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Zero Gravity Monorail

Because every generation needs an engineering project to get their teeth into.
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My parents' generation had all the excitement of the moon launches. The generation before that saw the car enter popular use and the change in landscape brought by massive roadbuilding programmes. Going back just a little further the Victorians built all sorts of things. No, really. What have I seen in my life: the growth of the Internet. I want something *big* and *physical* to look back on and the Millennium Dome just didn't cut it for me.

What I feel we need is a huge engineering project of the scale that seems impossible when you start and just plain great by the time you've finished. What I want is a Zero G monorail.

When you go over a hump-backed bridge a little fast you start lifting off the seat. That's the forces exerted by the way you are moving counteracting the force of gravity. Go too fast over a curved bridge and you'll take off. Pick your speed just right and the wheels will only just touch the ground.

The Zero G monorail works on exactly the same principle. Build a monorail all the way around the equator. Make it two it's always good to have a spare. (I know that's most of it's ocean but I did say that this was a *big* engineering challenge). Build an enclosing tunnel around the monorail and suck all of the air out (the train will be going *really* fast and we will need to cut out wind resistance).

Start the train off and keep accelerating. The faster you go the lighter the train will seem to get until, at a staggering 17,500mph, the train and everything on it appears to become weightless. Taa daa and viola. The Zero Gravity Monorail is born.

st3f, Nov 30 2001

The (Millennium) Clock http://www.longnow.org/
[phoenix, Nov 30 2001, last modified Oct 21 2004]

(?) Newton's apple: weightlessness http://www.ktca.org...ons/12/gravity.html
Weightlessness of objects in a free-falling plane and an orbiting craft. [pottedstu, Dec 10 2001, last modified Oct 21 2004]

(?) Force, mass and acceleration http://www.physics....venture/gravity.htm
Orbit = free fall = going round the earth fast enough. [pottedstu, Dec 10 2001, last modified Oct 21 2004]

(?) Misconceptions about satellites and weightlessness http://www.jal.cc.i...icki/ex_satell.html
There is no forward force and no centrifugal force operating on bodies in orbit. [pottedstu, Dec 10 2001, last modified Oct 21 2004]

(?) Misconceptions about inertia http://www.jal.cc.i...wicki/ex_inert.html
For completeness. [pottedstu, Dec 10 2001, last modified Oct 21 2004]

Lofstrom's Launch Loop http://www.launchloop.com/
A earth to orbit launch system megaproject. [bdh, Jul 14 2005]

Earth-Space Web Earth-Space_20Web
What you could build if you splatted people to the ceiling. [baconbrain, Aug 20 2006]

[link]






       I guess all the cocktails in the dining car would be served in sippy cups
bristolz, Nov 30 2001
  

       No, I'd rather if we were to waste all that money that we went to Mars or terraformed Venus or ended world poverty or something.   

       I take it that asking "why?" would be superfluous.
pottedstu, Nov 30 2001
  

       How about a 10,000 year project? (see link)
phoenix, Nov 30 2001
  

       the problem with a deadline like that is it is a bit hard to motivate yourself... I'll do it later, there is still plenty of time... an hour passes... I'll do it tommorow... etc. until you have to do a milleniums work the night before the deadline...
RobertKidney, Nov 30 2001
  

       Actually, everyone would be pasted to the ceiling from centrifugal force, if the track didn't collapse and fling them into orbit...
StarChaser, Dec 01 2001
  

       No it's not. You have to balance the cetrifugal force to exactly cacel gravity. It's just a question of picking the right speed.
DZ, Dec 03 2001
  

       Indeed: you're effectively orbitting the earth, in free fall just metres above ground.
pottedstu, Dec 03 2001
  

       Then little Johnny knocks a fly ball over the fence, hitting the tube. A weak weld (hey, it was Monday) gives way to microfissures. As air slowly leaks into the tube all the passengers die from Chernekov radiation and/or heat from friction.
phoenix, Dec 03 2001
  

       ooh, aren't you Mr The-Glass-Is-Half-Empty today!
lewisgirl, Dec 03 2001
  

       How're you going to get Cerenkov radiation? You're not going *that* fast. The possibility of air getting in is a worry, though. Bound to upset the path of the car and send you smashing into the walls of the tube.
pottedstu, Dec 03 2001
  

       Put it underground, then little johnny couldn't hit it with a hand grenade over the fence. Besides, I doubt the alloys involved would be damaged by a baseball.
salmon, Dec 05 2001
  

       I like this idea (though I initially thought it was going to be a through-the-earth evacutated-tunnel free-fall thingy, that wouldn't give the impression of zero-G though except for trips to the antipodes). But how do you get on and off, when the thing is travelling at just under 8 kilometers/second? (that's 17900 MPH or so).
wiml, Dec 06 2001
  

       And you'll still be splatted to the ceiling.
StarChaser, Dec 06 2001
  

       Yeah, but what a way to go ...
1percent, Dec 07 2001
  

       About wiml's anno, what exactly would you experience on a fall thru the centre of the earth? I seem to remember that a body dropped in such a tunnel oscillates in simple harmonic motion, but I've no idea how to tell if you'd feel gravity.   

       There seems a lot of confusion about gravity with other people in here as well. If you were flying round a ZGM at the right speed, you *would* be weightless because you'd effectively be in orbit, and you wouldn't be splatted to the ceiling -- is that because centrifugal force at the right speed would cancel out the force of gravity? But I can't find a formula for centrifugal force -- I think it's an illusory force (a property of the inertia of matter). Help!
pottedstu, Dec 07 2001
  

       You will have zero - g. You are effectively in orbit, and gravitational and centrifugal forces balance each other.
neelandan, Dec 07 2001
  

       You are correct [pottedstu]. Centrifugal force is illusionary and simply represents the one of the appearent effects of centripital force. And the effect wouldn't be constant. Occupants would occasionally have to make contact with the train to maintain velocity (and therefore weightlessness).
phoenix, Dec 07 2001
  

       This is off topic a bit, but if you did step into a shaft through the center of the earth, you'd initially be travelling "eastwards" at about 24,000 miles per hour. I can't figure out what force would diminish that "horizontal" component of your trajectory, and I wonder if you wouldn't eventually run into the eastern side wall of the shaft when you got to depths where the eastward rotation is considerably slower. Or, since the angular momentum would be converted to straight-line "horizontal" speed, would you end up contacting the western wall of the shaft first?
beauxeault, Dec 07 2001
  

       [StarChaser]: You're missing the entire point. You would be in freefall inside the train.   

       [pottedstu]: The formula for centripetal acceleration (centrifugal force's more pedantic name) is acceleration = velocity-squared / radius. So, if you plug in Earth's radius (6378000 meters) and acceleration due to gravity (9.8 meters/second^2) you get the velocity I posted, 7.9 km/ sec. Too slow for Cerenkov radiation, alas. But you'd make a nice fireball if someone let air into the tube.   

       [beauxeault]: I think you'd brush the side of the shaft on the way down and up (since you have to match velocity with the far surface, which is travelling in the opposite direction from the surface you're standing on, you'll need to have *some* sideways acceleration). Coriolis force, I guess. Put wheels on the side of the elevator car and/or coat the shaft with margarine.
wiml, Dec 08 2001
  

       I don't think you would be in freefall, because the train is not falling, it's accelerating. If you're moving at the same speed as the train, then you are at rest with respect to it, and gravity will work on you normally. If you are not moving at the same speed as the train, you are quickly going to be reduced to a thin smear along whatever vertical surfaces happen to impinge upon you first.   

       The Vomit Comet works because the plane is actually falling, and the people inside are thus free-falling. Gravity is working normally, but the support structure is now falling at the same speed as the people inside it.   

       The 'lightening' felt when going over a bump in a car is acceleration away from the direction of gravity; your ass has been swatted with enough energy to counteract gravity momentarily. Gravity still brings you right back down, even though the car is still moving. If you continued the motion, you would hit the roof of the car, or if it was a convertible and you weren't wearing a seatbelt, people with a loud truck would be asking you stupid questions.   

       The point I'm making, I guess, is that if you achieve escape velocity, you will attempt to escape. If something resists this <the track for the monorail, the seatbelts, etc> then the centrifugal force* will be felt as thrust away from the center, and splat.   

       *I feel the scientists are now saying that 'centrifugal force doesn't exist' for the same reason that it's no longer hyperactivity, it's ADD, except now that it's ADHD...that when people not of a profession begin using the same words as the professionals, the words change so the pros can look down on the out of date. So don't bother 'correcting' me about centrifugal force.
StarChaser, Dec 08 2001
  

       Thanks you all for the comments, fishies and the spelling correction (noted). Garry (sp?) Rhodes had a really good recipe for a fish pie last week. A couple more fish and I may give it a try. pottedstu, wiml, DZ thanks for putting the science out there even if you didn't all like the idea.   

       I hate being fishboned because people don't understand an idea (however silly) so I'll make an attempt at the physics on this (StarChaser - stop hissing and waving your tail about -- it's very distracting. UnaBubba - stop singing and put MammaBubba down - just for a moment.)   

       Firstly, I think that pottedstu said it all precicesly and clearly when he said, "you're effectively orbitting the earth, in free fall just metres above ground." That's absolutely it. I can't say it clearer or more concisely than that but I'll try to come at it from a different angle. (If anyone wants to take this offline please feel free to email me)   

       First things first: Teachers discourage students to think about centrifugal force because it only exists in a rotating frame of reference (which makes the maths very complicated). The maths for non-rotating frames of reference is *far* easier and centrifugal force does not exist in those frames of reference.   

       As an example to the above paragraph, picture a man sticking to the inside of a large rapidly spinning drum - the wall of death as found in some fun-fairs. From the inside (rotating frame) he can feel himself being pushed into the wall (centrifugal force). From the outside his friend sees the drum accelerate him but instead of letting him continue in a straight line the drum keeps pushing him into a circle (centripetal acceleration).   

       Having cleared that one up let's try an earth orbit. Imagine a 'plane with ridiculously powerful jet engine in it. The pilot is test flying it when suddenly the wings fall off. He is in flat, level flight when this happens. His engine is so powerful that if he ramps it up to maximum he will spiral out of the atmosphere. Instead he uses the curve of the earth to maintain his altitude. As the 'plane moves forwards (in a straight horizontal line if it weren't for gravity) the earth's surface drops away from him (because it's curved). If he gets his speed just right the earth drops away below him at exactly the same rate that he gets pulled in due to gravity and he flies around the earth in a perfect circle. He orbits.   

       Looking from the outside (non-rotational frame), the 'plane maintains a constant altitude without any lift - the 'plane and pilot are in free-fall. Looking from the inside (rotational frame) the pilot experiences a centrifugal force upwards exactly countering the force of gravity and finds himself weightless in the cockpit.   

       If you're happy with this example and haven't nodded off yet you can picture the pilot cutting back on thrust and letting himself curve into an inward spiral until he is just above ground level running parallel to the zero-g monorail tunnel. He can wave as they will both be doing the same speed - about 17,500mph - the speed required to orbit the earth at its surface.   

       Another pointer to understanding this is to know that the pilot need his engine to get up to speed and to counter air resistance once he is up to speed. If the 'plane were in a vacuum he could turn the engine off once he reaches orbital speed. This is why satellites orbit outside the atmosphere. Once they have attained orbit in a vacuum they need no thrust to keep going*.   

       *This is a lie but is so close to the truth that I refuse to worry about it.
st3f, Dec 10 2001
  

       How do the general public get on and off the 17,500mph monorail?
If it halts to pick people up, what G do the passengers feel on acceleration and deceleration?
Over what travelling distance is this a viable mode of transport?
How was your holiday?
lewisgirl, Dec 10 2001
  

       kvestions, kvestions.   

       Who said anything about the general public?
Halts? Nah. Getting this behemoth up to speed and down again would take a lot of energy. You'd probably take a smaller craft in the same tunnel (or a parallel one), accelerate it up to speed and dock.
Probably not a viable mode of transport. It will only travel around the equator. I saw this as a (once it's built) cheaper alternative to space-flight for experiments and production methods that require zero gravity, the training of astronauts, zero-g luxury cruises etc.
And finally: Great, thanks for asking. Lindesfarne is lovely (if a little cold, wet and muddy) this time of year.
st3f, Dec 10 2001
  

       'cold, wet and muddy'. Welcome to the north-east. (You must have passed by my home town on your way.)
angel, Dec 10 2001
  

       This IS a rotating frame of reference. The monorail is going around and around the equator. Standing outside it doesn't change that one is going to be flung away from the center of rotation. Exactly as your 'wall of death' <I love those things, by the way...>, you must be attached to what is going on somehow, which produces a force on you. Gravity still works on those, incidentially; I have slid down the walls when they didn't spin it fast enough after they removed the floor.   

       It isn't going to just magically make everything float. If you're on the floor of the car, you will be thrown outward; if you're on the ceiling you will be pressed against it. The direction of the force is away from the center, and because the train will always be turning toward the center, there will never be free fall.   

       Objects in orbit are not being accelerated, they're falling such that the speed at which they are falling is matched by the rate at which the Earth curves away from beneath them.   

       Everything in the train has inertia; the train is resisting inertia to keep things in the train. If the train suddenly disappeared, everything would continue in a straight line; straight out away from the Earth. THEN it might go into orbit and free fall, but until then, it is not.
StarChaser, Dec 10 2001
  

       Oh well 'Chaser, I've tried to explain this to you and you've tried to explain it to me. We both know we're right which is an unfortunate contradiction. This is going to get really dull if we carry on here.   

       If you want to keep going I'm happy to do so by email. If we break this down bit by bit we'll find the exact spot where we disagree (and hopefully resolve it). If not, I guess we leave it here.
st3f, Dec 10 2001
  

       Objects in orbit *are* being accelerated. Although the magnitude of the velocity is constant (in a circular orbit), the direction of the orbit is changing. Recalling Newton's first law of motion, objects move in a straight line at constant speed without a force acting on them. A force, with consequent acceleration (F = ma), is required to curve the path of an orbitting body. This force is gravity, which produces an acceleration, in this case towards the centre of the earth.   

       An object moving in a circle tends to fly outwards because of its inertia. There is no force pushing it outwards, just the tendency of a body to seek to move in a straight line (if there was a force where would it come from?). In a wall of death, there is no force pulling you towards the centre. Your inertia directs you to move in a straight line, but you are prevented from leaving the wall of death by the wall itself pressing on you. However in the monorail, there is a force pulling you towards the centre: gravity. This force produces the same acceleration on the train and its contents, meaning that they follow the same trajectory (the only external forces acting on the train and its contents are gravity and friction with the tunnel walls, and we are assuming that friction has been eliminated).   

       The principle of Newton's laws of gravity is that the forces acting on a body on the earth's surface are the same as those acting on a body in orbit. Therefore, it does not matter if you are orbitting in a train within the atmosphere or in a rocket outside the atmosphere, the forces you feel will be the same.   

       If the train suddenly disappeared, objects in it would not fly out into space, because gravity would be acting upon them. They would continue to move in the same path as before, until friction reduced their speed, at which time they would fall to the floor of the tunnel.
pottedstu, Dec 10 2001
  

       They are not being accelerated, they're moving at the same speed. If they were being accelerated, they would be going faster, and thus even more outward pressure.   

       'Pushing them outward' is a manner of speaking. Try tying a piece of string to a rock and swinging it around. This is what the train is doing. Now let go of the string <'the train has disappeared'>. What happens? It does not continue in the same path. It flies off in the direction it was moving last.   

       Now take a bottle and put some pebbles in it. Tie the string to the neck of the bottle and swing it around your head. Do the pebbles float in the center of the bottle, or do they end up stuck to the bottom of the bottle by centrifugal force? This is exactly the same thing as this train, except for scale.   

       The difference between orbit and going-really-fast-in-a-train is that in an orbit, one is falling toward the Earth, but the curvature of the Earth is such that you always miss. <Hitchhiker's Guide to the Galaxy flight>
StarChaser, Dec 11 2001
  

       [StarChaser] You got the first part right.
  

       "Do the pebbles float in the center of the bottle, or do they end up stuck to the bottom of the bottle by centrifugal force?" They *would* if _you_ exerted a gravitational pull about equal to that of the centrepital force the pebbles are experiencing. The train is moving at 7.9 kps (per [wiml]) which means the people are too. If the train disappeared, the directional velocity of the people would tend to carry them away from the Earth. However the speed has been calculated such that the Earth exerts and almost identical pull on the people, pulling them back to it. The result is that the people on the train will be on a curved trajectory around the planet. As long as they stay at about a certain speed (I pointed out earlier that they would need to make contact with the train periodically), they will effectively remain weightless.
phoenix, Dec 11 2001
  

       How large does the tunnel have to be?   

       A satellite orbits at the altitude where the acceleration due to gravity of the Earth-Moon-Sun matches the (convenient but fictitious) centrifugal force.   

       You will need to make the tunnel large enough to contain the perturbations in orbit caused by the changing positions of the moon and sun, or in other words, the tides.
neelandan, Dec 11 2001
  

       neelandan: We can calculate the difference in acceleration due to the Sun's gravity at the earth's centre and at the point on the earth's surface nearest to the sun: this comes out as a variation of 5E-7 metres per second squared over the 6200 km radius. The equivalent figure for the force exerted by the moon is 1.1E-6 meters per second squared, since the moon is smaller but much closer.   

       An orbit at 7.9 km per second takes about 84 minutes. If we assume the body faces the above lunar-produced acceleration relative to the earth for half of its orbit (an over-estimate), we can calculate that this would produce a total displacement of around 3.2 metres. You can add half of this for the displacement produced by the sun. So we'd maybe allow 5 metres around the train for safety's sake.
pottedstu, Dec 11 2001
  

       I give up.
StarChaser, Dec 12 2001
  

       Don't give up StarChaser. Drop me an email and I'll wax lyrical about high velocity bullets on earthlike planets without an atmosphere.   

       pottedstu: thanks for the gravity calculations. I thought we'd just have to nudge the train with elecromagnets (creating a slight gravity) but 5 metres is good. We could either make the tunnel big enough or move it on hydraulics. <homer>Mmmmmmm hydraulics.</homer>
st3f, Dec 12 2001
  

       The diameter of the Earth is around 6000 km. The atmosphere extends to around 400 km.   

       Just by increasing the radius of orbit by around 7%, you are out of the atmosphere which, at ground level, you have to fight to keep out by maintaining an airtight tunnel.   

       Is there an advantage? I mean, what amount of fuel is saved by delivering payload to an orbit "400 km below near earth orbit"? And remember you cannot use rockets - they would destroy the vacuum.
neelandan, Dec 13 2001
  

       Some figures.   

       The orbit at the Earth's surface takes about 84 min at a speed of 8 km/sec. Accelerating at a comfortable 1 G, you will need to travel a distance of about 3200 km taking about 15 min to reach that speed.   

       The orbital tunnel will be about 40000 km long. At a diameter of 10m that is an area of about 1300 sq.km to build airtight. Once it is finished 12.5 teralitres of air will need to be pumped out.   

       How would it be like to take a trip?   

       You enter through air locks the start of an acceleration loop about 4000 km long. You will travel in a capsule which is also taking raw materials out for zero-G processing. You are strapped in. No windows. Nothing to see, except metal walls rushing past. There is the hum of pumps as the air in the lock is pumped out. You have 'launch windows' of a few minutes every 84 minutes.   

       Jerk. You seem to be suddenly moving upwards at an angle of 45 degrees. Slowly, over the next fifteen minutes, the direction will seem to change to vertically upwards.   

       Jerk. You are weightless. End of acceleration, and docking manouevours begin. Clank. You are docked. You float through the opened hatch to the main "Orbiter".   

       The orbiter drifts at 8 km/sec through the 10m dia tunnel for the most time. Occcasionally it will enter a narrower section of tunnel for orbital corrections. These sections are fitted with electromagnetic linear motors, and moved by hydraulics to match the wandering orbit due to the tides.   

       Your visit over, you take your seats in the capsule once more. It has been stocked with zero-G products. The hatch is closed and a spring ejects you clear of the orbiter.   

       The capsule is electromagnetically guided into the deceleration loop. With a jerk, gravity gets hold of you as deceleration starts and you emerge into daylight about fifteen minutes later.
neelandan, Jan 04 2002
  

       <Prys teeth off engineering project>Who are you calling a jerk? The waitress? Oh, I see - weightless - my bad.
Given that 17,500 mph is point of weightlessness. Given that it does actually take a bit of time to reach that speed. Given that resistance to gravity - I'll call this 'negative' - occurs during the acceleration process. And during the process of deceleration gravitational pull is 'positive'. What, pray tell - is the method of restraining passengers in this (relative to space orbit) smaller arc? Straps have to be able to restrain without physical discomfort whether on the musculoskeletal or organic properties of humans. Welds, Rivets, Nuts and Bolts have to be able to withstand repeated use. Offal has to be able to stay where it was relative to the human body 1/17,500 of a minute ago while moving in neither an upward or downward manner. I like my intestines, but I prefer they stay out of sight.
thumbwax, Jan 04 2002
  

       neelandan: I like the description you've added (may even nick some of it later).   

       If you travelled lying down withe your feet facing the back of the train you woud feel as if you were lying down at the start of the trip and slowly being pushed puright until the bed you were lying on seemed to be a wall and the wall you put you feet on seemed to be the floor. When the acceleration stopped you would be weightless.   

       thumbwax: It's not a big acceleration you'd experience, just an extended period of light acceleration. Neelandan's model only accelerates and decelerates you at 1g.   

       While writing the original posting I tried to find out what the rate of change of acceleration was called. I found it nowhere on the web but an email to the Oxford English Dictionary got me a word. The rate of change of acceleration is called the 'jerk'. Ho hum.
st3f, Jan 04 2002
  

       [StarChaser] I've realised a few things about the use of language here and some unspoken ideas/assumptions that might help us convince you that you'd be safe* on board the train (I say *us* because I agree with St3f).   

       1) In every day life when people use the word 'acceleration' they mean a change in speed. Some people on this page, however, are using it in the technical sense of 'the rate of change of velocity'. Velocity includes both speed and direction, so if an object is changing direction but not speed, it is still said to be being accelerated. A change in speed or direction only occurs when an object has a force applied to it. When pottedstu says 'Objects in orbit *are* being accelerated' he is refering to the fact that everything (even things in orbit) have gravity acting upon them, only in the case of an object in orbit, it doesn't change speed, only direction. Or to put it another way, as you noted, things in orbit do fall towards the Earth but they always miss. They don't speed up, but they do change direction, thus, in a technical sense, they are accelerating.   

       2) You don't have to be in space to be in orbit, it's just that it's easier to fall and miss the earth when you're further away from it. If you're close to the ground (like the train is) you have to be going very fast so that the ground slopes away from you just at the same rate as you are falling towards it. (It doesn't matter to the argument if the train is connected to tracks, floating somehow or is in a tunnel)   

       3) Weightless does not mean that gravity is not acting upon you. It's just that everything around you is falling/moving at the same speed in the same direction so you don't notice gravity and tend to float away from things.   

       4) People don't move away from the earth unless they are pushed up. In the case of the train, there is nothing pushing the person up. The only way you would be splatted against the ceiling is if the whole train moved down suddenly and came down on your head. This would happen if the train went fast enough so that as it went along the ground, the ground fell away slower than it was falling, ie even faster than the 8kps needed to make the train 'orbit' the earth (ie fall but miss it). In that case it would look very much like centrifugal force had lifted you out of your chair and up to the roof.   

       I hope that makes things clearer and I appologise if you and St3f have had this out over email already. Even if you have, perhaps it will be instructive to others for me to point out that words are tricky little buggers that mean different things to different people :)   

       * Safe as in 'not splatted to the ceiling'. I, personally, wouldn't consider travelling at 8kps in a vaccuum in a tunnel safe, whether it was above or below ground...
tobythepig, Jan 04 2002
  

       If they could only design a golf course with my swing in mind.
thumbwax, Jan 04 2002
  

       Sorry, I'm out of this one. I know what I'm talking about and this is wrong, but I give up trying to explain.   

       Thumbwax, mini golf?
StarChaser, Jan 05 2002
  

       [StarChaser] Fair enough
tobythepig, Jan 05 2002
  

       One last try ... [SC], think of it this way. If you build the track and go around it very slowly, you will be "plastered" to the floor by gravity. If you go very quickly, you will be plastered to the ceiling as you say. But if you start out slowly, and gradually speed up, then at some intermediate speed the two effects must cancel out, and you will float freely in the middle of the car. This is what an orbit is.
wiml, Jan 06 2002
  

       Isn't this argument just another reason why we should build the thing, to find out?
pottedstu, Jan 06 2002
  

       I'm not entirely sure what cool-poole is getting at, but a couple of points.   

       1. Time will run slower at the poles than the equator because the equator is moving faster.   

       2. The frequency of light from a spinning LED will change constantly due to doppler shifting.   

       3. Centrifugal force doesn't exist.   

       4. "Theoretically, a gravitational coordinate system can be set-up using a gyro." Gravity, like any other field, can be mapped in three dimensions. Gravity causes time dilation, but can also be measured by looking for tidal forces.   

       5. This doesn't allow you to manipulate gravity in any practical way. Aside from the difficulty of solving the Einstein's equation in a moving system, and hence the problem of predicting relativistic influences on gravity, the amount of energy required would be prohibitive.   

       6. I'm not convinced that spinning an object would correspond to removing gravity from that object, but as I said above the equations are rather complex.   

       7. I propose a moratorium on physics-related ideas.
pottedstu, Jan 07 2002
  

       <deleted post>   

       Never mind. I've given up, I'm out of it. This is, as I have said and pointed out, completely wrong.
StarChaser, Jan 07 2002
  

       I would endorse this if only for the opening paragraph
NickTheGreat, May 17 2002
  

       [Begin Rant]   

       Well, it seems starchaser has finally thrown in the towel. SC, I give you full credit for sticking with your guns, but you have several glaring holes in your understanding. Perhaps I can patch a couple for you.   

       You state several times that "Everyone would be pasted to the ceiling from centrifugal force, if the track didn't collapse and fling them into orbit." In one post, you likened the train to a rock on the end of a string, noting that if you let go of the string, the rock goes flying off directly away from you.   

       First, let me clear this up by saying that the rock doesn't go flying off directly away from you. The rock is now free to continue on a straight path (which happens to be tangent to the circular path that it was originally following), and does so. When on the string, the rock has a constant SPEED, but a changing VELOCITY, I.E. the rock is always accelerating toward the person swinging it, but since the acceleration vector is constantly changing direction, the rock doesn't get any closer. (SPEED is a SCALAR, VELOCITY is a VECTOR.) When you let go of the string, you simply set the acceleration vector to zero, and the rock continues on in a straight line. (this is all assuming no gravity, which serves to pull the aforementioned rock to the ground, or air resistance, which serves to slow it down.) Hopefully, I haven't confused you too much.   

       Here's where that rock-on-a-string analogy comes in to play and your logic falls apart. You liken the TRAIN to the string holding the passengers in. In reality, the train and the passengers can all be considered as part of one big rock, and GRAVITY, provided by dear old mother earth, is the string. Take away the train at cruise velocity, and indeed, we'd all be in dire straits, as we'd quickly become a bunch of lifeless blobs of floating decompressed jelly, still travelling along through the airless tube at a leisurely 17900 mph, but we'd all still be weightless, as the giant string that is Earth's gravitational pull, would still be there to counteract the centrifugal force (or for all of you die-hard physicists who insist that there is no such thing as centrifugal force, the Earth is still providing the centripetal acceleration needed to keep the floating blobs of decompressed jelly travelling in a CONSTANT SPEED circular path around the Earth.)   

       To sum it up, as has been done by many before me, the train would be in orbit around the Earth while cruising through the tube.   

       [End Rant]   

       All logistical problems of constructing such a monorail (one that comes to mind is the bridge over the ocean, while the train inside would be in free fall, the bridge itself would still need to be supported somehow, and I don't think a flexible structure would quite cut it...), it sounds like a wonderful idea. I hereby award to St3f one jumbo-sized tube of Freeze-dried, re-hydrated Croissant paste.   

       In a final note to SC, Please remeber the following quote before you make your next post:   

       "Knowledge is proud that he has learned so much;   

       Wisdom is humble that he knows no more." --William Cowper
Freefall, May 17 2002
  

       Discussions of orbit above the atmosphere, and on the surface of the Earth have taken place. But what about an orbit through a similar tube, but below the surface? What would be the characteristics of the hypothetical orbit then?   

       From what I can remember of orbital equations, the smaller the radius of your orbit, the faster you must travel. This is assuming a constant mass around which you'd orbit. In an orbit underground, a signifigant portion of the mass (say 1/4 if at half the radius of the earth) would be above you at any given time. Would such an orbit take place in a figurative Larange point? How would this effect your ultimate required speed?
rapid transit, May 10 2003
  

       Starchaser, think of it this way: At zero speed, there is no centrifigal force and gravity keeps us in our seats. At some hypothetical high speed, there is enough centrifigal force to squash us to the ceiling. Therefore, there must be some speed between these two extremes at which the two forces balance.   

       Imagine our voyage. As the train speeds up, the centrifigal force builds up, and we begin to feel lighter. At a certain speed, centrifigal force (measured, for example, at the mid-height of the cabin) will exactly cancel gravity, creating a weightless condition at the centerline of the cabin. At floor level, centrifigal force will be slightly less, and gravity wins. At the ceiling, centrifigal force is higher (due to larger radius), and gravity loses. For safety reasons, the train should probably be operated at a speed which creates the zero G locus above the center of gravity for the tallest occupant, so that the net force on any rider will be a downward, microgravity.   

       Croissant! Our great-great-great grandkids will be arguing about the aliens that must have helped us build it.
gardnertoo, Sep 04 2003
  

       Y'know people. I think it would be better if we cut out the idea of the train altogether and just put in another tube closely fitting the other one, supported by maglev with some Linear Synchronous Motors to compensate for magnetic drag. Once you get over the initial price (in a few centuries) it willl be cheaper than using rockets for Zero-G industries, plus us flatlanders wouldn't have to worry about spacemen having a revolution and leaving us without zero G.
Cats Whiskers, Oct 06 2003
  

       Neeladan, You wouldn't have to be in contact with the train now and then just as astronouts don't need to be in contact with a space shuttle's walls/ceiling/floow in orbit. Since the atmosphere in the train always has the same velocity as a person in the train you can float free in the centre of the train without any friction. All the friction is by the way taken care of by the outer shell of the train since it's impossible to have a complete vaccum.   

       Also, the centrifugal force is never the same force as the centripetal force even though its magnitude may be (as in this train scenario) where centripetal force is gravity and centrifugal force is just the geometrical curvature as experienced within the system...
astronomer, Nov 15 2003
  

       Could be used to launch to space, but it already been considered. It's basically just a bad version of the Lofstrom Launch Loop.
bdh, Jul 14 2005
  

       Good link, bdh. Thanks. But tell me, do you also criticise cars for being bad versions of aeroplanes?
st3f, Jul 14 2005
  

       Nice. I don't know why I didn't see this first time it came up. Would it have to go straight round the Earth, or would minor deviations be OK? I'm thinking of a London-New York-San Francisco-Tokyo-Moscow route, with the ability to live in San Francisco and commute to my job in London.
hippo, Jul 14 2005
  

       I've kept the maths simple by going for an equatorial loop and avoiding any coriolis complexity. If I have a sit-down with some numbers and a look at some engineering so I can work out how significant those numbers are, I'll let you know.   

       Than again I'm sure that trains that go from New York to London and so on are massively halfbaked in science fiction. I'm going after what I hope is originality in getting a zero gravity lab on the surface of the earth without having to go even a smidge out of the gravity well and in an environment in which you can stop the vehicle without having to think about landing it.
st3f, Jul 14 2005
  

       No, but I do criticize cars for not being aerodynamic and for having inefficient engines. ;-)
bdh, Jul 17 2005
  

       Yeah, [bdh] but you used that exact same phrase about [Vernon]'s idea for another, larger centrifugal loop-around-the-Earth, the Earth-Space Web. Perhaps Lofstrom's idea is a bad version of this. Lofstrom may have been first, but he didn't go far enough and large enough.   

       I must admire [Starchaser] for his determination in defending his point of view. I say that he is wrong, though, and hope that others will learn something from him about how to be polite, which he was, and also about re-examining one's own views.   

       [Starchaser]'s errors got me to thinking, and I envisioned filling this monorail with cars and running them faster and faster. After orbital velocity was exceeded by enough, the whole monorail would lift off the ground, and, if steadied with cables, would become a structure standing above the Earth. Which turns out to be [Vernon]'s idea for an Earth-Space Web. Which is the logical expansion and improvement on Lofstrom's Launch Loop that [bdh] mentions.   

       It is odd how some ideas go around and connect in unforeseen ways. This one (and [Starchaser]) inspired me to a lot of creative thinking, that took me to [Vernon] and Lofstrom. Plus, it is a damn good idea in its own right. It will work, and could be built. [+]
baconbrain, Aug 20 2006
  

       //The point I'm making, I guess, is that if you achieve escape velocity, you will attempt to escape.//
Funny. And I guess that's why his handle is StarChaser.
ldischler, Aug 20 2006
  
      
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