ulyssessword

The cannonballs' velocity is measured relative to your body, so they should be affected by everything that affects the body, including gravity. Yeah, probably, I thought of that, but ignored it in favour of clarity. This example would probably require a pewterarm to do as described, both to get the speed at high mass, as well as possibly avoiding dislocating your shoulders in step 3 (due to decreased material strength). In hindsight, the example is more of a proof of concept than a description of what would actually happen.

Momentum = velocity * mass Inertia = mass #3 should work, even though nothing changes that the entire system is pulled to the ground at a rate of 9.81 m/s^2, it should be able to get enough acceleration to overcome it. More numbers and calculations! I'll do a step by step breakdown of the entire process, (hopefully) showing that it could work. Notes: I pulled the numbers for the time it takes to bring the cannonballs out of thin air, but 1/3 of a second seems reasonable, if a bit strenuous. The velocities were derived based on an arm length of 1 m. Changing from tapping and filling ironminds happens instantly between the steps where applicable. The cannonballs' velocity is measured relative to your body, not the ground. Because of this, the gravity calculation for the body also applies to it. The positive vector is up. Gravity is 2% stronger (10.00 m/s^2 vs. 9.81 m/s^2) on Scadrial, because it is better for math. EBV (ending body velocity) formula is gravity added to an expansion of conservation of momentum, The momentum of the body and the momentum of the cannonballs at the start = The momentum of the body and the momentum of the cannonballs at the end. Momentum is mass multiplied by velocity, so expanding that sentence, you get: Body mass * starting body velocity + cannonball mass * starting cannonball velocity = Body mass * ending body velocity + cannonball mass * ending cannonball velocity to abbreviate: (BM * SBV) + (CM * SCV) = (BM * EBV) + (CM * ECV) => reorganize a bit... BM * EBV = (BM * SBV) + (CM * SCV) - (CM * ECV) => divide both sides by the body mass... EBV = ((BM * SBV) + (CM * SCV) - (CM * ECV)) / BM => simplify some... EBV = (CM / BM) * (SCV - ECV) + SBV => and voila an easy to use formula for momentum transfer. change in velocity due to gravity is simple, it is time * acceleration: (t * g) Acceleration due to gravity (g): -10.00 m/s^2 Cannonballs mass (CM): 20 kg Step 1: Above head to shoulder level Body mass (BM): 5 kg Starting Body Velocity (SBV): 0 m/s Starting Cannonball Velocity (SCV): 0 m/s Time elapsed (t): 0.33 sec Ending Cannonball Velocity (ECV): -6 m/s Ending Body Velocity: ??? Ending Body Velocity (EBV) = loss from gravity + change from cannonballs + starting velocity EBV = (t * g) + ((CM / BM) * (SCV - ECV)) + SBV EBV = (0.33s * -10.00 m/s^2) + ((20 kg / 5 kg) * (0 m/s - -6 m/s)) + 0 m/s EBV = (-3.3 m/s) + (4 * 6 m/s) + 0 m/s EBV = -3.3 m/s + 24 m/s + 0 m/s EBV = 21.7 m/s Step 2: Shoulder level to waist level Body mass (BM): 300 kg Starting Body Velocity (SBV): 21.7 m/s Starting Cannonball Velocity (SCV): -6 m/s Time elapsed (t): 0.33 sec Ending Cannonball Velocity (ECV): 0 m/s Ending Body Velocity: ??? Ending Body Velocity (EBV) = loss from gravity + change from cannonballs + starting velocity EBV = (t * g) + ((CM / BM) * (SCV - ECV)) + SBV EBV = (0.33s * -10.00 m/s^2) + ((20 kg / 300 kg) * (-6 m/s - 0 m/s)) + 21.7 m/s EBV = (-3.3 m/s) + (1/15 * -6 m/s) + 21.7 m/s EBV = -3.3 m/s - 0.4 m/s + 21.7 m/s EBV = 18 m/s Step 3: Waist level to shoulder level Body mass (BM): 300 kg Starting Body Velocity (SBV): 18 m/s Starting Cannonball Velocity (SCV): 0 m/s Time elapsed (t): 0.33 sec Ending Cannonball Velocity (ECV): 6 m/s Ending Body Velocity: ??? Ending Body Velocity (EBV) = loss from gravity + change from cannonballs + starting velocity EBV = (t * g) + ((CM / BM) * (SCV - ECV)) + SBV EBV = (0.33s * -10.00 m/s^2) + ((20 kg / 300 kg) * (0 m/s - 6 m/s)) + 18 m/s EBV = (-3.3 m/s) + (1/15 * -6 m/s) + 18 m/s EBV = -3.3 m/s - 0.4 m/s + 18 m/s EBV = 13.3 m/s Step 4: Shoulder level to above head Body mass (BM): 5 kg Starting Body Velocity (SBV): 0 m/s Starting Cannonball Velocity (SCV): 6 m/s Time elapsed (t): 0.33 sec Ending Cannonball Velocity (ECV): 0 m/s Ending Body Velocity: ??? Ending Body Velocity (EBV) = loss from gravity + change from cannonballs + starting velocity EBV = (t * g) + ((CM / BM) * (SCV - ECV)) + SBV EBV = (0.33s * -10.00 m/s^2) + ((20 kg / 5 kg) * (6 m/s - 0 m/s)) + 13.3 m/s EBV = (-3.3 m/s) + (4 * 6 m/s) + 13.3 m/s EBV = -3.3 m/s + 24 m/s + 13.3 m/s EBV = 34 m/s The cycle would then begin again, with each cycle of the four steps adding a total of 34 m/s to the feruchemist's upward velocity.

Yes, and they would drag him down quite a bit, but with skill and practice (and possibly allomantic pewter), it might be possible for a person to do. Yes, that would be a much easier way of doing it, but what you are describing isn't flying, it's just jumping really high and fast. The cannonball method can be controlled into hovering, turning, going up, down, and to the sides by using small adjustments. This could be useful if you needed to jump around a corner or two without touching the ground, and it breaks physics more (which is what I was trying to show).For a better (but less thematic) example for introducing momentum (#2), replace the traincar with a space shuttle or escape pod (lower mass, no friction), and it could theoretically reach relativistic speeds, eventually. This also works for flight (#3). I'll try to break cannonball-flight (#3) down more, as I didn't describe it very clearly the first time. In hindsight, it might require a pewter/iron twinborn to work effectively. Starting stats: feruchemist: normal 75 kg, cannonballs always 10 kg each. Start with the cannonballs held high, at low mass. 1. With mass still low, and bring the cannonballs downward as quickly as possible. This will pull you upward at 4x the speed of the balls (discounting gravity) because you are lighter than the two cannonballs. 2. Increase mass to 300 kg, and stop the cannonballs as they pass your shoulders and reach waist level. This will cause you to lose ~1/15 of the speed of the cannonball, as you are much more massive than the cannonballs. 3. With your mass still high, pull the cannonballs upward. Again, this will slow you ~1/15 of their speed because you are much more massive than the cannonballs. 4. Decrease mass to 5 kg again as the cannonballs reach your shoulder level, heading up. Slowly extend your arms as they pull you upward to ~4/5 their speed (they are more massive than you). 5. repeat from step 1. To hover, use slightly less effort, to turn, add a slight curve to their path, to travel, don't be parallel to the ground. I agree with darnill's observation about quickly changing between filling and tapping metalminds, it seems to be quite possible.

Nah, you wouldn't be able to splatter a RAFO Monster over a few kilometres, maybe two or three rooms, max. It wouldn't be much worse than being hit by a fast moving train. (Weak argument alert!) Thankfully, I don't think that anyone on Scadrial has figured this out, or else there would be a lot more people scattered over the landscape. By my reading of a couple situations, it has been shown to work, albeit used in a much less extreme way. I'm recycling quotes here, but pretty much every example of iron Feruchemy is in this thread already.From WoA ch. 53 pp 679 Sazed was running at the gate, and tapped iron just before hitting it with his body, otherwise his weight wouldn't "crash" into it. Pretty weak, but it's the best I have. HoA ch. 78 pp 678 I think that this is an example of your thought experiment (but less extreme, and only one half of it), and that he tapped the ironmind in the middle of the swing, but it could have been at the start too.I think that it is a consequence of conservation of velocity, but I don't think it will ever show up. There are some more things that would work with conservation of velocity, but would never show up in a book. If they did, I imagine it would go something like this: Free energy (aka perpetual motion): Wax is trapped by a koloss tribe, and they will only release him if he can make their windmill turn, so they can grind bones to make their bread (that's what koloss do, right?). Unfortunately, the only problem with it is that there is no wind on it, as it was built underground. The koloss confiscated his Guns, Germs, and Steel, stripping him of his colonial powers, and leaving him nothing but his wits, metalminds, and oddly enough, a physics textbook that Wax "traded" to him. Reading the book, he quickly sees a solution, and ties two or three hundred pounds of weight to one of the blades of the windmill (koloss build things well, very, very well). Climbing out to the end of the opposite blade, he taps his ironmind, increasing his weight tenfold and forcing it to drop to the ground and lift the counterweight. As he swings past the bottom, he starts filling his ironmind, allowing the counterweight to pull the other side down and bringing him to the top, where he begins tapping his ironmind again, swinging over the top and repeating the process. The koloss are grateful, and lock him in a train bound for civilization. Introducing momentum to a (otherwise) closed system: Wax is trapped inside of a locked train car, again with nothing but his wits, his metalminds, and the physics textbook. The traincar is not attached to any other cars, as the Vanishers unhitched it while stealing the rest of the train. Wax desperately needs to move, but knows that he would need to push off of something outside to move the car, but has no steel to burn. He sketches the problem on the walls, complete with Wayne's mistwraiths and stickmen, then comes up with a solution. He stands at the back of the car, fills his metalmind as much as he can, then heaves himself forward as fast as he can. While in midair, he taps as much weight as possible and lands, shoving the train along the nearly frictionless rails a small amount. He then repeats this dozens of times, and gets the train car going fast enough (about walking speed) to go along the tracks and follow the Vanishers' path. Unfortunately, they switched the track behind them, and it now leads to an abandoned factory. Flying: Wax is trapped in the basement of an abandoned cannonball factory (it was abandoned because of lack of demand, nobody had invented the cannon yet), still with nothing but his wits, his metalminds, and the physics textbook. The stairs are broken, but the hole from the falling train car is directly above him, from which he can reach the surface. Remembering how he moved the train before, he grabs two cannonballs, one in each hard. He then raises them above his head, fills his ironmind, and jumps while pulling the cannonballs down towards his waist. The cannonballs weigh nearly as much as his body, so he pulls himself up off of them. When they are level with his shoulders, he taps iron, and becomes more massive, stopping them at waist level. He then pulls the cannonballs up from his waist, not losing any speed because he is so much more massive than they are. When they are at his shoulder level again, he fills his ironmind, and lets the cannonballs tow him upward before repeating it again, and again, and again.

If physical strength is directly proportional to change in mass, then it should not make you any faster or slower no matter how much you tapped or filled an ironmind. This can be seen by applying Newton's second law of motion (force = mass * acceleration, or f = ma). Rearranging the formula, you can get (acceleration = force / mass), which is what we are looking for. Doubling (or reducing by 1/4, or increasing a hundredfold) would not change the acceleration. Calculations: For a standard person, at a point in time while jumping Force = f = 2000 Newtons (random number, I have no idea what a good one is) Mass = m = 100 kg (Wax put on some weight) Acceleration = a = ??? m/s^2 a = f/m a = 2000 N / 100 kg a = 20 m/s^2 For a feruchemist at 1/10 normal mass (with proportional physical strength), at the same point in time while jumping Force = f = 200 Newtons Mass = m = 10 kg Acceleration = a = ??? m/s^2 a = f/m a = 200 N / 10 kg a = 20 m/s^2 For a feruchemist at 5X normal mass (with proportional physical strength), at the same point in time while jumping. Force = f = 10000 Newtons Mass = m = 500 kg Acceleration = a = ??? m/s^2 a = f/m a = 10000 N / 500 kg a = 20 m/s^2 As you can see, it would not make you able to move faster, as every bit of strength that you gained would be countered by the resistance of moving it, and every bit of freedom you gained from lower mass would be countered by lost strength. I think it would work well with Allomancy though.

Hmm, I was almost thinking I had a pretty airtight case for each of the three questions, oh well, what fun would that be ? I can't think of any good way to reconcile this with my theory, but here goes anyways: 1) The 100 fold figure could be hyperbole from an unreliable narrator. His mass could be as high as 30-50x normal or so without it being impossible for a strong person to do. This would still be difficult, even considering that a large part of the weight is near the shoulder/core muscles, so it would take less effort to lift than dumbbells (as good of a comparison as we can get here on earth). 2) Him lifting his guns above his head happened at the same time as him tapping his ironmind, and it was narrated in that order because it would be disjointed to the reader if it was described in any other order. There are huge problems with these, most notably that they both rely on the assumption that what was written wasn't what happened. Some combination of the two is possible, even though it doesn't necessarily take any time to tap or fill a metalmind, he may have taken it. #2 is especially weak, as it would have said "He had raised his guns..." if it was the case. I'm still in support of material strength without physical strength, but not as sure anymore.

Now for motion, I am fairly sure that it is velocity that is conserved, because of the many exceptions that are required for momentum conservation to not seem completely broken. In addition to the two examples before (my second post, 5th in total), here's a new one, that illustrates the need for yet another exception. You are jogging at 10km/h, while at 1.5x normal density. At this point in time, your right foot is on the ground (going 0 km/h) and your left foot is reaching forward for the next step (going 20 km/h). Your arms are similar, with your left arm staying still relative to the ground, and your right arm travelling forward at 20 km/h. You suddenly fill your ironmind, reducing your density to 1/10 of normal. This increases the speed of every part of your body 15 fold (to maintain the same momentum). Your right foot (and left arm) is still going 0 km/h, but your left foot (and your right arm) are now travelling at 300 km/h. Similar things happen with your blood travelling faster etc... Because of this, you explode into a feruchemist mist, scattered over the next few kilometres of road. I see three possibilities regarding this situation: a) This is literally how it works, but despite never mentioning it to the reader, Sazed and Wax both take precautions to avoid this, such as slowly increasing/decreasing density, and stopping moving as much as possible. Not very likely at all. b)This is not how it works, because although momentum is conserved for your entire body compared to the outside world (a reference point that is convenient to you, like in the train example), there are special rules for what happens within your body, which prevents this. I don't like it, as it adds unnecessary complexity. c) Velocity is conserved, not momentum. Can be understood intuitively, even if it breaks (Earthly) physics more. Iron is also sometimes used in the middle of a swing or charge, which would be counterproductive if momentum is conserved, as it would slow down the motion. Also, Iron Feruchemy is never used as an alternative (or in addition to) to steel's speed for fast travel, as I'm pretty sure Sazed would have figured out/known the trick below (which works with momentum, but not velocity conservation.) Jog (5 m/s) at normal weight, and jump, similar to a longjump. Decrease your density to 1/10 as much immediately after jumping, increasing your velocity to 50 m/s, angled slightly upward. Maintain this for about a second as you are in the air, then go back to full density as you land. Jog 1 or 2 more steps to regain balance and recover the lost momentum from air resistance, then repeat. This allows travelling about 5x as fast for the same effort, which can be further boosted with steel and pewter Feruchemy or increasing your mass above normal on the ground, albeit at a slight loss due to increased drag.

I believe that the "increased strength" from tapping an ironmind (finally found an unambiguous way of saying it!) is material strength (how much force it takes to break something), as opposed to physical strength (how much force you can exert with your body). Musicspren, if I understand correctly, you are saying that the ability to move your body remains constant whether tapping or filling an ironmind (this would be due to physical strength increasing proportionally with mass). I don't think that that is the case, examples/analysis below: (AoL prologue, also in first post) As noted in my first post, this would not make you any faster to react (with your body, Allomancy is another matter) if it had a proportionate decrease in physical strength as well. WoA Ch. 58, pp 747-748 If iron gave increased physical strength as well, pewter would not have been necessary. Not a very good example, but it helps to show how iron would make pewter redundant if it gave physical strength as well.In Sazed's fight with the kandra guards (HoA ch. 78, pp 677), he had iron but no pewter. If iron let you move your body at the same speed as normal, punching and kicking would have been much more effective than falling, as your fists/feet travel faster than a drop of 5 feet gives. Iron feruchemy is also described in a couple of places (WoA Ch. 58, pp 747, AoL Ch. 6, WoA ch. 12, pp 124/125, among others.) as feeling light or heavy to the feruchemist. If you gained physical strength along with density/mass, it should not feel any different to you.

Nope, I think the philosophers Sazed is referring to are wrong there . Slower falling could be explained by reduced mass (or density, which is a better way of saying the same thing), for an example, think of dropping a balloon vs. a cannonball. Quotes: AoL Ch. 6: (emphasis added throughout) Direct contradiction with Sazed's quote, but this is an "it is" statement, instead of an "other people say that..." statement. Even with the unreliable narrator, I put more value in it. From WoA ch. 53 pp 679, also in my last post. Weight cannot "crash against" anything, only mass can.WoA Ch. 58, pp 747 Another direct contradiction, as gravitational force and density are not related at all. Also, Marsh pushed against Sazed's arms hard enough to be "addled" when he hit the wall. Only having more grip against the floor would not prevent Sazed from being thrown a near equal distance (or more, as Marsh is presumably bracing himself somewhat), added mass is required.WoA Ch. 58, pp 747-748 Same argument as above, but he is tapping Pewter as well, which counters some of it if he is bracing/leaning slightly.HoA ch. 78, pp 677 From Sazed's fight with the kandra guards. Another reference to "density" which, as noted above, is related to mass, not gravity.HoA ch. 78 pp 678 Unless he was swinging the hammer very oddly, the effect of added weight would only be to add the weight of his arms to the blow, added mass would allow a more efficient swing (with no need to brace against its momentum) in addition to both weight and momentum of his arms. I don't think it would be worthwhile for just weight.

Thanks for the feedback, I agree 100% on mass = density, it is true by definition. This also solves the strength issue for me, as increasing your mass would lead to increased "toughness"(resistance to force, including the force of gravity that would crush you), but not increased "strength" (the ability to move things, either your body or things outside of it). This fits with both of the examples, as well as working logically. The problem that I have with momentum being the basis of motion is that it would require the frame of reference to constantly change in order to not have a bunch of unintended side effects, a couple of examples: 1. You are standing on the equator of a planet that is spinning such that the velocity of the surface is 1,674.4 km/h East relative to an outside point (it's oddly like Earth in that way). You tap an Iron Metalmind, becoming 100x as massive for 5 seconds. During this time, you maintain your momentum, and are now travelling at 16.744 km/h East relative to a point that is not rotating. This sends you West (on the world) at a rate of 1657.656 km/h relative to the rest of the planet. If unimpeded (and no air resistance etc...), you will travel about 2.3 km, or roughly a mile and a half, to the west in those 5 seconds. This can be done in the reverse direction by filling the metalmind instead of tapping it. This is an absurd example, the reference point (if it is momentum) would need to be the surface of the planet to avoid things like this (or worse the farther away you put the point). The next example is much more likely to happen 2. You are standing inside of a train that is going 100 km/h North (it's faster than most, but it's a math train, so it's OK). You fill your metalmind, becoming 3/4 as massive. To maintain momentum, you are now going 133.3 km/h North as opposed to the train's 100 km/h. This sends you towards the front of the train at a speed of 33.3 km/h, or roughly sprinting speed. I also found some quotes, but none of them are very illuminating. From WoA ch. 53 pp 679 (paperback) Emphasis added throughout. This seems to suggest that he was using the increased mass for momentum in addition to grip. He then continued to use the increased mass for grip to finish pushing it closed and keep it there. I couldn't find any good quotes from Sazed's fight with Marsh or anywhere else, but there are a couple in AoL. From Ch. 1: Unless he started filling it at exactly the peak of his jump, he would shoot to the ground if momentum was conserved, so this suggests that it is velocity.Ch. 6: This one seems to support momentum, but it could just as easily have been attempted to be used for grip (which wouldn't work, as the coefficient of friction would be unchanged).Another from Ch. 6: I think that this is fairly strong evidence that momentum is not conserved. He pushed with a huge amount of force against the chandeliers, enough to break off an entire row of them, but did not go shooting backwards when he decreased his weight to normal. I don't think that his push against the harp would be enough to counteract that much momentum.In conclusion, I think that velocity is conserved, even though it could have more problems than momentum. NOTE: this was written before reading zas678's post. In response to it: Iron Feruchemy affects mass, not weight. Changing the gravitational constant would result in having more gravitational acceleration (so you would press to the floor harder, and fall faster, which is directly contradicted in the previous quote), but not more mass (aka inertia. It is resistance to acceleration, such as from steelpushing against another object.)

I haven't been able to pin down exactly what changing your mass by using Iron Feruchemy does, but I've gotten close. The three main questions I have are what happens to your mass/weight, strength, and motion. Mass/weight Iron Feruchemy affects mass, and does not affect the acceleration of gravity. The increase in mass would lead to you pressing harder on the floor, but would not make you fall faster (discounting air resistance). This is pretty much confirmed in a few places. Strength From AoL Ch. 6 This seems to suggest that increasing your mass increases your strength by a proportionate amount, but it would negate the logic behind lowering your mass to make yourself lighter on your feet, as below (AoL prologue): The only way I could see this working is if he was referring to being able to push off of lighter objects, as opposed to physical reactions.Motion I think that velocity (or speed) is conserved instead of momentum. This seems to fit best with what is written, with 100 fold increases or decreases in mass not resulting in instantly stopping or shooting off wildly in one direction; the problem I have with this is that it breaks conservation of momentum, and can change the motion of an otherwise closed system. This would require creating energy, which goes against the principles of Feruchemy. I think I pretty much have the mass/weight part down, but what are your thoughts on motion and strength from Iron Feruchemy?

I put a higher standard on observation than this. I don't think that it is good enough to not notice anything, I think it has to be a direct observation of the spren, and not seeing anything does not make a logical link in your mind. That being said, we really don't know anything about spren, so I could be way off here.

I worded my last post wrong, that isn't quite what I was trying to say. There are two key pieces to binding a spren: observation and documentation. A property must be observed before it is made into a binding record. Even the two parts are done by different people, there is still a cognitive link present. In the case of a spren that is currently invisible to an individual, I don't think that any observations could be made. I don't think that it is possible to make a mental connection between "I don't see anything" and "that specific spren is invisible". This holds true whether other people who can see the spren communicate with the (non)observer or not. The only way I could see this happening is if a character could know both that the spren is there, and that it is invisible, but I don't see how this would work without mindreading or a sixth sense. I agree that they are different, but I have no solid ideas on how they are different. I guess we'll just need to wait for a few more books.

I see two solutions to this: they are only visible to certain people (like Rock), or else they were visible randomly, but very infrequently. This is one of the weakest points in my theory. I don't think that you can record spren on someone else's behalf, and I am confidant that you wouldn't be able to have a binding record of something without observation, so I don't think that spren could ever have a binding record of being invisible. When else could their appearance be observed or documented? It would never be written that "I don't see a flamespren" or something to that effect, because that doesn't seem like it is the type of statement that could lock a spren. Also, I think that spren are somehow linked to change, (spren of the goblet changing it to blood, Syl constantly changing/evolving, other types appearing around changes that are happening, will occur soon, or have occurred recently (pain, rain, fire, wind, anticipation, glory, fear, death, rot), so I'm not sure that spren aren't all in a constant state of change.

I interpreted this differently than you did. Being able to see something is not a requirement for being aware of it, and even then, I think that someone who is bonded with a Spren would be able to see it regardless of its normal state. This knowledge of spren wouldn't need to be shared with the public, and there could easily be a taboo about describing the spren that you are bonded with as it is very personal. I don't believe that spren were visible to the general public, as there were no mentions of fearspren when Dalinar fought the midnight essence, or anticipationspren at feverstone keep, or anything else, while in the present day, they are mentioned the majority of the time in situations like that. Also, I seem to remember Dalinar remarking that there were no spren, but that could just be me confusing his visions with Rysn in Shinovar.

I was thinking about this some more, and this quote from Syl sparked a chain of thought. (emphasis added) What if "flamespren", "windspren", and the rest are not fundamentally any different from one another? There could be one type of Spren, whose appearance would change based on the conditions. This fits well with my visibility theory, as it just adds appearance to it. I don't think it is contradicted by anything in the books either.

I have found it strange that Spren are visible in some areas/times, but not others. Specifically, they are visible in the present day Vorin and Iriali areas, but not in Shinovar, the Purelake, or during the time of the Desolations. I believe that this is because nothing is/was recorded about Spren in those times/areas. Recording information about Spren constrains their behaviour, as shown in Geranid's interlude. Spren can be invisible, as shown by Syl usually being invisible to most people, but there are situations that they can be seen under (such as by someone with Rock's ability, or possibly other ways). They would never be recorded as "not seen," so any records of their presence would increase their visibility. During the cycle of Desolations, records and information would be very difficult to keep, as well as abstract research (such as Spren) being less of a priority in times of war, leading to a very long dark age. There would be no records in Shinovar either, as the Shin seem to avoid Spren through their resistance to Fabrial technology. The more modern areas have a system of research and record keeping in place, allowing information about Spren to be kept for a long time.This would lead to Spren being more constrained in their traits, such as visibility. Other, random pieces of evidence I haven't worked into coherent thoughts: -Axies the Collector said that intoxicationspren only appear in Iri, possibly because that is the only place they are recorded in? -The Protector Spren appears to have the time that it appears at recorded, locking it to a specific time, and possibly countering issue #2 (below), although it is an individual instead of a group, so may follow different rules. -Axies also has difficulty attracting common Spren. This could be because only invisible individuals come, or not. This theory also has several (big) issues: 1. The Spren may not be there. Instead of being invisible, they could be absent. This works in the Desolation era because Honour has not been shattered (IIRC, and also if they are Splinters of it), and from Shin because Cultivation rules there. EDIT: there were Spren during the time of the Desolations. 2. There is nothing to indicate that types of Spren are bound to follow inductive reasoning. For example, there is nothing to indicate that "that firespren is red, that other firespren is red, and that third firespren is also red" can lead to "all firespren are red," and each individual Spren of a given type being recorded the same way is unlikely. 3. No specific mention of their absence on areas outside of Shinovar. The visibility of Spren in developed areas is because of science and research, but if they are seen in less civilized areas as well, this theory is less likely. I've lurked around for a while, and read some of the theories (and will link the ones in my counterarguments later), but I may have missed some things.