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

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 1 '18 at 3:03
• 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 1 '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 2 '18 at 21:10
• @sharur: It’s static. It appears at 12:00 am, at the limit – DT Cooper Aug 2 '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. – a CVn Aug 3 '18 at 6:07

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.

• @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
• 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
• 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
• @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
• @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 1 '18 at 13:04

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.

• 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 0:21
• 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 9:39
• @JohnMontgomery that's going to bite you when writing a sequel in which you need to go back further in time. ;) – JJJ Aug 2 '18 at 6:26
• @JJJ You can then invent some kind of hyper-time which will allow to pass that blast :) – Alissa Aug 3 '18 at 12:14
• @BlindKungFuMaster What happens if they crash another one in the future? :O – Devsman Aug 3 '18 at 19:29

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.

• "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? – a CVn Jul 30 '18 at 5:59
• 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
• 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
• @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.. Jul 30 '18 at 18:34
• @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

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).

• 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
• 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
• @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
• 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*(0.1t*v)^2 where f is fuel consumed, t = time traveled (years), v = containment field volume – Doktor J Jul 31 '18 at 19:17
• 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

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 (en.wikipedia.org/wiki/Tunguska_event). +1 for the beacons idea. – theGarz Jul 30 '18 at 9:11
• 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 2:51
• 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 7:18

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.

• 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 1 '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 1 '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 1 '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 1 '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 1 '18 at 15:41

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.

• 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
• @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 1 '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 1 '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 1 '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 1 '18 at 11:32

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.

• 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 5:07
• @RonJohn That sounds like a reasonable conclusion. Let's see what tomorrow brings. – a4android Jul 30 '18 at 5:20
• @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 5:29
• 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 5:36
• @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 5:47

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)

• 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 2 '18 at 15:13
• @Carl I don't see the problem, further tests just further loosened the fabric of spacetime. – Emilio M Bumachar Aug 2 '18 at 16:06

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.)

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
• Skud and Patriot missiles and 1991 in Israel. – KalleMP Aug 3 '18 at 19:32

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).

• Brilliant! Nice lateral thinking. – Patrick McElhaney Jul 30 '18 at 16:03
• 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 4 '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 7 '18 at 13:22

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
• – ShadowRanger Aug 7 '18 at 6:27
• @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 7 '18 at 13:20

### 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.)

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.

• 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. – a CVn Jul 30 '18 at 5:57
• @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 6:01
• @MichaelKjörling less plausible than time machines? – jwenting Aug 1 '18 at 5:51
• @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. – a CVn Aug 1 '18 at 6:15
• @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 1 '18 at 10:51

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.

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.

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 3:02
• @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

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.

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.

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 3 '18 at 15:07

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

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.

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

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.

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.

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 1 '18 at 15:07

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.

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)

@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 2:43

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.

## protected by FrostfyreJul 31 '18 at 12:29

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