There are other Groundhog Day questions and other Simulation questions. I believe this differs from all the others.

[This movie features a] ...TV weatherman who, during an assignment covering the annual Groundhog Day event, is caught in a time loop, repeating the same day ... [over and over]. https://en.wikipedia.org/wiki/Groundhog_Day_(film)

The point is that only one character is aware that same day is repeating. Others all start their day as normal and continue unless their day is altered by interacting with the main character.

If you woke up repeatedly like the weatherman Phil Connors and could convince* yourself you weren't merely dreaming or hallucinating, wouldn't your only other conclusion have to be that you were (a) in a simulation of some kind and (b) it is designed just for you?

The reason I say this is that if your universe is the 'real' one then it would have to reset itself purely for your benefit to provide your repeating experience. Resetting the actual universe is presumably much more difficult than resetting a simulation based on one viewer's point of view - yours.

  • When I say 'convince yourself' I don't mean prove it indisputably, I mean decide in your own mind - perhaps to preserve your sanity!.
  • $\begingroup$ Stanisław Lem wrote a novel about such experience. AI was experiencing and creating "on the fly" it's own world based on all human experience recorded on wax rolls. The Dejavu effect was a result on rolls skipping and replaying the same content twice. And that experiencing dejavu was a proof of you not being real and just using provided content. $\endgroup$ Dec 5, 2018 at 15:04
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    $\begingroup$ Did you just start your thousandth repetition of this day, but suddenly realized that having access to the internet means you can crowdsource your philosophical issues? $\endgroup$ Dec 5, 2018 at 17:51
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    $\begingroup$ @chaslyfromUK I notice you seem to ask a lot of questions with interesting concepts, which lead to some fascinating answers, but always start with pages of back-and-forth clarifications and refinements in the comments. Have you considered writing your questions down, and reviewing them at a later date or bouncing them off an individual, to try and get them into the more "polished" form before they get posted to Worldbuilding? $\endgroup$ Dec 6, 2018 at 8:24
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    $\begingroup$ [SPOILERS] See the webnovel "Mother of Learning" for a groundhog day story very close to your question. [/SPOILERS] $\endgroup$
    – Mindwin
    Dec 6, 2018 at 11:40
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    $\begingroup$ @chaslyfromUK -- That's exactly what the Sandbox is for: helping you formulate your queries before posting them! $\endgroup$
    – elemtilas
    Dec 10, 2018 at 5:52

11 Answers 11


No. It would not prove that this was a simulation for you alone, as there are other strong possibilities.

  1. This is a simulation, but not for you. Your memory of previous loops are due to a bug. Where your state should be reset at the beginning of each loop, for some reason your experience has been carried over to the next loop.
  2. This is not a simulation, you (or your consciousness) are really travelling back in time (to a preset point).
  3. This is a simulation and it is for you, but it is shared with others who you have not encountered (it isn't just for you).
  4. You're not in a simulation - but all of the "past iterations" you remember are artificially implanted memories which were simulated. (As suggested by Chronocidal in the comments. A particularly horrific one to realise as you choose to skip out on "this go round").

There are many other possibilities, many of which are included in other answers. The key thing is that your experience doesn't rule out enough other possibilities to conclude this is a simulation just for you.

  • $\begingroup$ (1) Fair point, (2) If you are really travelling back then you are affecting everyone's future maybe even setting off a butterfly effect - possible though, (3) Also possible. $\endgroup$ Dec 5, 2018 at 17:12
  • $\begingroup$ @chaslyfromUK In the movie, the main character did wind up changing people's lives by doing drastically different things the final time around than the first time. $\endgroup$ Dec 5, 2018 at 18:43
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    $\begingroup$ 4. You're not in a simulation - but all of the "past iterations" you remember are artificially implanted memories which were simulated $\endgroup$ Dec 6, 2018 at 8:21
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    $\begingroup$ On 1. - In reality, the "groundhog day" trope is a lot older than the movie in science fiction stories. One particular story using it was about 1. - one person fell asleep under his boat (or car?) as he realised everybody was repeating their day. Turns out that if he was under the vehicle at night, his memories remained, while everybody else was apparently "reset". He eventually found out his entire city was a (non-virtual) simulation to test ads on humans and see how they respond. Every they'd sweep and wipe all memories of the day, unless you were protected by a sheet of metal. $\endgroup$
    – VLAZ
    Dec 6, 2018 at 8:59
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    $\begingroup$ @Chronocidal Or you're just going crazy. The "past memories" are just a figment of your imagination and you are not of sound enough mind to be able to tell the difference (e.g. the same way that sometimes you are convinced of very obviously false things being true while in a dream) $\endgroup$
    – SamYonnou
    Dec 6, 2018 at 19:14

There are a few flaws in your logic...

1). How would you convince yourself that you weren't merely dreaming or hallucinating? The simplest explanation for what you are experiencing is that you are dreaming. Ruling that out just for argument, the next most likely explanation is that your brain is broken; you are delusional and what you are perceiving is the result of something wrong in your head.

2). Assuming that it is really happening, there is no proof that it is happening outside of the portion of the world that you are directly interacting with. In Phil's case, the time reset need only encompass the town he is in and everything within one day's driving distance (remember that he steals a pickup truck). The rest of the universe could be cruising forward through time at its normal pace. There could even be another instance of the town and surrounds which are traveling forward through time. Phil could have fallen into a pocket universe with an extremely short, circular timeline.

3). Even if you somehow rule out every possible explanation except for virtual reality, there is still nor proof that you are alone in the simulation. There is only proof that, so far, the software running the simulation has not allowed you to interact with any other real people.

There is nothing wrong with your conclusion that you are trapped in a simulation, but it is far from an exclusive and thus proven certainty, given the available facts.

  • $\begingroup$ Thanks - (1) I kind of specified that you could convince yourself (whether correctly or not). (2) In the movie they broadcast the live event to the nation so anyone capable of picking up the broadcast would have to be reset. This would cause chaos unless the whole world was reset to match. (3) That is a flaw. As you say, you might not be the only one. $\endgroup$ Dec 5, 2018 at 14:17
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    $\begingroup$ Okay, agreed on #1, you did specify that. On #2, again you are right, but that still only limits the reset zone to the planet. But then you have to deal with the gravitational relationships of the solar system and the motion of that solar system relative to our galaxy, and that of our galaxy relative to all others, so... yes, I guess the universe is the minimum scope of the rest. Glad that at least 1/3 of my answer was useful. $\endgroup$ Dec 5, 2018 at 14:33
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    $\begingroup$ No wait! On #2,we only know that Phil is broadcasting out to the rest of the nation. We don't know if the nation outside the time-reset-zone is receiving anything. So it could be a town size pocket universe... I think. $\endgroup$ Dec 5, 2018 at 14:35
  • $\begingroup$ I'm not sure if I've understood that last bit. Surely when the snow melts and the film crew all go home, they will have been missing from work for many days. $\endgroup$ Dec 5, 2018 at 16:48
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    $\begingroup$ @chaslyfromUK They could just fade from reality half a mile down the road. Who says they have to have any reality in the outside world? $\endgroup$ Dec 5, 2018 at 19:36

It's a possible theory, and maybe even a strong one but to be your only other conclusion, it has to be the only theory. Another possible theory, albeit a bit sci-fi'ish (but probably no more than the virtual reality), is that person may be slipping through alternate universes, each one exactly like the previous except it was 24 hours behind and the universe he left carries on as normal as for the person he replaces, he could have ended up anywhere.

I'm not saying that's a scientifically possible answer or the only answer but it does make the point that no hard conclusion for it being a virtual reality is possible (without other information).

Thinking about it, I remember that in a specific scene he got hit by a train and died. Maybe that's a place to start if you want to convince a story character that it was a simulation. His physical body is being reset, injuries healed. This would at least suggest his body isn't traveling between universes. And if the person in question is versed in virtual reality, it may be enough to remove reasonable doubt.

  • $\begingroup$ Yes, parallel universes could do it. However it seems a bit suspicious that he always slipped back 24 Earth hours. It still makes me think someone is more likely to be doing a reset. $\endgroup$ Dec 5, 2018 at 14:21
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    $\begingroup$ it could just be a natural phenomenon that happens to be 24hours,(it might not even be strictly 24hours i.e 23.59) or as you say it could be controlled by a conscious entity in the way that old thought experiments along the lines of a simulated world before computers where controlled by "demons". if your creating a story where you want the character to come to the conclusion of a virtual reality your going to have to provide more information to him $\endgroup$ Dec 5, 2018 at 14:27
  • $\begingroup$ He also never ages, despite apparently repeating the day long enough to be granted an honorary medical doctorate. This also casts doubt on the parallel universe hypothesis. $\endgroup$ Dec 5, 2018 at 20:01

You couldn't know that time was resetting flawlessly each time, because your own senses and memory aren't perfect, and if you used any kind of external instrument (like, trying to memorise patterns from the cosmic background radiation to see if they repeat) then someone could be manipulating that.

All you know for sure is that you remember this day happening, and external events appear to be happening the same way you remember.

The Occam's-razor hypothesis would be that it's all in your head. The fact that you seem to know the future does not disprove this, because you could just be misremembering your previous memories. You say "the next card will be a three of spades", and then the card is drawn, and you think "my god, the jack of diamonds – just as I predicted!".

If you discount that possibility regardless, then you might think a simulation (VR, Truman Show, etc.) is more likely than the whole universe, outside of your mind, being in a loop. I am not sure that follows. Any of these explanations would require phenomena or technology well beyond your experience, so it's pure speculation to say that a flawless simulation of the whole universe is "more likely" than a phenomenon that teleports a human mind back through time.

In fact, I'm not sure there is even any difference from your point of view. If the simulation is so good that you could live in it forever, and have great-grandchildren who colonise the stars for a billion years and so on, then the only practical difference between that and the "real" universe is the label. All the time loop tells you is that, whatever the nature of the universe you live in, something is happening that is outside your previous understanding of how that universe works.

  • $\begingroup$ But when your character dies, maybe all your simulated children and grandchildren are also deleted. I think the point about the Truman Show is a good one but I suppose you could classify that as 'a simulation'. $\endgroup$ Dec 5, 2018 at 17:16
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    $\begingroup$ In the movie, the character was acting successfully on past memories, such as having the timing down exactly correctly to steal money from a delivery, and knowing where to be when to prevent accidents. That would argue against fake memories. $\endgroup$ Dec 5, 2018 at 18:49
  • $\begingroup$ @DavidThornley those things would disprove the hallucination theory for an outside observer (such as you, the movie viewer). But from Phil's point of view, he doesn't know if he really averted the accidents, or if his memory of that is false. As far as he knows, it's all just a vivid dream. (As movie viewers, we don't know whether the movie is in the first person or the third person; we might be watching a movie of his dream) $\endgroup$
    – bobtato
    Dec 5, 2018 at 23:29
  • $\begingroup$ @bobtato Theoretically possible, but if I'm assuming that I'm just having a vivid dream, or subject to Descarte's demon, all bets are off. For practical purposes, I have to assume that a world I am in for a long period of time, that surprises me, is real, although I may be greatly mistaken about parts of it. Then again, did you see the movie Total Recall? $\endgroup$ Dec 7, 2018 at 3:06

Speaking as a scientist, it would not preserve my sanity to believe the world and all the people in it were a simulation for only me. It would quite likely make ME a simulation too! (Particularly if, by investigation and/or observation, I cannot find any notable physical difference between my own body and others -- why should I believe they are simulations and I am not?)

If not, I'd be the only sentient in the world, and that would be the ultimate loneliness, and make me a rat treading water forever. Since I couldn't kill myself, I think I'd go insane.

Also, speaking as a scientist, there is no telling how much energy it would take to reset the universe to 24 hours ago, with only my brain and memories changed. It is entirely possible every moment of the universe exists simultaneously, so no "energy" is required at all. More generally, if you have no idea how something is accomplished, you have no idea how easy or difficult it is.

For example, before the existence of gunpowder or other explosives, somebody trying to compute how to accelerate a quarter-ounce stone (about the weight of a 9mm bullet) to supersonic speeds with a handheld device may conclude it is impossible to do with any combination of springs or levers. Does that make a handgun impossible? Of course not, they are just too uninformed to know that there is a pretty cheap, safe and easy way to do it; in fact it is so cheap and safe some people will shoot a few hundred bullets in a day for the fun of it, and never notice the expense.

As a scientist, if I can't figure out how something is done, I can't estimate the cost of doing it. Also, I will believe my own eyes, ears and senses: If everything looks like I traveled back in time with my memories intact, I will believe that is what I am doing, and that neither me (or my senses) or the people are simulations. Also as a research scientist, I am perfectly fine saying "I don't know", and I don't need even a possible explanation for a phenomenon, I am comfortable with no explanation. Most of the things science is investigating have no answer, nobody knows why they happen or how to predict them or how they work. It is our job to come up with testable ideas to give us clues to that.


The real question is the question of proof. If a person was in a scenario where everyday was an absolutely flawless repetition of Groundhog Day, the question is really one of how many possible solutions are there for such a situation.

As a standard of proof this answer will use Ockham's Razor (please note the spelling of "Ockham" does vary). Namely, establishing what is the minimal explanation for any phenomenon.

One possible explanation is that the recursive world is the result of a simulation. Certainly the person is the only one aware that each is exact repeat and for him it is Groundhog Day every day.

But it is not sole explanation. Also, as an explanation the simulation hypothesis has to make a number of assumptions. (1) The world is a simulation, (2) whoever or whatever is in charge of the simulation has taken the person and allowed them to continue advance through the simulation gaining new memories and experience while everybody else does not or they lose that in the daily reset, and (3) what is the purpose of subjecting only one person to the Groundhog Day recursion.

Actually of those assumptions itself requires their own explanation. In fact, all three explanations are needed to explain the simulation as such.

Simpler explanations for Groundhog Day can be devised. The person is caught in a time-loop type of phenomenon. When he reaches the end of Groundhog Day he is transported back to its beginning. While the nature of the time-loop itself cannot be explained, its consequences conform to those experienced by someone caught in a time-loop. Basically the person experiences the Groundhog Day effect.

The person has become adrift in the multiverse. At the end of each Groundhog day the person is shunted "sideways" and one day "backwards" in time to the beginning again.

The person is a brain in a vat experiencing an exceptionally fidelity virtual reality which is, of course, Groundhog Day. Simulating the world for one person is much more economical hypothesis than the simulation of the whole world.

Voltaire's demon. This concept was a philosophical proposition that reality as we know it was created by a being, possibly a supernatural entity, by manipulating our senses to generate the impression that we were experiencing an actual world. Anyone could be effectively a sort of brain in a vat experiencing an artificial reality from synthetic perception. This means we could be simply conscious spirits floating in formless void with Voltairean demons fooling us into believing we exist in a physical world.

This makes Voltaire's demon the precursor of both the brain in the vat and the universe as a computer simulation.

In conclusion, with the application of Ockham's Razor and the realization that it is possible to construct simpler counter-examples as explanations for a Groundhog Day than the presumption that the existence of a Groundhog Day would be proof for the universe as a simulation. The answer is no. Groundhog Day does not prove the universe is simulation. Because other simpler explanations are also possible. Also, it is not possible to choose conclusively between the alternative explanations. This makes the suggestion of Groundhog Day proves the simulation hypothesis only one possible explanation among many.


Although there is not enough evidence, nor opportunity, to make conclusive reasoning, a Simulation Hypothesis would be a major cause you should consider

The issue, in a way, with any natural cause is that it is only happening to you.

Furthermore, current scientific thought is based on observations of our physical world. 'Everything is physics' some people say. So how come all around you everything resets, except the atoms in your brain? After all, your memories (which are still intact) are just collections of chemicals and processes, for which if you remember would have to have been continued unaffected for you to even perceive your days are repeating.

This gives credence to the suspicion (and I say that word sparingly) that all is not well, that some alien or other force is artificially interfering with your world.

As you are immortal, one way to test this hypothesis is to conduct brain surgery on yourself. Do removing parts of your brain responsible for various functions also carry over? What if you take a drug such that effects should carry over, do these? If memories are recorded and you wake up the next morning, how come your brain is ok suddenly, retaining memories, when last night you removed those memories? Test the boundaries.

In a way, this is where physics thrives. It is at its most exciting when investigating things that don't line up, and finding the point at which it does or doesn't.

(By the way, I do not condone anyone doing brain surgery on yourself - this is just a theoretical thought experiment - Kids don't do this at home).

  • $\begingroup$ In the uncut movie he did get injured and even killed. The physical damage was always repaired and so presumably was his physical brain. The problem is that he still remembered the previous day(s) despite this. Therefore I say his whole physical body including his impression that he had a brain to operate on, must be part of the illusion and just his memory is preserved somewhere in the simulation and immune from being reset. $\endgroup$ Dec 5, 2018 at 14:25
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    $\begingroup$ @chaslyfromUK The point is though that memory and brain are not separate normally - they are the same. We take drugs that make us forget. Parts of our brains affect the way we think. If these are reset, but our memories are retained, then this is proof that our Brains and Memories are separate, and thus our bodies do not really exist. Our memories, and therefore our brain, must be therefore existing 'elsewhere'. $\endgroup$
    – flox
    Dec 5, 2018 at 14:30
  • $\begingroup$ @flox, there is no proof that memory and brain are the same. In fact, drugged amnesiac experiences can be recovered by specific techniques (though it's a lot better not to take those drugs, obviously). Your conclusion in comments that "thus our bodies don't really exist" in the hypothetical Groundhog Day scenario is completely non sequitur and bad science. $\endgroup$
    – Wildcard
    Dec 5, 2018 at 20:49
  • $\begingroup$ @Wildcard Apologies - I skipped a step for you. The conclusion was that memories are stored in our brain, and therefore they are dependent on it. If your brain is affected, so too are your memories. If your brain is altered or damaged, yet your memories are intact, it stands to reason this relationship is not what you thought it was. If something is not recoverable, yet you recovered it, we need to reassess the assumptions made till that point. What I am advocating is rigorous experimentation, conclusion, and testing that conclusion. $\endgroup$
    – flox
    Dec 6, 2018 at 11:56

No. Consider HBO's Westworld. For the hosts, everyday is "Groundhog Day", but the simulation is not for them, it's for the guests of the park.

  • $\begingroup$ Good point. I can't remember whether the hosts have their memories wiped or whether they are just paid actors. $\endgroup$ Dec 5, 2018 at 22:47

While Phil has no way of knowing this, the second conclusion (that it was for Phil's benefit) is demonstrably untrue.

Groundhog day, as a movie, IS a simulation, and it is NOT for Phil's benefit. It is for the benefit of the movie studios, and the actors, and the people who paid to watch the film.

While I cannot rigorously show that a simulation is never for the benefit of that simulation's occupant, no simulation I have ever heard of matches that criteria.

Your premise that resetting a universe is difficult is a baseless assumption. We also have no idea how difficult it is to "reset a universe", it may be happening all the time.

And while a NEARLY resetting universe (after all, his memory is in the universe, and that doesn't reset) does have benefits for Phil in the short term, that benefit is only real INSIDE that universe - it does not automatically carry over to experience OUTSIDE that universe (unless Phil, himself, is outside the simulation of Groundhog Day).

Even if he was "patched in" to the simulation, the things he learned would not be of significant benefit once he came out. His memory is partitioned (he does not recall going into the universe), and the people inside the simulation are presumably NPC (since they don't remember). If he is a player controlling Phil, then it is not for Phil's benefit (and, since he's controlling Phil, Phil wouldn't notice unless the player wanted him to). If the player IS Phil, then Phil would remember going in (or would have his experience partitioned), and the skills Phil learned while in the simulation, could just as easily have been learned outside the simulation.


Whilst that's possible, even perhaps the most rational, sadly it's a matter of the design decisions of God. If one takes on the assumptions of deliberate design and of retro-active fixes, the possible mechanics of the underlying system become as varied as imagination.

In simulations we have the concept of instancing, that is to say that whilst the general system is ongoing and universal, sections of it can be duplicated and isolated (dungeons in mmorpg's, remote user sessions on vm's.) Or in general, a program's continued operation may be delayed or operate using a broad(temporary) assumption whilst a given clackerlation/process/function is completed to derive the data with which the program will operate going forward.(aka local history)

Using an instancing implementation every one of those individuals may have experienced their own closed simulation simultaneously, algorithms may delete the memories of those who 'failed the mission' or any other such criterion if a preferred progression path is accepted.

There's also the possibility of 'timeline' divergence to consider, common in anything dealing with time travel, each individual user may have had their own realities spun off until they came up with the 'correct' solution/series of events, so far as they're concerned I guess then 'this' reality actually is just for their benefit.


A ground hog day experience (GHDE) is a backup or a glitch in a simulation for 1 person. Say when a simulator hardware fails etc a full server goes does or/ Lets say the simulator for 1 person is as large as the matrix is but its just for 1 person. But say that computer was the size of a whole world okk... but lets just say it was even 6 or 60 times bigger than that... And just this one persons simulator expanded 6 worlds and each world was just a mirrored server of its brother and sister worlds.

Now say the GHDE was one world crashing or dying so maybe the one person who owns this simulation was on that world when the crash happened but because they were locked into the simulation ( or maybe thats how the simulation works you just move from planet to planet for a few years at a time, then they recover ( it might be a very eccological system that we have invented in the distant future (so nothing like the matrix)

The GHDE is the backup of that person just waiting in the loop till either the simulation is fixed or linked back up at the cloud. Or maybe its just the backup that is stuck in a loop that will never be fixed ? but because the simulation is designed to run for milenia with no assistance it might just run that loop for thousands and thousands of years and the movie like that movie 50 dates and we are actually drew barrymore.


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