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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?

SirTechSpec
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wcarhart
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    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. – SE - stop firing the good guys Aug 09 '16 at 09:53
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    ^ Yep depending on how you make the traveling mechanism then going back in time can result in something like creating matter out of time i.e in 2010 there appears an apple from 2016 out of nothing, where did the universe get the matter to create this apple? Either something was destroyed for the sake of the apple or something was created. –  Aug 09 '16 at 09:57
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    @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 ~8.98710¹⁶J energy. (That is ~1.5 Hiroshima per kilogramm*.) – mg30rg Aug 09 '16 at 11:56
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    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 09 '16 at 12:29
  • If you want some ideas you could buy an history book and go back in time to predict the ''future'' then truly you become god on earth... – Charon Aug 09 '16 at 12:41
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    @渡し守シャロン Until you changed the future by making a couple of predictions and then suddenly ceased to exist. – Mad Physicist Aug 09 '16 at 13:01
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    If time travel were possible, bacteria would be the least of your worries – Bradley Thomas Aug 09 '16 at 13:14
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    @άλεξμιζέρια The conservation of mass applies to time travel. No new mass is created nor is mass annihilated. It's not a problem. Just keep track of the matter involved in any trip through, back and forth, and you'll see it's conserved. – a4android Aug 09 '16 at 13:28
  • @a4android Actually it could be done that way; extracting matter from the target timeframe could be a solution, therefore "exchanging" matter instead of moving it through time, but it really increases the risk of causing big troubles by the means of butterfly effect. – mg30rg Aug 09 '16 at 13:36
  • @a4android I mean you can travel back in time stealth around take pictures etc. without changing much, but if you actually take away matter it must have consequences. – mg30rg Aug 09 '16 at 13:43
  • aren't you yourself as time travel matter that's been taken away from present and thrown in the past? – Charon Aug 09 '16 at 14:06
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    You might want to worry more about bringing back something from the past that we declared "exterminated" and don't bother with vaccinating anymore. I'd imagine the future (knowing there exists a time machine) would keep things constant in case someone from the past showed up. – David Starkey Aug 09 '16 at 15:57
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    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. – frIT Aug 09 '16 at 16:31
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    Viruses and bacteria are not the same sort of thing. You're using the terms interchangeably, which is wrong. Pick one and stick with it -- presumably bacteria, since antibiotics don't work on viruses. – Mike Scott Aug 09 '16 at 17:37
  • Wow, I asked this before I went to sleep and woke up to see that it had completely exploded – wcarhart Aug 09 '16 at 18:02
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    By the time we have time travel expect nanobots that can solve such problems. – Donald Hobson Aug 09 '16 at 19:45
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    The way bacteria mutated 100 years ago is still the same way as today. The only trouble is that bacteria may have probably developed accumulative resistance to antibiotics (and similar drugs), but if you set that in the future the use of antibiotics is limited, I don't see how bacteria in the future can be worse than bacteria in the present, except that there are probably more types but not more serious types of bacteria that exist in the present world. – SOFe Aug 10 '16 at 09:39
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    @mg30rg Actually the SF author William Tenn wrote several time travel stories where matter had to be exchanged between the two times involved following conservation of mass. But that's not what I had in mind. Conservation of mass seemed to be an objection against time travel. i thought so, until I realized I was wrong. Mass traveling backwards in time is effectively 'negative mass' so it balances the mass when it returns to the future. Sorry. Too few words to explain here properly. It also depends on time travel mechanism. Not a problem with general relativity based time travel. – a4android Aug 10 '16 at 12:27
  • Why does viruses from the present not also wreck the future? Given a sufficiently long timeline, immunity will cease to be present in the future populace. In fact, we are currently experiencing this problem. Also this. – Nathaniel Ford Aug 11 '16 at 00:22
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    As your audience/reader/whatever I find it disingenuous when time has been spent on trying to resolve minor obscure issues like this with time travel while ignoring the big fundamental problems (relativity, conservation laws, causal paradoxes, etc.) that you can't get rid of. If you're telling a time travel story it's already utter bullshit from any rational standpoint, so just handwave issues like this away too. – R.. GitHub STOP HELPING ICE Aug 11 '16 at 03:01

11 Answers11

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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.

hmakholm left over Monica
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    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 09 '16 at 12:50
  • This would not help much as the diseases which we are resistant to, did not exist previously (as they've mutated). Also, back then, some diseases exist for which we do not have vaccines for. This answer is somewhat difficult to understand lol. – King of Snakes Aug 09 '16 at 12:56
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    @KingofSnakes. No it's not. This answer's straight forward. Our bugs don't have any advantage over earlier bacteria in the past. However, you made a good point, earlier strains of disease, if brought into the present, could be a problem. – a4android Aug 09 '16 at 13:39
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    @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 09 '16 at 14:05
  • Your answer also reminded me of the thought ingrained into my head from The Andromeda Strain:

    Any given strain of infectious bacteria is more likely to survive if it does not kill or even harm its host. Other factors ignored, current bacteria might be stronger than former bacteria precisely because it has evolved to be less deadly, hence surviving inside the hosts longer.

     – Tyrannosaur Aug 09 '16 at 17:55
  • I will give you props for figuring out this issues with going to the past (I still do not entirely believe your answer, but for the moment being I will take it as the truth), but you still do not discuss the future. If we were to bring back a mutated bacteria from the future to a time that are heavily dependent on antibiotics, would we not all perish? – wcarhart Aug 09 '16 at 18:05
  • @HenningMakholm If you expand on that in your answer, it'll probably be selected as the best answer – wcarhart Aug 09 '16 at 18:19
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    (@ThePickleTickler: Long comments moved into answer proper). – hmakholm left over Monica Aug 09 '16 at 18:26
  • @HenningMakholm and well-deserved rep has been awarded – wcarhart Aug 09 '16 at 18:27
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    @ThePickleTickler Usually you should wait $\approx$ 7 hours before accepting an answer and, for most cases, at least 48 hours to give everyone around the world a chance. – wizzwizz4 Aug 09 '16 at 19:57
  • @wizzwizz4 Good thing it's my question, so I don't have to wait that long – wcarhart Aug 09 '16 at 20:24
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    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 06:08
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    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 06:41
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    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
  • @Dunk: That may be a valid argument against time travel -- I'm responding more narrowly to the "future bugs are deadlier due to resistance" reasoning in the question. – hmakholm left over Monica Aug 11 '16 at 18:11
  • Please, could you clarify what was meant by I immune system will do? – Bugs Oct 15 '20 at 16:09
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    @Bugs : It's ‘a plain old Mk. I immune system’, or ‘a plain old Mark I immune system’ without the abbreviation. Here, Mark I (or Mark 1) is the original model (see Wikipedia). So you could leave that out and just say ‘a plain old immune system’. – Toby Bartels Apr 23 '21 at 00:59
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Clean the time travelers and boil their clothes then keep them in isolation until the traveling back in time. This way they won't infect anybody.

Though even if the time travelers don't wash away the bacteria and manage to kill the majority of humans, a small percentage will survive and adapt to it causing the virus to not exist at all in future, which creates a paradox.

If the virus/bacteria was killed in the past by adapting people to it before it comes to existence, then humans would not have adapted to this ''non existing'' virus which means nobody will die in the past even if they get contaminated, and the bacteria and virus simply stops to exist.

(This was just a theory about time paradoxes, technically a paradox could even just simply destroy the universe, but who really knows what happens when the laws of physics stop working to allow backward time traveling ...)

For traveling forward in time you only need this, a simple and cheap hazard suit with an air filter mask to survive new viruses. Oh, and don't worry about paradoxes this time.

An air filter mask costs about only 4 euro here which is like 6-7 dollars in the United States, while an entire suit costs about 9.08 dollars on Amazon.

enter image description here

(image source)

A.L
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Charon
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    This will probably cause panic, as people will think you are some sort of alien. Or god. Which will be amusing. (atleast, if they don't realize you are wearing a suit) – King of Snakes Aug 09 '16 at 09:56
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    If they are convinced you are a god, you can make them bury treasure in a area that you can safely take (later in the future, this might be the reason how you got the funding to go time travelling in the first place ☺ ) – King of Snakes Aug 09 '16 at 10:10
  • Bacterion or virus? Can't be both! – Lightness Races in Orbit Aug 09 '16 at 14:40
  • one can have both bacteria and viruses like flu... – Charon Aug 09 '16 at 14:44
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    nine dollars for a hazmat suit? That's ten times cheaper than an actual suit! – John Dvorak Aug 09 '16 at 15:16
  • I could understand a human-shaped paper bag (though $9 might be too much in that case), but that's hardly any use against tomorrow's diseases, and certainly wouldn't look like what's in the picture. – John Dvorak Aug 09 '16 at 15:20
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    depends, the ones for radiation protection can cost from as cheap as 1600 dollars to 3000 dollars depending on the model, the chemical resistant ones can cost from 80 dollars to 300+ while the bio resistant cost less than 10 dollars. – Charon Aug 09 '16 at 15:24
  • Can I have a link, please? – John Dvorak Aug 09 '16 at 15:25
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    https://www.amazon.com/Radiation-Shield-Technologies-XS-Demron/dp/B007XI3WI4/ref=lp_8402290011_1_3?srs=8402290011&ie=UTF8&qid=1470756377&sr=8-3 – Charon Aug 09 '16 at 15:27
  • Yep, disposable :-) – John Dvorak Aug 09 '16 at 15:40
  • kind of expensive if it's disposable, can you imagine buying a new suit every day at work? – Charon Aug 09 '16 at 19:11
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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).

King of Snakes
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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).

Byte Commander
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T.Nel
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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.

Devsman
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  • 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 09 '16 at 18:50
  • ...but we didn't. – n00dles Aug 10 '16 at 05:30
  • I am imagining the first unfortunate time traveller causing the Black Plague. – IT Alex Sep 19 '18 at 20:11
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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?

a4android
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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.

Byte Commander
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  • 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
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    @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
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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.

M. A. Golding
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    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. – hmakholm left over Monica Sep 19 '18 at 20:07
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    @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 05:11
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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.

Zac Crites
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  • 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 09 '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 09 '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 09 '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 09 '16 at 16:33
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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.

Bio Toxin
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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.

Tiffany
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