For Questions That Don't Merit Their Own Thread

[K^2]

1 hour ago, Jacke said:

Proper motion

Does look that way. I was expecting something precise based on the fact that everyone's pretty confident that collision will happen, and that parallax methods have garbage precision at intergalactic distances, even for close neighbors, and indeed, best figure I found is 80km/s ± 40km/s from this ref. And with the outside result of 120km/s transverse, -300km/s radial, it's a lot less "straight on collision" than I was picturing, but I guess, it's close enough for gravity to do the rest.

I'm disappointed, though. I was hoping for something like second-order red shift measurements, based on periodic changes as Earth moves around its orbit for some very precise numbers... Measuring proper motion from parallax was exciting two centuries ago.

[JoeSchmuckatelli]

@K^2Thanks for the explanation!

Okay - I think I understand my confusion.  I was trying analogize 'background' to a visual background rather than 'background noise'.  Background noise makes sense - especially given how Penzias and Wilson discovered it. 

 

So the CMB photons are everywhere - and, effectively, always have been. 

 

On to the thing I'm curious about:

Given the vast distances, and that the greater the distance the higher the chances of admittedly rare photon-photon scattering... Is it possible that our current measure of the redshift of distant galaxies might be less due to expansion (and/or rate) and possibly due to photon scattering? 

b/c from what I can tell, the more scattering you have, the redder light appears.  https://www.optics4kids.org/what-is-optics/scattering/why-is-the-sky-blue-why-are-sunsets-red#:~:text=Within the visible range of,red and yellow light remaining.

(note: this mathematically challenged jarhead is not discounting some tenured physisists' conclusions - but rather asking whether photon scatter is accounted for in the explanation of expansion rate / redshift - or a gentle explanation of why it would not, could not contribute to redshift) 

Edited February 26, 2021 by JoeSchmuckatelli

[Jacke]

5 hours ago, K^2 said:

Does look that way. I was expecting something precise based on the fact that everyone's pretty confident that collision will happen, and that parallax methods have garbage precision at intergalactic distances, even for close neighbors, and indeed, best figure I found is 80km/s ± 40km/s from this ref. And with the outside result of 120km/s transverse, -300km/s radial, it's a lot less "straight on collision" than I was picturing, but I guess, it's close enough for gravity to do the rest.

I'm disappointed, though. I was hoping for something like second-order red shift measurements, based on periodic changes as Earth moves around its orbit for some very precise numbers... Measuring proper motion from parallax was exciting two centuries ago.

There is a minuscule relativistic transverse red-shift effect, but I'd think it's impossible to see in practice.

This isn't parallax, which is back-and-forth, but to reduce data for that is challenging at any range, especially as aberration of light has to be accounted for first.  (Fun fact: if "aberration of gravitation" existed, there'd be no stable orbits.  Because it doesn't exist, Newtonian Gravitation had to assume its speed of propagation was instantaneous.  General Relativity solved this issue.)

Anyhoo, proper motion has to be measured against more distant objects that can be safely assumed to be near static.  It can be tricky to confirm this and properly reduce the data, but it has been done in some cases out to the Local Group of Galaxies, as discussed here.

[K^2]

9 hours ago, JoeSchmuckatelli said:

Given the vast distances, and that the greater the distance the higher the chances of admittedly rare photon-photon scattering... Is it possible that our current measure of the redshift of distant galaxies might be less due to expansion (and/or rate) and possibly due to photon scattering?

Scattering can create smooth attenuation in spectrum. To a human eye, yes, this can cause the light color to change, but when we measure the red shift, we aren't looking at spectrum overall. We are looking at spectral lines, which are rather narrow gaps in spectrum due to absorption by various atoms in the star's atmosphere. Since it's a sharp gap, even if the overall spectrum is distorted, the gaps will still be quite identifiable, and their position cannot change due to scattering or absorption in interstellar medium.

[JoeSchmuckatelli]

Your post reminds me of a lecture that I had on black body emissions, God, 30 years ago.

 

Okay - so all the breaks shift together? 

 

That would certainly discount what I was thinking. 

 

 

[Jacke]

9 minutes ago, JoeSchmuckatelli said:

Okay - so all the breaks shift together?

The light at a given frequency is effectively transformed by the expansion of space as if it had a red shift.  I can't at the moment remember how the frequency changes (because I can't recall the exact formulae), but the frequencies of the light in the original rest frame of emission become transformed down in frequency and up in wavelength as the formulae specify.  This means the dark lines (which are the light absorbed by cold elements and compounds), the lack of frequencies, are also transformed.

Edited February 27, 2021 by Jacke

[JoeSchmuckatelli]

Physics is fascinating - partly because the specificity of meanings used defy the common understanding of the use of the same words.  I wonder if it is the same with German.  Are, perhaps, Einstein's ideas better expressed in German?  Certainly, the maths are universal (you have to learn that language to converse in it)... but my Internetting without the language of maths tends to leave me with a remarkably incomplete understanding.

 

Again - thanks for 'splaining.

[ARS]

Does a cartoonish cone-shaped drill actually makes sense or practical to be used to dig through earth? Because as far as I know, no drill in real life looks like that, it's either long, spiralling drill or a massive flat-headed tunneling cylinder with crushing teeth lining up the front part

Edited March 1, 2021 by ARS

[magnemoe]

1 hour ago, ARS said:

Does a cartoonish cone-shaped drill actually makes sense or practical to be used to dig through earth? Because as far as I know, no drill in real life looks like that, it's either long, spiralling drill or a massive flat-headed tunneling cylinder with crushing teeth lining up the front part

The long spiraling part of an drill is to push the material out of the hole, standard for handheld tools but not so much for large diameter wood ones. 

I don't think the cone makes sense, if you drill deep you drill trough rock, rock drills  5-15 cm tend to have an rounded head and they use compressed air to blow the dust out. 
Drilling for oil you drill so deep you don't rotate the entire pipe, you use hydraulic to rotate the head and more so spin 2-4 cutting heads, the hydraulic removes the dust, again the head is rounded but more like an first.
Last the flat full profile rigs for drilling 2 meter and larger tunnels. 
 

[JoeSchmuckatelli]

1 hour ago, magnemoe said:

The long spiraling part of an drill is to push the material out of the hole, standard for handheld tools but not so much for large diameter wood ones. 

I don't think the cone makes sense, if you drill deep you drill trough rock, rock drills  5-15 cm tend to have an rounded head and they use compressed air to blow the dust out. 
Drilling for oil you drill so deep you don't rotate the entire pipe, you use hydraulic to rotate the head and more so spin 2-4 cutting heads, the hydraulic removes the dust, again the head is rounded but more like an first.
Last the flat full profile rigs for drilling 2 meter and larger tunnels. 
 

Deep sea, they pump mud into the drill pipe sleeve to push out the rock and other material they don't want.

[kerbiloid]

"roadheader conical pick"

Spoiler

 

[Guest]

Can anyone help with the following:

There is a wormhole by Kerbin and the other end is by Eeloo. You enter at the Kerbin end and exit by Eeloo.

General consensus seems to be that you would conserve your momentum and therefore be doing Kerbin orbital speeds at Eeloo and would need a massive change in delta-v to orbit without escaping into interstellar space.

However, is there any theory to support the the act of bending spacetime and moving through the result could transfer some of that energy thus resulting in you exiting with the correct orbital velocity and not needing the massive dv change? 

Would the gravitational effect of kerbin also ‘travel through’ and therefore slow your velocity as you moved away from it (at a massive rate, possibly tearing the ship apart?) If half your ship was pointing out of each side of the wormhole could the ship be subjected to different rates of gravitational ‘pull’ from the same body at the same time?

[JoeSchmuckatelli]

On 2/26/2021 at 5:45 PM, K^2 said:

Scattering can create smooth attenuation in spectrum. To a human eye, yes, this can cause the light color to change, but when we measure the red shift, we aren't looking at spectrum overall. We are looking at spectral lines, which are rather narrow gaps in spectrum due to absorption by various atoms in the star's atmosphere. Since it's a sharp gap, even if the overall spectrum is distorted, the gaps will still be quite identifiable, and their position cannot change due to scattering or absorption in interstellar medium.

Follow up on my previous question: 

When we look at the sun at zenith, it's bright yellowwhite.  When we look at the sun at either end of the day we get the oranges and reds. 

 

So if we presume that the spectral lines above are from the Sun at zenith, if we were to take measurements again at sunset /sunrise would the atmospheric scattering result in the blue and green portions of the plot just not printing?  (lower half of the image cut off?) 

Edit - apologies to FruiGoose... Apparently we were typing at the same time. 

 

Answer his first if you can! 

[kerbiloid]

14 minutes ago, FruitGoose said:

thus resulting in you exiting with the correct orbital velocity and not needing the massive dv change?

https://wiki.kerbalspaceprogram.com/wiki/Tutorial:_interplanetary_warp

[Jacke]

7 hours ago, FruitGoose said:

However, is there any theory to support the the act of bending spacetime and moving through the result could transfer some of that energy thus resulting in you exiting with the correct orbital velocity and not needing the massive dv change?

The real world papers that attempt to consider worm-holes have to assume crazy states of matter to maintain them and I don't know if any even touched upon creating them.

Considering how difficult it's been to even approach good theories of quantum gravity and how hard it is for them to recreated our relatively flat spacetime, I'd say that complicated linked spacetimes like worm-holes and the like are limited to speculative mathematical physics.

So if you want to put them in a game, well, simpler is likely better but more from a game stand-point.  Because it's verging on magic.

[DDE]

8 hours ago, FruitGoose said:

However, is there any theory to support the the act of bending spacetime and moving through the result could transfer some of that energy thus resulting in you exiting with the correct orbital velocity and not needing the massive dv change? 

One thing that comes to mind is a fictional hard SF setting from one of the Atomic Rockets people, where shining a big laser down a wormhole did work as a photon rocket for its other end.

Was very necessary to insert it into a target stellar orbit after yeeting it out of the Solar System at relativistic velocities. Being on the home end of the wormhole also meant being affected by velocity-induced time dilation, and thus any trip through the wormhole technically took you into the far future.

[K^2]

9 hours ago, FruitGoose said:

There is a wormhole by Kerbin and the other end is by Eeloo. You enter at the Kerbin end and exit by Eeloo.

General consensus seems to be that you would conserve your momentum and therefore be doing Kerbin orbital speeds at Eeloo and would need a massive change in delta-v to orbit without escaping into interstellar space.

However, is there any theory to support the the act of bending spacetime and moving through the result could transfer some of that energy thus resulting in you exiting with the correct orbital velocity and not needing the massive dv change? 

You can have entrance and exit to the wormhole traveling at literally any local velocity you want. But then you get to plug in the numbers for your wormhole geometry and compute the stress-energies required to maintain that wormhole and for anything that doesn't conserve momentum you are likely to end up very unhappy with the results. The compensation will come from somewhere, and if you don't engineer it with some matter to deflect to compensate for recoil, then the compensation will come from gravity waves, and that means you'll be paying photon drive equivalent energy costs. It's going to be a lot cleaner if you require the ship to accelerate to a very specific speed before going through the wormhole.

9 hours ago, JoeSchmuckatelli said:

So if we presume that the spectral lines above are from the Sun at zenith, if we were to take measurements again at sunset /sunrise would the atmospheric scattering result in the blue and green portions of the plot just not printing?  (lower half of the image cut off?) 

The image is based on Sun's radiation as seen from space, AFAIK. The atmosphere is going to make a complete mess of it even if the Sun is directly overhead. But it's not going to be a sharp cutoff anyways. When you see the Sun near the horizon, there is just a lot less of the blue light getting to you, but you'd still be able to make spectrum measurements based on light that makes it through.

[JoeSchmuckatelli]

5 minutes ago, K^2 said:

The image is based on Sun's radiation as seen from space, AFAIK. The atmosphere is going to make a complete mess of it even if the Sun is directly overhead. But it's not going to be a sharp cutoff anyways. When you see the Sun near the horizon, there is just a lot less of the blue light getting to you, but you'd still be able to make spectrum measurements based on light that makes it through.

I guess what I'm trying to figure out now is what happens / what the difference is between Redshift and scattering.  As I understand it, once redshifted the whole spectra goes towards red, but the breaks (black lines, whatever they are called) maintain the same spacing(?) - which is how they know the source is receding/ expansion is occurring.  

 

If that is correct... what happens when a light source hits dust or an atmosphere that scatters part of the total spectra?  How does that look?

[K^2]

2 hours ago, JoeSchmuckatelli said:

If that is correct... what happens when a light source hits dust or an atmosphere that scatters part of the total spectra?  How does that look?

It might be that the ratio in spacings is preserved in red shift, not the absolute spacing, but same principle.

In scattering, you lose some of the intensity of light, and you might lose more at some wavelengths than others. So the overall shape of the spectrum, the envelope if you will, can actually end up similarly distorted, but the gaps will stay exactly where they were.

Edit: This is a very artificial example. I made up all of the parameters, but it's good as an illustration.

You have intensity on the Y axis and frequency on the X axis. So redder light to the left, bluer to the right, but you can pretty much ignore the actual numbers. The dashed curve is the original spectrum. It's the black body radiation with three gaps.

Red curve simulates red shift. Everything is proportionally shifted to the left, which includes both the overall shape of the curve and all the gaps.

Blue curve simulates scattering. I did compensate for the intensity loss, because you can't be certain about how bright the source is, so it's valuable to compare the light at the same relative intensity, but other than that, this is pretty realistic to what you might get. You can see that  the overall shape is not that different from red shift, and to the eye, if this is in visible range, they'd look close to identical. Even if you were measuring, given various sources of noise, you might not be able to distinguish between the two all that well. But the gaps in spectrum stay exactly where they were with the original. There is almost no light coming in at these frequencies, so scattering doesn't affect it at all.

Edited March 5, 2021 by K^2

[JoeSchmuckatelli]

Is there a reason that most US rockets I see these days are single stacked, while several other nations add boosters attached to the central stack?

[DDE]

1 hour ago, JoeSchmuckatelli said:

Is there a reason that most US rockets I see these days are single stacked, while several other nations add boosters attached to the central stack?

I disagree with your assessment. Until SpaceX gobbled up the launch market, all operational US LVs had boosters.

There are several factors I can list off the top of my head:

• the Soyuz uses boosters because of doubts in the possibility of mid-air ignition, an old consideration that also affected the original Atlas
• late-century US SLVs had access to large solid-propellant boosters that could be used to augment the first stage; liquid-propellant boosters are much more complex by comparison
• a related concept are attempts to produce extra-heavy boosters for special missions instead of new SLVs, hence stacks of common first stages like Angara, Delta IV (where they became the norm), Falcon Heavy, and proposed for Atlas V
• Ariane, Energia and the Shuttle all use a hydorlox first stage for reasons that honestly aren't entirely clear to me, and so need boosters to drag themselves off the pad
• finally, increasing stage diameter to hold more propellant and more powerful engines  is really difficult (tooling requirements) if not impossible (logistics requirements), which lead to, originally, various oddities like Saturn I and Proton, and later, once again, clusters of multiple first stages

Edited March 5, 2021 by DDE

[kerbiloid]

36 minutes ago, DDE said:

the Soyuz uses boosters because of doubts in the possibility of mid-air ignition, an old consideration that also affected to the original Atlas

The engine could hardly lift a double-mass post-V-2 rocket even with four of it, so the whole rocket was made of five such subrockets.

The mid-air ignition problem affected the central core ignition moment - right on start.

***

USA had SRB manufacturers (Thiokol, UA) who wanted to eat, and who got used to manufacture ballistic rockets.

USSR wasn't so much advanced in solid ICBM, so didn't have a need to feed them when no new ICBM are required at the moment.

And the obvious advantage of the liquid ones (apart from better ISP) is that you don't need to manufacture and fuel it in advance.
Aluminium sheets are aluminium sheets, fuel is fuel, feel free to not combine them until the need.

***

Pre-Saturn US rockets were ICBM and IRBM, like Atlas, Titan, and Thor.

So, they were just up to 3 m in diameter, and this limits the payload size and mass with several tons and 3+ meters, and needed assistance for heavier than 3 t payloads.

This brought funny solutions like MOL with its absolutely empty 8 meter long telescope compartment, because the forward end was obstructed by the crew, so the poor telescope had to look out through a 2 meter wide hole and a mirror.
USSR had built a 4 m wide ICBM (UR-500), so similar telescope was just put perpendicularly. The originally designed lateral boosters were cannibalized by the central core in the early draft project.

***

So, basically, USSR had the UR-500, USA didn't.

Currently they just made bigger central cores, so SLS _{is just to feed the miltary manufacturers} looks utilzing the Space Shuttle ancestry.

Other countries make smaller rockets because they don't lift heavy payloads. When they need, they attach boosters.

Edited March 5, 2021 by kerbiloid

[Jacke]

6 hours ago, kerbiloid said:

...SLS....

...is just another symptom of when Boeing acquired McDonnel-Douglas, somehow all the bad features of both corporations, especially McD-D, became prominent in the merged mess that was produced.

[JoeSchmuckatelli]

1 hour ago, Jacke said:

...is just another symptom of when Boeing acquired McDonnel-Douglas, somehow all the bad features of both corporations, especially McD-D, became prominent in the merged mess that was produced.

Sounds like when Allied-Signal sold itself to Honeywell and paid for it as well.

[ARS]

18 hours ago, JoeSchmuckatelli said:

Is there a reason that most US rockets I see these days are single stacked, while several other nations add boosters attached to the central stack?

Because other nations knows how to do asparagus staging