# Why haven't aliens contacted Earth? New Fermi Paradox analysis suggests we're not that interesting yet

Page 2 - Seeking answers about space? Join the Space community: the premier source of space exploration, innovation, and astronomy news, chronicling (and celebrating) humanity's ongoing expansion across the final frontier.

#### rod

Artemis would have to be re-engineered for a planet with a higher escape velocity than ours. This would entail reducing the payload mass, increasing the combustion chamber pressure or adding stages.

I just depends upon what the force of gravity is on the planet and how much weight you want to lift off.

A combustion chamber like ours, with 1,000 PSI inside it and, say, a ten square foot surface at the top has an upward force of 1,440,000 pounds. If the weight of the rocket is more than 1,440,000 pounds it won't get off the pad. If the weight of the rocket is 720,000 pounds it will acellerate upwards at 32 ft/sec^2.
Okay, so presently the NASA SLS as designed will not work on an exoplanet with higher gravity and faster escape velocities than our planet. That is good to know because when discussing why E.T. phoning home has not buzzed our planet and said hello, perhaps what Unclear Engineer brought up should be part of the probability creation story equation too.

Another note about weight for rockets. I watched a number of NASA TV online shows about rockets. Something like 80-85% of their weight is the fuel supply needed.

So, summing up what Unclear Engineer mentioned that jarred my old brain, our current rocket programs in use since 1957 through present, likely would not work on an exoplanet with higher surface gravity and greater escape velocities.

Last edited:

#### billslugg

You are correct, our current rockets would not work on a planet with a higher escape velocity. If launched as is, they would fall short of the orbit they were designed for. A redesign would be called for. If, for example, the gravity was 10% higher, the payload would have to be cut by 10%.
The limit is based on how much pressure the combustion chamber can handle. I think it would be reasonable to think that payloads could be put into orbit with an escape velocity twice that of Earth, using the type of technology we have. Might have to go to 4 stages but it could be done.

What is the ultimate limit for chemical propulsion? Current combustion chambers are made of steel at around 100,000 psi. Carbon fiber might get you 2,000,000 psi. Twenty times the payload at Earth gravity as current rockets or same payload at twenty times the force of gravity. (Numbers are "order of magnitude" only)

#### rod

You are correct, our current rockets would not work on a planet with a higher escape velocity. If launched as is, they would fall short of the orbit they were designed for. A redesign would be called for. If, for example, the gravity was 10% higher, the payload would have to be cut by 10%.
The limit is based on how much pressure the combustion chamber can handle. I think it would be reasonable to think that payloads could be put into orbit with an escape velocity twice that of Earth, using the type of technology we have. Might have to go to 4 stages but it could be done.

What is the ultimate limit for chemical propulsion? Current combustion chambers are made of steel at around 100,000 psi. Carbon fiber might get you 2,000,000 psi. Twenty times the payload at Earth gravity as current rockets or same payload at twenty times the force of gravity. (Numbers are "order of magnitude" only)

I think about it like this. On Earth, 100 lbs. weighs 100 lbs. because of 1 g surface gravity. On an exoplanet with 2 or 3 g, that 100 lbs., now 200-300 lbs. NASA lifting off Artemis on an exoplanet with 2 g, Artemis weighs 2x as much as it does on Earth. On an exoplanet with 20 g, that 100 lbs. weights 20x more now or 2,000 lbs. Liftoff and escape velocities change quickly when exploring exoplanets and for any postulated E.T.s phoning home

#### rod

This report may be interesting for some readers.

Astronomers scanned 12 planets for alien signals while they were in front of their stars, https://phys.org/news/2022-12-astronomers-scanned-planets-alien-front.html

The ref paper link is 18 pages. https://arxiv.org/pdf/2212.05137.pdf

"Table 1. Stellar and planetary properties for the twelve transiting Kepler planets observed in this work. We use the properties from Morton et al. (2016) for all planets, except Kepler-446b which we pull from Muirhead et al. (2015)."

Some of these look-like large exoplanets, Kepler-723 b. Others with radii larger than earth and thus likely more massive exoplanets with higher surface gravities and higher escape velocities.

#### JamesK

The Fermi Paradox always bugged me.

"If life is so easy, someone from somewhere must have come calling by now." That's the Femi paradox in a nutshell, but is that really true?

I look at it this way. Yes, there are billions and billions galaxies (maybe a trillion), and each galaxy has billions and billions of stars, so the likelihood of intelligent life elsewhere seems undeniable. There could be billions of advanced civilizations in the universe.

However, if we focus on just the stars within 1,000 light years of us, then it's quite possible that we are totally alone. There is simply nobody in a reasonable distance to talk to or visit us. I picked the 1,000 light year distance because assuming we had Star Trek Voyager technology, those are the stars we could actually visit within a reasonable time-frame (yes, sort of silly, but it is a good point of reference).

The universe might be teeming with life, but in our cosmic neighborhood (possibly within 50,000 light years), we could be totally alone. The sheer distances are the reason why someone from somewhere hasn't come calling yet.

Last edited:

#### billslugg

Yes, the sheer distances are probably why we have never been visited. To shorten the travel time in accordance with a human lifespan requires enormous amounts of energy. There is no known source that could provide it. Aliens who travel between stars would need lifespans on the order of thousands of years. It is possible, I suppose, but not very likely. Also would need to learn to prevent damage to themselves from cosmic rays. Another very difficult problem to solve. Thus, very, very few civilizations could be capapble of interstellar travel.

#### Helio

Bill, don't we need to add momentum conservation to your pressure differential in the tank?

The velocity of the ignited fuel becomes very important.

#### billslugg

Momentum is conserved in a rocket. The mass of the propelled gasses times their velocity is equal to the mass of the rocket times its velocity.

The combustion chamber is an example of Bernoulli's law. The sum of the static head and velocity head in a fluid is constant. At the top of the chamber the gasses are stagnant but with high static pressure. At the nozzle end the gasses have high kinetic head but no static pressure.

#### Helio

The combustion chamber is an example of Bernoulli's law. The sum of the static head and velocity head in a fluid is constant. At the top of the chamber the gasses are stagnant but with high static pressure. At the nozzle end the gasses have high kinetic head but no static pressure.
Yes, this gives it a forward push.

Momentum is conserved in a rocket. The mass of the propelled gasses times their velocity is equal to the mass of the rocket times its velocity.
Right, this is the part that needs added to the net thrust. M1v1 = M2v2.

If M1 is the rocket fuel mass and it is given, over time, a velocity, v1; then the velocity of the ship (mass of M2) will have a velocity of:
v2=M1v1/M2, IIRC.

billslugg

#### billslugg

Yes, and here is another interesting fact. The center of mass of an isolated rocket and its exhaust gasses never moves. As the rocket arrives at a star 4 light years away, the exhaust gasses are located 4 light years in the opposite direction. The center of mass of that system is still located here in the Solar System.

Helio

#### Unclear Engineer

A few corrections on rocket engine performance are needed:

First, you can't calculate the thrust of a rocket engine simply by looking at the chamber pressure and the area of the chamber top exposed to it. You would need to integrate the pressure forces over the entire interior surfaces of the chamber and the exhaust nozzle. And, the exhaust at the end of the nozzle does not have zero static pressure. (That would require an infinitely long diverging exhaust nozzle.) If it is designed to run at a particular altitude, then it will be designed to have that same static pressure as the atmospheric pressure at that altitude. For rockets that start near sea level and go into orbit (or beyond), the exhaust static pressure is usually optimized to produce the largest time integral of the thrust through the powered part of that rocket's designed ascent path. So , at sea level, it is over-expanded, and the static pressure of the atmosphere is more than the static pressure of the exhaust at the end of the diverging nozzle. That pressure difference creates a diagonal shock wave to compress the exhaust gases as soon as they get out of the nozzle, which ends up over-compressing the exhaust gases once that conical shock meets iteslf at a point in the center of the exhaust stream and progesses back out to the edge. At that distance from the nozzle, the shock reflects back into the exhaust gases as an expansion fan (spreading group of waves) and when that gets to the center, it then over-expands the exhaust gases again as it progresses back out to the edge, where it makes the exhaust gas static pressure again lower than ambient atmospheric pressure. So the compression shock/ expansion fan pattern repeats, and what we see it that familiar diamond shock wave pattern in the exhaust gases. If you look at a rocket exhaust in space, you will note that this diamond shock pattern is not occuring in the "vacuum of space".

Next, while it is correct that a rocket and its propellant in free space will always maintain the same postion for their combined center of gravity, it is not so simple as saying that
As the rocket arrives at a star 4 light years away, the exhaust gasses are located 4 light years in the opposite direction.
The exhaust gases will be moving at different velocities with respect to the center of mass, depending on when they were expelled from the moving rocket vehicle. Initially, the first gas expelled will be traveling much faster than the rocket, so it will move father from the center of mass than the rocket (and remaining propellant it is still carrying) than the rocket does in a period of time. Unless that rocket ultimately exceeds the relative exhaust velocity of its gases, the first exhaust gases released will always be farther away from the center of mass than the rocket propelled vehicle that released them. And, the last released propellant gas might not be moving at all with respect to the center of gravity if the final speed of the rocket just reaches its exhaust gas relative velocity. Or, if the rocket's velocity with respect to the center of mass does eventually exceed its propellant relative exhaust velocity, then the last exhaust gas will actually be moving in the same direction as the rocket away from the center of mass.

Finally, I should point out that planets that are more massive than Earth may have atmospheres that are more dense and deeper that what we have here on Earth. If so, then rockets launced from the surface need to expend more propellant to overcome the higher atmospheric drag, and do so longer to get higher to reach the top of the atmosphere, and do those things while attaining an even higher orbital velocity due to the heavier planet. So, there are multiple compounding effects that need to be taken into account.

And, if the planet's atmosphere is always cloudy enough, intelligent life on its surface may not have seen the stars, at least not for a lot longer in their evolution than us humans.

Last edited:
Helio and billslugg

#### karlp295

We are basically hypothesizing without enough information. I believe, but don't know for sure, that aliens are either already here observing or that they visited in the past. If we take everything our governments say to us at face value and do not question, we deduce we have no signs of aliens yet. Perhaps this is simply not true.

I have no doubt in my mind that there must be life out there somewhere and if they have found us, they can remain hidden for as long as they wish. Or maybe they just don't want to bother with us. They could be billions of years ahead of us in technology, we may not be capable of detecting them.

On the other hand, any great filters that exist must take effect before that alien race can travel beyond their own planet and galaxy. It seems that somewhere in the vast distances of the Universe there must be enough opportunity for that to happen several times. The question of time and distance is all prevailing.

Not only do we need a race to survive and develop interstellar travel, we also need them to do it in a time period that enables us to contact them or vice versa.

We are just pondering... but I believe one day the evidence will be there for all to see.

#### Edward Coulter

<<Content removed by moderator>>

I saw the movie Contact. Why doesn't science present this evidence as factual and confirmed to the public showing *alien spacecraft* are here on Earth? In the movie Contact, this was widely proclaimed. I would think the entire scientific community would be onboard here, loudly proclaiming we now have confirmed alien intelligence outside of Earth and our solar system demonstrated by *intact* alien spacecraft. I use the Galileo standard of science. Galileo could show others the tiny lights moving around Jupiter using his telescope. I use my telescopes and more than 400 years later, can still see these tiny lights moving around Jupiter.

Applying this simple test to the aliens are here and visited leaving behind the evidence, should be no problem and show the entire world that this is true.
Did you really think the movie "CONTACT" was factual? How about Bugs Bunny then too?

It does not take much of a telescope to see the 4 Galilean moons moving about Jupiter. They are not aliens of course, but rather just moons and large ones too. Much larger than anyone would expect necessary to traverse the galaxy.

As a Scientist /Engineer I see no compelling reason to think that FTL travel is ever going to be possible. It is not just a matter of research time or wishful thinking but a matter of fundamental physics. Does the physics exist? If not we will never even reach the next star, nor vice-versa for any supposed extra-terrestrial inhabitants of other star systems. I am not one for suspended animation to pass the time. Who really is going to be willing to do that, even if it becomes possible? Once on that journey you will never see or speak to people you know again. You can't come back to the world you knew either. Such a journey would seem to be pointless as you are not going to colonize until you know it is possible and you can't find out if it is habitable until machinery makes the journey and reports back first.

#### Edward Coulter

A few corrections on rocket engine performance are needed:

First, you can't calculate the thrust of a rocket engine simply by looking at the chamber pressure and the area of the chamber top exposed to it. You would need to integrate the pressure forces over the entire interior surfaces of the chamber and the exhaust nozzle. And, the exhaust at the end of the nozzle does not have zero static pressure. (That would require an infinitely long diverging exhaust nozzle.) If it is designed to run at a particular altitude, then it will be designed to have that same static pressure as the atmospheric pressure at that altitude. For rockets that start near sea level and go into orbit (or beyond), the exhaust static pressure is usually optimized to produce the largest time integral of the thrust through the powered part of that rocket's designed ascent path. So , at sea level, it is over-expanded, and the static pressure of the atmosphere is more than the static pressure of the exhaust at the end of the diverging nozzle. That pressure difference creates a diagonal shock wave to compress the exhaust gases as soon as they get out of the nozzle, which ends up over-compressing the exhaust gases once that conical shock meets iteslf at a point in the center of the exhaust stream and progesses back out to the edge. At that distance from the nozzle, the shock reflects back into the exhaust gases as an expansion fan (spreading group of waves) and when that gets to the center, it then over-expands the exhaust gases again as it progresses back out to the edge, where it makes the exhaust gas static pressure again lower than ambient atmospheric pressure. So the compression shock/ expansion fan pattern repeats, and what we see it that familiar diamond shock wave pattern in the exhaust gases. If you look at a rocket exhaust in space, you will note that this diamond shock pattern is not occuring in the "vacuum of space".

Next, while it is correct that a rocket and its propellant in free space will always maintain the same postion for their combined center of gravity, it is not so simple as saying that

The exhaust gases will be moving at different velocities with respect to the center of mass, depending on when they were expelled from the moving rocket vehicle. Initially, the first gas expelled will be traveling much faster than the rocket, so it will move father from the center of mass than the rocket (and remaining propellant it is still carrying) than the rocket does in a period of time. Unless that rocket ultimately exceeds the relative exhaust velocity of its gases, the first exhaust gases released will always be farther away from the center of mass than the rocket propelled vehicle that released them. And, the last released propellant gas might not be moving at all with respect to the center of gravity if the final speed of the rocket just reaches its exhaust gas relative velocity. Or, if the rocket's velocity with respect to the center of mass does eventually exceed its propellant relative exhaust velocity, then the last exhaust gas will actually be moving in the same direction as the rocket away from the center of mass.

Finally, I should point out that planets that are more massive than Earth may have atmospheres that are more dense and deeper that what we have here on Earth. If so, then rockets launched from the surface need to expend more propellant to overcome the higher atmospheric drag, and do so longer to get higher to reach the top of the atmosphere, and do those things while attaining an even higher orbital velocity due to the heavier planet. So, there are multiple compounding effects that need to be taken into account.

And, if the planet's atmosphere is always cloudy enough, intelligent life on its surface may not have seen the stars, at least not for a lot longer in their evolution than us humans.
We are fortunate to have a large moon companion to help point the way to space travel. If our planet were only a little larger with a corresponding increase in atmospheric depth and pressure it would likely be impossible for humanity to get off the earth.

#### GM23

perhaps they are here and contacted earth, they just don't contact humans in mass fora bigger reason

#### Unclear Engineer

perhaps they are here and contacted earth, they just don't contact humans in mass fora bigger reason

Well, it's not possible to completely disprove a negative statement like "No space aliens have ever visited Earth."

There are certainly a significant number of people who have claimed to see aliens, and even be aliens, but credible investigations tend to find no objective evidence and those people are often categorized as crazy.

So, if you ever do have a flying sauce land near you and some obviously non-human life forms get out and "contact" you, would you tell anybody about it? What evidence would you hope to have that would override the overwhelming belief in the majority of the population that those who see space aliens and ghosts are crazy?

That situation makes it just a bit harder to prove that negative about space aliens never having contacted/visited Earth.

But, count me in that majority who believes it is highly unlikely that they have.

#### billslugg

You nailed it:

"...but credible investigations tend to find no objective evidence and those people are often categorized as crazy." Unclear Engineer

The proof of alien visitation is trivial. Simply obtain but a bit of matter brought here, such as a fingernail clipping, a matchbook cover or a paperclip, turn it over to any large scientific university and have them do a laser ablation mass spectrometry. The isotopic readout will conclusively determine the origin, be it Earth, elsewhere in the Solar System or any of the nearby stars. All locations have known, well documented isotopic signatures. Such signatures cannot faked practically*.

*One would need to commandeer an isotopic enrichment plant such as at Oak Ridge or Savannah River, then develop gaseous forms of each element, conduct long runs for each gas then convert the gas back into solid forms. The cost would be in trillions of dollars, involve thousands of people and take hundreds of years. This from a government that can't stop spam calls.

"But, Bill, half a million people kept the faked Moon landings secret."
"So, you believe in the Moon, huh?"

fermifan

#### Pogo

Even a used beer glass, the residual DNA should prove to be different enough.

billslugg

#### murgatroyd

Yes, the sheer distances are probably why we have never been visited. To shorten the travel time in accordance with a human lifespan requires enormous amounts of energy. There is no known source that could provide it. Aliens who travel between stars would need lifespans on the order of thousands of years. It is possible, I suppose, but not very likely. Also would need to learn to prevent damage to themselves from cosmic rays. Another very difficult problem to solve. Thus, very, very few civilizations could be capapble of interstellar travel.
I loved the Jetsons cartoons and Star Trek, but the lightspeed limit nixes interstellar travel for humans in our present form. However, I am optimistic that genetic engineering techniques will come up with a solution. Tardigrades are extremely resilient to cold and radiation. They also weigh next to nothing, reducing the propulsion requirements. One possibility worth exploring is to create tardigrades with some human DNA (for sentimental reasons to preserve the thread to humankind's ancestors, and because reusing genes coding for intelligence is easier than designing new ones from scratch) and the ability to form a hive mind linking together each individual specimen's limited (due to size) brainpower.

At the destination planet, thaw the tardigrade-human hybrids and let colonization begin. These hardy creatures do not require much terraforming to be done, and they should feel right at home in conditions that Humans 1.0 would find inhospitable.

#### billslugg

DNA cannot survive long periods due to radiation damage that comes from within. A key chemical is potassium, 0.012 percent of which is radioactive Potassium-40 with a half life of 1.25 billion years and undergoes beta decay with electron energies of 1.2 MeV which are very damaging.
No amount of shielding can fix this problem. The DNA would have to be made from potassium that had the radioactive component removed by isotopic separation.
Then there would be the problem of creating an artificial womb, and artificial babysitters, teachers, etc.
Better to send a bot. ChatGPT has already reached the point it can pass a medical entrance exam and the bar exam. Won't be much longer it is smarter than any human.

#### fermifan

I'm not sure we'll ever receive an answer to this paradox, as interesting as it is. I agree that there are more pressing matters in terms of space research and our developing knowledge of the Universe, but this paradox is an interesting thing to think about if you believe in intelligent life across other planets and solar systems as I do. The only way to answer this paradox (hence the word 'paradox') is for intelligent life to visit us and for us to ask them. Even then, that would only be a small piece of the puzzle.

But, even if we never get an answer (likely, considering there don't seem to be any active plans to seek out intelligent life), I still think this is worth discussing. It can open up theories about the value of our solar system, our planet, and the chances of our planet seeming to perfectly align with the criteria to create intelligent life.

And perhaps aliens have the same paradox too? Maybe they're just waiting for us to make first contact as we are. Maybe they're wondering if they're alone in the Universe, too. But that's just indulgent speculation on my end.

Replies
0
Views
1K
Replies
5
Views
5K
Replies
0
Views
2K