The universe is vast, old, and contains billions of potentially habitable planets. If intelligent life is common, why haven't we found any evidence of it?

Two identical twins: one stays on Earth, the other travels at near-light speed and returns. The traveling twin has aged less than the Earth-bound twin. But from the traveler's perspective, Earth was moving fast, so Earth's twin should be younger.

In quantum mechanics, particles exist in superposition until observed. But what constitutes an "observer"? A human? A cat? A camera? A rock?

It's more likely for a single brain to spontaneously and briefly form in a void (complete with a memory of having existed in our universe) than it's for the universe to have come about as the result of a random fluctuation in a universe in thermal equilibrium.

Feynman inverse sprinkler is a sprinkler-like device which is submerged in a tank and made to suck in the surrounding fluid. The question of how such a device would turn was the subject of an intense and remarkably long-lived debate.

It was a reductio ad absurdum argument which was used when the universe was believed to be eternal. If the universe was really infinitely old then why, according to the second law of thermodynamics had it not already reached thermal equilibrium.

It relates to the relativistic description of the geometry of a rotating rigid disk. Ehrenfest has formulated this paradox in 1909 in the context of Einstein's special relativity theory (SRT) published in 1905. The paradox is that there is a discrepancy between the Euclidean circumference vs. the circumference calculated by Special Relativity Theory based on Lorentz concentration.

The tea leaf paradox is a phenomenon where tea leaves in a cup of tea migrate to the center and bottom of the cup after being stirred rather than being forced to the edges of the cup, as would be expected in a spiral centrifuge.

A wheel is depicted in two-dimensional space as two circles. Its larger, outer circle is tangential to a horizontal surface, while the smaller, inner one has the same center and is rigidly affixed to the larger. Assuming the larger circle rolls without slipping for one full rev., the distances moved by both circles are the same. The distance travelled by the larger circle is equal to its circumference, but for the smaller it is greater than its circumference, thereby creating a paradox.

Concerning the two rows of bodies, each row being composed of an equal number of bodies of equal size, passing each other on a race-course as they proceed with equal velocity in opposite directions, the one row originally occupying the space between the goal and the middle point of the course and the other that between the middle point and the starting-post. This...involves the conclusion that half a given time is equal to double that time.

If everything when it occupies an equal space is at rest at that instant of time, and if that which is in locomotion is always occupying such a space at any moment, the flying arrow is therefore motionless at that instant of time and at the next instant of time but if both instants of time are taken as the same instant or continuous instant of time then it is in motion.

It's a thought experiment that illustrates a paradox of quantum superposition. A hypothetical cat may be considered simultaneously both alive and dead as a result of its fate being linked to a random subatomic event that may or may not occur.

The quantum Zeno effect (also known as the Turing paradox) is a feature of quantum-mechanical systems allowing a particle's time evolution to be arrested by measuring it frequently enough with respect to some chosen measurement setting.

The paradox involves two entangled particles according to quantum mechanics. Under the Copenhagen interpretation of QM, each particle is individually in an uncertain state until it is measured, at which point the state of that particle becomes certain.

Simply stated, Zeno's Dichotomy Paradox posits that it is impossible to travel from point A to point B because there are an infinitely divisible number of spaces in between, and it is impossible to traverse an infinite amount of space.