# How do you prove that mirrors aren't parallel universes?

Migrated from WorldBuilding SE $:)$

"Each mirror is a world of it's own," Joe explains.
"Joe, that's bulls***! Mirrors are just mirrors!" Rita proclaims.

Joe proceeds to lay out the following~

$10~Laws~of~Mirror~Universes$

1. Law of Rotation: For each one orientation of a mirror (rotated spherically or in 3D space) there is exactly one universe perfectly symmetrical to our own.
2. Law of Quantity: There are infinite orientations possible in a sphere, so there are infinite universes parallel to our own. (Sidenote: Why not just have one mirror universe? Why do we need infinite parallel ones? Because moving an entire universe when you pick up a mirror seems like a lot more work then connecting to a bunch of other ones)
3. Law of Non-Uniqueness: Every universe began at the same time and progressed in the exact same way, with the exception of being "flipped" as we observe in a reflection. That means we are nothing special.
4. Law of Boundaries: The line of symmetry in each case is defined by the mirror itself.
5. Law of Congruent Copies: Changing the orientation of a mirror links to another exact replica of that mirror universe - they are all congruent to each other through transitivity
6. Law of Interaction: We cannot exchange matter into these universes because for every one particle that attempts to cross the threshhold, exactly one congruent particle meets it at the barrier.
7. Law of Weak Reflectional Attraction: A weak force along the mirror accounts for substances "stuck" to each other across the threshhold. This is why mirrors can be dirty: the boundary has a slight pull that makes dirt, etc stay on the plane of the mirror instead of falling as if there was simply air.
8. Law of Rigidity and Flatness: Boundaries such as funhouse mirrors or reflections in water serve only as distorted windows; they must be flat and rigid to allow actual interaction.
9. Law of Heat Transfer: The medium of the mirror (metal, etc) moderates heat transfer, which is why you don't feel immediate warmth when you touch a mirror with your hand.
10. Law of Medium Maintenance: That which would break the mirror surface due to pressure or momentum will do so before trying to pass through. Each new shard is its own boundary.

"Joe," says Rita, "Your 'theory' doesn't account for this."
Rita picks up a household object and demonstrates why mirrors are simple reflections of light, and nothing more.

Her demonstration produces results that would be different if there were multiple, congruent universes bound by mirrors (for example hitting the mirror with something that should have gone through if there were actually an alternative universe on the other side)

## What does Rita do?

• Must resist... must... the urge to migrate "back" to Worldbuilding :) Commented Nov 25, 2016 at 21:21
• You don't - we all know that mirrors are parallel universes. Commented Nov 25, 2016 at 23:32
• Surely this is simple. Just ask yourself in the mirror if you strive for world domination. If you and your mirror-self agree on the answer then it must be a mirror. Everyone knows that individuals across parallels always exist as an evil/good pair of twins Commented Nov 27, 2016 at 10:12
• Not reputed enough to actually answer but - law 6 indicates that the only reason why matter cannot pass is that it is blocked by the same amount of matter with the same force passing through the other side. Also we can see the other side to electromagnetism seems to come through. I would argue that gravitational force would come through as well, thus turning the mirror to the ground would quickly result in potential cataclysmic event with 2 very close parallel Earths starting to pull each other with their gravitational fields. Commented Nov 27, 2016 at 16:51
• @DarrenH Tell your mirror counterpart: "If you strive for world domination, raise your right hand, if not, raise your left hand." Commented Nov 28, 2016 at 3:46

With the bare minimal force required to not drop it, Rita

holds a refrigerator magnet — not the lame flexible ones that just stick to a fridge, the good ones that can actually hold papers and photos — up against the mirror.

If there were different universes,

this would be placing the pole of two magnets against each other.

I contend that these would be the same poles, which repel each other; if you've ever done this, you know this repulsion tends to push the magnets sideways relative to each other.
@Dr Xorile, in comments, suggested the symmetry of the alternate universe would mean the poles would in fact be opposites. This makes no real difference; that would cause the magnets to "snap" together from their magnetic attraction.

Either way, the magnets will exert force on each other and they will move accordingly.

It is trivial to show that, in fact, nothing happens.
So Rita proves Joe's conjecture is false.

Joe might argue that Law 7, or Rita's grip itself, still explains this.

Strong enough magnets should demonstrate the effect before actual surface contact, keeping Law 7 out of play. Hanging the magnet from some string would prevent any argument that Rita's hold on the magnet is preventing it from moving. I have kitchen magnets strong enough (those shown in a comment below, for example), and a good few of my fridge souvenir magnets also qualify, and I doubt my household is particularly unique in this.

• I agree that something would happen! I marked your answer up and think it's great! Commented Nov 23, 2016 at 23:25
• I don't know, I think I found a more common household item as not everyone has "the good ones" - see my answer Commented Nov 23, 2016 at 23:28
• @Jay But magnets aren't blocked by mirror glass. (Not even when backed by an equal thickness of laminate wood.) See?
– Rubio
Commented Nov 25, 2016 at 6:08
• @MattiaNocerino I agree, but selecting an answer discourages new askers so I'm gonna wait a bit Commented Nov 25, 2016 at 11:13
• A perfect mirror (gamma waves to DC) would be a superconductor, and the magnet would repel its reflection. as predicted by the 10 laws. Commented Nov 27, 2016 at 2:27

She uses

A Wifi Access Point

She uses 6 mirrors to create a cube, and places the AP inside the cube. Then she uses her smartphone or PC and she notices she receives signal.

This works because

Mirrors are not windows to other worlds, otherwise the waves would go to these worlds. Instead, the waves pass through the mirror.

• They may be coming from the parallel universe? Commented Nov 25, 2016 at 3:28
• @dark32 No, because the waves coming from parallel universes would be inside the cube, and then go to another universe. They couldn't go outside the cube in our universe. Commented Nov 25, 2016 at 14:12
• Should be a battery operated wifi access point then (like a smartphone with wifi hotspot turned on). Great answer! Commented Nov 25, 2016 at 14:33
• @miva2 Yes, I thought that but at the end I didn't say it. The point is fully enclosing some 3D space between mirrors. So the wifi AP would need some kind of power also stored inside the cube. An UPS would work, or devices with their own battery like smartphones or laptops. Commented Nov 25, 2016 at 14:41
• Joe says "Each mirror..." and gives laws for "Mirrors". Not certain, special mirrors; just, "mirrors" — that implies he means all (generic optical) mirrors, not just special kinds or certain types. Answers that rely on specialness of the mirror to work should be suspect. Having said that, this answer works really well (+1) - a basic optical mirror allows WiFi signals to go through it, but at about 50% strength. In a multi-universe scenario, the WiFi signals are trapped in the cube in all 6 universes; you get no signal outside. If they're just mirrors, you get a 50% strength signal. Voila!
– Rubio
Commented Nov 28, 2016 at 4:22

*This is not a serious, scientific answer

Rita

Takes her handy household vampire and pushes them into the mirror.

If Joe were right,

The handy vampire would pass through the mirror, because of a lack of reflections in the other universes, but alas, they cannot, and so they just hit their head..

• I'd be somewhat concerned if many people consider this a common household object. :-)
– Joe
Commented Nov 23, 2016 at 23:29
• Certainly a common household item. +1?
– Rubio
Commented Nov 23, 2016 at 23:30
• Law 10 now clearly precludes this. In either case now a head bonk is forthcoming. -1. 😄
– Rubio
Commented Nov 24, 2016 at 20:18
• If mirrors are portals to other universes, and vampires don't appear in mirrors, then the simplest explanation is that we can only see vampires from our own universe. Rita's counterpart actually is throwing a counterpart vampire into the mirror, but Rita and Joe can't see him, just as their counterparts can't see the vampire that Rita is throwing. (Does that make any less sense than the idea that we can see vampires but not vampire reflections?) Commented Nov 28, 2016 at 1:23
• @ruakh That is true although the existence of vampires alone justifies Rita's claims because it goes against the "We are not unique" law Commented Nov 28, 2016 at 2:03

They shoot...

...a particle beam through the mirror, preferably at an angle away from the normal. X-Ray, Gamma, neutrinos, neutrons... any one that normally passes through matter and is not reflected by the mirror surface will do.

If the mirror was a window, then...

...the beam would not come out the back of it; we would not be able to detect the beam there. And the beam would also appear to be reflected as the corresponding beam from the other side passes into our universe.

Now where do you get a...

...particle accelerator...

...and a corresponding...

...particle beam detector...

Simple. You grab a screwdriver and pick apart...

...your microwave oven. The magnetron will suffice just fine. Place a glass of water behind the mirror and the magnetron in front of it. And place your oven thermometer in the water. Once you see that the water heats up, you will know that the photons from the magnetron have passed through the mirror instead of disappearing into the other universe.

...and with that we know that the mirror is not a window into another world.

• Clever but not necessarily using a household object Commented Nov 24, 2016 at 15:25
• @Zxyrra It will take some tinkering but she can use the microwave oven's magnetron for that. Magnetron on one side, a glass of water on the other. If the water heats up then the photons have passed through the mirror. I would consider a microwave oven as a "household object" Commented Nov 24, 2016 at 15:32
• And here is how you create a DIY particle accelerator. Sure, it will take a trip to the store... but you can carry this stuff home and build it there. wired.com/2012/07/diy-particle-accelerator Commented Nov 24, 2016 at 15:36
• Still not really a (standalone) household object eventhough I like the idea! Commented Nov 24, 2016 at 16:11
• try microwaving a mirror some time, it breaks the silver coating. Commented Nov 27, 2016 at 2:35

Putting a mirror in front of another mirror.
If they are the same size and right in front of each other there will be an infinite loop that makes those "two universes" are mostly void.
Or you could put them in front of each other with a different angle, which would make the symmetrical law questionable. And also, you could see mirror A inside mirror B, which mean the exact same universe is in our universe and inside the B universe, which would make the uniqueness and parallel to our universe laws quite questionable too.

• Or just the fact that you see an infinite repeated series of mirrors. If a mirror only made a single parallel universe, you'd have two extra universes with two mirrors, not an infinity of them. They would not reflect infinitely because parallel universe mirrors do not reflect. Commented Nov 25, 2016 at 12:03
• But there's nothing special about your universe, it is just one of the infinite parallel universes. So if you in universe A hold a mirror to show this effect, you are basically seeing a parallel universe B with another you holding a mirror showing you parallel universe C, and he can see your mirror showing him parallel universe D. Commented Nov 27, 2016 at 7:28

A simple way for her to disprove this is to use

another mirror.

If two mirrors were held together at an angle of 120 degrees (or turned so at some point they pass through that relative angle), the two parallel universes these mirrors show connect to each other. Take three universes connected like this. The image below shows this with black lines as mirrors and the universes tinted for distinction.

Imagine putting any object off-center in the grey universe. It would have copies in both other universes.

The grey/green and grey/pink universes are mirrored, but the green/pink universes aren't. Any asymmetry in any of the universes is impossible if all three are mirror images of the other two. Any loop with an odd number of universes causes this contradiction.

Holding two mirrors at 120 degrees to each other, for even an instant, and showing that the laws of the universe permit it, disproves the theory.

• This is the best answer so far. Commented Nov 24, 2016 at 4:18
• Seriously? Someone just ripoff my answer then get all the upvotes and I get downvotes? Commented Nov 24, 2016 at 4:32
• I don't think this works. There are two sets of infinite universes of each chirality. The object would be reflected as expected anyway. This would not demonstrate the case. Commented Nov 24, 2016 at 5:35
• @stackreader - I apologize; you submitted yours as I was writing my response. Not any angle would work, though; 'mirror A' doesn't have to lead to the same universe every time. If you see a copy of mirror A in mirror B, that copy could just lead to a fourth universe if the angle isn't right. I like the idea of having two mirrors facing each other, tho Commented Nov 24, 2016 at 7:05
• Sure. But all the mirror will reflect. You won't have a situation like you describe where there is something that is not reflected. Commented Nov 24, 2016 at 14:49

# This answer was perfectly valid before the edit to the question which added rule #10, nullifying this method.

This mainly takes advantage of Rule 6:

We cannot exchange matter into these universes because for every one particle that attempts to cross the threshhold, exactly one congruent particle meets it at the barrier.

## Rita picks up (and uses)

A brick (or any other handy, rather heavy object)

Because

If Joe were right, another brick from the mirror universe should be thrown with velocity and trajectory identical (but mirrored) to the real world. The result should be two bricks colliding, and falling down with relatively no interesting effect.

The brick would impact the mirror, breaking it apart with a lovely smashing sound. (Instead of being repelled away by a mirror brick)

The bonus to this is that it doesn't have to be that household item, any household item would do, provided

that it is heavy enough, blunt enough, or otherwise suitable for throwing and/or destroying a pane of glass.

## Essentially

Rule number 6 should cause a congruent brick from the mirror universe to stop the original brick form impacting the mirror. This does not happen, as the mirror breaks. Rita proves Joe wrong, at the expense of the poor mirror.

• The brick pushes the glass along ahead of it, so it never actually passes through. You end up with a bunch of shards of mirror, each facing some largely random orientation as they spin and move through the air in their own separate trajectories from the brick. Since the brick never actually moves through the glass to reach the reflective surface, Joe can still explain why it moves past the original plane of the mirror without vanishing or rebounding.
– Rubio
Commented Nov 24, 2016 at 0:29
• Clever answer but @Rubio accounts for why this would not work. Actually gonna edit in another Law to summarize Rubio's explanation - something like "That which would break the mirror surface due to pressure or momentum will do so, and thus cannot pass through" Commented Nov 24, 2016 at 19:27
• Related: When the brick (or other solid object) pushes against its counterpart in the other universe, the force is transmitted to that other brick, not to the mirror itself. So if you push on a hand-held mirror, you won't feel that force against your hand. Hello, reactionless drive. Commented Nov 25, 2016 at 7:01
• But is it not the whole point of this question that there is in fact not a plane that could even break? i thought the question was meant to argue that there is in fact no mirror but a hole and the only reason that we can not - say, put a hand through, is because another hand pushes back with the same force. So what exactly would shatter? Commented Nov 25, 2016 at 11:31
• @Timme The glass on the mirror would break. This would obviously disprove Joe, because he thinks (as you just pointed out) that it is just a hole to a mirrored universe. Commented Nov 25, 2016 at 15:47

My original answer which I didn't get to post on WB because the question was put on hold: So it’s a serious answer, not lateral thinking.

QED describes how a mirror will work, along with all electric, magnetic, optical, radio, etc. phenomina. Other tests on the material of the mirror will show the conductivity, and reflecting light of different wavelengths will cause electric fields to penetrate to different depths, which can be illustrated by looking at different thicknesses of coating, evencent waves, etc.

The mirror is reflecting via QED, so what would the mirror universe be doing? You aready have the functionality of a mirror without it, and since you are seeing reflected light you are not seeing light from the mirror universe.

A cool trick would be (as decribed in Feynmann’s book to mask off parts of the mirror. He described scraping the aluminium off, but I say cover it instead so the mirror is “still there” in the portal theory. You carefully mask off strips and create a diffraction grating which reflects in a completely novel manner, not at all like a familiar mirror.

This is a clear demonstration that the image in the mirror

is caused by photons. Compare the results with a window getting the same treatment. The transmitting grating will act as a prism, which is different from reflecting images as odd angles.

• And the household object Rita picks up in your interesting but convoluted answer would be ... what exactly? :)
– Rubio
Commented Nov 26, 2016 at 0:31
• @Rubio Feynman’s little book! Commented Nov 26, 2016 at 7:08
• @JDługosz Or masking tape. Commented Nov 26, 2016 at 11:54
• yeah microscopically fine masking tape, or a really sharp Sharpie. Commented Nov 27, 2016 at 2:37
• To really do it, a candle to make soot to coat the (first surface) mirror, and a razor to make slits. That is used to illustrate quantum phenomena with light, now. But you would have a hard time making the “erase parts of a mirror” described in QED without precision equipment. Commented Nov 27, 2016 at 8:25

Rita holds up

a speaker

If the mirror is a window into another universe, then

it would sound like there are two speakers. Sound waves are not particles and do not bounce off of each other. Since they are playing identical sounds, they would amplify each other instead.

Since it's not actually another universe,

it sounds like a single speaker held up to a pane of glass. A wall or pane of glass does reflect some sound, but it also absorbs some of it, so the reflected sound is muffled. You can quite easily tell the difference between a single speaker held up against a wall and two separate speakers.

• The speaker on the other side would produce waves whose pressure was in phase and whose velocity was out of phase. A mirror isn't a perfect reflector of sound, but if it were, measurement aparatus inside an anechoic box with a a speaker and a reflective surface would pick up the same sound as if the relective surface were replaced with a mirror-image setup which produced waves with the proper phase relationship. Commented Nov 26, 2016 at 18:15
• a mirror made of glass or thick-ish metal is a pretty good reflector of sound. Commented Nov 27, 2016 at 2:41
• @supercat Actually the pressure waves in a universe-boundary scenario would arrive at the boundary in matching phase and amplitude, but of course from opposing sides; at the boundary itself, pressure from the "left" would be exactly met by pressure from the "right", imparting no net force on the boundary. The pressure waves would then continue propagating beyond the boundary unhindered by it, and you would hear the sound coming directly from the alternate universe speaker, identically as if though it were located in the local universe at the relevant distance behind the mirror.
– Rubio
Commented Nov 28, 2016 at 3:59
• I actually really like this answer (+1) because I think it works. Since two sound sources cause an interference pattern, at pretty much any point not directly at the mirror/boundary it should be possible to tell if you have two true identical sources (even if one's sound waves are only coming through the aperture of the mirror) vs. a single source and its reflection; among other notable details, sound waves vibrate glass (this is how laser microphones work) and that means those sound waves gave up energy, and are thus lesser powered.
– Rubio
Commented Nov 28, 2016 at 4:04
• You're not going to measure that effect with stuff on-hand in your house, but ­— the human ear is remarkably sensitive to changes in volume (1 dB is the JND, or Just Noticeable Difference, in volume for human hearing), and between acoustic muffling from reflected sound and the decrease in sound energy caused by vibrating a mirror, you should be able to tell when you're listening to reflected sound vs. direct sound.
– Rubio
Commented Nov 28, 2016 at 4:15

She holds a

mirror, by pinching the front and back faces between her fingers.

Since

the force of the finger on the front (reflective) surface is counteracted by the other universe (rule #6), there is nothing counteracting the force of the finger against the back of the mirror.

• Simple yet great answer! $(+1)$ Commented Jun 28, 2018 at 0:07

Rita does this:

She rubs an eraser across the mirror.

According to Law 6, "we cannot exchange matter into these universes because for every one particle that attempts to cross the threshhold, exactly one congruent particle meets it at the barrier."

Therefore,

It can be logically determined that there is no physical matter separating the two universes -- the only thing that stops a particle from going through is itself.

If Joe was right:

Mirrors would therefore be frictionless. On the other side of the mirror, your clone would be exerting the same force you are, so there would be no resistance as you moved something across. Also, the eraser would leave no residue because two erasers touching each other do not leave residue in the air.

However:

There is a clear sense of friction that occurs when Rita slides it across the surface, something which makes no sense if you try holding two identical erasers together outside the mirror world. Also, the eraser leaves a smudge / residue behind. This also makes no sense, because the eraser should logically leave nothing behind: the only thing touching it is an identical copy of itself, with nothing in between. Therefore, there is no surface present for the eraser to leave particles on.

Therefore, Rita proves Joe wrong.

• See Law 7 - this accounts for the attraction / material "stuck" to the mirror - and Law 10 - which accounts for the friction to some extent Commented Nov 24, 2016 at 22:45
• @Zxyrra Law 7: even if the attraction was really strong the friction on the mirror's surface would be equal to 0. It only attracts already existing objects (like dust) preventing them from falling down. Law 10: It would indicate some minimum friction if there was any surface between the worlds and there isn't (according to Joe). Commented Nov 25, 2016 at 14:20
• And there is a glass pane in front of the mirror.
– Pere
Commented Nov 25, 2016 at 21:46
• @Pere according to law 6, the only reason something cannot pass through the mirror is because it meets itself at the entrance. Law 7 says "substances 'stuck' to each other. This contradicts Law 10, as there would logically be nothing to break.
– user14061
Commented Nov 25, 2016 at 21:53
• Well, it depends on what we call a mirror in the sense of this question. Anyway, even if we define a mirror as the reflecting surface, you have a point if we use a reflecting surface not behind a glass - for example, a polished inox piece.
– Pere
Commented Nov 25, 2016 at 22:20

She picks up

a metal spoon

and then

she shows Joe the reflection on the back of the spoon, and explains that actually, in a parallel universe, an extremely obese Joe is looking at his own reflection on a spoon. Joe doesn't like this thought very much, and abandons mirror universe theories completely.

• See law 8 - explains this answer Commented Nov 26, 2016 at 2:50
• I like your style! Commented Nov 27, 2016 at 2:46

Forgive my answer, if it's wrong, however I haven't done any science in years, nor do I have the item i'd need to test if this were true.

She uses

A Magnifying glass.

This works because

If you hold a magnifying glass up, to a parallel universe, you would essentially be magnifying something twice. Thus the you on the other side would be even more magnified than you would be normally. (To get the normal you would probably have to take a picture of yourself, or have your good friend Joe do it for you.)

Preferably the picture would be taken from the same distance away as the mirror for the test to be fair. (A length of string from a point at Rita's Center (between her legs on the floor would work.) This would mean the images from the camera were equal in magnification to the mirror (Perhaps atop the mirror for the camera.)

If it was a parallel world:

You would be magnified twice on the other side, as you would essentially be holding up two magnifying glasses rather than just the one, doubling the concentration of the 'zoomed in bit' and be more than that of the camera, as the camera only views the one magnification.

If it weren't:

The magnification of the camera and the glass would be equal. (As equal as they could be).

• This doesn't work. Looking at yourself on a mirror with a magnifying glass is just like looking at somebody else without a mirror but with two magnifying glasses. In this aspect, a mirror is not different from a parallel universe.
– Pere
Commented Nov 25, 2016 at 21:44
• The light bounces off the mirror in (almost) exactly the same way as it would travel through a mirror-portal. So... this doesn't work. Commented Nov 26, 2016 at 11:59
• But if the mirror was a dimensional portal that you couldn't pass.... then the light from your end would be showing what was happening on the other side, thus, you would see what was there. If not the mirror would simply bounce light off and... be a mirror, or be nothing. Meaning light has to pass through and back Commented Nov 26, 2016 at 14:30

Rita can use just another identical mirror and put it on front of it with surface touching. At that point if Rita does not disappear that means laws of Joe does not holds anymore. However if Joe is right, Rita's universe would stop existing, so who would take the risk?

Lightweight version:

Use a 50 w laser and point it on any objecy on the other "universe" (of course need protective eyewear). If Jow is right, then light pass but heat not (in reality light is heath), so the object on our room should receive light but remains cold. If Jow is wrong the object would heat up (and also the mirror). The best item to test that is a ballon, because it immediatly pops if hitted by laser disprooving any heat regulation.

Physical version:

If touching a mirror should be countered by your same hand pushing in opposite direction then the mirror itself should not be subjet to anyforce. Instead if you hang the mirror on a wire and you push it it will oscillate, meaning that the mirror opposed resistence and not the hand in the other universe.

The nice thing is that the last 2 answers have a nice property: If you fix them by an additional law you get inconsistencies with other laws so no matter what, If you have an additional universe it has the following properties:

1. It is either a invisible universe
2. Or a Universe that you can communicate with (by passing through the mirror)

(Assuming most Joe's rules are correct, and you want to "force fix" to have a un-counterable axiom set, you find you cann't actually make a un-counterable axiom set, which is a good point because no one can lie about multiple dimension in mirrors).

If you fix the laser heath you get a perfectly adiabatic mirror,

.

If you fix the pushing force then you obtain a mirror that can't be moved anymore

Yay!

• Laws 1,2,5 argue against Rita disappearing. Mirrors are the intersections between universes, not the universes themselves, or those laws wouldn't make sense. Lightweight: Light is NOT heat. It is a stream of photons and might reflect (real mirror) or rebound (Law 6 effect), indistinguishably. See comments on Ray Wu's answer also. Physical version: +1 I think this argument is valid.
– Rubio
Commented Nov 29, 2016 at 6:38
• those are intersections, but some laws claims the intersection is not null so it is not clear what happens if mirrors are touching. Basically it happens a situation that is not covered by laws so a contradiction to me (universe disappear is a funny explaination used in immaginary physics when laws does not hold) Commented Nov 29, 2016 at 10:38
• Light is heath indeed. heath is transmitted as infrared waves, while heath by vibration should anyway be transmitted instantly if atoms are touching and opposing resistance. Otherwise atoms are not pushing any force and hence your finger could trapass the mirror (or atoms are repelling each other or not, there' cant be any heath regulation device inside the mirror). I'll edit the answer when I have time :) Commented Nov 29, 2016 at 10:41

She gets,

Anything at all, and compares its brightness in the reflection to its brightness alone.

This works because,

All physical solid mirrors are at least slightly absorptive (silver, a great conductor, only reflects around 95% of the light that hits it), and the theory doesn't involve a nether-world to send absorbed light off to.

It would fail if,

The mirror was made of plasma, which is perfectly reflective until it dissipates. (This is why good fictional laser weapons are pulsed!)

similar to another answer but should enable matter transfer between the worlds.

rita combs her hair to produce a static charge and puts her head to the mirror.

if there is a mirror world, the individual hairs repelling each other as they approach rather than touching, should be able to cross the barrier.

• Law 6 will stop matter transfer between worlds. That's more or less exactly what it says. Your underlying premise is reasonable though.
– Rubio
Commented Nov 25, 2016 at 0:38
• if the em field penetrates the worlds then it will move the 'congruent particles' out of the way. if it doesn't then its just a mirror
– Ewan
Commented Nov 25, 2016 at 0:50
• although to be fair you would expect the same behaviour mashing any springy pointy thing against its mirror world opposite. rule 6 isnt really what you would expect to happen if there was a mirror world
– Ewan
Commented Nov 25, 2016 at 0:52

Hold up a lit match to a mirror and touch the flame to it. You would expect the flames to get bigger as they interact together. Instead the flames snuff out.

• Smart although the law about heat accounts for this Commented Nov 24, 2016 at 22:42

How about a concave mirror, like some of the ones found in cosmetic or vanity kits? Unless one wanted to speculate that such a mirror provides views into an infinite multitude of universes that just so happen to line up perfectly, I don't see any way to explain what's going on on the other side of the mirror. If mirror laws are said to apply only to flat mirrors, that would raise the question of how flat mirrors would need to be to make the laws apply, and whether any real-world mirrors are actually that flat.

Rita can use

Literally any small object that will fall if you drop it.

Here's how to use it.

Hold the mirror above your head with it's reflective side downward and place the object against the mirror and let go. Gravitational waves do not penetrate the mirror. If they did, the object would not fall (or would fall slower) because it would experience a gravitational force of equal magnitude from our Earth and the mirror Earth and these forces would cancel out.

• Gravitational waves carry changes in gravity and propagate that change through space at the speed of light. Gravity per se is already present in space, it does not propagate through space; it is the warping of space-time caused by the presence of mass, and is a field present throughout space, not a point attraction radiating from the mass. Thus it does not propagate through the mirror-portal, any more than it propagates anywhere else. (Gravity waves may or not propagate through the mirror-portal, but without insanely sensitive equipment you'd never know which was the case.)
– Rubio
Commented Dec 29, 2016 at 8:07

Rita picks up

a magnetic compass, a battery, and a piece of wire. She coils the wire to make an electromagnet and puts the compass inside it.

If the mirror were a parallel universe,

the mirrored compass would have its direction inverted in addition to being mirrored.

But since it's just a mirror,

the mirrored compass does not have its direction inverted.

This works because

magnetic fields are pseudovectors.

She holds up

two strips of velcro, one hook and one loop

If the mirrors where parallel universes then

if you held the hook and loop strips side by side then the edges in the middle would catch in the parallel universes and the pieces would stick together.

But

Because it's just a mirror, when she lets go the pieces fall of the mirror instead of sliding down being stuck to the piece of Velcro in the mirror.

• I think this won't work because of rule 6. They will basically be too far apart to stick together. Commented Nov 24, 2016 at 15:15
• I agree with @miva2 - if congruent material meets it, it will fall. Commented Nov 24, 2016 at 15:27

If the mirror universe is defined purely by the orientation of the mirror rather than also its position. then simply moving the mirror towards or away from you disproves the theory.

ie. if you were looking into a parallel but flipped world you would expect the mirror to behave more like an empty picture frame than a mirror.

similarly if you move a mirror rapidly it pushes the air in front of it. If it was a doorway the air would move through it because the congruent air would be moving away from the surface.

If you try to get around this problem by saying that the mirror moves in the opposite way in the mirror world, then you no longer need to suppose a seperate universe per orientation of the mirror. The mirror becomes a camera in the mirror world and this same property explains 'flipping', mutiple mirrors etc. indeed this is how mirrors are modeled in computer games.

• On the contrary, it simply moves the door between two places of fixed location Commented Nov 24, 2016 at 22:40
• if you follow that logic then you only need a single mirror dimension
– Ewan
Commented Nov 24, 2016 at 22:44
• Not true. Moving position of the door allows the universes to remain in the same place; moving the orientation while keeping 1 dimension requires shifting the entire universe Commented Nov 24, 2016 at 22:47
• not at all. if the mirror is a window to a parallel universe from which you look out of in the opposite direction to the way you look into it in the 'real' world. Then you get the expected moving the mirror away from you also moves it away from your twin effect. but you can also change the orientation without problem and dont need to flip the mirror world
– Ewan
Commented Nov 24, 2016 at 22:53
• @Zxyrra Although it seems counter-intuitive at first, this is a valid way of looking at mirrors. If you own the video game "Portal 2" you could (using the Source SDK) make a mock-up of two mirrored rooms and tie two surfaces together, enable moving portals and place a portal on each. Moving and rotating both surfaces simultaneously will show a "mirror effect" when looking through the portals. Commented Nov 26, 2016 at 12:26

She

puts some water in a container. She proves that the water's surface mirrors the room,

and therefore it is a mirror.

Then she shows that

the container can be seen through the mirror but does not exist on the other side of the water. So something in the parallel universe exists which does not exist in her room.

Therefore the reflection is not a congruent parallel universe.

• Could you give an example? I don't think this is possible; generally mirrors reflect everything visible to them Commented Nov 24, 2016 at 20:01
• @Zxyrra I've edited to make it a little bit clearer; do you understand the answer now? Commented Nov 26, 2016 at 12:13
• @lolno I've made a (suggested) edit to your answer; if it changes your intention feel free to roll it back. Commented Nov 26, 2016 at 12:14
• See law 8 ~ the mechanics change in these circumstances Commented Nov 26, 2016 at 12:54

She picks up:

a battery powered mirror polishing machine and polishes another surface to create a new mirror.

This implies that:

an entire new universe has been created. Meaning that new universe must contain all the energy of an entire universe.

The only way this is possible:

Due to conservation of energy - is if the battery contained all the energy of the entire universe, which it did not. (Or do I have to prove it didn't?)

• I disagree - see laws 1 & 2. She merely accessed one of many that already exists Commented Nov 25, 2016 at 13:31

She

shines a flashlight at a mirror.

This works because

she then notices that the person on the other side of the mirror or her reflection turns on the flashlight a few nanoseconds later. If it was a parallel universe, that delay would not exist. This delay comes from the fact that light travels at 300,000km/s, not instantly.

• I don't think this works; the distance light travels to the mirror and back to your eye is the same as the distance from the virtual image to your eye, do the delay would be the same. Commented Mar 16, 2023 at 12:51