This might be more fit in physicsSE, but since I don't even know if my premise is physically sound, I'll post it here.

One day, something caused the universe to get mirrored, meaning a left-right swap, relative to any arbitrary view point. Let's ignore how this is done, how it's possible to move things over more distance than the observable universe in an instant etc., I only care about before and after.
The laws of physics are not affected by this, it's just a change in location and orientation for all particles, energy and so on.

Let's say you already suspected that this might happen one day (for whatever reason), so you're regularly doing some kind of test that should change its result after the universe got mirrored.

What kind of test could this be? What kind of experiment would give a different result in a mirrored universe?

My own ideas so far:

  • Of course you wouldn't just see everything being mirrored, since your eyes, brain and so on also got mirrored.
  • There are some molecules that only work the way they do in one orientation and not if mirrored, but in the mirrored universe, everything they react with would be mirrored as well, so there should be no difference.
  • There is the Lorentz force which can create a circular electric field (or was it magnetic?) from a straight motion, but it goes the opposite direction when the charge is opposite. This might be a good candidate. Is enough known about electrons and other charged particles to know whether they would get an opposite charge if they got mirrored? I know that there is always an anti-particle with opposite charge, maybe that is exactly this? I've also heard about theories that this means that it travels backwards in time, but I haven't looked much into that so far. Could this reverse time travel (if that's even what it is) cause some side effects if the universe is mirrored?

Related questions: I've seen some questions that ask about interactions between a normal and a mirrored universe, the answers to those always come down to doing something on the border. In my case, there is no such border, except in time. And you can't do experiments in an infinitesimally short timespan.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jan 8, 2019 at 16:51
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    $\begingroup$ Are past memories mirrored too? If not, then I would think that everyone will realize very quickly. What about digital pictures? $\endgroup$ Jan 8, 2019 at 21:58
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    $\begingroup$ This feels kind of like those "how do you know you see the same blue as everyone else?" questions. $\endgroup$ Jan 9, 2019 at 0:45
  • $\begingroup$ @GrumpyCrouton It's not just you that gets mirrored, it's everything in the universe. "Are past memories mirrored?" I guess, since your eyes, the connection of neurons and everything else is mirrored. There's nothing specials about humans in this case. $\endgroup$ Jan 9, 2019 at 7:02
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    $\begingroup$ @GrumpyCrouton If something was previously left, it is now right. But your previously left eye is now also right. So you see it the same as before. $\endgroup$ Jan 10, 2019 at 16:19

5 Answers 5


In physics mirror symmetry is called parity, so the question is whether any of the fundamental interactions depend on the parity.

It turns out that the electromagnetic and strong interactions do not depend on the parity, but the weak force does. In fact parity is maximally violated by the weak interaction - only left handed particles and right handed antiparticles interact via the weak force and their mirror images (right handed particles/left handed antiparticles) do not.

In everyday life this is not going to make any detectable difference, but the difference is easily measured and indeed this was first done over six decades ago by the Wu experiment.

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    $\begingroup$ but if "everything" gets mirrored... An answer needs to define what mirrored means exactly! $\endgroup$ Jan 8, 2019 at 14:27
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    $\begingroup$ @FabianRöling no, life for the inhabitants of the mirror universe would be the same as life for us. Only the mirror physicists would detect a difference, and even then only when they were specifically looking for it. $\endgroup$ Jan 8, 2019 at 15:17
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    $\begingroup$ @FabianRöling No. The strong force also holds neutrons and protons together. The weak force mediates various forms of nuclear decay e.g. beta decay. Basically anything involving neutrinos is mediated by the weak force. But note that mirroring does not change the strength of the weak force, so for example it wouldn't change beta decay rates. It just changes the quantum spin direction of the products. Bear in mind we didn't even notice the weak force was chiral until three decades after particle scattering experiments got going. It is a remarkably subtle effect. $\endgroup$ Jan 8, 2019 at 15:33
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    $\begingroup$ @Jean-BaptisteYunès: Chien-Shiung Wu was one of the greatest American experimental physicists. In 1956, she proved experimentally that left and right are not equivalent, and, as a consequence, it is "possible to distinguish between a mirrored version of the world and the mirror image of the current world". $\endgroup$
    – AlexP
    Jan 8, 2019 at 17:12
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    $\begingroup$ @AlexP Again, don't tell this "as is", you must define what mirrored means! There exists definition of this word that make both world as indistinguishable the way OP asked. WU's result has been made under assumptions. $\endgroup$ Jan 8, 2019 at 17:44

This answer is supplementary to Rennie's answer. I try to explain the long discussion in the comments to that answer.

The problem is defining what is meant by mirroring. In your question you end your description with "and so on". Depending on what is included here the situation changes.

For more discussion of this, you can look up "CP violation" on Wikipedia or other places.

The basic question is "What is the mirror image of an elementary particle like the electron?"

If you answer "just an electron", then Rennie gave the right answer: Wu discovered in 1956 that certain things are different in such a mirror.

If you answer "a positron (anti-electron)", then things look more similar in the mirror, but it turns out that there are still differences. This was discovered in 1964 by Cronin and Fitch.

If you answer "a positron going backwards in time" there are currently no known experiments that can tell the difference, but we don't really know. Maybe, just maybe, there might be an obscure difference somewhere.

I assume you didn't mean to include time reversal in your mirroring, and if so there there are experiments you can do. As the World-Builder, you have to decide which type of mirroring you want and thereby which experiment is needed.

  • $\begingroup$ Well, we don’t know only as long as we are willing to abandon QFT, no? (That is, not really, barring foundational issues and energies orders of magnitudes higher than currently accessible.) There’s the Weinberg argument for QFT being the unique low-energy relativistic theory of finite number of degrees of freedom, and just about every current formalization of QFT proves CPT. [cont.] $\endgroup$ Jan 9, 2019 at 12:59
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    $\begingroup$ The assumptions of the former are special relativity (which, granted, rules out cosmology) and that there is a low-energy regime where the high-energy DOFs don’t bleed through (true unless we missed something really badly—“our bodies not glowing”, in McGreevy’s words). The latter is, again, SO(3,1) invariance, plus you can complexify SO(3,1) to SO(4,C) without the theory falling apart, and then of course CPT complexifies to central symmetry, which in four dimensions lies in the identity component. [cont.] $\endgroup$ Jan 9, 2019 at 13:07
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    $\begingroup$ Finally, I’d advise against CP: it’s historically important (as in “Landau believed in it and was wrong” and “people assumed a real mixing matrix for no reason at all and were wrong”), but a red herring overall, I think. The worldline formalism makes it clear that the physically meaningful set is target space reversal P, target time reversal CT (sic!) and worldline time reversal C, see Siegel IA5 (painful but worth it). $\endgroup$ Jan 9, 2019 at 13:14
  • $\begingroup$ I wish I knew enough Physics to understand these three comments… $\endgroup$ Jan 9, 2019 at 19:04

There is no way of telling. If everything is mirrored, then the means to detect changes are also mirrored.

The left-handed particles are now right-handed. But your understanding of left and right have also switched. So even though there is a switch in the handedness of particles, the mirrored-left particle still corresponds with your mirrored-left hand.

You look in a text book to check. But instead of the particle looking left handed, it looks right handed, which confirms that everything is "normal".

  • $\begingroup$ Welcome to Worldbuilding, Sparhawk! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Gryphon
    Jan 9, 2019 at 1:40

I don't possibly see how there could be a scientific, empirical way of telling. If everything is swapped, so are the means to measure it. For all intents and purposes, you could have swapped the definition of right and left. And while the Wu Experiment seems to have proved that a mirror image would be distinguishable, if everything was reflected, would that not include the (previously) left-handedness of the neutrinos and the right-handedness of the anti neutrinos? Really it depends on the OP definition of "everything," and in the end it would really be determined by how this relates to the scenario the world-building is for.

  • $\begingroup$ Well, what determines the handedness of neutrinos? Is it some underlying particle/energy distribution/whatever that would also be flipped? Or is it a physical law? $\endgroup$ Jan 9, 2019 at 7:05
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    $\begingroup$ @Andy If you change all existing (original-)left neutrinos into (original-)right ones, then suddenly the cosmic neutrino background (CvB, which used to be original-left) is now original-right, but beta decay (measured in the Wu experiment) still produces original-left neutrinos and electrons by @Fabian’s ban on changing physical laws. (Also, edible fructose used to rotate light in the same direction that beta-decay electrons and neutrinos are spinning, but now it’s in the opposite one, &c.) $\endgroup$ Jan 9, 2019 at 16:39
  • $\begingroup$ The law in question is the Standard Model Lagrangian, because it doesn’t matter if people call the weakly interacting fermions “left” or “right” (and write their Lagrangian accordingly), they still presumably stay original-left. The “left” and “right” labels are not just one-kind-of-stuff and the-other-kind-of-stuff, there is indeed a handedness about them. $\endgroup$ Jan 9, 2019 at 16:42
  • $\begingroup$ @Alex Does that mean you could also just look at a bowl of fruit through a polarising filter to check whether the universe got mirrored? $\endgroup$ Jan 9, 2019 at 18:51
  • $\begingroup$ @Fabian Bowl of fruit doesn’t work (try it), you need to dissolve the sugar in water (also not straightforward, but doable with help from a physicist or chemist or a good YouTube video). And—depends on whether you flipped sugars (i.e. all the Earth’s biochemistry) and circular polarizers (probably both or neither: they both work due to non-mirror-symmetric atomic structures): yes (i.e. the flip is detectable with home equipment) if you didn’t flip them, no (i.e. the flip is either detectable only with a beta-decay experiment) if you didn’t. $\endgroup$ Jan 9, 2019 at 20:14

Well, I was left handed yesterday, but somehow that changed.

Pseudo vectors are an excellent way of describing what you want.

If all the coordinate axes of a system are inverted, a pseudo vector pics up a negative sign. From wikipedia "(A pseudo vector)is a quantity that transforms like a vector under a proper rotation, but in three dimensions gains an additional sign flip under an improper rotation such as a reflection."

Lorentz forces are a good example of pseudo vectors. This is as a result of the cross product involved in the equation. A cross product is a pseudo vector if both its inputs are of the same type (E.g. both vectors or both pesudo vectors) and a polar vector if they are different.

If electromagnetism is not something you'd like to use, consider angular momentum and torque. For example, the vector of precession torque of a spinning disk or top might cause a clockwise precession of a top which was given a counter-clockwise angular velocity (looking from above) if the coordinates of the entire universe were flipped. (If the universe hasn't been flipped, you would see counter-clockwise precession as expected)

See http://hyperphysics.phy-astr.gsu.edu/hbase/top.html for more.

Also, if said top never stops spinning, you're dreaming or in limbo. Wake up neo, you're in the matrix.

Perhaps someone else can verify this as well, but I believe any physical phenomenon described by at least one polar vector and at least one pseudo vector would be useful as an indicator of the flipping of the universe. (Otherwise, if they were all the same you'd see them ALL picking up the flipped sign and then it would cancel with the flipping of the universe)


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