Setting: Modern day Earth, with technology no more advanced that what's foreseeable within the next 30 years.

Premise: An impending disaster which will completely obliterate the entire solar system in a few centuries has been detected. The only possible way for humanity to survive is throwing all available resources into perfecting a nascent, barely-understood portal technology.

Technicals: When the portal is open, scientists can look through it and observe the position of stars in the sky to verify the destination portal's time and location: Earth, 2 billion years ago.

While the portal is open, the scientists can also observe that it offers no apparent obstruction to wind on the past-side, and nothing from the past-side ever comes over to the present-side (aside from light). So, the portal appears to be one-way.

Problem: If the past-side of the portal opens into the same timeline as the present-side, then the entire timeline could be overwritten as soon as someone is sent through. In this case, they'd want to wait until the portal's size could be maximized, then send as many people through at once to ensure humanity's survival.

Alternatively, if the past-side of the portal opens into a new timeline which cannot affect the current timeline, then they'd be better off sending people through immediately, to begin setting up a colony.

Question: How can the scientists safely determine whether the portal opens into a separate timeline, or whether there is a risk of altering their current timeline?

By "safely", I mean in regards to the timeline (ie, the test itself shouldn't run the risk of altering the timeline to the extent that the time portal no longer exists).

  • 1
    $\begingroup$ If sending someone back through the portal creates a new timeline, then each person you send through will create another timeline, which is bad because everyone except the last person will live in a timeline where subsequent people never arrived (the first person through, for example, will live alone for the rest of his life). So either way, you want to send everyone through in one massive lump. $\endgroup$
    – komodosp
    Mar 18, 2016 at 9:49
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    $\begingroup$ @colmde It is worse than that. If we assume a time line overwriting model which the OP proposes (as opposed to multiple parallel timelines), then as soon as a single particle travels through the portal, then the sending timeline will be overwritten. The subsequent people who were hoping to go through the portal won't even exist anymore, and won't have ever existed, in any sense of the word. $\endgroup$
    – JBentley
    Mar 18, 2016 at 10:05
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    $\begingroup$ @colmde In fact, because light is coming through the portal from the past, then the past was altered the instant the portal was created, and the timeline would be overwritten immediately. $\endgroup$
    – JBentley
    Mar 18, 2016 at 10:07
  • $\begingroup$ @JBentley Yeah and with the multiple timelines, billions of new universes are being created every second as the particles stream through! But why would allowing a particle through would cause the people not to exist? Would they not now still exist but just live in a world whose history contains this particle? $\endgroup$
    – komodosp
    Mar 18, 2016 at 10:11
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    $\begingroup$ If the portal allows light through, it's not one way. $\endgroup$
    – aslum
    Mar 18, 2016 at 14:13

4 Answers 4


You write that the viewing portal is one-way “except for light” — well, that's not one-way: it provides for communication, and it is having an effect in the past even without you sending light/messages through because it casts a shadow due to the light showing up at the future end rather than where it would have landed had the portal not gotten in the way..

The basic idea to make the test is to send a probe back and then see if it's in the present from that past. That is, a direct test of your portal's past having effects that carry through to our timeline.

But if you don't find the probe, maybe that just means it got lost somewhere, especially over geologic time. No matter where you burry it, the ground will have been recycled and changed.

So, change the aim a tiny bit, and leave your artifact on the far side of the moon. It will have no effect on any butterflies, and it will still be there after 2 billion years.

As for your plan, you will find the Earth an alien world. You would not be able to breathe the air, for example.

You mentioned looking at the stars: the great galactic year is only a quarter billion years, so after 8 time around the stars will be totally scrambled and you would not be able to tell how much time passed from such a look. You need something more subtle, such as sending through advanced observation instruments and sighting the position of outside galaxies. You would send through space-based observation platforms (so it would not affect the ancient Earth) and allow the fact that light comes through the other way to receive telemetry. This is what you'll find on the moon today when you then go look where you left it.

Addendum: you can obviously use this technology to reach different planets and different times. So you might explore other worlds and find a different place to live, avoiding the paradox problem.

Why are you looking specifically at 2Ga? Maybe there is some relationship about the distance vs time of the portal? The fact that there was a solution to look at Earth's position in the past what with the motion of the galaxy relative to the cosmic microwave background, suggests at least that you have some control over things. Clearly you are reaching for a targeted position in space, quite far from “here” in the past. Other stars in our galaxy will be at approximatly the same distance, so you could just as easily look there.

  • $\begingroup$ +vote for the scientific method. Plus, I was expecting you would mention some paradoxes too :D. Which you alluded to, but did not discuss directly. Very nice answer! Also, the moon then, would have been seriously large and the tides would have been gigantic! $\endgroup$ Mar 18, 2016 at 7:49
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    $\begingroup$ Thanks, this answers my question and gives me a lot of other things to think about; great answer! $\endgroup$
    – Liesmith
    Mar 18, 2016 at 8:04
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    $\begingroup$ If the portal casts a shadow in the past and the scientists doesn't immediately disappear, then they never will. Colonize away! $\endgroup$ Mar 18, 2016 at 8:48
  • $\begingroup$ Now we know what triggered the Great Oxygenation Event: Liesmith's scientists opened a portal and let the wind blow through, carrying spores of photosynthetic plant life back in time 2.3 billion years. $\endgroup$
    – Cyrus
    Mar 18, 2016 at 15:56
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    $\begingroup$ Plants didn't come 'till much later. They use oxygen as well as produce it. The anaerobic oxygen producers were things like bacteria that eventually became endosymbiotes in plants. $\endgroup$
    – JDługosz
    Mar 19, 2016 at 0:55

Problem: If the past-side of the portal opens into the same timeline as the present-side, then the entire timeline could be overwritten as soon as someone is sent through.

If you're worried about an "overwriting" model, as opposed to changing the past just creating a new parallel timeline which coexists with your own, then the answer is: Nothing. According to chaos theory, there's absolutely nothing you could send through that wouldn't be overwhelmingly likely to radically change the subsequent history of that world millions or billions into the future.

The reason has to do with the so-called "butterfly effect", also known as "sensitive dependence on initial conditions." The butterfly effect gets its name from the idea of re-running history twice starting from initial conditions that are nearly identical but differ in some very small way, like whether a butterfly flaps or doesn't flap at that moment, or even just a shift in the trajectory of a few air molecules. Since the weather is a chaotic system, this very small difference between the initial conditions of the two histories will lead to greater and greater divergence in the weather conditions of the two histories, until about two weeks or so later the weather conditions will be basically completely unrelated (aside from what commonalities you can predict just knowing about the average weather at that time of year, i.e. the climate). For example, in one history there may be a major hurricane hitting the east coast of America two weeks after the initial state, in another history there may be no hurricane at all two weeks after the initial state.

And it's been shown that the systems that undergo repeated collisions, including air molecules, are themselves sensitive to extremely small differences in gravity. According to this page, a mathematician calculated that a difference in initial conditions as small as the presence or absence of a single electron 100,000 light years away would lead to high levels of unpredictability in the trajectories of air molecules, or even billiard balls colliding on a frictionless surface, after 100 collisions or less:

Now, those of you who never had time for billiards because of homework will immediately say: "This is absurd." After all, the edge of our galaxy is said to be about 100,000 light-years away. Not only that, an electron is unimaginably small - about one thousand billion, billion, billionth of a kilogram. In fact, celebrated mathematician Michael Berry of England calculates that the angle through which the cue-ball would typically be deflected by the distant electron's gravity would be so small that it would be represented very roughly as a fraction of a degree by a decimal point followed by 100 zeros, then a one. Such a minuscule deflection is impossible to measure. Yet, if snooker balls kept rolling for more than a minute after being struck, its effect would become profound.

And this is the nub of chaos theory. Little things mean a lot. Incredibly minute uncertainties in the initial state of a system lead to total uncertainty in our best conceivable predictions of the futures of those systems. For instance, if you knew more or less a point in space that marked the edge of the solar system at its birth, it would be impossible, using that slightly inexact information to plot where the edge is now. It would therefore never be possible to predict the weather accurately beyond a few weeks, or the behaviour of the planets more than a few hundred million years from now (that may seem like a long time to humans, but it is a trivial amount of time astronomically and geologically). The theorists tell us it is because minute effects may grow exponentially in such non-linear systems. While this is difficult to explain in a simple way for the weather or the solar system, the snooker illustration is far simpler to grasp. It is all because of the way snooker balls bounce off each other. Because of the balls' curved ivory-like surfaces, the incredibly tiny effect of the electron at the edge of the galaxy is multiplied by 10 every time the balls collide, according to Dr. Berry. So after 101 collisions, the decimal point I referred to earlier will have marched 101 positions to the right and the effect of the distant electron will have grown to 1 degree. One more collision means a 10- degree change in direction and two more gives 1,000 degrees, an enormous effect in a short time. Snooker balls, of course, don't run for a couple of minutes and the effect is therefore hypothetical.

However, there is an essentially frictionless form of snooker taking place all around us in the air we breathe. Imagine dancing oxygen molecules to be minute snooker balls, unceasingly bouncing off each other, forever moving in new directions. Such imaginary molecular snooker balls would have incredibly tightly curved surfaces, leading to an error amplification factor of roughly 600, instead of 10 for real snooker, per molecular collision, according to Dr. Berry. He therefore calculated that after about 50 collisions, which occur in less than a blink of an eye for molecules, not taking into account the gravitational pull of one electron at the edge of the galaxy would lead to enormous error in estimating what directions an oxygen molecule went. So even assuming a purely classical Newtonian world, as Dr. Berry did to make a point, we see that there is an extraordinary and unavoidable unpredictability all around us. This is the meaning of chaos. When the limits to predictability of the smallest particles of matter are added to this, it is remarkable that on a large scale there is so much order.

So, sending a single particle through the portal from the future to the past is going to totally randomize the day-to-day weather patterns starting a week or two afterwards, assuming the past can be changed at all (i.e. assuming the universe doesn't obey the Novikov self-consistency principle). And of course, this is going to have effects on which individual animals live and die, along with where they happen to be standing at different moments which affects which of their DNA molecules happen to get struck by photons and other particles that can cause mutations. So, in the long term, evolution should be totally changed.

Very small differences in the initial conditions of small bodies like asteroids and comets also lead to unpredictability on scales of a few thousand years--for example, Fig. 1 on the second page of this paper graphs 11 simulations of the changing orbital size of a comet, and finds that with only small deviations of the initial conditions--one part in a million--they get "gross divergences of trajectories" in less than 10,000 simulated years. So any tiny gravitational perturbation 2 billion years ago is going to very likely change which asteroids and comets end up hitting the Earth over the next 2 billion years, including especially important ones like the one that's thought to have hit Earth 65 million years ago and killed off the dinosaurs.

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    $\begingroup$ There is one way of escaping the butterfly effect - abuse the speed of light. If you open the portal 2 billion light years away from Earth and then beam a radio message through the portal so it arrives on Earth today, it is impossible for the events caused by that message to propagate and affect present-day Earth. That is, of course, assuming that opening the portal at such distances is allowed. $\endgroup$ Mar 20, 2016 at 8:23
  • $\begingroup$ @IndigoFenix - Good point. The question said the portal opened to "Earth, 2 billion years ago" but if we assume the same technology could somehow be used to open a portal sufficiently far away, outside of our past light cone, this would be a way (probably the only way) to send something through and avoid the worry about butterfly effects. $\endgroup$
    – Hypnosifl
    Mar 20, 2016 at 13:36

If your only concern is figuring out a way to tell if this goes to our past or a different timeline, then the solution is simple. Go and take the portal to somewhere that historically, hasn’t really been habituated by humans, or pretty much any life, ever. Someplace like the Arctic, or the marinara trench. It’d also help if you could find someplace that isn’t susceptible to a lot of geological activity, no earthquakes or volcanoes, but given that this is 2 billion years ago, the continents are going to move around anyway, so there’s really no fixing that.

Anyway, take a box, and make it as indestructible as you can. Make it basically everything-proof, and of some non-rusting, non-corroding metal. Inside the box, put a small gamma-wave transmitter, that can be turned off and on using a remote control. Sterilize the box (to prevent seeding with any microorganisms), huck the box through the portal, press the button on the remote, and go to the place you left the box. (don’t forget to account for tectonic plate shift!). If that actually was the past, then you should detect some gamma-wave radiation coming through the earth’s crust, and the only change is that there’s now a small box, deep underground, hurting no one. If it’s not the past, then there’s nothing. PROBLEM SOLVED!

Except for the big disaster. That’s still on you.


It does not result in a new timeline. That would create 2 types of paradoxes.

The Loop Paradox

If I return to the past and shoot my grandparents before they made any babies, it would mean that my parents did not exist. Which implies that I (who went to the past and shot my grandparents) never existed. Which raises the question: if I never existed, who shot my grandparents?

The Matter Paradox

2 billion years ago, iron existed as rust particles, most of them at seafloors and some at the surface of land. If you send a car back 2 billion years ago, it would create two copies of the same atoms.

A lot of iron (and other metallic atoms) would be like "Hmmf! That's what I would become 2 billion years later. Oh wait! My future version already exists now! So which atom of iron exists? The one in the car or the one at the seafloor?


Thing is, if/when you send a car back 2 billion years, the matter forming the car would return to its state as it was 2 billion years ago. So sending the car would result in not having any car at all.

If all the humans return to 2 billion years ago, all of our atoms would return to their states 2 billion years ago. We would cease to exist.

... and all the evolutionary history would be repeated in precisely the same detail, all over again ...

  • $\begingroup$ The loop paradox means that it does form a different timeline. You seem to have confused things. As for the world-line of the atoms, again if something like that was happening I think it would speak against the alternate timeline, in that the new arriving atom was the same one as what's in the past. As for the whole assertion of that point, you're basically saying that time travel doesn't happen! The car that "goes back" becomes what it already is at that time, e.g. nothing happens to the past. $\endgroup$
    – JDługosz
    Mar 18, 2016 at 8:04
  • $\begingroup$ A portal, if anything like a General Relativity wormhole, would not do anything like that. I don't think it's possible to distinguish the atoms, or track them in this manner. And don't forget the electons move around. Worse, what of some electrons were created since that time, via cosmic rays and beta decay and whatnot? It's identity can't be traced to a specific thing in the past but is just energy spread out in many places. $\endgroup$
    – JDługosz
    Mar 18, 2016 at 8:10
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    $\begingroup$ @JDługosz What he calls the "matter paradox" is in fact a possible and valid scenario, but is usually called a causal loop. The point is that when the car goes back in time, it doesn't "become" the iron atoms that later turn into that car, it always was those atoms. That's why it is a loop. The car goes back, and 2 billion years its atoms are made into a car, which goes back, etc. $\endgroup$
    – JBentley
    Mar 18, 2016 at 10:34
  • $\begingroup$ Yes, both examples do suggest that the timeline would change. And they also create physical (and logical) paradoxes which cannot be solved. For example, if past travel was possible, how would you explain the grandfather paradox? And in case of the matter-timeline paradox, the mass-energy of the universe is constant. So that if new electrons were created after 2 billion years, sending those electrons back to 2 billion years would only transform them back into photons which are energy carriers (electromagnetic ones, that is. otherwise everything is energy carrier). @JDługosz $\endgroup$ Mar 18, 2016 at 10:34
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    $\begingroup$ "New timeline" is invented specifically to solve the grandfather paradox. I think you are not reading the same meaning of this term. You kill a different peeson from a different universe which is not really your grandfather; this other timeline will not hace you in it. In this timeline you exist to the point where you left. Get it? You're explaining it exactly contrary to its normal meaning. $\endgroup$
    – JDługosz
    Mar 18, 2016 at 10:39

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