# Ask Ethan: Is zero gravity possible at all?

The plane of reduced gravity by NASA, which is officially called the "weightless miracle", and unofficially - the "emetic comet" [Vomit Comet]. In the photo - a modified version of the model Douglas DC-9

It is said that astronauts in space, flying outside the Earth’s atmosphere, experience zero attraction - a real lack of weight. However, they are still attracted by the Earth, the Moon, the Sun and all other masses in the Universe. So how does zero gravity work, and does it happen? This question was asked to us by one of the readers:

I hear about experiments conducted on the ISS in zero gravity, as well as about studying people existing in zero gravity. But does zero gravity happen?

Very good question, despite the fact that this feeling you can experience not only in space.

Attraction "free fall" in the amusement park " Six Flags over Georgia ", 2006

Consider the difference between what you are doing now - and you are most likely sitting - and free falling. The difference is not in the chair, not in the seat and not in the force of gravity acting on you. All this can be preserved during the flight - as on an attraction with a free fall. The difference is that you feel like a seat (or floor, or bed) are pushing you in the opposite direction, resisting gravity. The fact that you do not feel the sensation of falling is connected not with gravity, but with normal force. It is normal not because it is the opposite of the abnormal, but because it is directed along the normal, that is, perpendicular to the surface on which you are standing, sitting or lying down.

Gravity force (red) and normal force (blue), equal in magnitude and opposite in direction, affect any mass on Earth

Let's eliminate this power. This can be done, for example:

1. Removing the surface that has it - floor, chair, whatever.
2. Turning off the gravity, because of what you lose weight.
3. Turning off the electromagnetic interaction between you and the atoms of the surface.

In the first case, you just fall down, as if you were suddenly in the air and began to fall. In the second, you will understand that normal force acted only because gravity pulled you to Earth, and the floor was in its way and pushed you back with the same magnitude as opposite in direction , which is why you remained in place. If you turn off gravity, its effect will be zero, and the same, oppositely directed force will be the same. In the third case, gravity will remain, and you will be dragged to the center of the Earth, but without electromagnetic forces between the atoms of the floor and your atoms, you will fall down.

The force of gravity acts on a falling body, and it seems that it describes a parabola, but in fact it is a small part of an ellipse, one of the focuses of which is in the center of the Earth. If you turn off electromagnetism, the ball would describe a full ellipse in about 90 minutes

These three cases are very different. In the first, gravity would drag you down to the center of the Earth, with acceleration that would stop either due to friction with the air, or because of the action of the Earth or another solid body on you. In the second case, you would become weightless, and you would not accelerate at all - that would be zero gravity. In the third, you would accelerate when moving to the center of the Earth, describing an ellipsoid with the center of the Earth in one of the foci, and return to the starting point in about 90 minutes

But in all three cases at the very beginning you would experience the same sensation.

Astronauts and fruits aboard the ISS. Gravity is not turned off, but everything accelerates the same, which leads to a feeling of zero gravity

This feeling of lack of weight, elevation of the stomach, disorientation - this is the feeling of zero gravity. Even more strange is the fact that you could not distinguish between the second and third options without opening your eyes. It is impossible to tell the difference between turning off gravity and continuous acceleration with a free fall. This may seem counterintuitive, but this is the essence, the basic principle that led Einstein to the formulation of the General Theory of Relativity. And this is what we know today about Einstein's equivalence principle.

The same behavior of a ball falling on the floor in an accelerating rocket and on Earth is a demonstration of the Einstein equivalence principle

The principle postulates that it is impossible to distinguish between gravitational acceleration and any other form of acceleration — for example, rocket thrust. If you, along with your reference system, simultaneously and evenly accelerate, then your position will be equivalent to the absence of acceleration [being in a gravitational field - approx. trans.]. The force that you feel on the Earth, rotating around its axis and moving around the Sun, is identical to the force that you would feel if the Sun disappeared and the Earth would simply rotate alone in the depths of space.

Speaking of zero gravity, we do not mean that gravity disappears. There is no place in the Universe where one could hide from gravity, and we don’t need it. We only need to turn off the normal force, which means to allow us and our frame of reference to fall evenly together. We only need to accelerate with our environment. This principle also works during the flight on the “emetic comet”, when the engines of the aircraft are turned off and it falls freely for about 40 seconds and you are on board.

Passengers gag comet in 2006 during the jump with zero gravity

Your sense of zero gravity may look like a fall, only without the air rushing past. Just between you and what you feel, as the direction of "down", there is no relative gravity. If you permanently expose yourself to zero gravity in this sense, you will begin to suffer from various physical ailments: bone loss, muscle atrophy, vision loss or even blindness, loss of calluses on the legs (which leads to loss of skin on the feet) and, in the absence of tightening your insides force, increased gas formation. And all these are the risks that astronauts take. Apollo 16 expedition member John Young is known to have suffered particularly from flatulence on the Moon, which he attributed to Florida orange juice.

Literally, there is no zero gravity in the Universe, for it is impossible to hide from a long-range force. But from the point of view of the physical sensations of your body, as long as your environment is evenly accelerated along with you, you cannot find a difference. Zero gravity and free fall are synonymous if we evaluate them in terms of your experience. So, while in the universe nothing can experience zero gravity, if you just let gravity pull you down - until you run into anything that resists you - you will get the best possible replacement for this effect. At least, this was enough for Einstein, for astronauts this was enough, and for us this too should be enough.

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! He wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology ].

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