Starship Asterisk*

Chris Alex wrote: ↑Wed Nov 01, 2023 4:01 pm

Chris Peterson wrote: ↑Tue Sep 12, 2023 3:13 pm

Chris Alex wrote: ↑Tue Sep 12, 2023 2:55 pm


I'm all for spacetime! I only meant replacing the word in that sentence alone, to help the 6 year old in me understand a little more.

You ask: how is it different from other fields like the electric and magnetic? This might help clarify my clumsy question. With those fields, we can say a few things about what's going on, e.g. with electrons and photons etc. But can we say what's going on when a planet pulls on a moon? How does gravity connect the two bodies, by which medium?

No, you can't say what's "really" going on with electric and magnetic fields. You're just pushing the problem back one level. In the end, the only way we have to understand them at the lowest level is with mathematical descriptions involving "fields" created by "interactions" between obscure "particles". It really is no different.

Gravity connects two bodies by virtue of the shape of spacetime. The math is perfectly clear.

Sorry for returning to this (and you have been more than generous with your answers as it is!) but one clarification regarding your "it really is no different" - are you implying that gravity too is at the lowest level a field with obscure particles interacting, in other words are you of the view that gravity must be quantized? Or of the view that GR and the curvature of spacetime is a complete theory of gravity which needs no quantum mechanics?

Chris Peterson wrote: ↑Tue Sep 12, 2023 3:13 pm

Chris Alex wrote: ↑Tue Sep 12, 2023 2:55 pm

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

I'm all for spacetime! I only meant replacing the word in that sentence alone, to help the 6 year old in me understand a little more.

You ask: how is it different from other fields like the electric and magnetic? This might help clarify my clumsy question. With those fields, we can say a few things about what's going on, e.g. with electrons and photons etc. But can we say what's going on when a planet pulls on a moon? How does gravity connect the two bodies, by which medium?

No, you can't say what's "really" going on with electric and magnetic fields. You're just pushing the problem back one level. In the end, the only way we have to understand them at the lowest level is with mathematical descriptions involving "fields" created by "interactions" between obscure "particles". It really is no different.

Gravity connects two bodies by virtue of the shape of spacetime. The math is perfectly clear.

Ann wrote: ↑Wed Sep 13, 2023 1:22 pm

johnnydeep wrote: ↑Wed Sep 13, 2023 12:58 pm

Ann wrote: ↑Wed Sep 13, 2023 3:58 am


Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group Pablo Carlos Budassi.png

The Local Group. Illustration: Pablo Carlos Budassi

Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

---

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

The Virgo Cluster. Image Credit & Copyright: Fernando Pena

---

Ann

Thanks for that impressively detailed diagram of the Local Group. So the Local Group seems to contain only two large galaxies - the MW and Andromeda - one much smaller galaxy - M33 (aka Triangulum) - and 70+ dwarf galaxies, with the MW and Andromeda dominating their respective "lobes" (of the dumbbell shape that Wikipedia describes the LG as looking like). Is the Local Group part of any larger Galaxy Cluster, or is there no grouping between the LG and the much larger Virgo Super Cluster that it apparently is a part of?

I think the Local Group is considered to be an outlier of the Virgo Cluster. Or maybe that should be the Virgo Supercluster.

One of the absolutely closest groups to us is the M81 Group. I don't know if it considered to be associated with the Virgo Supercluster (but yes, it is). Another very nearby galaxy is NGC 5128, or Centaurus A. Not sure if NGC 5128 is considered to be a member of a group, or if it is an isolated galaxy (I think not, though). IC 342 is also very close, and it belongs to the IC 342/Maffei group.

I guess anything that is really close to us is considered to be a part of the Virgo Supercluster!

I'm too lazy to google. But I guess I did google just a little a bit anyway.

Ann

The Local Group and other neighboring galaxy groups
I. D. Karachentsev
Special Astrophysical Observatory of Russian Academy of Sciences, N.Arkhyz, KChR, 369167, Russia
ikar@luna.sao.ru

ABSTRACT
Over the last few years, rapid progress has been made in distance measurements for nearbygalaxies based on the magnitude of the tip of red giant branch stars. Current CCD surveys withHST and large ground- based telescopes bring∼10%-accurate distances for roughly a hundredgalaxies within 5 Mpc. The new data on distances to galaxies situated in (and around) thenearest groups: the Local Group, M81 group, CenA/M83 group, IC342/Maffei group, Sculptorfilament, and Canes Venatici cloud allowed us to determine their total mass from the radius ofthe zero- velocity surface,R0, which separates a group as bound against the homogeneous cosmicexpansion. The values ofR0 for the virialized groups turn out to be close each other, in the rangeof 0.9 – 1.3 Mpc. As a result, the total masses of the groups are close to each other, too, yieldingtotal mass-to-blue luminosity ratios of 10 – 40M⊙/L⊙. The new total mass estimates are 3 –5 times lower than old virial mass estimates of these groups. Because about half of galaxies inthe Local Volume belong to such loose groups, the revision of the amount of dark matter (DM)leads to a low local density of matter, Ωm≃0.04, which is comparable with the global baryonicfraction Ωb, but much lower than the global density of matter, Ωm = 0.27.  

johnnydeep wrote: ↑Wed Sep 13, 2023 12:58 pm

Ann wrote: ↑Wed Sep 13, 2023 3:58 am

johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group Pablo Carlos Budassi.png

The Local Group. Illustration: Pablo Carlos Budassi

Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

---

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

The Virgo Cluster. Image Credit & Copyright: Fernando Pena

---

Ann

Thanks for that impressively detailed diagram of the Local Group. So the Local Group seems to contain only two large galaxies - the MW and Andromeda - one much smaller galaxy - M33 (aka Triangulum) - and 70+ dwarf galaxies, with the MW and Andromeda dominating their respective "lobes" (of the dumbbell shape that Wikipedia describes the LG as looking like). Is the Local Group part of any larger Galaxy Cluster, or is there no grouping between the LG and the much larger Virgo Super Cluster that it apparently is a part of?

Ann wrote: ↑Wed Sep 13, 2023 3:58 am

johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group Pablo Carlos Budassi.png

The Local Group. Illustration: Pablo Carlos Budassi

Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

---

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

The Virgo Cluster. Image Credit & Copyright: Fernando Pena

---

Ann

johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Roy wrote: ↑Tue Sep 12, 2023 5:05 pm There’s a lot of information in the “Frontier Fields” site mentioned. The picture subtends approximately 1 square arcminute; a grain of sand held at arm’s length. How many square arcminutes make up the whole sky globe? I don’t know how to figure this out.

Roy wrote: ↑Tue Sep 12, 2023 5:05 pm There’s a lot of information in the “Frontier Fields” site mentioned. The picture subtends approximately 1 square arcminute; a grain of sand held at arm’s length. How many square arcminutes make up the whole sky globe? I don’t know how to figure this out.

Chris Alex wrote: ↑Tue Sep 12, 2023 2:55 pm

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

I'm all for spacetime! I only meant replacing the word in that sentence alone, to help the 6 year old in me understand a little more.

You ask: how is it different from other fields like the electric and magnetic? This might help clarify my clumsy question. With those fields, we can say a few things about what's going on, e.g. with electrons and photons etc. But can we say what's going on when a planet pulls on a moon? How does gravity connect the two bodies, by which medium?

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

Chris Alex wrote: ↑Tue Sep 12, 2023 1:51 pm

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Spacetime is being distorted, which alters the path light takes. Because that is how gravity works. There is nothing circular here. Gravity is easy to understand if you can understand general relativity.

Thanks for your answer! Yes I get it, spacetime is being distorted. What I don't get is what spacetime is! Metrically I get it, concretely no. It surely is not a "thing" like, say, dark matter, so if we are not to reify it, what's gravity distorting in the reality of things? That is my question. "Gravity distorts spacetime" - can you replace the word "spacetime" with anything else?

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Spacetime is being distorted, which alters the path light takes. Because that is how gravity works. There is nothing circular here. Gravity is easy to understand if you can understand general relativity.

Tszabeau wrote: ↑Tue Sep 12, 2023 12:52 pm

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am I can see four stars with diffraction spikes, so are ALL the other objects in that photo beyond the Milky Way?

If so, well, Gosh!
But another question, why do distant object not raise spikes?
Thanks,
John

The galaxy on the bottom/middle of the frame edge seems to have faint spikes unless… it’s superposed by a star that just gives that impression.

Chris Alex wrote: ↑Tue Sep 12, 2023 12:46 pm There it is again, the very familiar idea of gravity (be it lensing or just gravity in general) as "warped spacetime", which describes what we see, but does it tell us what is actually going on? I remember Feynman saying somewhere that general relativity does not tell us what the "machinery" of gravity is. And I am itching to understand this! How does mass communicate its effect to its surroundings?

When gravity distorts light, we actually mean that it is not light per se that is affected by gravity but its path (if that is a correct way to put it). Same with physical bodies: planets don't have forces that "pull" on their moons, they just distort their path with their mass. Fair enough. What I am itching to understand is: WHAT exactly is being distorted? Please don't say "spacetime", this is going around in circles! Please don't say "this is how gravity works", this is saying: gravity works by virtue of the workings of gravity!

If spacetime is not some bendy elastic fabric that gets bent and curved, if this is just a naive explanatory image, then I ask again: WHAT is bending? Because light's path indeed gets bent, so what is that path "made of" in order to bend?
I'm desperate to understand gravity...

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am I can see four stars with diffraction spikes, so are ALL the other objects in that photo beyond the Milky Way?

If so, well, Gosh!
But another question, why do distant object not raise spikes?
Thanks,
John

Knight of Clear Skies wrote: ↑Tue Sep 12, 2023 11:34 am

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am But another question, why do distant object not raise spikes?

It's because stars are point sources whereas distant galaxies have a measurable angular size, with their light spread out over a larger area.

(In reality any light entering the telescope will diffract but most of the light ends up in a point in the centre, which is why we don't notice diffraction spikes on extended objects or on really dim stars. Some very bright galaxy cores do also show diffraction spikes, especially in JWST images.)

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am But another question, why do distant object not raise spikes?