Time travel and energy

I was reading this neat summary of time travel rules in fiction and thinking about a couple of things basically angles and effort. The idea that a small change at one point leads to a big change in the future (aka two different kinds of things both known as a butterfly effect) predates modern science fiction. The proverb of consequences that typically starts with “for want of a nail” dates back to at least the 13th century and describes a causal chain of circumstances where a small issue (the nail in a horseshoe) leads to a major outcome.

Put another way: a small amount of effort in the past can lead to a result that would require a huge amount of effort if you were to attempt the same outcome in the present. I think that gives a neat rationale for fiction where you want time travel that allows changes to the future but where you don’t want an oops-I-stepped-on-a-butterfly-now-Donald-Trump-is-president situation.

If we see the initial act of time travel is the cause of the “future” change (i.e. the change to the present time of the time travellers before they set off) then we can (fictionally) say the amount of energy used has to match the amount of energy needed to change the original present to the changed present. So while a non-time traveller can make a relatively low-energy course correction to the timeline, a time traveller’s actions impact the future only as much as the same action would have changed their original present. So, for example, if a time traveller from 2020 stamps on a butterfly in the Cretaceous the net impact on 2020 can’t be any greater than stamping on a butterfly in 2020. The old ethical conundrum of going back and killing Hitler as a baby would then require you to (say) drop a comet on late 19th century Austria if you wanted to stop World War 2 etc.

Note that isn’t actual physics as it would require some kind of cosmic bookkeeping of energy but it might work as a tidy rationale for a time travel story in which there can be future consequences but not big future consequences.


16 responses to “Time travel and energy”

  1. Well that got me thinking.

    Two immediate thoughts.

    First, energy isn’t really expended so perhaps entropy is the issue? Changes in the timeline are paid for by the universe running down quicker.

    Second, isn’t time travel itself an apparent violation of conservation of energy? Travelling into the past represents a temporary gain of energy while travelling onto the future represents a temporary loss. That ought to mean something, but I’m not sure what.


    • //Second, isn’t time travel itself an apparent violation of conservation of energy?//

      Yes but it is temporary, the past borrowing energy from the future. If I go back in time to Tuesday then there’s two of me until we reach the point on Wednesday were one of me goes back in time to Tuesday. So by the end of Wednesday there’s been no net change.


  2. Arguably, the energy being expended would be involved in how much it takes to get back there in the first place, as opposed to how much must be expended to perform an action? Killing one person would be still killing one person.

    But if you make changes, perhaps you need to expend even more energy (above and beyond whatever baseline amount is required) to get back to ‘your’ timeline?

    Though I suppose then you get the possibility of traveling backwards (at whatever energy cost) and then living forwards… something like James P. Hogan’s The Proteus Operation, that might be, which got a little confusing with how many time lines were intersecting.

    Liked by 1 person

    • It would (in this fictional physics) still take the same amount of energy to actually, say, shoot a person but if you wanted it to stick and actually cause a change in the timeline, you’d need to do something more energetic.

      So, if you were just a regular guy you could shoot my grandfather and he’d die and I wouldn’t exist. But if I try and do that as a time traveller, shooting him or even bribing you to shoot him, wouldn’t be enough to actually impact the timeline. The gun shot wouldn’t be fatal or some such. It would still change the future (maybe my grandfather now has a scar and a fear of guns that he didn’t have before). To kill him and make a difference I’d have to expend the same effort I’d need to use in the present to kill all his descendants and erase all his impacts on history (including myself).


  3. There’s also the idea that a change to the past makes a change to the present which may result in a different change to the past, ’round and ’round until the system stabilizes–possibly by making a change big enough that the time machine never gets invented in the first place. Hence the invention of a time machine is never more than a temporary aberration.

    Liked by 2 people

    • There was a great short story like this, I believe it was published in 2020 but sadly I don’t remember where or by whom.

      Anyway, different people marginalized in the present break the rules of the time travel department to try to prevent some past injustice.

      And it works! But now there’s a new set of people at the time travel department, marginalized in different ways, who travel back in time to solve still more problems. They prevent the enslavement of Africans, for instance, but now it’s Native Americans being enslaved.

      It wasn’t entirely clear at the end whether they were slowly approaching a more just universe, or only trading out oppressions. But it was a great story.

      Liked by 1 person

  4. It’s really hard to equate energy and historical impact though – stamping on a butterfly probably requires no more energy than nudging young Adolf onto the tracks of an oncoming train. And trying to force physics to care about human history when any kind of formulate the latter in terms of the former would probably need to regard any two events as equivalent regardless of their moral implications.

    You might have more luck approaching it from the perspective of thermodynamics as Paul King suggests above. The universe should resist attempts to decrease its entropy, so anything you try to do to eg prevent WWII would ultimately be unsuccessful but you could successfully assassinate a leader whose reign left to a long era of peace and prosperity. Thermodynamics is very depressing.

    Liked by 1 person

    • Yeah but there can be local decreases in entropy (e.g. us!) at the cost of greater disorder ‘elsewhere’, so Cam is fine to go ahead (Maybe that explains the state of the current timeline)


  5. That could make sense. I’m reminded of the machine in the Oxford Time Travel series which refused to work if it could tell you would make a change in the timeline. Or it would land you somewhere not quite when or where you asked it to, to prevent changes to the timeline. Somehow the stories are just as exciting even though you know they won’t change history.

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  6. I am in the if time travel (a big if) than you can’t chance the past, because the time travel was always part of the history and it has allready happened from a certain point of view.
    I am not enterly confortable in what this says about free will.
    Of course for a story I am in the use whatever serves the story best camp.


  7. My take is that time travel is possible because of the level of quantum uncertainly within normal space-time; because events are ‘fuzzy’ at the fundamental level, there is the possibility for matter and energy to be displaced, breaching local conservation of mass-energy.
    However the level of uncertainly is itself variable, sometime unpredictably so.
    The “nothing” of the vacuum of space actually consists of subatomic space-time turbulence at extremely small distances measurable at the Planck scale, the length at which the structure of space-time is dominated by quantum gravity. At this scale, the Heisenberg uncertainty principle allows energy to briefly decay into particles and anti-particles, thereby producing “something” from “nothing.”
    And nothing is unstable. (just try tearing toilet paper)

    The ability of time to be modified is referred to as it’s plasticity and conversely the resistance to alteration is known as Temporal Inertia.. This varies across time and space; at some locations the flow of time may be wide and shallow, allowing for relatively easy minor alternations to “reality” but ones that have little futureward consequences. Other locations are narrower, deeper and faster flowing parts of the timestream where it’s difficult to alter events at all, but if you do manage it things tend to flood outwards and change reality greatly.
    Not knowing the difference can be bad for your health.

    Observer Effect
    The more you know, the less you can do. Observing an event reduces the ambient uncertainty and hence makes it more difficult to change the event. So if one time traveller sees her companion fall off a cliff she may not run over to see the impact because that make it more difficult for her future self to pop back and save their life. If she wanted the companion plummet down and small themselves on the rock below and doesn’t see (say) her differently dressed self grabbing him a few metres below the edge using a contragrav saucer and tractor beam then it becomes much more difficult to save them in this may in the future.
    The better an event is known or studied the more difficult it is to alter it. Sometimes ignorance is good.

    Limelight effect
    If the level of uncertainty is itself variable (sometimes unpredictably so) then there are potential problems with time travel; repeated attempts to travel to a certain location in space-time alters the level of useful local uncertainty and hence makes it more difficult for others to appear there. Thus well known historical events are difficult to access, due to quantum effects caused by large numbers of efforts to travel there.

    While time isn’t exactly alive, sentient or sapient it’s a sufficiently complex phenomena that, rather like the Gaia Hypothesis, it does have certain characteristics of a living system. Hence there are situations when “time” finds it easier to interfere , sometimes violently, with a time traveller’s actions in order to preserve the “normal flow” of time. Among seasoned temporal voyagers (those who’ve experiences and [more importantly] survived this phenomena with their personal reality reasonably intact) this is referred to this as “getting clockhammered” or (sigh) “clock-blocking”. It can be unpleasant, irritating or fatal.

    Some time travellers of a more mystical bent (or recovering mathematicians) attempt to avoid this unpleasantness by providing Time with an easier way to signal them than, say, causing them to be hit by a passing meteorite whose passage through the skull strangely resembles a .32 bullet wound. They have a set of dice or random number generator or comm app that they use. For example; “If all six of these dice show zero I’ll change my plans”. The theory is that altering the probability is “easier” for Time than other methods of deterring them from tampering with something they shouldn’t.
    This may or may not actually work; it certainly doesn’t work if the person doesn’t believe and intend to adhere to the belief sincerely.
    It’s an interesting quirk for a time traveller; rolls dice before important decisions.

    Artron energy.
    Temporal or Artron eneregy is analogous to electromagnetic or gravitational energy.
    Objects have a certain natural level of chronons, of certain energy levels, depending on their natural position in space-time and it’s ambient chronon level; elsewhen in the universe this is different from the ambient level and may be detectable.
    With the right equipment time travellers glow.

    In extreme cases objects, especially living beings with their great quantum potential (from the myriad of potential futures they can select from) can interact disastrously with objects from very different periods. Meeting yourself, as your Artron signatures will be very similar but different in intensity, is a particularly bad idea. This is known as the Limitation Effect.

    Within the quantum temporal framework there is the concept of the Jonbar Hinge. This is a point in space-time where the temporal potentials are reduced drastically; instead of a multitude of fairly similar possible outcomes from a certain action there are only a few, classically two, and they are very different. The probability wave is, if not collapsed, highly constrained. Altering history becomes both rather easy, and very dangerous.

    Something that has no creation, i.e. an object that is part of a causal loop or a stable, standing flicker or whose creation has been erased by alterations to the timestream. Always slightly odd, sometimes quite powerful and always beacons of Artron energy.
    Such an artefact may be a person of course (e.g. Gandalf Grey). It’s not actually common for time travellers to alter events to erase their own birth/creation or their access to time travel but it does happen.

    [Re-posted from the AITAS forum]

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  8. There’s this massive problem of scale and egotism, reminiscent of Douglas Adams’ fairy-cake extrapolator.

    Space is like, really big (also see Adams). There’s a lot of stuff in it. Anything that suggests the specialness of humans, or even Earth, or the Milky Way, is carrying a ridiculous load. Of course we like hearing stories about ourselves or people like us. But we had better not believe in them as a theory of how the universe works.

    So one of these things is likely to be true:

    – There is no method of backwards time travel.

    – Backwards time travel exists but somehow never changes anything, which makes it not so much travel as information gathering.

    – Backwards time travel exists and always changes everything, but we don’t notice because by definition we don’t have any experience in other time-lines. This opens up the notion of cross-time travel.


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