are accessible to people who don’t speak the math.

I’ll just follow along quietly in the back.

Same. I find this stuff fascinating, and love YouTube videos that explain this stuff with demonstrations or visual affects that help me understand the concepts.

 

[donbosco]

I used to listen to these sometimes...never really understood much of anything but there was something intriguing about the lecture audios just the same. Feynman was a card.

Relativity is #15.

https://www.feynmanlectures.caltech.edu/

https://www.feynmanlectures.caltech.edu/flptapes.html

 

[ChapelHillSooner]

My question isn't about general relativity itself but about the motivation to come up with it. My understanding is that one motivation is that Einstein wanted to bring gravity in line with special relativity. I'm getting way over my skis but one problem is that changes in gravitational fields would be immediately apparent everywhere. Ok, so, I get that.

Yes, “action-at-distance” violated the finite speed limit set forth by light traveling, well, at the speed of light.

Where I have a problem is when people claim that a motivating factor is that we always feel an acceleration (or instruments can detect it) but we don't feel acceleration caused by gravity when in orbit or free fall.

But looking back at Newtonian physics, we understand that every single particle is accelerated at the same rate. If every particle in my body is experiencing the exact same acceleration, then it seems obvious that we could not feel that acceleration. It is clearly a different situation than being accelerated in a car where the seat is pushing only on the surface of your back and those forces/accelerations have to work their way through your body.

The analogy would be if I were using a spring to detect acceleration, if the force is applied to one end of the spring then that would be noticeable but if it were applied to every single particle within the spring (and the acceleration is identical for every particle) then the spring would not be compressed.

So my argument is that from a physicist in the 1800's, I see no contradiction in saying that I can't feel/detect gravity.

There is no contradiction with Newtonian mechanics, and a physicist from the 1800’s, or even prior to then, would not gripe. They would say, of course you don’t feel gravity in free-fall.

Now, the question is why does gravity accelerate every particle at the same rate. That is because of the dual nature of mass. It both resists changes in momentum and causes the force of gravity. Put those together and mass/gravity is somewhat unique.

The reason is that energy (stress, mass, etc.) curves the spacetime around it. Particles follow along geodesics, curved by this curvature of the spacetime. To add on to what you’ve said and to answer a few of your questions, in Newtonian physics you assume that the inertial mass (the m appearing on the RHS of Newton’s second law) is equivalent to the gravitational mass (the m appearing in the formula for the gravitational force) are equivalent. Many folks have measured the two and found them to be identical up to many orders of significant digits, but it is still an assumption. Einstein made it a principle, m_in = m_gr, a starting point per se. What lead him to this was thinking of a worker falling off of a roof during his morning coffee (it was apparently a story in the newspaper he was reading). As the worker falls, he looks over and sees his hammer falling with him. Einstein realized that the worker, being accelerated, would still serve as an inertial frame (defined via Newton’s first law) w.r.t. to the hammer. So, to the worker, it would appear that the hammer is not experiencing any gravitational field. That is, locally, free fall is indistinguishable from inertial motion, while uniform acceleration is locally indistinguishable from being in a constant gravitational field. Now, if you’re not “local” then you would be able to tell because of tidal forces. So, locally, our spacetime is flat (this is special relativity) and we can cover it with a single chart. However, this cannot happen globally due to tidal forces (if it could then there would be no way to distinguish gravity from acceleration). This leads directly to proposing that spacetime is a curved manifold, and particles follow geodesics on this manifold - the force of gravity is their inertial mass as they’re moving through spacetime.

What are your questions? (This is something that I really enjoy talking about, so please ask your questions.)

First, I understand what everyone is saying about general relativity and how gravity is not a true force and due to mass curving spacetime, when falling in a gravitation field we are in fact an inertial frame. Obviously I only understand this as a layman’s level.

Let me try to simplify my question.

I am a physicist from 1850. I assert, because of the unique nature of of gravitational mass being the same as inertial mass, a closed system in free fall can be treated as if it is an inertial frame of reference. I wouldn’t asset that it is but that it can be treated as such.

That professor’s claim is perfectly valid and consistent with known science at that time. So I think people are wrong when they claim that treating freefall as an inertial frame of reference somehow breaks Newtonian/Galilean physics. It is just a quirk and the root of that quirk is the equality of gravitational mass and inertial mass.

Even if it might not be Einstein’s thought process, I think the first big question is why the quirk which leads to everything else.

But, more importantly, I don’t agree with the proposition that the falling man thought experiment somehow breaks the idea of Galilean frames of reference. It just means that that quirk exists.

 

[ChapelHillSooner]

That just depends on what we mean by feel. I mean, particles can't "feel" anything in that sense. I meant that a particle's state would be invariant in free fall or gravity.

I was using feel both to describe human perception but also the ability to detect the acceleration via some experimentation.

I consider former as just a subset of the latter.

But, apologies if I wasn’t clear.

 

[ChapelHillSooner]

Oh, I actually came back to this thread to delete it as I expected there would probably be no responses to it. (I do that sometimes when I get kind of embarrassed that I posted something that nobody cares about.)

I was surprised at all of the comments.

My guess is that we’re all looking for a distraction from the serious things going on. ;-)

 

[ChapelHillSooner]

Also I am today old when I found out Galileo is older than Newton.

And I even took a history of science course as an elective - though it was up to Newton.