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I was thinking how to have a situation on Earth of feeling weightlessness for an indefinite time, not for a few minutes like on a diving plane

Not reduced gravity, weightlessness as I feel in a swimming pool.

Is it possible to imitate a state of free fall inside the Earth's atmosphere?

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    $\begingroup$ Ellipses (orbits) which never touch the ground and are inside the atmosphere require orbital speeds (around 7 km/s); not practical. Or a 0 radius orbit at the center of the Earth, which is even less practical. $\endgroup$ – Juancho Mar 13 '15 at 14:04
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    $\begingroup$ Note that what you feel in a swimming pool isn't free fall. If you want to have what you feel in a swimming pool, try a swimming pool. $\endgroup$ – Russell Borogove Mar 13 '15 at 16:33
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    $\begingroup$ You could suspend an enormous mass, perhaps of neutronium, above you so that its gravitational pull would cancel out the Earth's. The details of how to do this are left as an exercise for the student. $\endgroup$ – Organic Marble Mar 13 '15 at 17:23
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    $\begingroup$ Welcome to the site. It is difficult to understand what is your exact question. The title mention "on the ground", then your closing sentence asks "something which remain in state of free fall but never touch the ground". You'll receive accurate answers if you rethink to it and adjust your post accordingly. Also do not forget that the topic must be about space exploration, not physics only. $\endgroup$ – mins Mar 13 '15 at 17:55
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    $\begingroup$ The floating frog in the superconducting electromagnet experiences "true" microgravity, because his weight is supported throughout his volume. This is different to a swimming pool or a vertical wind tunnel, where the weight is supported on the the object/occupant's surface. $\endgroup$ – pericynthion Mar 13 '15 at 20:18
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The way I see it, you have 4 options:

  1. Free fall. As pointed out, you won't do this for very long before hitting the ground.

  2. Go in one direction really fast (Orbit). This will result in extreme air resistance if you don't hit anything else, causing intense heat and probable incineration. The energy required to achieve this kind of acceleration would also extraordinary, so it would just be easier to orbit in space.

  3. Go to the center of the earth (Where gravity pulls equally in all directions, so net zero). If you do this, you will promptly be crushed by the immense weight of everything else around you.

  4. Compress the air around you so that it has about the same density you do. This is why swimming feels weightless. We are talking several orders of magnitude compression here, so in this case, you will be crushed almost instantly and your "weightless" crushed body will float in the special tank you built.

Not sure if any of the options are very appealing, but as far as I can tell, there aren't any others

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    $\begingroup$ With regard to #4, some weight will still be apparent because of density variations and absence of continuous fluid throughout the body. A submarine has the same average density as the fluid around it, but the crew still sink to the deck... $\endgroup$ – DJohnM Mar 13 '15 at 15:41
  • $\begingroup$ I don't see how #4 works. That's basically what diving is, but with water instead of air. It's still the same pressure $\endgroup$ – raptortech97 Mar 13 '15 at 21:59
  • $\begingroup$ @User58220 I agree, but you still feel somewhat weightless if you swim. #4 suggests you "swim" in highly compressed air. It won't be exactly like weightlessness, but that isn't the biggest problem with that suggestion $\endgroup$ – neelsg Mar 16 '15 at 7:17
  • $\begingroup$ @raptortech97 There is a difference between pressure and density. I'm no expert, but for comparison, air pressure at sea level is about 101 kPa, but have a density of only 1.225 kg/m3. If you dive 1 m into water, the pressure only increases by about 10 kPa, but you are in a substance with a density of about 1000 kg/m3, which is roughly the density of a human. To increase air to 1000 kg/m3, you need to compress it by something like 816 times, which means 816 times more pressure = 82712 kPa (keeping temp constant). This is about the pressure from diving at something like 8218 m below sea level $\endgroup$ – neelsg Mar 16 '15 at 7:39
  • $\begingroup$ Any mammal will be killed by the compressed air long before the density of the body is the same as the air around it. $\endgroup$ – Uwe Jul 25 '17 at 12:17
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I think a Vertical Wind Tunnel could do it.

If you could build one which adapt its power to your current drag, you would be like in micro gravity, - minus a strong wind all around you.

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    $\begingroup$ I've been indoor skydiving. It's not free fall; the weight of your body is supported by the force of the air being driven up into you, not that dissimilar from lying on a waterbed. (But then, a swimming pool isn't free fall either.) $\endgroup$ – Russell Borogove Mar 13 '15 at 16:32
  • $\begingroup$ @RussellBorogove Yes I understand that very well. But I think that's somewhat close to the impossible thing that Akaks is asking for :) $\endgroup$ – Antzi Mar 13 '15 at 16:44

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