Could the Universe Have Been Created Last Thursday?
There’s a well-known philosophical thought experiment called the Last Thursday hypothesis. It suggests that the universe might have been created very recently—last Thursday, five minutes ago, or at any arbitrary moment—fully equipped with fabricated memories, fossils, historical records, and cosmic background radiation, all arranged to make it look billions of years old. The hypothesis is constructed so that no observation could ever rule it out: any evidence you point to is simply declared part of the fabrication.
Traditionally, this idea is dismissed on philosophical grounds. It’s ad hoc, unhelpful, and explains nothing. But those are preferences about how we choose theories—not statements about what is physically possible. The question this paper asks is sharper and more modern: could such a universe actually exist under the physical laws we know?
To answer that, the paper takes seriously what a fact is in physics. A fact isn’t just a pattern in matter or a string of bits—it’s something that was made. Facts are the result of irreversible physical processes: nuclear reactions fixing isotope ratios, photons scattering off matter, memories being written into brains. Every one of these processes consumes energy, generates entropy, and takes time. In this view, the past isn’t a narrative—it’s the accumulated cost of irreversible commitments the universe has already paid.
Once you see facts this way, the Last Thursday hypothesis runs into a hard physical limit. You can’t simply “print” a universe full of facts without doing the work required to create them. The paper makes this precise by deriving a fact capacity bound: a limit on how many irreversible commitments any finite event can produce, grounded in well-established physics such as Landauer’s principle (which ties information erasure to energy dissipation) and quantum speed limits (which cap how fast physical systems can change). No matter how generous the assumptions, a short creation event cannot generate anything close to the amount of structured information the universe contains.
This is where the argument connects naturally to the simulation hypothesis. In popular discussions, a simulation is often imagined as a shortcut: instead of computing billions of years of cosmic history, a simulator could simply load a finished state containing all the records and memories. The paper shows that this idea is physically equivalent to the Last Thursday hypothesis. A simulation that injects a fully formed universe without computing its history is not avoiding the cost—it is attempting to bypass it.
By contrast, a simulation that genuinely computes the universe forward from simple initial conditions—step by step, paying the energy and time costs at each stage—does produce real history. In that case, the distinction between “simulated” and “real” largely evaporates. The history happened; it just happened on a different substrate. What the argument rules out is simulation-without-computation: a shortcut that tries to obtain the output of a deep computation without performing the computation itself.
The same logic also dissolves familiar worries about Boltzmann brains—observers that supposedly fluctuate into existence with fake memories. A random fluctuation can assemble a brain-shaped object, but it cannot generate the vast web of irreversible, redundant records that make memories genuine facts about a shared past. You can get the pattern of memory without the substance of memory, but not both for free.
The conclusion is simple but powerful. A universe that looks old must actually be old, because the evidence of its age is made of the same irreversible processes that produced it. You can’t have the records without the work, and you can’t do the work without the time. Whether framed as divine creation, radical skepticism, or futuristic simulation, the idea of a fully fabricated past isn’t just philosophically unsatisfying—it’s physically inadmissible.
The past isn’t a story we tell about the present. It’s the bill the universe has already paid.