# 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 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.

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.

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).

• 1941 is not 150 years ago – L.Dutch - Reinstate Monica 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

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.

• 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

# 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)

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!)

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.

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...

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.

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.

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.)

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.

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 3 '18 at 8:49

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.

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).

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.