Gfilui

Similar to ltmauve's answer, what if the tears only allowed energy to pass through. Any physical/solid object traveling through the tear will disrupt it, causing the magician to have to re-stabilize it or allow it to dissipate. I think this could add some interesting combat techniques as magicians try to make the tears as small or hidden as possible so they can't be disrupted, while non-magicians will try to search for and hit the tear with something (rocks, arrows, swords, etc...).

You could even make the amount of disruption proportional to the size/number of items disrupting the tear, so a single small rock or arrow is trivial to re-stabilize, but a boulder or stream of arrows will make it nearly impossible to stabilize the tear before it dissipates.

Edit from comments:
This is also a good explanation of why non-magicians would be on a battlefield at all, since it would otherwise just turn into who has more/better magicians. Non-magicians are present on the battlefield to protect their magicians' tear(s) and to disrupt the tears of enemy magicians.

If you made the tears self-healing, requiring that the wizard maintain concentration on the tear to keep it open and draw energy from it, then anything that disturbed that concentration would end the casting.

Strategic wardrobe malfunctions as a defense mechanism!

You could make it so the tear can't be in the same place twice for however many days/weeks/months is needed. You could even make an economy of wizard guards that do nothing but open and close tears everywhere to mess up the area and make it impossible for any wizard to open another tear.

You could even turn this into a form of battle sabotage or traps by having tears opened and closed in an area to create a magic-less battlefield.

The first "actual magic" a magician is taught (for safety reasons) is how to close a rift that their mentor opens and contains. Learning to open a rift comes much later, when the student has demonstrated the ability to control magic to a suitable degree

Many Law Enforcement officers are taught how to close any of the 6 types of rift, so that they can subdue belligerent spellcasters - however, the conflicting forces/concepts this exposes them to prevents most of them from learning/performing more than minor cantrips with open tears.

Disrupt the tear

Anytime a wizard attempts to open a tear, poke it with a stick. This disrupts the tear before it forms, causing it to not work right. This stick may need to be made out of some certain material (cold iron does follow from older tales, is expensive enough that it's not just everywhere but not so rare that only the wealthy can have it)

I think it would be amusing and appropriate that "Poke it with a stick" is the response to someone trying to rip a hole in reality. It would be a metaphor for how people who don't know what they're doing often manage to disrupt or destroy carefully engineered technologies.

Direct line of magic flow from Tear to Wizard

Magic is a physical flow of energy going from wherever the tear was opened to the Wizard. Thus, they are constricted to be within a certain radius to the tear, as well as a line of sight.

Limits the power of wizards

• They cannot be very mobile and powerful at the same time




• Re-positioning requires closing the tear and opening a new one




• They are weaker in tighter spaces, not very effective around corners,
  etc.

Gives civilians options to combat

• Just disrupt the link: run through it with a wall, battering ram,
  person, whatever.




• If you have mobile units, you can stay out of range until the wizard
  tries to make a new, closer tear, then close quickly before she has
  
  her power back.




• You can safely confine a wizard by keeping them in a space too small
  to open a tear without it being on top of them, which is suicidally risky.

As an exercise in imagination, here is a stream-of-consciousness list of possibilities for "stopping magicians from using magic without resorting to the old 'special anti-magic metal' cliche"?

• 
  

  The ordinary people can use a new 'special anti-magic metal'. Maybe the new anti-magic metal has only recently been discovered, and it's expensive.

  
  
  

  • Or the manufacturing of the new anti-magic metal is a closely guarded commercial secret.

  
  

  • Or to make the new anti-magic metal a special ore is needed which can be found only in Ultrasylvania.

  
  


• 
  

  The ordinary people can use a reality tearing retardant, sold in pressurized spray cans so that they can create a cloud of tearing-retardant around them.

  
  
  
  • In palaces, rich town halls and upmarket shops a fine mist of tearing retardant is sprayed continuously (and recycled) so that in such places magic is generally ineffective.
  
  


• 
  

  The government can initiate diplomatic relations with one or more magic realms, prohibiting the importation of magic energy unless customs dues are payed and a license is obtained.

  
  
  
  • Or a merchant house strikes a monopoly agreement with one or more magic realms, allowing only some magicians to import energy.
  
  



• People can pay trainers to train them in the rudiments of magic so that they can undo any (or maybe only some) tear in reality.




• Maybe the tears in reality cannot be made close to one another in time and/or in space, and thus people can pay a magician to make a harmless tear and render the vicinity useless for magic for a certain time.




• Some people have a reality reinforcement field (sort of an inverse Steve Jobs) and such people can be hired to loiter around Very Important Persons like anti-magic bodyguards.



• 
  

  Or else people can resort to old-school tactics:

  
  
  

  • Drunk magicians cannot work. Have all public houses in town offer aggressive discounts to magicians.

  
  

  • Exhausted magicians cannot work. Have all the brothels in town offer aggressive discounts to magicians.

  
  

  • Magicians are people, and have weaknesses, or shameful events in their past. Blackmail them.

  
  



• If nothing else works, maybe the Noble Guild of Assassins has a special promotion for contracts on magicians.

Some idea come to mind

1. Normal people are resistant to magic, the reason normals can't open tears is because they resist the flow of magic, meaning mages have to work extra hard to make magic work on normal people. Mages can resist the flow but that also cuts them off from using their magic.




2. Ownership, You can't open a tear in a space owned by someone else, like a home for instance, or you can but it is much much harder.




3. Tears are visible as is opening one, this makes countering them easy IF you are prepared, like preventing someone from drawing a gun. Surrounded by armed guards with a standing order to put a bullet in you if you try to open a tear. This makes mages dangerous but not OP nor useless. Smart/prepared enemies can be counter you kinda like a gun. Works even better if being a mage leaves some kind ao visual mark on you, just walking around with such a mark is like walking around with a gun on your hip.

I like the idea of a carefully balanced staff in the local constabulary covered in stones physically repelled by the 6 different types of tears. An open rift within a certain distance will push the staff over onto a bell housing and both alert the guards to the presence of magic but also it's general direction. Smaller versions of the staff would be deployed on the anti-magic squad wagon. The constabulary may have magicians or special anti-magic items or even specific tactics for dealing with magicians (think flash bang or tear gas like devices made by magicians).

This would open up interesting plot points with town too small for these services being vulnerable as well as use of distraction tears to get the anti-magic squad on a wild goose chase.

Dogs can detect tears significantly before they reach a useable size. Most people don't cast magic too well when a large mastiff is chewing on an arm.

Ergo, training and keeping mastiffs becomes quite fashionable.

I'll assume for the sake of this answer that the tear closes once the magician has finished drawing energy from it, and I'll leave aside the case where they have to focus on the tear to keep it open as Henry Taylor suggested in his answer.

If instead, the tears close naturally on their own because of they violate the nature of the earth, then it stands to reason that exposing the tear to something fundamentally of the earth would accelerate the closing process. So any sort of natural solid (e.g., wood, rock, earth, unrefined metal, water) brought in contact with an inactive tear could snap it shut. If a magician is actively drawing from the tear, then things could become more difficult, as both the foreign energy and the tear would need to be disrupted. In this case, perhaps certain materials that can withstand the energy flow (e.g., rocks) would be strong enough.

Do the tears need to be in the vicinity of the magician? If yes, here are some thoughts:

What happens if objects enter the tear? The tear might block are from affecting it via some kind of surface tension but what if an object with more mass tries to collide/go through said tear? My suggestions:

1) The tear collapses into itself just vanishing or exploding etc. (what happens could even be realm specific)

2) The tear expands drawing more energy possibly overwhelming the magician when he tries to use it. It could also become unstable since they are expanded to fast once again collapsing... The magician could in return create a tear that is smaller than what he could possibly handle leaving handling room for unforeseen collisions.

3) The realm that the tear is aligned to could shift (maybe based on the material of the object) rendering the tear useless for the magician that summoned it.

4) The tear could convert some of the mass into energy (E=mc^2) causing unpredictable surges of heat (explosions), electricity, etc. (might once again be realm specific)

The intensity could vary based on the amount of mass involved. These could also be taken advantage of by other magicians by flinging other objects at the tear of their opponent.

PS: As an afterthought: Are magicians of the same/opposite realm able to expand each others tear to make them to big for the enemy to handle? Is the size/form of the tear limited or does it indicate something about the magician that summoned it? Is it usual for magicians to use already available tears or is the tear something personal to the magician (just as we don't let other people just use our phones for no apparent reason)? Are they in a fixed position after their creation?

There are several ways how magic users and non-users can be balanced. Unfortunately, these ways are not numerous and you will have to fall into one or another cliche.

1. 
   Magic-canceling material, like Kryptonite. Depending on the fantasy world, it can be extremely rare like Kryptonite, somewhat rare, like Steven Erikson's Otataral or very common, like iron in Robin Hobb's "Soldier Son" series. Also, magic-cancelling areas, like Robert Jordan's steddings fall into this category.


2. 
   Anti-magic device designed specifically to nullify other's ability to cast magic. It can be a restraining device, like Robert Jordan's a'dam or Terry Goodkind's Rada'Han, or a device that gives its user protection, like foxhead medallion.


3. Personal anti-magic ability, which may make a person immune to magic, like Terry Goodkind's "Pillars of Creation", or generate a large magic cancelling field like Glen Cook's White Rose or Brian McClellan's Magebreakers.


4. Sword and sorcery balance. If the magic is not very powerful, or difficult to cast when in combat, non-magic warriors (or law enforces, or just commoners) can effectively keep mages in check. for any real or perceived wrongdoings, mages would have to answer like everyone else.

For mundane folks, the best way to stop a mage is to stop the mage concentrating.

Set them on fire. Gas them. Electrocute them. Harpoon them (if the mage has to stay in proximity to the tear then a harpoon roped to a horse is a good way of dealing with them). Distract them with loud noise or bright light (e.g. flashbang grenades).

The best offence for the common folk would be flasks of nasty chemicals orf explosive grenades.

I think your constraint that

they take a few seconds to form after being 'opened'

can help with this.

You can apply two constraints to the tear-opening magicking:

1) Opening a tear takes time and concentration. This lengthy and exhausting setup is required before you can do anything of tangible effect.

2) Keeping a tear open for too long takes a bad toll on the caster. It's sort of like holding your breath. Doing so for too long can kill you. Even if you don't do it long enough to kill you, you can still do severe irreversible damage. But, unlike with holding your breath, humans aren't evolved to instinctively release the tear if they're in the "danger" zone (imagine if we lacked the instinct to gasp for air uncontrollably when we deprive our brain of too much oxygen; people could accidentally kill themselves by holding their breath for too long).

That way, when a mage fights, they have a delicate balance. If they are unprepared for a fight, and are just opening their tear on the spot, a warrior could just pummel them before they can do anything. If a mage is caught in a spontaneous confrontation, their best bet is to try and fight normally or run. But if they prepare for the fight too far in advance, they will be putting themselves in serious danger by holding open a tear for too long.

An adept combat mage has to develop a very strong instinct of timing, knowing when it's too early or too late to open a tear. Even an advanced mage who knows how to use a tear extremely adeptly, can be a novice when it comes to combat timing, and can be beaten by a drunken brawler, or a novice mage who has a better sense of timing. (I imagine magic "academics" who spend their lives studying it in an ivory tower would fall under this category.) But even as mages get better at timing, warriors will adapt and get better at thwarting that timing, by launching more surprise attacks, or being more alert of possible aggressive mages.

Magic involves all sorts of things, from material components to gestures. One of the things involved is vibrations. The tears give out a hum at a given frequency - the larger the rift, the longer the wave, hence the lower the pitch.

If you produce a sound that is a note at the same pitch, you mess up the rift. The louder and more pure your note is, the greater the effect, until you shut the rift close. Therefore, a pitch giving an E₂ can be closed by strumming the 6th string of a guitar.

Musicians will be able to shut rifts with lutes, flutes and other instruments. People who have a good ear and can sing will be able to do it with their voice.

So all in all, bard trumps wizard.

You say that wands and similar special items are not required to do magic, but in many other stories magic requires hand gestures, drawn out symbols, words, or mundane materials (something as simple as water or ash, for instance). If that will be the case for your world, then the answer is clear.

• If gestures are required, bind them.


• If symbols are required, bind them tightly and blindfold them.


• If words are required, bind them and gag them.


• If materials are required, bind them, strip them, and search them thoroughly.

This is an opportunity to really let your sadistic side get creative, because you can just think of all the cruel ways that real people have created to restrain others and realize that those people didn't have to worry about being turned into a frog if their prisoner managed to move their hand in a certain way or draw an arcane symbol in the dirt.

Alternatively, magic is usually portrayed in stories as something that requires focus and concentration. Perhaps in your world the people have discovered a combination of herbs that can reliably keep an individual docile, euphoric, dizzy, in pain, or otherwise distracted for hours at a time. Just keep forcing the drugs on them, and the captive magicians can't even see straight much less cast a spell.

Tears could require incantations to open. Stopping a magician from casting magic would be as simple as gagging them, or otherwise stop them from doing the incantation uninterrupted.

Self closure for entropic reasons

Transfering Power/Energy from a Realm to another increase it. Therefore It can not exist a loop between the Realms for entropic reasons : if so, Power/Energy would flow in circle and would increase to infinity.

A B C D E F -> Your realms

You can open as much tears as you want bewteen A and B.

As much other tears can be open in the same time between A and C.

But the exact moment a tear is opened bewteen B and C, all the tears that linked A and B and A and C will close ! That is a self-limitation inherent to the nature of your Realms and their Power/Energy.

Therefore, as your magiciens are not an organisation with very strict rules about tears opening, they constantly close the tears of others magiciens ! It is then very difficult for them to keep a stable access to Power/Energy as the probabilty for a loop to be created whenever a tear is randomly created by a "colleague" is very hight !

If they are organised enought to be a problem for a government or anything else,
it would be enough for a team of 6 renegats (one for each Realm) working for this government/organization to open tears continuously, canceling all other existing tears!

All the answers here are pretty neat but they also ignore some of the most practical ways of dealing with wayward wizards that requires barely any preparation: Violence.

Bear with me here. As mentioned in the comments, it takes some effort and time to open up one of these magical rifts. Unless the Mage is a remarkably composed and sneaky individual, the effort is going to show to an observant bystander. So once you notice a mage tearing a hole in the fabric of reality, you PUNCH THEM IN THE FACE. Clobber that robe-wearing, reality-warping fool until they stop doing whatever magical nonsense is turning the frogs purple. Millennia of experience have taught the human race that it's remarkably difficult to accomplish anything when you're being kicked in the ribs by an angry peasant, let alone something delicate. So apply that knowledge.

Now, you may ask yourself, that doesn't help me against a wizard with a tear in reality already open. Let me offer you a different piece of ancient wisdom: If violence doesn't solve your problem, you're not using enough violence. Shoot that thunderbolt-throwing, fire-balling fool with a crossbow. The power to incinerate an entire village is meaningless against a silent chunk of wood and metal flying towards the back of your head at a few hundred kilometers per hour. Stab them with a sword if you're close enough. Whatever that wizard is planning with the grand cosmic power at his beck and call is likely bad news, so make them stop. Shoot them from different directions, run them down with a horse, taser them until they stop forming consistent words or hold their head underwater. If the choice is between being the living guinea pig for Malkor the Blackened's latest spell and spilling a little blood, it's not a very hard one.

So get crackin.

Reality is stubborn.

Reality doesn't like to be altered. But also, reality needs witnesses to exist: if no one is experiencing reality then reality is nothing more than just meaningless causality (if a tree falls in a forest and nobody is around to hear it, does it make a noise?).

So, when magicians tears reality they should do it in an unnoticeable way, otherwise, if any bystander casually notices the tear on reality, reality itself will fix the tearing. This is like any other magic trick: once you know the trick, the magic vanishes.

Why magicians do not trigger the reality self-fixing?

That's a good question, but the answer is simple: they trained very hard to widen their reality awarenes in a way that doesn't interfere with it. You don't get it? That's because you're not a magician! (yet)

Countermeasures.

So, as a countermeasure against magicians, any 'common people' need to peek the suroundings looking for unusual stuff; for example: a magician with expertise on fire Realm may try to hide the tear he/she's feeding from as a cozy flame but our commoner suddenly says/thinks: "Hey, Isn't weird that this fire doesn't heat the suroundings? anyways, how did you create this campfire in the middle of this soaked alley?" and then, reality realizes the oddity and fixes itself.

This countermeasures may be tough for other magicians trying to close enemy magicians tearings, after all is quite hard to make reality aware of some tearings while keeping other ones unnoticeable, for this reason magicians try to overwhelm their enemy with power instead of blocking the oponent power source; that makes those fights quite interesting.

Counter-countermeasures

This kind of countermeasures make skeptical people more effective against magicians than gullible and clueless people, then: if somehow the magician persuades the commoner that "everything is fine", then this commoner wouldn't be able to alert reality hence closing the tear.

No disrupting metal but what about other disruptions.

You need a usable, stable disturbance in the fabric of reality to draw power and cast. To stop it being usable you have some; item, person, animal or device that:

1. Causes too much leak. Small holes rapidly become larges ones unless closed. Your choice on whether this; pops the wizards, summons things from the unseen universe, causes lightning, fire or spontaneous mutations. A well-practised wizard might use such a device in a controlled situation to get more power but having someone just kicking it off at random is going to be messy.




2. Causes too little leak. Some kind of repelling field that causes no energy to flow or even a reverse of energy. Suddenly wizards get drained, the area becomes cold, lightning goes off but the current flows the other way.




3. Causes a turbulence or disturbance in the fabric of reality. Those without magic ability are unaware of the issue or perhaps feel some small effect. Hairs on end, the smell of ozone, dogs or cats act weird. Those with magic ability are left in the mental equivalent of a turbulent ocean. Open tears suddenly close or expand then snap shut. Brief moments of power might be possible in a great effort but it becomes swimming against the tide.

Your choice of what creates these effects is limitless. As there are natural magic users there could be natural magic sumps and sources. People that although rare, cause great disturbances even if they are not able to use magic; whether they have some physical trait to signal things is up to you. The usual metals, crystals and plants are all there as options. Devices become fun if you want a technology vs. magic theme or they could be magical themselves. Up to you if the device is powered or passive. The great sacrificial alter fed by the blood of slain wizards is going to appeal to a different set than the pleasing arrangement of crystals that sometimes glows or gets hot when a mage is nearby.

In the real world, people "prevent" bad magic from harming them with charms, amulets, written prayers, gargoyles, etc.

Perhaps specially constructed or magically imbued items, such as articles of clothing, jewelry, tattoos or drawn symbols, could counter-act magic. This doesn't make the common person a magic user, like they would wield wands or staffs, but instead they do need to acquire protection beforehand from a magic-user, less they wander out unprotected.

Likewise buildings could be constructed with particular geometry, painted and decorated with certain symbols, or blessed and imbued with magic to protect them.