# Gravity and Space

## Recommended Posts

The gravitational effect between two objects of mass seems apparent. Even uncomplicated until you start to think about it in detail, but gravity is said to warp Space which I assume can be attributed to mass. The assumption being that the effect is an attribute of mass.

Generally there is no clear explanation of what Space is. It's this, it's that, know one really knows, yet it expands and warps.

When a photon is affected by gravity we can say that it's velocity gives it just enough mass to be gravitationally effected.

If Space is expanding it could be said that it has velocity. My understanding of term accuracy becomes convoluted at times, but Space is said to be expanding at an increasing rate, so I think velocity is the correct term.

Is this velocity of Space, what allows gravitational warping of Space, much like a photon is effected because it has velocity, and wouldn't this account for most, if not all of the matter that is said to be missing in the universe?

Another thought, is that the photon is simply following a geodesic of curved space but that simply brings us back to why space warps without an explanation.

Edited by jajrussel
##### Share on other sites

Gravity is is a "curvature" of space-time.  But this doesn't mean that anything is physically "bent".  It is a term used to express that the geometry of space-time is non-Euclidean.

Non-Euclidean geometry is often described as being like doing  plane (2 dimensional) geometry on a  3 dimensional curved surface ( like a sphere), But this is just an easy way to visualize it. Non-Euclidean geometry doesn't require an added dimension.

##### Share on other sites

42 minutes ago, Janus said:

Gravity is is a "curvature" of space-time.  But this doesn't mean that anything is physically "bent".  It is a term used to express that the geometry of space-time is non-Euclidean.

Non-Euclidean geometry is often described as being like doing  plane (2 dimensional) geometry on a  3 dimensional curved surface ( like a sphere), But this is just an easy way to visualize it. Non-Euclidean geometry doesn't require an added dimension.

Would it be helpful to say "the spacetime coordinate  system" instead of just "spacetime"?

Would  people be less tempted to interpret it physically  ?

##### Share on other sites

47 minutes ago, Janus said:

Gravity is is a "curvature" of space-time.  But this doesn't mean that anything is physically "bent".  It is a term used to express that the geometry of space-time is non-Euclidean.

Non-Euclidean geometry is often described as being like doing  plane (2 dimensional) geometry on a  3 dimensional curved surface ( like a sphere), But this is just an easy way to visualize it. Non-Euclidean geometry doesn't require an added dimension.

2 minutes ago, geordief said:

Would it be helpful to say "the spacetime coordinate  system" instead of just "spacetime"?

Would  people be less tempted to interpret it physically  ?

I was actually puzzling over the Janus post, and maybe yours helped to clarify, but I'm still puzzle because I thought that relativity was a physical phenomena that stated specifically that the effect is physical. One clock runs slower than another, due to gravity and or acceleration. I could possibly plot a curve, but no I wasn't speaking specific to geometry. I did use the term geodesic, but that was primarily because some like to define the path taken gravitationally between two objects as a geodesic.

If relativity predictions are as correct and as real as it has been pointed out to me time after time, and probably a few dozen times in this forum, then I would assume any physical effect that presents is real, physically.

Personally, I would prefer to believe that gravity is a force and that the dance between two massive objects as they move through space is simply point, and counter point due to that force.

I am not the one who says that relative effects are real. I am the one who is accepting that they are, and if they are then maybe if an object of zero mass gains vertual mass through acceleration then possibly space if it is in fact expanding, might do the same.

I mean no disrespect, but when you say that nothing is physically "bent", I am confused. Of course if Space doesn't warp, or expand, then I have no reason to be confused, but personally 🙂 I'm confused, maybe when you said that nothing is physically "bent" you weren't referring to space warping, and expanding. It's possible I took it out of context 🤔? Could you please clarify.

And I do tend to get wordy. Knowing so I try not to, still it happens. To that extent I apologize.

##### Share on other sites

39 minutes ago, jajrussel said:

I was actually puzzling over the Janus post, and maybe yours helped to clarify, but I'm still puzzle because I thought that relativity was a physical phenomena that stated specifically that the effect is physical. One clock runs slower than another, due to gravity and or acceleration. I could possibly plot a curve, but no I wasn't speaking specific to geometry. I did use the term geodesic, but that was primarily because some like to define the path taken gravitationally between two objects as a geodesic.

If relativity predictions are as correct and as real as it has been pointed out to me time after time, and probably a few dozen times in this forum, then I would assume any physical effect that presents is real, physically.

Personally, I would prefer to believe that gravity is a force and that the dance between two massive objects as they move through space is simply point, and counter point due to that force.

I am not the one who says that relative effects are real. I am the one who is accepting that they are, and if they are then maybe if an object of zero mass gains vertual mass through acceleration then possibly space if it is in fact expanding, might do the same.

I mean no disrespect, but when you say that nothing is physically "bent", I am confused. Of course if Space doesn't warp, or expand, then I have no reason to be confused, but personally 🙂 I'm confused, maybe when you said that nothing is physically "bent" you weren't referring to space warping, and expanding. It's possible I took it out of context 🤔? Could you please clarify.

And I do tend to get wordy. Knowing so I try not to, still it happens. To that extent I apologize.

I would have to defer to others who have greater knowledge than I do on this subject.Hopefully Janus will  reply in due course.

##### Share on other sites

22 hours ago, jajrussel said:

which I assume can be attributed to mass

No, the source of gravity is energy-momentum, which includes many more things other than just mass. For example, and electromagnetic field (in otherwise empty space) would also be a source of gravity, as would be stresses and strains in the interior of a planet (e.g.).

22 hours ago, jajrussel said:

Generally there is no clear explanation of what Space is

It needs to be spacetime, not just space. It is not meaningful, in the context of gravity, to separate space from time, and vice versa.
As to what spacetime is - it is quite simply the set of all events, meaning the set of all spatial locations at all instances in time. It is thus a mathematical model.

22 hours ago, jajrussel said:

When a photon is affected by gravity we can say that it's velocity gives it just enough mass to be gravitationally effected

No. When any test particle - irrespective of whether it has mass or not - is affected by gravity (and only gravity, for simplicity), then that means that its world line in spacetime is a geodesic of that spacetime. It is a purely a geometric phenomenon.

22 hours ago, jajrussel said:

If Space is expanding it could be said that it has velocity.

When space is expanding, that means that the separation between any two points within that space increases over time. However, locally those points remain at rest - you can attribute a relative velocity to these specific points, but not to space (that would be meaningless).

23 hours ago, jajrussel said:

Is this velocity of Space, what allows gravitational warping of Space

Relative motion is not a source of gravity, so it does not 'warp' spacetime.

15 hours ago, jajrussel said:

If relativity predictions are as correct and as real as it has been pointed out to me time after time, and probably a few dozen times in this forum, then I would assume any physical effect that presents is real, physically.

What GR does is model the motion of test particles in the presence of sources of energy-momentum; as such, its predictions are quite physical indeed.

15 hours ago, jajrussel said:

Personally, I would prefer to believe that gravity is a force and that the dance between two massive objects as they move through space is simply point, and counter point due to that force.

This is just what Newtonian gravity does, and such a model works quite well in the low-velocity, weak field domain. However, once you venture further into the strong field regime, the predictions of Newtonian gravity are no longer accurate.
And even in the everyday low energy domain - consider putting an accelerometer into free fall (drop it off a tower etc). It will read exactly zero at all times while it is falling - and zero acceleration means no force is present. And yet, the falling accelerometer is very clearly still affected by gravity. So gravity cannot be a force in the Newtonian sense. There are also deeper, more technical reasons why gravity cannot accurately be modelled by a vector field.

15 hours ago, jajrussel said:

if an object of zero mass gains vertual mass through acceleration

You cannot accelerate a massless test particle.

15 hours ago, jajrussel said:

I mean no disrespect, but when you say that nothing is physically "bent", I am confused.

You have every right to be, because the way GR is generally presented does indeed make it confusing, once you give it more than just a passing glance.
Spacetime is not a mechanical medium, so curvature is not any kind of mechanical 'bending'. As mentioned above, spacetime is simply the set of all events, and the geometry of spacetime can be thought of as how these events are related to one another. If the geometry is flat, then that means the relationship between any pair of neighbouring events will be the same, regardless of where/when in spacetime you are (like on a flat sheet of paper). If spacetime is curved, then this is no longer true - the relationship between a given pair of neighbouring events depends on where that pair of events is located in space and time. That's the meaning of curvature - a change in the relationship between events. It's a geometric property, not a mechanical action.

This is analogous to the longitudinal lines on a globe - at the equator, they are spaced apart by a specific distance, but as you go north (or south), that distance will change, even though these lines are perfectly straight within the surface. That's because the relationship between points on those lines changes depending on where you are, since the surface has intrinsic curvature. Spacetime is the same, just in two more dimensions.

##### Share on other sites

44 minutes ago, Markus Hanke said:

No, the source of gravity is energy-momentum,

Indeed. A bunch of massless particles can in principle generate curvature.

45 minutes ago, Markus Hanke said:

It needs to be spacetime, not just space.

Indeed. The acceleration parameter the OP is talking about refers to time-space relative curvature.

45 minutes ago, Markus Hanke said:

[...] its world line in spacetime is a geodesic of that spacetime. It is a purely a geometric phenomenon.

Indeed. Intrinsic geometry are the key words, as Janus suggested.

48 minutes ago, Markus Hanke said:

When space is expanding, that means that the separation between any two points within that space increases over time.

Indeed. No velocity there, just dilation, measured by expansion parameter.

48 minutes ago, Markus Hanke said:

Relative motion is not a source of gravity, so it does not 'warp' spacetime.

Indeed. Only energy-momentum, as said.

49 minutes ago, Markus Hanke said:

This is just what Newtonian gravity does, and such a model works quite well in the low-velocity, weak field domain. However, once you venture further into the strong field regime, the predictions of Newtonian gravity are no longer accurate.

Indeed. Action at a distance is no longer accepted as accurate.

50 minutes ago, Markus Hanke said:

You cannot accelerate a massless test particle.

Indeed. m=0, v=c. Period.

50 minutes ago, Markus Hanke said:

You have every right to be, because the way GR is generally presented does indeed make it confusing, once you give it more than just a passing glance.

Indeed. It's not an easy theory. A cylinder appears 'bent' to you --as @Janus said-- only because you're looking at it as embedded in a 3-dimensional space. But a cylinder is actually flat.

##### Share on other sites

1 hour ago, Markus Hanke said:

No, the source of gravity is energy-momentum, which includes many more things other than just mass. For example, and electromagnetic field (in otherwise empty space) would also be a source of gravity, as would be stresses and strains in the interior of a planet (e.g.).

It needs to be spacetime, not just space. It is not meaningful, in the context of gravity, to separate space from time, and vice versa.
As to what spacetime is - it is quite simply the set of all events, meaning the set of all spatial locations at all instances in time. It is thus a mathematical model.

No. When any test particle - irrespective of whether it has mass or not - is affected by gravity (and only gravity, for simplicity), then that means that its world line in spacetime is a geodesic of that spacetime. It is a purely a geometric phenomenon.

When space is expanding, that means that the separation between any two points within that space increases over time. However, locally those points remain at rest - you can attribute a relative velocity to these specific points, but not to space (that would be meaningless).

Relative motion is not a source of gravity, so it does not 'warp' spacetime.

What GR does is model the motion of test particles in the presence of sources of energy-momentum; as such, its predictions are quite physical indeed.

This is just what Newtonian gravity does, and such a model works quite well in the low-velocity, weak field domain. However, once you venture further into the strong field regime, the predictions of Newtonian gravity are no longer accurate.
And even in the everyday low energy domain - consider putting an accelerometer into free fall (drop it off a tower etc). It will read exactly zero at all times while it is falling - and zero acceleration means no force is present. And yet, the falling accelerometer is very clearly still affected by gravity. So gravity cannot be a force in the Newtonian sense. There are also deeper, more technical reasons why gravity cannot accurately be modelled by a vector field.

You cannot accelerate a massless test particle.

You have every right to be, because the way GR is generally presented does indeed make it confusing, once you give it more than just a passing glance.
Spacetime is not a mechanical medium, so curvature is not any kind of mechanical 'bending'. As mentioned above, spacetime is simply the set of all events, and the geometry of spacetime can be thought of as how these events are related to one another. If the geometry is flat, then that means the relationship between any pair of neighbouring events will be the same, regardless of where/when in spacetime you are (like on a flat sheet of paper). If spacetime is curved, then this is no longer true - the relationship between a given pair of neighbouring events depends on where that pair of events is located in space and time. That's the meaning of curvature - a change in the relationship between events. It's a geometric property, not a mechanical action.

This is analogous to the longitudinal lines on a globe - at the equator, they are spaced apart by a specific distance, but as you go north (or south), that distance will change, even though these lines are perfectly straight within the surface. That's because the relationship between points on those lines changes depending on where you are, since the surface has intrinsic curvature. Spacetime is the same, just in two more dimensions.

Okay... Thank you. The reason I asked the question is because I read one of those trending articles, well, actually I skimmed through it because I was distracted in the article there was a quote attributed to a person named Penrose. That using my bad paraphrasing went something like, "the universe is expanding, and will continue to expand until the last bit of matter deteriorates."

I thought, well so much for energy conservation. Then I starting trying to figure out where where does all the energy go? My question was what I came up with.

Now I see that joigus has replied so I'll stop and see what he has said, again thank you.

##### Share on other sites

20 hours ago, jajrussel said:

I thought, well so much for energy conservation.

One of the more counterintuitive things about General Relativity is that there is no law of energy conservation for regions of spacetime with non-zero curvature. It is possible to formulate other, more general conservation laws by taking into account curvature itself, but they are not quite the same as what we traditionally understand by 'energy conservation'.

20 hours ago, jajrussel said:

Then I starting trying to figure out where where does all the energy go?

It doesn't actually go anywhere, it just changes form. What Penrose likely means (you didn't link the article) is that the end result of an accelerating expansion is a universe that expands so fast that it eventually 'rips apart' atomic structures, and even composite particles. What you end up with is just vacuum and a thermal bath of particles that is incapable of doing work.

##### Share on other sites

On 11/2/2020 at 6:46 AM, jajrussel said:

When a photon is affected by gravity we can say that it's velocity gives it just enough mass to be gravitationally effected.

Photons have no mass, and even if they did, the effect would not be due to velocity, since all photons travel at c. They have energy and momentum, owing to their frequency (or wavelength).

On 11/2/2020 at 2:33 PM, jajrussel said:

I was actually puzzling over the Janus post, and maybe yours helped to clarify, but I'm still puzzle because I thought that relativity was a physical phenomena that stated specifically that the effect is physical. One clock runs slower than another, due to gravity and or acceleration. I could possibly plot a curve, but no I wasn't speaking specific to geometry. I did use the term geodesic, but that was primarily because some like to define the path taken gravitationally between two objects as a geodesic.

...

when you say that nothing is physically "bent", I am confused.

The effects on our observations is real, but spacetime is a coordinate system. Like latitude and longitude on a globe, the coordinate system is not flat, it is curved, because that's the proper coordinate system to use to describe the very real effects. When you look at them on a flat map (with a projection that lacks distortion), the lines are not straight. But latitude and longitude are not physical objects.

##### Share on other sites

On 11/4/2020 at 6:21 AM, swansont said:

Photons have no mass, and even if they did, the effect would not be due to velocity, since all photons travel at c. They have energy and momentum, owing to their frequency (or wavelength).

Okay. But I guess I’ll have to pay Dr. PhysicsA another visit because I made an assumption  that energy divided by momentum would give a mass value. My other understanding was that a photon would only have zero mass at rest.

On 11/4/2020 at 6:21 AM, swansont said:

The effects on our observations is real, but spacetime is a coordinate system. Like latitude and longitude on a globe, the coordinate system is not flat, it is curved, because that's the proper coordinate system to use to describe the very real effects. When you look at them on a flat map (with a projection that lacks distortion), the lines are not straight. But latitude and longitude are not physical objects.

This part is also confusing. I’ll have to go back and read it again, but I thought that the mention of spacetime being a coordinate system was meant to convince me that the very real effects you mention are not real.

On 11/4/2020 at 3:26 AM, Markus Hanke said:

One of the more counterintuitive things about General Relativity is that there is no law of energy conservation for regions of spacetime with non-zero curvature. It is possible to formulate other, more general conservation laws by taking into account curvature itself, but they are not quite the same as what we traditionally understand by 'energy conservation'.

It doesn't actually go anywhere, it just changes form. What Penrose likely means (you didn't link the article) is that the end result of an accelerating expansion is a universe that expands so fast that it eventually 'rips apart' atomic structures, and even composite particles. What you end up with is just vacuum and a thermal bath of particles that is incapable of doing work.

Markus, I’m not exactly sure how to put my understanding of GR into a few sentences. My understanding is that we live the life. We exist within the coordinate system. Plenty of people understand it better. They worked out laws of energy conservation that that I have been convinced by others work. 🙂 I’m not claiming to completely understand them, but I thought that the genius of Einstein’s general relativity was that there would be no actual need to formulate, other more general conservation laws.

I’ll go out on a limb here and say that my understanding is that even if I see your  clock  as keeping time more slowly than mine that my understanding suggests that if each of us measures the amount of energy that our clock  is using, then should we switch places taking our respective clocks with us.

Any new measure should only vary by the norm that would be present had we not switched places. But, if I again look at your clock from my new perspective it is going to still be running slower than mine, because from my perspective you are moving, and the moving clock always presents as being slower. Hmm, I seem to have switched sides.

So now, maybe I’m getting somewhere?

The effect is real, we have to allow for it. Is this what everyone is saying? Or, is someone going to heap coals to my confusion by saying that the actual amount of energy used by each clock is dependent on its position, and my relationship to each clocks position, without bothering to explain that relativity actually adjusts the measure to the extent that should you join me our measures would be as nearly equal as two clocks in the same frame can be.

And I apologize, I didn’t link the article. I wasn’t actually planning on thinking about it. But sometimes a statement stays with me, or maybe it was my thought of, “Oh well, so much for energy conservation”, linked with a desire to understand where does the energy go as each bit of matter deteriorates? If in fact expansion is linked to matter deterioration as the statement  seemed to suggest.

One thing I am sure of with near absolute certainty is that since I want it so bad, as soon as I Google it, Google is going to play dummer than dumb.

Markus, at the risk of once again putting my ignorance on display. How does an expanding universe expand so fast that it rips atomic structures apart. I’m assuming that if that is a possible reason for the statement I remember being attributed to Penrose. Then there is a way it happens?

I will try to find the article.  Thank you 🙂

found it. https://futurism.com/the-byte/physicist-other-universes-before-big-bang The statement is right after the bolded statement ( Hard Reboot ) in the article.

I have no idea why the print became so small. I tried to select it to increase the font size. Apparently I’m not allowed. I assume that the link will work. Note there seem to be a lot of advertisements in the article. Annoying advertisements that suggests that the original trending version was stripped down, but if you can stand the advertisements it is the same article.

Note - It doesn’t really say much more than I stated to begin with either.

Edited by jajrussel
##### Share on other sites

41 minutes ago, jajrussel said:

Okay. But I guess I’ll have to pay Dr. PhysicsA another visit because I made an assumption  that energy divided by momentum would give a mass value. My other understanding was that a photon would only have zero mass at rest.

The rest mass is the only mass that matters.

E/p doesn’t give you the mass. For massless particles, p = E/c (so E/p = c)

in general E^2 = p^2c^2 + m^2c^4

41 minutes ago, jajrussel said:

This part is also confusing. I’ll have to go back and read it again, but I thought that the mention of spacetime being a coordinate system was meant to convince me that the very real effects you mention are not real.

Coordinate systems are a tool to describe real effects. You need a coordinate system to know if e.g. a collision is expected. A collision is a real effect.

Time dilation is a real effect, too.

##### Share on other sites

1 hour ago, jajrussel said:

Markus, at the risk of once again putting my ignorance on display. How does an expanding universe expand so fast that it rips atomic structures apart. I’m assuming that if that is a possible reason for the statement I remember being attributed to Penrose. Then there is a way it happens?

Expanding phases of the universe don't rip atomic structures apart because the expanding term is very small when relative distance is very small. Even if the expanding phase is a DeSitter phase (exponential expansion) as it is now (in cosmic time). IOW, atoms don't notice universe expansion. Universe expansion is only noticeable at the galactic super-cluster scale. If Penrose said such a thing, well.. he was wrong. I don't think he did.

And I, for one, don't see any ignorance in your question. Ignorants are those who don't know that they don't know, don't ask, or don't want to know.

##### Share on other sites

9 hours ago, jajrussel said:

How does an expanding universe expand so fast that it rips atomic structures apart. I’m assuming that if that is a possible reason for the statement I remember being attributed to Penrose.

Ok, Penrose didn’t actually say this in the article you linked. He said that eventually, all matter will decay - that’s not quite the same.

8 hours ago, joigus said:

Expanding phases of the universe don't rip atomic structures apart because the expanding term is very small when relative distance is very small. Even if the expanding phase is a DeSitter phase (exponential expansion) as it is now (in cosmic time). IOW, atoms don't notice universe expansion. Universe expansion is only noticeable at the galactic super-cluster scale. If Penrose said such a thing, well.. he was wrong. I don't think he did.

I wrote that post before I saw the article, so I only guessed that it was referring to the “Big Rip” conjecture. I’ve never actually looked at this in any detail, but my understanding is that it postulates a changing Hubble constant, in which case the effective scale on which expansion is “felt” decreases over time, until even subatomic scales will be affected. This would happen in a finite time. Or am I getting this wrong?

##### Share on other sites

56 minutes ago, Markus Hanke said:

I wrote that post before I saw the article, so I only guessed that it was referring to the “Big Rip” conjecture. I’ve never actually looked at this in any detail, but my understanding is that it postulates a changing Hubble constant, in which case the effective scale on which expansion is “felt” decreases over time, until even subatomic scales will be affected. This would happen in a finite time. Or am I getting this wrong?

Maybe you're right. I haven't studied the Big Rip conjecture in any detail. Right now I must confess I fail to see how it would affect the subatomic scales. Is that a claim of the theory?

As the Hubble parameter is not really a velocity, but comes from a stretching out of the $$dt^2$$ term over $$da^2$$ term in the metric, for any finite value of,

$H \left( t \right) = \frac{\dot{a}}{a}$

$$\dot{a}$$ must go to zero when $$a$$ goes to zero. But maybe you could have a sizable $$\dot{a}$$ for $$a$$ of subatomic scale...

I do remember Lenny Susskind dismissing the 'Big Rip' idea on similar grounds to what I'm saying here, but he did make some disclaimer at the end. I don't remember.

You probably have a much more accurate picture of this than I do.

PD: My intuition is that the whole set of gauge fields would re-scale with the rest of space-time and the equilibrium would be unaltered, but I could be wrong.

##### Share on other sites

On 11/12/2020 at 9:26 AM, joigus said:

Is that a claim of the theory?

Yes, that’s my understanding. That also seems to be consistent with what is stated on Wiki.

On 11/12/2020 at 9:26 AM, joigus said:

You probably have a much more accurate picture of this than I do.

Not really, this is something I have never looked into in any detail.

##### Share on other sites

There seems to be a bit of a mix-up.
Big Rip Cosmology was mot modelled by R Penrose.
Conformal Cyclic Cosmology was, and it also makes use of particle decay.

"The conformal cyclic cosmology hypothesis requires that all massive particles eventually vanish from existence, including those which become too widely separated from all other particles to annihilate with them. As Penrose points out, proton decay is a possibility contemplated in various speculative extensions of the Standard Model, but it has never been observed. Moreover, all electrons must also decay, or lose their charge and/or mass, and no conventional speculations allow for this. "

Edited by MigL
##### Share on other sites

4 hours ago, MigL said:

There seems to be a bit of a mix-up.
Big Rip Cosmology was not modelled by R Penrose.
Conformal Cyclic Cosmology was, and it also makes use of particle decay.

[...]

Thank you. I wasn't aware of this. It seems that massive particles vanish into nonexistence in Penrose's model, rather than decaying or being ripped apart. I see a problem with particle-antiparticle asymmetry with it though.

## Create an account

Register a new account