AFAIK, inanimate bodies and life forms are in the end a large set of subatomic particles (and atoms, and molecules) and energy. Unless my basic knowledge of Physics fails me, you can return a particle or an object to its original position. Also, many chemical reactions can be reverted. Could it be possible that in a very far future, people are able to track every single particle-subatomic particle that constituted a dead person, and return each particle-subatomic particle to its original position/state, reshaping the person back from death, or is there any known law in physics that makes impossible to track and/or return a particle to its original position/state, making this general idea physically impossible?

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    $\begingroup$ The uncertainty principle has something to say about this idea. In a nutshell, it is impossible to know precisely both the position and the momentum of a particle. Or, as old Heraclitus put it in the 5th cenury BCE, you cannot bathe twice in the same river. $\endgroup$
    – AlexP
    Mar 22, 2018 at 18:35
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    $\begingroup$ "Also many chemical reactions can be reverted." Only by adding lots of energy. $\endgroup$
    – RonJohn
    Mar 22, 2018 at 19:12
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    $\begingroup$ The uncertainty principle has everything to say about it. And entropy, and the fact that we don't even know why living things are alive. $\endgroup$
    – RonJohn
    Mar 22, 2018 at 19:15
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    $\begingroup$ we don’t? $\endgroup$
    – JDługosz
    Mar 22, 2018 at 19:18
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    $\begingroup$ @RonJohn entropy has nothing to say about this. Overall entropy must go up, yes, but can be locally overcome. With respect to entropy, the proposed method would be no different from a fridge or air conditioning. $\endgroup$
    – Kevin
    Mar 23, 2018 at 0:49

6 Answers 6


The most interesting issue is that of the Ship of Theseus, which is a long-standing philosophical quandary regarding identity of a thing when you take pieces off and put them back on. The question as to whether your reanimation process has a meaning is an interesting philosophical one.

Philosophy not withstanding, there is no law which prevents a form from reoccurring. Not even the mighty uncertainty principle can prevent it, though uncertainty may prevent you from measuring the reanimated entity so that you may say "yea verily, this is precisely the same person as they were before." The Pauli Exclusion Principle could potentially get in the way if the person was already alive and you needed an exact copy, but if you're reviving dead people from the past, it won't apply, as there is nobody in existence with precisely the same state at that moment.

The tricky bit is the tracking thing you mention. Getting the information required to make this reanimation happen on purpose is tough. There are a lot of dispersive effects and chaotic effects in the universe, which take the information about what happened before and mix it all up so thoroughly that it is mighty difficult to piece the parts back together again.

The real problem would be electromagnetic radiation. Once dispersive effects have disseminated the information across a lot of objects, there's a good chance that some of their interactions will produce EM radiation, like light waves and radio waves. The information contained in these waves propagates outward at the speed of light. This information may not be retrieved. If you need this information (and you likely do), you may not be able to pull together all of the information you sought.

Of course, you can always weave in a little magic. If the long-dead person intentionally entangled themselves with an object, and it was believed that the "essence" of what made that person them was still contained in the object, there might be a process to spawn a new body from that artifact, like a tree sprouting a new branch to replace a dead one. The meaningfulness of this is also very hard to analyze, but I find it to be a rather interesting approach.

  • $\begingroup$ I always wondered that "Ship of Theseus", but I never knew where to look about it. Thanks!. In regards with a person, I think most people would agree a person who lacks a member of the body remains the same, and that the essence is somehow related to the brain, but in that point is complicated because after a hangover and after losing neurons we remain the same, so how much / which parts of the brain exactly do you need to have to be you and not someone else. This question makes me remember asian thinkers which says we are one with the universe and the sensation of being "I" is an illusion $\endgroup$
    – Pablo
    Mar 22, 2018 at 19:01
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    $\begingroup$ Pauli Exclusion Principle only says that two fermions cannot be in the same "state". Note that "state" here includes location in spacetime. So all you need to do is make sure that they are not in the same place. $\endgroup$ Mar 22, 2018 at 20:58
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    $\begingroup$ @CortAmmon Two humans can never occupy the same place, even when they are different people. Pauli exclusion already applies to the individual electrons, which is why matter is hard and solid. Exclusion doesn't mean that two identical systems can't exist. $\endgroup$
    – JDługosz
    Mar 22, 2018 at 21:49
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    $\begingroup$ The no-cloning theorem doesn't 'apply because the human body isn't a quantum state for which you mind if it entangles itself with the environment - in fact it does so on a regular basis. More specifically, even if you didn't know the exact momentum of someone's hand, and you decided to cloning them anyways, it would be perfectly fine if both copies were entangled to have the exact same momentum. The no-cloning theorem is fine with that. $\endgroup$ Mar 22, 2018 at 22:53
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    $\begingroup$ If the long dead person intentionally entangled themselves with an object, and it was believed that the "essence" of what made that person them was still contained in the object, there might be a process to spawn a new body from that artifact, like a tree sprouting a new branch to replace a dead one. I think you just explained why the Doctor regenerates... $\endgroup$
    – Michael
    Mar 23, 2018 at 0:55

There is a limit to knowing the exact state, but reprodicing a living body and brain does not require that degree of accuracy — thermal motion is jostling things around all the time and the body is made to work under these conditions.

Consider that getting an MRI with no effects whatsoever even though it changed the state of all your hydrogen atoms. The proton spin just doesn’t mean anything to the construction of biomolucles.

Consider that you get a dental x-ray as a matter of course, but the shadows mean that molecules absorbed some rays. This actually does cause some damage! But the body has redundancy and doesn’t need every single atom to be just so.

It is well within the laws of physics to scan and reproduce a human body with the necessary fidelity. As you hoped, it’s just an engineering problem. A person can be scanned and the information stored passively, serving as a backup. After death, the most recent backup can be vivified.

(If the body can be recovered, it might be customary to use the atoms from the old body to build the new, giving a psycological ease that you’re just being repaired and are somehow more continuous with your previous incarnation.)

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    $\begingroup$ You just changed the state of all my neurons that relate to MRIs. $\endgroup$ Mar 23, 2018 at 3:26
  • $\begingroup$ So if you created an exact clone of a person at the same point in time, they will both be indistinguishable. But due to small differences (such as the ones you mention) they will go on to have different experiences, and by the time they die they will have become qualitatively different people - no longer the same. At that point, which one is the "real" one? So if you reanimate a person inexactly, are you truly reanimating them, or creating a simulacrum that is similar to them? $\endgroup$ Mar 23, 2018 at 6:00
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    $\begingroup$ @LoganPickup Fun story fodder is having a character who is such a reanimated clone pondering this line of argument, while they are considering a course of action that requires them to die and be reanimated again. $\endgroup$
    – Ben
    Mar 23, 2018 at 7:26
  • $\begingroup$ -1 here too because — just like Cort Ammon — you are answering the wrong question. Yes a body can be reproduced, if its state was known when it was alive. But OP asked: can a body be reproduced by back-tracking from its known state when it is already dead. $\endgroup$
    – MichaelK
    Mar 23, 2018 at 10:29
  • $\begingroup$ @MichaelK i'll clarify the last paragraph. $\endgroup$
    – JDługosz
    Mar 23, 2018 at 15:46

Yes there is a show-stopper to that

Ponder the following gedanken-experiment:

I had two numbers. I then added them. The sum of the numbers is 73.

What were the numbers?

You cannot answer this question.

Your proposed procedure suffers from the same problem, in that you are looking at the results of physical processes. But an infinite number of different interactions can give the same result. So you cannot backtrack. Because even if every physical process in theory is reversible, you cannot know which physical process it is that you need to reverse when looking at the results.

Another example

Here is a picture of a pen in a table.

enter image description here

Even if you could measure every atom in that pen, their direction, energy states and so on... there is no way you can determine how that pen was put there. You cannot — ever — backtrack how that pen ended up there only by knowing the state of the pen.

You are about to say "But what if I knew the information about every atom and sub-atomic particle in the universe!". No, you cannot do that... because then you would have to use those same particles to record that information and then do backwards calculations on them. This is impossible.

This involves some pretty deep maths and logic but in short: the Church-Turing thesis says you cannot do it. The only way you can do this is to flip the arrow of time around. Unless you are a supernatural extra-universal deity, you cannot do that.

You also cannot prove that this has not already happened... maybe we have played out this exact conversation over a million times already. We cannot tell.

  • $\begingroup$ How about electromagnetic radiation? Doesnt that give an unique trace? (may be not because it could be blocked by other objects?) I mean, at a macroscopic level if I watch a person from the distance I'm seeing how he was in the past $\endgroup$
    – Pablo
    Mar 23, 2018 at 12:44
  • $\begingroup$ @Pablo No, because those photons can come into existence for innumerable reasons. They do not — in any way — provide a unique signature. $\endgroup$
    – MichaelK
    Mar 23, 2018 at 13:00
  • $\begingroup$ Suppose that you are looking at a half-silvered mirror and you see someone in it. From the electromagnetic radiation (light) you don't know whether you are looking at his reflection in the mirror or directly at him through the mirror. $\endgroup$
    – DMPalmer
    Mar 23, 2018 at 17:10
  • $\begingroup$ @michaelK ok, but suppose that I have a way to look at the Earth from 10 years light away (something similar to this is being conceptualized these last years by a NASA developer by the way, to see the surface of other planets outside the solar system) and a see the silhouette of a person, I could safely assume most of those photons belong to the person and they didnt come into existence for another reason, couldnt I? $\endgroup$
    – Pablo
    Mar 23, 2018 at 17:17
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    $\begingroup$ @Pablo, just stop. I say again: the sum is 73. What were the terms of the summation? That(!) is the kind of problem you have to solve in order to succeed with your scheme. This you cannot do. So your idea is dead and cannot be resurrected. This cannot be done. You cannot turn the arrow of time around. $\endgroup$
    – MichaelK
    Mar 23, 2018 at 17:43

There are some sources of uncertainty that mess up this technology. The prime one is the Heisenberg uncertainty relations in quantum mechanics: the product of the uncertainties about the position and momentum of a particle will always be larger than a certain (small) value. This is not just a lack of instrumentation or even that the measurement will jostle the particle, but seems to be a deep part of how quantum mechanics works. Hence your measurement of the state of the world will by necessity have some small uncertainties.

These would not be a huge problem except for chaos (and perhaps quantum randomness). When backtracking the particles the uncertainty in location will grow linearly with time due to the momentum uncertainty as long as they do not interact with each other. But of course they do, and that produces a much faster growth of uncertainty. On one hand nonlinear interactions amplify uncertainties exponentially, and on the other hand individual particle reactions look random on the quantum scale. So go back far enough and you will have little clue where all those particles were.

(This is really annoying since quantum mechanics and all the other laws of physics appear to be time-reversible in the small: in a sense the information is there, it is just spread out to such a degree that you cannot reconstruct it)

To add to the annoyance, your measurements need to be stored somewhere. Each particle needs 6 values to denote their position and velocity (plus a few more for other particle states). There are about $10^{27}$ molecules in a body, each with about 30 particles (most are water). So if you want to simulate just a body you need $1.8\times 10^{29}$ numbers (each with a certain number of bits). Note that this is an annoyance, not a showstopper. I recently estimated that using all silicon and carbon in the solar system you could get up to $10^{46}$ bits - more than enough for that data.

Now, there is another approach to the problem. Instead of trying to scoop up all atoms and accurately predict where they truly were, make a lot of plausible scenarios instead. Not every past is likely: a cloud of air molecules could have been a toy that spontaneously dissolved into nearly nothing, but it is not as likely as past air. A being who wrote an email in English probably had a brain that understood English, and so on. This will not guarantee finding the one true past state. In fact, it might find a near endless number of plausible pasts that could have happened. No problem, just resurrect copies of all of them.


So, the problem you're going to run into before you hit on the uncertainty principle or issues of "the spark of life" or whatever else is probably a fundamental limit of thermodynamics. In simple terms, the information you want to store and process is too large to be handled feasibly.

There are, on average, about 7*1027 atoms in the human body. Converting that to a binary yields something on the order of 292 bits of information. Of course, we need to store more about those atoms than whether or not they exist. For starters, what atom they are. There are 118 elements on the periodic table, so we can do that in 7 bits (27, or 128), so add 7 to that exponent, and we're up to 299 bits of information. We also need to know about ionization. A potassium ion is very different from a potassium atom. My memories of high school chemistry tells me we're not going to see anything more than a -/+8 state for ionization, so we can do that in 4 bits, which takes us to 2103. High school chemistry also reminds us that an atom sitting beside another atom is very different from two atoms bonded together, so we need to know and store something about the bond state of our atoms - what type of bond it is (like ionization states, we should be able to store this in 4 bits), and which atom(s) it's bonded to, which will take at least 92 bits (because it could be any of the atoms we originally stored information on), which will take us to 2199. There's definitely more we need to know (like positional information, for example), but let's stop there for a moment, because this brings us into the same ballpark as one of my favorite computing tidbits.

One of the consequences of the second law of thermodynamics is that a certain amount of energy is necessary to represent information. To record a single bit by changing the state of a system requires an amount of energy no less than kT, where T is the absolute temperature of the system and k is the Boltzman constant. (Stick with me; the physics lesson is almost over.)

Given that k = 1.38×10-16 erg/K, and that the ambient temperature of the universe is 3.2 K, an ideal computer running at 3.2 K would consume 4.4×10-16 ergs every time it set or cleared a bit. To run a computer any colder than the cosmic background radiation would require extra energy to run a heat pump.

Now, the annual energy output of our Sun is about 1.21×1041 ergs. This is enough to power about 2.7×1056 single bit changes on our ideal computer; enough state changes to put a 187-bit counter through all its values. If we built a Dyson sphere around the sun and captured all its energy for 32 years, without any loss, we could power a computer to count up to 2192. Of course, it wouldn't have the energy left over to perform any useful calculations with this counter.

(Bruce Schneier, from his book Applied Cryptography)

2199 is 128 times larger than 2192, so by Mr. Schneier's math, we're at the point where we'd need a 100% efficient Dyson sphere around the Sun for ~4100 years to have enough power to even flip all those bits, to store that information, never mind running any calculations, or actually doing the work of putting things back into place.

So before we've even addressed the fundamental issues that might make this idea of yours impossible, we've discovered that storing this information for a single body would require 4 millennia of energy output from our star, before we even get to calculations or doing the actual work (or even storing all the information we need), or getting the materials we'd need to construct our computer with.

Which brings us to the end of that Schneier snippet, which is:

These numbers have nothing to do with the technology of the devices; they are the maximums that thermodynamics will allow. And they strongly imply that brute-force attacks against 256-bit keys will be infeasible until computers are built from something other than matter and occupy something other than space.

He wasn't talking about storing information on all the atoms in a human body, but it sure looks to me like it applies here as well.

  • $\begingroup$ How much data can we store today worldwide? 2^20 ? $\endgroup$
    – Pablo
    Mar 23, 2018 at 12:17
  • $\begingroup$ @Pablo Beats me, but that's not the point. The point is that there is a fundamental limit on how much information can be stored, thanks to thermodynamics. When simply writing a partial set of the information you need requires an ideal computer and 4000 years of energy output from our star, it's safe to say you're in that "prevented by the laws of thermodynamics" territory. We haven't even looked at how you'd actually gather all that information, do calculations on it or use it do to work. Even storing some of the information we'd need requires so much energy as to be infeasible. $\endgroup$ Mar 23, 2018 at 21:34
  • $\begingroup$ Actually all information is retained forever--it cannot be destroyed. There was some debate as to how it survived a black hole and I think Hawking solved that with light emitted from the black hole as matter was consumed. Anyway, you can't destroy information. The energy to re-gather the information, however, would be immense! $\endgroup$
    – Bill K
    Mar 23, 2018 at 23:14
  • $\begingroup$ @BillK Yeah, but that's using "information" in way that's very specific to quantum mechanics, not in a way that's all that related (or useful) to the general meaning of the term. It's like "spin" states or quark "color" charges in that sense. Neither of those words means (remotely) the same thing in general usage as it does in quantum physics jargon. The "information cannot be destroyed thing" is a consequence of, or another way of stating that time evolution is unitary, and is a theoretical statement, not a practical one. ... $\endgroup$ Mar 23, 2018 at 23:26
  • $\begingroup$ ... as this answer shows, it is, in fact, possible to destroy information (in the general sense of the term), because you can make retrieving it impossible according to other physical laws. That doesn't change the fact that time evolution is unitary, and in theory, you could run time backwards to a state before you destroyed the information or theoretically collect all the information needed to reconstruct it ... but if that's made practically impossible, it's fair to say that information is destroyed, even if it's not strictly true according to quantum physics jargon. $\endgroup$ Mar 23, 2018 at 23:29

There are a couple quantum theories of interest.... The first is that all information is preserved which means that you could at least calculate any previous state of the universe from any future state, so you could figure out pretty much where everything was including every atom in a person's body...

However, another aspect is quantum state: Matter has various qualities that are un-measurable.. This does not mean it's difficult to measure, it means that the information does not exist in our universe. One example of this is how long it takes for a radioactive atom to decay--which is why decay is stated as an approximated half-life instead of a specific amount of time.

So these two pieces of data lead me to believe that you could reproduce an approximate physical state, possibly even down to the molecule or maybe atom, but could not reproduce the exact quantum state.

This last bit is more important than you'd think--every time a neuron decides to fire or not in your brain it is adding a little bit of this quantum state into the decision. It's like a super-computer built with each CPU operation adding in a little completely random factor so that even if you re-created the computer, program and data bit-for-bit the output of your program would still deviate every time.

Because of this I don't think it would be possible to get the exact same behavior even if you could reproduce the physical state exactly--at least not from within our current universe/reality/dimension/??.


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