The only thing that must be absent for two objects of drastically different masses to fall at the same speed under the influence of gravity is air resistance.

Here's why:

* Newton's Law of Universal Gravitation: This law states that the force of gravity between two objects is directly proportional to the product of their masses. So, a more massive object experiences a greater gravitational force.

* Acceleration due to Gravity: However, the acceleration due to gravity (g) is the same for all objects, regardless of their mass. This is because the force of gravity is countered by the object's inertia, which is also proportional to its mass.

* Air Resistance: Air resistance, also known as drag, is a force that opposes the motion of an object through the air. It depends on factors like the object's shape, surface area, and speed. The more massive object, with its greater inertia, will have a smaller change in speed due to air resistance compared to the less massive object.

In a vacuum (where there's no air resistance), a feather and a bowling ball will fall at the same rate and reach the ground simultaneously. This was famously demonstrated by Apollo 15 astronaut David Scott on the moon.

Let me know if you'd like to explore other aspects of gravity!