I'm quoting from the article about the neutrino's lower mass bound; I am not a particle physicist.

While I was intentionally making that pun, the uncertainty principle applies here only if you want to locate the photon more and more precisely in space. It's actually a fascinating phenomenon, since knowing the location means constraining its waveform, which increases its energy (smaller wavelength = higher frequency = higher energy). ^note More precisely, since you are using a photon to measure the photon, the photon used to make the observation has to be extremely high-energy to localize its position enough. Even more technically, you can't use a photon to measure a photon, since they don't interact, so you'd have to detect the photon interacting with another particle and measure that particle's position... yeah.

If you measure the photon so precisely that its wavelength is smaller than the Planck length, the local energy density becomes so high that it creates a black hole... we think. This is where we start to run into the limits of both QM and GR and need a better, unified model.

Edit: I'll wait for some more science before I seriously consider the woo about "photonic crystals", although if anyone has links to more info, I'd be happy to check them out.

Edited by Fighteer on Sep 20th 2019 at 9:11:22 AM

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

How does a laser system that emits beams at multiple wavelengths simultaneously work, compared to a single-wavelength laser? It surprises me that I couldn't find a clear answer within a few minutes of Googling.

Fiat iustitia, et pereat mundus.

Laser only emits light on a single wavelength. Like, that's its whole thing. You can only get that level of focus and intensity when beam is fully coherent.

You can kind of emit multiple wavelengths from a single device if you use a "gain" medium that can do that - like, I don't know, maybe metal nanoparticles of several different sizes dissolved in potassium bromide - then filter the output through a beam splitter and project it through multiple lenses. But I'm guessing that the energy output of such a device would be extremely weak compared to a regular laser with an equivalent energy source. You might as well tape two different devices together and save yourself the trouble.

Echoing hymn of my fellow passerine | Art blog (under construction)

Sound waves may fall up in gravity instead of down.

Huh, I never knew sound was affected by gravity. Then again, it only makes sense. After all, if waves like light are affected by gravity, why would sound be immune?

I'm a (socialist) professional writer serializing a WWII alternate history webnovel.

I was going to say that's absurd, but then I read the article and it's interesting. At a fundamental level, all energy has a mass equivalence, even potential energy; this is why a compressed spring is heavier than a slack one, although by so little that it's extraordinarily difficult to measure. Sound waves represent mechanical energy, so of course they should have some gravitational effect.

The idea that their effect is negative however, seems wrong on its face, like something I'd mark down on a student's paper without a second glance if I were a teacher. The unique medium in which researchers were able to observe this also makes me wonder if it's not an artifact of the way materials behave at near absolute zero rather than something inherent to sound itself. Still, it'll be really cool if they manage to demonstrate it under more normal conditions.

Negative mass-energy is one of those Jabberwockies of physics that would allow us to do insane things, like create perpetual motion machines or travel faster than light, which is why it attracts ridicule when someone claims to have found it.

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

I have a couple of hypothetical physics problem that requires an answer.

Problem #1

A truck-sized robot is designed such that it mimics the anatomy of a spider, with the main body split into two subdivisions, the anterior of which bears the robot's legs; furthermore, the posterior body division is deliberately designed to be twice as heavy as the anterior one. This Spiderbot is ordered to maneuver itself such that the anterior body portion is above a suitably sized weighing scale, lower itself onto said scale, lift all of its legs such that their feet no longer contact the ground (and thus do not bear any of its weight) while keeping the posterior body portion both lifted in the air (thus not in contact with the scale or the ground) and roughly parallel with the Spiderbot's anteroposterior axis.

The total weight of the Spiderbot is 3 metric tons. What would be a roughly accurate estimate of how much of that weight would be recorded by the scale?

Problem #2

The same Spiderbot from before is ordered to stand with its forebody portion above a hyperdurable force-measuring device. Another robot is rolled out, this one with a singular, undivided body of the same total mass on an identical set of legs to the Spiderbot's; this one is ordered to stand with its body above another force-measuring device. Both robots are then ordered to slam themselves into the device with as much power as they can; the Spiderbot would be slamming with their forebody alone, just to be clear.

Would the force-measuring devices record different values from each other? If so, which one would be higher?

Edited by MarqFJA on Jul 10th 2020 at 1:14:12 PM

Fiat iustitia, et pereat mundus.

1. That's a simple one: 3 metric tons. If the entire body of the spider-bot is off the ground except the part contacting the scale, then the entire weight will rest on the scale. It doesn't matter how the bot is proportioned or contorted.

Of course, very simple lever mechanics means such an arrangement could not be stable, and the bot would tip off the scale. Indeed, it should be impossible for the anterior legs to keep it from falling over backwards if the posterior weighs twice as much. I'm trying to think of some way to avoid this and coming up blank.

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2. Assuming that in the case of the first bot its anterior portion contacts the measurement device before any other portion of the bot, then the device should record the exact same force as the second. It falls the same distance (and thus is at the same velocity) and weighs the same amount.

Now, in reality, the bot with the uneven weight distribution would experience lever force that would cause it to begin to pivot the moment any part of the anterior touches something, but the force recorded on initial contact should be the same.

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In neither of these cases, as you have presented them, does the weight distribution of the robot make any difference.

Edited by Fighteer on Jul 10th 2020 at 6:21:50 AM

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

That's very interesting. From my admittedly amateur understanding of the physics of motion forces, I was under the impression that dividing the mass between two separate divisions connected by a flexible articulation would have a tangible impact on how the weight contributes to the force exerted by the legs if the latter are all located on only one body division (specifically the one that's significantly less massive). Then again, any sensible design of such robots would make the articulation capable of limiting its range of movement, if only to avoid wild swings of the rear body portion that could easily wreck unwanted collateral damage on anything in its way.

Uh, just to be sure there are no misunderstandings (your repeated use of "anterior legs" is confusing me), the robots have more than one pair of legs.

Fiat iustitia, et pereat mundus.

The spiderbot should be fine as long as its back legs extend far enough (ideally beyond the abdomen.) There's going to be a lot of unnecessary stress on its joints, though.

Edit : the mass distribution will determine how much weight is supported by each legs (in this case the back legs will support more) but in your example the entire mass rests on the scale, so that's what is measured.

Though if you account for the fact that the whole thing will be in the process of tipping over backward, it might actually measure less.

Edited by Aetol on Jul 10th 2020 at 1:53:14 PM

Worldbuilding is fun, writing is a chore

From your scenario, I assumed that the spider-bot would hold its heavier posterior in such a position that it would not contact the ground, thus placing its entire weight on the scale. Of course, it would immediately begin to pivot and fall off the scale because its center of mass would not be supported. If the posterior section is left to swing freely along its connecting joint, then it would fall to the ground and pivot at the same time. Calculating what portion of its weight would remain on the scale after it fell over would require precise knowledge of the bot's shape and position.

In your second scenario, both robots would be falling at the same velocity and would contact the force sensor with the same momentum. What happens next depends on where the sensor is relative to the center of mass, but if it is not precisely on that center, the robot would begin to pivot. This would change the force in ways that are difficult to calculate, especially if one body is rigid while the other is not, but you specified the initial state, not the state after lever forces came into play.

Edited by Fighteer on Jul 10th 2020 at 8:06:14 AM

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

I assume that the tipping/pivoting happens as a byproduct of the Square-Cube Law's effect on scaling up spider anatomy to such large proportions, and could only be counteracted by having disproportionately large hind legs?

Fiat iustitia, et pereat mundus.

You took the legs out of the equation by having it pull them up all at the same time. The tipping happens because the back is heavier than the front. The center of mass is behind the legs.

Edited by Fighteer on Jul 10th 2020 at 10:07:03 AM

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

Oh, right. I forgot about that. I only specified that the legs was raised because it was the simplest way I could think of to unambiguously say that the legs don't bear any appreciable fraction of the weight, and so we can get the most accurate reading on the scale. Maybe I shouldn't just said that the legs are still on the ground but not actively pushing against it anymore than they absolutely need to for postural stability.

So... in a normal situation, the spiderbot wouldn't suffer any tipping/pivoting due to the heaviness of its rear in comparison to its front?

Edited by MarqFJA on Jul 10th 2020 at 5:59:38 PM

Fiat iustitia, et pereat mundus.

It would certainly want to tip. Having the legs attached to the lighter forward section makes for a very difficult engineering challenge, since they'd have to be constantly fighting that torque. There would be significantly more strain on the posterior legs.

Regardless, my original point stands. At the very moment the body of the bot contacts the scale (or the force sensor), its full weight/momentum will bear on that sensor. However, that will last only for the first instant of contact; afterwards the body will begin to pivot, the joint between body sections will pivot, or both, changing the force distribution in ways that would require some advanced calculations to figure out.

Edited by Fighteer on Jul 11th 2020 at 6:33:17 AM

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

If the legs are still there to keep it from tipping over, then most of the weight will still be supported by the hind legs - unlike the front legs, it's impossible to let the weight off them - and the scale will only register part of it.

Worldbuilding is fun, writing is a chore

Heh, I gave that exact answer to Marq in another thread, but apparently he didn't believe me...

"We learn from history that we do not learn from history."

It's less "didn't believe you" and more I realized that this is far more of a physics question than a biology one.

... Wait, are you talking about this? Because I only saw it just now.

Would that part of the weight form a majority or a minority of the total?

It just hit me why the whole pivoting/tipping thing has been bothering me: why would that happen if the rear portion is being constantly subjected to a lifting force through the articulation (via motor/hydraulics/whatever in the spiderbot, and muscular action in biological versions like a Giant Spider)? My understanding of Newton's third law says that whatever generates the lifting force to keep the rear body in its elevated position would cause an equal downward force on the other end (i.e. within the front body).

Edited by MarqFJA on Jul 12th 2020 at 10:59:13 PM

Fiat iustitia, et pereat mundus.

Depends on the mass distribution and geometry of the whole thing.

The problem is that these forces aren't aligned. If you look at the body as a whole (and assuming there are no legs), the normal force opposing gravity is applied at the point of contact with the scale, under the thorax, while the weight is applied at the center of gravity, closer to the abdomen (since it's heavier). Opposed but misaligned forces are called a couple, and they cause rotations.

It's just like trying to balance a book on the edge of a table with two-thirds hanging out: it's going to fall, not because the table can't support its weight, but because the supportive force is off-center.

Now the interesting thing (which didn't occur to me at first) is that since the body will immediately start tipping over, its center of gravity will be accelerating downward. It won't be a full 9.81 m/s², but it won't be zero either. That means the normal force applied by the scale (which is really what the scale measures) will be less than the full weight of the body. How much less depends, again, on the mass distribution and geometry of the whole thing. Though since all this will be happening very fast, possibly faster than the scale's own inertia and dampening, you might not get any useful reading.

Worldbuilding is fun, writing is a chore

Yes, what you get is a very rapidly changing force applied to the scale, and one that's very difficult to calculate.

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

It's not that difficult, at least for the first instants where the angle is small (so you can simplify cos(θ) = 1 and sin(θ) = θ)

Worldbuilding is fun, writing is a chore

That's exactly what I said. The initial contact will bring the full weight to bear on the scale. It's after it begins to pivot that the difficult calculations start.

"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"

No, since the rotation/acceleration begins immediately, you don't have F = mg even at t = 0.

Worldbuilding is fun, writing is a chore

Okay, so if the legs are still in contact with the ground to brace the rest of the body, and the articulation is actively exerting enough force to match the downward one caused by the abdomen's mass, would the system still tip over backwards?

Fiat iustitia, et pereat mundus.

The only way the legs can brace anything is by exerting a supportive force of their own. Forget about the articulation, the internal forces of the robot are just that, internal, and therefore not relevant. The problem is:

The only possible way a single contact point can support the entire weight of an object, with the object remaining stable, is if the contact point is directly under the object's centre of gravity.

In the situation you've set up, with the scale under the thorax and most of the mass in the abdomen, this cannot happen. The scale is not under the centre of gravity, so it cannot be a single contact point supporting (and therefore measuring) the entirety of the robot's weight. Other contact points (in this case the hind legs) have to support part of the weight to prevent the robot from tipping over backward.

Worldbuilding is fun, writing is a chore

Given the forces, less a tip and more a terrible backflip.

That is, if the hind legs don't snap and cause a rebound faceplant.