Szydagis' point 3: Interstellar travel is too hard

[Mauro]

Seriously, all one is saying is that QM was a discontinuous (not incremental) scientific paradigm shift. It's hardly an earth-shaking or controversial statement

But if this is just what you intended, in the context of this thread:

So what?

 

[John J.]

when we posit that progress in Science is plateauing

I liked your post, and to paraphrase the American philosopher Donald Rumsfeld there are probably unknown unknowns.
We might discover something (perhaps many things) that makes interstellar travel easier than it would otherwise appear to be.

I don't think anyone here is claiming that science or technology will plateau, though- just that advances might not be limitless.

But, increased knowledge might find a factor or factors that make interstellar flight even more difficult than many might think it to be at present. The universe is under no obligation to make things easy for us.

One finding which might have implications for long duration crewed spaceflight in the near future might be
spaceflight-associated neuro-ocular syndrome (SANS).

Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth.

Content from External Source

J.S. Lawley, L.G. Petersen et al. (2017), "Effect of gravity and microgravity on intracranial pressure", The Journal of Physiology 595 (6)
https://physoc.onlinelibrary.wiley.com/doi/pdfdirect/10.1113/JP273557

Most of the astronauts onboard the International Space Station (ISS) develop visual impairment and ocular structural changes that are not fully reversible upon return to earth.

Content from External Source

N. Alperin, A.M. Bagci (2018), "Spaceflight-Induced Visual Impairment and Globe Deformations in Astronauts Are Linked to Orbital Cerebrospinal Fluid Volume Increase", Acta neurochirurgica supp. 216 (link here)

Spaceflight associated neuro-ocular syndrome (SANS) is common amongst astronauts on long duration space missions and is associated with signs consistent with elevated cerebrospinal fluid (CSF) pressure. Additionally, CSF pressure has been found to be elevated in a significant proportion of astronauts in whom lumbar puncture was performed after successful mission completion.

Content from External Source

W.H. Morgan, J. Khoo et al. (2023), "Correlation between retinal vein pulse amplitude, estimated intracranial pressure, and postural change", npj Microgravity 9
https://www.nature.com/articles/s41526-023-00269-0

In the past, many terms were used to describe the symptoms of malaise, nausea, vomiting, and vertigo, though longer duration spaceflights have increased the prevalence of overlapping symptoms of headache and visual disturbance. Spaceflight-induced visual pathology is thought to be a manifestation of increased intracranial pressure (ICP) because of its similar presentation to cases of known intracranial hypertension on Earth as well as the documentation of increased ICP by lumbar puncture in symptomatic astronauts upon return to gravity.

Content from External Source

A.P. Michael, K. Marshall-Bowman (2015), "Spaceflight-Induced Intracranial Hypertension", Aerospace medicine and human performance 86 (6),
https://pubmed.ncbi.nlm.nih.gov/26099128/

There is some evidence that effects on vision may persist post-mission, at least in some astronauts.
Spaceflight-associated choroidal folds (wrinkling of the tissues behind the retina) does appear to persist, albeit without detected effects on vision so far.
Anecdotally, there is concern at NASA that intracranial pressure may also remain elevated post-mission, and/or might become more prevalent on missions of longer duration:
At least one of the authors of "Mean intracranial pressure monitoring by a non-invasive audiological technique: a pilot study"
(A. Reid, R.J. Marchbanks et al. 1989, Journal of Neurology, Neurosurgery and Psychiatry 52 (5), https://jnnp.bmj.com/content/52/5/610) continued researching non-invasive ICP monitoring and has worked with NASA in more recent years (2010s+) in an attempt to develop reliable non-invasive ICP monitoring for use in-flight because of this concern.

“SANS represents a critical risk for deep space exploration, where astronauts will experience spaceflight for longer periods of time and cannot easily return to Earth to address a medical emergency,” says HRP chief scientist Jennifer Fogarty.
“The idea of embarking on a long-duration flight to Mars without having made this discovery is mind-boggling,” Barratt adds. “We would have had people taking voyages of up to three years and coming back with all these problems that would have taken us forever to figure out.”

Content from External Source

NASA website article International Space Station Research Keeps an Eye on Vision Changes in Space, Melissa Gaskill, 04 August 2020 https://www.nasa.gov/humans-in-spac...arch-keeps-an-eye-on-vision-changes-in-space/

Should be said, while effects of SANS on the eye are indisputable, there isn't a clear consensus that raised ICP is the cause.

In "Long-duration spaceflight alters estimated intracranial pressure and cerebral blood velocity", 2021,
The Journal of Physiology 599 (4), 2021 https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280318,
Ken-Ichi Iwasaki, Yojiro Ogawa et al. found post-flight ICP lower in 9 out of 11 subjects (only 2 S's demonstrated optic disc oedema; both had temporarily raised ICPs.) This appears to contradict related studies.
The authors acknowledge their technique for estimating ICP was not definitive. Michael and Marshall-Bowman (ibid.) and Morgan, Khoo et al. (ibid.) refer to documented raised cerebrospinal fluid pressures found on lumbar puncture of returned astronauts, a better indicator of ICP.

Alperin and Bagci (ibid.) argue that increased orbital (anatomic, not astronomical) CSF pressure might cause the eye deformations seen in astronauts without a general increase in ICP.
Y. Martin Paez, L.I Mudie and P.S. Subramanian (2020) point out

Two main hypotheses are proposed for the pathophysiology of SANS. The first being elevated intracranial pressure and the second compartmentalization of CSF to the globe. These hypotheses are not mutually exclusive...

Content from External Source

"Spaceflight Associated Neuro-Ocular Syndrome (SANS): A Systematic Review and Future Directions", Eye and Brain (2020)12
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7585261/

Martin Paez, Mudie and Subramanian later refer to the more recent paper
"Visual changes after space flight: is it really caused by increased intracranial tension? A systematic review",
R. Elwy, M.A. Soliman, A.A. Hasanain et al. (2020), Journal of Neurosurgical Sciences 64 (5),
https://pubmed.ncbi.nlm.nih.gov/32347675/, whose authors are more certain:

Conclusions: Exposure to microgravity increases ICP possibly precipitating ocular changes. Whether other factors come into play is the subject of investigation. Further randomized studies and methods of direct ICP measurement during spaceflight are needed.

Content from External Source

I read somewhere- and can't find again- that the ISS has a small number of adjustable pairs of glasses for astronauts whose vision deteriorates. If long duration missions can cause raised ICP, that might be harder to mitigate, particularly if ICP continues to rise over time.
One factor which might help a little is reducing the carbon dioxide content of the atmosphere of crewed spacecraft, high blood carbon dioxide can increase ICP.
On the ISS, atmospheric CO might be 5000 ppm, higher at some times. NASA believes this cause some lethargy and headaches amongst crew (not surprised!)
"In-Flight Carbon Dioxide Exposures and Related Symptoms: Association, Susceptibility, and Operational Implications", a NASA technical publication (NASA/TP–2010– 216126), J. Law, S. Watkins, D. Alexander (2010), found via Internet Archive/ Wayback Machine,
PDF attached.

 

[Scaramanga]

On a more serious note, one argument above seems to be that any arriving interstellar visitors would have to be decelerating energetically and in the modern era we'd have at least passively detected a radiation source approaching and arriving in the solar system. For which I guess the believer response would be that the visitors arrived before modern radio astronomy -- the CMB was identified in 1965 -- and/or used a mechanism that decelerates without any emissions.

Well, yes, again we have the basic conservation laws and don't even need to hypothesise weird and wonderful stuff. Any ship decelerating is going to have to use energy to do so. A starship is going to have to use a huge amount of energy to decelerate from light speed, which would make it highly visible. And that's just travel below light speed. At above light speed the warp energy builds up in front of the craft and all gets released on deceleration.....possibly frying the planet they intended to travel to, like some comedy scene out of Hitchhiker's Guide.

 

[Scaramanga]

Lets see what the next 50, 100, 200 years will bring

I guarantee that even the next 200 million years of advance wont overcome the temporal paradoxes inherent in faster than light travel. No amount of scientific advance will do so. Some things will simply always be impossible.

 

[Kacmarek]

I guarantee that even the next 200 million years of advance wont overcome the temporal paradoxes inherent in faster than light travel. No amount of scientific advance will do so. Some things will simply always be impossible.

Entirely possible thay superluminal speed travel is impossible in principle even in the far future .... though, in principle, you, me or Alan Guth cannot guarantee it

But, as inflation theory has demonstrated, nature can have surprises (how the horizon problem was solved)

 

[Kacmarek]

But if this is just what you intended, in the context of this thread:

So ... one cannot dismiss other discontinuous discoveries and innovations in the (near or far) future in principle
Which some of us seem to be arguing in this thread or at least arguing that interstellar travel presents such serious challenges that not only

- we cannot conceive as of now that it would be ever be solved (quite reasonable logical position)
- it would never be solved since various laws of physics and engineering challenges make it virtually impossible (which seems a stretch)

 

[Kacmarek]

Well, yes, again we have the basic conservation laws and don't even need to hypothesise weird and wonderful stuff. Any ship decelerating is going to have to use energy to do so. A starship is going to have to use a huge amount of energy to decelerate from light speed, which would make it highly visible. And that's just travel below light speed. At above light speed the warp energy builds up in front of the craft and all gets released on deceleration.....possibly frying the planet they intended to travel to, like some comedy scene out of Hitchhiker's Guide.

I liked your post, and to paraphrase the American philosopher Donald Rumsfeld there are probably unknown unknowns.
We might discover something (perhaps many things) that makes interstellar travel easier than it would otherwise appear to be.

I don't think anyone here is claiming that science or technology will plateau, though- just that advances might not be limitless.

But, increased knowledge might find a factor or factors that make interstellar flight even more difficult than many might think it to be at present. The universe is under no obligation to make things easy for us.

One finding which might have implications for long duration crewed spaceflight in the near future might be
spaceflight-associated neuro-ocular syndrome (SANS).

Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth.

Content from External Source

J.S. Lawley, L.G. Petersen et al. (2017), "Effect of gravity and microgravity on intracranial pressure", The Journal of Physiology 595 (6)
https://physoc.onlinelibrary.wiley.com/doi/pdfdirect/10.1113/JP273557

Most of the astronauts onboard the International Space Station (ISS) develop visual impairment and ocular structural changes that are not fully reversible upon return to earth.

Content from External Source

N. Alperin, A.M. Bagci (2018), "Spaceflight-Induced Visual Impairment and Globe Deformations in Astronauts Are Linked to Orbital Cerebrospinal Fluid Volume Increase", Acta neurochirurgica supp. 216 (link here)

Spaceflight associated neuro-ocular syndrome (SANS) is common amongst astronauts on long duration space missions and is associated with signs consistent with elevated cerebrospinal fluid (CSF) pressure. Additionally, CSF pressure has been found to be elevated in a significant proportion of astronauts in whom lumbar puncture was performed after successful mission completion.

Content from External Source

W.H. Morgan, J. Khoo et al. (2023), "Correlation between retinal vein pulse amplitude, estimated intracranial pressure, and postural change", npj Microgravity 9
https://www.nature.com/articles/s41526-023-00269-0

In the past, many terms were used to describe the symptoms of malaise, nausea, vomiting, and vertigo, though longer duration spaceflights have increased the prevalence of overlapping symptoms of headache and visual disturbance. Spaceflight-induced visual pathology is thought to be a manifestation of increased intracranial pressure (ICP) because of its similar presentation to cases of known intracranial hypertension on Earth as well as the documentation of increased ICP by lumbar puncture in symptomatic astronauts upon return to gravity.

Content from External Source

A.P. Michael, K. Marshall-Bowman (2015), "Spaceflight-Induced Intracranial Hypertension", Aerospace medicine and human performance 86 (6),
https://pubmed.ncbi.nlm.nih.gov/26099128/

There is some evidence that effects on vision may persist post-mission, at least in some astronauts.
Spaceflight-associated choroidal folds (wrinkling of the tissues behind the retina) does appear to persist, albeit without detected effects on vision so far.
Anecdotally, there is concern at NASA that intracranial pressure may also remain elevated post-mission, and/or might become more prevalent on missions of longer duration:
At least one of the authors of "Mean intracranial pressure monitoring by a non-invasive audiological technique: a pilot study"
(A. Reid, R.J. Marchbanks et al. 1989, Journal of Neurology, Neurosurgery and Psychiatry 52 (5), https://jnnp.bmj.com/content/52/5/610) continued researching non-invasive ICP monitoring and has worked with NASA in more recent years (2010s+) in an attempt to develop reliable non-invasive ICP monitoring for use in-flight because of this concern.

“SANS represents a critical risk for deep space exploration, where astronauts will experience spaceflight for longer periods of time and cannot easily return to Earth to address a medical emergency,” says HRP chief scientist Jennifer Fogarty.
“The idea of embarking on a long-duration flight to Mars without having made this discovery is mind-boggling,” Barratt adds. “We would have had people taking voyages of up to three years and coming back with all these problems that would have taken us forever to figure out.”

Content from External Source

NASA website article International Space Station Research Keeps an Eye on Vision Changes in Space, Melissa Gaskill, 04 August 2020 https://www.nasa.gov/humans-in-spac...arch-keeps-an-eye-on-vision-changes-in-space/

Should be said, while effects of SANS on the eye are indisputable, there isn't a clear consensus that raised ICP is the cause.

In "Long-duration spaceflight alters estimated intracranial pressure and cerebral blood velocity", 2021,
The Journal of Physiology 599 (4), 2021 https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280318,
Ken-Ichi Iwasaki, Yojiro Ogawa et al. found post-flight ICP lower in 9 out of 11 subjects (only 2 S's demonstrated optic disc oedema; both had temporarily raised ICPs.) This appears to contradict related studies.
The authors acknowledge their technique for estimating ICP was not definitive. Michael and Marshall-Bowman (ibid.) and Morgan, Khoo et al. (ibid.) refer to documented raised cerebrospinal fluid pressures found on lumbar puncture of returned astronauts, a better indicator of ICP.

Alperin and Bagci (ibid.) argue that increased orbital (anatomic, not astronomical) CSF pressure might cause the eye deformations seen in astronauts without a general increase in ICP.
Y. Martin Paez, L.I Mudie and P.S. Subramanian (2020) point out

Two main hypotheses are proposed for the pathophysiology of SANS. The first being elevated intracranial pressure and the second compartmentalization of CSF to the globe. These hypotheses are not mutually exclusive...

Content from External Source

"Spaceflight Associated Neuro-Ocular Syndrome (SANS): A Systematic Review and Future Directions", Eye and Brain (2020)12
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7585261/

Martin Paez, Mudie and Subramanian later refer to the more recent paper
"Visual changes after space flight: is it really caused by increased intracranial tension? A systematic review",
R. Elwy, M.A. Soliman, A.A. Hasanain et al. (2020), Journal of Neurosurgical Sciences 64 (5),
https://pubmed.ncbi.nlm.nih.gov/32347675/, whose authors are more certain:

Conclusions: Exposure to microgravity increases ICP possibly precipitating ocular changes. Whether other factors come into play is the subject of investigation. Further randomized studies and methods of direct ICP measurement during spaceflight are needed.

Content from External Source

I read somewhere- and can't find again- that the ISS has a small number of adjustable pairs of glasses for astronauts whose vision deteriorates. If long duration missions can cause raised ICP, that might be harder to mitigate, particularly if ICP continues to rise over time.
One factor which might help a little is reducing the carbon dioxide content of the atmosphere of crewed spacecraft, high blood carbon dioxide can increase ICP.
On the ISS, atmospheric CO might be 5000 ppm, higher at some times. NASA believes this cause some lethargy and headaches amongst crew (not surprised!)
"In-Flight Carbon Dioxide Exposures and Related Symptoms: Association, Susceptibility, and Operational Implications", a NASA technical publication (NASA/TP–2010– 216126), J. Law, S. Watkins, D. Alexander (2010), found via Internet Archive/ Wayback Machine,
PDF attached.

Extremely detailed citations, seems health effects on human astronauts even in long-term near earth orbit is quite debilitating, interstellar travel would have much worse health implications ....

 

[Mauro]

So ... one cannot dismiss other discontinuous discoveries and innovations in the (near or far) future in principle
Which some of us seem to be arguing in this thread or at least arguing that interstellar travel presents such serious challenges that not only

- we cannot conceive as of now that it would be ever be solved (quite reasonable logical position)
- it would never be solved since various laws of physics and engineering challenges make it virtually impossible (which seems a stretch)

Eh, @Kacmarek, you should really try to hone a bit your logic. Let me help you a bit, if I can.

In your post #156 you argue Quantum Mechanics...

QM was/is not incremental at all, marked a significant departure from the deterministic description of the world till that point and moved physics in a completely different direction. And its experimental predictions have been repeatedly verified experimentally over many many years.
Similar step changes in other branches of physics followed - QED, QCD, modern cosmology, elementary particle physics, chaos and complexity to name a few.

... which are ideas no one here disputes, at all (for sure I don't!).

But then, why did you say that, in the context of this thread, where your position is (if I'm not mistaken) that new scientific discoveries happen to remove previous limits to what we can do, while my position is that it never happened, rather the contrary? It shouldn't have been necessary because I had already talked about QM in post #67...

Quantum theory (one of the examples you proposed), with its fundamental randomness, put a nail in the coffin to the idea one could acquire perfect knowledge (Gödel, and then the discovery of chaos put in more nails).

...nonetheless in post #157 I showed you again that with Quantum Mechanics you choose another bad example for your thesis.


Now I guess you realized you had better not have talked about QM at all, because then you answer in post #159:

Seriously, all one is saying is that QM was a discontinuous (not incremental) scientific paradigm shift. It's hardly an earth-shaking or controversial statement

Ah well, okay, from you answer I get I was wrong to think you posted #156 in support of your thesis, you were instead just adding an assertion for color or for effect, so I feel free to ask (post #161):

But if this is just what you intended, in the context of this thread: so what?

Ah, gosh, but no, I was mistaken again, because immediately later (post #166) you are yet at it:

So ... one cannot dismiss other discontinuous discoveries and innovations in the (near or far) future in principle
Which some of us seem to be arguing in this thread or at least arguing that interstellar travel presents such serious challenges that not only.... etc. etc.

Can't you see how you're constantly banging you head against the same wall, round and round?


I'm not enjoiying any more answering you, @Kacmarek. It's straining to have to repeat again and again the same things in an endless circle when you propose again and again the same rebutted arguments. So I'm sorry, but I'm going to put you on ignore for a little while: nothing personal of course but I just got tired and I need some rest. I hope in the meantime you'll learn a little about how logical reasoning works, so our future forums interaction might become more fruitful and less tiresome (for me, at least). I suggest you to start from Wikipedia: fallacy, and pay special attention to cherry picking, straw man, moving the goalposts and non-sequitur. You look seriously interested in learning (that's the reason I engaged with you in the first place), and learning how to reason is the most important skill of all. I bet you'll thank me in the future, probably years from now, but it took years to me too (decades, actually, and I'm not done yet): I sincerly wish you to do better. Cheers!

 

[FatPhil]

Seriously, all one is saying is that QM was a discontinuous (not incremental) scientific paradigm shift. It's hardly an earth-shaking or controversial statement

As it seems your comment was directed at one of my earlier comments, let me be the first to say that your view is certainly supportable. In the 2400+-ish years that are relevant (I'm starting at Aristotle and Epicurus explicitly pondering about the nature of cause and effect, and the implications of a presence or an absence of causality), comparing the viewpoint of the mid-1800s to the mid-1900s certainly *looks like* a paradigm shift.

However, if you zoom into the 1850-1960 area, it certainly looks more continuous. The photoelectric effect was certainly known to be experimentally true, but inexplicable, what would later be called quantisation of energy was noted, but remained a mystery. Planck solved the ultraviolet catastrophe by introducing "bundles" of energy which were quickly renamed to "quanta". The great divine-dice denier himself ran with that, applied it to the photoelectric effect and won a nobel prize. However, this contradicted Maxwell's laws - a crack had appeared in classical theory. A "leap" had already been performed. However, the "aha!" moment that introduced apparent non-determinism was still a *long* way off, we're only two steps of the way to a fully-formed theory, but those to steps are definitely the introduction of the quantum into widely accepted science.

So I'd argue that the science (the process of discovery) of QM was incremental. It's just that implications do seem to require a somewhat binary "go quantum and believe the wierdnesses or reject it" choice. However, because of our very shallow understandings of the "why" of quantum mechanics, even that's no longer a compulsory yes/no choice. Interpretations exist that put determinism back in, such as pilot wave theory, many worlds (from the perspective of one world, but that's the perspective we have), and super-determinism. Feynman would just brush it off as not even bothering to make any choices, just do the maths, get the result, stop worrying about what the implcations are (or "shut up and calculate").

 

[FatPhil]

So ... one cannot dismiss other discontinuous discoveries and innovations in the (near or far) future in principle
Which some of us seem to be arguing in this thread or at least arguing that interstellar travel presents such serious challenges that not only

- we cannot conceive as of now that it would be ever be solved (quite reasonable logical position)
- it would never be solved since various laws of physics and engineering challenges make it virtually impossible (which seems a stretch)

I refrain from dismissing the existence of future paradigm-shifting advancements in science in principle, certainly, but I request that you refrain from assuming them. All such assumptions are sci-fi at this point. The description of the implications of an undiscovered scientific theory carries no more weight to me than the description of the properties of a deity.

I'm glad you used the word "stretch" though; I don't mind my perspective appearing to you as "a stretch" - you've left it within the realms of possibility rather than excluding it absolutely.

 

[Kacmarek]

As it seems your comment was directed at one of my earlier comments, let me be the first to say that your view is certainly supportable. In the 2400+-ish years that are relevant (I'm starting at Aristotle and Epicurus explicitly pondering about the nature of cause and effect, and the implications of a presence or an absence of causality), comparing the viewpoint of the mid-1800s to the mid-1900s certainly *looks like* a paradigm shift.

However, if you zoom into the 1850-1960 area, it certainly looks more continuous. The photoelectric effect was certainly known to be experimentally true, but inexplicable, what would later be called quantisation of energy was noted, but remained a mystery. Planck solved the ultraviolet catastrophe by introducing "bundles" of energy which were quickly renamed to "quanta". The great divine-dice denier himself ran with that, applied it to the photoelectric effect and won a nobel prize. However, this contradicted Maxwell's laws - a crack had appeared in classical theory. A "leap" had already been performed. However, the "aha!" moment that introduced apparent non-determinism was still a *long* way off, we're only two steps of the way to a fully-formed theory, but those to steps are definitely the introduction of the quantum into widely accepted science.

So I'd argue that the science (the process of discovery) of QM was incremental. It's just that implications do seem to require a somewhat binary "go quantum and believe the wierdnesses or reject it" choice. However, because of our very shallow understandings of the "why" of quantum mechanics, even that's no longer a compulsory yes/no choice. Interpretations exist that put determinism back in, such as pilot wave theory, many worlds (from the perspective of one world, but that's the perspective we have), and super-determinism. Feynman would just brush it off as not even bothering to make any choices, just do the maths, get the result, stop worrying about what the implcations are (or "shut up and calculate").

Agreed that we do not, at this stage, fully grasp or comprehend what QM is all about, and what is the picture of reality it truly represents.

Also agreed that Planck and Enstein tiptoed into it with quantization of energy.
(still not a grand departure from physics as we know it for say the last 1000 years)

But wave-particle duality, the Heisenberg uncertainty principle and the Schrodinger wave equation (incl the famous Schrodinger's Cat gedankenexperiment) makes it amply clear the QM describes a world (microscopic, for sure, with imperceptible effects in our macroscopic world) which are far from the world of Newton and Maxwell.

Whether the Copenhagen or many-worlds interpretation is correct, both have significant mind-altering implications for reality.

It is a true paradigm shift in my opinion, from a deterministic worldview to a probablistic one (except the earlier statistical formulation of the 2nd law of thermodynamics)

 

[FatPhil]

But wave-particle duality, the Heisenberg uncertainty principle and the Schrodinger wave equation (incl the famous Schrodinger's Cat gedankenexperiment) makes it amply clear the QM describes a world (microscopic, for sure, with imperceptible effects in our macroscopic world) which are far from the world of Newton and Maxwell.

You could still hypothetically have all of those three things without quantum mechanics.

The easiest of the three to show that with is the middle one - a cognate to Heisenberg's uncertainty principle applies to all waves in all media, because, like Noether's Theorem, it's a mathematical law, not a physical one. (And because both are applied in nature to the same ideas of time, space, and "action", you end up with the same pairings - time with energy, and position with momentum.) You can't tell the wavelength of a tsunami from brief footage of a boat rising. There's a chance that's a terrible analogy, but I can't prove to myself it's useless, so is left in so that others can improve on it. Shannon-Nyquist keeps polluting my thought patterns presently, but that's a different theorem (but again related, everything can be mapped onto fourier transforms).

As an aside, back to my earlier argument pro small incremental steps from above, the schroedinger wave equation isn't that much of an evolution from the normal heat equation with potential; the only twist, quite literally, is that the "i" multiplier causes wave-like behavior to drop out in the solutions. Yes, this implies that it technically doesn't have the form of a *wave equation*; someone, de Broglie?, chose the wrong nomenclature about the time they were trying to model matter waves. Removing the word "wave" from its name is tricky, as it's been there too long and seems to have stuck. But if Planck's comment about scientific truths applies to names of things too, the users of the old name will eventually die out.

 

[Kacmarek]

You could still hypothetically have all of those three things without quantum mechanics.

The easiest of the three to show that with is the middle one - a cognate to Heisenberg's uncertainty principle applies to all waves in all media, because, like Noether's Theorem, it's a mathematical law, not a physical one. (And because both are applied in nature to the same ideas of time, space, and "action", you end up with the same pairings - time with energy, and position with momentum.) You can't tell the wavelength of a tsunami from brief footage of a boat rising. There's a chance that's a terrible analogy, but I can't prove to myself it's useless, so is left in so that others can improve on it. Shannon-Nyquist keeps polluting my thought patterns presently, but that's a different theorem (but again related, everything can be mapped onto fourier transforms).

As an aside, back to my earlier argument pro small incremental steps from above, the schroedinger wave equation isn't that much of an evolution from the normal heat equation with potential; the only twist, quite literally, is that the "i" multiplier causes wave-like behavior to drop out in the solutions. Yes, this implies that it technically doesn't have the form of a *wave equation*; someone, de Broglie?, chose the wrong nomenclature about the time they were trying to model matter waves. Removing the word "wave" from its name is tricky, as it's been there too long and seems to have stuck. But if Planck's comment about scientific truths applies to names of things too, the users of the old name will eventually die out.

Yes the Schrodinger wave equation is your basic PDE (heat/fluid dynamics), but the implications of all three things (uncertainty/wave particle/Schroedinger) put together ..... now thats a bit weird from reality perspective.

Some say that John Bell's "Speakable and Unspeakable in Quantum Mechanics" gives a good introduction to many of the key issues of QM vs reality (havent read it myself though)

"Shut up and calculate" as you or someone said sometime back may of course be v useful from the physicist's perspective by ignoring the philosophical issues raised by QM

 

[FatPhil]

Some say that John Bell's "Speakable and Unspeakable in Quantum Mechanics" gives a good introduction to many of the key issues of QM vs reality (havent read it myself though)

Bell proved Einstein wrong. Bell's a hero, Einstein's a stuck-in-the-mud.

What else did you expect me to say. Even the smartest geniuses can be - perhaps - wrong. I sympathise very strongly with Albert, but I don't know enough to make any judgements. So I'm prepared to just go hands off and say "shut up and calculate" even though I know it doesn't answer any "why" questions. I said "prepared", not "happy".

 

[John J.]

Quantum mechanics is obviously a vital cornerstone of modern physics. It is of extraordinary importance.
But in 99 years, it hasn't provided a new form of transport, or enabled any existing mode of transport to move faster or further.
(Skating over claimed "teleportation" of particles at quantum scales here.)

QM doesn't weaken the obstacles that relativity puts in the way of near-c or >c flight.
Nor does QM offer clues to overcoming the 1st and 2nd law of thermodynamics, at least in any way useable on the macro scale.

Maybe a conceptual breakthrough/ paradigm shift on the scale of QM, or greater, will lead to a physics which mitigates or nullifies the above obstacles, which would be cool, but if so current scientists have no idea where that advance will come from.
Such an advance might never be found. It might not exist to be found.

All reliable evidence so far is that >c transport is impossible, not because we aren't smart enough but because it is an inherent and inviolable quality of this universe. This makes me unhappy, and I hope one day humans get to explore, and live on, other planets. But I don't think there's any reason to assume that this will happen .

 

[Robert Sheaffer]

This question was discussed in depth decades ago. Among those calling interstellar travel "preposterous" were Nobel physicist Edward Purcell, radio astronomer Sebastian von Hoerner, and astrophysicist William Markowitz.

Purcell, 1963: “For our vehicle we shall clearly want a propellant with a very high exhaust velocity. Putting all practical questions aside, I propose, in my first design, to use the ideal nuclear fusion propellant… I am going to burn hydrogen to helium with 100 percent efficiency; by means unspecified I shall throw the helium out the back with kinetic energy, as seen from the rocket, equivalent to the entire mass change. You can’t beat that, with fusion. One can easily work out the exhaust velocity; it is about 1/8 the velocity of light. The equation of Figure 13 tells us that to attain a speed 0.99c we need an initial mass which is a little over a billion times the final mass.”

Of course, that leaves us whizzing past our destination at almost the speed of light. There is no way to stop! Unless we accelerate sufficient fuel to stop. So that's a billion times a billion times the payload mass.

Some people ask, why study such old analyses? Maybe something new has come up? Sorry, but the laws of physics have not changed since the 1960s. All this follows from Einstein's relativity.

https://badufos.blogspot.com/2012/12/is-interstellar-travel-preposterous.html

 

[FatPhil]

Purcell, 1963: “For our vehicle we shall clearly want a propellant with a very high exhaust velocity. Putting all practical questions aside, I propose, in my first design, to use the ideal nuclear fusion propellant… I am going to burn hydrogen to helium with 100 percent efficiency; by means unspecified I shall throw the helium out the back with kinetic energy, as seen from the rocket, equivalent to the entire mass change. You can’t beat that, with fusion. One can easily work out the exhaust velocity; it is about 1/8 the velocity of light. The equation of Figure 13 tells us that to attain a speed 0.99c we need an initial mass which is a little over a billion times the final mass.”

Thanks for hunting that out. I Fermi'd the numbers similarly a few years back on another forum. I forget the exact question I was attempting to answer, but it was something like the requirements for getting an to alpha-centauri in a couple of decades, using only known-to-work thrust technologies, rather than the "best possible" assumptions above. The ratio I came out with was 1:10^14, IIRC - one hundred megatons of fuel for each gram of payload: that's seventeen great pyramids of Giza.

 

[Scaramanga]

So ... one cannot dismiss other discontinuous discoveries and innovations in the (near or far) future in principle
Which some of us seem to be arguing in this thread or at least arguing that interstellar travel presents such serious challenges that not only

- we cannot conceive as of now that it would be ever be solved (quite reasonable logical position)
- it would never be solved since various laws of physics and engineering challenges make it virtually impossible (which seems a stretch)

It is a stretch, as some things clearly are impossible. No matter how much one postulates human advancement and discovery in science, some things will always be impossible.

Faster than light travel is quite simply impossible. One can state so with certainty, regardless of potential future science discoveries, for the very simple reason that faster than light travel violates THE most basic principles of causality.

Faster than light travel or even communication violates cause and effect. One could have someone arriving at their destination before they set out. You could have someone go back and shoot Hitler and thus alter the whole of history such that they themselves would never have been born and thus would never exist to go back in time anyway. You cannot get round the temporal paradoxes. A million or even a billion years of human advancement will not get round them.

 

[Eburacum]

I can provide a limited number of examples of 'faster-than-light' travel where causality cannot, in theory, be violated. For instance, it would, in theory, be possible to create a wormhole to a distant location which allows travel to that location at a speed much faster than light; but so long as that location is not inside the light-cone of your present location, no reversal of causality would be possible. As an example, there would be no causality overlap between a wormhole which links our galaxy to one beyond the Hubble Horizon; these two locations would be permanently outside each other's light cones, whether we have a wormhole link or not. So there could be no temporal interaction through normal space, and no possibility of the creation of a CTC (closed timelike curve).

However there is no indication in the real universe that such wormholes are possible in reality, and they almost certainly aren't.

 

[John J.]

If causality were violated, would we necessarily be aware of it?

I'm not aware of any plausible mechanism (whether natural phenomenon or manufactured) whereby causality could be violated, and if no such mechanism is possible, causality will remain inviolate.

But is there anything about causality in itself that means it can't be violated?

(Actual questions, not rhetorical).

 

[Kacmarek]

It is a stretch, as some things clearly are impossible. No matter how much one postulates human advancement and discovery in science, some things will always be impossible.

Faster than light travel is quite simply impossible. One can state so with certainty, regardless of potential future science discoveries, for the very simple reason that faster than light travel violates THE most basic principles of causality.

Faster than light travel or even communication violates cause and effect. One could have someone arriving at their destination before they set out. You could have someone go back and shoot Hitler and thus alter the whole of history such that they themselves would never have been born and thus would never exist to go back in time anyway. You cannot get round the temporal paradoxes. A million or even a billion years of human advancement will not get round them.

Well, for one

1) interstellar travel /=/ (not equal to) FTL travel, other possibilities do exist (including those possibilities which we cannot conceive of right now, with our 120 years approx understanding of Special Theory of Relativity), a mere blink of an eye in modern human existence, not to speak of the age of the universe

Also

2) Relativistic jets emanating from AGN of some galaxies (e.g. AGN of M87) "seem" to be moving at velocities of 6c +
What about that
Obviously that has a explanation (and it a jet of elementary particles and not any artificial propulsion)

 

[FatPhil]

I can provide a limited number of examples of 'faster-than-light' travel where causality cannot, in theory, be violated. For instance, it would, in theory, be possible to create a wormhole to a distant location which allows travel to that location at a speed much faster than light; but so long as that location is not inside the light-cone of your present location, no reversal of causality would be possible. As an example, there would be no causality overlap between a wormhole which links our galaxy to one beyond the Hubble Horizon; these two locations would be permanently outside each other's light cones, whether we have a wormhole link or not. So there could be no temporal interaction through normal space, and no possibility of the creation of a CTC (closed timelike curve).

However there is no indication in the real universe that such wormholes are possible in reality, and they almost certainly aren't.

That does assume H won't decrease.

I don't have any mathematical problems with wormholes, as I like doughnuts. The rule that there must be one and only one shortest route between two places is not a rule, and clearly false in GR. Maybe if a light ray could "cheat" by going through a wormhole to a distant location, then maybe that was actually the correct shortest path to take, and the one that we had naively presumed was the shortest route simply wasn't, it was simply another solution to the equations that we didn't know about because we didn't know about the wormhole. The wormhole's not the shortcut, the boring route is just a pointlessly long way of traversing that same segment of spacetime.

 

[Eburacum]

2) Relativistic jets emanating from AGN of some galaxies (e.g. AGN of M87) "seem" to be moving at velocities of 6c +
What about that
Obviously that has a explanation (and it a jet of elementary particles and not any artificial propulsion)

Yes, it has an explanation, which involves perspective and light travel time. Although the particles 'seem' to be travelling at superluminal speeds, they actually aren't - it is an optical illusion.

Remember we are not seeing the actual particles as they travel, just a different selection of particles as they travel across a distant segment of space, and illuminate different parts of that segment sequentially.

 

[captancourgette]

It is a stretch, as some things clearly are impossible. No matter how much one postulates human advancement and discovery in science, some things will always be impossible.

Whilst I do sorta agree, A couple of things

1. Quantum entanglement, now if info can be transmitted faster than the speed of light could be build something like a star trek teleporter in the distant future? Though wouldn't that just leave you with 2 of the object no matter if they were identical (*)
2. the universe is something like 80 billion light years across even though its 'only' ~12.6 billion years old. OK I believe the reasoning for this is during just after the initial bigbang the normal laws didn't apply but the fact is there was a brief time when things did move faster than the speed of light.

(*)Which gets me thinking of that idea for immortality if you could clone yourself exactly (memories and all) would you 'the original person' be OK with the idea of, now just kill me now as I will just live on in this new body. Hmmm a bit of a philosophical muse

 

[jarlrmai]

1. Quantum entanglement, now if info can be transmitted faster than the speed of light could be build something like a star trek teleporter in the distant future? Though wouldn't that just leave you with 2 of the object no matter if they were identical (*)

Entanglement does not transmit information faster than light, this is a common misunderstanding of the implication of non locality in QM.

https://en.wikipedia.org/wiki/Superluminal_communication
https://en.wikipedia.org/wiki/No-communication_theorem

Quantum mechanics is non-local in the sense that distant systems can be entangled. Entangled states lead to correlations in the results of otherwise random measurements, even when the measurements are made nearly simultaneously and at far distant points. The impossibility of superluminal communication led Einstein, Podolsky, and Rosen to propose that quantum mechanics must be incomplete (see EPR paradox).

However, it is now well understood that quantum entanglement does not allow any influence or information to propagate superluminally.

Practically, any attempt to force one member of an entangled pair of particles into a particular quantum state, breaks the entanglement between the two particles. That is to say, the other member of the entangled pair is completely unaffected by this "forcing" action, and its quantum state remains random; a preferred outcome cannot be encoded into a quantum measurement.[4]

Content from External Source

 

[Kacmarek]

Whilst I do sorta agree, A couple of things

1. Quantum entanglement, now if info can be transmitted faster than the speed of light could be build something like a star trek teleporter in the distant future? Though wouldn't that just leave you with 2 of the object no matter if they were identical (*)
2. the universe is something like 80 billion light years across even though its 'only' ~12.6 billion years old. OK I believe the reasoning for this is during just after the initial bigbang the normal laws didn't apply but the fact is there was a brief time when things did move faster than the speed of light.

(*)Which gets me thinking of that idea for immortality if you could clone yourself exactly (memories and all) would you 'the original person' be OK with the idea of, now just kill me now as I will just live on in this new body. Hmmm a bit of a philosophical muse

The size/extent/diameter of the universe being much larger than age is supposed to be an artifact of inflation (assuming that the theory of inflation is correct) where "things" i.e. space itself expanded much faster than light (for a very very brief period just after the big bang).
But what expanded was space itself and not particles/matter

Alan Guth's book "The Inflationary Universe" (he is one if the original theorists, along with Andrei Linde abd others) gives a historical picture of how the theory was developed and how it solved certain age old problems in cosmology e.g. the horizon and flatness problems

 

[Kacmarek]

Yes, it has an explanation, which involves perspective and light travel time. Although the particles 'seem' to be travelling at superluminal speeds, they actually aren't - it is an optical illusion.

Remember we are not seeing the actual particles as they travel, just a different selection of particles as they travel across a distant segment of space, and illuminate different parts of that segment sequentially.

It does seem that, if the particle jets are highly oriented towards earth (+- 19 degrees?), then a trigonometric relation involving a (1+cos theta) term ensures that "apparent motion" can be much higher than c while "proper motion" still doesnt exceed c

Yet .... some of the jets esp in M87 seem to be not directly oriented towards earth - may be about 39 degree off-boresight - need to study further
There is also something about speed of the particles in the jet and speed of the bulk in the jet - are they additive?
Definitely a bit complex

Also, these jets are very large astrophysical objects (dimensions in light years or parsecs) which can be detected by us

Sub or superluminal velocities of any artificial craft/structures are probably not likely to be viewable by us at these distances. .....

 

[John J.]

interstellar travel /=/ (not equal to) FTL travel

I think that's a fair point. Although it's hard to imagine a non-relativistic form of transport that would allow practical interstellar travel within the lifetimes of human crew. Relativistic- (near-c) craft aren't forbidden by known physical laws, but they might still be very, very difficult to build (particularly, to propel).
Maybe that old SF standby, a hibernation/ suspended animation technology? As far as I understand, the more that low-temperature possibilities have been studied, the more difficult the problem seems; tissues thaw at different rates (and often sustain considerable cellular damage). And you'd need to shield crew from years of cosmic ray exposure, more mass to add to your craft. Not impossible in principle as far as we know, though- yet.
Even if the wetware- the neurons and all they depend on- can be held in stasis for a prolonged period, perhaps the arrest and reactivation of inter-neuronal chemo-electrical activity, an uninterrupted multi-million-fold parallel process in the living brain, presents problems.

Which sort of brings us back to the Fermi paradox, "Where is everybody?"
-Fermi's assumptions were based on the premise of sub-luminal flight speeds.

In the billions of years that the Milky way has had solar systems with sufficient metallicity to allow life-supporting planets able to host technological civilisations, not one has arisen which has colonised the galaxy (probably just as well for us- it's unlikely we'd be here).
We have no (serious) evidence that any sort that ETI has ever visited this solar system, even though it must be an attractive target for exploration by beings with a biology even remotely similar to ours.
Not a single orbiting calling-card, perhaps transmitting a brief section of Fibonacci sequence once a year, every year.

In billions of years, amongst perhaps tens of millions of solar systems similar to ours
(and that might be an underestimate by an order of magnitude, "How Common are Solar Systems Like Ours?", Universe Today website, Nancy Atkinson 2010 https://www.universetoday.com/49605/how-common-are-solar-systems-like-ours/),
either no technological civilisations that want to colonise other worlds, or directly contact other species, have arisen, or such civilisations have arisen but couldn't overcome the problems of interstellar flight on a meaningful scale.

 

[Ann K]

either no technological civilisations that want to colonise other worlds, or directly contact other species, have arisen, or such civilisations have arisen but couldn't overcome the problems of interstellar flight on a meaningful scale.

And then there's the problem of timing. We have been a technological society capable of something as simple as radio transmission for not much more than a century out of our (perhaps) half billion years of protohuman development, the mere blink of an eye. If civilization proceeded on much the same timeline but a small fraction faster or slower than we have, we could easily miss that teeny tiny window during which any kind of communication would be possible. Actual face to "face" meeting is, I'm confident, MUCH less probable than mere beeps passing in the night.

 

[captancourgette]

Entanglement does not transmit information faster than light, this is a common misunderstanding of the implication of non locality in QM.

True (actually I was gonna amend my post afterwards) but didn't, must of got distracted
Though would you be surprised if a breakthrough happens in a lab where its possible to alter A and it to influence B at a distance, I know I wouldn't then again I am old.

Since I'm spitballing in this thread WRT fermi paradoxe I believe its been shown that once some species achieves a van neumann ability (provided we dont kill ourselves we will achieve in the next millennium) then they (the machines) can visit all the planets in a galaxy within a few million years, thus why hasn't it happened?

 

[jarlrmai]

Though would you be surprised if a breakthrough happens in a lab where its possible to alter A and it to influence B at a distance

If you mean faster than the speed of light then yes I would be incredibly surprised. It's a fundamental limit of the laws of physics, not a technology thing. It would break causality.

 

[Eburacum]

But is there anything about causality in itself that means it can't be violated?

I've read a few attempts to imagine a universe where causality can be violated. One is the Novikov Self-consistency principle; where effect can precede cause, but only if it is self-consistent. That is to say, if you go back in time and try to change events, you can't change them. So you would not be able to kill your own grandfather before he conceived your father, and so on. This one seems quite popular with physicists, for some reason; they often have doubts about the concept of free-will, so going back in time and acting like a self-consistent automaton seems reasonable.

The other attempts to imagine a universe with reversed causality, where effects precede causes, all involve many worlds and multiple timelines. You can go back and change history, but this creates a new timeline. What happens to the old timeline is another problem, and there are several variations on that theme. But each observer in each timeline can only perceive one set of possibilities, so the effects of these changes are difficult or impossible to detect.

Both of these concepts are so difficult to envisage as a working model of the universe that I am fairly sure that causality is inviolable.

 

[John J.]

The other attempts to imagine a universe with reversed causality, where effects precede causes, all involve many worlds and multiple timelines. You can go back and change history, but this creates a new timeline.

Reminds me a bit of the Everett Many-worlds interpretation: Any possible outcome of a quantum event is realised in one of a countless number of perpetually-branching universes.
Arguably solves "the observer problem". Doesn't explain how each quantum event instantaneously generates many, many universes, identical except in the outcome of that one quantum event, each universe a minimum of maybe 93 billion light-years diameter, with very little drama.

Or maybe each branching "daughter" universe radiates out from the source of the quantum event at c, with some yet-to-be-identified effect "painting in" the surrounding reality over the ensuing aeons, as if it had a memory of the universe from which it sprang.
If a specific quantum event occurred in a nice, naturally-lit setting on Earth, in all the daughter universes that setting (and its sunlight) would exist eight minutes before the Sun physically existed... (perhaps. The beauty of this conjecture is that I am unencumbered by any understanding of the maths, or indeed much else, underlying the many-worlds interpretation ).

Does seem to involve a lot of "multiplying entities unnecessarily", though https://en.wikipedia.org/wiki/Occam's_razor.

 

[Asherah]

As has been already noted, travel at near-light speed (and ignoring fuel requirements) enables transversing very vast diatances - but how would a destination be picked? If we could observe a planet 1 million light years away, the data would be 1 million years old - and by the time we arrived, it would be 2 million years old. Without a specific destination in mind, the probability of reaching an appropriate destination is infinitisessimal.

 

[Eburacum]

If we could observe a planet 1 million light years away, the data would be 1 million years old

A common misperception. None of the visible stars we can see in the sky are anywhere near as distant as a million light years. Alpha Centauri is only four light years away, Sirius is eight - even a really distant star like Deneb is only 1500 light years way, and it is a relatively simple calculation to find where it will be 1500 years from now.

If we have to travel a million years to find another civilisation we might as well be alone.

 

[FatPhil]

A common misperception. None of the visible stars we can see in the sky are anywhere near as distant as a million light years. Alpha Centauri is only four light years away, Sirius is eight - even a really distant star like Deneb is only 1500 light years way, and it is a relatively simple calculation to find where it will be 1500 years from now.

If we have to travel a million years to find another civilisation we might as well be alone.

I'm a bit of an astro nerd but I have a terrible memory, even for numbers, and most of these dimensions have been just a jumble in my head. I think part of the problem is the persistence of two out-by-a-factor-of-three units for measuring astronomical distances - and 3's a pretty terrible factor for error either way. Scrapping the parsec mostly solved the problem, as it simplifies the numbers and I've finally got them fixed. To astronomically-suitable accuracy:

The Milky Way's 100000 ly across
We're half of the way out
=> Our furthest MW neighbours will be 75000 ly away
Andromeda's 2.5 Mly away
Andromeda's 150000 ly across

So yes, the "1Mly" selected is right in the sparse zone that's in neither galaxy where there will only be dwarf galaxies (of which there are many, but with few exceptions they're small and dim) and rogue stars. But worse than that, as you note, the "visible stars", to mere humans without the ability to detect x-rays outside earth's atmosphere and lacking arrays of multi-metre radio-wave-detecting ears, are mostly within few thousand ly. (Those interested in how mostly is "mostly" are free to deep dive down https://en.wikipedia.org/wiki/Lists_of_stars_by_constellation , and report back: Casseopeia seemed to have only a couple out of the top couple of dozen; Gemini and Orion I got bored searching for any; Canis Major had a few in the first few pages, ... so "mostly" seems fair.)

So in summary the human-oriented approximate grand scale of space is:
Visible stars: up to 1000s of ly
Milky Way: up to 100000 ly
MW's dwarf galaxies: 10000s to many 100000s of ly
Neighbouring galaxies: several millions of ly

 

[Eburacum]

Kappa Cassiopeiae (4000 ly) and Rho Cassiopeiae (8100 ly) are two of the most distant stars which are visible to the naked eye - both are unusually bright and large, and extraordinarily distant. From our viewpoint they are right next to Eta Cassiopeiae, which is a sun-like star 19 light years away, and is one of the better candidates for a life-bearing system.

We don't really want to bother with big, bright, short-lived stars like Rho Cassiopeiae - they only last a million years or so, which doesn't give enough time to allow the evolution of intelligent life - and they emit vast amounts of radiation, which would make any environment nearby uncomfortable, to say the least. So those large, relatively distant stars are not currently targets for SETI or for planet hunters.

 

[jdog]

I'm a bit of an astro nerd but I have a terrible memory, even for numbers, and most of these dimensions have been just a jumble in my head. I think part of the problem is the persistence of two out-by-a-factor-of-three units for measuring astronomical distances - and 3's a pretty terrible factor for error either way. Scrapping the parsec mostly solved the problem, as it simplifies the numbers and I've finally got them fixed. To astronomically-suitable accuracy:

The Milky Way's 100000 ly across
We're half of the way out
=> Our furthest MW neighbours will be 75000 ly away
Andromeda's 2.5 Mly away
Andromeda's 150000 ly across

So yes, the "1Mly" selected is right in the sparse zone that's in neither galaxy where there will only be dwarf galaxies (of which there are many, but with few exceptions they're small and dim) and rogue stars. But worse than that, as you note, the "visible stars", to mere humans without the ability to detect x-rays outside earth's atmosphere and lacking arrays of multi-metre radio-wave-detecting ears, are mostly within few thousand ly. (Those interested in how mostly is "mostly" are free to deep dive down https://en.wikipedia.org/wiki/Lists_of_stars_by_constellation , and report back: Casseopeia seemed to have only a couple out of the top couple of dozen; Gemini and Orion I got bored searching for any; Canis Major had a few in the first few pages, ... so "mostly" seems fair.)

So in summary the human-oriented approximate grand scale of space is:
Visible stars: up to 1000s of ly
Milky Way: up to 100000 ly
MW's dwarf galaxies: 10000s to many 100000s of ly
Neighbouring galaxies: several millions of ly

I always recall the sense of scale from Star Trek: Voyager, where Janeway's ship is sent most of the way toward the far end of the galaxy (the "Delta" quadrant, the federation being in "Alpha") and it's going to take them 70 years at ~high warp to return 70,000 light years across 70% of the width of the galaxy.