Sciencemadness Discussion Board - Antimatter question

There's nothing odd about the idea of photons being released - sure, they do have particle properties but the bit that matters (pardon the pun) is the fact that they have neutral charge. I.E, the antimatter counterpart of a photon is, well, the photon. Also, the conservation of energy refers to that alone - matter is a form of energy and it is well documented that certain processes (such as pair production and nuclear fusion) readily convert one to the other. You can create and destroy matter but energy can only be converted to other forms. Hope that helps and I'll be happy to answer any other questions.

[Edited on 17-10-2017 by LearnedAmateur]

Actually, I've got one. If it's the charge that matters then what about neutrons and neutrinos?

My knowledge of this is very sketchy, but I thought they were both matter.

Texium - 17-10-2017 at 07:54

Actually, I've got one. If it's the charge that matters then what about neutrons and neutrinos?

My knowledge of this is very sketchy, but I thought they were both matter

Isn't a neutron essentially a proton with an embedded electron, hence its very slightly larger mass?

j_sum1 - 17-10-2017 at 08:00

Quote: Originally posted by zts16

Actually, I've got one. If it's the charge that matters then what about neutrons and neutrinos?

My knowledge of this is very sketchy, but I thought they were both matter

Isn't a neutron essentially a proton with an embedded electron, hence its very slightly larger mass?

Close.
A neutron can decay into a proton and an electron with the release of some energy and the loss of a little of the total mass.
In what form the energy is...? I forget.

Texium - 17-10-2017 at 08:15

Quote: Originally posted by j_sum1

Quote: Originally posted by zts16

Actually, I've got one. If it's the charge that matters then what about neutrons and neutrinos?

My knowledge of this is very sketchy, but I thought they were both matter

Isn't a neutron essentially a proton with an embedded electron, hence its very slightly larger mass?

Close.
A neutron can decay into a proton and an electron with the release of some energy and the loss of a little of the total mass.
In what form the energy is...? I forget.

Ah, right, beta minus decay... seems strange and counterintuitive too that it can go the other way, with a proton decaying into a neutron with the emission of a positron.

My current knowledge is limited to what I remember from AP Chemistry in high school. I'll probably get a rigorous schooling in this subject next semester in Physical Chemistry II.

DraconicAcid - 17-10-2017 at 08:33

I believe (but don't cite me) that a neutron will only undergo beta decay when it's in the nucleus of an atom that has too many neutrons, and the increased nuclear binding energy makes up for the fact that ordinarily a neutron is more stable (and less massive) than a proton and electron. Nuclei with too few neutrons can undergo beta capture, taking an electron from an inner orbital to turn a proton into a neutron, and I think the same process happens in neutron stars. And planet killers made of pure neutronium.

LearnedAmateur - 17-10-2017 at 12:09

Actually, I've got one. If it's the charge that matters then what about neutrons and neutrinos?

My knowledge of this is very sketchy, but I thought they were both matter.

Charge is just one factor, others like spin can be opposite in matter-antimatter pairs but it really depends on what class of subatomic particle you're discussing. I can't comment for neutrinos because their properties are above my knowledge but neutrons have an antimatter pair because their constituent quarks (udd) have antimatter counterparts with opposite charge.

saphireblue - 17-10-2017 at 21:46

"Photons are a form of energy, and since they have no mass, they are not matter."

How is this possible?

1. " two photons travelling in opposing directions to satisfy the conservation of momentum" -> surely mass is also required to conserve momentum?

2. Photos can and do exert force (think of those black and white weather vane things in vacuum bulbs, or more recent experiments of "tractor" beams of photons acting on individual atoms, pushing space sail ships with solar/laser light, etc). The classic expression for this is surely FORCE = MASS x ACCELERATION.

(Please excuse my ignorance on this subject, but have always found it interesting... even though after 15 years of pondering I still can't wrap my mind around the 2 slit experiment or particle vs. wave thing)

Assured Fish - 17-10-2017 at 21:57

Ok my mind is a little bit blown.

Suddenly the universe is starting to make sense.

So the actual products of the collision is very dependent on the amount of energy put in. The way i worded my question is that both particles had no momentum and so they would loose all their mass to form momentum energy, if we give the positron and electron momentum then we could make conceivably larger particles.
But then theoretically if we were to give the electron and positron the same momentum as the mass energy of a positron and electron then we could basically cancel the reaction but still have the collision take place. thus the positron and electron would just collide and then fly apart again without annihilating each other conserving both momentum and mass but is this possible?

Also what they do at the LHC makes more sense now, they are not just smashing particles together, they are smashing particles and anti particles together, really big and heavy particles and anti particles at insane speeds, or maybe just moderately heavy particles at even more insane speeds since as we get closer to the speed of light the mass of the particles is increased until its infinite at the speed of light.

What particles and anti particles did they collide together to get the higgs then and at what speeds? I must research this.

wg48 - 18-10-2017 at 00:33

Briefly correcting what seems to be some misconception of the most recent posts.

The rest mass of a neutron is greater than the rest mass of a proton and greater than the total rest mass of a proton plus the rest mass of an electron. Neutrons are not stable out side of a nucleus. Protons are stable outside of a nucleus. Their stability inside a nucleus is dependent on them having or not a decay path available (the short version).

Neutrons are not considered to contain electrons. They only contain quarks and gluons and I would guess/speculate photons (quarks are charged)

Photons have no rest mass but they do have mass/energy, momentum, and spin. Yes how can something have momentum with no rest mass? The short reply is well photons do. Its tied up with the way we think about momentum in the low velocity (< < c) cases and the simplifications made when we first learn about momentum and mass(rest).

I don’t know if an electron and a positron can bounce of each other but if they do its not because their momentum cancel. If their momentum cancel it just means that the total momentum of the products must be zero ie the momentum of particles going one way is equal to the total momentum of the particle going the opposite way so the total is zero. Momentum and mass/energy are always conserved.

I think all this is explained in the link I gave in my first post in this thread.

PS: thinking about, yes an electron and a positron can appear to bounce of each other because just as their energy/mass can convert to other particles it could convert back in to an electron and a positron. Note: if sufficient energy/mass available they can convert into a shower of various particles.

[Edited on 18-10-2017 by wg48]

LearnedAmateur - 18-10-2017 at 00:44

Momentum is more of a vector concept here, not so much the transformation of mass-energy. When a hypothetical object is stationary, it'll have zero velocity thus zero momentum. Suddenly, it emits two particles travelling in opposite directions with equal velocity, say +2 and -2 units per second along a single plane. These particles, both with identical mass and/or energy, must then have the same but opposite momenta - if you look at the maths behind it, 2 + (-2) equals zero, therefore momentum is ultimately conserved. A lot of things work this way and it isn't readily apparent from the surface, like two cars colliding head on to create a stationary wreck, an explosive detonating, even jumping will push the Earth a little bit but the difference in mass is so huge that the velocity will basically be zero.

Back to the annihilation, the products will always be electromagnetic radiation but it is expected to be gamma due to the amount of energy produced (c^2 is one helluva number). Increasing the kinetic energy/momentum of the particles will only serve to make the photons more energetic - this extra energy has to go somewhere after all, so it becomes part of the only products. The only way you can generate an electron-positron pair is by passing high energy gamma rays near an atomic nucleus, so annihilation itself is an irreversible reaction. BUT, if you had large atoms nearby to a high kinetic energy (close to the speed of light) annhilation, then you can form another pair with just one of the produced photons if it has an energy at least that of both the antiparticles. Mass isn't a property that needs to be conserved but energy is, so that's why antimatter pairs can form photons - since momentum equals mass times velocity, it is correct to assume that the mass can be substituted by a form of energy, E=mc^2, giving the ultimate equation p = (E*v)/(c^2). We then use this to describe the photon's momentum, simplifying the equation to p = E/c, since v now equals c.

[Edited on 18-10-2017 by LearnedAmateur]

Melgar - 18-10-2017 at 02:16

Quote: Originally posted by saphireblue

2. Photos can and do exert force (think of those black and white weather vane things in vacuum bulbs, or more recent experiments of "tractor" beams of photons acting on individual atoms, pushing space sail ships with solar/laser light, etc). The classic expression for this is surely FORCE = MASS x ACCELERATION.

Radiometers are actually a bit sneaky in the way they work. They actually don't work at all if the bulb has a total vacuum in it. They need a low-pressure gas to work. What happens is, the dark sides of the vanes absorb energy faster than the white ones. Gas molecules hit the vanes, absorb some of their energy, then bounce away with most of that energy converted to kinetic energy. With all these molecules hitting the dark side at a low velocity and ricocheting off of it with a higher velocity, some momentum is imparted to the vane, and obviously this effect would be larger on the darker side.

A few weird aspects of physics that it took me some time to wrap my head around:

1) Cosmic microwave background radiation. This radiation dates back the Big Bang, and corresponds to black-body electromagnetic radiation. For the first few micro/nano/pico/whatever seconds, the universe was too dense for photons to form, since they'd just immediately get absorbed by matter. At around a temperature of 3000K, matter was spread out enough that photons could form, and did simultaneously throughout the universe (which wasn't very large at the time). Now what always threw me off is the fact that even though nothing can travel faster than the speed of light, the universe can and does EXPAND faster than the speed of light. It's a good thing too, otherwise it would have had an event horizon that it couldn't have escaped. So the universe expanded faster than the photons could catch up to it, and those photons have been traveling outward from every point in the universe since then. As the universe stretched, the photons' wavelengths have stretched too, to the point where they're now corresponding to a temperature of about 2K. This means their energy is very, very low.

2) Particle wave duality makes a lot more sense if you imagine that the photon has to stop behaving like a wave and start behaving like a particle when it stops moving or slows to matter's speed. What happens when this slowdown takes place? Well, that's what quantum mechanics is all about!

LearnedAmateur - 18-10-2017 at 03:34

Can anyone offer any reasonable explanations as to why light is influenced by gravity despite being massless? Not only do black holes have event horizons which represent the point where gravitational potential equals the speed of light, but massive objects are also able to bend light as gravitational lenses outside of the EH if one is present. I'm guessing this has something to do with the curvature of space time associated with gravity but is there actually a more logical, mechanical explanation?

wg48 - 18-10-2017 at 05:51

Can anyone offer any reasonable explanations as to why light is influenced by gravity despite being massless? Not only do black holes have event horizons which represent the point where gravitational potential equals the speed of light, but massive objects are also able to bend light as gravitational lenses outside of the EH if one is present. I'm guessing this has something to do with the curvature of space time associated with gravity but is there actually a more logical, mechanical explanation?

Light has no rest mass but it does have mass/energy.
Irrespective of light having rest mass or not or aything else, it along with all other particles and objects not subject to a force (not gravity) move along geodesics (straight lines in space time).

So yes it’s the curvature of space.

wg48 - 18-10-2017 at 12:10

A few weird aspects of physics that it took me some time to wrap my head around:

Now what always threw me off is the fact that even though nothing can travel faster than the speed of light, the universe can and does EXPAND faster than the speed of light. It's a good thing too, otherwise it would have had an event horizon that it couldn't have escaped. So the universe expanded faster than the photons could catch up to it, and those photons have been traveling outward from every point in the universe since then. As the universe stretched, the photons' wavelengths have stretched too, to the point where they're now corresponding to a temperature of about 2K. This means their energy is very, very low.

That’s a common myth perpetuated by popular science writers and even by some people who should know better. Because of relativistic effects you cannot just add up (the normal everyday low velocity way) the velocities between galaxies in a given line of sight to arrive at the velocity of a distant galaxy.

To get the correct velocity the relativistic addition of velocities called Einstein velocity addition (google it for details) must be used. In which case the final velocity can never exceed c.

Simply put there are no galaxies, not even the microwave background (surface of last scattering), receding away from us faster than the speed of light.

Below is a graphic of how the velocity varies with distance. The horizontal axis is distance. Its is assumed that on average the universe is isotropic meaning that on average each galaxy is receding from its neighbour by a fixed velocity. The line marked Newtonian motion plots the velocity of a galaxies with distance, a simple proportionality. The further they are away the faster they are receding, It must exceed c at some distance that’s the myth.

The plot marked relativistic motion is correctly calculated and can never exceed c.

receding-galexy-velocites.jpg - 5kB

Edited: to improve the explanation

[Edited on 18-10-2017 by wg48]

[Edited on 18-10-2017 by wg48]

Melgar - 18-10-2017 at 19:45

Although people often assume that Einstein's Theory of Relativity is the "special" one, there are actually two of them: general relativity and special relativity. General relativity covers scenarios at more ordinary velocities, and predicts that one consequence of gravity bending space-time is that there should be no observable difference between an acceleration due to gravity and an acceleration due to accelerating. Any phenomenon you'd see if you were in a constantly-accelerating elevator, you should also see while under the effects of a gravitational field. So far, this has proven to be true in every experiment that's been done.

The main problem with general relativity is that even though it's one of those near-perfect theories of nature, gravity doesn't tend to work nice with the other fundamental forces and their quantum mechanics. At least not on paper, and not yet.

Melgar - 19-10-2017 at 04:00

I've been trying in vain to figure out what part of my post this was a response to, and I honestly don't know. The CMB radiation obviously HAS been "catching up" with us over time, or else we wouldn't be able to detect it. The strange thing about it though, is that we're still detecting that radiation 13 billion years later, and it seems to be emanating from every point in the universe more or less simultaneously. Of course, any radiation that would have originated less than 13 billion light years away would have passed us already, and that would have included any radiation originating between us and any distant galaxies. But the universe is 100 billion light years across, despite being only 13 billion years old, which was the weird part that I initially had trouble understanding. (I think I have a pretty good grasp of it now, though.)

What you're referencing seems to just be an answer to the common mistake of summing velocities without using the appropriate Lorentz transformations. (ie https://xkcd.com/675/)

JJay - 19-10-2017 at 04:24

You know, a rock actually falls faster than a feather in a vacuum due to the greater gravitational force exerted by the rock (if you're confused, assume that the rock is the size of Jupiter).

I know absolutely nothing about relativity. I always had a hard time seeing how I would ever use anything beyond Newtonian physics.

wg48 - 19-10-2017 at 04:55

Quote: Originally posted by JJay

You know, a rock actually falls faster than a feather in a vacuum due to the greater gravitational force exerted by the rock (if you're confused, assume that the rock is the size of Jupiter).

.

I like that

wg48 - 19-10-2017 at 05:20

Ok ignoring your reference to a cartoon joke.

I perplexed, what then did you mean by your statement:

"the universe can and does EXPAND faster than the speed of light"

In particular what did you mean by "the universe" and "EXPAND" ?

Fulmen - 19-10-2017 at 13:10

General relativity covers scenarios at more ordinary velocities

Not quite. Special Relativity came first (1905) and explained the constant speed of light, the matter/energy-duality and such. General Relativity came later (1916) and incorporates gravity/space-time into SR.

LearnedAmateur - 19-10-2017 at 14:07

General relativity covers scenarios at more ordinary velocities, and predicts that one consequence of gravity bending space-time is that there should be no observable difference between an acceleration due to gravity and an acceleration due to accelerating. Any phenomenon you'd see if you were in a constantly-accelerating elevator, you should also see while under the effects of a gravitational field. So far, this has proven to be true in every experiment that's been done.

But isn't there also no discernible difference for electromagnetism? I mean charged objects still experience an exponential increase in acceleration with decreasing radius, like masses with gravity, yet is many orders of magnitude more powerful. Mass is simply exchanged for charge - the equations are even the same format albeit with different constants, however we know that electromagnetism is facilitated by photons and not space time curvature.

Consider an observer sitting in a positively charged sphere several thousand kilometres from either a planet or a highly negatively charged metal sphere with a negligible mass. They wouldn't be able to tell whether they were accelerating towards a massive object because of gravity, or a far lighter charged object because of an electromagnetic attraction. In either case, they would simply be accelerating towards it, and assuming that both objects are very large, they would have similar force-distance relationships at a sufficient radius from the surface.

Melgar - 19-10-2017 at 16:24

Quote: Originally posted by JJay

You know, a rock actually falls faster than a feather in a vacuum due to the greater gravitational force exerted by the rock (if you're confused, assume that the rock is the size of Jupiter).

I know absolutely nothing about relativity. I always had a hard time seeing how I would ever use anything beyond Newtonian physics.

Ah, but the "rock" would then have the lowest velocity of any of the objects. The feather would actually fall faster, assuming that "fall" wasn't so narrowly-defined as to exclude certain components of motion that were a result of gravity.

I never much cared for physics because the easy stuff is too intuitive, and even the hard stuff is usually just proving stuff that's already intuitive. And with the really hard stuff, there's usually not much you can do with it on an amateur budget.

Also, physics is perhaps the one branch of science that's nearly exhausted, with the possible exception of astrophysics. We're still learning plenty of new stuff in the field of chemistry. Even amateurs here have learned things that have previously never been documented, using only their own resources.

Melgar - 19-10-2017 at 16:32

I was referring to the expansion of space, according to Hubble's law. After all, if the expansion of the universe was limited by the speed of light, it could only be 27 billion light years across, since the universe is only 13 odd billion years old.

https://en.wikipedia.org/wiki/Observable_universe#Misconcept...

Quote: Originally posted by Fulmen

Eh, I never actually contradicted any of that, but I concede that I could have explained it better.

Not really. The force is squared for mass and cubed for electromagnetism. That odd power actually makes a pretty big difference, since it allows for negative forces and charges. It also makes the force increase a lot faster with smaller distances. I think (though I could be wrong) that the strong nuclear force is to the power of 4.

The difficult part for physics is that mass is responsible for both gravitational force and inertia, and yet there isn't really a good reason that those two should be exactly proportional like they are. So mass plays two totally different roles in two different models. And for some reason, the rules are very different in the two models, but it's virtually impossible to tease them apart, because the only experiments that could be done to test hypotheses would require the use of a black hole. I think. This is not my area of expertise, and maybe someone else understands it better.

Consider an observer sitting in a positively charged sphere several thousand kilometres from either a planet or a highly negatively charged metal sphere with a negligible mass. They wouldn't be able to tell whether they were accelerating towards a massive object because of gravity, or a far lighter charged object because of an electromagnetic attraction. In either case, they would simply be accelerating towards it, and assuming that both objects are very large, they would have similar force-distance relationships at a sufficient radius from the surface.

Here's a clue: if you're disintegrating because of the electrical repulsion caused by the ionization of your entire body, it's the electrical one.

[Edited on 10/20/17 by Melgar]

Fulmen - 19-10-2017 at 21:16

Eh, I never actually contradicted any of that, but I concede that I could have explained it better.

I could have been clearer myself it seems, saying that GR deals with "more ordinary velocities" is wrong. GR covers all of SR plus gravity.

LearnedAmateur - 20-10-2017 at 04:36

I was under the assumption that for electromagnetic interactions, F=k*Qq/r^2 where k is the reciprocal of 4π(ε0), hence follows the inverse square law like gravity does (F=-G*Mm/r^2). Negative charges are accounted for in the equation, if both Q values have opposite signs then the force is negative/attractive, whereas the same sign indicates a positive/repulsive force. For gravity, the value of G is always negative hence gravity always attracts since you can never have a negative mass without negative energy.

wg48 - 20-10-2017 at 06:35

Melgar wrote:

"I was referring to the expansion of space, according to Hubble's law. After all, if the expansion of the universe was limited by the speed of light, it could only be 27 billion light years across, since the universe is only 13 odd billion years old".

That’s just an other way concluding that very distant galaxies are moving fast than light.

I can not give you the full version but the short version is:

Thanks to relativity in an expanding universe there are no common sets of coordinates of position and time that allow you to do the every day simple velocity distance time calculations between different parts of the universe and get correct answers. Its much more complicated than that.

Fulmen - 20-10-2017 at 14:14

IIRC space isn't limited by the speed of light, so space could expand (and according to inflation theory, already has expanded) faster than the speed of light.

Melgar - 20-10-2017 at 15:10

That’s just an other way concluding that very distant galaxies are moving fast than light.

Well, they actually are, and relative to us, no less. Special relativity doesn't hold true for objects so far distant that the expansion of the universe is significant. Here is an article that I skimmed and seems to explain it well:

http://curious.astro.cornell.edu/about-us/104-the-universe/c...

Also, do you understand the "dark energy" problem very well? Because that's one of the few big problems that physicists don't know what to make of yet.

wg48 - 21-10-2017 at 04:59

That’s just an other way concluding that very distant galaxies are moving fast than light.

The article you reference explains very little. I would give it a popsci score of nine out of ten. It did caveat the statement “there are galaxies moving away from each other faster than light” but failed to detail the caveat. However when discussing receding galaxies approaching the speed of light he did make the following statement “After that, we will observe them to freeze and fade, never to be seen again”, presumable SR effects. That statement could be reworded to: “in our inertial frame we observe no galaxies moving faster than the speed of light”.

We are apparently making no progress in our discussion I can do no more than repeat my previous statements with more detail and you could stand at the interacting point of beams of the LHC and say see the protons that don’t collide are moving away from each other at almost twice the speed of light just like some galaxies do.

My understanding of dark energy is that it’s the mechanism that is accelerating the expansion of universe.

Is space expanding at the LHC? If its diameter is expanding will it end up in my garden at some point?

Sorry I could not resist adding the sarcastic and humorous comment.

[Edited on 21-10-2017 by wg48]

Melgar - 22-10-2017 at 09:09

I was under the assumption that for electromagnetic interactions, F=k*Qq/r^2 where k is the reciprocal of 4π(ε0), hence follows the inverse square law like gravity does (F=-G*Mm/r^2). Negative charges are accounted for in the equation, if both Q values have opposite signs then the force is negative/attractive, whereas the same sign indicates a positive/repulsive force. For gravity, the value of G is always negative hence gravity always attracts since you can never have a negative mass without negative energy.

Err, oops. I was wrong. I was thinking of magnetic attraction. I guess the only real difference with electrostatic attraction is that it actually can be negative, unlike gravity that's only positive. Also, electrostatic forces tend to cancel each other out on macroscopic scales, and would destroy the matter that they occupied if they became too large, due to the large value of the constant.

Is space expanding at the LHC? If its diameter is expanding will it end up in my garden at some point?

Sorry I could not resist adding the sarcastic and humorous comment.

Large Hadron Collider? Um, no. Space is expanding uniformly, and the expansion is only noticeable for points that are separated by very large distances. Seriously, I think you're dismissing the expansion of the universe as just being an illusion due to relativistic effects, when that isn't the case at all. This is a very real phenomenon that nobody completely understands to this day. And the cosmic microwave background radiation? And dark energy? And the fact that the universe* is proven to be 100 billion light years across, more or less, despite being less than 14 billion years old? This all has to do with actual, real phenomena regarding space expansion; it's not just some delusion of physicists that don't understand relativity.

* meaning everything that came into being during the Big Bang, including photons and the space that everything occupies

wg48 - 23-10-2017 at 16:07

Don’t you find it odd that the LHC can get protons to separate at more than the speed of light without expanding space or violating SR but the universe has to create space and ignore SR?

Have you ever read a technical paper explaining the difference or discussing the details of expanding space or how it stretches photons? Popsci does not count. It’s not hard to find ones calling it a myth and bitching about it.

These are rhetorical questions, which hopefully kick start the critical thinking parts of your brain. This is pervasive myths it will not die easily. It will be like admitting you believe in spontaneous human combustion or perpetual motion.

Incidentally the size of the universe at present was perplexing to me. Not because it expanded (separated) faster than light but because the numbers did not make sense. I thought it was a linear but apparently it expanded exponentially similar to how hot gas would. That should not have been a surprise. There is nothing mysterious about it, no violation of any physics including sr. But it is a lot more complicated to explain or understand than the myth.

If your serious comment was serious I wasted my words on you. Perhaps they will help others exorcise the myth.

Melgar - 24-10-2017 at 06:36

Have you ever read a technical paper explaining the difference or discussing the details of expanding space or how it stretches photons? Popsci does not count. It’s not hard to find ones calling it a myth and bitching about it.

Well, here's a classic paper on the subject. (Attached)

I sort of understood your joke, but you seemed to be referencing some event at the LHC that I'm unfamiliar with, involving speeds faster than light.

With galaxies moving away from us faster than light (when universe expansion is added into their velocity relative to us), they actually seem to be moving away from us more slowly than they are, in a way that's remarkably similar to what an object would look like as it crossed the event horizon of a black hole. If you witnessed that, the object would actually never seem to cross the event horizon, rather, it would stop there, then it would become redder and redder, until it was only visible in infrared, then microwave, then radio spectra. Finally, there would be no electromagnetic radiation coming from it that would be detectable, although this would take a very long time to happen. Galaxies that are moving away from us faster than light show the same phenomena, just on a longer time scale. They just become redshifted out of the visible spectrum. Because of how much light galaxies emit though, they'd probably be visible it at least some spectra for trillions of years.

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wg48 - 24-10-2017 at 12:16