CeleryKills

How a Casual Claim About Gravity Sent Me Down a Physics Rabbit Hole

I first heard the phrase “entropy equals gravity” over a beer that was aggressively hoppy, the kind that tastes like a physics dare. We were at our local micro‑brewery, the kind with reclaimed wood and a chalkboard menu, and one of my friends, who designs chip architectures for a living and casually understands physics at a level that makes my brain itch, dropped it into the conversation like it was no big deal. Entropy equals gravity. He said it the way you say “oh yeah, rain’s coming,” and then took a sip.

I nodded, the universal signal for “I will absolutely have to look this up later.”

That later turned into a rabbit hole, and this essay is me climbing back out with muddy boots and a few useful maps.

At a high school level, gravity is simple. Things with mass attract each other. Newton gives you a clean equation, , and for most everyday purposes, it works beautifully. Einstein complicates the picture by saying gravity isn’t really a force at all, but the curvature of spacetime caused by mass and energy. Objects follow the curves because that’s what straight lines look like in a curved space. It’s elegant, weird, and experimentally rock solid.

Entropy, meanwhile, shows up in a different wing of the physics building. It’s the measure of disorder, or more precisely, the number of microscopic ways a system can be arranged while still looking the same macroscopically. High entropy means lots of possible arrangements. Low entropy means very few. The second law of thermodynamics tells us entropy tends to increase. Coffee cools. Ice melts. Bedrooms drift toward chaos.

So why are these two ideas even in the same sentence?

The short answer is black holes ruined everything, in the best possible way.

In the 1970s, Jacob Bekenstein noticed that black holes behave suspiciously like thermodynamic objects. They have entropy proportional to the area of their event horizon, not their volume. Stephen Hawking then showed black holes have a temperature and emit radiation. Suddenly gravity, quantum mechanics, and thermodynamics were all sitting at the same table, glaring at each other awkwardly (Bekenstein, “Black Holes and Entropy,” 1973; Hawking, “Particle Creation by Black Holes,” 1975).

Ted Jacobson took this discomfort seriously. In a 1995 paper, he showed that if you assume spacetime has entropy and obeys the basic laws of thermodynamics, you can derive Einstein’s field equations themselves. Not approximate them. Derive them. Gravity starts looking less like a fundamental rule and more like an equation of state, the way pressure emerges from molecular motion in a gas (Jacobson, “Thermodynamics of Spacetime,” 1995).

That’s the moment when “entropy equals gravity” stops sounding like bar talk and starts sounding like a legitimate, if unsettling, idea.

Later, Erik Verlinde pushed this further, arguing that gravity is an entropic force. The classic analogy is a stretched polymer. The force that pulls it back isn’t fundamental; it emerges from the system’s tendency to maximize entropy. Verlinde suggested gravity works the same way. Matter moves because information rearranges itself, and what we perceive as gravitational attraction is the macroscopic result (Verlinde, “On the Origin of Gravity,” 2011).

Here’s where it gets fun and uncomfortable. If gravity is emergent, then spacetime itself might be emergent. And if spacetime is emergent, then at the deepest level, the universe might be made of information, not geometry.

Quantum entanglement strengthens this suspicion. In modern approaches like AdS/CFT, spacetime geometry appears to be tightly linked to patterns of entanglement. Change the entanglement structure, and spacetime itself stretches, pinches, or disconnects. Some physicists now say entanglement is the glue that holds space together. No entanglement, no space (Van Raamsdonk, “Building up spacetime with quantum entanglement,” 2010).

Listening to Curt Jaimungal’s interview with Jacobson, what struck me was how cautious the language actually is, despite the click‑friendly framing. Jacobson never claims gravity is “nothing but” entropy. He treats thermodynamics as a robust, fault‑tolerant framework that often points toward deeper microscopic truths before we fully understand them, much like thermodynamics preceded statistical mechanics (Jaimungal, The Physicist Who Proved Entropy = Gravity, 2023).

That caution matters, because the idea has real weaknesses.

Entropic gravity reproduces familiar gravitational behavior in some regimes, but it struggles in others. Precision tests like gravitational lensing and galaxy cluster dynamics are not cleanly explained without adding extra assumptions. Dark matter does not quietly disappear just because you invoke entropy. Worse, the microscopic degrees of freedom that supposedly give rise to spacetime remain unspecified. Saying “information” does a lot of work here, and sometimes too much.

Philosophically, the idea is both powerful and unsettling. It demotes gravity from fundamental law to emergent behavior, which feels like progress until you ask, emergent from what? Information stored where? Entanglement of what underlying system? At some point, the explanation risks becoming a restatement. Gravity happens because entropy increases, entropy increases because information rearranges, information rearranges because… well, here be dragons.

Still, the strengths are hard to ignore. The entropy–gravity connection explains why gravity seems to “know” about thermodynamics, why horizon areas matter, and why black holes are such profound laboratories. It reframes gravity not as an actor, but as a symptom. That shift alone changes how you think about quantum gravity, cosmology, and the early universe.

Recent cosmological data only sharpens the tension. Dark energy might not be constant. The universe’s expansion history may be more subtle than we thought. If spacetime itself is emergent, then cosmic acceleration might be telling us something about the thermodynamic bookkeeping of the universe as a whole, not just about a mysterious fluid filling space (DESI Collaboration, 2024).

So where does “entropy equals gravity” sit? It’s not a theory in the textbook sense. It doesn’t yet give a full predictive framework with unique, testable signatures. It’s better described as a deep hypothesis with partial derivations, a lens that reorganizes existing facts and exposes pressure points in our understanding.

Which brings me back to that beer.

If gravity really is an emergent entropic phenomenon, then what we call spacetime might be more like temperature than like atoms. Real, measurable, indispensable, and not fundamental. If that’s true, what else do we treat as bedrock that is actually a large‑scale average? And if information underlies geometry, what does that say about causality, locality, and the idea that the universe is made of stuff at all?

I don’t have answers. That’s the point. The idea doesn’t settle the conversation. It opens it. And the next time someone casually drops “entropy equals gravity” over a drink, you’re allowed to nod again. But this time, you’ll know why your brain itches.

References

Bekenstein, J. D. “Black Holes and Entropy.” Physical Review D, 1973.
Hawking, S. “Particle Creation by Black Holes.” Communications in Mathematical Physics, 1975.
Jacobson, T. “Thermodynamics of Spacetime: The Einstein Equation of State.” Physical Review Letters, 1995.
Verlinde, E. “On the Origin of Gravity and the Laws of Newton.” Journal of High Energy Physics, 2011.
Van Raamsdonk, M. “Building up spacetime with quantum entanglement.” General Relativity and Gravitation, 2010.
DESI Collaboration. “Dark Energy Spectroscopic Instrument Results.” 2024.
Jaimungal, C. The Physicist Who Proved Entropy = Gravity. Theories of Everything, 2023. https://www.youtube.com/watch?v=3mhctWlXyV8