Plausible reason why my time machine can only go back a certain amount of time?

Question

So, I have built a Time-Traveling machine. It’s a prototype, and I’ve noticed a few kinks in the machine. If the control pad gets jammed in home mode, the universe will repeat the same day for all eternity. Also, I noticed that my machine can only go back in time within a limited range. I can go anywhere in time between August 12th, 1941 and the present. This is very important to the plot of my story, and I want to give a logical reason why it is that way.

My question is, what is a plausible reason why my time machine is limited like this?

Comments are not for extended discussion; this conversation has been moved to chat. — Monica Cellio, Aug 01 '18 at 03:03
You may wish to note that while current scientific thinking does allow for travel through time, one can (theoretically speaking) only travel between two fixed points, one of which is the initial creating of a wormhole (with the other being the current time of the wormhole). The answer to your question would be "Because that's how time travel actually works in reality" — Richard, Aug 01 '18 at 16:06
Is the date static, or does it move with normal time? E.g. In 10 years, will the limit be August 12, 1951? Also, what happens if you attempt to exceed the range? Do you just fail, blow up, disappear, or appear at the limit? — sharur, Aug 02 '18 at 21:10
@sharur: It’s static. It appears at 12:00 am, at the limit — DT Cooper, Aug 02 '18 at 21:46
@Richard If you have an answer that hasn't been posted yet, then please post it as an answer, not as a comment (which bypasses the normal quality control systems of the site). If it has already been posted as an answer, and you agree with it, just upvote that answer. — user, Aug 03 '18 at 06:07
@MichaelKjörling - I would do except that A) Other answers seem to have this covered and B) I don't think this answers the question of why his time *machine* wouldn't work. — Richard, Aug 03 '18 at 06:15
Too short for an answer: The subatomic particles used in the time travelling science, e.g. tachyons, decay. So you can not ride them anymore — Ander Biguri, Aug 07 '18 at 08:40

score 132 · Accepted Answer · edited Jun 16 '20 at 11:03

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There's an old fashioned option

You can't travel back in time to a point before the invention of the first time machine.

That means of course that the first person to invent a time machine couldn't travel back in time to point before his own invention, and possibly initially thought it didn't work, which is true for a given value of true.

edited Jun 16 '20 at 11:03

Community

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answered Jul 30 '18 at 06:48

Separatrix

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This is a good one. The great-grandpa actually built the time machine. Less sure about true for a given value of true, but right on. – Willk Jul 30 '18 at 12:08
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@Willk, the original worked, but it couldn't be effectively used, hence "didn't work" because of the constraints on it. Ignoring the 1min time machine. – Separatrix Jul 30 '18 at 12:41
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Or, alternatively, it's only possible to go back to moments in time that have some particular characteristic which got established somehow, possibly without anyone recognizing its significance to time travel. The only thing I can think of that would have been going on then historically would have been naval mine sweeping efforts using large-area magnetic fields, but maybe one can come up with an excuse as to why those would facilitate time travel. – supercat Jul 30 '18 at 15:11
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I assumed it was a programming error caused by using the wrong data type which couldn't handle the result of a calculation. – B540Glenn Jul 30 '18 at 17:41
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Or see how the time machine in Primer worked. In order to travel back in time, you had to turn it on and have it running, and then you could get in at some point and travel back to when you first turned it on. – Shufflepants Jul 30 '18 at 18:58
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It seems to me that nothing prevents going back in time before the invention of the first time machine, just going back in time before the invention of the time machine that holds US Patent Number One. – Codes with Hammer Jul 30 '18 at 19:45
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Time travel requires unobtainium, and traveling back will send you to the location (then) of the rock used. The only unobtainium on earth comes from a meteor that landed in 1941 – Arcanist Lupus Jul 30 '18 at 20:15
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@ArcanistLupus: Do you have a cite to that claim? I have only one reference work that discusses the supply of unobtainium [Cameron (2000)] and would be thrilled to add a second good source. – Codes with Hammer Jul 31 '18 at 13:19
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@CodeswithHammer the best source is undoubtedly Richardson (2027), but I can look for others if you'd like. – Arcanist Lupus Jul 31 '18 at 13:34
I once read a story much like this; in that story the USSR had created an artificial time warp or black hole or something at a certain point in the 1980s(?). It had a limited lifespan and you could travel from any time in the lifetime of that artifact, back to the moment it was created. Other timewarps were built later so you could take a sequence of them back to that earliest date. – workerjoe Jul 31 '18 at 19:39
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@ArcanistLupus, The potential reasons are many and varied, consider this one as an example: Time travel is not possible in the primary timeline, inventing time travel branches your timeline from the primary but you can't travel to a point before your branch was created as that would lie in the primary timeline. – Separatrix Aug 01 '18 at 09:02
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@ArcanistLupus: Why would it transport you to where/when the comet landed, and not where/when the comet was flying around in deep space? What specifically defines the "fixed past" of the unobtainium when using it in the present? – Flater Aug 01 '18 at 10:30
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@Flater, you pick the time, the unobtainium controls the location. you could go back in time to when the comet was in deep space, but doing so isn't very useful for Earthly time travel. – Arcanist Lupus Aug 01 '18 at 13:04
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@ArcanistLupus: Then it's not impossible to travel to the past, just impractical. Might be enough for OP, might not be. – Flater Aug 01 '18 at 13:06
Maybe as a supplement for your answer, one issue with FTL travel is that we haven't invented brakes yet. Once you start moving the speed of light, everything around you stops, so... what's going to stop you? Nothing in the normal reference frame can do it; it's frozen in time. You'd need something which can interact with your super-lightspeed-capable machine, and what better than another super-lightspeed-capable machine (conveniently built on August 12, 1941)? In other words, that's the last stop; if you miss it, you've just bought a 1-way ticket to the Big Bang. Hope you brought popcorn. – Lord Farquaad Aug 02 '18 at 17:44
Wasn't that part of the plot of Quantum Break? The activation of the original time machine is the epoch of time travel? – Michael Brown Aug 06 '18 at 02:22

score 112 · Answer 2 · answered Jul 30 '18 at 08:37

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A previous time machine crashed and exploded on August 12th 1941 during a test flight. Maybe sabotage or a design fault. The explosion and shockwave will exist forever at that point in time.

The explosion acts like a reverse black hole. The closer you get to the point of time of the explosion, the more power you need to bypass the shockwave emanating from it. The power required to bypass it increases exponentially and generating that amount of power is impossible with your current technology.

answered Jul 30 '18 at 08:37

user3161729

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I like this answer, though rather than exponentially I'd say asymptotically - you can get really close to that point in time, but passing it would require infinite energy, not just a prohibitively large amount, so you don't have to worry about the possibility of a better power source from the future (fits the black hole analogy better too). – John Montgomery Jul 31 '18 at 00:21
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I think this answer provides the best opportunities to spice up the plot. Who crashed the time machine? Was it intentional, to seal off the past? Can that seal be broken? Or can the trick be used again to seal off the more recent past, after it has been changed? – BlindKungFuMaster Jul 31 '18 at 09:39
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@JohnMontgomery that's going to bite you when writing a sequel in which you need to go back further in time. ;) – JJJ Aug 02 '18 at 06:26
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@JJJ You can then invent some kind of hyper-time which will allow to pass that blast :) – Alissa Aug 03 '18 at 12:14
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@BlindKungFuMaster What happens if they crash another one in the future? :O – Devsman Aug 03 '18 at 19:29
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Bonus points if the energy released from the explosion in 1941 is the energy that makes time travel possible at all. – mbomb007 Aug 03 '18 at 21:10
@JJJ Or maybe trying to go further back in time creates the explosion in the first place ;) – user3161729 Aug 04 '18 at 13:08
If exponentially, then if you have enough energy to burn then you can recover it on the other side of the reverse black hole. Wait... you wanted to slow down? I thought you were aiming to get to the start of the universe? Oh, well you're in for the ride now. Enjoy your -80 000 000 second / second journey! – wizzwizz4 Aug 04 '18 at 18:32
It wouldn't need to be a previous time machine, now would it? – Aug 05 '18 at 21:52
That suddenly becomes like the Flash's speed force. – Michael Brown Aug 06 '18 at 02:21
IMO infinite energy doesn't make sense, since the other time machine did only have finite energy, meaning the explosion couldn't emit/contain an infinite amount of energy – somebody Aug 06 '18 at 05:07

score 61 · Answer 3 · answered Jul 30 '18 at 03:49

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You need to know where the Earth is. If you think about it, the Earth is spinning around the sun, in a spinning galaxy in an expanding universe. If you just headed back in time, you'd appear in the void of space. Not only do you need to know when to send someone, you also need to know where.

To have a limit on how far you can go back, your time machine has only calculated where the Earth is to that point. If you try to go back further, you'd end up floating in space.

answered Jul 30 '18 at 03:49

Thorne

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"You need to know where the Earth is." Well, we sort of covered that in my old question How can I explain that a time travelling apparatus moves itself through time but appears in the same location? – user Jul 30 '18 at 05:59
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At first I was going to comment that this was a very soft limit, since you simply need to handle larger errors to go further back. Then I started thinking... The Earth moves a bit erratically due to solar wind and such These are unpredictable sources of error. HOWEVER, for the recent past we have very accurate astronomical data telling us about the errors. I think the first atomic clock, which is actually from 1941, was a key to getting these astronomical data. – Stig Hemmer Jul 30 '18 at 10:50
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Kudos for the atomic clock answer! Baking in limitations to the time machine based on how accurate it can be is smart design and provides a "soft" limit that actually makes a heap of sense (providing hard limits in time travel discussions gets very difficult, depending on the medium of travel). Go for a soft limit like this :) – GrayedFox Jul 30 '18 at 11:45
Needing to know where earth is depends on how the time machine works. See, for example, the movie primer where they absolutely dont need to know, and are simultaneously greatly limited in how far back they can go. – Matt Jul 30 '18 at 12:49
Or maybe the machine cannot move in space, so you can only go back to times when Earth was exactly where you are now. – algiogia Jul 30 '18 at 13:01
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@algiogia The Earth has never before been where it is in the present. The Sun itself is actually moving along its orbit at a rather high rate of speed. – Michael Richardson Jul 30 '18 at 15:06
@MichaelRichardson I know the Sun moves... but hadn't realized it's orbit was sooo long – algiogia Jul 30 '18 at 15:27
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@algiogia: There's no such thing as "exactly where you are now" because there's no such thing as absolute position. All you have are relative positions in 4 dimensions with the added complexity that it's warped by gravity, etc. – R.. GitHub STOP HELPING ICE Jul 30 '18 at 18:34
@StigHemmer But how can you account for the movement of the Milky Way itself, and the movement of the cluster it moves around, etc? Do we really know all that, and in relation to what? What is the reference point? And can we really know it with enough accuracy? I imagine a tiny error could easily bring one to 10 km up into the atmosphere or deep into the Earth's crust? – Fiksdal Jul 30 '18 at 19:15
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@R.. There is such a thing as "exactly where you are now" relative to a given reference frame, e.g. the Sun. You just need a reason as to why the time machine prefers that reference frame over the Earth. – Ray Jul 30 '18 at 21:08
Related to what R and Ray are saying, assuming the time machine uses the Earth's inertial reference frame, you only need to worry about where it goes if you travel to a time before or after the Earth experienced a significant acceleration. Sedimentation, erosion, and plate tectonics would be much bigger concerns than Earth's movement through space. – StackOverthrow Jul 30 '18 at 22:49
Any answer along these lines, depending on how hard-sciency you get, means that the time machine has some knowledge of the "universe-relative location." Or, it has to be able to perfectly calculate from {current_location} and {current_time}, where your destination is. Sounds fun to think about :) – Caleb Jay Jul 31 '18 at 22:58
@StigHemmer (and Thorne): while minor fluctuations happen over time, how big are they? Why not simply use a time travelling submarine and point it at the middle of the Pacific Ocean? Plenty of wiggle room there. Disregarding that, why would it be impossible to travel back, rather than inadvisable? You're still leaving the door open for someone to decide travelling back further and gambling on whether they survive the arrival. – Flater Aug 01 '18 at 10:36
See http://edition.cnn.com/2011/WORLD/asiapcf/03/12/japan.earthquake.tsunami.earth/index.html – Thorne Aug 01 '18 at 12:08
@StigHemmer The idea about atomic clocks is nice, but it doesn't work out: The earth is a pretty damn heavy object, and the non-gravitational forces acting on it are really low. Thus, once you know its exact position in space at any given time, the error margins on where it was the day before become very small. So, after landing and measuring the error in earths position, you can simply go back another day, or week, or year, depending on how much error you are able to tolerate in a single jump. And my gut feeling would definitely see a year at a time as feasible. – cmaster - reinstate monica Aug 02 '18 at 21:16
@StigHemmer It's impossible not because of the error in accumulating "all" sources of motion, it's impossible because there are no universal reference frames, there is NO underlying grid of the universe, there is NO universal origin of position, there does not exist an object you can measure relative to for all time and space. For all we know the entire visible universe is a bubble moving at 99.99999% the speed of light relative to some "nearby" other bubble and you could never account for that, but it's irrelevant in the first place. – Quantic Aug 03 '18 at 21:18

user · Answer 4 · 2019-07-14T10:19:31.550

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If you're willing to accept that the machine (as in this specific apparatus for time travel; not as in any apparatus capable of time travel in your universe) cannot travel further back in time than some specific amount of time, rather than to some arbitrarily selected date, then there's an easy option that might even make a modicum of sense scientifically (to the point that anything about time travel can be said to make sense scientifically in the first place).

Make it so that time travel requires power. Tons, and tons, and tons of power. Not just energy, which you can store (think batteries), but pure, unadultered, raw power, which is the instantaneous flow of energy. (Energy is measured in watt-hours or multiples thereof; power is measured in watts or multiples thereof.)

The farther (back or forward) in time you go, the more power it requires. So a ten day time jump requires more power than a five day time jump. The relationship between the two could easily be anything from sublinear to a tetration, depending on how time travel works in your universe.

Something about the design of the machine causes it to not be able to handle arbitrarily large amounts of power. There is nothing strange about this part; not even a simple electrical wire can handle an arbitrarily large amount of power.

There might also be some design or physical reason why components need to be below a certain size to work correctly, so you can't just make them bigger in order to handle more power; if you try, they fail to work for this other reason. Modern computer CPUs fall into this category; if the die was much larger, then the speed of light propagation delay in terms of clock periods becomes prohibitively large, thus putting an upper limit on how fast a given CPU design can be clocked. (Note that the physical chip is much larger than the die.)

It just so happens that this power limit works out such that on the first day that the machine works, it can't travel back in time further than to your chosen date. If you were to try, it'd burn out a critical component, resulting in anything from just a stranded protagonist with a spare part to complete destruction of the machine and anything near it. (Your choice of value for "near".)

Obviously in that case, the next day, the machine will only be able to travel back in time to the day after your chosen date, because the target time window moves as time goes on.

To keep the protagonist from making multiple smaller time jumps instead of a single large one, limit the machine's carrying capacity and the energy density of whatever devices power it. Maybe in your universe, there's a hard upper limit to the amount of mass that can be transported to a different time; much like how in our universe, there's a hard upper limit on speed (that is, the fact that nothing can exceed the speed of light, and that nothing with a non-zero rest mass can attain the speed of light).

edited Jul 14 '19 at 10:19

answered Jul 30 '18 at 06:07

user

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To extend on this, if you use handwavium as your power source that could only be refined using a technique/technology only available after a specific point in time, you can take multiple shorter jumps back as far as that point and refuel, but prior to that point in time there is no more handwavium available, and the maximum time you can travel back from that point is to August 12, 1941. That way, even 100 years from now you can continue jumping back to 1941, but no earlier. As an added twist, eventually even the handwavium produced at that point will be depleted by prior time travelers! – Doktor J Jul 30 '18 at 16:54
@DoktorJ Interesting idea! – user Jul 30 '18 at 17:26
@DoktorJ: You'd need to explain why someone wouldn't establish a successful chain of handwavium filling stations for time travelers, with handwavium "tanker trucks" replenishing their supplies every so often. It's not an insurmountable obstacle, but it's something to consider. – Michael Seifert Jul 31 '18 at 15:31
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combine this with the accuracy of moving in space answer and you get a lot of options. Also, Just a plain cool answer, I was formulating something similar, but you beat me to it. And said it better. – Paul TIKI Jul 31 '18 at 16:55
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@MichaelSeifert but how much handwavium can you take in a time machine? Plus, you could introduce an element where excess handwavium in the time machine (other than in the reactor's containment unit) can destabilize the time machine with catastrophic effects... so sure you could pack some extra in your tank or enlarge the tank a bit, but it wouldn't be cost effective to keep transporting it back in time. – Doktor J Jul 31 '18 at 19:07
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also, perhaps operation of the containment field consumes power in an exponential manner in relation to both volume contained and amount of time traveled, so while a 1L containment field might consume 0.1L of handwavium to go back 10 years or 0.4L to go back 20, a 2L containment field might take 0.4L to go back 10 years, and would take 0.9L to go back 15 (making that close to the maximum you can go back, and still return). f=0.1(0.1tv)^2 where f is fuel consumed, t = time traveled (years), v = containment field volume – Doktor J Jul 31 '18 at 19:17
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Using that formula, a 4L containment field would consume 1.6L to go back 10 years; it'd take just shy of half the "tank" to go back 11 years and a month. Of course the equations can get much more complex (add a linear component for actually transporting the time machine and its nonvolatile contents), but it's trivial to concoct a formula – Doktor J Jul 31 '18 at 19:42
This still leaves the door open to improvement. Depending on OP's story, is there more than one time machine? Is there someone able to build a new (and possibly better) one? You need to tick a lot of boxes before this answer is fully applicable. – Flater Aug 01 '18 at 10:40
@Flater True, but OP asked about the one specific time machine, not time travel in general in their universe. For one specific machine, my answer can apply; for time travel in general, probably not necessarily. – user Aug 01 '18 at 11:47
@MichaelKjörling: Even if we assume only one machine will ever exist, it still leaves the door open for someone to improve the machine. I'm not saying your answer is wrong, but OP will have to accept that there is no one better at time machine building than the inventor, and that the inventor is incapable/unwilling to improve the machine. It's also unintuitive for the machine to be unable to travel past a given point in time which just also happens to be a significant point in time for the plot, while the machine's limitations are not directly related to the plot. – Flater Aug 01 '18 at 13:15

LSerni · Answer 5 · 2018-07-30T10:41:59.330

The simplest way would be to limit the machine's range (see Asimov's classic Chronoscope). But this way you wouldn't have a "precise start date", as the start date would move forward in time.

You could have some strange mechanism by which the range extends gradually backwards - each day, you can reach exactly one day farther in the past. So the farthest date remains the same.

Otherwise, you need some key event that never occurred before, or never occurred in a reachable range before (i.e. if you have a range limit of 5,000 years, it's enough that it only occurred the once in the last 5,000 years - the 7129 BCE event isn't reachable anyway).

So we need a plausible unique event to have happened no earlier than August 12th, 1941, and a reason why it's crucial.

The time machine requires orienting in 4-D spacetime, which can be accomplished only by either specially constructed beacons or by exploiting point-like natural phenomena whose location in spacetime must be known with great precision. Once the beacon is locked on, any spatial coordinate within a reasonable range in the same timeframe can be reached. Interpolating two beacons allows reaching any timeframe between the two.

The earliest known such phenomenon, Beacon Prime, was a self-contained, unreported small-scale criticality accident inside a six ton uranium oxide pile located in New York, on the 7th floor of University of Columbia's Pupin Hall laboratory, in the morning of August 12th, 1941.

Something very similar appears in Dave Freer's Pyramid Scheme, where an alien probe targets the exact point where the first large-scale nuclear fission reaction took place, in Chicago, on December 2nd, 1942 (where the New York pile had been transferred since 1941).

It is a choice designed to give Earthmen the idea of hitting the Krim probe with nuclear energy, thus giving it enough energy to engulf the whole Earth. The evil Krim's plan backfires spectacularly.

Or the first atomic explosion (during the Manhattan Project or the two bombs actually delivered) or the Tunguska event (https://en.wikipedia.org/wiki/Tunguska_event). +1 for the beacons idea. — theGarz, Jul 30 '18 at 09:11
Or the machine measure time from an epoch, like unix time, going back before would require modififcation (assuming it doesn't accept offsets or negatives) — Wilf, Jul 30 '18 at 10:36
@PunctualEmoticon thank you. Yes, that's because I'm Italian :-) — LSerni, Jul 30 '18 at 10:43
Arthur C Clarke's "All the Time in the World" (1952) had a better version of limited access for time travel. Alien archaeologists in the future reach back in time to the only point they can access. This is the end of the world. Ours. — a4android, Jul 31 '18 at 02:51
I knew I remembered your name from somewhere... It's a long way to here from it.discussioni.ufo , but it sure brings back memories :-) — ChatterOne, Jul 31 '18 at 07:18

Jasper · Answer 6 · 2018-07-30T06:46:44.400

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The time machine comes with a lifetime warranty, but with a twist. It is warranted for the lifetime of its inventor.

The inventor wanted to make sure that the grandfather paradox would not apply to himself. So he put lockouts in the control system, to prevent any attempt to jump to before the inventor's birth.

Perhaps the software lockout is reinforced by a hardware lockout, such as this:

The control system includes quantum entangled qbits.
Some of the qbits are part of a positronic brain.
Their twin qbits are part of a regular electronic brain.
The inventor sought out quantum entangled qbits that use positron-electron pairs that were made on his birthdate in a MeV (Mega Electron Volt) particle accelerator. (Somehow somebody stored and kept track of these particle accelerator products.)
If you go back to before the inventor's birthdate, each pair of critical qbits is replaced by a 1 MeV photon.

As Michael Kjörling♦ points out, the time machine needs a lot of power. In particular, it needs a lot of power to open the doors. This power is obtained via a controlled matter-antimatter reaction. An electronic brain controls the matter side of this reaction; the positronic brain controls the antimatter side. The two are linked via the quantum entangled qbits. If you go back too far, the link between the two brains dissolves, and you cannot open the door. At the story author's option, if you then return to the valid operating range of the time machine, the link is reinstantiated, and you can open the door.

The positronic brain is not a user-serviceable part. If the user tries to physically tamper with it, the antimatter is likely to be catastrophically released.

As a bonus (for the inventor), the warranty terms give customers an incentive to extend (rather than reduce) the inventor's lifespan.

edited Jul 30 '18 at 06:46

answered Jul 30 '18 at 05:59

Jasper

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I'm pretty sure that the average 3-day-old is still not able to defend himself from being killed. So does the inventor of the time machine really care that he gets born (= preventing you from killing his ancestors) if a time traveller is able to kill him minutes, hours, days, ... after he is born? – Flater Aug 01 '18 at 10:42
As a bonus (for the inventor), the warranty terms give customers an incentive to extend (rather than reduce) the inventor's lifespan. That entails selling time machines who are magically attuned to the inventor's life force since they need to independently be able to assess at what point in time the inventor dies. That opens up a whole slew of magic and inexplicable features (can we track others? Is my missing husband really dead? What if the inventor is comatose? How much of the inventor needs to be alive for the machine to consider him alive? Do his descendants count?) – Flater Aug 01 '18 at 10:46
@Flater -- No, there is no magical connection between the death date of the inventor and the working of the time machine. Instead, there is a contractual relationship between whether the inventor is alive, and whether the machine is covered under warranty. This means that if the machine's operator wants the machine to be covered under warranty, then the machine's operator will avoid taking actions that reduce the inventor's lifespan. As you point out, some methods of extending a person's lifespan are not good for the person whose lifespan is extended. – Jasper Aug 01 '18 at 15:37
@Flater -- You have a good point about the vulnerability of 3-day-olds. The answer's logic can be used to implement any particular first valid date (that is on or before the date that the time machine is completed, and after suitable particle accelerators become available), with both hardware and software lockouts. – Jasper Aug 01 '18 at 15:40
But I don't get the point of the warranty. What is stopping me from travelling to the past in order to get "past inventor" to repair the time machine? As long as I only travel in the time period between the invention of the time machine and the inventor's death, I effectively have an infinite warranty. The inventor's short lifespan may limit the time window in which I can travel and maintain the infinite warranty, but the warranty remains infinite if I stay within the window. – Flater Aug 01 '18 at 15:41
@Flater -- It sounds like you have some good hooks (or twists) for stories. The point of worldbuilding is to explain plausible backgrounds for stories, so that authors can write good stories. – Jasper Aug 01 '18 at 15:46
Cool, but too easy for Mr. 31337 |-|@XX05 to override. – Carl Witthoft Aug 02 '18 at 15:12
@CarlWitthoft: On the other hand, it's the first remotely plausible solution posted yet. Something about the interlink between time travel and space travel makes this ridiculously hard to do otherwise. – Joshua Aug 03 '18 at 02:07

score 21 · Answer 7 · answered Jul 30 '18 at 09:26

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You did not build a time machine, you built something that force loads the save state of the simulated world you are trapped in, which is a mirror of the world on August 12th, 1941. The illusion of time travel is maintained by the "time machine" inserting you into the world after the simulation has run for a specified amount of time.

Or perhaps time travel cannot actually violate causality. A time machine cannot actually travel back in time, it is instead a beacon that summons things from the future. Your "time machine" is just one of many possible objects and phenomena that can interact with this beacon, which was created on August 12th, 1941.

answered Jul 30 '18 at 09:26

Teleka

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Augh. A forced reload and rerun of the universe every time you travel. Some overworked cosmic cider is going to have one hell of a time debugging that! – Joe Bloggs Jul 31 '18 at 20:08
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@JoeBloggs: It's easier to debug, not harder. There's a reason we focus on reproducing bugs before we set off on a hunt for the mystical Heisenbug. – Flater Aug 01 '18 at 10:48
@Flater: you’re assuming that the original state of the universe is stored somewhere. If it’s more akin to a database (without a proper backup strategy) then you can’t recreate the conditions leading up to the bug by any means other than just letting the simulation (or universe) run. For years.. – Joe Bloggs Aug 01 '18 at 10:52
@JoeBloggs: That's not what I meant. Reproducing a bug does not mean starting from zero. It means taking a snapshot of the data when the bug exists, and then being able to repeatedly tinker/rinse/repeat the same data snapshot over and over (if needed). We don't need to store the original state of the universe, because this question inherently focuses on the inability to travel beyond 1941. Therefore, the oldest accessible backup is 1941. (Similar: you can't go further back in the commit history of your Git repo than the day you did your first commit in the repo). – Flater Aug 01 '18 at 10:55
@Flater: ah, you might have misunderstood the joke I was making. If the bug is ‘Universe database occasionally resets to 1941 for no discernible reason’ an external engineer is going to have one hell of a time working out that the reason for that bug is that part of the simulated universe has somehow gotten hold of the ‘reset’ code, given that all the changes that led to the bug have been erased and set back to their 1941 state. The only way to do it is to rerun the simulation (but now there’s a rogue bit of data in the database that can arbitrarily reset the whole thing). – Joe Bloggs Aug 01 '18 at 11:32
@JoeBloggs: Yep, missed the joke (well, I thought it was a different one). My bad :) – Flater Aug 01 '18 at 11:33
+1 Way cool. Versions of the simulation software before the date did not include variables and data tables that were required for the time travel patch hence no way to return to before that. No restore points before that date. – KalleMP Aug 03 '18 at 19:26

a4android · Answer 8 · 2018-07-30T05:27:41.500

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If your time machine can only travel into the past. Presumably it must create an effective "anchor" for its initial launch point. That is, the present. Therefore, if the time machine can access any moment in time between the anchor point time and 12 August 1941, its range of travel is from now, backwards in time, to 12 August 1941. And with all stops in-between.

This is a time machine with a range of seventy-seven (77) years backwards in time. The time machine is also capable of travelling to any time within that range.

It is left as an exercise to the querent (or OP, if you prefer) to calibrate the exact limits of its backwards range.

EDIT:

There is another possibility suggested by @RonJohn's comment, but related to the rationale presented above, namely, that the time machine created a fixed "anchor" point in the past at 12 August 1941. There is an effective range for its setting a past "anchor" point, but once set this will be the limit of travel into the past and any points in-between the present and 12 August 1941.

In this case, the past limit will remain 12 August 1941, but the upper temporal limit will always be the present.

Any time machine on start-up will create its past temporal "anchor" point seventy-seven (77) years in the past, but its upper temporal limit will continue to stretch forward in time.

edited Jul 30 '18 at 05:27

answered Jul 30 '18 at 05:01

a4android

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A range of travel would imply that tomorrow the farthest back you could go is 13 Aug 1941. And then the 14th, 15th, etc. – RonJohn Jul 30 '18 at 05:07
@RonJohn That sounds like a reasonable conclusion. Let's see what tomorrow brings. – a4android Jul 30 '18 at 05:20
@RonJohn Thanks to your comment I have added to the concept. See edit. Very grateful for you for spurring me into thinking further about it. – a4android Jul 30 '18 at 05:29
Why just 77 years in the past? Because the ambiguities of motion means that you can only accurately calculate relative space-time locations that far back (very similar to how Chaos Theory prevents us from forecasting the locations of the planets too far into the future). – RonJohn Jul 30 '18 at 05:36
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@RonJohn Because of the physical nature of time travel itself. If the limit was 77 years due to calculating relative positions, then a time machine could leap frog its way back through in 77 year hops. The 'anchor" point concept assumes an unspecified something in a deeper physical nature inherent in time travel itself. I cannot say more, the time police might be on to me. – a4android Jul 30 '18 at 05:47
Well, if you "anchor" to the present, it immediately becomes an anchor to the past, because the present immediately becomes the past. So if you place this "anchor" and travel, you can never really travel forward to the exact moment that you started traveling. You'd be back a split second before that moment and most likely re-live it time and again. Or fuse with yourself. Or create infinite copies of yourself. You'd need some sort of compensation when trying to come back. But this actually gives some hints for a nice plot-twist, let's say. – ChatterOne Jul 30 '18 at 13:48
If a time machine comprises two parts - a base and a vessel - then the construction of the base could serve as the original anchor point. This would allow the past limit to indefinitely remain 12 August 1941. Also, the limit could be 100 years if the "present" time in OP's world is 2041 rather than 2018, which might be desirable and could allow there to be some tension leading up to that point where people wonder if the machine will be done in time. – corvec Jul 30 '18 at 17:10
@ChatterOne You've got some nice ideas there.Although possibly the time machine halts an infinitesimal moment into the future after its original departure. You are right about the compensating when returning to the present. All sorts of interesting possibilities. Much appreciated. – a4android Jul 31 '18 at 03:09
@corvec I had considered the possibility of a two-part time machine. A fixed unit in the present & a travel unit that together had a range limited at 12 August 1941 (1941-08-12). The date acted as an "anchor" but the fixed unit allows travel to any date between itself, in the present, & 1941-08-12.This imposed too many constraints on the design & construction of the time machine. So I went for a temporal nature based limit. I am delighted you thought of something similar. Please go ahead & post your idea as an answer. Thanks! – a4android Jul 31 '18 at 03:16
Perhaps the first anchor was created on that date (by an inadvertent test at a nuclear cyclotron facility that caused a brief singularity that time machines can use to point towards while travelling back. Going forward you can aim for the same anchor point in the future. – KalleMP Aug 03 '18 at 19:30
@KalleMP That seems too complicated, for my taste. Also, the physics of a "nuclear cyclotron" and "a brief singularity" is too problematic for credibility. However, what I called an "anchor" might be "a brief singularity", generated by the time machine itself when first activated, that acts as a barrier, preventing further travel into the past. Despite the hurricane-strength hand-waving it feels more "plausible pseudoscientifically". – a4android Aug 04 '18 at 02:22

score 10 · Answer 9 · answered Jul 30 '18 at 08:39

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A lot of nuclear research was happening at that time.

Some event in that context weakened the fabric of spacetime enough for time travel to work. Before that event, spacetime is just woven too tightly to wade through in any feasible time machine.

So research some event that either happened on that day or can be plausibly argued to have happened on that day and been misrecorded in History. Or make up an event that was lost to history, due to intense secrecy and other causes. Optionally, this could be unknown to everyone, and possibly discovered as a plot point.

(If you could push your limit date to 1945, the first detonation of a nuclear weapon would fit the bill perfectly)

answered Jul 30 '18 at 08:39

Emilio M Bumachar

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To chime in with @Emilio's answer - they have actually used caesium-137 dating for detecting wine forgeries. You could make it that cesium-137 interferes with the time "navigation" and that the OS is programmed to protect the user, else they are never heard from again. – OldTinfoil Jul 30 '18 at 15:40
The only problem with this is that subsequent nuke tests were far more powerful, so you'd think you couldn't even get past 1961 or so. – Carl Witthoft Aug 02 '18 at 15:13
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@Carl I don't see the problem, further tests just further loosened the fabric of spacetime. – Emilio M Bumachar Aug 02 '18 at 16:06

score 8 · Answer 10 · answered Jul 30 '18 at 11:48

On 9 October 1941, President Roosevelt approved the atomic program after he convened a meeting with Vannevar Bush and Vice President Henry A. Wallace.

https://en.m.wikipedia.org/wiki/Manhattan_Project

A few weeks before that some kind of experiment happened that’s crucial to time travel. Maybe it depends on an isotope that came out of that lab.

Maybe it acts as some kind of beacon. (And since then new beacons were created at an increasing rate, to the point that travel to any precise moment from mid 1940s is possible.)

WernerCD · Answer 11 · 2018-07-30T15:55:57.320

Epoch Time

The Time Machine is built and runs using a variation of Linux and the epoch date - 0 date - is set to August 12th, 1941.

Normal Linux machines of a "0" date-time of January 1st, 1970... but for some reason, the Time Machine has a different "starting" datetime set as epoch.

Trying to use a date before "0" in the operating system causes a system crash.

https://en.wikipedia.org/wiki/Year_2038_problem

I'll wager the version of Linux uses an unsigned integer though... using a signed integer would make the limit of the machine 68 years (1941 to 2009)... making it unsigned would stretch that 32bit field to 138 years (1941 to 2079).

That would mean any attempts to go before Zero... or after 2079 (Whatever the exact math boils down too) would either crash (going backwards) or overwrap (going forwards).

Sure, you might be able to recompile the Operating System or port the Code that runs the Time Machine... but, sadly, the original programmer isn't with us anymore (How do you think we learned about the issues with the code? Woops). So the effort to rebuild and rewrite are prohibitively expensive - and getting more so.

Ever try to hire a Cobol programmer? same issue, times 1000...

Variation on the theme

The epoch time is set for 2009 with a signed integer. So, the furthest back you can go - before over-wrapping integers occur - is 68 years: 1940. And the furthest forward you can go is 68 years: 2079.

All too probable. I have seen systems that used an epoch other than 1 Jan 1970 0000Z. — Codes with Hammer, Jul 31 '18 at 13:11

score 7 · Answer 12 · answered Jul 30 '18 at 16:55

Time is weaker in mid-to-late 1900s due to frequent time travel

Your time machine is presumably the first time machine ever invented and is considerably weaker than other time machines that will undoubtedly be invented in the future. As such, it cannot fully "punch holes" in the fabric of spacetime to jump to any arbitrary point. It only works on regions of history that have already been "worn down" by more powerful future time machines.

As it happens, the buildup towards the atomic era / World War II and the aftermath thereof is by far the most well-studied part of (currently past) history and is therefore "weak" enough for low-powered prototype time machines to travel to.

August 12th 1941, while not important in and of itself, happens to be the limit of how far back you can go with your current technology and power source due to wear and tear from travelers to subsequent months and years. (There is a slight blurring; travelling to time X also weakens down a surrounding area of history inversely proportional to temporal distance.)

The hypothesis here is plausible. There has been so much time travel to the 1936 Olympics that all the time machines may interfere with each other, and hence with all future (and past?) attempts to cross that lynchpin. It is possible that the OP's prototype cannot overcome the interference any closer than the posted date. — Codes with Hammer, Jul 31 '18 at 13:15
@CodeswithHammer: Please read IATT Bulletin 1147 regarding the killing of Hitler before your next excursion. Failure to do so may result in your expulsion per Bylaw 223. :-) — ShadowRanger, Aug 07 '18 at 06:27
@ShadowRanger: I am a member of ACN not IATT. Because many of the ACN missions to 1936 attempt to prevent or undo the other missions, ACN supports the intent of IATT Bulletin 1147. — Codes with Hammer, Aug 07 '18 at 13:20

score 6 · Answer 13 · answered Jul 30 '18 at 09:17

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Your main character bought an entry level time machine. August 12, 1941 is its earliest setting. Travel to an earlier point in time requires either a different, better, time machine, or an expensive upgrade (and the time machine will be in the shop for six weeks to be able to perform that upgrade).

answered Jul 30 '18 at 09:17

Abigail

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Brilliant! Nice lateral thinking. – Patrick McElhaney Jul 30 '18 at 16:03
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I don't see how the time machine being in the shop for six weeks is a problem. It's a time machine, after all... – wizzwizz4 Aug 04 '18 at 18:47
@wizzwizz4: It depends on frame of reference. The machine will be in the shop for six weeks subjective. Who knows how long it will take from the point of view of the owner? – Codes with Hammer Aug 07 '18 at 13:22

PunctualEmoticon · Answer 14 · 2018-09-01T20:44:04.297

It's a result of the computer's programming.

In many modern computers (Linux, macOS, iOS, Android, basically everything except Windows), computers keep track of the time by counting the number of seconds since January 1, 1970 (because this scheme was created in the early 70's). This is called Unix time.

Somewhere in your time machine's code, the time is limited to -895,852,800. Maybe that was the programmer's phone number. Maybe they were lottery-winning numbers. Most realistically, if you're flexible with the date, they rounded it off to -900,000,000 (June 25, 1941). (Unfortunately, there's no powers of 2 that are sufficiently close.)

score 5 · Answer 15 · answered Jul 30 '18 at 05:41

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Summarizing from the H2G2

Long ago, the people of Krikkit attempted to wipe out all life in the Universe, but they were stopped and imprisoned on their home planet;

August 12th, 1941 is the day Bobby Peel, a cricketer, died.

he became well known for liking alcohol

Actually, he didn't really died, he simply hitchhiked to a different dimension to keep hold of the people of Krikkit. Traveling back in time to before that day will trigger the people of Krikkit into unleashing their evil plan.

The Universe itself conjure to protect life, and tweak its own laws preventing time travel earlier than that date.

answered Jul 30 '18 at 05:41

L.Dutch

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While it does make for great entertainment, I'm not sure I'd consider much of The Hitchhiker's Guide To The Galaxy to be particularly plausible. – user Jul 30 '18 at 05:57
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@MichaelKjörling, I agree, and I have checked that the OP didn't use the science-based tag before posting this. – L.Dutch Jul 30 '18 at 06:01
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@MichaelKjörling less plausible than time machines? – jwenting Aug 01 '18 at 05:51
@jwenting Just because we're dealing with time travel doesn't mean the rest of what's going on can't be allowed to make a little sense. But to each their own. – user Aug 01 '18 at 06:15
@MichaelKjörling: If we're already considering a universal temporal road block at a particular point in time; it's not as far-fetched to allow for other species to have caused this temporal road block. Even if they live across the galaxy and will never ever interact with us. The universal temporal road block is order of magnitudes larger in scope than alien life. – Flater Aug 01 '18 at 10:51
@jwenting Oh time machines are not implausible. In fact, we are all sitting in one. Pity, it only moves into a single direction at constant speed... Jest aside, afaik, general relativity does allow time travel via wormholes. And those definitely have the feature of disallowing time travel to times before the time traveling wormhole has been created. – cmaster - reinstate monica Aug 02 '18 at 21:28

score 5 · Answer 16 · answered Jul 30 '18 at 09:27

The August 12th 1941 limitation is one of range. Not temporal range, the machine can theoretically go anytime you need it to go. No, the problem is in the other three dimensions.

The time machine in question has two separate engines: One for translating the equipment in the fourth dimension, which due to properties of time travel itself, can go basically anywhen. The other is a 3D transversal device that ensures that when the machine moves in the 4th dimension, it ends up in roughly the same 'place'. Unfortunately, this 3D transversal device has to struggle against galactic inertia to make sure that when we travel we don't end up in the empty space between solar systems after making a small hop back to 1972. It's remarkably efficient at this, but is limited by the antimatter fuel tank that powers the device.

So we've built in a safety (margin ~10%) into the travel so we cannot accidentally get stranded in interstellar space. The big advantage is that we don't need to find antimatter in the past, as we can piggyback on the existing velocity to ensure we end up in the same place when traveling forward.

As for your home screen jam, are you sure you didn't accidentally engage the timer function? The timer function is a safety feature to make sure the device isn't captured by hostile forces in the past, it lets you either make the machine make jumps of a specific time (so every friday, at 10PM, you can find the machine in the same place). If you set the loop to a day in the past and are wearing your temporal anchor (you are wearing your temporal achor, aren't you?), it would certainly look like a groundhog day type scenario.
Good news though, it won't repeat the same day forever, just until the antimatter fuel talk that powers the 3D transversal device runs out, which should be somewhere in a hundred years or so.

Ruadhan · Answer 17 · 2018-07-30T14:11:14.250

Limited Temporal Scouting

Before you acquired the time machine, someone had to map space-time coordinates to feed into it, this took a lot of trial and error and a lot of ending up in deep-space before they got a short list of viable data out of it.

Getting new data is really really hard. Or at least takes very specialised equipment that you don't have.

The onboard computer can also interpolate the data it has to produce a sliding range of viable coordinates so that you can essentially pick any date within its range. However if you want a wider range, someone is going to have to map earlier or later coordinates.
As it happens, the earliest date you have access to is august 12th 1941.

Explanation B: The initial jumps for the machine were disasters, the machine consistently wound up in deep space and after much spinning of dials and random coordinates you found just one viable space-time coordinate that worked. August 12th, 1941.

By interpolating your starting space-time coordinates and the viable one, you managed to get a range of coordinates that you can travel safely within, but if you want more then you'll have to deal with the random-number-generators again.

Home mode is very simple, the machine's guidance systems detect what day it is (by the 24 hour period). And if you jam the guidance systems into going home, then at midnight that night, the machine detects it's no longer on the correct day and jumps back in time 24 hours automatically to compensate.

score 4 · Answer 18 · answered Jul 30 '18 at 05:38

There is an obvious possibility. The clue can be found here in this part of the question.

If the control pad gets jammed in home mode, the universe will repeat the same day for all eternity. Also, I noticed that my machine can only go back in time within a limited range.

Like many prototype machines, the time machine has a fault or most likely several faults. If the control pad is jammed in home mode, the machine is stuck in a time loop.

Therefore, part of the time machine's circuitry or whatever kind of mechanism is either to control or propel the time machine has gotten itself stuck on 12 August 1941. While the machine can travel between then and the present. (It is only backward and then forwards again type of time machine.)

The good thing is this can be fixed. Take the largest spanner you can find and strike the side of the time machine several times with great force. This should unjam its mechanism and the past before 12 August 1941 should become accessible. Either that or it's back to the workshop to fix the fault.

Levity aside, a plausible explanation for the limited time travel could simply be a technical fault.

The issue is, the user doesn't need to perform percussive maintenance on the time machine, the user needs to whack whatever is stuck in 12 Aug 1941. Fortunately they get to repeat that day as often as necessary until they whack the correct item. — Codes with Hammer, Jul 30 '18 at 19:44
@CodeswithHammer This is the problem with using humour in an answer. Obviously whatever specifically is struck needs to be unstuck. This is why the suggestion about taking the time machine to a workshop is included. The problem is a technical fault, prototypes are hag-ridden with them, and it needs to be fixed. This isn't funny. Sorry about that. See my last paragraph above. It says it all. — a4android, Jul 31 '18 at 03:02
@a4androd: I was trying to be only partly humorous; the other part was an attempt to provide hardware tech support, but I'm a software person. Nevertheless, when a time machine has a problem anchored to a particular date, we must consider the possibility that the problem is at least partially temporal in nature, and hence can only be fixed on that date. — Codes with Hammer, Jul 31 '18 at 13:08

score 4 · Answer 19 · answered Jul 30 '18 at 08:35

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While booting your machine for the first time, you thought you've need to enter your birthday, while it was the limit time range, here for security purpose.

answered Jul 30 '18 at 08:35

fabunazov

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AmiralPatate · Answer 20 · 2018-09-13T07:39:54.297

The solution I'm offering doesn't limit to a specific date but rather to a specific range. It also allows travel beyond that limit, which will be either extremely inconvenient or unsafe, or likely both. This was inspired by real life, turns out whether you should use float, double or decimal sometimes matter a whole lot.

Time is much like space: better explored when you know where you're going. Anybody who've seen Stargate knows you need two things to travel: the coordinates of the destination, and the coordinates of the point of origin. Well, consider it's called space-time continuum for a reason, any point in time is therefore identified by coordinates. To find your way between now and then, you need to calculate a trajectory through time. Double the fun if time travel implies defining the location in time and space.

The basic conundrum is you can either do things fast or precise. You can travel between any two points with tip-top accuracy by using arbitrary-precision arithmetics (aka as infinite-precision, which sounds way cooler), a supercomputer and three years of your time. Note that you'll have to predict how long the calculation takes, and input the precise time of your departure in advance and stick to it with rigorous precision. If the calculation fails (computer crash or other calamity), you'll have lost all that time for nothing.

And then you'll have to do it again to calculate your way back in advance, since it's unlikely you'll find a supercomputer in the 1940s. If anything goes wrong, if you meet an unexpected obstacle, and if you miss your mark going back or forth, there is zero guarantee you'll be anywhere near where/when you want.

All in all, you can dismiss that possibility by denying access to the appropriate resources, supercomputer time ain't cheap, nor is the power plant to get the lights on, or by making your characters sane enough to not consider doing it that way.

So instead you'll aim for something fast using built-in floating-point arithmetics. It will take a couple of minutes at most but the trade off is accuracy. There is a range of time where you're 95% certain that you will be where and when you want, give or take a day or two. Beyond that, the machine might send you to the Moon and miss the date by a century.

Essentially, the further you go, the worst your odds are. As a good software dev, you've put blocks that limits use to a defined safe range. It just happens 1941 is the limit if you were to start travelling from today.

Obviously, that opens the door for someone to travel by increments. If you have a 10 year range and want to travel 100 years back, you'll just do it in 10 steps. Well, to close that door, the machine can have an autonomous power supply that is good for two travel and a half, one in, one back and some reserve for rainy days, and then you are stuck in time forever. Also allows you to sacrifice your time machine to do something really important.

You might also have an unsafe mode hidden and accessed by typing Ctrl+Shift+C and enter rosebud, but it's called unsafe for a reason. It might just transform you into a fly instead of doing what you ask. So again, you would not consider doing that unless you had really important and dramatic reasons for it.

Compiler bug. Programmer wrote the code to subtract the limit from the current time. The compiler “optimized” that by doing the calculation at compile time and putting the answer into the executable. — WGroleau, Jul 30 '18 at 21:19
Several people have studied the source code and concluded it is correct. A compiler bug didn’t occur to anyone, so they assumed some unidentified thing/event has created a barrier at that moment. — WGroleau, Jul 30 '18 at 21:29
you'll just do it in 10 steps Note you will need to determine accurately where you are after each step. Plus, you will need 10 safe points at those years to "land" safely (ie. appear from nowhere) plus wait during the 40h required for the machine to prepare for the next jump. — Ángel, Aug 03 '18 at 15:07

score 4 · Answer 21 · answered Aug 01 '18 at 20:08

Lots of great answers already, but one possibility not yet even implied:

Your time machine "finds" the correct spot on Earth and in time by triangulating multiple stars and solar system bodies. Unfortunately the astronomical reference library you downloaded includes a star which had a dramatic phase change whose light reached the Earth in 1941; earlier than that date your geotemporal library fails to resolve.

(In 17 years when time travel becomes widespread you will open-source your control software and a 14-year-old from Nigeria will find and fix the bug.)

Alternate bug: there was a leap second you left out of your time math routines.

score 3 · Answer 22 · answered Jul 30 '18 at 08:09

The time machine abuses the effects of a wormhole linking any point in time along the circumference of the wormhole to any point on the other end of the wormhole. But any point of time that isn't somewhere along this circumference cannot be reached via the machine.

This would work both ways. You have an earliest date you can reach and a latest date that you can reach. But this is less of a problem since you can go some place in time that's close and just wait for time to pass normally. You can't exactly wait for time to go backwards

score 3 · Answer 23 · answered Jul 30 '18 at 21:44

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You are not the first inventor of a time machine. In an ironic twist a future inventor created the first time machine and traveled back to August 12th, 1941 and became trapped. He brought with him future technology, or at least knowledge and took advantage of it to survive.

Your time machine is dependent on inventions and discoveries made possible by this future time traveler bringing back this future information.

answered Jul 30 '18 at 21:44

Michael J.

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Plausible, and consistent with the (presently) accepted answer. – Codes with Hammer Jul 31 '18 at 13:21

score 2 · Answer 24 · answered Jul 30 '18 at 09:28

There's a biological resonance effect. Your protagonist can only go back to periods where they as a person existed (ie. anytime after you were born, August 12th 1941)

Yes, this makes them about 77 by the time of the story, but if you can do time travel in any fashion, biological aging might be simple by comparison.

The Square-Cube Law · Answer 25 · 2018-08-01T16:55:26.010

It's due to the operating system of the machine.

When working with bits and bytes, dates and times are stored as an amount of milisseconds from a base date. For many programming languages, the base date is somewhere in the 20th century. You can have a negative amount of miliseconds to represent earlier dates, but due to integer size constraints, you can't go too far into the past (nor into the future as well).

In Javascript, dates use 64 bits and can go all the way back to 285,616 years before its base date (January 1, 1970). Perhaps you designed your time machine with a base date around like your present and a date type that is only 11 bits long - that will allow you to go back or forward in time 84 years from your base date.

Better yet... Your base date IS the fiethest you can go into the past. Due to a one character typo in the source code, your machine does not tale negative values. You can go anywhen in the future, but you can't go anywhen before the base date. Add to that you lost the source code and the compiled executable you have is too complex to reverde engineer.

This is hillarious! The time machine can't go back any further due to a hardware limitation or coding error equivalent to the Y2K bug.... — JBH, Aug 01 '18 at 15:07

score 2 · Answer 26 · answered Jul 30 '18 at 14:42

In the eternam words of Mr. Spock, "Nature abhors a vacuum"

And nothing creates a vacuum like moving something or someone away from when they're supposed to be. The further away you move in time, the stronger the "time vacuum" gets. The curve is exponential over time, and it hits its practical "infinite force against the vacuum" point at time X from the origin.

Further, you could say the more critical the need for the person to be at his or her appointed time, the stronger the vacuum created when they leave their appointed time. After all, time is trying to contine back when they were, but now it's being held back by the lack of one of its parts. And if your baby's being born and you were expected to be by your wife... oooohhh... nature really hates that kind of vacuum.

The value of this solution is that you could easily create some pseudo-math to support it.

score 1 · Answer 27 · edited Jul 30 '18 at 09:42

You could use this event:

The Royal Air Force conducted the heaviest daylight bombing raid against Germany since the war began. The Germans could not offer as much opposition as they once did because many of their planes had been diverted to the Eastern Front.

The royal air force had used a wormhole, that day in order to attack; the wormhole is still open to this day through space and time and it is the only way the time machine can travel. Beware! In order to be consistent, your time machine should stop working if the wormhole somehow stops working, or being cut.

score 1 · Answer 28 · answered Jul 30 '18 at 13:11

While the Royal Air Force Bombing Raid on Germany, they hit a Special Nazi Project on Researching of the possibilities of Time Machines.

Accidentally one Specific Particle was released that blocks every interference with Time before that Particle was release. Like a Barriere in the Time.

Another Possibility would be that on that event, a Particle was created (due to the bombing) that make it possible to travel in Time. So it isn't possible to travel before that Particle was create because of the Grandfather-Paradox (change the Time so that the Particle was never created so timetravel was never possible )

https://en.wikipedia.org/wiki/August_1941#August_12,1941(Tuesday)

score 1 · Answer 29 · answered Jul 30 '18 at 14:03

@Separatrix Basically summed it up but there, but one theory holds that the time machine doesn't travel through time like a car travels through space, but rather like an elevator travels through an ever increasing building. Imagine if you will that time is a perpetually growing tower... a new story is added at the rate of one second. There are no stairs or entrances to another level other than the one you are presently on.

If an elevator was created on August 13, 1941 at exactly midnight local time, than that elevator shaft will continue to receive an addition every second of an extra floor... But it cannot create floors retroactively.... So from our present time, we could take the elevator all the way back to the date that it exists and all the way forward to the date that it ceased to exist... but we could never go beyond those limits... there is no floor to go to. The time-elevator would also be spacially locked... it can only exist in one place while turned on and cannot be moved... thus if you built this in... lets say area 51 because we always build the really kooky machines there... than you could travel to the past, but it would be area 51 in the past (Think 4th dimensionally, Marty!).

Spatially locked? This neglects the orbital motion of the Earth the Sun, the Sun around the galactic centre etc. etc. Think all four dimensions including three dimensional spatial movement too. — a4android, Jul 31 '18 at 02:43

score 1 · Answer 30 · answered Jul 30 '18 at 15:49

The first atomic test wasn't in 1945, but clandestinely, on August 12, 1941. Your time machine uses the radiation 'tag' of the miniscule amount of Carbon-14 that was added to every living thing after that first test to effectively 'date' you for your return. Without it, it can't bring you home again to the appropriate causal 'branch' you came from.

score 1 · Answer 31 · answered Jul 31 '18 at 02:30

There are a lot of answers here; but I have one more. Heisenberg's Uncertainty Principle gets in the way after a certain point.

The uncertainty principle is that ΔρΔx ≥ ħ/2π ; that is (in plain English) our certainty in momentum, times our certainty in position, is (by natural law!) limited to a very small, but importantly non-zero, number.

Momentum is mass times velocity, which is distance over, importantly, time; and the more dramatic the velocity or momentum gets, the less certain we can be about what it is. (I'm glossing over a lot of statistical physics there, but assume that uncertainty in x or ρ is always a certain percentage of its value.)

This is, again, not because of design flaws; it's proven natural law, and one of the corner stones of quantum mechanics. That perfect certainty does not exist.

That said, beyond a certain point, the navigational mechanism of the time machine may not be able to calculate, or store, where it's going. Anything before 1941 may physically be out of reach of any time machine built in the modern day.

Of course, there are potentially ways around this--like going back in time, building another time machine in 1941, and repeating the process--but that does sound like a lot of work and massive investment in 1941-money.

RonJohn · Answer 32 · 2018-07-30T13:30:42.317

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Any further back in time would cause you to create a catastrophic time paradox. Thus, the Invisible Hand of General Relativity prevents you from going back any further.

edited Jul 30 '18 at 13:30

answered Jul 30 '18 at 03:59

RonJohn

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This is a bit short. Why would a long jump be worse than a short jump? You can set up a paradox with a very short jump too. – Stig Hemmer Jul 30 '18 at 10:59
@StigHemmer very true. But it just so happens that in this circumstance that the jump length is 77 years. – RonJohn Jul 30 '18 at 13:30

score 0 · Answer 33 · answered Jul 30 '18 at 07:32

There could be an issue('issue' is somewhat strong wording, previous answers are excellent) with giving the machine a limited 'range' as some have suggested. It 'implies' that as the present moves forward the machine range follows along.

So if the main plot takes place during a longer time span then at the begining of the story the time machine goes back to 1941. After that time jump which makes the story go forward a couple of years the time machine only goes back to 1943.

As an alternative to 'range' i will pitch an external event limiting the time-space-continuum. The main character either discovers the limit of his machine and searches the astrohistory vaults or is simply knowlegable in science history. The articles he finds tells of the observatories registering a supernova with several anamalous characteristics. This August 12th, 1941 an unusual supernova seem's to have ruptured the techno-chromatic field of the time flow.

Disclaimer: I don't know much about astronomy but wikipedia states 'The word supernova was coined by Walter Baade and Fritz Zwicky in 1931.' So that's in the clear.

score 0 · Answer 34 · answered Jul 30 '18 at 08:34

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I have read it in one book:

Imagine time is a spiral, and you can only jump sideways to close points of that spiral. So you can go 150 years forward and 150 years back (and maybe 300 with some more effort).

answered Jul 30 '18 at 08:34

alamar

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1941 is not 150 years ago – L.Dutch Jul 30 '18 at 10:09
@L.Dutch so change it to any number of years you like. Actually this idea was already shown in the Dark series, so perhaps forget it :) – alamar Jul 30 '18 at 11:33

score 0 · Answer 35 · answered Jul 30 '18 at 16:32

Your machine travels towards a beacon

Time travel works because a time travel beacon was created on August 12th, 1941; the time machine is able to travel towards the beacon through time, but is unable to actually pass it by.

(this idea is inspired by Harry Potter and the Temporal Beacon)

score 0 · Answer 36 · answered Jul 30 '18 at 17:25

Your protagonist and his time machine exist inside a closed loop of time (likely tangentially related to the fault with the home button). Via imprudent meddling or technical fault they have significantly altered the time line in such a way that they have effectively severed their connection to the rest of the universe and created their own, limited-duration pocket dimension. In this pocket of time, nothing before August 12th 1941 exists. The distance into the future may also be limited. A plot point could be that the protagonist eventually realises his mistake and that he's created a doomed universe that will pop like a soap bubble if his time machine ever runs out of power, or that is slowly collapsing, and trying to figure out the best way to fix the problem since he doesn't know for sure if the "original" universe actually still exists or not (and besides, he rather likes the one he lives in thank you!)

DoubleDouble · Answer 37 · 2018-07-30T21:52:06.077

Another Time Machine, or even this Time Machine further into its future or past, either crashed or landed at that date/time and is set in home mode, repeating the day forever.

Perhaps time machines need a locking mechanism of some kind similar to multi-threaded applications. The time machine which is furthest in history has a lock on all of time (before it arrived) while it is activated.

This is necessary because otherwise the arrival of another traveler before that point would very likely cause a de-synchronization of the two timelines (at which point we don't know what happens, possibly very bad things)

Since time naturally resolves all on it's own in a forward direction, they don't have to worry as much about having a lock on time after the arrival.

score 0 · Answer 38 · answered Jul 31 '18 at 01:24

The entire universe runs on Galactic Standard Time Units, each one equivalent to roughly seventy-seven of our “earth years”. Any and all time machines built are easily capable of traveling further back than August ‘41, but simply requires coding in for the second or subsequent GSTU coordinates values. Since your protagonist is unaware of this universal convention, it becomes a built in limitation of their particular machine, and any dates entered into it simply are not synced properly, reverting automatically to the ‘safest’ value of ‘1’ expressing itself as 8/12/41.. Attempting to go back further by using multiple will not work, since the mechanics of time travel in a living, aware universe will always keep a machine built and setup incorrectly inside of the ‘1’ (77-year) value. This is both a safety feature for the user and the universe at large, and also an integral part of ‘how time travel works’ for beginners 101...

score 0 · Answer 39 · answered Jul 31 '18 at 03:41

The location where your time travel machine is constructed is actually your traditional family home that you've been able to reacquire as a now empty building for your temporal experiments. Your parents/grandparents/great grandparents and ancient family before that lived in that location up until August 12, 1941. They were driven out by the Nazis or similar unknown force on that date. To go back in time to August 12, 1941 or before would cause a time paradox that would prevent your own existence.

score 0 · Answer 40 · answered Jul 31 '18 at 19:18

It's just the way the math works out.

The math behind time travel is based on a pseudo-octonion algebra and is a function of the strong and weak nuclear forces. The result is there are "islands of stability" as the 8-dimensional tensor collapses into the four we can perceive. One of those "islands" occurred on August 12, 1941, and another will occur in the not-too-distant future (cosmologically speaking).

Of course, it's possible to construct a time travelling machine that will go past that date, but each consecutive "island" hop requires factorially more energy. There are practical limits to how much energy you can transfer without catastrophic heat loss, not to mention your electricity bill will be an unpleasant surprise on your return.

score 0 · Answer 41 · answered Jul 31 '18 at 23:28

Your time machine isn't a time machine.

1) I call this approach the time road. This is similar to Separatrix's answer but he based in on the first time machine, I'm saying that the road is part of this machine. Something that was made that day actually starts the road on which the machine operates. Just like a train is completely incapable leaving it's tracks, the time machine is likewise constrained. For examples in fiction to give you ideas, Thrice Upon a Time by James Hogan (they only have communications, you can only communicate with the machine); Dragon's Egg by Robert Forward (this time a communications machine that actually extends through time, we only see an early prototype); Timemaster by Robert Forward (actual travel through wormholes where one end has been relativistically aged. You can't go back to before the first wormhole was aged--exactly Separatrix's answer)

2) If making something that day isn't a practical solution then there's the alternate issue that a given machine can't appear in the same time twice. It's sitting there on whatever it's road is, if two copies try to sit there you get a low grade approximation of a hydrogen bomb. (Normal atomic repulsion won't apply to something approaching through time, thus when the second machine emerges some of the nuclei will overlap.) Of course safeties prevent this. For some reason the time machine was sitting around for a long period of time and only departed at your cutoff time. Note that this answer means you can't revisit a spot in time unless you have another machine!

There's one big advantage to time road machines. I think it was Larry Niven who pointed out the big problem with time machines that can alter the past--people will alter the past. You will keep getting alterations until you end up with a universe in which the time machine is never invented--that's the only stable state. Thus if time travel is possible no time machine will be invented.

A time road, however, inherently precludes going back to a time before it's creation and thus can't be altered out of existed. Thus in practice it's the only type of time machine that can exist. (Unless you're dealing with a situation where you end up in a copy of the universe if you time travel.)

score 0 · Answer 42 · answered Aug 01 '18 at 15:16

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As in The End of Eternity: someone is blocking it. Somebody has a reason for preventing travel beyond that point, and the technology to do so.

answered Aug 01 '18 at 15:16

Mark Wood

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score 0 · Answer 43 · answered Aug 02 '18 at 00:14

Also, I noticed that my machine can only go back in time within a limited range. I can go anywhere in time between August 12th, 1941 and the present.

It's a software limitation.

The time machine software is programmed in 32-bit code. The largest number 32-bits can contain is 2,147,483,647, and if you subtract 2,147,483,647 seconds from the present calendar the furthest back you can go is August 12th, 1941.

This requires a specific present date.

We can calculate what the present date is by adding 2,147,483,647 seconds to August 12th, 1941.

JavaScript

const pastInSeconds = (new Date("August 12, 1941)).getTime() / 1000;
const present = pastInSeconds + 2147483647;
new Date(present * 1000);

The above will print out "Aug 30, 2009" as the present date.

That could be a reason, but isn't a plausible reason since the software developers could trivially fix that if the machine itself would otherwise be capable of going virtually anytime. — forest, Aug 03 '18 at 08:49

score 0 · Answer 44 · answered Aug 03 '18 at 15:47

Since nobody reads bulletin 1147, your time machine has been altered to prevent first-time travelers to go back and kill baby Hitler. In fact, it was future travelers who locked your time machine (well, they locked every time machine ever invented), as it was much cheaper than restoring the history every time.

(They could have locked it up even further, but time machines will be used -in a quite controversial campaign- to evacuate people from Operation Barbarossa attacks starting from August 1941, so they couldn't place it later)

Moreover, some of those kinks may be the consequence of it being a c. XXI machine patched by c. DI time travelers. But any attempt to further fix it is likely to make it more unestable. If at least you could read all those error messages that appear when you try to jump further back... Maybe there's even a confirmation prompt somewhere. Sadly, language at year 5025 is quite different than the one read and written by your main character.

You may not discover all of this until the day you meet a time-traveler from DI century, though.

score 0 · Answer 45 · answered Aug 04 '18 at 12:29

Your time machine depends (somehow - power, navigation etc) on a large nuclear explosion. The earliest nuclear weapon detonation was July 16, 1945

Option A: the explosion, in addition to having a blast radius in the usual 3 dimensions, also has a temporal blast "radius" of approx 4 years forward & backwards in time.

Option B: By accelerating towards the nuclear explosion, you can get just slightly past it (approx 4 years - the time you can get past depends on the size of the explosion).

score 0 · Answer 46 · answered Aug 05 '18 at 14:37

Time travel requires some kind of fuel. Let's call it "Eternium".

Eternium can only be created at a rate equal to the amount of travel it facilitates (e.g. it takes a year to generate enough fuel for one year's travel back in time). Because eternium can only be used to fuel the machine that generated it, you cannot combine two sources of eternium to double your range.

If you travel back one year with one year's worth of fuel. It will take you one year to generate enough fuel to return to your original timeline (but one year later).

So, think carefully about travelling back more than six months if you only have enough fuel for one year.

Plausible reason why my time machine can only go back a certain amount of time?

46 Answers46

You can't travel back in time to a point before the invention of the first time machine.

It's a result of the computer's programming.

Your machine travels towards a beacon

Linked

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