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joseph h
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234 votes
20 answers
38k views

What exactly is a photon?

182 votes
13 answers
28k views

Why does space expansion not expand matter?

  • 1,935
142 votes
7 answers
25k views

Why are there only derivatives to the first order in the Lagrangian?

  • 2,278
95 votes
9 answers
14k views

What is time, does it flow, and if so what defines its direction?

89 votes
12 answers
14k views

What is spontaneous symmetry breaking in quantum systems?

53 votes
7 answers
8k views

Why isn't the Euler-Lagrange equation trivial?

  • 1,706
48 votes
5 answers
12k views

Noether charge of local symmetries

44 votes
4 answers
14k views

Do virtual particles actually physically exist?

37 votes
8 answers
8k views

Why is there this relationship between quaternions and Pauli matrices?

  • 2,273
34 votes
6 answers
10k views

If string theory is inconsistent with observations, why hasn't it been rejected yet?

  • 16.9k
30 votes
8 answers
7k views

Deterministic quantum mechanics

27 votes
3 answers
14k views

Intuitive understanding of the elements in the stress-energy tensor

  • 7,640
23 votes
6 answers
5k views

How do we know that entanglement allows measurement to instantly change the other particle's state? [duplicate]

  • 247
20 votes
7 answers
12k views

Why was quantum mechanics regarded as a non-deterministic theory?

  • 5,966
16 votes
1 answer
3k views

Quivers in String Theory

  • 585
16 votes
2 answers
6k views

What does a non-perturbative theory mean?

  • 161
14 votes
3 answers
36k views

How is a Hamiltonian constructed from a Lagrangian with a Legendre transform

  • 243
12 votes
2 answers
906 views

Is my interpretation of a quantum field correct?

  • 497
11 votes
5 answers
2k views

Why do eigenvalues correspond to observable quantities?

  • 689
9 votes
3 answers
4k views

Counting degrees of freedom of gauge bosons

  • 727
9 votes
1 answer
7k views

Correspondence between wave function and state vector

7 votes
2 answers
1k views

What is the exact meaning of Lorentz invariance for a quantum scalar field?

7 votes
1 answer
144 views

What developments have been made in this unsolved problem? [duplicate]

6 votes
2 answers
424 views

Complex Lie algebra vs Real Lie algebra in Physics

5 votes
3 answers
2k views

Generalising a Dirac Delta function formula in General Relativity

4 votes
4 answers
494 views

Forces acting on rope [duplicate]

4 votes
6 answers
593 views

How exactly does an electron falling back to its original state from an excited state produce electromagnetic waves?

4 votes
1 answer
726 views

Do Dirac field states belong to a Hilbert space with spinor coefficients?

  • 3,107
4 votes
1 answer
109 views

Is there a mathematical model of the process of "getting entangled with"?

  • 99
4 votes
3 answers
238 views

Why does the spectrum of Sun follow a black-body distribution curve?