A day late - yesterday in #PhilosophyOfCosmology we asked how probability arises in Many-Worlds. We looked at both the decision-theory and self-locating-uncertainty approaches. But my main message was that (1) subjective uncertainty necessarily arises, and (2) ...
December 3, 2025 at 6:25 PM
A day late - yesterday in #PhilosophyOfCosmology we asked how probability arises in Many-Worlds. We looked at both the decision-theory and self-locating-uncertainty approaches. But my main message was that (1) subjective uncertainty necessarily arises, and (2) ...
Today in #PhilosophyOfCosmology we rolled up our sleeves and thought about the metaphysical implications and puzzles associated with Many-Worlds. How many worlds are there? Where does the energy come from? Which future self will I be? Stuff like that.
November 20, 2025 at 8:29 PM
Today in #PhilosophyOfCosmology we rolled up our sleeves and thought about the metaphysical implications and puzzles associated with Many-Worlds. How many worlds are there? Where does the energy come from? Which future self will I be? Stuff like that.
After having explained the nuts and bolts of quantum mechanics, this week in #PhilosophyOfCosmology we turn to one possible formulation of it: Everettian quantum mechanics, or the Many-Worlds Interpretation. Unlike some, I don't mind the "many-worlds" label, but the worlds are not the point.
November 18, 2025 at 6:50 PM
After having explained the nuts and bolts of quantum mechanics, this week in #PhilosophyOfCosmology we turn to one possible formulation of it: Everettian quantum mechanics, or the Many-Worlds Interpretation. Unlike some, I don't mind the "many-worlds" label, but the worlds are not the point.
Today in #PhilosophyOfCosmology we finished up the intro to QM. We talked about Hilbert space as a vector space, quantum states as superpositions of measurement outcomes, spin, entanglement, and the EPR puzzle. That's a lot, now that I think about it.
November 13, 2025 at 6:48 PM
Today in #PhilosophyOfCosmology we finished up the intro to QM. We talked about Hilbert space as a vector space, quantum states as superpositions of measurement outcomes, spin, entanglement, and the EPR puzzle. That's a lot, now that I think about it.
Starting our final big topic in #PhilosophyOfCosmology: quantum mechanics. We'll be looking at Many-Worlds, in particular, as that's the approach that opens up the most obvious philosophical questions (and opportunities!).
November 11, 2025 at 7:06 PM
Starting our final big topic in #PhilosophyOfCosmology: quantum mechanics. We'll be looking at Many-Worlds, in particular, as that's the approach that opens up the most obvious philosophical questions (and opportunities!).
Sounds sus to me, at least based on the summary here: www.sciencealert.com/physicists-j...
#philosophyofcosmology #simulationhypothesis #simulation #quantumgravity
#philosophyofcosmology #simulationhypothesis #simulation #quantumgravity
Physicists Just Ruled Out The Universe Being a Simulation
A question that has vexed physicists for the past century may finally have a solution – but perhaps not the one everyone was hoping for.
www.sciencealert.com
November 6, 2025 at 3:01 AM
Sounds sus to me, at least based on the summary here: www.sciencealert.com/physicists-j...
#philosophyofcosmology #simulationhypothesis #simulation #quantumgravity
#philosophyofcosmology #simulationhypothesis #simulation #quantumgravity
Today in #PhilosophyOfCosmology we finished up our exploration of the arrow of time by taking a brief tour of the relationship between entropy and complexity. Very high or low entropy implies simplicity; medium entropy can either be simple or complex. Dynamics determine whether complexity develops.
November 4, 2025 at 9:13 PM
Today in #PhilosophyOfCosmology we finished up our exploration of the arrow of time by taking a brief tour of the relationship between entropy and complexity. Very high or low entropy implies simplicity; medium entropy can either be simple or complex. Dynamics determine whether complexity develops.
Hectic day yesterday, forgot to update everyone on #PhilosophyOfCosmology! More arrow-of-time discourse: memory (we remember the past, not the future) and causality (causes precede effects). In both cases the ultimate reason for temporal asymmetry is the low-entropy boundary condition in the past.
October 31, 2025 at 2:19 PM
Hectic day yesterday, forgot to update everyone on #PhilosophyOfCosmology! More arrow-of-time discourse: memory (we remember the past, not the future) and causality (causes precede effects). In both cases the ultimate reason for temporal asymmetry is the low-entropy boundary condition in the past.
Hectic day yesterday, forgot to update everyone on #PhilosophyOfCosmology! More arrow-of-time discourse: memory (we remember the past, not the future) and causality (causes precede effects). In both cases the ultimate reason for temporal asymmetry is the low-entropy boundary condition in the past.
There's a huge amount of speculative physics involved, which means the whole scenario is shaky at best. But to my mind it's still the only idea on the market that accounts for the low-entropy past of our observable universe without cheating by breaking time-symmetry explicitly.
October 31, 2025 at 12:51 PM
Hectic day yesterday, forgot to update everyone on #PhilosophyOfCosmology! More arrow-of-time discourse: memory (we remember the past, not the future) and causality (causes precede effects). In both cases the ultimate reason for temporal asymmetry is the low-entropy boundary condition in the past.
Today in #PhilosophyOfCosmology we finished up our discussion of cosmological implications of the Past Hypothesis. This paper by Huw Price was formative for me: if something is "natural" about initial conditions, it should also be "natural" about future conditions.
arxiv.org/abs/gr-qc/93...
arxiv.org/abs/gr-qc/93...
Cosmology, Time's Arrow, and That Old Double Standard
It is widely accepted that temporal asymmetry is largely a cosmological problem; the task of explaining temporal asymmetry reduces in the main to that of explaining an aspect of the condition of the e...
arxiv.org
October 28, 2025 at 7:10 PM
Today in #PhilosophyOfCosmology we finished up our discussion of cosmological implications of the Past Hypothesis. This paper by Huw Price was formative for me: if something is "natural" about initial conditions, it should also be "natural" about future conditions.
arxiv.org/abs/gr-qc/93...
arxiv.org/abs/gr-qc/93...
Today we turned over the keys of our #PhilosophyOfCosmology class to guest lecturer Saakshi Dulani, a postdoc in physics and philosophy here at JHU. She talked about the Past Hypothesis of primordial low entropy: should we simply take it as a law, or try to invent dynamical explanations for it?
October 23, 2025 at 6:26 PM
Today we turned over the keys of our #PhilosophyOfCosmology class to guest lecturer Saakshi Dulani, a postdoc in physics and philosophy here at JHU. She talked about the Past Hypothesis of primordial low entropy: should we simply take it as a law, or try to invent dynamical explanations for it?
Today in #PhilosophyOfCosmology we went over some good and bad reactions to the Boltzmann Brain problem in eternally-fluctuating universes. It's bad to say "I'm not a BB so I don't care if most people are," but also bad to say "I'm not a BB so most people must not be."
October 21, 2025 at 6:16 PM
Today in #PhilosophyOfCosmology we went over some good and bad reactions to the Boltzmann Brain problem in eternally-fluctuating universes. It's bad to say "I'm not a BB so I don't care if most people are," but also bad to say "I'm not a BB so most people must not be."
Today in #PhilosophyOfCosmology we learned that an eternally-fluctuating universe doesn't really work because, given any anthropic criterion for what is an "observer," the theory predicts most such observers will be surrounded by thermal equilibrium. I.e. they will be Boltzmann Brains.
October 14, 2025 at 6:28 PM
Today in #PhilosophyOfCosmology we learned that an eternally-fluctuating universe doesn't really work because, given any anthropic criterion for what is an "observer," the theory predicts most such observers will be surrounded by thermal equilibrium. I.e. they will be Boltzmann Brains.
Today in #PhilosophyOfCosmology was one of my favorite topics: Boltzmann’s justification(s) for the Second Law, especially in response to Lohschmidt’s Reversibility Objection and Zermelo’s Recurrence Objection. Boltzmann was both incredibly clever and somewhat shifty!
October 9, 2025 at 10:14 PM
Today in #PhilosophyOfCosmology was one of my favorite topics: Boltzmann’s justification(s) for the Second Law, especially in response to Lohschmidt’s Reversibility Objection and Zermelo’s Recurrence Objection. Boltzmann was both incredibly clever and somewhat shifty!
Today in #PhilosophyOfCosmology we started in on statistical mechanics proper. It was a bold move to think that the rules of thermodynamics could be derived from an assumption of underlying particulate structure, plus an appropriate coarse-graining and some assumptions about probabilities.
October 7, 2025 at 8:01 PM
Today in #PhilosophyOfCosmology we started in on statistical mechanics proper. It was a bold move to think that the rules of thermodynamics could be derived from an assumption of underlying particulate structure, plus an appropriate coarse-graining and some assumptions about probabilities.
Mindscape 331 | Solo: Fine-Tuning, God, and the Multiverse. In which I shamelessly steal material from the #PhilosophyOfCosmology course I am teaching to talk about some big questions. #MindscapePodcast
www.preposterousuniverse.com/podcast/2025...
www.preposterousuniverse.com/podcast/2025...
October 6, 2025 at 11:42 AM
Mindscape 331 | Solo: Fine-Tuning, God, and the Multiverse. In which I shamelessly steal material from the #PhilosophyOfCosmology course I am teaching to talk about some big questions. #MindscapePodcast
www.preposterousuniverse.com/podcast/2025...
www.preposterousuniverse.com/podcast/2025...
We talked a bit about that today in #PhilosophyOfCosmology, before switching gears and starting on entropy and the arrow of time. Today was just groundwork: Laplace, Carnot, Clausius. The beginnings of thermodynamics and its tensions with classical mechanics. Lots of excitement ahead.
September 30, 2025 at 5:57 PM
We talked a bit about that today in #PhilosophyOfCosmology, before switching gears and starting on entropy and the arrow of time. Today was just groundwork: Laplace, Carnot, Clausius. The beginnings of thermodynamics and its tensions with classical mechanics. Lots of excitement ahead.
Today in #PhilosophyOfCosmology we talked about two puzzles of anthropic reasoning. One was the cosmological measure problem in eternal inflation. It's a manifestation of the difficulty of getting a sensible answer when dividing infinity by infinity.
en.wikipedia.org/wiki/Measure...
en.wikipedia.org/wiki/Measure...
Measure problem (cosmology) - Wikipedia
en.wikipedia.org
September 25, 2025 at 7:45 PM
Today in #PhilosophyOfCosmology we talked about two puzzles of anthropic reasoning. One was the cosmological measure problem in eternal inflation. It's a manifestation of the difficulty of getting a sensible answer when dividing infinity by infinity.
en.wikipedia.org/wiki/Measure...
en.wikipedia.org/wiki/Measure...
Today in #PhilosophyOfCosmology we pointed to some of the unexpected conclusions one reaches by starting with the assumption that we are typical observers in some distribution.
September 23, 2025 at 5:53 PM
Today in #PhilosophyOfCosmology we pointed to some of the unexpected conclusions one reaches by starting with the assumption that we are typical observers in some distribution.
Today in #PhilosophyOfCosmology we continued our exploration of anthropics, but pushed things in a more quantitative direction.
First up: the possibility that cosmological/physical fine-tuning of parameters provides evidence for Theism over Naturalism. The old Argument From Design.
First up: the possibility that cosmological/physical fine-tuning of parameters provides evidence for Theism over Naturalism. The old Argument From Design.
September 18, 2025 at 5:34 PM
Today in #PhilosophyOfCosmology we continued our exploration of anthropics, but pushed things in a more quantitative direction.
First up: the possibility that cosmological/physical fine-tuning of parameters provides evidence for Theism over Naturalism. The old Argument From Design.
First up: the possibility that cosmological/physical fine-tuning of parameters provides evidence for Theism over Naturalism. The old Argument From Design.
Today in #PhilosophyOfCosmology we tackled "fine-tuning" in a somewhat more systematic way. But I emphasized that it's not very systematic. As examples I gave the flatness problem, cosmological constant problem, hierarchy problem, neutron/proton mass difference, & low entropy of the early universe.
September 16, 2025 at 6:01 PM
Today in #PhilosophyOfCosmology we tackled "fine-tuning" in a somewhat more systematic way. But I emphasized that it's not very systematic. As examples I gave the flatness problem, cosmological constant problem, hierarchy problem, neutron/proton mass difference, & low entropy of the early universe.
Today in #PhilosophyOfCosmology: Guest lecture by Joe Silk! One of our leading contemporary theoretical cosmologists. And a vocal sceptic of the idea that the multiverse and anthropic reasoning have true explanatory power.
en.wikipedia.org/wiki/Joseph_...
en.wikipedia.org/wiki/Joseph_...
Joseph Silk - Wikipedia
en.wikipedia.org
September 11, 2025 at 9:03 PM
Today in #PhilosophyOfCosmology: Guest lecture by Joe Silk! One of our leading contemporary theoretical cosmologists. And a vocal sceptic of the idea that the multiverse and anthropic reasoning have true explanatory power.
en.wikipedia.org/wiki/Joseph_...
en.wikipedia.org/wiki/Joseph_...
Today in #PhilosophyOfCosmology we went over the basics of probability theory -- what probabilities are, joint/marginal/conditional probabilities, and Bayes's Theorem. Hilariously superficial as an overview of deep ideas, but at least we got the notation on the table.
September 9, 2025 at 6:43 PM
Today in #PhilosophyOfCosmology we went over the basics of probability theory -- what probabilities are, joint/marginal/conditional probabilities, and Bayes's Theorem. Hilariously superficial as an overview of deep ideas, but at least we got the notation on the table.
Today's #PhilosophyOfCosmology class was about self-locating uncertainty. We talked about David Lewis's paper "Attitudes De Dicto and De Se." (It's the only course reading that Canvas gave a medium readability score; all the others were not-very-readable.)
www.asc.ohio-state.edu/roberts.21/N...
www.asc.ohio-state.edu/roberts.21/N...
www.asc.ohio-state.edu
September 4, 2025 at 5:30 PM
Today's #PhilosophyOfCosmology class was about self-locating uncertainty. We talked about David Lewis's paper "Attitudes De Dicto and De Se." (It's the only course reading that Canvas gave a medium readability score; all the others were not-very-readable.)
www.asc.ohio-state.edu/roberts.21/N...
www.asc.ohio-state.edu/roberts.21/N...
Today in #PhilosophyOfCosmology we talked about fine-tuning and naturalness, and introduced inflation and eternal inflation.
So many juicy philosophical questions that physicists don't take as seriously as they should! What does it mean for parameters/initial conditions to be "unnatural," anyway?
So many juicy philosophical questions that physicists don't take as seriously as they should! What does it mean for parameters/initial conditions to be "unnatural," anyway?
September 2, 2025 at 9:42 PM
Today in #PhilosophyOfCosmology we talked about fine-tuning and naturalness, and introduced inflation and eternal inflation.
So many juicy philosophical questions that physicists don't take as seriously as they should! What does it mean for parameters/initial conditions to be "unnatural," anyway?
So many juicy philosophical questions that physicists don't take as seriously as they should! What does it mean for parameters/initial conditions to be "unnatural," anyway?