Very persuasive article explains why space travel is impossible. The main reason is gravity. Written by Allan Milne Lees; I found it on Medium.

Allan Milne Lees5 days ago·7 min read

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Despite the populist hype of billionaire Sci-Fi fanboys and a perpetual stream of Hollywood entertainments to the contrary, humans will never explore the galaxy in person. In fact, we won’t even explore our own solar system up close and personal. This is not merely because robotic missions can do the job 1,000% better for 1/1000th the cost. It’s because of two fundamental biological reasons.

The first is gravity. Everything about our bodies is evolved to function under a gravitational acceleration at sea level of approximately 9.8 meters per second squared (9.8m²). Our hearts pump blood up to our heads, fighting gravity every centimeter of the way. Our muscles and bones are as strong as they are because every part of our bodies is fighting gravity every moment of our lives. Our sense of balance, which orients us spatially, depends on gravity being constant in one direction only: straight down.

Without gravity, very bad things happen: the heart pumps too much blood to the head and too little to the lower extremities, leading to ocular distortions, crushing headaches, and nausea as the inner ear loses all sense of up and down. Our bones and muscles atrophy dramatically, even when hours each day are dedicated to exercises specifically designed with the intention of slowing down this decay. Put simply, our bodies are incapable of handling microgravity and despite the pictures of smiling astronauts merrily enjoying microgravity on the ISS, the harsh reality is that every single one of those astronauts pays a price very few of us would wish to incur.

The Sci-Fi fanboy response to this fundamental problem is either (a) to ignore it entirely, as per Musk and Bezos, or (b) claim that artificial gravity is the answer.

As Musk and Bezos are ignoring the problem we can likewise ignore them. So what about artificial gravity?

There are only two ways to create artificial gravity. The first is called “constant-g” which means that we accelerate our hypothetical space ship at a constant 9.8m² for the first half of the trip and then flip it around and decelerate it at a constant 9.8m² for the second half of the trip. Einstein’s insight that over areas too small to experience tidal effects such acceleration would be indistinguishable from regular gravity means that in theory Earth-style gravity could be induced in such a manner. Better yet, because the acceleration is constant, relativistic speeds will eventually be attained. In just 12 years (in the reference frame of the spacecraft) we could travel across our Milky Way galaxy. In a single human lifetime (in the reference frame of the spacecraft), under constant acceleration, we could reach the edge of the universe that’s observable from Earth. An Earth upon which, in that frame of reference, billions of years would have passed.

So with constant acceleration we get a “twofer.” Earth-identical gravity and the ability to traverse vast distances within a human lifetime. Problem solved!

Except that there is no way, theoretical or otherwise, to achieve constant acceleration of this magnitude. No propulsion mechanism, theoretical or otherwise, can overcome the problem of mass. In order to power the continual acceleration, our imaginary space ship is constrained by Newton’s observation that any action in a vacuum requires an equal and opposite reaction. In other words, to accelerate a mass of X by some amount of velocity we will need to discharge an equivalent amount of energy in the opposite direction. And that energy can only come from fuel. Which adds to the mass of our space ship. So now we need to expend more energy, which means we need more fuel, which means we’re now carrying even more mass, which means we need to expend even more energy, which means…

In other words, even with some imaginary technology that could convert matter into energy with 100% efficiency, there’s simply no way to get to 9.8m² constant acceleration for any meaningful amount of time. Sure, we can talk about things like an Alcubierre drive but then we’re just as entitled to say that Hogwarts will invent the Spaciamus drive to solve our problem instead. In other words, running off to hide inside imaginary “solutions” is no solution at all.

If constant acceleration can’t provide artificial gravity, what about centrifugal force? We all remember the rotating space station in 2001 A Space Odyssey and everyone knows that this was the only Sci-Fi movie ever to have utilized a science-based series of technologies. Plus, it’s easy to find on the Internet lots of schemes to create artificial gravity in this way, from tethering ships together and spinning them around a central axis to building enormous hollow rotating cylinders on the inside of which humans will experience Earth-like gravity. So, problem solved!

Except the movies and the Sci-Fi books mislead us, as is the way of popular entertainments.

First, the good news: if a person stood perfectly still and did not move in any way whatsoever, then centrifugal force could seem to mimic Earth-style gravity. Unfortunately, here’s the bad news: if they made any movement whatsoever, they would instantly be overcome by nausea and be disoriented.

Why is this? Imagine throwing a ball up into the air here on Earth. If you throw it straight up, it will come straight down, pulled by gravity toward the center of the Earth we’re standing on. But under conditions of “gravity” induced by centrifugal force, a ball thrown straight up will arc and fall away from the person who threw it because unlike here on Earth there’s a second force acting on the ball: centripetal force. As our inner ear orients us by means of reference to the constant downward force of gravity, this means that any movement at all — even something as minor as turning one’s head — would result in signals from the inner ear (responding to the centripetal force) jarring dramatically with the signals from our eyes. At best this would lead to our hypothetical human vomiting in a majestic arc; at worst it could render them incapable of any controlled movement whatsoever.

The diagrams below show the difference between gravity (or constant acceleration at 9.8m²) and a rotating object. On Earth there’s only one force acting on us: gravity. On our imaginary rotating artificial gravity environment there are two forces: centrifugal, and centripetal. And that makes all the difference in the world.

Perhaps this is why Bezos prefers to ignore the problem; it can’t be solved just by throwing money at it. As for Musk, he makes people with ADHD look like paragons of sustained concentration so he probably doesn’t even know the problem exists. But even if you don’t know a brick wall exists, it still kills you if you slam into it at 1,000 kilometers per hour.

Gravity, therefore, is one reason why human beings will never be a space-faring species. It’s also the reason why it’s highly unlikely any other species capable of developing suitable technologies would ever become space-faring either. All organisms are highly adapted to the environments in which they evolve and it is extremely difficult to sustain organisms outside of their natural environments for any significant period of time. Add it the problems of solar radiation, the deleterious effects of microgravity, and everything else associated with space travel and it’s apparent that Sci-Fi fanboy dreams are a very poor guide to the future.

There is a second major reason why we humans will never be a space-faring species: psychology.

Our brains are as much the result of selection pressures as our bodies. Like our bodies, our brains are highly adapted to life on Earth. As a primate group species adapted to foraging, we’re not well-suited to being cooped up in tiny cages. We become obese and we develop all manner of mental problems. Without access to natural cues like water and grass and trees, we become stressed. When forced to interact with the same small group of people for years without respite, we become irrational and angry, or conversely withdrawn and depressed. Worse still, our emotional hardwiring makes us competitive even when cooperation is the optimal strategy, and our intellectual limitations lead us to acquiring and then strongly defending irrational and harmful beliefs.

Imagine, therefore, a space ship upon which 200 hapless humans attempt to exist for years or even decades. Instead of looking to Star Trek as our inspiration, a more probable vision is depicted in One Flew Over The Cuckoo’s Nest or perhaps the concluding episodes of some trash reality TV show.

It is difficult to imagine any species capable of making spacecraft not having equivalent psychological limitations, albeit likely somewhat different from those that control our own behaviors.

There are many other reasons why humans will never spread across the galaxy, but these two should suffice to prove the contention. This does not mean, however, that there won’t be money to be made in enabling space tourism. A few days in microgravity, ensconced in a modestly comfortable environment with a small number of others, could be a very congenial way for the wealthy to break up the monotony of holidaying in the Hamptons or on a private island in the Bahamas. Sheltered in low orbit by the Earth’s magnetic field, the dangers of solar radiation are reduced to a perfectly acceptable level and likely no worse than a dozen trips in a private jet. Microgravity sex will no doubt become this century’s equivalent of the Mile High Club that was so popular among the early jet-setters of the 1960s and 1970s.

But beyond a few amusing days spent orbiting the Earth while watching one’s champagne bubble around one’s head, and after the inevitable disaster of Mars Colony One, we will accept the fact that robotic missions are the real future. And then we will expand our knowledge of the universe exponentially instead of wasting hundreds of billions of dollars on futile dead-end fanboy dreams.

A few days ago, we silvered an 8” diameter 43” FL mirror that had previously been aluminized, and applied the Angel Guard coating.

We did a Ronchi test and some Foucault-Couder knife edge tests before stripping the aluminum and after the silver was applied.

To my amazement, we found that the mirror’s figure was about the same in both cases. How that works, especially how the Angel Guard coating is laid down so even and smooth over the entire mirror, is beyond me. But it DOES work.

Prior figure (aluminized mirror), seen with Ronchi grating of 100 lines per inch: https://share.icloud.com/photos/0LyqGC35cx0QfWKcd08aI0vzw…

Final figure (silvered mirror), with same Ronchi grating:

https://share.icloud.com/photos/03mdTeZD3dZ-bqGNoDZXSqpjw…

This is a video of us washing off the Angel Guard coating.

Here is a video of the finished mirror after drying. Notice that the very edge of this mirror did not take the silver coating, but the area uncoated is probably on the order of one or two percent of the total area.

Ive been doing the aluminization process for telescope mirrors at the NCA ATM workshop with a 55-year-old military surplus aluminizer at a DC rec center for about 20 years. (I’ve had a lot of help!!) This involves high vacuum, a noisy pump, voltage both very high and very low, and quite a lot of time.

Today, I had the opportunity to silver a random piece of glass, in my driveway, with the aid of another longtime ATMer and some chemicals from Angel Gilding. I had seen this demonstrated at Stellafane by Howard Banich and Peter Pekurar in 2019.

Doing it myself was quite eye-opening.

Here’s what I wrote on FB:

Success with our first attempt at silvering a piece of glass under a tent canopy, and then stripping off the silver quickly and easily with PCB etchant (FeCl?).

I’ve aluminized many mirrors with the NCA’s vacuum chamber, using a modified version of John Strong’s method from the 1930s.

I must admit that this method was faster, easier, quieter, and much more low-tech, compared to depositing aluminum. In the latter case, sometimes you have to wait an hour or more for the dual-stage vacuum chamber (the primary pump is VERY noisy!!) to finish all the preparatory steps and pump down low enough that a hot atom of gaseous aluminum can travel two or three feet before striking any other remaining air molecule! (That is one hell of a vacuum!)

With the silvering process, you can do any size mirror you can fit on your cleaning jig — and you can make it out of pieces of scrap wood, a few nylon chair legs, two old hinges, and some screws! !

With our NCA-ATM-CCCC aluminizer, we are limited to 12.5” max. I’m currently working on a 16.5” thin Pyrex mirror; the price I’m quoted for aluminizing it is about $600 at Majestic Coatings, which is about three times what I paid for the blank!! And that doesn’t even include shipping!

Today Alan T and I tested the silvering process in my driveway using the screen tent canopy that we use at the ATM workshop to stop dust particles from landing on mirrors that are being polished. (After getting permission to go retrieve the canopy from the Covid-closed rec center, we immediately went into the parking lot to hose off a decade of dust!!)

We unfortunately do not have Angel-Guard overcoating on hand. It should arrive Wednesday. As most folks know, bare silver, unlike bare aluminum, tarnishes very quickly (in weeks or months) when exposed to ordinary air, whereas a I have seen many bare Aluminum layers last a decade. This overcoating is said to extend the life of the coating to about a year, but obviously conditions will vary.

We used a 6″ float glass mirror blank to try out the process today – not an actual, parabolized mirror.

How does it work?

This is basically a five step process:

1. Get prepared and mix the tinning solution afresh;

2. Clean off the mirror properly with precipitated CaCO3 and/or Alconox; rinse;

3. Sensitize the mirror with an invisible layer of tin (Sn); rinse;

4. Spray on the silver solution and its reducer at the exact same time with two separate brand-new one-pint hand squirt bottles, until fully silvered & shiny; rinse;

5. Spray on the Angel-Guard overcoat; rinse; dry.

The amount of chemicals used is minimal. The nastiest stuff was the reducer. I’m glad we did this outside.

Btw: a number of people have bench-tested mirrors before and after this process. Some report no change in figure; somebody I trust, who has a Zygo interferometer, says there is a little degradation, but not much: a mirror that was 1/10 lambda (excellent) might go to 1/4 lambda, which is certainly usable for a big Dob, iirc.

And it’s cheap! And fast! And easy! And quiet!

We were able to fully and completely strip the brand new silver off with the PCB etchant in under 3 minutes. Aluminum takes much longer.

Thanks to: Léon Foucault; Steinheil; Howard Banich; Peter Pekurar; Angel Gilding; Alan Tarica; and my wife, Gail.

Can you do 994 times 997 in your head?

‘Shruti” will explain how to do it, but she doesn’t explain why it works:

Personally, I had never come across this one in my over 40 years of teaching math. She is correct, it does work, but WHY?

I’ll try to explain.

994 is 6 away from 1000, or 10^3 minus 6.

And 997 is 3 away from 1000, or 10^3 minus 3.

If we multiply (10^3 – 6) by (10^3 – 3), we get 10^6 – (6+3)*10^3 + 6*3

So as she explains, we count down from a million by nine thousand, and then we add on 18.

or, if we take away any small number from a thousand, say, a, and subtract any other small number b from a thousand, you have a situation like this if you use the area model for multiplication: