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This is how time travel in my question works:

You go back to a certain point in time, physically, regardless of means. Be it a spaceship or a time machine.

So you've reached to the past. You know certain big things are going to happen because you are from the future, you've had your history class.

You've decided not to do anything about it, just being an observer. You just eat and sleep everyday.

Will this kind of behavior change the past?

If so, is it because daily activities still involve interaction with people in the past and thus triggers butterfly effect?

Also, I've read it somewhere saying that even "do nothing just watch" would change the past. It doesn't obey the physical law because the person who travels backwards in time absorbs light and thus is interacting with the past universe and therefore changing it.

I don't quite understand this whole "the person who travels backwards in time absorbs light and thus is interacting with the past universe" thing. What light is this person absorbing? Why is this person absorbing light? Could someone explain it? Thanks!

(There is only one single timeline of course. I'm not going into a parallel universe)

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    $\begingroup$ Related - Previous question, "You cannot change the past. You just watch history unfold as a spectator locked in your own body. " - worldbuilding.stackexchange.com/questions/136901/… $\endgroup$ – chasly from UK Feb 28 at 19:27
  • $\begingroup$ @chaslyfromUK Thank you for giving me this information. I assume that you were referring to that answer about wave function collapse due to an observer. But I don't know if that answer is correct or not since there are people who don't agree with it. $\endgroup$ – user61906 Feb 28 at 19:49
  • $\begingroup$ Yes, it was about collapsing the wave function. I haven't studied quantum mechanics so I can't answer that myself. You might be able to formulate a question to ask about this on the Physics site - not sure. physics.stackexchange.com $\endgroup$ – chasly from UK Feb 28 at 19:55
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    $\begingroup$ Welcome to the site! Note that, without a proven model of time travel, any answer to this question would require either A) an explanation of how time travel works in your universe, or B) pure speculation on the part of those answering. Since (B) will likely result in this question being put on hold for being primarily opinion-based, would you mind editing your post to provide details on your time travel mechanics? Also, feel free to take the tour and check out our site culture to get a better understanding of the site. $\endgroup$ – Frostfyre Feb 28 at 19:55
  • $\begingroup$ @Frostfyre Sure! I will edit my question. Thank you. $\endgroup$ – user61906 Feb 28 at 20:04
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It is theoretically possible to observe the past without changing it. After all, when we look at the sun, we are (given a standard frame of reference) looking at the sun as it was eight minutes ago, and we aren't changing what happened then.

But if you're observing the past by actually going back to the past, then simply being present in the past can change the past. You mention eating, but that means getting food (either stealing it, or somehow getting money to buy it), which means altering the past. If people can see you, then that will affect the past. Everything you do moves molecules around, and that can have unpredictable effects.

As far as absorbing light is concerned, "looking" at things means interacting with the light they give off. To detect a photon, that photon must interact with matter, and the way photons interact with matter is by being absorbed. The human eye has chemicals that absorb photons, and those absorption set off chemical reactions that result in signals to the brain. A film camera has chemicals that can absorb light, and change color when they do so. A digital camera has chemicals that give off an electrical signal when they absorb a photon. And so on.

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Any, ANYthing you do in the past changes things to incredibly tiny amounts. The air you breathe was supposed to have been breathed by somebody else - maybe it contained a cold virus that you caught instead. The food you eat would have been eaten by someone else. Maybe the hamburger had a bone chip in it that another person was originally to eat, chip a tooth, and meet their future spouse, who was a dental hygienist.

How likely such coincidences are is an unknown. But they exist.

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    $\begingroup$ You really don't need to stretch weird events, our reproduction and thus our personalities, talents, and health, are all based on pretty much random chance that the tiniest even can alter $\endgroup$ – Andrey Mar 1 at 17:52
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If you breath in once and breath out once in the past, you will mix past and future airborne germs and viruses.

You will leave some bacteria and viruses from the future in the past, and you will bring some past bacteria and viruses to the future when you return to the future.

And the bacteria and viruses transported through time are likely to have different genes and introduce those different genes into the populations of bacteria and viruses in their destination eras. Thus they will h change the gene pools of bacteria and viruses in their destination eras, and that will change the future evolution of new strains and species of bacteria and viruses.

These new species of bacteria and viruses will mostly be harmless but will include some harmful ones and some deadly to humans.

Thus some humans will die that would have lived, and some humans will live who would have died. Some of the humans who would have lived would have died without offspring anyway, and others would have had offspring who would have died out within a few generations, but others would have had descendants that would continue for as long as the human race survives.

Some of the humans who would have died will live longer but die without offspring anyway, and others will have have offspring who will die out within a few generations, but others will have descendants that will continue for as long as the human race survives.

If a human has a line of descendants that never dies out as long as the human species exists, his descendants will multiply and eventually include every single human being alive at some specific future date, and then all the rest of the humans who will ever be alive in the centuries and millennia and tens of millennia and hundreds of millennia of the future existence of the human species.

And the time it takes for a human's descendants to spread and multiply until every single human is descended from them should be only a few thousand years, perhaps 5,000 or 10,000 years.

So if a human from AD 2019 goes back in time to, for example, 10,000 BC or 20,000 BC or 100,000 BC, and breathes even once back then, they will cause all humans alive in AD 2019 (including their self) to vanish and never have been born, replaced by a totally different set of people alive in AD 2019.

So if a human from AD 1,000,000 goes back in time to, for example, AD 950,000 or AD 900,000 or AD 500,0000 and breathes even once back then, they will cause all humans alive in AD 1,000,000 (including their self) to vanish and never have been born, replaced by a totally different set of people alive in AD 1,000,000.

And if a time traveler brings bacteria and viruses from the past back into the future time of AD 2019 or AD 1,000,000, or whenever, that will eventually cause all of the people who are alive a few thousand yeas in their future to vanish and never have been born and be replaced by a totally different set of people for the rest of human history.

I have an idea for a story where the protagonist is sent to different times and places, including some on Earth and other planets at various times, and I have been wondering about how to avoid making that character the "Typhoid Mary" of the universe.

One idea is to somehow make that character's body and immediate area a "death zone" for bacteria and viruses, so none of them survive coming close to that character, and so he cannot transport living bacteria or viruses to the past or present of their world.

But if the character kills all bacteria and viruses they come near, every time they are sent to a new location in space and time, that should kill a lot of viruses and bacteria that would have otherwise lived, thus changing the future evolution of viruses and bacteria, thus causing some people to live and others to die and thus changing the entire future histories of those worlds.

One way to avoid that is to postulate that the beings who send the protagonist to various locations in space and time know from their time travelling that those are the locations in space and time they are destined to send the protagonist to, and thus the protagonist only kills the bacteria and viruses the protagonist was always destined to kill.

So you can see that I am interested in the best answers you get to your question about what is necessary for the time traveler to avoid changing the fate of worlds. Perhaps some of them will help me with my story idea.

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    $\begingroup$ our modern diseases are most likely to decimate any populations they touch as our evolved immune systems are not even comparable to prehistoric man. Look at what the Europeans did to the native Americans. It was not from pox blankets. $\endgroup$ – Andrey Mar 1 at 17:53
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About your question regarding light absorption (I’m going to go ahead and replace light with energy) - Lets say the total amount of energy available from a point in the past till the present moment is x. Now to move back in time you require some amount of energy and lets call that y and y is less than x. So this certain amount of energy y, which you’re using to turn back the clock will most likely be pulled from the past timeline. This energy might have been used for something else had you not intervened, so now you have inadvertently affected the past simply by traveling to it!

But my theory won’t hold under the following conditions:

  1. You’re using energy from the present to jump back in time - kind of like a time catapult ( But won’t that release excess energy into the past????)
  2. You are stuck in some kind of paradoxial loop. You are indefinitely traveling from this particular moment in time to a point in the past
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  • $\begingroup$ Thank you for your clear and well-written answer! Is this whole energy mechanism the law of conservation of matter? $\endgroup$ – user61906 Feb 28 at 20:39
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    $\begingroup$ @user61906 Yes! The fact that Energy can neither be created nor destroyed is what I based this on. But this is just a theory that made logical sense to me. I could be wrong :’) $\endgroup$ – Lord of the Larks Feb 28 at 20:42
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    $\begingroup$ It also made logical sense to me. Also, since changing the past by a time traveler would give rise to a paradox in a single timeline, could the paradox be avoided by the following: once a paradox occurs, the current timeline automatically destroys and a new timeline is created with a future that's being changed. Does this make sense to you? I personally don't think so because I don't think a timeline that's already existed can just vanish along with all the people in it. $\endgroup$ – user61906 Feb 28 at 20:49
  • $\begingroup$ But if parallel timelines exist, then all possible timelines must be simultaneously existing. If you moved from one timeline to another, would it not result in two of you in the same timeline? ( Time travel related stuff always ends up leaving me confused ) P.S You don’t have to select my answer because its the first one that made sense to you. There are lots of people with a lot of good ideas here :) $\endgroup$ – Lord of the Larks Feb 28 at 20:57
  • $\begingroup$ Well, it's not necessarily a parallel universe. It's like one that doesn't make sense so another one that makes sense pops out. But I don't think it works. Plus, I thought I could select multiple answers, I'm new here but looks like I can't. I prefer yours because you answered my question about "light". That's what I really wanted to know at first place. $\endgroup$ – user61906 Feb 28 at 21:04
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As you wish.

There are basically two different theories about changes in time travel. And the answer depends on which one you subscribe to.

The first sees time as self-healing in the way that a pendulum works or something else that is resting in a stable position: If you disturb it, it will swing this way and that, but unless your disturbance is large enough to cause a phase change, it will eventually return to its original position. Of course, if you swing it strong enough that the string breaks, or push it strong enough to go to another local minimum, then not. But in this theory, the past will absorbe and self-correct small changes. Rivers work like that: If I push some water to the side or take a couple buckets out, a little bit further down you won't notice a difference.

The second theory sees time as chaotic. Any disturbance can have extreme consequences. Anything can happen. A small change can throw the whole thing completely off track. This is where the term "butterfly effect" originates.

Since there is no time travel, there is no saying which of these theories (or a completely different one) are true. So you can pick the one that fits your world or story better.

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Yes, two reasons:

Butterfly Effect: You go to a tavern to eat, you take a seat which was free later on in the original timeline, so somebody who in the original timeline was at your seat takes seat somewhere else, which makes him not overhearing some conversation he did overheard originally, or overheard some other he did not in the original, thus potentially changing his perceptions and decitions, which will have an influence on others and so on. Mind, not everytime you have lunch you change fate, it is just the accumulation of minimal probabilities everytime you are there where nobody was at originally.

Your very presence might change decitions on the spot: You want to observe interesting events in the past, but those many times are stressful moments for everyone involved, how would their decition making be influenced by somebody unknown staring at them ? Extreme example, say you decide to observe one of the Ripper's killings to learn his identity and he spots you, he might well flee, or go for you giving the victim a chance to flee. Less extreme, you decide to witness Christ's cruxifixion. How will the Centurion leading the guard react to another body in the crowd ? Maybe he will not notice at all, maybe it will be enough to make him nervous about the amount of people gathered and change his decitions. How will the very crowd react to you? It is expecting for everyone to shout insults and start a row directed at you if you do not join, thus making the centurion even more nervous? Or the other way around, it is all friends and they expect everyone there to be lamenting the event, and you're only there staring like a statue. Again, your mere presence does not warrantee fate is changed, but it is something taken into account by others when deciding what to do, and might be enough to decdie for a subtly different line of action. Or an extremely different one if the Centurion gets angry enough to charge down the hill pilum-first to dissolve the row you caused.

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Ofcourse it would change the past.

Imagine if you were sent to the farthest edge of the observable universe. You'd die within a few minutes, but you would still be a point of mass that wasn't there before, pulling on everything and everything around you.

Now for something important: The universe doesn't care if you changed the course of human history, or that of the tiniest molecule by pulling it with your mass for even the smallest moment imagineable. Any time travel to any time, any moment, anywhere will result in the same thing: A changed time.

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  • $\begingroup$ Actually, that is not true. The universe is quantized in all aspects we have sufficiently researched. Which means that there is an actual smallest amount and there is an amount below which no interaction occurs because it's less than one unit. According to current theory: One graviton. $\endgroup$ – Tom Mar 1 at 13:45
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    $\begingroup$ @Tom could you refer me to those sources? I looked here: en.m.wikipedia.org/wiki/Graviton and it confirmed once again that Gravity has unlimited range and travels at the speed of light. Hence I said at the edge of the observable universe as anything beyond will need to travel faster than light to reach us due to the expansion of the universe. $\endgroup$ – Demigan Mar 1 at 16:52
  • $\begingroup$ I'm not arguing about distance or speed, but unit. If gravity is quantized it means that there are units of it, e.g. gravitons. If a limited number of gravitons radiate from a gravity source in all directions, then by the power laws there's a point where the effect would have to be less then one graviton, which can't be. So the wave function (assuming quantum mechanics) could collapse to zero. $\endgroup$ – Tom Mar 1 at 17:38
  • $\begingroup$ @Tom a hypothetical particle with hypothetical units of which the math isnt even complete yet? Could you point to any theories proven or not that a quantity could get so low it'll be considered zero? $\endgroup$ – Demigan Mar 1 at 19:08
  • $\begingroup$ The double-slit experiment shows that if you have a single photon, it will after collapse of the wave function be in one place or the other, but not both. The graviton is hypothetical, but if gravity is quantized then the same must be true of it. The number of gravitons sent out by an object is not infinite, so its effects cannot be infinite in distance and time. $\endgroup$ – Tom Mar 2 at 2:47
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Everything you do changes the past.

  • You breath, exchanging O2 and CO2 with the enviroment.
  • You mere body occupies space, so all the air molecules in that space where moved to the sides when you appear there. Each time you move, you also move the surrounding air.
  • Your body is solid, it stops the flow of wind.
  • Your body weights, an increase in mass decreases the speed of Earth, even at the slightest amount.
  • Your body has mass, even the slightest mass produce a gravity well. Your own presence if affecting all the particles of the universe.
  • You see, so the light which collides with your skin and eyes is absorbed/redirected, and thus changing its pattern.
  • Your body is warm, so it irradiates thermal energy.
  • You move, which produce kinetic energy. Even when you walk you hurt the grass or left footprints.

All of this are small things that your body does to the past, so yes, you are altering past with your mere presence.

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The "absorbing light" part refers to quantum backaction. This is a fundamental part of quantum mechanics, that there are no disturbance free measurements. To detect anything, you have to interact with it in some way, to see anything, you have to absorb the photons you see.

So for your time traveller to notice anything, he has to interact with the past, it is impossible to simply observe. The effect will be tiny, but it most definitely will be there on a very fundamental level.

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