In my story, there is a VR world. A victim is chosen to be put into that world, and his/her job is to decide whether he/she is actually in the real world.

The VR world is implemented is a way such that the victim feels extremely real. So the victim does not only carry headsets and motion sensors. He/she also has some cognitive changes, for example, under the influence of drugs. So even though the victim feels wide awake, he/she may not actually be.

Now, I am asking how the victim can decide that he/she is actually in the real world. I can think of some conditions/constraints:

  1. The victim is originally in the real world. When the VR developers have to put the victim into the VR world, they have to ensure that the victim is unaware of such an action. Therefore, the developers can only do so when the victim is unconscious (e.g. when he/she is drunk or sound asleep).

  2. The drugs have non-lasting effects. To prevent overdosage the developers must stop injecting the drugs after several (perhaps 3-4) days of the first intake. After that the victim thinks that he/she wakes up normally in the morning.

  3. The victim can keep some secrets, perhaps in his/her memory. There is no way the developers can get them in the real world, but they can fully control the VR world so that they obtain such secrets when the victim disclose them in the VR world.

  4. For the victim to proof that he/she is in the real world, he/she doesn't need a logical proof. Instead he/she accepts a probabilistic or cryptographic proof. For example, I consider it to be a valid proof when the victim has 99.9% confidence that he/she is in the real world.

  5. The victim has no responsibility to keep him/herself staying in the real world. In other words, it is OK if the victim found him/herself in the VR world in 10 consecutive days. His/her only job is to distinguish one world from another.

  6. You can assume the victim is powerful (a normal guy having good physique, knowing how to fight/use weapons, knowledgeable, fluent in major languages, smart etc), as the idea of such a VR world comes from a fiction series I've read before.

Is it possible for the victim to do so?

Edit: After reading some answers I am going to clarify something. I will not downvote existing answers, but amendments are welcome. As I have mentioned earlier, the idea comes from a fiction series. Some drugs were injected into a guy. He did find something special/unusual, but the information was insufficient. He was put into a situation without communication devices and weapons. He saw her (dead) girlfriend as a clone, and eventually he was tricked to disclose something. So essentially he (1) was emotionally controlled (2) appeared to be less analytic (3) was "identical" to the one in the real world under the influence of drugs. I mean something like dreaming -- you aren't going to think that that self in the dream is different from yourself. Originally I wanted to figure out a probabilistically secure way to verify the world's integrity, and so I didn't intend to make it too philosophical. Anyway, thanks for any help and sorry for any misleading information.


The classical approach is to ask someone (preferably someone you know well) something personal about you. If they answer incorrectly, you can have a reasonable suspicion to be in VR.

If you know a telephone number from memory, call that number. Is the right person taking the call? Are they reacting like you know them to usually react? You can take a similar approach with social media or texting in general, but that can be analyzed and simulated too easily.

Next is your own body. In VR, your whole body has to be simulated. Are all of the old scars accounted for? The hangnail that developed just yesterday? The little bump where you hit your head? If you know you might be selected to enter the VR-Test, you could inflict a small wound in a very unusual or hidden spot, then check if it's still there in VR.

The feeling of your own body is especially hard to simulate. You know exactly how is feels to pull at your skin, to crack your knuckles or your neck, to pull your hair. You know exactly how hard your muscles feel, how soft your skin, how rough the calluses on your hands and feet. In VR these sensations would need to be simulated (since your body doesn't do the same actions). If you concentrate more on the inside than on the environment, you should be able to tell VR from reality.

  • $\begingroup$ In addition, you could inflict a small hidden wound on yourself and use it to track which world you are in. $\endgroup$ – Clay Deitas Aug 17 '18 at 11:29
  • $\begingroup$ @ClayDeitas I don't quite follow... If I cut myself in the real world, I feel pain, there may be blood and the wound will take several days to heal. If I cut myself in VR, the pain and blood will be simulated, just like the healing of the wound. How would this help me distinguish VR from reality? $\endgroup$ – Elmy Aug 17 '18 at 11:31
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    $\begingroup$ Remember your comment about scars? The point is to make intentional scars, basically. if you know you could be put into VR at any point, you just need to make a secret mark somewhere that is unlikely to be properly simulated. Just like a hangnail, but intentionally made so that you always have a determining factor. $\endgroup$ – Clay Deitas Aug 17 '18 at 11:41
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    $\begingroup$ "The classical approach is to ask someone (preferably someone you know well) something personal about you. If they answer incorrectly, you can have a reasonable suspicion to be in VR." I'd be more suspicious of paranoia and schizophrenia in such a case. $\endgroup$ – Renan Aug 17 '18 at 12:17

The victim can keep some secrets, perhaps in his/her memory. There is no way the developers can get them in the real world, but they can fully control the VR world so that they obtain such secrets when the victim disclose them in the VR world.

The first idea that came in my mind was to use this to have a "flag" in the real world that you're the only one to know about, and that you check every morning to see if you're still in the same world (i.e. a hole in your bedsheet or a scratch on a wall behind a bookcase). If it isn't there the next morning then you're in the simulation.

This would work as long as the programmer doesn't know about it. And of course, it works only once with the same flag.

  • $\begingroup$ Welcome to Worldbuilding, Aegis! If you have a moment, please take the tour and visit the help center to learn more about the site. You'll also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods. Have fun! $\endgroup$ – FoxElemental Aug 17 '18 at 12:27

I would read the question as a decidability question, posed in number 5. Is one able to distinguish between being in a real or a virtual world?

The answer would be no, unless the VR world has imperfections. According to the solipsistic view (see Solipsism), it is already impossible to prove that an external real world exists outside the mind. If that is true, then it would certainly be impossible to to distinguish between a real world and a stimulation and simulation of all the senses that is sufficiently rich and detailed. This would include a simulation and corresponding stimulation of consequences: if you fall, you will feel pain, if you cut yourself, you will feel pain and see a cut which will then simulate itself to heal over time. If you commit a crime, you will be persecuted and punished and so on.

For this to work, the victim has to be unaware of being brought into the virtual world (or the real world) so that the victim cannot prepare themselves or their environment. Your conditions 1 and 2 take care of that: the victim is unprepared.

The only loophole in this setup is stated in condition 3. The victim may have prior secrets shared with his environment that the VR world builders have no knowledge about and where thus unable to incorporate into VR world. This could be mundane things like passwords but also shared knowledge between them and significant others. The trick is then to assert that these secrets still exist, which would entail disclosing them to the builders of the VR world. Memory being fickle, shared knowledge may be denied by significant others but substantial denial of multiple secrets is then a probabilistic proof (condition 4) that the victim is in a virtual reality. Passwords and so on are insufficient as validation as the passwords may have been acquired through hacking and key-loggers by the builders of the VR world prior to posing the decidability question.

For an infant brought up in this VR world the proposition is truly undecidable.


Take the red pill, not the blue pill.


The red pill and its opposite, the blue pill, are a popular cultural meme, a metaphor representing the choice between:

Knowledge, freedom, uncertainty and the brutal truths of reality (red pill) Security, happiness, beauty, and the blissful ignorance of illusion (blue pill) The terms, popularized in science fiction culture, are derived from the 1999 film The Matrix. In the film, the main character Neo is offered the choice between a red pill and a blue pill by rebel leader Morpheus.


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