Brian Keating's Video Alert and String Wars Background

• Friend/colleague Brian Keating alerted about video claiming Loop Quantum Gravity (LQG) disproven
• String wars occurred 20 years ago
• LQG faction positioned theory as string theory competitor
• Lee Smolin (LQG founder) most vocally led faction
• Major string theory criticism: practically untestable
• Smolin's gang claimed LQG better because testable with upcoming experiments
• Speaker warned Lee he'd regret this - experiments would look for LQG evidence and not find it

Loop Quantum Gravity's Lorentz Invariance Violation Prediction

• LQG violates key ingredient of Einstein's special/general relativity
• Speed of light supposedly not constant - violates Lorentz invariance
• LQG predicted speed of light depends on color of light
• Small effect but accumulates over astronomical distances
• Should cause noticeable delay of some colors over others

Experimental Testing Results

• Tested few dozen times with increasingly high precision
• No deviations from Einstein's predictions found
• New paper (2022) examined light from brightest gamma-ray burst ever detected
• Found no indication speed of light depends on color
• Brian concludes LQG in big trouble
• Paper from China group poses significant/fatal blow to theories predicting Lorentz invariance violations

Internal LQG Community Disagreement

• Most LQG researchers disagreed with Lee Smolin back then
• They said theory wouldn't have such effect
• Community split on testability claims

Loop Quantum Gravity Technical Explanation

• LQG attempts to combine quantum physics with Einstein's general relativity
• Candidate for quantum gravity theory
• String theory more ambitious: unifies gravity with other interactions (theory of everything)
• LQG less ambitious: just quantizes gravity, doesn't unify interactions
• Problem with quantizing gravity: treating space as collection of zero-size points
• Points introduce infinities in mathematics
• String theory avoids points using strings
• LQG describes space as collection of loops (not spacetime - just space)
• Treats space differently from time (Einstein said not to do this)

Area Quantization Problem

• Loops result in quantization of 2D areas of space
• Areas discretized with smallest possible value
• Minimum area = square of Planck length × constant
• Illustrated in widely shared LQG visualizations
• Problem: smallest area incompatible with Einstein's length contraction
• Length contraction: object length depends on velocity
• Length can become arbitrarily small as speed approaches light speed
• If area has minimal size, some observers see it shortened below minimal
• Having smallest possible area incompatible with Einstein's theory
• Requires modifying Einstein's mathematics
• Modification makes speed of light no longer constant

Two Camps Within LQG

• Smolin's camp: argued for Lorentz invariance violation (speaker thinks they had point)
• Carlo Rovelli's camp: claimed could recover Einstein's symmetries
• Rovelli: average over all possible ways to divide space into areas
• Speaker's view: this effectively gets back to zero-size areas
• Always some observer for whom area arbitrarily small
• Returns to singularity problem
• "Can't have your cake and eat it too"

Current Status and Personal Views

• Rovelli's camp won because testability proponents became quiet
• People literally yelled at speaker at conferences about this topic
• Speaker wants nothing to do with LQG now
• Uncertain whether Rovelli or Smolin correct
• LQG empirically unsupported theory
• As old as string theory but less successful
• Pro-LQG: can claim not disproven
• Anti-LQG: can claim not testable
• Speaker doesn't understand why people waste time on this

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