# gravity vs freefall **URL:** \ **Category:** Factual Questions\ **Created:** [April 19, 2017, 3:24pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792 "2017-04-19T15:24:08Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [April 22, 2017, 1:46pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/41 "2017-04-22T13:46:50Z")
> [@sbunny8](#): > > I respectfully disagree about jumping off a high dive. Being in freefall is not what makes the Vomit Comet an amazing experience. What makes it amazing is the illusion that you are stationary because the room which is around you is also in freefall and yet you do not drift toward the floor of the room. Jumping off a high dive does NOT give the same illusion because you don’t have a room around you which is also falling. > > Even if you had a giant cardboard box with you when you jumped, the illusion would not be the same because wind resistance would slow down the box, making it appear that your feet are in fact drifting toward the floor of the box. This is a flaw with the weightless-in-a-falling-elevator illusion too. You would not feel totally weightless b/c the elevator itself would be falling slightly slower than 9.8 m/s2 due to wind resistance. Your feet would tend to stay in contact with the floor. > > What makes an orbiting space ship an amazing experience is that you look at the walls, you feel like you aren’t moving, you don’t drift toward the floor, in fact there is no “floor”. This gives the illusion that gravity has let go of you. But this illusion can be shattered if you look out the window and see cities and continents whizzing by, revealing that you are actually moving very very very fast and neither you nor the room you are in is stationary. Then you feel like a person shot out of a cannon, moving so fast that you are going to miss Earth entirely (which is basically true, except for the part about the cannon). If you had total situational awareness, you’d realize that you actually are falling quite rapidly and that gravity has almost as strong a grip on you as it ever has during your mundane life up to that point. > > A falling elevator would also lose the “no gravity” illusion if the elevator had glass walls and you could see the fact that you are actually falling. Another difference is that the end of your journey is a tad more abrupt when jumping from a high dive. 🙂 ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [April 22, 2017, 3:59pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/42 "2017-04-22T15:59:29Z")
> [@jharvey963](#): > > I just read a Sci-Fi novel that featured a space elevator: i.e., a cable or stalk running from the earth to geosynchronous orbit. > > In the story, they stopped at the elevation of low earth orbit (same as the ISS). The story said that the passengers still felt a significant amount of gravity at this height. Me: “wait, but the astronauts in the ISS don’t feel gravity. Why would the characters in the story?” > > They made a distinction between gravity and freefall. The ISS is in freefall around the earth, hence no apparent gravity in the ISS, but the space elevator is not in freefall so they do feel gravity. > > So my questions: > > 1. Is this correct? > 2. If so, how much gravity would a passenger feel at the height of the ISS? (Call it 250 miles high.) > > Thanks, > J. While I don’t have time to do the math for #2 I wanted to address #1 as you are asking about correctness and truth. The concept that astronaut in orbit feel weightless because they are in “free fall” is based in Classical physics. It is important to remember that while extremely useful and accurate enough more most uses classical physics has been superseded by much newer theories for over a century and is not descriptive of our modern understanding of fundamentals. What we view as “weight” is a what is called a fictitious force or a pseudo force. The force we perceive as “weight” can be measured but it is only due to our reference frame. Weight is like other fictitious forces like centrifugal force in that it is an artifact of a particular frame of reference and not descriptive of a fundamental reality. As orbits are circular, if classical physics was descriptive of the fundamental nature of the universe the curved path would result in the astronaut’s observing a acceleration to the outer edge of the spacecraft. To explain the current best understanding of this phenomenon requires the use of Einstein’s theory of general relativity and 4D spacetime. But as we as humans do not possess the ability to visualize in four dimensions it would be useless for a SciFi novel. In relativity the IIS is following a geodesic or the shortest path through spacetime. Due to the mass of the earth curving time and space the IIS is really traveling in what should be considered a straight line at a constant speed. It is this lack of acceleration that results in weightlessness and not a free fall. The “Einstein equivalence principle” is the name for the thought experiment relating to falling elevators or boxes and it states. > [@](#): > > The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime. But there are some assumptions there. While I will not go into the implications of a space elevator the critical thing to remember about that thought experiment is that the acceleration needs to match the gravitational acceleration so any form of wind resistance or friction which would slow the decent of a falling elevator/laboratory/box. There are other assumptions like being able to shut off the gravitational attraction of items that are falling, charge and a problem with locality as “falling” would be radially inward. While this may sound absurd the reason you experience “weight” is because unlike the IIS you are not able to follow the shortest path through space time because the earth gets in your way. What we feel as weight is actually due to being constantly accelerated away from our shortest path in space and time. It will seem weird but it is best described as an effect of the earth running into you. As a space elevator in low earth orbit would also have a net force from the tether it will also be prevented from taking it’s shortest path through spacetime and their would be some resulting fictitious force. TLDR; 1: no 2: requires a lot of math. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 22, 2017, 4:37pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/43 "2017-04-22T16:37:07Z")
Whether you call gravity a “real force” or not is a matter for philosophy, not physics. But regardless of what you call it, a body in orbit is in the same state as one falling straight down. (and as a nitpick, as the word “classical” is used by physicists, GR is still considered “classical physics”. “Classical” just means “not quantum”.) ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [April 22, 2017, 4:37pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/44 "2017-04-22T16:37:41Z")
As I cannot offer a way to visualize spacetime this will show how what appears to be a curved path can be due to ones reference frame. [![]( " - YouTube") ](https://www.youtube.com/watch?v=uMN4L94O4HE) While I encourage anyone with a curiosity about science to watch that full video I have started it at a time that will only require a few min of watching to get the point. At a minimum stick with it until 18:50. While it will not cover how it is us on the ground that are being accelerated and not the IIS it will show how we can can perceive what is really a straight line path as a circular one. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [April 22, 2017, 4:38pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/45 "2017-04-22T16:38:21Z")
> [@Chronos](#): > > The feeling of acceleration (or lack thereof) is far stronger than the effect of visual cues. If you’re in that completely-sealed elevator, or on the ISS, or in deep intergalactic space, you don’t feel like gravity has been turned off. How could you? We’ve never experienced gravity being turned off, so we have nothing to compare it to. In any of those situations, _even with all visual cues removed_, you still feel like you’re falling. I’m pretty sure this is incorrect. If being on the ISS felt like continuous falling I think that astronauts who’ve been up there would talk about it in those terms. But they don’t: they talk about floating, not falling. I also disagree that “the feeling of acceleration” is the predominant component in the feeling of falling. We’re not actually very good at sensing acceleration, except insofar as we sense the apparent weight of our bodies. When an elevator begins dropping, the predominant sensation is the initial decrease in apparent weight, followed by a return to close to normal weight, followed by the increase in apparent weight as the elevator stops. In these cases the experience is not of acceleration, but of _change_ in acceleration. Δa, if you will, rather than Δv. This is because we are _always_ accelerating relative to the gravitational field we’re in. If you’re sitting “motionless” in a chair, your experience is that of gravity trying to accelerate you downwards at 9.8m/s[sup]2[/sup], and your chair pushing back with an equal and opposite force. What you experience is your “normal” body weight with a downward direction. Ah, you say, but if you get in a car and drive an extended curve, you feel the centripetal acceleration. No. Not in the sense that you feel the acceleration of an elevator car. You again just feel weight. So long as you are driving a steady curve (say, in continuous circles on a test track) you simply feel the sum of gravitational and centripetal acceleration vectors, i.e. weight at something over 1G in a direction tilted away from vertical. If we tilt your chair to match the direction and blindfold you, you won’t be able to tell the difference between driving in a circle sitting in an elevator being _continuously_ accelerated with the same force. (Obviously it wouldn’t be feasible to build an elevator that could sustain that sort of thing for any amount of time.) So if we move into the Vomit Comet, as you enter the weightless flight segment you’ll feel the elevator-like bottom-dropping-out sensation of Δa, but you won’t experience the weightless segment itself as acceleration, but simply as not having any weight. Unless I’m completely misreading your post and when you say “you’ll feel like you’re falling” you mean “you’ll feel like you’re floating with no weight.” In that case I simply disagree with the use of language. What most people conventionally mean by “it felt like falling” is that sensation of Δa at the beginning of a fall (or the beginning of an elevator descent). Most falls we experience are over before this sensation ceases, and so we don’t bother to distinguish between the initial kinematic sensation and the subsequent sensation we would experience if the fall continued long enough to reach a steady-state. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [April 22, 2017, 4:41pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/46 "2017-04-22T16:41:38Z")
> [@Chronos](#): > > Whether you call gravity a “real force” or not is a matter for philosophy, not physics. But regardless of what you call it, a body in orbit is in the same state as one falling straight down. > > (and as a nitpick, as the word “classical” is used by physicists, GR is still considered “classical physics”. “Classical” just means “not quantum”.) QFT and GR are incomplete, both of them will be classical at some point in time but no a Newtonian “free fall” orbit is absolutely not the same state as a radially inward falling path. Unless you are referencing the weak equivalence principle with objects who interact with the earths gravity but not charge or mass of other objects. But as I mentioned in my reply the OP was asking if something was “true” and the free fall model of orbit would result in an imbalanced force. Note my cited video above. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [April 22, 2017, 4:52pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/47 "2017-04-22T16:52:31Z")
> [@Chronos](#): > > The feeling of acceleration (or lack thereof) is far stronger than the effect of visual cues. If you’re in that completely-sealed elevator, or on the ISS, or in deep intergalactic space, you don’t feel like gravity has been turned off. How could you? We’ve never experienced gravity being turned off, so we have nothing to compare it to. In any of those situations, _even with all visual cues removed_, you still feel like you’re falling. The “feeling of acceleration” is called weight. “Weightlessness” in a general context is the sensation of not being accelerated. This is fairly easy to demonstrate even without resorting to Relativity by just looking at the unit of standard gravity which is, by definition, 9.80665 m/s^2 Note the change in velocity over time, that is by definition acceleration. Experiments have confirmed the Einstein’s weak equivalence principle or EEP to ~10−7 even with massless particles. [https://arxiv.org/pdf/1608.07657.pdf](https://arxiv.org/pdf/1608.07657.pdf) The Weak Principle of Equivalence states all the laws of motion for freely falling particles are the same as in an unaccelerated reference frame. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 23, 2017, 1:44am UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/48 "2017-04-23T01:44:40Z")
Yes, it does feel the same as gravity being turned off, but that’s not the label that our subconscious would give to that sensation, because “gravity being turned off” is a situation totally foreign to our experience and evolution. Falling, however, is not a totally foreign situation. And yes, in fact, astronauts do say that it feels like falling, and that it’s quite uncomfortable as a result until they get used to it. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [April 25, 2017, 5:37pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/49 "2017-04-25T17:37:33Z")
> [@jharvey963](#): > > I just read a Sci-Fi novel that featured a space elevator: i.e., a cable or stalk running from the earth to geosynchronous orbit. Just curious about the engineering feasibility of this. Geosynchronous orbit altitude is something like 22,000 miles. I know that there are serious discussions about space elevators but could one that long really be built? ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [April 25, 2017, 5:44pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/50 "2017-04-25T17:44:17Z")
Currently no, because there’s no material strong enough. I saw a documentary (Sky Line) in which a researcher said we need a material that’s 15 to 20 times the tensile strength of any currently existing material. He said it’s like the difference between balsa wood and titanium. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 25, 2017, 6:49pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/51 "2017-04-25T18:49:05Z")
> [@markn\_1](#): > > Currently no, because there’s no material strong enough. I saw a documentary (Sky Line) in which a researcher said we need a material that’s 15 to 20 times the tensile strength of any currently existing material. He said it’s like the difference between balsa wood and titanium. I think it’s not so much a matter of “strong enough” as it is a matter of a high enough strength-to-weight ratio. [Carbon nanotubes](https://en.wikipedia.org/wiki/Mechanical_properties_of_carbon_nanotubes) are getting there, with a specific strength several hundred times that of steel. Even with the inevitable crystalline defects that reduce the strength by a factor of ten, it would still meet your stated requirement, having several dozen times the strength-to-weight ratio of steel. We would just need to make a whole lot of 'em. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 25, 2017, 7:17pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/52 "2017-04-25T19:17:00Z")
To make a space elevator we need three things in this order: 1. A high strength to weight material. 2. A way to manufacture the above by the thousands of tonnes, not the tenths of grams. 3. A way to do the above cost effectively. Right now we’re at about stage 0.9. When we get to stage 3.0 then design can begin in earnest. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 25, 2017, 9:17pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/53 "2017-04-25T21:17:24Z")
Yeah, we can currently produce nanofiber in lengths of about 10 cm. You could, in principle, construct a space elevator cable out of 10 cm-long fibers, and still have a realistic taper ratio, so in that sense, I’d say that we’re already at stage 1 on that scale. But we’re nowhere near being able to scale up enough. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [April 25, 2017, 10:07pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/54 "2017-04-25T22:07:19Z")
I don’t think we’re quite at stage 1 yet, even if we could produce long lengths of carbon nanotube. [This article](https://www.newscientist.com/article/2093356-carbon-nanotubes-too-weak-to-get-a-space-elevator-off-the-ground/) in New Scientist says that when a carbon nanotube has one single atom out of place, the tensile strength of the whole fiber drops by more than 50%. We would need a new method of synthesizing nanotubes of a quality vastly better than we can produce today. The author says “Unless great breakthroughs on CNT synthesis can be achieved, using CNTs to build a space elevator would be extremely challenging”. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 25, 2017, 11:02pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/55 "2017-04-25T23:02:05Z")
How big a problem that is would depend on the mean length between such single-atom defects. If that length is macroscopic, then that’s about as good, on the whole, as making perfect strands of that same length. Does anyone know what that mean length between defects is, for current production methods? ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [April 25, 2017, 11:44pm UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/56 "2017-04-25T23:44:59Z")
> [@sbunny8](#): > > …snip… > > A falling elevator would also lose the “no gravity” illusion if the elevator had glass walls and you could see the fact that you are actually falling. I did the “falling elevator ride” at MGM Disney. You can see out the front over the park, at least for big chunks. You go down and up and down… I lost the falling illusion and gained the ball at the end of a rubberband illusion. Luckily, no paddle. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [April 26, 2017, 1:36am UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/57 "2017-04-26T01:36:19Z")
Individual sheep hairs are only a few cm long, but you can spin then together to produce a thread of any length. Can you, in principle, do the same with 10cm carbon nanotubes? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 26, 2017, 1:46am UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/58 "2017-04-26T01:46:46Z")
Yes, if you have enough of them. We don’t, nor is any technology we have now for making them sufficient to the task under any reasonable assumptions. It’s be like trying to weave one of those football-field-sized flags out of the wool of one sheep. ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [April 26, 2017, 2:45am UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/59 "2017-04-26T02:45:37Z")
Is the problem just the prohibitive expense of making so many? Or is there some other limiting factor? ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [April 27, 2017, 12:47am UTC](https://boards.straightdope.com/t/gravity-vs-freefall/784792/60 "2017-04-27T00:47:35Z")
IANA structural expert, but ISTM we’re not going to “spin” individual short carbon fibers into a strand that’s both long and strong. Or at least we’re not going to spin individual 10cm fibers into strong 10,000km yarns. Individual sheep hairs are very, very barby. They aggressively catch and snag on each other. The stremght with which they grabtheir neighb ors is a large fraction of their own strength. Conversely, a proper molecularly pure carbon fiber is \*extremely \*slick and smooth. It wants to slide past its fiber-mates like graphite plates want to slide past one another. the one thing it does \*not \*want to do is snag on it’s neighbors. Success comes when the molecular strands are long compared to the length of the next higher level yarns in the overall elevator cable. I’m guessing molecular strand lengths to 100s to 1000s of meters are needed. A large number of those will be able to be braided or bound together to form yarns which in turn can form cables which can in turn form 3D fabrics. Said another way, a 3D fabric column a meter across composed of 100,000 molecular strands won’t be strong with 5-10 joints in every strand in every meter of length. It will be strong when it has but a handful of joints in every meter. Which implies a very long average strand length. That requisite average strand length is a few orders of magnitude beyond present tech. Regardless of price. [Previous page](https://boards.straightdope.com/t/gravity-vs-freefall/784792.md?page=2) [Next page](https://boards.straightdope.com/t/gravity-vs-freefall/784792.md?page=4)