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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.

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.

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.

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".

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.