**Quantum mechanics** is a branch of physics that explores the smallest building blocks of the universe in a way that's very different from how we understand bigger objects in the macro-universe.

- Quantum mechanics tells us that quanta (like particles of light) can sometimes behave like waves or particles, and therefore exhibit duality.
- Uncertainty: In the quantum world, there's always some uncertainty. For example, we can know a particle's position, but then its momentum becomes uncertain, and vice versa.
- Superposition: Particles can exist in multiple states at the same time. It's like they're doing multiple things simultaneously until we measure them and "force" them into one state.
- Entanglement: When particles interact, their properties can become linked or entangled, so that changing one particle instantly affects the other, no matter how far apart they are.
- Quantum leap or jump: Particles can suddenly "jump" from one state to another without passing through the states in between.

According to the **Copenhagen interpretation** of quantum mechanics, whenever we measure an eigenvalue of a quantum system, this causes the associated wavefunction of that quantum system to "collapse." Before the measurement is made, the state of the quantum system exists in a superposition of all possible states. For example, before measuring the position (an eigenvalue) of a *random electron*, it could be located anywhere else in the universe.

However, according to the **Many-Worlds interpretation** of quantum mechanics, the wavefunction never actually "collapses." This interpretation predicts that we live in one of many universes and that every possible measurable position of the *random electron* is measured in different parallel universes. Therefore, the total number of universes containing that *random electron* in the multiverse would be the total number of all possible measurable eigenvalues (position, momentum, energy, etc.) associated with that system. Physicist Hugh Everett estimated the total number of possible states of every particle in the universe could equal the total number of universes in the multiverse, which is about 10^{500}. String theorist Leonard Susskind also spoke about this here (2nd minute). Ours is just one of those 10^{500} possibilities, apparently "fine-tuned" to support life - discussed in the anthropic principle - and giving rise to the laws of physics that we observe. Many of these universes would have different laws of physics, such as not having electrons, and these would not be able to support life, or not exist for long enough.

The **De Broglie–Bohm theory** is a deterministic interpretation of quantum mechanics that describes the behavior of particles in terms of both waves and particles. Particles are guided by a "pilot wave" or a quantum wave, described by the Schrödinger equation. While the wave evolves and spreads out in space, the particle itself always has a specific position and follows a trajectory. There's no need for the mysterious "collapse" of the wavefunction.

When quantum particles like photons of light interact with their surroundings, their quantum states get mixed up and entangled with the environment. This loss of quantumness is called quantum decoherence. It makes the quantum system lose its probabilistic properties, like being in multiple states at once, and behave more like a regular, classical system that follows predictable and deterministic rules. When we try to observe or measure the quantum system, we only see one state because the other states have become "entangled" with the environment and effectively disappear from our view. It's a big challenge in the field of quantum physics because it limits our ability to harness the full potential of quantum mechanics for things like quantum computing or communication.

Quantum mechanics is complex with laws that challenge human intuition but it's essential for understanding everything from the behavior of atoms to the functioning of electronics and even the nature of reality itself.