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Playing Tennis in a Space Station

Internal View of an O’Neill Cylinder

External View of an O’Neill Cylinder (Pair)

On the Tennis Court

What if you were to play tennis in outer space?

Imagine living in a space station and standing on a tennis court. Your feet remain firmly planted on the ground due to gravity. The court curves slightly upward on the left and right instead of being flat. It’s your serve and you strike the ball, visualizing its trajectory. May your ball land in the service box; otherwise you’ve committed a fault. The ball flies from your racket yet veers from the direction of your strike. What happened?

Welcome to the world of centrifugal gravity.

Becoming a Space Faring Civilization

In the future, humans may venture beyond planet Earth. Several reasons could drive us to do this. Reasons range from an expanding population requiring additional resources to escaping repression on Earth. Otherwise, why leave a ‘paradise’ home to which we had evolved to thrive within? This assumes we won’t ultimately destroy our paradise to become inhabitable. An unfortunate consequence of our technological capabilities is that we could make our world uninhabitable.

Existential risks abound. And let us hope none of these happen. We’d rather not destroy our Earth or society. Otherwise, we couldn’t enjoy playing sports such as tennis.

If a risk became reality, we would be forced to settle elsewhere. Alas, outer space is a deadly environment where we could not live without significant technological assistance. Other planets are also unsuitable for multitudes of reasons. Mars is the closest to being Earth-like—that we know of. Even there, we would need to build pressurized shelters with advanced life support systems. The costs to build a suitable infrastructure are extremely high, especially when considering the costs to deliver materials from Earth.

Alternatively, we could try a ‘live off the land’ approach as proposed by the Mars Society (Or also known as In Situ Resource Utilization.) Essentially, settlers could use local Martian resources and if required, one could mine additional resources from ‘nearby’ asteroids and our moon, where escaping Earth’s gravity wouldn’t be a factor.

Can we do this? Eventually yes but certainly not now.  It could take a century or centuries to build up the outer-space mining and manufacturing capability. Opinions vary, ranging from optimism by the Mars Society and the NSS to the less optimistic. For a sobering viewpoint, check out “A City on Mars.”

The Challenge of Non-Earth-Like Gravity

A big unknown for Mars settlement is to live in a lower gravity of one third of Earth’s. Experience with astronauts has shown that zero gravity can cause bone loss among other effects. And we do not yet know the impact of non Earth-like gravity. In particular, we do not know whether human procreation in partial or heavier gravity is possible. And without procreation, what future is there for tennis?

One prudent approach is to settle where gravity is similar to Earth’s. But few other planetary alternatives potentially exist. Being light years away, we do not know yet if other factors may rule out these alternatives. Besides, all of them have gravity differing considerably from Earth’s.

Why Would We Live in a Space Station?

If one rules out planetary possibilities, this leaves us with artificial possibilities. Since the early 20th century, some theorists have suggested space stations. The type we will focus on here is the O’Neill cylinder,

Welcome to the O’Neill Cylinder

This animation will give you an idea of the interior of an O-Neill cylinder. Certainly looks like one can build a tennis court in it.

A pair of O’Neill cylinder consists of two counter-rotating cylinders. The cylinders would rotate in opposite directions. Without the counter rotation, it would be difficult to obtain the sunlight required for power. And without power we cannot sustain a tennis-playing culture. For now, we’ll ignore alternative power sources, such as nuclear reactors. Besides, we need a sufficiently large population of people for sustaining a long-term society.

Generally each cylinder would be 6.4 kilometers (4 mi) or 8.0 kilometers (5 mi) in diameter and 32 kilometers (20 mi) long, connected at each end by a rod via a bearing system. Their rotation would provide artificial gravity.

Among other things, the large dimensions are necessary for reasonable gravity generation.  To understand how gravity is generated, we’ll need to consider some physics. Sorry about that.

Remember Newton? He told us that force causes mass to accelerate. More precisely, the amount of force on an object equals an object’s mass multiplied by its acceleration. Acceleration is not just speeding up or slowing down. An acceleration can also be a change in the direction of motion.

About Space Station Rotation

Guess what? Rotation is a change in direction. This means rotation is also acceleration. It turns out that acceleration causes one to experience a fictitious force, where the term fictitious force is a physics term referring to a force resulting from acceleration, instead of a force which causes acceleration.

If you’ve ridden a Graviton at a carnival, you’ve certainly felt pushed against the inside wall of the ride. Nothing fictitious about that feeling! That force is centrifugal force. Similarly when an O’Neill cylinder spins, a centrifugal force pushes you against its inner wall. The force feels like and sort-of acts like gravity pulling you to the ground.

The strength of centrifugal force increases with the radial distance from the axis of spin and with the square of angular velocity. You probably know from experience that spinning at high speeds can make you throw up. So how fast can people spin comfortably? (See answer #13)

According to some research, a revolving speed of one revolution per minute (RPM) can be sustained comfortably. This means that in order to achieve a force sufficient for Earth-like gravity, one needs a larger radial distance. Hence the size of the station must increase, for example a cylinder diameter of 6.4 km.

Although living in an O’Neill cyclinder would be like living in gravity, there are some differences.

The Weird World of Spinning

When you walk on the Earth, you don’t notice a force pushing you sideways. It exists though, but it is too small to notice. Whenever something moves on a rotating surface the object experiences a coriolis force. Because the Earth rotates, this force affects ocean currents and cyclones. The preceding link shows the coriolis force as acting sideways when one is standing on a spinning disk. When standing on the inner surface the coriolis force direction will depend on spin direction and on which direction the object moves within the cylinder.

This means a sufficiently fast tennis ball will experience a coriolis force, unless the ball moves parallel to the spin axis. As the ball’s angle of direction (away from parallel) increases, the coriolis force increases for any given ball speed. When the ball moves perpendicular to the spin axis, it will experience the maximum possible coriolis force for any given ball speed. 

If the velocity is parallel to the rotation axis, the Coriolis force is zero. For example, on Earth, this situation occurs for a body at the equator moving north or south relative to the Earth’s surface. Within an O’Neill cylinder, the spin axis runs along the centre of the cylinder. If one places the tennis court to run along the side, to make longer sides parallel to the spin axis, the coriolis force becomes zero if the ball travels purely parallel along the side of the cylinder. Making the ball fly at an angle however causes a non-zero coriolis force.

Simulation

Here is a simulation of a ball sent flying from within a spinning cylinder. Left click to shoot the ball. Press escape to stop the simulation. To shift the position of start location use one of W, S, A or D keys. Try to shoot the ball. Notice how the ball deflects.

You can also use the Tom Lechner simulation to see some of the effects of throwing a ball along the wall.  A tennis court should be meshed in completely during play. Otherwise some balls may escape to inward within the cylinder, or otherwise bounce within the cylinder.

For a more detailed look at coriolis force, including direction, refer to a UBC physics note which also describes the right hand rule. In summary, use your right-hand thumb to point along the axis of rotation. By physics convention, the positive rotation axis would be spinning counter clockwise. The index finger points in the direction of the object (ball in this case) velocity. The middle finger would point in the direction opposite of the coriolis force (due to a negative sign in the mathematical derivation).

For a spinning cylinder spinning counter clockwise, hitting the ball towards the end results in a coriolis force pointing left, resulting in a leftwards deflection.

Would the Coriolis Force be Noticeable?

When we walk on Earth, we don’t notice the coriolis force due to our slow speed relative to the large size and spin rate of the Earth. Would the effects be noticeable while playing tennis in a ‘typical’ O’Neill space station?

I did a crude calculation to ascertain this. I used a cylinder spinning at 0.6 revolutions per minute and with a diameter of 6 kilometers. I considered a player hitting the ball from corner to corner in a standard doubles court. Furthermore this person sent the ball flying at 225 kilometers per hour.

The resulting acceleration from the coriolis force would be about 0.14 g (g being acceleration due to gravity on Earth). After one second the ball would have diverted from a straight line of travel by about 70 centimeters. This would be noticeable.

I imagine a player would also notice the force pushing sidewise upon them while running about on the court, but I haven’t done any calculations in order to assess this.

Peter Spasov. Last updated Saturday October 04, 2025

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Nanites – or – There’s Plenty of Room at the Bottom

Nanite Attacking Cancer CellNanite Heart Surgery

What if ultra-small robots could crawl into your skin? Literally. Like crawling into the space between your cells? What if they can grab some atoms and rearrange them? What if each robot had the smarts of a contemporary smart phone and could replicate? Your hero gets ripped apart by laser fire, arteries torn and gaping wounds.

How could your hero save herself? Simple. Grab a tube and squirt. Out come the nanites, propelling themselves within the hero’s body. Repairing the damage.

Such is the stuff of seeming magic. Remember Clarke’s Law?

So-oh. How real can this be? Could it?

Hold on. Let’s first check out how science fiction already uses nanites.

Nanites in Science Fiction

If your hero needs a wonder of technology to get out a scrape, she or he (or it) could use nanites. (Or the hero could use magic.) These little beasties can spread around and do all sorts of stuff. Heal the injuries as mentioned before. They could infiltrate the impenetrable.

Let’s say the locks of the castle gate are made of tougher-than-diamond graphene and your hero doesn’t have the key. She needs to sneak in to save Mr. Lucy. The hero whips out a thimbleful of power, and behold, the nanites get to work. She flings the powder onto the lock.

The beasties grab on the carbon lattice which makes up graphene and tear the sucker apart. Atomic bond by atomic bond. Invisible to the naked eye they crawl from link to link of the carbon atomic lattice. Voilà, the lock disintegrates. Mr. Lucy is saved.

For more: see Nanotechnology in Fiction. Fiction writers tend to use the nanite word rather than nanotechnology, nano-robots or whatever. In the link, nanite appeared fifteen times, the last time I looked.

Let’s hear from the pioneer of nanotechnology.

Introducing Doctor Feynman

“Hi folks, I’m Richard Feynman. I used to work on the Manhattan Project. I had a blip role in the Oppenheimer movie. For a second or so, I played the bongos while all us scientists were watching Oppie’s first atomic bomb explosion. Later, I wrote ‘There’s Plenty of Room at the Bottom,’ suggesting folks directly manipulate atoms mechanically. Imagine swallowing the surgeon. I tell you, the surgeon will be a swarm of tiny-tiny bots healing you of injuries or disease. Sounds fantastic, doesn’t it?”

“Eh,” you may say. “Sounds too good to be true.”

“Hold on there, buckette or buckaroo. We’re just getting started. Wait until you hear what Drexler’s got to say.”

Drexler and the Engines of Creation

“Thank you for that introduction, Richard. Guilty as charged. I wrote ‘Engines of Creation.’ Hey, this work spun out the nanotechnolog gig. For instance:

  • Use carbon nanotubes to make smaller microchips.
  • Build better solar panels.
  • Attack cancer cells without harming the healthy.
  • Use nanofiltration to remove heavy metals from polluted water.
  • Make textiles that don’t stain or wrinkle.

“Sounds like the cat’s meow.”

“But beware the gray goo, … and the grey goo.”

“Eh, what’s that?”

“Click the links, Luke.”

“Fine, what next?”

Could Nanites actually exist?

Good question.

A Nobel Prize winner in Chemistry has questioned the practical feasibility of molecular assemblers as proposed by Drexler, leading to a big spat between the two parties. More a more in depth view, see https://www.hyle.org/journal/issues/10-2/bueno.htm. The argument generally pits mechanical robotic manipulation at the molecular scale versus chemical. Smalley states: “you don’t make a girl and a boy fall in love by pushing them together.” Alright, the secret to love is good chemistry.

This doesn’t mean molecular scale manipulation is impossible. Nature already does this by using biochemical reactions in living organisms. The question is whether mechanical systems can do so.

There appear to be limits to how much smaller can mechanical type systems can be miniaturized. Some suggest the solution lies in emulating portions of living beings. In nature, miniature organisms already operate effectively.

As far as I can tell, the debate is not yet settled. The Institute for Molecular Manufacturing (IMM) begs to differ. IMM argues that molecular assemblers and nanorobots are theoretically feasible.

The IMM refers also to another of Clarke’s Laws [internal link] (Italics are mine).

When a distinguished but elderly scientist states that something is possible, the scientist is almost certainly right. When the scientist states that something is impossible, the scientist is very probably wrong.

Shall we conclude that Nanites are possible, but may require (natural and/or synthetic) biological mechanisms as well? Or are purely electromechanical nanites handwavium and unobtainium?

For more, dive into the following … https://philosophy.institute/philosophy-of-technology/perspectives-on-nanotechnology-debate/ https://peterallenlab.com/2022/05/28/distractions-drexler-smalley/

And finally, what does nano mean?

A nanometer is tiny-tiny small. Line up three water molecules side and side and the length approaches one nanometer. Line up four, the length exceeds one nanometer. A nanometer is one millionth of a millimeter. Nano derives from the Greek nanos for dwarf. Nano means billionth, more precisely, a billionth of a meter. Nana, on the other hand, may refer to grandmother.

Peter Spasov. Last updated Tuesday April 01, 2025

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Magic and Clarke’s Third Law

What if an extraterrestrial could turn you into a sentient tree? Imagine how you might stand fixed upon a grassy knoll unable to move, always looking at the same place day after day, rain or shine (assuming you could see). Birds could roost upon you. Wind could rustle your leaves. Would you sense the birds and wind? How would your mind react during the day? Would you sleep at night? And dream? If so, what?

 “Sure thing,” you may say to me, “what have you been smoking? Tell me another fairy tale.”

How would you consider such a scenario? You may conclude: Must be fantasy, bro. It ain’t got the science to be science fiction.

Maybe so. But just wait a minute. Let’s say, you’re part of a group Neanderthals who managed to escape detection over thousands of years. Granted, this is a big stretch. Okay, let’s switch this around. Say time travel became possible, and you visited Europe at, say, 150,000 years ago. You’ve brought your e-bike, one of those fancy electrically powered mountain bikes some game hunters use such those blogged about here. Okay, I don’t hunt for sport but you get the idea. In this scenario you go for a ride. Eventually you encounter some Neanderthals. How would they regard your ability to ride around? Your appearance, your clothing, you riding your bike, would be a major shock, completely foreign to their world view. To the Neanderthals, you are magical.

This brings us to Clarke’s Third Law. Science fiction author Arthur C. Clarke wrote 2001: A Space Odyssey. The law states: Any sufficiently advanced technology is indistinguishable from magic. If you brought your mobile phone and played your favorite (downloaded) cat video, the Neanderthal might wonder at the technology and the content. She or he may think: what’s the deal with cozying up to these mini cats? Do these strange visitors need some trepanning with a sharp pointed stick?

For the sentient tree scenario, on the other hand, needs more thought. First, would a sentient tree be possible? Second, could a scientist transform a normal human into such a tree? Let’s tackle the first.

Some scientists claim that trees can communicate with others in the same species by sending certain chemicals from root to root. However, other scientists claim the evidence isn’t conclusive. This is how science works.  Test the evidence and if other ‘peers’ think the evidence isn’t strong enough, they remain skeptical. Regardless, such tree-to-tree communication wouldn’t be a sign of sentience. Similarly with the claims of plants being capable of feeling pain, the evidence remains dubious. So, for this post, I’ll suggest that in our real world, trees aren’t sentient. But could they become sentient?

Trees, being plants, are able to photosynthesize, meaning to convert sunlight into the chemical energy necessary to fuel their metabolism. As for food, trees must absorb water and other chemicals from the ground using their roots. The transport of nutrients means going uphill against gravity. Trees don’t have hearts like we do, to pump said nutrients throughout their body. Instead they depend on a transpiration process. Essentially, water evaporation at the leaves causes a negative pressure which can pull water up from all the way down to the roots. As an aside, how tall can trees grow on Earth? By the way, the tallest height appears to be about 140 meters.

Think of what this mean in terms of being sentient or not? Sentience requires brains as far as we know, and brains use lots of energy. The human brain uses about 20% percent of the body’s energy and animal bodies use more energy than for plants because they have higher metabolism requirements. From this, we can conclude that a sentient tree would need significantly more energy than the currently non-sentient versions, in addition to a brain equivalent.

According to my admittedly simplistic calculations, a human could employ an advanced solar panel on each arm of about 60 cm square. The human ‘tree’ could stand up to expose the panels like a tree would expose leaves on its branches. This could theoretically work provided the panel efficiency was seventy two percent or more. The hypothetical panels would act like artificial leaves in order to convert the sunlight into the human’s need for oxygen and food. This doesn’t yet exist, but I argue it could in the future.

As an aside, prototype artificial leaves already exist. In this case, the sole function of the ‘leaf’ is to take carbon dioxide out of the air in order to cut down on greenhouse gases—and produce fuel. This technology has promise and could potentially expand to increased efficiency and produce other organic molecules required by our hypothetical tree-human hybrid.

Therefore, it’s reasonable to suppose a sentient tree could theoretically exist provided it had a brain and a pump to feed the brain with energy nutrients. Perhaps the person could become a plant-like being akin to the Venus Fly Trap but in an expanded form. This obviously is highly speculative but I would argue clever biohackers could create such a tree equivalent from a human. Please note this is a thought experiment because of ethical considerations.

Whether such a tree could evolve via natural evolution is another matter, and probably not—unless animals were wiped out. Food for thought here for you speculative fiction writers who might be reading this. But I will leave this aside for now to move on to point two. Could an advanced extraterrestrial transform a normal human into such a tree?

Point two is a tall order. It is one thing to biohack someone in a lab over a period of time, it’s another to sprinkle pixie dust or whatever that over a few hours or days transforms a human into a tree without any other intervention. In the lab scenario, surgeons may need to operate on the human.

“Eh,” you may say. “What could be this pixie dust? Sounds like magic.”

“Aha,” I may reply, “remember Clarke’s third law. Use a technology so advanced that, today; our smartest scientists can’t even understand it.”  

“Cop-out,” you may say, “Pure handwavium. Sounds like unobtainium.”

“Well, er, um, you may be right.” I shrug my shoulders with embarrassment. Then the metaphorical bolt of lightning strikes, red hot, searing red hot. (Enough with the melodrama you exclaim.) “Hey. How about nanites?”

Oh yes, the nanite word. How many stories have I read which employed nanites? Many. Hence, let us examine the nanite. Click the preceding link. Alas, that’s all I’ll give you for now. Stay tuned for a possible follow-up post.

And for the diehards, who want to see my calculations, try the download button.

ViabilityOfSentientTree-1Download

Image Credits

This is a poster for Harry Potter and the Chamber of Secrets.
The poster art copyright is believed to belong to the distributor of the film, Warner Bros., the publisher of the film or the graphic artist.

https://img.buzzfeed.com/buzzfeed-static/static/2018-08/22/16/asset/buzzfeed-prod-web-06/sub-buzz-24234-1534969658-5.jpg

Cover of I am Groot vol 1 issue 1, 2017.

https://www.marvel.com/comics/issue/62828/i_am_groot_2017_1 https://upload.wikimedia.org/wikipedia/en/1/17/I_am_Groot_vol_1.jpeg

Peter Spasov. Last updated Tuesday February 25, 2025