How can weightlessness be created on Earth?

Weightlessness does not mean gravity disappears; it cannot be switched off. What people experience in orbit is free fall: the spacecraft and its occupants accelerate at the same rate, so there is nothing left to press against the seat or the floor. The technical term microgravity describes exactly that state, along with small residual accelerations. On Earth it is created by dropping something, and that is precisely what limits the duration, because the ground is reached quickly or the air slows the motion down.

The best known is the parabolic flight: a modified commercial aircraft climbs at a steep angle and then, with thrust nearly cut off, follows a parabolic path that provides about 20 seconds of weightlessness before returning to normal flight. A single flight strings together dozens of those parabolas, each separated by phases of higher load. Drop towers, such as the one in Bremen, offer only a few seconds, but with very few disturbances, because the drop tube is emptied of air. Suborbital rockets extend that window to minutes, though only for experiments without a crew.

Everything else is an imitation, and that is not a weakness but its purpose. In the water tank, thrust and weight balance each other out, so grips and spacewalk procedures can be trained; but gravity keeps acting, tools fall downward, and water slows every movement. Slightly tilted bed rest studies reproduce the shift of fluids in the body, and rotating chairs reproduce the imbalance of the inner ear. Small samples can be levitated using magnetic fields. For people, free fall remains the only real path.