I stand corrected that there are a lot of puzzles within physics that are unsolved.

Yet I still think there will be a finite number of them and if not yet more answers than questions as long as we keep trying the proliferation of new question will diminish whilst those answered will increase. Identifying the puzzles is a big step to solving them. Like many truisms, I think it will prove incorrect that the more we learn, the more we realize how much we don't yet know.

More specifically to the OP I'm not convinced solving the known unknowns is going to lead to FTL or other workarounds to the physics underpinning the time and resources issues for interstellar travel and I remain skeptical the set of unknown unknowns will reveal any. I would not be disappointed to be wrong - but possibly alarmed with it.

I will add that I think that for some of these puzzles resolving one may provide resolution to others - that the number of solutions needed will be less than the number of questions.

Edited Saturday at 01:48 AM4 days by Ken Fabian

Oh, you're such an optimist 🙂 .

I agree about our assumptions on 'sentient' beings, also didn't NASA find fungus growing on the outside of the ISS. There is also a reason why space probes are made in sterile environments to reduce the risk of contamination of planetary bodies.

It is possible we have already had visitors to Earth, this could explain why there are depictions found on different continents. I think on the ST voyager episode Tattoo (season 2) the inhabitants of the planet the crew visited said they took several generations to travel to Earth.

13 hours ago, TheVat said:

I was asking in regard to his saying "collimate" light from the sun. Am I mistaken that you need coherent light to succeed in collimating over any distance we're discussing? I may have misunderstood @DanMP in thinking that he was implying some way to make the sun emit coherent light. If I'm wrong on either point, happy to learn more about this. (Also thought the inverse square law meant sails weren't much use beyond Pluto, so were usually considered in context of solar system travel)

Collimation isn’t really necessary; you could be focusing the light near where the sail is. I think being monochromatic is the more important factor.

On 6/18/2026 at 11:05 AM, exchemist said:

On 6/18/2026 at 11:05 AM, exchemist said:

vanishingly unlikely

These are the key words, in this vast universe "vanishingly unlikely" becomes a certainty over large scale time and space. Unless it can be shown to be impossible it will happen eventually. The very fact that we can speculate ways it could be done that are not impossible points to this. Winning the lotto might be improbable but try to take away the winnings of the man who won with that excuse will no doubt be a problem.

I will admit that alien visitation is highly improbable, I have changed my approach to the subject having admitted that the data we currently have is does not meet the standards of science and the jet stream of bullshit currently emanating from the Trump regime cannot be trusted and so far is no better than lights in the sky being reported by Billy Joe Jim Bob after his friday night anal probe!

Edited Saturday at 03:22 PM3 days by Moontanman

1 hour ago, Moontanman said:

These are the key words, in this vast universe "vanishingly unlikely" becomes a certainty over large scale time and space. Unless it can be shown to be impossible it will happen eventually. The very fact that we can speculate ways it could be done that are not impossible points to this.

No it does not. Vanishingly unlikely in this case means our physics has to be wrong for it to occur. Our physics is not going to just become suddenly wrong if we wait long enough.

20 hours ago, MigL said:

Oh, you're such an optimist 🙂 .

Not really - societies and economies face some serious problems over the not entirely foreseeable future. If high levels of funding and support continue to flow to fundamental physics without discontinuities I expect more puzzles will get solved. I'm not entirely convinced that will be the case.

On 6/20/2026 at 1:11 PM, exchemist said:

No it does not. Vanishingly unlikely in this case means our physics has to be wrong for it to occur. Our physics is not going to just become suddenly wrong if we wait long enough.

How does physics have to be wrong for interstellar travel to occur? I see no reason why matter from one planetary system cannot travel to another planetary system. Can you elaborate on this?

1 hour ago, Moontanman said:

How does physics have to be wrong for interstellar travel to occur? I see no reason why matter from one planetary system cannot travel to another planetary system. Can you elaborate on this?

I guess he means FTL (e.g. stable traversable wormholes, etc) is disallowed, but current physics doesn't expressly forbid slower than light travel or finding ways to preserve living passengers on such long journeys. The question is more regarding what can feasibly be engineered. And there are collateral questions e.g. at relativistic speeds what level of radiation would a ship encounter and what amount of shielding needs mounted in the direction of travel.

Suppose we somehow discover a propulsion system that gets us to relativistic speeds in the range of 0.3 c. That seems a minimum for regular manned starflight given the times and distances involved, but suddenly attaining it doesn’t end our problems. Interstellar space isn’t empty, and when we accelerate to cruising speed at a substantial percentage of the speed of light, our encounter with interstellar gas becomes a nightmare. Indeed, this haze of gas between the stars acts as a flow of nucleonic radiation bombarding the starship as we push ever higher into relativistic realms....

https://www.centauri-dreams.org/2009/04/03/shielding-the-starship/

29 minutes ago, TheVat said:

Interstellar space isn’t empty, and when we accelerate to cruising speed at a substantial percentage of the speed of light, our encounter with interstellar gas becomes a nightmare. Indeed, this haze of gas between the stars acts as a flow of nucleonic radiation bombarding the starship as we push ever higher into relativistic realms....

It sure would be nice if you had gigantic Electromagnetic scoops that gather the interstellar gas/dust in your way, and use it as ejected mass, once energized by a matter-antimatter reactor, to generate forward thrust.
Something like a Bussard ramjet ...

2 hours ago, Moontanman said:

How does physics have to be wrong for interstellar travel to occur? I see no reason why matter from one planetary system cannot travel to another planetary system. Can you elaborate on this?

Firstly, I refer you to the opening post. Secondly, any journey would take so long that it would be pointless. No information could ever be sent back unless the senders were willing to wait for hundreds of thousands of years. And then what would they do with it?

In space travel the numbers are awful. That’s why scenarios for alien visitation necessarily involve some magic macguffin that violates the principles of current physics.

Edited Sunday at 08:11 PM2 days by exchemist

6 hours ago, exchemist said:

Firstly, I refer you to the opening post. Secondly, any journey would take so long that it would be pointless.

Depends on what the pont was. I see no reason why such a journey would have to include living humans. Self replicating machines could make the journey, the time scale would be meaningless. Humans could be constructed once the ship arrives... assuming that humans are still the point.

6 hours ago, exchemist said:

No information could ever be sent back unless the senders were willing to wait for hundreds of thousands of years. And then what would they do with it?

Information travels at the speed of light, a great many stars are less than "hundreds of thousand years" away at light speed, info could be transmitted if it was wanted. Why is information be the point of interstellar travel? Maybe actual colonization is the point, information would just be secondary.

6 hours ago, exchemist said:

In space travel the numbers are awful. That’s why scenarios for alien visitation necessarily involve some magic macguffin that violates the principles of current physics.

No, no magical mcguffin is necessary, just technology and the desire to expand human civilization. I would have to think that controlled fusion is necessary as well. Lack of practical fusion power, IMHO, would be a deal breaker, but fusion power is just 20 years away.

Don't assume why future humans... or aliens would want to travel to another star. I still think that generational ships are a possibility for colonization of other star systems.

Once we colonize the solar system with space habitats, and living in those habitats are the norm, travel to the nearest star would just be like a normal life.

No planets are necessary for colonization of the galaxy once we live in space habitats.

I honestly think that underestimating human perseverance and technology will not age any better than early scientists who were sure flight would never be possible or that trips to the moon are a crazy fantasy.

The things I am suggesting are not impossible, they may seem unlikely from our current goals but goals change and space habitats are possible. Development of space habitats will change our zeitgeist, living in space will allow us to expand into the solar system in the trillions, living on millions of space habitats.

1 hour ago, Moontanman said:

  7 hours ago, exchemist said:

Firstly, I refer you to the opening post. Secondly, any journey would take so long that it would be pointless.

Depends on what the point was. I see no reason why such a journey would have to include living humans. Self replicating machines could make the journey, the time scale would be meaningless. Humans could be constructed once the ship arrives... assuming that humans are still the point.

Did you actually read the OP or the linked article? It wasn't really dealing with the problems of transporting humans, and therefore "self-replicating machines" doesn't address the problems stated in the article. The article was about, among other things, the amount of a given type of fuel required to achieve a speed of 0.1c, given that the mass of that fuel adds to the amount of fuel required.

1 hour ago, Moontanman said:

  7 hours ago, exchemist said:

In space travel the numbers are awful. That’s why scenarios for alien visitation necessarily involve some magic macguffin that violates the principles of current physics.

No, no magical mcguffin is necessary, just technology and the desire to expand human civilization. I would have to think that controlled fusion is necessary as well. Lack of practical fusion power, IMHO, would be a deal breaker, but fusion power is just 20 years away.

The article dealt with nuclear fusion, and to achieve a speed of 0.1c would require a total mass of fuel that is 150 times the mass of the ship itself.

The article also points out that the longer the mission, the more likely it is for disasters such as system malfunctions to take place. And even at 0.1c, colliding with interstellar particles could erode even the most resilient engineering materials.

Edited Monday at 04:06 AM2 days by KJW

3 hours ago, Moontanman said:

Depends on what the pont was. I see no reason why such a journey would have to include living humans. Self replicating machines could make the journey, the time scale would be meaningless. Humans could be constructed once the ship arrives... assuming that humans are still the point.

Information travels at the speed of light, a great many stars are less than "hundreds of thousand years" away at light speed, info could be transmitted if it was wanted. Why is information be the point of interstellar travel? Maybe actual colonization is the point, information would just be secondary.

No, no magical mcguffin is necessary, just technology and the desire to expand human civilization. I would have to think that controlled fusion is necessary as well. Lack of practical fusion power, IMHO, would be a deal breaker, but fusion power is just 20 years away.

Don't assume why future humans... or aliens would want to travel to another star. I still think that generational ships are a possibility for colonization of other star systems.

Once we colonize the solar system with space habitats, and living in those habitats are the norm, travel to the nearest star would just be like a normal life.

No planets are necessary for colonization of the galaxy once we live in space habitats.

I honestly think that underestimating human perseverance and technology will not age any better than early scientists who were sure flight would never be possible or that trips to the moon are a crazy fantasy.

The things I am suggesting are not impossible, they may seem unlikely from our current goals but goals change and space habitats are possible. Development of space habitats will change our zeitgeist, living in space will allow us to expand into the solar system in the trillions, living on millions of space habitats.

First, as @KJW points out, the opening post is purely concerned with the physics of propulsion (and deceleration), which apply regardless of what is transported.

Second, My comment about “”vanishingly unlikely” was a response to you suggesting the point of the exchanges on this thread could be to account for some aspect of current UAPs. In that context “colonisation” can be dismissed, since we have not been colonised. That leaves only reconnaissance. Which does require communicating findings back to the sender.

There is a vast difference between the possibility of interstellar travel, and the advantages/reasoning for interstellar travel.

We are all in agreement that at our current level of technology we can barely do interplanetary probing/travel, but that doesn't discount the possibility of doing so in the future, however, even if technology does advance to the point where we can, the reasons and advantages to interstellar travel may never be worth it.

This assumes, of course, that advanced alien civilizations, if they exist, reason, and have value scales, similar to ours.

On 6/19/2026 at 6:30 PM, swansont said:

The issue with discussions based on imagined physics is where the line is drawn. ...

OK, we don't discuss imagined physics, but we should acknowledge that there are many things we don't really understand ... and this should change/improve with time, especially in thousands or even billions of years. Also the technology would advance a lot.

On 6/20/2026 at 3:38 AM, TheVat said:

I was asking in regard to his saying "collimate" light from the sun. Am I mistaken that you need coherent light to succeed in collimating over any distance we're discussing? I may have misunderstood @DanMP in thinking that he was implying some way to make the sun emit coherent light.

What I mean is to use the Sun in order to propel the starship. We can do that by orienting & focusing large mirrors, in orbit near the Sun, towards the ship light sails. I also mentioned a mirror that would both protect and propel the starship.

A variation would be to use laser beams from solar power stations orbiting near the Sun.

On 6/19/2026 at 6:35 PM, TheVat said:

Good thing the aliens have no ethical qualms about that. Get some women in a room and ask for a show of hands on who wants to donate or sell their future child to be raised by robots many light-years away. I'll bring popcorn.

Men are donating sperm. Why women cannot donate/sell eggs? For a good price they would be happy to do it. Especially if there are signs/concerns that the Sun is dying or a supernova or some other catastrophic event would wipe out the civilization. To have your DNA selected for a colony meant to preserve the species and all its knowledge (in a database) would be, I think, desirable.

Also, in millions of years, we may be able to 3D print the embryos, plus animal embryos, seeds, etc.. So we only send the database and few sophisticated devices, plus robots, etc..

And the embryos would get to be transferred into an artificial womb only after the robots would build a safe environment, with air, water, food, etc..

On 6/19/2026 at 10:26 PM, MigL said:

The biggest issue with these multi-century colonization schemes is that they cost a lot, and there is zero return to the people who finance it, but have to remain behind.

It is small consolation to us on Earth, that, after having spent Trillions and Trillions of dollars, we may have a colony of humans on a distant planet, if they managed to survive, who we will never see, and we may not be able to communicate with within a human lifespan.

So what is the cost to benefit ratio ?

In my proposal, with the use of lasers from solar power stations orbiting close to the Sun, those power stations should be already deployed as a better way to produce/harvest energy. The lasers would be used to transport the energy to/near the Earth. This would be a miniature Dyson sphere.

To use, briefly, part of this power stations in order to propel a ship meant to preserve the species and all its knowledge from annihilation would be, I think, acceptable.

I mentioned such power stations before, when we discussed how to deflect an incoming asteroid. With such a power station near the asteroid, receiving energy, through laser beams, from the ones near the Sun, we may deflect the asteroid.

I proposed this scenario as a way to explain alien presence on/near Earth. It is not very probable, but it is possible.

If the payload is still too big, an inteligent species might send a LUCA (Last Universal Common Ancestor) in order to seed a planet with the kind of life they originated. So, we may be the aliens ...

11 minutes ago, DanMP said:

Men are donating sperm. Why women cannot donate/sell eggs? For a good price they would be happy to do it. Especially if there are signs/concerns that the Sun is dying or a supernova or some other catastrophic event would wipe out the civilization. To have your DNA selected for a colony meant to preserve the species and all its knowledge (in a database) would be, I think, desirable

Fair point. While many might have qualms about children raised by robots, a civilization survival scenario would really raise the stakes. If it's either robot-raised first generation or species extinction then, yes, donors would set their reservations aside. (though I can see a lot of potential conflicts over whose DNA is selected and how priority is given to maximal genetic diversity for a starter population in a colony)

In any case, the thread topic is on the possibility of aliens visiting here. Whatever the nature of UAPs, they don't seem to colonizing Earth. (and we would be a poor choice for a colony - imagine if we humans went looking for a terrestrial planet to colonize; overpopulated, polluted, bristling with nukes, and constantly having wars would be our last choice)

On 5/30/2026 at 1:20 PM, studiot said:

Well it's your thread so I will bow out with the observation that you are limiting the scope.

For instance why is it necessary to leave ?

The Founding Fathers of New England had nothing to go back to and did not expect to leave so what would be wrong with a one way journey ?

Further why should the journey not outlast the lifespan of those who started, if proper provision for continuity was made ?

Everyone seems to be discounting slow, but slow could be goog, given these details.

I agree, slow is the key, and IMHO planets would not be the point, this makes it much easier to colonise another star system.

To me this discussion is assuming quite a bit too much. If any star is the destination instead of only star systems with a habitable planet the possibilities and the practicality for slow travel rises significantly.

Once a space habitat reaches its destination, let's say 400 years to the alpha centauri system, then 500 years to get to the pointy of being able to launch two colony ships to the nearest stars to alpha centauri and so on. Let's say our numbers double every 2000 years. how long until we occupy all the stars within 100 light years from earth.

Also I can see the possibility of launching colonies like multistage rockets from earth. The first stage accelerates the colony to it's speed then another stage, slightly smaller decelerates the space colony. The two stages are used and discarded... The stages would be much larger than the cargo but still within the scope of technology?

It's really difficult to predict what future tech will be able to do but I remember reading how one of the original problems with attaining Earth orbit was that the rocket would have to be gigantic to even reach orbit but staging the rockets solved that problem.

19 minutes ago, TheVat said:

Fair point. While many might have qualms about children raised by robots, a civilization survival scenario would really raise the stakes. If it's either robot-raised first generation or species extinction then, yes, donors would set their reservations aside. (though I can see a lot of potential conflicts over whose DNA is selected and how priority is given to maximal genetic diversity for a starter population in a colony)

If... huge word, if robots raise the children, and I have trouble seeing how much worse they could do than we already do, I can see how how this could work.

19 minutes ago, TheVat said:

In any case, the thread topic is on the possibility of aliens visiting here. Whatever the nature of UAPs, they don't seem to colonizing Earth. (and we would be a poor choice for a colony - imagine if we humans went looking for a terrestrial planet to colonize; overpopulated, polluted, bristling with nukes, and constantly having wars would be our last choice)

Hence my idea of colonising the Kuiper belt of oort cloud with space habitats, they would want to keep track of any planet in the planetary system they are colonising. So... they visit and screw with us in improbable ways for no apparent reason... lol

Robert L. Forward proposed a laser propulsion system that would both accelerate and decelerate the spacecraft from lasers in our solar system.

Rocheworld

1 hour ago, Moontanman said:

To me this discussion is assuming quite a bit too much.

One of the arguments presented is "Why bother ?"

Well that has a point if the 'planet' approached is like Pluto, which is now considered as equal top of the most likely to have developed or be developing life, following the discoveries of the new Horizons spaceprobe.

But other than the satisfaction of curiosity what does Pluto have to offer humans ?

But on the other hand we could argue "What if leaving Earth is forced ?"

Scifiction has plenty of stories of aliens forced to leave their world for a variety of reasons.

1 hour ago, studiot said:

One of the arguments presented is "Why bother ?"

Well that has a point if the 'planet' approached is like Pluto, which is now considered as equal top of the most likely to have developed or be developing life, following the discoveries of the new Horizons spaceprobe.

But other than the satisfaction of curiosity what does Pluto have to offer humans ?

But on the other hand we could argue "What if leaving Earth is forced ?"

Scifiction has plenty of stories of aliens forced to leave their world for a variety of reasons.

More along the lines of debri is valuable, planets not so much.

5 hours ago, DanMP said:

What I mean is to use the Sun in order to propel the starship. We can do that by orienting & focusing large mirrors, in orbit near the Sun, towards the ship light sails. I also mentioned a mirror that would both protect and propel the starship.

A variation would be to use laser beams from solar power stations orbiting near the Sun.

The limitation is that reflective surfaces are not 100% reflective, and the high power needed for any appreciable acceleration makes absorptive heating a huge obstacle

1 hour ago, Moontanman said:

More along the lines of debri is valuable, planets not so much.

I don't follow how this is a response to either of my separate points ?

5 hours ago, DanMP said:

OK, we don't discuss imagined physics, but we should acknowledge that there are many things we don't really understand ... and this should change/improve with time, especially in thousands or even billions of years. Also the technology would advance a lot.

What I mean is to use the Sun in order to propel the starship. We can do that by orienting & focusing large mirrors, in orbit near the Sun, towards the ship light sails. I also mentioned a mirror that would both protect and propel the starship.

A variation would be to use laser beams from solar power stations orbiting near the Sun.

How would you decelerate the spacecraft at its intended destination?

1 hour ago, exchemist said:

How would you decelerate the spacecraft at its intended destination?

From the Rocheworld article in wiki: To catch the energy, Forward uses a 1,000-km-diameter circular aluminum sail. The sail resembles a flattened disk with a 300-km-diameter removable center portion. When traveling to Rocheworld, the entire sail is used. When the ship needs to decelerate, the smaller sail is separated from the larger outer sail. The large sail is used as a reflecting lens, focusing light onto the smaller sail, slowing the craft. (end quote)

Aside from the absorptive heating issues, I am unclear how this double-reflection gimmick would be sufficient delta-v. I recall Forward as the sort of SF writer to really thrash out all the RW science issues, so maybe reading the novel would tell me more. Maybe there's other shipboard mass that gets dumped.

In the book Forward says something about changing the wavelength of the laser after separation, Forward is a physicist and an aerospace engineer., somewhat know for his books scientific accuracy.

5 hours ago, studiot said:

One of the arguments presented is "Why bother ?"

But other than the satisfaction of curiosity what does Pluto have to offer humans ?

Why bother? Good question and Pluto?

My idea is that debries like the kuiper belt or the oort cloud, asteroids would be used, large planets would be avoided and the debri would be used to build space habitats, millions of space habitats could be built with just the small objects, ice, carbon, metals, all are there with no gravity well to deal with. it would no doubt take thousands if not hundreds of thousands of years to exhaust the resources of a planetary system. Space habitats, once they become the norm and people are used to living in them, using them to travel to the nearest star would be much like taking your home with you. ie slow boats!

Using resources available in system to reach out to any and all stars becomes more of a lifestyle than an adventure.

The idea is that what to do with pluto becomes moot... unless you can break it up for use but breaking up even a small planet like pluto might be too much trouble.

Edited Monday at 11:53 PM1 day by Moontanman