String Theory vs Loop Quantum Gravity: The Ultimate Showdown

at the heart of a black hole the math of general relativity predicts a point of infinite density a singularity physics as we know it just breaks it completely short circuits and that failure is the whole game last week on the show we asked if dark matter was a missing particle or a sign we needed new physics well today we're at the absolute bedrock of that question the clash between einstein's universe and the quantum world and the two leading contenders to solve it string theory and loop quantum gravity could not be more different okay let's start with my camp because i think string theory offers the most compelling vision the idea is almost absurdly elegant what

if every fundamental particle an electron a quark a photon isn't a point but a tiny vibrating string of energy a string a string and its vibration determines what it is a certain frequency gives you an electron a different one gives you a photon and here's the magic one specific vibration gives you the graviton the particle that carries the force of gravity so it unifies gravity with everything else i get the appeal it's not just appeal it's mathematical necessity the equations are so rigid so interconnected they basically force this unification on you it's a theory of everything okay but let's be real it comes with a lot of baggage like what ten eleven dimensions we only

see four where are the others they're curled up compactified too small to see that's a convenient answer for something we can't test and here's the bigger problem for me string theory assumes a background it needs a preexisting stage a fixed space time for its little strings to dance on that's a total betrayal of einstein's biggest insight what insight is that that spacetime isn't a stage it is the actor gravity is the curvature of spacetime it's dynamic and that's where loop quantum gravity begins alright so sell me on lqg what does it do differently it takes both general relativity and quantum mechanics seriously from the start it doesn't add new stuff like extra dimensions or undiscovered

symmetries it asks a simpler question what if spacetime itself is quantized meaning what exactly meaning it's not a smooth continuous sheet it's made of discrete chunks atoms of spacetime woven together in a network of loops space isn't infinitely divisible there is a smallest possible length the planck length it's like a digital photo zoom in far enough and you see the pixels so space itself is granular exactly and there's no background the loops don't exist in space the loops are space that's what we call background independence and it's the core of general relativity that string theory kind of ignores okay but that feels less ambitious lqg is really just a theory of quantum gravity right

it doesn't have this grand unifying story for all the other forces and particles it's a theory of one thing not a theory of everything i see that as a strength it's focused it's trying to solve the hardest problem first without making a bunch of unproven assumptions and string theory i mean you have to admit it has a terrible track record with predictions that's the weak spot i'll give you that one forty years and not a single testable prediction has been confirmed it's a beautiful mathematical object that might have nothing to do with our universe at least lqg makes a concrete prediction that space time is discrete and we might just maybe be able to test

that by looking at how high energy gamma rays travel across the cosmos you say might and maybe that's not exactly a smoking gun either and lqg has its own massive problem trying to show how our smooth classical world emerges from these weird loopy pixels it's been a struggle it has but i'd rather struggle with the physics we know than invent 10 new dimensions just to make the math work that feels like we're just telling ourselves a nice story and i'd rather have a story that includes all the characters all the forces from the beginning even if we can't see the whole stage yet so we're back to the start one theory has this beautiful complete blueprint for a castle but it's floating in the clouds with no connection to the ground

and the other has dug a really really solid foundation right here on earth but doesn't have the full blueprint for the castle yet maybe just a guard tower yeah that's about right so the question for you listening is which bet do you take the one that adds new undiscovered ingredients to the universe in pursuit of a total theory or the one that tries to rewrite the most fundamental rules of reality space and time itself using only the ingredients we already have