# How could we time travel backwards without killing everyone with germs from the future?

Consider a world in which time travel has been made viable possibility. Initially, many are ecstatic at the new technology and the knowledge it will unlock. However, biologists and other scientists soon come forward to warn about time travel.

The danger is bacteria. Due to our rapidly developing antibiotics, bacteria have evolved over time to combat our medicine. Were we to travel forward, we would likely contract a deadly mutation of a bacteria from the future and spread it to the present upon our return (thus killing everyone). Were we to travel backwards, we would bring our present evolved super-bacterias to the past, which would infect the then not-vaccinated population (thus killing everyone).

How would we be able to enjoy the gift of time travel without wiping out humanity?

• What problems that arise are highly dependent on how exactly time travel works in your setting. Possibly removing things like conservation of energy or momentum may have even worse consequences. – Hohmannfan Aug 9 '16 at 9:53
• @άλεξμιζέρια I always thought if time travel would be possible it's energy requirement would cover the costs of the new matter appearing in the past like Einstein told us E = m * c². So to bring one kilogram of apple back in time you should release E = 1kg * 299 792 458²m²/s² = ~8.987*10¹⁶J (1J = 1kg * 1m²/1s²). And that wouldn't even be the hard part. The hard part is the returning. When the matter returns to its own timeframe (even by natural means) 1kg of matter is annihilated, which will release ~8.987*10¹⁶J energy. (That is ~1.5 Hiroshima per kilogramm.) – mg30rg Aug 9 '16 at 11:56
• I would rather go back to the past. Far easier to convince others that Im a god (especially if you are wearing a full body suit which makes you look badass) rather than go to the future and get caught up in some sort of war. The best part is, you can quickly run off with the gold if some fanatic church declares war on you XD – King of Snakes Aug 9 '16 at 12:29
• If time travel were possible, bacteria would be the least of your worries – Brad Thomas Aug 9 '16 at 13:14
• I guess it is pretty safe to travel to the future if you do not go above the speed limit of 3600 seconds per hour. – fr13d Aug 9 '16 at 16:31

I don't think the premise of the question holds water. It seems to assume that pathogens have a single scalar "deadliness" score (like hit points in a game) which is higher for modern germs, such that we only survive because our drugs are correspondingly more potent. But that's not how it works.

Suppose a time traveler brings a population of penicillin-restistant staphylococci with him to the distant past. Those bugs are a problem for us because they don't respond to the drugs we use to kill them. However, in the past (before 1928) there is no penicillin to be resistant against, so the fancy resistance genes the staphs invested so much in is a no-op!

The resistant strain won't be any deadlier than old-style staphs for a patient who doesn't get antibiotics -- and it won't even have a selective advantage over the pasts's indigenous strains, because the thing they're better at simply doesn't exist in that environment. So there's not even a reason to expect that the resistance gene will have spread throughtout the population by the time penicillin is invented.

As for germs imported to now from the future, it's mostly the same story. We're not really that heavily dependent on antibiotics as a society. About one in five of us carry Staphylococcus aureus on our skin and mucous membranes, and it's not thanks to antibiotics that we don't die from them most of the time -- just a plain old Mk. I immune system will do. In particular, if all our antibiotics stopped working overnight we'd still survive as a species just as well as we did in the 1800s -- which is, not too well by modern standards, but it's not as if everyone would suddenly keel over. We need antibiotics for most of us to live till 85, not for the species to survive.

In particular, for your your hypothetical resistent-to-everything germ from the future to kill us all, it would need to spread through the general population of today -- where antibiotics are still not widespread in the usual case. You may get an antibiotic treatment from your GP if you happen to get sick, but most of the time you don't get sick, even tough you're not regularly dosing yourself with antibiotics. Future antibiotic-resistent bugs would find it no easier to spread in today's population than today's bugs do, because ordinary healthy people in our society have no artificial antibiotics in them anyway.

Things may look a little bleaker if the future holds virulent germs that can survive disinfectants such as hydrogen peroxide, bleach or simple alcohols. But that would be a much harder trick for them to pull off than mere antibiotic resistance -- those small-molecule disinfectants are poisonous to pretty much everything, including ourselves, whereas the challenge of an antibiotic is that it has to be selectively toxic to bacteria but harmless to us.

• Or to paraphrase: before antibiotics and vaccination there were already so many diseases infecting the population and killing "everyone", that a couple more brought from the future would barely signify. – Steve Jessop Aug 9 '16 at 12:50
• @KingofSnakes: the point of this answer is that they aren't "stronger". They're just more resistant to antibiotics. This is highly relevant today, but completely irrelevant once taken into the past, because there are no (human-administered) antibiotics for them to resist. So for all the "strength" it gives them in the past as human diseases, they might as well be good at ballroom-dancing as resistant to antibiotics. They're no stronger than their non-antibiotic-resistant ancestors. They may even be weaker, if their antibiotic-resistance "costs" them anything to achieve. – Steve Jessop Aug 9 '16 at 14:05
• It is also worth noting that in general, organisms that develop a useful quality (such as antibiotic resistance) often sacrifice other qualities (they may use more energy or reproduce slower). This is why useless body parts tend to disappear over evolutionary timescales. In a world without antibiotics, antibiotic-resistant bacteria will likely be less fit than their non-resistant cousins. – IndigoFenix Aug 10 '16 at 6:08
• Wouldn't a time traveler from the Americas of the present (or future) traveling to the Americas of the pre-Columbian past risk infecting and decimating native American populations, just as the European explorers did? It seems to me that bringing pathogens to isolated populations (whether isolated by distance or by time) has a strong potential to be bad without having anything to do with antibiotic resistance. – jamesdlin Aug 10 '16 at 6:41
• While I do not hold viral expertise, I think that focusing on the drug resistance is a bit of a red-herring. It is my understanding that part of the reason humans can readily fight off new and different viruses every year is because they've developed immunity to similar viruses already. And viruses do mutate quite rapidly. So it is likely that a virus brought back from the future could have mutated so much from viruses in the "present" that there may not be any form of immunity already present in most humans. Thus, killing hundreds of millions or even billions is possible – Dunk Aug 10 '16 at 17:14

For the past: Wear a full-body suit + gas-mask combo, and add your own stuff to make it look badass (Hopefully you won't be taken as some sort of whacko), which will make people think you are some sort of God (or alien). Then you can easily claim that getting to close to you will cause people to die (as they cannot handle your 'mighty power'), and the other people will believe it. That takes care of problems on how to prevent others from getting infected. The gas-mask and full body suit prevents you from getting infected with diseases that you have not vaccinated yourself against. And it makes you look like a awesome god.

Bonus: - If you wear something particularly badass, going back in time will convince others that you are a god (though what may look badass for us now, might be terrifying for others. Still, the others will worship you, and you will be safe for the time being).

• Oooooor they just burn you as a witch. The more probable outcome IMO. – Wolfie Inu Aug 10 '16 at 15:23

Can't we consider that if we invent time travel in the future, then people travelling from future already came to the past?

So if we try not to kill people from the past with bacteria, it didn't happened. But if we decide not to care, the past already had bacteria from future.

It mean that if I go back in time (in 400 After J.C. for the example) with all my germs, I won't change anything to what already happen since I was in the past in our history (in year 400 After J.C. in that example).

• Being able to travel in the future doesn't mean you can travel in the past, they are two different things. One consists of skipping time while the other means rewriting the history of the universe. – Charon Aug 9 '16 at 14:03
• I think your point her is valid: Even if something could go bad, we know it did not. – Hohmannfan Aug 9 '16 at 14:03
• I think the OPs implication is time travel is two way both to the past and the future. – Thucydides Aug 9 '16 at 14:11
• @Thucydides novikov self-consistency principle. Look it up. – John Dvorak Aug 9 '16 at 15:37
• Multiple histories I think is one theory. – n00dles Aug 10 '16 at 5:25

The Novikov self-consistency principle states that any time travel is mathematically required to be self-consistent. That is to say, the traditional time paradox isn't possible.

Consider drawing your situation to its ultimate conclusion. We travel back in time, taking supergerms with us, which kill everyone. Now, since everyone is dead--and since we traveled far enough into the past for the germs to kill everyone--we could never have been born. Traditionally, this is a time paradox. If we were never born, we could have never time traveled and killed everyone. There are a number of suggested "resolutions" to the time paradox, almost none of them good for us.

Novikov's solution is different. Novikov suggests that none of this could have happened in the first place. For some reason, regardless of whether it the reason is known for any particular instance, it is simply not possible to create a time paradox, much in the same way it is not possible to create any other kind of paradox.

If I work out a mathematical proof that shows that $1 = 2$, then there is something wrong with my proof, because we already know $1 \ne 2$. What is wrong exactly? Well I dunno, it depends on the "proof." Being unable to determine the reason does not mean that the paradox is allowed to exist.

In the case of our supergerms, we can't take supergerms to the past because we've already been to the past and there were no supergerms there. The only way we could take supergerms to the past is... if we had taken the supergerms to the past already.

Now, if we had taken the supergerms to the past already, clearly not everybody died or else we wouldn't have been around to take the supergerms to the past in the first place.

There is a pretty detailed example given at the above link involving a billiard ball traveling into a wormhole with the precise trajectory that will cause it to strike itself and knock itself off course, preventing it from entering the wormhole. Novikov refuted this example by redoing the mathematics himself and finding a number of self-consistent solutions.

So what happens if we try to take supergerms to the past? I dunno. But I do know that we mathematically can not... unless we did.

• How do you know that there haven't been any superbugs in the past? Perhaps there have been, but most of them lost their abilities because they have been no use to them. But some of the strains remained unmutated, quietly waiting for antibiotics to be discovered so that they could dominate the population again. Quite scary if you ask me. – John Dvorak Aug 9 '16 at 18:50
• ...but we didn't. – n00dles Aug 10 '16 at 5:30
• I am imagining the first unfortunate time traveller causing the Black Plague. – IT Alex Sep 19 '18 at 20:11

No! This won't wipe out humanity. Time travel and the possibility of importing new and old strains of diseases from the past and the future will provide an enormous boost to research into microbiology and virology. Pharmaceutical companies will benefit from having so many new diseases to combat with their medicines and drugs. The market potential is gigantic! This will increase employment for scientists and give a boost to the economy.

Who wants to live safe? Every species goes extinct sooner or later! Embrace the new paradigm. Live dangerously, travel through time, and get infected with interesting new diseases. Do good to your society, your time, create jobs and wealth, and die unexpectedly of an ancient illness. How else can anybody fund time travel?

• Ah! Downvoted by a person missing a sense of humour. What fun! – a4android Apr 12 '17 at 2:31

The problem with backwards time travel is that every minimal modification to our world in the past would drastically change the future and therefore our present.

Just imagine you travel around hundred years back in time and accidentally kick a tree seed to a different place. Now the tree starts to grow there instead of at another place. Maybe now the car of your father will crash into the tree at that new position later, before he met your mother. You could not have existed in your present any more because your modification to the past caused your father to die. That however means that as you can not exist in the present any more, you could not have travelled back to the past and modify it. Therefore the modification has never been made, your father did not die and you are happily alive.

Infecting people in the past with any disease could of course have the same effect, it isn't even necessary that anybody dies, even if somebody just feels ill and stays at home instead of going out and doing something already is a huge difference and leads to a completely different future.

This little story I imagined is just a variation of the well known grandfather paradox (you travel back in time and kill your grandfather before he met your grandmother) which could be resolved e.g. using one of those two theories below. Of course we don't know whether any of them is correct yet, it's still mostly philosophy and speculation.

This theory states that everything that has already happened is fixed, which means even your time travel and all actions you do in the past you're visiting have already been predetermined. If we go into quantum physics a bit, all possible quantum events and therefore all possible future paths have a specific probability to occur. However, events that would generate paradoxes get a probability of zero which means it is impossible for them to happen.

That would mean if you travel back in time to kill your grandfather, no matter how hard you try, the universe would always find a way to prevent you from succeeding, no matter how obscure it might get.

Coming back to your question about bacteria, if this theory is true, nothing would change as everything you do to the past you visit has already happened to the past you come from, as they are identical. It is impossible to perform (or not to perform) any actions that would change history in any way.

This theory states that all possible pasts, presents and futures coexist at the same time, forming a hue or even infinite number of alternative parallel universes. Every time a decision is made in "our" universe, it branches into two universes, so that "our" timeline contains the decision we made and the alternative universe goes on with the other possible decision we did not make.

That would mean, if you travel back in time to kill your grandfather, you could simply do it, creating another parallel universe where your grandfather is dead and your father and therefore you have never existed. This alternative universe does not affect "your" universe though, so you still exist.

Coming back to your question about bacteria, if this theory is true, nothing would change, as the past you travel to is not the history of your own universe, but a branched timeline that contains these modifications made by your time travel. You can do everything in this alternate past, including destruction of the entire planet earth, without causing any changes to your own present universe.

• Hold up, what about the "Old Faithful" paradox: you just kill a parent, or some ancestor, so therefore don't exist, creating a paradox, because as you never existed you never killed them, so you are alive, so you did kill them, etc. – DevilApple227 Aug 10 '16 at 12:40
• @DevilApple227 You describe the grandfather paradox which I addressed in my answer. If the Novikov principle will turn out as true, it is impossible for a time traveller to change the past and therefore he can not kill any ancestor of himself, or if MWI is right, the murder will happen in a parallel universe and not affect your own history. – Byte Commander Aug 10 '16 at 14:23

You could argue that, if civilization still exists in the future, the civilization of the "present" couldn't have been wiped out by a virus from the future, since that civilization had to continue to exist to create the future that you traveled to.

Personally, I subscribe the the belief that if time travel is possible, any action taken in the past has already occurred, and so cannot change the present. Since the present is the future's past, the present cannot be destroyed if the future still exists, since any events happening in the present "already happened," as far as the future is concerned.

If the civilization of the future does not exist, and your explorers emerge from their time machine onto a barren, lifeless planet, then all bets are off.

• While you can't change the future once you've seen it, the sole fact that you're going to the future without a hazmat suit might mean you arrive to a desolate landscape inhabited by nothing but supergerm spores - some of which you then take home and wipe out humanity with. – John Dvorak Aug 9 '16 at 15:43
• Sure, and I would take the hazmat suit with me the first time I went. After that, it's safe. – Zac Crites Aug 9 '16 at 16:28
• That said, once all the humans are gone, chances are that all the human pathogens are as well. – Zac Crites Aug 9 '16 at 16:29
• "Sure, and I would take the hazmat suit with me the first time I went. After that, it's safe." - I believe commiting to this strategy will gain you a barren wasteland on your first visit - and it will be too late to apologise to the universe as you will have already seen the future and can't change it now. Either that, or you'll be prevented from making a second visit (without a hazmat suit), or its corresponding return trip. That's the better option. – John Dvorak Aug 9 '16 at 16:33

two factors to consider are first and foremost do we even interact with these times or is it merely observational? and if we do interact how does resistance equate to deadliness? Many diseases are drug resistant but not deadly nor does developing or mutating resistance increase overall deadliness. It's very particular as to what you contract that could have devastating effect if any at all, the act of time travel might have an unintended side effect of killing all germs. Unless you catch something highly contagious it's unlikely that it will even spread no matter how deadly it is.

I think that many answers to this question are illogical.

The question itself is illogical. It assumes that the problem would be caused by super drug resistant bacteria from the present that have evolved resistance to antibiotic drugs of the present. But as someone has pointed out there wouldn't be any antibiotic drugs in the past anyway.

Instead the problem is caused by the very short generations of bacteria which means that bacterial generations are many times as numerous common as mammal generations - even many times as numerous as mouse generations. So bacteria evolve many times as fast as humans evolve.

So if a person with billions of mostly harmless bacteria in and on him travels to the past and releases some of those bacteria into the environment, those mostly harmless bacteria will have different genes than the mostly harmless bacteria living the past, and will introduce new genes into the bacterial population of the past.

This will change the genetic makeup of many different bacterial species. Changing the genetic makeup of those bacteria species will change the way they evolve in the future. Thus those bacteria species will evolve different new strains and species in the future. Most of those different types of bacteria will be harmless, but some will be deadly diseases. Different deadly diseases than the deadly diseases which would have evolved without the time travel.

Everyone knows that the main influences on how long people live are viruses and bacteria. Time travel would change the viruses and bacteria that people face. Thus time travel will result in some people dying who would have lived, and some people living who would have died.

A person living enough years in the past, perhaps 5,000or 10,000 years, will fall into one of three categories:

1) dies without any children.

2) Has children, but their descendants die out after just a few generations.

3) Has children, and their descendants never died out, but increase both in number and in percentage of the human species year after year, generation after generations, century after century, millennia after millennia, until every single living person is descended from them at least once. And then their descendants will continue to flourish over many millennia as long as Homo sapiens or any biologically descendant species lives.

So if you go far enough into the past, the germs you release will soon cause the evolution of different diseases. Different humans will live or die than would have lived or died without time travel. And some of those who live or die would be ancestors of everyone alive in your era, including yourself. Everyone in your era, including yourself, will disappear and be replaced by an entirely different human population.

A prime example of "grandfather paradoxes".

The Novikov self-consistency Principle and the many worlds interpretation have been offered in attempts to show that there wouldn't be any "grandfather paradoxes" in time travel. But it seems to me that it would take a lot of faith that the universe happens to be structured in a way that makes time travel harmless for anyone to dare to travel in time.

I have ideas for a series about a space/time traveler, and finding ways to make him biologically sterile as far as viruses and bacteria are concerned is vital to making his travels safe for the societies he travels to and their future descendants.

the traveler might be surrounded by a bacteriological "death zone" that somehow exterminates all bacteria within it. Thus he will be incapable of transporting bacteria thousands of years into the past or future of a world. But of course killing all the bacteria that enter the "death zone" will change the the future evolution of their bacteria species and thus the evolution of future diseases.

• This sounds like a generic butterfly argument (a la A Sound of Thunder), just with bacteria instead of butterflies. Clearly an author who wants to posit time travel has to decide on some response to that -- but it had better be one that works for all manner of other sources of timeline contamination than virulent bacteria in particular. – Henning Makholm Sep 19 '18 at 20:07
• @Henning Makholm I would say that a space traveler who walks on another habitable planet, or a time traveler who travels thousands of years into the past or the future, is likely to commit the most deadly dangerous type of contamination with every breath that he takes. At least on an alien planet there will be a high probability that Earth germs will be incapable of infecting native life forms, and vice versa. But in the past and the future of Earth introducing Earth germs from the future or the past can have devastating results. – M. A. Golding Sep 21 '18 at 5:11

There are holes in the question, I think. Nevermind the bacteria and antibiotic stand point; what about the butterfly effect? If you even introduced bacteria from this point in time to a previous point in time, you would change everything that happened as a result of that bacteria NOT existing throughout the course of history (aka the butterfly effect), and still - kill a lot of people. Too convoluted scientifically to mix together bacteria resistance and time travel, and even begin to guess at how it would affect us/our planet/the universe, etc.

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