How Would You Prove You Are a Time-traveler From the Past?
SF/F fiction frequently considers the case of a time-traveler from the future to the past, who can prove himself by use of advanced knowledge and items from the future. In the reverse case, a time-traveler from the past to the future wishes to prove he is from the past and that time-travel is real. How can he do this when all past knowledge is already known or whose chain of custody being broken is more likely than time-travel being real? I suggest 8 methods: carbon-14 nuclear isotope dating of their body as isotopes cannot be removed; sequencing of their genome to check consistency with pedigree as human genomes cannot be synthesized or edited on a large scale; selection & mutation clocks, likewise; immune system signatures of extinct or rare diseases such as smallpox, and accumulated pollution such as heavy metals, difficult & dangerous to fake. While these proofs may not offer conclusive proof since any human system can be subverted with enough effort, they can provide enough evidence to launch research into time travel and a definitive finding.
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An accidental time-traveler from the future to the past, the Connecticut Yankee in King Arthur's Court scenario, has it easy; even though they will have a hard time adapting to the past and may die untimely and will discover most of their knowledge is too vague or relies on a vast invisible infrastructure, they may possess impressive technological artifacts & will still remember enough to refute contemporary theories & provide simple experimental proofs of their claims & help people avoid dead ends.
Aside from accelerating scientific and technological development by possibly centuries (possibly more if the time travel can be repeated), they also prove the existence of usable backwards time travel, which would have cosmic implications.
What about the other way? What about an accidental time-traveler from the past to the future?
This scenario is not so immediately consequential but would have the same implication, as most forward time travel mechanisms ought to be impossible in our universe or for people of the past (wormholes, relativistic vehicles, Tipler cylinders, FTL travel, exotic space-time geometries with closed timelike curves) and so a verified instance of forward time travel implies either one of those remote possibilities or some entirely unknown way, all of which imply our understanding of the world is deeply wrong (eg relativistic vehicles implies aliens or a totally wrong human history).
The forward scenario is much harder. For concreteness, let's imagine a mad steampunk scientist from 1850s Victorian England.
How would we distinguish them from a committed con artist or prankster or lunatic?
The time traveler doesn't benefit from temporal asymmetry in their education or ambient tacit knowledge, since all science in the 1850s is well-known now, and anything about Victorian England that they know which we could verify, presumably they could have looked up in the same databases.
Likewise, accents can be trained with sufficient effort, polygraphs and other forms of lie detectors like fMRIs are beatable with training or unreliable, and any backstory like a scientist disappearing in an explosion in the 1850s proves nothing.
If they had traveled forward deliberately, they could have attempted to create hidden but verifiable knowledge like objects buried in particular places or letters left with lawyers, but even that may not provide sufficient evidence - how can one be sure that the objects were not buried more recently or the law offices were not broken into, or a genuine long-term letter on some mundane issue like an inheritance has not been swapped with a time-traveller-related letter, or that the excavators themselves can be trusted (eg Mormonism's "golden plates")?
Verifying the age of a letter is inadequate as there is plenty of genuine paper surviving from Victorian England which can be used in a forgery.
And of course they may have traveled forward accidentally or involuntarily, so it would be suboptimal to require verification which must be planned in advance.
What proof could a time-traveller offer of being from Victorian England which could not be faked for reasonably small sums of money like <$1b?
In general, Victorian people know and can do a subset of what a modern person could know or do, and hence while a modern person could prove to Victorians that they are from the future, how could a Victorian person prove to moderns that they are not a modern?
This may seem impossible because of the asymmetry, but there are in fact a few ways in which Victorian people are not subsets of modern people.
The key insight is to think about ways in which things have been lost or become unavoidable with the passage of time which might distinguish Victorians from moderns:
1. nuclear isotope dating
Atomic bomb testing in the atmosphere has endowed all people worldwide from 1945 to ~2030 with excess (the "bomb pulse") carbon-14. Carbon isotopes cannot be filtered from food and cannot be removed from bodily tissues such as bones nor can those bodily tissues be replaced by any method; all time-travelers from before 1945 will thus exhibit very anomalously low levels of carbon-14. This test would provide proof of time-travel from an era predating atomic testing (or backwards time travel from an era post-dating ~2030 assuming no future atmospheric bomb tests or nuclear warfare). The only way to cheat this test would be to either falsify the entire test, or perhaps spend billions of dollars over several decades on ultracentrifugation (gas centrifuge) raw ingredients to build a closed ecosystem to support a woman and grow the fake time traveler from conception to adulthood^[Ultracentrifugation since other approaches, like thermal diffusion or laser separation, are even more expensive and/or technically challenging.]. Given appropriate care in performing the test akin to those used in testing claims of psi (taking multiple samples at multiple times under the supervision of stage magicians, distributed to multiple blinded independent testing centers etc), a positive result would offer striking evidence in support of time travel.
2. DNA: human DNA remains effectively immutable as of 2017 as state of the art human CRISPR editing allows only a few edits of high but <100% comprehensiveness before the viral vector becomes ineffective, and full human DNA synthesis would cost >\$1b (and would be impossible to conceal, would affect commercial synthesis price curves, and any entity engaged in human DNA synthesis would use it for vastly more important things than hoaxing); thus any test based on DNA would indicate either being from the past or future. There are several possibilities; in decreasing order of strength:
- family pedigree tree: due to recombination and the presence of many variants, any particular human genome can exist in few places in a tree without being astronomically unlikely; a time traveler's relatives/descendants/ancestors should be easily located given their claimed biography & samples taken from them or their grave, or searched for & extracted from existing large DNA datasets like 23andMe or UK Biobank. Their sequenced genome will either fit perfectly into the claimed position & imputed genomes, or it will not. (It would be astronomically unlikely for just the most common 1 million SNP markers to randomly fit, and even less likely for full genome sequencing revealing family-specific mutations to be fooled by chance.) Like carbon-14 levels, the only way to cheat this test would be to fake the test.
- complex trait polygenic scores: in Western countries, particularly the UK, there has been select for & against certain traits (such as height or education, respectively) which will increase/decrease the respective polygenic scores. The polygenic scores are individually weak evidence but aggregated across many traits, may provide a worthwhile amount of evidence.
- ancestry admixture: the UK has experienced considerable immigration since the 1850s. Immigration admixture is evidence for a modern origin and thus its absence for a Victorian origin.
- mutational clock: more generically, as mutations are always accumulating, if only neutral ones, a Victorian genome will look somewhat different than, and have somewhat fewer mutations than, a contemporary genome (especially given relaxed selection/dysgenics). This test is probably too weak to be worth considering.
- gut & skin microbiota mutation clock: microbes have much shorter generation times, so while individual microbiomes are modifiable and not nearly as reliable as a test based on one's unalterable genome, it may be possible to try to measure whether the time traveler's microbiomes look properly archaic and similar to what would be estimated with a Victorian diet.
3. immune system: one of the signature accomplishments of public health, and major forms of progress since Victorian England, is the development of germ theory, a wide array of vaccinations, and the suppression or extinction of many infectious diseases & parasites.
This offers a variety of checks. Blood samples can be taken to measure antigen levels and immune system response to various agents. The time traveler may have been infected with smallpox and now resistant; I don't know if it is possible to distinguish a past smallpox infection from a cowpox inoculation or smallpox vaccination (cowpox is a different species, as is the modern smallpox vaccine's Vaccinia, but they are similar enough to offer protection so perhaps they are similar enough that a vaccinated person cannot be distinguished from a survivor), but if it is, it would show that either they are a time traveler from a period where smallpox is endemic (such as the 1850s) or the Russian/American biowarfare stockpiles have leaked (catastrophic global security news). Other diseases are still prevalent, so only imply a weaker conclusion that the time traveler, if fake, was willing to hazard their health.
4. pollution: another major change from Victorian England to modern England is massive improvements in the quality of the environment.
Lead, mercury, fine coal dust, heavy metals - all serious issues among even the wealthy of Victorian England, present at levels radically unacceptable & unseen except in toxic Superfund sites. Tests of their blood, lungs, and bones would indicate levels hardly ever seen in modern times. Similar to nuclear isotope dating, these poisons can accumulate over a lifetime; while it is possible for someone to deliberately poison themselves or a child for many years to provide verisimilitude for a fake time traveler, like the infection or isotope strategy, it would be a hazard to their health and evil.
Are there any others? Perhaps now-extinct plants' pollen or foodstuffs? Or some long-abandoned Victorian medical procedure which leaves unique traces? Maybe.
So that is ~8 ways to check whether a Victorian time traveler is really from Victorian England.
Most of these would work for other time periods as well, possibly even better (polygenic scores & ancestry, mutational clock, microbiota) although some would not (pollution may not be a major factor in various times & places, and the family pedigree tree doesn't work well if the family cannot be sampled).
Of these, the first 2 are extremely difficult or impossible to beat without spending billions of dollars, and several are dangerous to defeat.
On a more philosophical level, is it even possible to prove you are a Victorian time traveler?
Suppose one did pass all of these tests; given how incredibly unlikely time travel is based on a century of physics research, the absence of all other time travelers or traces thereof, how it would upend our understanding of the universe & human history, isn't time travel (forward or backwards) like psi in that a flaw in the proof will always be more likely than it being true?
Wouldn't it be more likely that passing the ancestral DNA test was due to a sophisticated hacking campaign targeting the lab's equipment or reports, or a breakin to tamper with calibration samples, than the person actually being a bewhiskered bewildered scientist accidentally blown 2 centuries ahead?
Wouldn't it be more likely that they'd discovered a flaw in carbon dating or some ultra-low-carbon-14 foodstuff or some other issue in standard isotopic tests than our understanding of physics so broken & incomplete that a Victorian gentleman might be still around and donating blood?
With psi, the more rigorous the experiments became, the weaker the effect becomes.
What once was secreting ectoplasm & communicating across time with ancestral spirits & moving tables with the power of one's mind has become reduced to a slight excess of 1 bits in a radioactive random number generator's output stream - if even that.
The sheer puniness of the effect disproves any claim to being an evolved power of the human mind which could accomplish any of what has been ascribed to psi.
With these time travel signatures, however, the effect should be large and easily observed.
Nevertheless, I would say that it may not be possible based purely on the Victorian time traveler's body to 'prove' time travel in some sense like rationally leading to P>0.99.
What such tests can do is offer enough evidence to pursue time travel.
Given the implications, time travel need not be anywhere near proven to be worth researching, just not be _P_~=0 as at present and with no feasible research program.
Knowing it may be possible is half the battle; and what has been done accidentally should be doable intentionally.
And accomplishing it will then provide all the proof necessary.
SF/F fiction frequently considers the case of a time-traveler from the future to the past, who can prove himself by use of advanced knowledge and items from the future. In the reverse case, a time-traveler from the past to the future wishes to prove he is from the past and that time-travel is real. How can he do this when all past knowledge is already known or whose chain of custody being broken is more likely than time-travel being real? I suggest 8 methods: carbon-14 nuclear isotope dating of their body as isotopes cannot be removed; sequencing of their genome to check consistency with pedigree as human genomes cannot be synthesized or edited on a large scale; selection & mutation clocks, likewise; immune system signatures of extinct or rare diseases such as smallpox, and accumulated pollution such as heavy metals, difficult & dangerous to fake. While these proofs may not offer conclusive proof since any human system can be subverted with enough effort, they can provide enough evidence to launch research into time travel and a definitive finding.
----
An accidental time-traveler from the future to the past, the Connecticut Yankee in King Arthur's Court scenario, has it easy; even though they will have a hard time adapting to the past and may die untimely and will discover most of their knowledge is too vague or relies on a vast invisible infrastructure, they may possess impressive technological artifacts & will still remember enough to refute contemporary theories & provide simple experimental proofs of their claims & help people avoid dead ends.
Aside from accelerating scientific and technological development by possibly centuries (possibly more if the time travel can be repeated), they also prove the existence of usable backwards time travel, which would have cosmic implications.
What about the other way? What about an accidental time-traveler from the past to the future?
This scenario is not so immediately consequential but would have the same implication, as most forward time travel mechanisms ought to be impossible in our universe or for people of the past (wormholes, relativistic vehicles, Tipler cylinders, FTL travel, exotic space-time geometries with closed timelike curves) and so a verified instance of forward time travel implies either one of those remote possibilities or some entirely unknown way, all of which imply our understanding of the world is deeply wrong (eg relativistic vehicles implies aliens or a totally wrong human history).
The forward scenario is much harder. For concreteness, let's imagine a mad steampunk scientist from 1850s Victorian England.
How would we distinguish them from a committed con artist or prankster or lunatic?
The time traveler doesn't benefit from temporal asymmetry in their education or ambient tacit knowledge, since all science in the 1850s is well-known now, and anything about Victorian England that they know which we could verify, presumably they could have looked up in the same databases.
Likewise, accents can be trained with sufficient effort, polygraphs and other forms of lie detectors like fMRIs are beatable with training or unreliable, and any backstory like a scientist disappearing in an explosion in the 1850s proves nothing.
If they had traveled forward deliberately, they could have attempted to create hidden but verifiable knowledge like objects buried in particular places or letters left with lawyers, but even that may not provide sufficient evidence - how can one be sure that the objects were not buried more recently or the law offices were not broken into, or a genuine long-term letter on some mundane issue like an inheritance has not been swapped with a time-traveller-related letter, or that the excavators themselves can be trusted (eg Mormonism's "golden plates")?
Verifying the age of a letter is inadequate as there is plenty of genuine paper surviving from Victorian England which can be used in a forgery.
And of course they may have traveled forward accidentally or involuntarily, so it would be suboptimal to require verification which must be planned in advance.
What proof could a time-traveller offer of being from Victorian England which could not be faked for reasonably small sums of money like <$1b?
In general, Victorian people know and can do a subset of what a modern person could know or do, and hence while a modern person could prove to Victorians that they are from the future, how could a Victorian person prove to moderns that they are not a modern?
This may seem impossible because of the asymmetry, but there are in fact a few ways in which Victorian people are not subsets of modern people.
The key insight is to think about ways in which things have been lost or become unavoidable with the passage of time which might distinguish Victorians from moderns:
1. nuclear isotope dating
Atomic bomb testing in the atmosphere has endowed all people worldwide from 1945 to ~2030 with excess (the "bomb pulse") carbon-14. Carbon isotopes cannot be filtered from food and cannot be removed from bodily tissues such as bones nor can those bodily tissues be replaced by any method; all time-travelers from before 1945 will thus exhibit very anomalously low levels of carbon-14. This test would provide proof of time-travel from an era predating atomic testing (or backwards time travel from an era post-dating ~2030 assuming no future atmospheric bomb tests or nuclear warfare). The only way to cheat this test would be to either falsify the entire test, or perhaps spend billions of dollars over several decades on ultracentrifugation (gas centrifuge) raw ingredients to build a closed ecosystem to support a woman and grow the fake time traveler from conception to adulthood^[Ultracentrifugation since other approaches, like thermal diffusion or laser separation, are even more expensive and/or technically challenging.]. Given appropriate care in performing the test akin to those used in testing claims of psi (taking multiple samples at multiple times under the supervision of stage magicians, distributed to multiple blinded independent testing centers etc), a positive result would offer striking evidence in support of time travel.
2. DNA: human DNA remains effectively immutable as of 2017 as state of the art human CRISPR editing allows only a few edits of high but <100% comprehensiveness before the viral vector becomes ineffective, and full human DNA synthesis would cost >\$1b (and would be impossible to conceal, would affect commercial synthesis price curves, and any entity engaged in human DNA synthesis would use it for vastly more important things than hoaxing); thus any test based on DNA would indicate either being from the past or future. There are several possibilities; in decreasing order of strength:
- family pedigree tree: due to recombination and the presence of many variants, any particular human genome can exist in few places in a tree without being astronomically unlikely; a time traveler's relatives/descendants/ancestors should be easily located given their claimed biography & samples taken from them or their grave, or searched for & extracted from existing large DNA datasets like 23andMe or UK Biobank. Their sequenced genome will either fit perfectly into the claimed position & imputed genomes, or it will not. (It would be astronomically unlikely for just the most common 1 million SNP markers to randomly fit, and even less likely for full genome sequencing revealing family-specific mutations to be fooled by chance.) Like carbon-14 levels, the only way to cheat this test would be to fake the test.
- complex trait polygenic scores: in Western countries, particularly the UK, there has been select for & against certain traits (such as height or education, respectively) which will increase/decrease the respective polygenic scores. The polygenic scores are individually weak evidence but aggregated across many traits, may provide a worthwhile amount of evidence.
- ancestry admixture: the UK has experienced considerable immigration since the 1850s. Immigration admixture is evidence for a modern origin and thus its absence for a Victorian origin.
- mutational clock: more generically, as mutations are always accumulating, if only neutral ones, a Victorian genome will look somewhat different than, and have somewhat fewer mutations than, a contemporary genome (especially given relaxed selection/dysgenics). This test is probably too weak to be worth considering.
- gut & skin microbiota mutation clock: microbes have much shorter generation times, so while individual microbiomes are modifiable and not nearly as reliable as a test based on one's unalterable genome, it may be possible to try to measure whether the time traveler's microbiomes look properly archaic and similar to what would be estimated with a Victorian diet.
3. immune system: one of the signature accomplishments of public health, and major forms of progress since Victorian England, is the development of germ theory, a wide array of vaccinations, and the suppression or extinction of many infectious diseases & parasites.
This offers a variety of checks. Blood samples can be taken to measure antigen levels and immune system response to various agents. The time traveler may have been infected with smallpox and now resistant; I don't know if it is possible to distinguish a past smallpox infection from a cowpox inoculation or smallpox vaccination (cowpox is a different species, as is the modern smallpox vaccine's Vaccinia, but they are similar enough to offer protection so perhaps they are similar enough that a vaccinated person cannot be distinguished from a survivor), but if it is, it would show that either they are a time traveler from a period where smallpox is endemic (such as the 1850s) or the Russian/American biowarfare stockpiles have leaked (catastrophic global security news). Other diseases are still prevalent, so only imply a weaker conclusion that the time traveler, if fake, was willing to hazard their health.
4. pollution: another major change from Victorian England to modern England is massive improvements in the quality of the environment.
Lead, mercury, fine coal dust, heavy metals - all serious issues among even the wealthy of Victorian England, present at levels radically unacceptable & unseen except in toxic Superfund sites. Tests of their blood, lungs, and bones would indicate levels hardly ever seen in modern times. Similar to nuclear isotope dating, these poisons can accumulate over a lifetime; while it is possible for someone to deliberately poison themselves or a child for many years to provide verisimilitude for a fake time traveler, like the infection or isotope strategy, it would be a hazard to their health and evil.
Are there any others? Perhaps now-extinct plants' pollen or foodstuffs? Or some long-abandoned Victorian medical procedure which leaves unique traces? Maybe.
So that is ~8 ways to check whether a Victorian time traveler is really from Victorian England.
Most of these would work for other time periods as well, possibly even better (polygenic scores & ancestry, mutational clock, microbiota) although some would not (pollution may not be a major factor in various times & places, and the family pedigree tree doesn't work well if the family cannot be sampled).
Of these, the first 2 are extremely difficult or impossible to beat without spending billions of dollars, and several are dangerous to defeat.
On a more philosophical level, is it even possible to prove you are a Victorian time traveler?
Suppose one did pass all of these tests; given how incredibly unlikely time travel is based on a century of physics research, the absence of all other time travelers or traces thereof, how it would upend our understanding of the universe & human history, isn't time travel (forward or backwards) like psi in that a flaw in the proof will always be more likely than it being true?
Wouldn't it be more likely that passing the ancestral DNA test was due to a sophisticated hacking campaign targeting the lab's equipment or reports, or a breakin to tamper with calibration samples, than the person actually being a bewhiskered bewildered scientist accidentally blown 2 centuries ahead?
Wouldn't it be more likely that they'd discovered a flaw in carbon dating or some ultra-low-carbon-14 foodstuff or some other issue in standard isotopic tests than our understanding of physics so broken & incomplete that a Victorian gentleman might be still around and donating blood?
With psi, the more rigorous the experiments became, the weaker the effect becomes.
What once was secreting ectoplasm & communicating across time with ancestral spirits & moving tables with the power of one's mind has become reduced to a slight excess of 1 bits in a radioactive random number generator's output stream - if even that.
The sheer puniness of the effect disproves any claim to being an evolved power of the human mind which could accomplish any of what has been ascribed to psi.
With these time travel signatures, however, the effect should be large and easily observed.
Nevertheless, I would say that it may not be possible based purely on the Victorian time traveler's body to 'prove' time travel in some sense like rationally leading to P>0.99.
What such tests can do is offer enough evidence to pursue time travel.
Given the implications, time travel need not be anywhere near proven to be worth researching, just not be _P_~=0 as at present and with no feasible research program.
Knowing it may be possible is half the battle; and what has been done accidentally should be doable intentionally.
And accomplishing it will then provide all the proof necessary.