Modern Physics

1. What is modern physics? Modern physics is the study of things that classical physics (the older type of physics) can’t explain. It deals with very big things like galaxies, very small things like atoms, and things moving extremely fast. The two main parts of modern physics are relativity and quantum mechanics.

2. Why did classical physics need to change? Classical physics, based on Newton’s and Maxwell’s ideas, works well for everyday situations. But when scientists looked at things very closely, they found some behaviors that classical physics couldn’t explain. This included how light behaves, how atoms are structured, and what happens at very high speeds or in strong gravity. These discoveries led to the development of modern physics in the early 1900s.

3. Relativity - special and general: Relativity, developed by Einstein, deals with how space and time work, especially when things move very fast or when there’s strong gravity.

   • Special relativity (from 1905):
     • It’s based on two main ideas: physics laws are the same for everyone moving at a constant speed, and light always moves at the same speed no matter who’s looking at it.
     • This leads to some strange effects: time slows down for things moving very fast, objects get shorter when they move fast, and mass can be turned into energy (that’s what E = mc² means).
   • General relativity (from 1915):
     • Einstein said that gravity isn’t a force, but instead it’s the bending of space and time caused by heavy objects.
     • This explains things like light bending around stars and why the universe is expanding.
4. Quantum mechanics: Quantum mechanics deals with very small things like atoms and the particles inside them. It’s very different from classical physics:
   • It says that tiny things like light and electrons can act like both waves and particles.
   • There’s a rule called the uncertainty principle that says you can’t know everything about a particle at once. If you know exactly where it is, you can’t know how fast it’s moving, and vice versa.
   • Instead of saying exactly where a particle is, quantum mechanics uses math to describe the probability of where it might be.
5. Quantum field theory: Quantum field theory is an idea that combines quantum mechanics with special relativity:
   • It says that particles are actually vibrations in fields that exist everywhere.
   • It explains how forces work between particles using special force-carrying particles.
   • This theory led to the Standard Model, which explains three of the four main forces in nature and all the basic particles we know about.

6. The problem with gravity: Even though we understand gravity well on a large scale (with general relativity) and we understand tiny particles well (with quantum mechanics), we still can’t combine these ideas. We don’t know how gravity works for very small things. Scientists are still working on this problem.

7. Modern cosmology: Modern physics also studies the whole universe:
   • The Big Bang Theory says the universe started as a very hot, dense point about 13.8 billion years ago and has been expanding ever since.
   • We’ve discovered that most of the universe is made of things we can’t see directly: dark matter and dark energy. We know they’re there because of how they affect things we can see, but we don’t know exactly what they are.

In summary, modern physics is our attempt to explain how the universe works at its most basic level, from the tiniest particles to the largest galaxies, using ideas that often seem strange compared to our everyday experiences.