13 hours ago, swansont said:

Collimation is a problem but the real issue is how do you not melt the sails or ship with the intensity you need for a light sail to work with any realistic payload. You need a really big sail.

For light F=Power/c; to get 1N of force (1g of acceleration for ~0.1 kg) you need ~150 MW - you get a factor of 2 from perfect reflection, but it’s not going to be perfect, so whatever absorption you have will heat the target.

You probably don’t want your laser to have to slice through the atmosphere, but if it’s on a craft in space it’s recoiling with that same photon momentum effect you’re using for the solar sail.

Not to mention the small problem of how to slow down at the end of your journey……

3 hours ago, exchemist said:

Not to mention the small problem of how to slow down at the end of your journey……

Or to mention, how to hop onto a planet like ours, then hop off again with all the swag...

17 hours ago, swansont said:

Collimation is a problem but the real issue is how do you not melt the sails or ship with the intensity you need for a light sail to work with any realistic payload. You need a really big sail.

For light F=Power/c; to get 1N of force (1g of acceleration for ~0.1 kg) you need ~150 MW - you get a factor of 2 from perfect reflection, but it’s not going to be perfect, so whatever absorption you have will heat the target.

You probably don’t want your laser to have to slice through the atmosphere, but if it’s on a craft in space it’s recoiling with that same photon momentum effect you’re using for the solar sail.

Yes materials technology would need some leaps forward and any kind of excess heat handling would likely involve the sail having a circulation of coolant and more mass from that. (also you got me thinking about imperfect reflection and IR radiating from the back of the sail...)

The recoil effect is another interesting problem. Put the laser on an airless planet and there's also issues with rotation and orbiting a primary, if continuous thrust is wanted.

As @exchemist comment about deceleration suggests, there's also no way I could see to tack and jibe like a sailboat.

In my opinion, the question of whether or not aliens are involved is irrelevant; what matters is the type of visit we’re talking about. After all, there is a huge difference between a fly-by and a large spacecraft.

The basic idea of solar sails does sound elegant. However, the counter-argument involving an extremely low payload, huge sail areas, heat from imperfect reflection, beam divergence, accuracy and, ultimately, the question of how to slow down is not long in coming. A fly-through of a neighbouring star system would be extremely difficult, but still within the realm of hard physics. A tourist visit with a return journey would be in a completely different league.

I therefore like the distinction between 'not prohibited by known physics' and 'practically feasible'. There's a world of difference between the two. Quite honestly, I'm out as soon as someone starts waving a 'Gravity Warp' around à la Bob Lazar, at least until it becomes verifiable physics. I’m not denying that one can speculate, though. But that doesn't make calculations involving energy, momentum, heat and braking any less important. They just get hidden behind a fog.

23 minutes ago, aliceinwonderland said:

the question of whether or not aliens are involved is irrelevant; what matters is the type of visit we’re talking about. After all, there is a huge difference between a fly-by and a large spacecraft.

Sure. Here we’re discussing visits that would generate a typical UFO/UAP report, i.e. enters our atmosphere, maneuvers, and then leaves.

The fuel burden of getting here at 0.1c was addressed in the article, but it did not address the considerable obstacle of also entering and leaving a gravity well.

What we do know is that we can't do it.
What we don't know is what a considerably more advanced civilization can do.

To quote former SoD D Rumsfeld
"There are things we do not know that we don't know"

This dilemma reminds me of proclamations made in the early days of science, going faster than a hundred mph or so is impossible, the earth cannot be more than a few million years old, going to the moon is impossible, life cannot arise naturally, rocks cannot fall from the sky, the ideas they thought to be nonsense were often just a lack of information.

I am not willing to say what is possible or not, I know a lot of science based speculation goes way to far for us really take it seriously... but rocks do fall from the sky, people can travel at speeds long thought to be impossible, the earth is 4.6 billion years old, life did indeed arise here, and we have visited the moon.

Saying something cannot be done is a good way to be wrong but one thing is sure... assuming something is impossible is a good way to make sure it is never pursued seriously.

Edited Saturday at 02:02 AM3 days by Moontanman
removing off topic opinion

4 hours ago, Moontanman said:

This dilemma reminds me of proclamations made in the early days of science, going faster than a hundred mph or so is impossible, the earth cannot be more than a few million years old, going to the moon is impossible, life cannot arise naturally, rocks cannot fall from the sky, the ideas they thought to be nonsense were often just a lack of information.

I am not willing to say what is possible or not, I know a lot of science based speculation goes way to far for us really take it seriously... but rocks do fall from the sky, people can travel at speeds long thought to be impossible, the earth is 4.6 billion years old, life did indeed arise here, and we have visited the moon.

Saying something cannot be done is a good way to be wrong but one thing is sure... assuming something is impossible is a good way to make sure it is never pursued seriously.

Which of those proclamations were predictions from the science of the day?

6 hours ago, exchemist said:

Which of those proclamations were predictions from the science of the day?

All of them.

29 minutes ago, Moontanman said:

All of them.

Are you sure, none of them seem scientific...

1 hour ago, Moontanman said:

All of them.

I doubt any of them were.

1 hour ago, dimreepr said:

Are you sure, none of them seem scientific...

All of them are real but I have not quoted them exactly, Dr. Dionysius Lardner in the 1820s is supposed to have said humans cannot travel faster than 40 mph.

Lord Kelvin said the Earth was a few million years old

This is just one claim about travel to the moon

" Dr. William Pickering
Director of the Jet Propulsion Laboratory (JPL), he famously declared in 1957 that the idea of sending a man to the moon was "absurd" and that it would not happen in the foreseeable future. [1]"

"The Famous Quote Origin

While Aristotle provided the ancient foundation, the most famous modern quote on the topic is often attributed to the third U.S. President, Thomas Jefferson. [1, 2]

When two Yale professors (Benjamin Silliman and James Kingsley) visited him to report a meteorite fall in Weston, Connecticut, Jefferson is famously reputed to have said:"

"It is easier to believe that two Yankee professors would lie than that stones would fall from heaven." [1, 2]

"The concept that life cannot arise naturally from non-living matter—known historically as spontaneous generation—was first challenged scientifically by Louis Pasteur in the 1860s. He disproved the ancient belief, leading to the Law of Biogenesis: the scientific principle that life only arises from pre-existing life. [1, 2, 3]

Several historical and modern figures are closely associated with arguing against the natural origin of life:

• Aristotle (c. 300 BC): He was the earliest to document and popularize the theory of spontaneous generation (e.g., believing fleas and maggots arose naturally from dust or mud), setting up the debate that later thinkers had to refute. [1, 2, 3]

• Louis Pasteur (1859): Conducted famous experiments using swan-neck flasks that conclusively proved even microscopic organisms could not arise spontaneously in sterilized broth, famously asserting "life only comes from life". [1, 2, 3]"

I apologise for the weird quotes, some kind of AI crap came with my new computer and it's driving me nuts.

2 hours ago, Moontanman said:

All of them are real but I have not quoted them exactly, Dr. Dionysius Lardner in the 1820s is supposed to have said humans cannot travel faster than 40 mph.

Lord Kelvin said the Earth was a few million years old

This is just one claim about travel to the moon

" Dr. William Pickering
Director of the Jet Propulsion Laboratory (JPL), he famously declared in 1957 that the idea of sending a man to the moon was "absurd" and that it would not happen in the foreseeable future. [1]"

"The Famous Quote Origin

While Aristotle provided the ancient foundation, the most famous modern quote on the topic is often attributed to the third U.S. President, Thomas Jefferson. [1, 2]

When two Yale professors (Benjamin Silliman and James Kingsley) visited him to report a meteorite fall in Weston, Connecticut, Jefferson is famously reputed to have said:"

"The concept that life cannot arise naturally from non-living matter—known historically as spontaneous generation—was first challenged scientifically by Louis Pasteur in the 1860s. He disproved the ancient belief, leading to the Law of Biogenesis: the scientific principle that life only arises from pre-existing life. [1, 2, 3]

Several historical and modern figures are closely associated with arguing against the natural origin of life:

• Aristotle (c. 300 BC): He was the earliest to document and popularize the theory of spontaneous generation (e.g., believing fleas and maggots arose naturally from dust or mud), setting up the debate that later thinkers had to refute. [1, 2, 3]

• Louis Pasteur (1859): Conducted famous experiments using swan-neck flasks that conclusively proved even microscopic organisms could not arise spontaneously in sterilized broth, famously asserting "life only comes from life". [1, 2, 3]"

I apologise for the weird quotes, some kind of AI crap came with my new computer and it's driving me nuts.

The trouble with these is they do not show these claims were actually underpinned by the science of the day.

Dionysus Lardner was just one rather opinionated populariser of science. There is nothing in Newtonian mechanics, which was the science of the day, to support his notion - and in fact he seems to have made an idiot of himself several times over in disputes with Brunel, who was a professional engineer.

Kelvin estimated the age of the Earth at a few million years. That is not at all the same as claiming it could not be older than that.

Pickering did not say sending a man to the moon was impossible. He said he thought it absurd and would not happen in the foreseeable future. That means he thought it impractical. He did not claim it was ruled out by science.

Jefferson was not any kind of astronomer, so his opinions on meteorites can't be taken to reflect a scientific consensus.

Pasteur's dismissal of spontaneous generation was to scotch the notion that, for example, "the sun breeds maggots in a dead dog" (Hamlet), i.e. the ancient idea that living things could arise spontaneously all the time. He was not expressing an opinion on the origin of life on the Earth.

So it seems to me that none of these examples is comparable to the conclusion that, according to current physics, aliens will not have visited the Earth and will not do so in the future. That is based on the scientific considerations outlined in some detail in this thread.

Edited Saturday at 05:41 PM2 days by exchemist

Well what to make of Lord Kelvins famous comment in 1897, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement."

One imagines the eminent physicist felt himself quite underpinned by the science of his day. (pins which quickly gave him some sharp pricks within the year)

3 hours ago, Moontanman said:

All of them are real

Nobody said they aren’t real. The question is if they are scientific.

Can you point to the scientific reasoning behind the claims?

3 hours ago, Moontanman said:

but I have not quoted them exactly, Dr. Dionysius Lardner in the 1820s is supposed to have said humans cannot travel faster than 40 mph.

Lord Kelvin said the Earth was a few million years old

Kelvin explained his reasoning; it suffered from not accounting for convection and not knowing about radioactivity. Scientific means one can critique the analysis and point out errors. If that analysis is not present, it’s not scientific. It’s a guess, or an appeal to conventional wisdom.

3 hours ago, Moontanman said:

This is just one claim about travel to the moon

" Dr. William Pickering
Director of the Jet Propulsion Laboratory (JPL), he famously declared in 1957 that the idea of sending a man to the moon was "absurd" and that it would not happen in the foreseeable future. [1]"

The site linked in the citation doesn’t contain this claim.

Your annoying AI might have hallucinated this

33 minutes ago, exchemist said:

The trouble with these is they do not show these claims were actually underpinned by the science of the day.

Dionysus Lardner was just one rather opinionated populariser of science. There is nothing in Newtonian mechanics, which was the science of the day, to support his notion - and in fact he seems to have made an idiot of himself several times over in disputes with Brunel, who was a professional engineer.

Kelvin estimated the age of the Earth at a few million years. That is not at all the same as claiming it could not be older than that.

Pickering did not say sending a man to the moon was impossible. He said he thought it absurd and would not happen in the foreseeable future. That means he thought it impractical. He did not claim it was ruled out by science.

Jefferson was not any kind of astronomer, so his opinions on meteorites can't be taken to reflect a scientific consensus.

Pasteur's dismissal of spontaneous generation was to scotch the notion that, for example, "the sun breeds maggots in a dead dog" (Hamlet), i.e. the ancient idea that living things could arise spontaneously all the time. He was not expressing an opinion on the origin of life on the Earth.

So it seems to me that none of these examples is comparable to the conclusion that, according to current physics, aliens will not have visited the Earth and will not do so in the future. That is based on the scientific considerations outlined in some detail in this thread.

@exchemist The science of the day? I have heard recently over and over that interstellar travel is impossible here on this site, there is nothing in physics that supports that, in Jefferson's day and many years before, rocks falling from the sky was explained away by saying that such rocks had to be blasted out of the earth by volcanoes or some other "earthly" explanation. Meteorite displays were removed from museums because the consensus was that rocks could not fall from the sky. That consensus can be wrong and I am for one glad it can be wrong, it's how we progress.

Pasteur showed that life did not arise spontaneously, he had no explanation for how life came to be only that life came from life.

All of these things in their time were dismissed by the "scientific consensus" of the time, not because the people were stupid or uneducated but because they didn't have the knowledge to say otherwise. Right now, we really don't know enough to say what is and what is not possible, we can only continue to further science as find new paths to knowledge.

We are all hamstrung by what we currently know, it just is what it is, if you look deep into what is being pursued in science and engineering there are many ongoing efforts to break past what is currently scientific consensus. Not in huge showy mind bending ways but in the slow increments that has always been the lifeblood of science. Very few scientists have the "I have become death destroyer of worlds" moments.

Trying to project our current understanding into the future are just as likely to be as accurate as the scientists or philosophers of centuries or even decades ago projecting their understanding onto our time.

Science breaks barriers by building on the works of others.

I am just not so sure that our current consensus of science is going to ever be the final consensus.

25 minutes ago, swansont said:

Nobody said they aren’t real. The question is if they are scientific.

At the time they were made they were considered as scientific as it got at the time. Science now is not the same as the science that rejected dinosaurs, or that Humans are related to apes. They were scientists they just weren't as advanced as they are today.

34 minutes ago, swansont said:

Nobody said they aren’t real. The question is if they are scientific.

Can you point to the scientific reasoning behind the claims?

Kelvin explained his reasoning; it suffered from not accounting for convection and not knowing about radioactivity. Scientific means one can critique the analysis and point out errors. If that analysis is not present, it’s not scientific. It’s a guess, or an appeal to conventional wisdom.

The site linked in the citation doesn’t contain this claim.

Your annoying AI might have hallucinated this

Kelvin explained his reasoning; it suffered from not accounting for convection and not knowing about radioactivity. Scientific means one can critique the analysis and point out errors. If that analysis is not present, it’s not scientific. It’s a guess, or an appeal to conventional wisdom.

25 minutes ago, swansont said:

The site linked in the citation doesn’t contain this claim.

Your annoying AI might have hallucinated this

You are correct,

No, Dr. William H. Pickering (the JPL director) never said that travel to the moon was unlikely; in fact, he actively built the spacecraft that made it possible. [1, 2]

Confusion around this claim typically stems from two main sources: a mix-up with a completely different scientist, or a historical quote about airplanes being misremembered as being about space travel. [1]

1. The Confusion with Professor A.W. Bickerton

The famous skeptical quote often floating around the internet—"This foolish idea of shooting at the moon is an example of the absurd length to which vicious specialization will carry scientists..."—was actually written by a New Zealand physicist named Professor A.W. Bickerton in 1926. Because Dr. William Pickering was also a famous scientist from New Zealand, internet trivia lists and quote repositories occasionally misattribute Bickerton's pessimistic quote to Pickering

2. A Mix-Up with a Different "William Pickering"

There was an older, unrelated Harvard astronomer named William Henry Pickering (1858–1938). While he spent his life obsessively mapping the moon (and incorrectly believing it was covered in changing vegetation and insects), he did make a famously wrong prediction in 1910 regarding aviation: [1, 2, 3, 4]

"The popular fantasy is to suppose that flying machines could be used to drop dynamite on the enemy in time of war..."
"By no possibility can the carriage of freight or passengers through mid-air compete with their carriage on the earth's surface." [1, 2]

Because this older William Pickering claimed commercial flight was a fantasy, his quotes are frequently lumped into "bad historical predictions" lists, which people later mistake as comments about space exploration. [1, 2]

Edited Saturday at 06:47 PM2 days by Moontanman

28 minutes ago, Moontanman said:

At the time they were made they were considered as scientific as it got at the time. Science now is not the same as the science that rejected dinosaurs, or that Humans are related to apes. They were scientists they just weren't as advanced as they are today.

How is something claimed by a non-scientist, or before science existed, be considered scientific?

Something rejected/resisted owing to religious dogma is not something you can ascribe to science

1 hour ago, TheVat said:

Well what to make of Lord Kelvins famous comment in 1897, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement."

One imagines the eminent physicist felt himself quite underpinned by the science of his day. (pins which quickly gave him some sharp pricks within the year)

Worth noting this sentiment was actually expressed by Albert Michelson in an 1894 address - but it's often misattributed to Kelvin.

Kelvin, however, did say in a newspaper interview in 1902, "No balloon and no aeroplane will ever be practically successful." Chuckle.

I, for one, don't believe aliens are visiting Earth.
If they are rational enough to have devised a means of doing so, they would realize that it isn't worth the effort ( unless they are really, really fond of anally probing southern American hicks ).

However I take issue with the stated claims that 'consensus' dictates our knowledge and methods will never improve much beyond the current.

I'll use one of my previously used examples ...

It was previously ( 1950s ) thought CP symmetry could not be violated, but in 1964 it was observed in the weak interaction ( Nobel in 1980 ), and has recently been observed in Baryons ( 2025 @ LHC ).
This may be extremely important to the problem of matter dominance in the present universe, and a better understanding of matter/antimatter asymmetry could eventually lead to methods of generating antimatter more efficiently, and the matter/antimatter reactor I previously mentioned ( 100% conversion to energy, as opposed to a couple of % for fusion ).

The more we know/understand in current science, the more questions arise, and sometimes, whole new fields open up ( just ask Max Planck ).

2 hours ago, swansont said:

How is something claimed by a non-scientist, or before science existed, be considered scientific?

Something rejected/resisted owing to religious dogma is not something you can ascribe to science

You make a good point, back then science wasn't what we consider science, I apologize for suggesting otherwise. But as others have said, Lord Kelvin evidently being a good example, what passed for science or maybe more appropriately authority in the past has made some assertions that simply didn't hold up. It's easy to point back and say they were wrong... will it be as easy to point back at us from 2 or 3 hundred years and consider what we have as not real science? I wouldn't be surprised if it was...

But I am not suggesting that aliens are visiting due to this, I am suggesting that making assertions of fact about things that can change is not a good idea.

I have backed far away from UFOs are run amuck in our skies... It was silly to assume such things without better evidence, how ever I see no reason to assume that because we can't currently star travel doesn't mean that other life forms cannot. I have no idea what aliens would be like or capable of, anything we say is simply speculation. Even if it's based on what we know it is still subject to the limitations of what we know currently.

I have seen some ideas from futurists that suggest a lot of what some say is impossible is simply very difficult for life forms like us to accomplish, other life forms might very well find ways around the biological problems that we simply cannot do and maybe even will not do.

These kind of conversations inevitably run up against speculation, speculation can make a great conversation, but it is still speculation and must be tempered by evidence.

I think that colonization of our own solar system might eventually result in technology that would allow star travel, slow boats in my estimation are most probable but again that is just my opinion. Colonization of our own solar system is, in my mind at least, paramount for the survival of humanity in the long term. I see no reason to seriously pursue star travel when the solar system is available for our exploitation. I would think we'll be busy near our own star for thousands if not millions of years, if that eventually leads to space habitats capable of slow boating to other star systems then we will probably do it.

But I see no driving force to get to another star anytime soon.

Personally I can conceive of ways to star travel that are not impossible, maybe not particularly probable, but such conversations are still just thought experiments of ifs, and maybes projected by us. Once you start speculating science starts retreating into the distance.

"If frogs had wings"

And remember Fusion Power is just 20 years away!

12 hours ago, Moontanman said:

You make a good point, back then science wasn't what we consider science, I apologize for suggesting otherwise. But as others have said, Lord Kelvin evidently being a good example, what passed for science or maybe more appropriately authority in the past has made some assertions that simply didn't hold up. It's easy to point back and say they were wrong... will it be as easy to point back at us from 2 or 3 hundred years and consider what we have as not real science? I wouldn't be surprised if it was...

But I am not suggesting that aliens are visiting due to this, I am suggesting that making assertions of fact about things that can change is not a good idea.

I’m trying to point out the differences between the kinds of arguments. I don’t think you can say a claim is scientific if there is no or scant mention of the science that allegedly buttresses it. If there is, then there is an opportunity to point out errors or omissions. (people - scientists included - can be wrong. Science relies on consensus because it’s less likely that errors are missed when multiple people check your work). So if there’s no science in it, it’s not scientific, even if a scientist makes it.

Further, I think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true. You can’t engineer your way around e.g. a violation of conservation of energy, or momentum

21 hours ago, Moontanman said:

All of them are real but I have not quoted them exactly, Dr. Dionysius Lardner in the 1820s is supposed to have said humans cannot travel faster than 40 mph.

But that's not based on science, just his particular form of bias.

Isn't there a maximum size of planet from which it's impossible to escape the planet with known propellant's?

Not much bigger than the Earth, IIRC it's less than !.5.

I think, even you, would have to admit that any visitor's have to come from our galaxy, so a finite number of seed planets; I think this would represent a further restriction on the Drake equation.

1 hour ago, swansont said:

Further, I think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true. You can’t engineer your way around e.g. a violation of conservation of energy, or momentum

I agree, but how do these things prohibit star travel? There are ways to star travel without violating the laws of physics. Slow boats are an example, we might not be able to do it now but it's mostly engineering problems not the laws of nature.

1 hour ago, dimreepr said:

But that's not based on science, just his particular form of bias.

Isn't there a maximum size of planet from which it's impossible to escape the planet with known propellant's?

Not much bigger than the Earth, IIRC it's less than !.5.

I think, even you, would have to admit that any visitor's have to come from our galaxy, so a finite number of seed planets; I think this would represent a further restriction on the Drake equation.

I don't think planets are necessary, in fact I doubt planets would be a factor at all in any equation. Planets are deep gravity wells, why bother when space habitats made from debrei like asteroids and kuiper belt like objects can be used to build space habitats. An inhabited planets would be at best a curiosity.

42 minutes ago, Moontanman said:

I don't think planets are necessary, in fact I doubt planets would be a factor at all in any equation. Planets are deep gravity wells, why bother when space habitats made from debrei like asteroids and kuiper belt like objects can be used to build space habitats. An inhabited planets would be at best a curiosity.

Really?

Intelligent life has to evole somewhere, that retains an atmosphere to live in, long enough to develope an intelligent approach to escaping a gravity well that retains an atmosphere, but prohibits an escape,,,

Seems like a fine balance...

2 hours ago, dimreepr said:

Isn't there a maximum size of planet from which it's impossible to escape the planet with known propellant's?
Not much bigger than the Earth, IIRC it's less than !.5.

A 'reaction' motor provides thrust by ejecting energized mass rearward, at velocity, to provide forward motion according to Newtonian mechanics ( essentially conservation of momentum ).
Escape velocity is proportional to the square root of the mass you are attempting 'escape' from ( assuming a fixed radius of departure ).

I don't see how your statement is valid.

Edited Sunday at 02:39 PM1 day by MigL