Jump to content

For Questions That Don't Merit Their Own Thread


Recommended Posts

19 minutes ago, magnemoe said:

The CMB is the leftover heat of the big bang and is pretty uniformly distributed

I've read that - it's interpreting the answer I'm asking about.  Is it a background that's believed to be behind what we can see?  Like the inside of an expanding shell? 

 

Because a fog that we cannot see through beyond a certain distance could give a similar impression of uniformity. 

Link to post
Share on other sites
35 minutes ago, JoeSchmuckatelli said:

I've read that - it's interpreting the answer I'm asking about.  Is it a background that's believed to be behind what we can see?  Like the inside of an expanding shell? 

 

Because a fog that we cannot see through beyond a certain distance could give a similar impression of uniformity. 

It might help to read this wikipedia article

https://en.wikipedia.org/wiki/Recombination_(cosmology)

 

Link to post
Share on other sites
5 hours ago, JoeSchmuckatelli said:

CMB question:  From what I understand, we have looked in every direction and detected the Cosmic Microwave Background everywhere.  Are the interpretations of this that by measuring the CMB that we are seeing 'the end' of the universe, or is it more like the CMB is a fog, and that's merely how far we can see through it (i.e the distance at which it becomes opaque from our point of view)?

CMB isn't coming from any particular source. It's not like a boundary or a medium we're seeing. It's literally leftover heat from the universe itself. Which, yeah, you can think of as every particle in existence having contributed to it, so it has a certain kind of foggy quality to it, but it also comes from the time that the universe was a lot denser, so right now it passes through without much scattering.

In terms of whether we see all of the universe this way? Well... Everything we can observe, including CMB, appears to have come from what's as close to a single point in space as physically possible. We can't say for sure if that's all there is, or if the universe is actually even bigger. What's even more concerning, if you are the sort of person who is uncomfortable with things staying unknowable, is that there is nothing that says that this one point in space was all there was to the universe. If there was more of the universe, then there is infinitely more universe out there now beyond our ability to detect in any way shape or form. On the positive side, nothing from out there can ever influence us for much the same reason, so whether or not it exists is an entirely a philosophical discussion. And does it even matter at that point?

One thing about CMB, though, is that it's not quite perfectly uniform. Besides some variation that's believed to be purely statistical, one side is ever so slightly warmer than the other. Because we're moving relative to CMB. The fact that you can't select the correct frame of reference is a pretty big deal in physics, but that's theory. In practice, CMB comes as close as possible to the rest-frame of the universe. If you want to measure how fast you are moving with respect to all of the observable cosmos, you just need to measure your velocity with respect to CMB. And we've done so and we have shockingly precise measurements, to within just a few km/s. Our Sun is moving at about 370km/s w.r.t CMB. So all that talk about, "No, Earth isn't moving in a circle around the Sun, but rather a helix as Sun is moving along through the Milky Way," is all kind of silly. We're all whizzing with such speeds relative to CMB that Earth's trajectory might as well be a straight line.

Link to post
Share on other sites

So... Today, I revisit my old collection of games for nostalgia. Among those are Call of Duty Modern Warfare 2 (2009 one, not the 2020 remaster). While playing it, something strikes my curiosity: In 2 consecutive mission (Contingency and Second Sun), a nuclear missile was launched from submarine base in Russia towards US east coast before detonating at high altitude to create EMP effect that cripples Russian troops on US soil. The one that interest me is the missile itself. Judging from what I've seen in-game:

1. Contingency started at August 14th, 2016 at 11:22:34. Assuming a normal playthrough, player would likely reached the submarine base within 10 minutes of playing, adding the obligatory defend objective at the end, at most this will take around 15 minutes, so let's assume the missile is launched at 11:37:34

2. Second Sun takes place at August 14th, 2016 at 18:57:20, (roughly 7 hours 19 minutes and 46 seconds after the missile has launched, let's assume it's 7 hours and 20 minutes). From the viewpoint of sat 1, he's likely in-orbit above southern portion of US east coast. At this point, shortly after the mission start, the missile can be seen coasting over horizon. After a short chatter with mission control, the missile detonates and knock the power across US soil and also destroyed the ISS

Judging from the submarine model in the sub base, it's either Yankee or Delta-class, since other Russian SLBM-capable submarine class are too different than what's seen in-game. There is a developer oversight however, regarding the missile flight path from the submarine at Petropavlosk, Russia to very high altitudes above Washington, D.C. The missile was depicted in mission briefing as flying away from Petropavlosk to northwest (the long path, with a distance of around 26,000 Km), where the correct angle for shortest flight path is northeast (the short path, with a distance around 8600 Km). The missile was later depicted from Sat1's view as approaching Washington D.C. from northeast (long path), where the correct approach angle is from northwest (short path). Now, assuming the technology level of 2016's ballistic missiles (the year the game takes place), does a distance of 26,000 Km from Petropavlosk to Washington DC is possible to cross in 7 hours and 20 minutes with parabolic trajectory? Is it way too fast or way too slow?

Edited by ARS
Link to post
Share on other sites

The whole book is free (as PDF):

https://www.nap.edu/catalog/25977/space-nuclear-propulsion-for-human-mars-exploration

 

Take away from the vid and paper:

Either NTP or NEP for human Mars missions by 2039 will take aggressive development (starting pretty much immediately, particularly for NEP) to accomplish.

I'm unsure why they chose the arbitrary Isp of 900 for NTP, I know the guys at Marshall (Houts, et al) have tested to 2200 K, and were gonna try for 2400 K exit temp, vs the 2700 K the above claims is required. Having a much harder time of it to gain an extra several 10s of seconds Isp might be good in the long term, but if they could do 830 with little more work compared to 900 with an Apollo level effort... 830 is pretty good.

Edited by tater
Link to post
Share on other sites

According to the wiki on Two-Photon Physics, 'In pure vacuum, some weak scattering of light by light exists' 

So what would this look like?  How would it affect our perception of a distant light source? 

 

 

(was surprised to read this - as I thought light generally passed other light unperturbed) 

Link to post
Share on other sites
30 minutes ago, JoeSchmuckatelli said:

According to the wiki on Two-Photon Physics, 'In pure vacuum, some weak scattering of light by light exists' 

So what would this look like?  How would it affect our perception of a distant light source? 

 

 

(was surprised to read this - as I thought light generally passed other light unperturbed) 

Light has non-zero energy, and as such, it has non-zero mass.  Any object with non-zero mass will warp space-time and thus bend light.

I think the key word in that statement is 'weak' as we would expect this scattering theoretically, but I do not think we could measure it...

Edited by Terwin
Link to post
Share on other sites
10 hours ago, ARS said:

does a distance of 26,000 Km from Petropavlosk to Washington DC is possible to cross in 7 hours and 20 minutes with parabolic trajectory?

https://en.wikipedia.org/wiki/Volna

The real SLBM-based launch vehicle (actually, itself but disarmed).

So, you can compare the payload with what you need

A 7 h long trajectory would mean 5 orbital turns (then not parabolic) or nearly GSO capability if launch it up (which obviously isn't possible).

Link to post
Share on other sites
11 hours ago, ARS said:

So... Today, I revisit my old collection of games for nostalgia. Among those are Call of Duty Modern Warfare 2 (2009 one, not the 2020 remaster). While playing it, something strikes my curiosity: In 2 consecutive mission (Contingency and Second Sun), a nuclear missile was launched from submarine base in Russia towards US east coast before detonating at high altitude to create EMP effect that cripples Russian troops on US soil. The one that interest me is the missile itself. Judging from what I've seen in-game:

1. Contingency started at August 14th, 2016 at 11:22:34. Assuming a normal playthrough, player would likely reached the submarine base within 10 minutes of playing, adding the obligatory defend objective at the end, at most this will take around 15 minutes, so let's assume the missile is launched at 11:37:34

2. Second Sun takes place at August 14th, 2016 at 18:57:20, (roughly 7 hours 19 minutes and 46 seconds after the missile has launched, let's assume it's 7 hours and 20 minutes). From the viewpoint of sat 1, he's likely in-orbit above southern portion of US east coast. At this point, shortly after the mission start, the missile can be seen coasting over horizon. After a short chatter with mission control, the missile detonates and knock the power across US soil and also destroyed the ISS

Judging from the submarine model in the sub base, it's either Yankee or Delta-class, since other Russian SLBM-capable submarine class are too different than what's seen in-game. There is a developer oversight however, regarding the missile flight path from the submarine at Petropavlosk, Russia to very high altitudes above Washington, D.C. The missile was depicted in mission briefing as flying away from Petropavlosk to northwest (the long path, with a distance of around 26,000 Km), where the correct angle for shortest flight path is northeast (the short path, with a distance around 8600 Km). The missile was later depicted from Sat1's view as approaching Washington D.C. from northeast (long path), where the correct approach angle is from northwest (short path). Now, assuming the technology level of 2016's ballistic missiles (the year the game takes place), does a distance of 26,000 Km from Petropavlosk to Washington DC is possible to cross in 7 hours and 20 minutes with parabolic trajectory? Is it way too fast or way too slow?

ICBMs have an average flight time of roughly 30 minutes, depending on their exact launch location and target. SLBMs have shorter flight times because generally they are launched from closer to their targets, but since in this case it's an SLBM being launched from Russia to the US, you could probably count on it being roughly the same flight time as an ICBM. (The question that would arise then would be if the SLBM actually has enough range to reach the intended target, since they usually have shorter range than ICBMs.) But in either case, a 7.5 hour flight time for an ICBM or SLBM would be ludicrous. If that were the case you might as well book the warhead a ticket in economy class.

However, an EMP burst is a high altitude explosion, preferably 400-500 km over the surface. So, maybe the warhead was propelled into orbit? (Instant Outer Space Treaty violation there, for anyone keeping score.) Even then, half an orbit would be 45 minutes. Some radically huge parabolic trajectory? Dunno, seems unlikely. When you combine these oversights with the awful recurring trope of "Space is so small that anything that blows up there destroys the ISS," I'd be much more inclined to attribute it to the producers not putting a lot of thought into it.

Link to post
Share on other sites
1 hour ago, TheSaint said:

However, an EMP burst is a high altitude explosion, preferably 400-500 km over the surface. So, maybe the warhead was propelled into orbit? (Instant Outer Space Treaty violation there, for anyone keeping score.) Even then, half an orbit would be 45 minutes.

I think it's technically only an Outer Space Treaty violation if the object actually completes a full orbit, not just that it was put into what would have been orbit if it hadn't been de-orbitted before completing the first orbit.  I think both the U.S. and U.S.S.R. worked on fractional orbit bombardment systems that planned to use just that loophole to at least keep the Outer Space Treaty from preventing the development.

Link to post
Share on other sites
2 minutes ago, JoeSchmuckatelli said:

Remember folks: after a nuclear war, you too can sue!

Message brought to you by Dewey Screwem & Howe, PLLC, Attorneys at Law.

Technically, it's an aggressive war by at least one side, so it breaks the U.N. Charter.

However, in a post-apocolyptic society (or rather lack thereof), you think anyone is going to admit they were a *lawyer* before ?

Link to post
Share on other sites
15 minutes ago, Jacke said:

I think it's technically only an Outer Space Treaty violation if the object actually completes a full orbit, not just that it was put into what would have been orbit if it hadn't been de-orbitted before completing the first orbit.  I think both the U.S. and U.S.S.R. worked on fractional orbit bombardment systems that planned to use just that loophole to at least keep the Outer Space Treaty from preventing the development.

Now that you mention it, I believe you are correct.

5 minutes ago, JoeSchmuckatelli said:

Not after Shakespeare ruined it for all of us!

It is the first thing we need to do.... ;)

Link to post
Share on other sites
54 minutes ago, Jacke said:

I think it's technically only an Outer Space Treaty violation if the object actually completes a full orbit, not just that it was put into what would have been orbit if it hadn't been de-orbitted before completing the first orbit.  I think both the U.S. and U.S.S.R. worked on fractional orbit bombardment systems that planned to use just that loophole to at least keep the Outer Space Treaty from preventing the development.

The term is being thrust back into the limelight with the Russo-Chinese Treaty on Prevention of Placement of [conventional] Weapons in Space.

Link to post
Share on other sites
On 2/24/2021 at 1:06 AM, K^2 said:

we're moving relative to CMB. The fact that you can't select the correct frame of reference is a pretty big deal in physics, but that's theory. In practice, CMB comes as close as possible to the rest-frame of the universe. If you want to measure how fast you are moving with respect to all of the observable cosmos, you just need to measure your velocity with respect to CMB. And we've done so and we have shockingly precise measurements, to within just a few km/s. Our Sun is moving at about 370km/s w.r.t CMB. So all that talk about,

Very interesting... do you have a citation for this?

Link to post
Share on other sites
1 hour ago, Jacke said:

it breaks the U.N. Charter.

As the big five nukes are the UN permanent members, the UN probably may be considered automatically dismissed in this case.

52 minutes ago, DDE said:

the Russo-Chinese Treaty

First Clancy, then Shakespeare, now Russo.
(I even didn't know the latter has some space treaty with China...)

Requesting The 100 in this thread.

Offtopic.

Spoiler

Why do they think Shakespear < shake + spear?
Maybe he is a peaceful shakes + pear ?

 

Edited by kerbiloid
Link to post
Share on other sites
On 2/23/2021 at 7:06 PM, K^2 said:

CMB isn't coming from any particular source. It's not like a boundary or a medium we're seeing. It's literally leftover heat from the universe itself. Which, yeah, you can think of as every particle in existence having contributed to it, so it has a certain kind of foggy quality to it, but it also comes from the time that the universe was a lot denser, so right now it passes through without much scattering.

In terms of whether we see all of the universe this way? Well... Everything we can observe, including CMB, appears to have come from what's as close to a single point in space as physically possible. We can't say for sure if that's all there is, or if the universe is actually even bigger. What's even more concerning, if you are the sort of person who is uncomfortable with things staying unknowable, is that there is nothing that says that this one point in space was all there was to the universe. If there was more of the universe, then there is infinitely more universe out there now beyond our ability to detect in any way shape or form. On the positive side, nothing from out there can ever influence us for much the same reason, so whether or not it exists is an entirely a philosophical discussion. And does it even matter at that point?

One thing about CMB, though, is that it's not quite perfectly uniform. Besides some variation that's believed to be purely statistical, one side is ever so slightly warmer than the other. Because we're moving relative to CMB. The fact that you can't select the correct frame of reference is a pretty big deal in physics, but that's theory. In practice, CMB comes as close as possible to the rest-frame of the universe. If you want to measure how fast you are moving with respect to all of the observable cosmos, you just need to measure your velocity with respect to CMB. And we've done so and we have shockingly precise measurements, to within just a few km/s. Our Sun is moving at about 370km/s w.r.t CMB. So all that talk about, "No, Earth isn't moving in a circle around the Sun, but rather a helix as Sun is moving along through the Milky Way," is all kind of silly. We're all whizzing with such speeds relative to CMB that Earth's trajectory might as well be a straight line.

This info about the CMB being as close to a rest frame as possible is intriguing.  I had seen papers describe the apparent temperature differences - but not described as us moving in relation to it.

 

If we have calculated that the sun, and presumably our galaxy is moving at such high speed relative to the CMB, have we also made measurements of say Andromeda or any other galaxy's speed / direction relative to it?

Also - if it is merely left over heat... does it permeate the visible universe (like a fog or like heat in an oven), or is it properly described as existing beyond the galaxies we can see?

 

...

 

If it does permeate... what impact would photon scattering (mentioned above) have on how distant galaxies appear?

 

**Edit: I just re-read through your response, and I'm not trying to be pedantic asking the same question again... I'm trying to grasp something here, so please humor a duffer!

Edited by JoeSchmuckatelli
Link to post
Share on other sites
10 hours ago, kerbiloid said:

(I even didn't know the latter has some space treaty with China...)

It's not bilateral. It's Russia's and China's proposal for a UN-level treaty. The US finds it distasteful because of the limited scope - it neither makes an effort to resolve high-faluting issues like what constitutes an act of war in space, nor something more mundane like prohibiting ASAT that is not permanently placed in orbit (and in all honesty, the treaty seems to be meant to head off US space-based missile defence efforts while leaving Russia's and China's bustling counter-space programs untouched).

However, I can't even find the US counter-proposal.

Link to post
Share on other sites
2 hours ago, DDE said:

It's not bilateral. It's Russia's and China's proposal for a UN-level treaty. The US finds it distasteful because of the limited scope

This  also involves Burundi and iirc several other countries

https://www.mid.ru/en/foreign_policy/news/-/asset_publisher/cKNonkJE02Bw/content/id/4094131

Also Angola is in game.

https://africanews.space/russia-and-angola-sign-agreement-on-space-research/

US also may join if they wish, why not.

Link to post
Share on other sites
6 hours ago, JoeSchmuckatelli said:

If we have calculated that the sun, and presumably our galaxy is moving at such high speed relative to the CMB, have we also made measurements of say Andromeda or any other galaxy's speed / direction relative to it?

Not directly, but we have measurements of Andromeda's movement relative to us that are fairly precise. It's moving towards us at about 300km/s, and this we can measure very precisely using red shift. Well, actually, it's one of the rare cases where it's actually blue shift, since it's moving towards us. I'm not sure what methods are used to estimate tangential velocity, but while they are definitely less precise in general, it's known that Andromeda is heading for us pretty much straight on. We can use our measurements of our own velocity relative to CMB and use this information to get Andromeda's velocity relative to CMB. Naturally, with cumulative errors.

6 hours ago, JoeSchmuckatelli said:

Also - if it is merely left over heat... does it permeate the visible universe (like a fog or like heat in an oven), or is it properly described as existing beyond the galaxies we can see?

I'm not entirely sure what you're asking. It's just heat photons that were emitted very long time ago, shortly after big bang, and are still "in flight," because universe has been expanding so rapidly. 14by might seem like a very long time, but since universe managed to expand to 90bly in that time, it's clear that some of the light simply has not had the time to cross from one end to another. And, well, most of that light never will make it all the way across now. These are fairly low energy photons with fairly long wavelength, so they pass through atomic hydrogen and helium in open space pretty much without effect. Sure, some of them will get absorbed by matter, but universe is mostly a very empty place.

Link to post
Share on other sites
40 minutes ago, K^2 said:

I'm not sure what methods are used to estimate tangential velocity....

Proper motion across the sky with respect to more distant features, which for the Andromeda Galaxy would be further galaxies.   As usual, a chain of steps to reduce the data from the raw motion on the sky to the best estimate of the actual proper motion with respect to the CoM of the Milky Way Galaxy, I would imagine.

As for what causes the CMB, from the Wikipedia article (3rd of 5 paragraphs is key):

Quote

The cosmic microwave background radiation is an emission of uniform, black body thermal energy coming from all parts of the sky. The radiation is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK,[8] after subtracting out a dipole anisotropy from the Doppler shift of the background radiation. The latter is caused by the peculiar velocity of the Sun relative to the comoving cosmic rest frame as it moves at some 369.82 ± 0.11 km/s towards the constellation Leo (galactic longitude 264.021 ± 0.011, galactic latitude 48.253 ± 0.005).[9] The CMB dipole and aberration at higher multipoles have been measured, consistent with galactic motion.[10]

In the Big Bang model for the formation of the universe, inflationary cosmology predicts that after about 10−37 seconds[11] the nascent universe underwent exponential growth that smoothed out nearly all irregularities. The remaining irregularities were caused by quantum fluctuations in the inflation field that caused the inflation event.[12] Long before the formation of stars and planets, the early universe was smaller, much hotter and, starting 10−6 seconds after the Big Bang, filled with a uniform glow from its white-hot fog of interacting plasma of photons, electrons, and baryons.

As the universe expanded, adiabatic cooling caused the energy density of the plasma to decrease until it became favorable for electrons to combine with protons, forming hydrogen atoms. This recombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old.[13] As photons did not interact with these electrically neutral atoms, the former began to travel freely through space, resulting in the decoupling of matter and radiation.[14]

The color temperature of the ensemble of decoupled photons has continued to diminish ever since; now down to 2.7260±0.0013 K,[4] it will continue to drop as the universe expands. The intensity of the radiation also corresponds to black-body radiation at 2.726 K because red-shifted black-body radiation is just like black-body radiation at a lower temperature. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering. This represents the set of locations in space at which the decoupling event is estimated to have occurred[15] and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background,[16] making up a fraction of roughly 6×10−5 of the total density of the universe.[17]

Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature.[7]

The energy density of the CMB is 0.3825 eV/cm3 (4.0204×10−14 J/m3) which yields about 400 photons/cm3.[18]

 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...