Abp

[ThePhy]

Bob sometimes uses a common and effective technique of presenting part of his argument in the context of a well-delivered silly dialogue. A classic case of this was his BEL program of April 4th, 2002, titled “Venus Spins Backwards”. His comments and then dialogue follows, starting about 11 minutes and 44 seconds in: (Whenever I point to a specific BEL show and give the time within the show, I heartily recommend you take the time to download it and listen for yourself. This one in particular).

And one of the stars that was formed in the big bang, it lived its long and happy life, and then billions of years ago, it became a supernova, and it exploded. And when it exploded, the results of that supernova – it created all the elements - the raw elements - that we have in our solar system. And some of those elements somehow, they all got together, and decided from the explosion, to form a swirling cloud, a swirling cloud of matter. And it swirled in space, and as gravity – you know gravity will – things will attract themselves to one another.

And so they began to condense. The cloud got denser and denser - heavier – well it just condensed. And it got smaller, and as it got smaller, it got denser, and pretty soon that cloud – that’s actually what became our solar system. And in the middle of it was the sun, of course. And as the sun – as the cloud was spinning, you have the sun, and then all the planets flew off one way or another, or they were the tentacles that weren’t quite spinning as tightly. And they coalesced and they became the planets, and a bunch of the planets – they had moons. And the moons spun around the planets, and the planets spun around the sun, and you have a happy solar system. That happened just by absolute random circumstances, with no design whatsoever.

Now if that were true – here is what we would find … everything should be spinning in the same direction. There is a law to that effect called the Conservation of Angular Momentum. Things are spinning and they keep spinning. If you have a merry-go-round spinning and you put a basketball on it, and the merry-go-round is spinning, the basketball will end up spinning. And if the basketball flies off the merry-go-round, it will be going in generally the same direction and will be spinning in the same direction as the merry-go-round was spinning.

It’s the Conservation of Angular Momentum. Well there is a problem for the atheists who believe in the big bang. The problem is this -- It’s that Venus is spinning backwards.

“Uh, Houston, Houston, we have uh, retrograde, retrograde rotation of Venus, Houston.”

“Uh, come in Galileo, come in, you are breaking up, come in.”

“Houston, we have retrograde rotation of Venus!”

Huh? Venus is spinning backwards! So is Uranus. They are spinning backwards. Now they cannot be spinning backwards if they coalesced off a spinning cloud from a supernova from the big bang. They can’t be spinning backwards.

Recently there was another conversation that Bob may not remember:

Uh, Houston, Houston, ThePhy here, priority message, come in please.

Houston here, go ahead ThePhy.

Houston, I have come across someone who claims that Venus can’t be spinning backwards because of the Conservation of Angular Momentum.

Roger, ThePhy, understand you to say you are in the Denver area.

Houston, what? I didn’t tell you where I am, I just said …

This is Houston, ThePhy, we heard you loud and clear. There are only a few places in the world where scientific nonsense like that is promulgated, so we assumed you were at the nearest one, were we wrong?

Negative Houston, you were right. Please advise.

This is Houston. Good to hear that, sometimes those silly things spread. Recommend plan ABP.

Roger Houston, understand. ABP.

ABP​

One of the primary objectives of physics teachers is to instill an understanding of physics in his students. But sometimes, it becomes starkly clear that a student just isn’t getting it. Then the teacher invokes plan ABP. “Dear student, for your own good, and mine too, I strongly recommend you change your course of study to Anything But Physics (ABP).

Conservation of Angular Momentum​

When the subject of Darwinian evolution arises in these forums, I severely limit the depth to which I participate. I am not a biologist or geologist and so any information I might offer in such areas is necessarily dependent on others. I have no doubt that in a debate with a qualified Creationist biologist or geologist, I would fare miserably. But I am less deeply dependent on the “authorities” when it comes to some basic principles of physics. And what Bob is expounding on is indeed one of the most basic principles of physics – the Law of the Conservation of Angular Momentum.

Here is my challenge. I understand the Law of Conservation of Angular Momentum, and it says nothing about the direction or rate of the spin of Venus or of any other planet in the solar system. If Bob has a book or an authority that says that a solar system condensing out of a gas or dust cloud means that Venus cannot be spinning backwards, then I want to see that book or talk to that person. That is sheer scientific nonsense. The Law of Conservation of Angular Momentum says no such thing.

And any physics student nearing the end of his freshman year in college who parroted what Bob has said about the conservation of angular momentum would in all seriousness be given ABP counseling.

Venus​

In clarification, astrophysics is not sure why Venus has a retrograde spin. But science is dead certain that Venus having retrograde spin in no way violates the Conservation of Angular Momentum under Bob’s scenario.

There are hints as to what might be occurring with Venus. Bob noted the backwards spin, but was conveniently silent on the rate of that spin. Venus takes a long time to spin once. A Venusian day is almost 8 months long on earth. But since Venus is closer to the Sun than earth, it takes less time to circle the sun. In fact, as viewed from the Sun, the rate and direction in which Venus is spinning almost makes it non-rotating. In a full year on Venus, you would be privileged to experience 2 sunrises. It is very close to not rotating at all as viewed from the sun.

Tidal Locking​

This occurrence of a satellite not rotating when viewed from the center of its orbit is actually very common in our solar system. (And the word “satellite” in this usage just means any body orbiting another. The earth is a satellite of the sun, and the moon is a satellite of the earth.) Almost every one of the major moons in our solar system keep one face towards the planet they orbit. Our moon does that. How the backside of the moon looked was completely unknown until the first spacecraft circled to the far side of the moon and took pictures – an event well within the lifetime of many of us.

Why do moons keep one face toward their planets? The earth doesn’t do that with the sun, nor do most of the other planets. Only Mercury and Venus come close. The rate at which a planet or moon spins on its axis has no intrinsic relationship to the time it takes to complete an orbit. Kepler’s Laws that describe orbits have no need to know how fast the moon or planet is spinning. And there is no known a priori reason that the creation of a planet or moon would result in its axial spin exactly equally its orbital period. In fact, that would be a highly anomalous expectation from a planet or moon that coalesced from a primordial cloud, or was ejected from a body.

Yet the no-spin situation is commonly seen - now – eons after the creation event. Why? In addition to the moons, Mercury and Venus are the two planets closest to the sun, and they have very little relative spin. Might that hold a clue?

For the moons, the cessation of spin is a process that took many millions of years, and is maintained by what is called tidal locking. Even moons and planets that are solid rocks experience tides similar to what we see in our oceans. For reasons beyond this discussion these tides slow the relative rotation rates of both the moon and the planet that the moon circles. But since the moon is always much smaller than the planet it circles, the moon has to lose far less angular momentum than the planet to come to a complete stop. Thus the earth still spins, while its moon has already locked one face toward the earth.

Mercury and Venus, being closest to the sun, may likewise have had their original spin stolen from them by the sun. I deliberately say “may”, because there are still technical questions in the rotation of Venus that are not well understood.

Angular Momentum​

The real meaning of the principle that Bob misconstrues is not difficult. Angular momentum is a property of a body (mass) that depends on the amount of mass in the body, the distance from the axis about which it is rotating, and the rate of rotation (how many degrees per second). The Law of Conservation of Angular Momentum just says that once you figure out the angular momentum for something, it will always have that same angular momentum as long as it does not interact with an external body (via gravity or magnetic force, or whatever). If angular momentum could be expressed by a simple numerical value, then that value would not be allowed to change. (Angular momentum is actually a vector, which is a bit more complex than a simple number. But vectors can be added in a process analogous to simple numbers.)

In the case Bob starts with, it is the angular momentum of the “swirling cloud of matter” that we start with. Admittedly, one might have trouble visualizing an ephemeral cloud as having angular momentum. But angular momentum has no requirement that the thing be a solid body. Even something like a non-spinning bullet whizzing past a fence post has angular momentum as viewed from the fence post. If we know its mass, distance from the fence post (at some specified point in its flight), and how fast your line of sight has to turn to watch it go by, we can compute the angular momentum.

For the condensing galactic cloud, the best reference axis would be the center of mass of the cloud. As the cloud condenses to form stars or planets or whatever, there may be all kinds of collisions, magnetic interactions, explosions, and so on within the cloud. And the angular momentum will not change. If some blob that that will someday be called Venus forms, it will contain a portion of the cloud’s angular momentum. If some huge galactic vagrant asteroid chances to smack into Venus, one result will be that Venus’s spin will be altered. We assume the vagrant asteroid is also a product of the condensing cloud, and even though the impact with Venus involves immense energy, the total angular momentum of the cloud is unchanged. In fact, if we consider just the approaching asteroid and Venus, we can look at just the total angular momentum of these two objects. Post-impact, that sum will be unchanged. We may have to chase down billions of rocks and gasses and whatever that were ejected from the impact and are now flying away, but after we have accounted for all of them and added up the angular momentum of everything that once was Venus or asteroid, presto – the conservation law holds.

Spinning Backwards​

I suspect what Bob might have really been referring to is why would Venus be spinning the “wrong way” (albeit very slowly). If the angular momentum of the original cloud shows an overall rotation in a direction counter to that of Venus, how could Venus have ended up spinning the “wrong way”? The safest answer is “We don’t really know”. But we do know that as long as we add up the angular momentum of Venus and everything else in the condensing cloud, its will total up to the value it had originally. Even if Venus ends up just whizzing around and around in a goofy direction. If we could backtrack and identify everything that interacted with Venus, and totaled all of the angular momentum from just those things, we could be confident that after Venus is found to be tumbling in whatever direction at whatever rate, the sum of Venus’s new angular momentum plus whatever is left of the things that hit it will add up to the starting value.

In a simple case, assume an interaction between Venus and an asteroid in which they approach, hit, and separate, both still completely intact. Pre-impact, Venus is spinning the way Bob thinks it should, in the direction of the clouds rotation. Post-impact, Venus is tumbling head over heels (pole over pole?) What can we conclude? That the asteroid must be tumbling in a direction and at such a rate that the sum of the angular momentum of Venus and the asteroid adds up to the same value it did pre-impact.

Skidding on the Ice​

A simple example of the Law of Conservation of Angular Momentum that is closer to everyday experience - think of standing with a partner on a very smooth sheet of ice. You both are wearing shoes that are the slickest ever. If you are not moving, you might starve to death even if only yards from the shore, since zero traction means zero ability to walk anywhere (Executing your enemies via exiling them to a sheet of ice with said shoes is not a good idea, since if they know a bit of physics, there is a way to circumvent the slick shoes problem. But that is for another day.) Anyway, you and your partner are motionless facing each other just inches apart. Total angular momentum (measured from the center of mass of the two of you) – zilch. Now you put out your right hand, and your partner their left. Your hands meet and you each give a mighty shove on the other’s hand. You recoil, sliding across the ice, and spinning to the left. You see your partner sliding away from you, spinning to their right. Total angular momentum – still zilch. If you are twice the size of your partner, you spin more slowly and you see them spinning at twice your rate. Even if you get desperate and pull a stick of dynamite out and detonate it between you, after the detonation you are spinning off in one direction on the ice in your armored suit, as is your partner in the opposite direction. Now catch up to all of the microscopic fragments of dynamites skittering away over the ice, add up the angular momentum of all, and you already know what the sum must be.

That pretty well sums up the Law that Bob doesn’t understand. If a gas and dust cloud condenses to form a planetary system, the Law of Conservation of Angular Momentum says nothing about how the individual planets are required to spin, other than the requirement that the sum of all the angular momentum be constant. Except when taught to one offbeat congregation in Colorado, where the pastor needs ABP counseling.

 

[sentientsynth]

The Phy,

I'm no Ph.D., but I have just finished two years of physics for engineers. Hopefully you and I can have a little intelligent dialogue about this topic.

You gave a good illustration about the conservation of angular momentum with the two folks on a sheet of ice, and with the collision of the planet and meteorite.

So I'll pose to you the same question I posed to my professor.

About the sum of net forces during a collision between a planet and a meteorite, does it not stretch credulity beyond the breaking point to assume that an impact of such magnitude to change a planet's net torque would in no way change it's net translation? It is to me. And it's a simple experiment to see why. Take any physics simulation software and simulate one large body with net torque and zero relative translation and strike it with a significantly smaller object with enough force to change its net torque. Is it's net translation changed? Every time. I demonstrated this to my professor with a simulator and it works like a charm.

So it seems reasonable that if a planet is smacked hard enough to have it's overall spin changed, then it was smacked hard enough to change its orbit about the sun. And with a tiny planet like Venus, I'd say it would easily get smacked right into the sun if it were hit that hard.

Hey, Phy, I've been chewing on this question for a while now and have found absolutely zero literature in any camp. Maybe you can add some insight into this, or point me to a nice article or something.

Peace,

SS

 

[Johnny]

does it not stretch credulity beyond the breaking point to assume that an impact of such magnitude to change a planet's net torque would in no way change it's net translation? It is to me.

Perhaps for you. But that is no reason to invoke the supernatural. We find things we cannot immediately explain in nature all the time. But that is no reason to invoke the "God did it" principle.

For example, I have a carpeted porch outside my apartment. A vase with a plant and some soil sits upright on the carpet. Sometimes when the wind gets bad, it blows up under the carpet, knocks the vase over while flinging the carpet out from under it. The other day the wind was bad. I walked outside, and my carpet is flung back as usual. There is soil spilled all over the ground (indicating the vase did actually tip over). But my vase was, incredibly, sitting upright. There are two possibilities that I could think of: 1) My neighbor set the vase upright, or 2) The wind just happened to blow the vase the right way. "God did it" was not included in my line of reasoning.

The universe we live in is governed by natural law. Thus, I attribute Venus retrograde spin to a natural phenomenon. I may not know exactly why. But that is not a reason to invoke "God did it".

 

[Yorzhik]

Not that I'll involve myself with this conversation, but as a sidenote, I have it on good authority that it is extremely hard to hit the sun in one shot (without orbiting first).

 

[sentientsynth]

Hey Johnny, I don't take a "Well if I don't understand, that means God did it" approach. I'm asking ThePhy about some particulars about the evolution of our solar system, and how astrophysicists explain certain things. My Ph.D. professor couldn't give me an answer. I was wondering if ThePhy could. But thanks for your invaluable input. Moron.

 

[sentientsynth]

Not that I'll involve myself with this conversation, but as a sidenote, I have it on good authority that it is extremely hard to hit the sun in one shot (without orbiting first).

Yeah, Yorzhik, I think it is, especially from a moving launching pad, like the Earth. Odds are it'll orbit a little bit. But the Sun has no qualms about sucking whatever into itself.

My overall question is how is it possible for the momenta of an impact to be converted to 100% torque and 0% translation. And remember we're talking about planets here. Huge impacts. It just seems to me, with my limited physics background, that the planets wouldn't hold thier orbits for billions of years if they're hit even once with a collision of such magnitude as to be able to change their net torque. That takes huge amounts of energy. And for someone to tell me that such an impact wouldn't affect its an object's net translation at all stretches credulity beyond the breaking point.

So I'm taking this opportunity to ask the resident physicist (you are that, aren't you, ThePhy) what term in the equation I'm overlooking. And I trust he'll be able to give me some answer, and hopefully we can talk about it. Why? Cuz that's just the kind of guy I am.

SS

 

[koban]

Not that I'll involve myself with this conversation, but as a sidenote, I have it on good authority that it is extremely hard to hit the sun in one shot (without orbiting first).

Yorzhik - what does hitting the sun have to do with this discussion?

If you're trying to make the point that direct impacts are rare, might I suggest you take a look at the moon?

 

[Yorzhik]

Yorzhik - what does hitting the sun have to do with this discussion?

Nothing

 

[fool]

Yeah, Yorzhik, I think it is, especially from a moving launching pad, like the Earth. Odds are it'll orbit a little bit. But the Sun has no qualms about sucking whatever into itself.

My overall question is how is it possible for the momenta of an impact to be converted to 100% torque and 0% translation. And remember we're talking about planets here. Huge impacts. It just seems to me, with my limited physics background, that the planets wouldn't hold thier orbits for billions of years if they're hit even once with a collision of such magnitude as to be able to change their net torque. That takes huge amounts of energy. And for someone to tell me that such an impact wouldn't affect its an object's net translation at all stretches credulity beyond the breaking point.

So I'm taking this opportunity to ask the resident physicist (you are that, aren't you, ThePhy) what term in the equation I'm overlooking. And I trust he'll be able to give me some answer, and hopefully we can talk about it. Why? Cuz that's just the kind of guy I am.

SS

Why don't you put the current telemetry into your program and run it backwards, do the planets suddenly blink out of existence 6000 yrs. ago?

 

[ThePhy]

From sentientsynth:

About the sum of net forces during a collision between a planet and a meteorite, does it not stretch credulity beyond the breaking point to assume that an impact of such magnitude to change a planet's net torque would in no way change it's net translation?

You are right.

So it seems reasonable that if a planet is smacked hard enough to have it's overall spin changed, then it was smacked hard enough to change its orbit about the sun. And with a tiny planet like Venus, I'd say it would easily get smacked right into the sun if it were hit that hard.

Orbit changed – definitely. Into the sun - rarely.

To understand why, we need to understand some fundamental concepts.

Energy​

Energy is something that most of us have at least an intuitive understanding of. If someone takes a swing at your face because you glanced at his date, the pain in your jaw is due to him causing damage by depositing energy in it. If you bend a rod back and forth, it gets hot where the bend is because of energy in the form of heat building up there. Basking in the sun is enjoying energy from the sun. X-rays, explosions, radio waves, and on and on – they are all energy. So energy exists in many forms. Many machines are just devices to change energy from one form to another.

But there is an aspect to energy that is a bit more abstract, and is only vaguely understood by most people. And that is potential energy. The term “potential energy ” is well embedded in the world of physics, yet I dislike that term, because it carries the incorrect inference that the “potential” energy is something that might exist in the future, but is not now. Rather like the meaning of “potential” in talking about a "potential" customer or "potential" girlfriend or a "potential" sale – it may or may not happen in the future. Potential energy is very real. In more advanced physics there are fields of study that go into depth in talking about the reality of potential energy. But for us, we simply need to realize that energy can be the obvious kind that we feel and see everyday, and potential energy. We could speak of the energy of deformation (like happened to the bones and tissues in your jaw), or radiation, or electrical energy, but to understand the question you pose we need to look at the energy due to a body’s motion – its kinetic energy and its rotational energy.

The kinetic energy is the energy due to its straight-ahead velocity. Without burdening us with the underlying physics, I will simply state that the kinetic energy of a given body varies as the square of its speed. Double the speed, the kinetic energy goes up by 4 times. In practical life, using your brakes to slow your car from 60 to 30 puts 4 times as much wear on your brakes as slowing the remaining 30 down to zero. Ever notice at big airports that large jets reverse the thrust on their jets for a few seconds after landing, and only when they have slowed way down do they start using the mechanical brakes? To use the mechanical brakes for the whole job would really put a lot of wear and tear on the brakes. (A related aside – part of the FAA certification process for a new airliner requires that one be loaded to its maximum takeoff weight – preferably with sandbags in place of passengers – and then it is accelerated to takeoff speed. If at that moment the airplane suffered loss of engine thrust, the plane would have to come to a stop before the end of the runway using only mechanical brakes. So, that’s what they do in the FAA test. The plane is going to be approved for runways of a certain length, so the airplane manufacturer actually has to bring a fully loaded plane to a full stop using only the mechanical brakes within the distance that would be remaining on a typical runway. The result is usually bad news for the tires, because the massive kinetic energy of the half-million pound airplane traveling at 150 mph has to be rapidly converted into some other form of energy – usually heat. So the entire landing gear mechanism gets blistering hot and the rubber melts and the tires blow. But if it was properly designed, nothing more catastrophic happens.)

Back to planets. Rotational energy is how much energy it would take to spin up a planet to its rate of spin, or conversely, how much energy it would take to stop it from spinning. Similar to the kinetic energy, it varies as the square of the speed of rotation – spinning twice as fast means four times the rotational energy.

Now lets apply this to the question at hand – wouldn’t an impact that was strong enough to stop the rotation of a planet also radically affect its speed (or its kinetic energy)? Let’s look at the earth, since it’s the planet we are most familiar with, and in orbit and size it is comparable to Venus. The earth rotates once every 24 hours. The distance around the earth at the equator is about 24,000 miles. So the speed of rotation at the equator is close to 1000 miles per hour. The earth orbits the sun in 365.25 days (8766 hours), and travels a 600 million mile path around the sun in doing so. That means its orbital speed (translation speed) is about 68,000 miles per hour.

So now let’s think about something that hits the earth with enough energy to effectively stop its rotation, like might have happened to Venus. That collision needs to dump enough energy into the earth (or Venus) to change it from its 1000 mph rotation to zero rotation. Since its orbital speed is about 68 times its rotational speed, then the kinetic energy is 68 * 68 = 4600 times the energy due to its rotation. (Really the difference is significantly greater than this, but the underlying reasons are not important to us). So if half the energy of the collision went into stopping the earth’s spin, and the other half went into affecting its orbital speed, the orbital speed change would be on the order of one fiftieth of one percent. The kinetic energy of the earth is massively greater than the rotational energy. Smacking Venus hard enough to stop its rotation would not necessarily make much difference to its translational movement.

Now to the second part of your question – hitting the sun.

Orbits, Kepler, and Newton​

Orbits are interesting things. Assume you were an astronaut doing a spacewalk outside the space station, and your screwdriver is floating next to you. You turn to grab it, and bump it. The direction you bumped it is straight down, towards the earth, and it drifts out of reach before you can grab it. Does it leisurely drift down and start to interact with the gradually thickening atmosphere of the earth, soon to be seen as an exceptionally bright meteor for some amateur backyard astronomers? No, in fact if you have an hour or so of spare time, you might want to watch very closely to see your screwdriver drifting back up to you a half-orbit later. Reach out and grab it and finish your job. (This is an idealization.)

The essence of what Kepler observed in his orbital laws - and Newton later proved mathematically - was that orbits are ellipses, and that on the high point of the orbit the object is traveling more slowly, and on the low point it is traveling most rapidly. This means that when the object is in high orbit, its kinetic energy (energy of motion) is minimum, and it has maximum kinetic energy when it is lowest. But where does the change in energy come from? There are no rockets firing as a planet (or screwdriver) moves from low to high orbit. The answer is the energy provided as it descends through (or climbs out of) a gravitational field. Gravity pulls it down, or slows it as it rises. This is a form of energy that we call potential energy. Anyway, Newton’s laws show that the energy in an orbit does not change. Any increase in speed as an object goes from the high part of its orbit to low results in the potential energy being reduced by exactly the amount the kinetic energy increased. We say orbits are conservative – meaning the sum of PE + KE is constant.

A circular orbit is when the curve in something’s path as it falls is exactly the same as the curvature of the earth beneath. If its speed is decreased, its path will curve just a little sharper, towards the planet, which will cause it to pick up speed as it descends. But this faster speed will build until finally it will be fast enough that its path curvature is too little to match the curvature of the earth, and it seems to gain altitude (really it is just trying to go straight ahead, and the earth is curving away as it flies past).

So as your screwdriver drifted down, its decreasing potential energy as it went lower resulted in increasing its speed. Literally, it was falling, just not directly down. This speed finally was high enough (for the screwdriver – only a change of a couple of mph) that it reached the low point in its new orbit and starts swinging on the outward leg. For small changes from the speed of the space station, as in the screwdriver, the time to go around the earth is almost unchanged, thus you might meet your screwdriver again as renders obedience to Kepler on the upward swing in its orbit.

In the case of the earth, the space station is in low earth orbit. That means it is really almost grazing the upper atmosphere, and when you look outside the space station windows, the earth takes up almost half the sky. If you decrease the orbital speed of the space shuttle by a rather modest amount, it will change its orbit in accord with Kepler, so its current position is the high point of the ellipse. But the low point is low enough that the space shuttle will start feeling friction from the earth’s atmosphere as it nears what would have been the low part of the orbit. This friction slows it more, and the orbit changes yet again, lowering the low point. Thus it descends more into thicker air, and soon the drag of the air becomes more influential than the shape of the orbit, and the shuttle is inexorably on its way down. Note that to come down it first needed to slow its orbital speed, which means it briefly fires its rockets with the rocket nozzles pointed “forward”, so it slows its speed, not in a direction that would push the shuttle directly towards the earth.

But when we are talking about solar orbits – orbits around the sun – the sun is usually so far away that it is only a miniscule portion of the sky. To come close enough that its atmosphere will start to drag on something requires a vastly greater decrease in orbital speed. If you are orbiting the sun in a circular orbit, even slowing the orbital speed by a hundred times as much as the space shuttle slows to come down will only result in you changing your orbit so the low point – the nearest approach to the sun – is a few thousand, or perhaps a million, miles down. Hardly noticeable.

This means that for the sun, even radical changes in orbit will seldom set the object on a path to hit the sun. A very radical change in orbit may result in a very long and narrow elliptical orbit, similar to those experienced by comets. If the change in orbital speed is in decreasing the orbital speed, then the new orbit will have its low point closer to the sun, but seldom so low that the sun gobbles it up. If the change in orbital speed is to increase the orbital speed, the result will be that the orbit swings higher for a while. Thus the place where the impact happened becomes the new low point of the orbit.

It is pretty darn hard to actually hit the sun. To do so, you effectively need to completely stop the orbital speed (which means a really big kinetic energy change – see above) and just let it fall – straight down into the sun. Any appreciable motion to the side will result in it swinging past the sun and back out to the place it started (where its speed will be very slow once again, so it falls again, picks up speed, ad infinitum). As Yorzhik observed - it’s hard to hit the sun in one shot (I like that phrasing.)

This thread was initiated to discuss a gross misrepresentation of the physical laws pertaining to momentum. Yet almost all of the above dialogue has been about energy, with hardly a mention of momentum. That is because the answer to your question is found not in momentum considerations, but in the energy.

Koban made a valid observation that deserves consideration. He said

If you're trying to make the point that direct impacts are rare, might I suggest you take a look at the moon?

Looking at the moon shows that it has been smacked, over and over and over. But the moon has almost nothing that would erase the evidence left by an impact – no water, no air, no plate tectonics. An impact made a billion years ago will look little different than a recent one, except that the recent one might have smacked into the moon close enough to the old one to throw some of the new ejecta on top of the previous layer. Even that only shows the order in which the impacts occurred, not the time frames involved. If the two impacts were one day apart a billion years ago, or one day apart ten years ago, or a billion years apart, you can only tell their sequence in that way.

So many impacts on the moon does not mean they were not rare. One impact every ten thousand years would leave a hundred thousand craters over a billion years (or in the case of the moon, 500 thousand craters over its existence).

 

[Johnny]

I'm asking ThePhy about some particulars about the evolution of our solar system, and how astrophysicists explain certain things. My Ph.D. professor couldn't give me an answer. I was wondering if ThePhy could. But thanks for your invaluable input. Moron.

I apologize for making assumptions that I shouldn't have.

 

[sentientsynth]

Why don't you put the current telemetry into your program and run it backwards, do the planets suddenly blink out of existence 6000 yrs. ago?

Naw, fool, that takes a few billion years. And that only after they lose cohesion and become a homologous cloud of dust. :dunce:

ThePhy, thanks for your awesome reply. I'll have a response later on today, when I can devote some time to it.

Peace

SS

 

[bob b]

As usual ThePhy tends to go on and on with an elementary explanation (presumably to convince others and himself of his superior intellect) instead of getting to the point of BobE's argument, which was simply that a swirling cloud of gas which then condensed to form the Sun and planets should consistently form them spinning in the same direction.

Of course there are other difficulties which would seem to rule out such a scenario in the case of the Solar System, but there is no better "natural" explanation so this is all that gets mentioned in the textbooks.

BTW, there are difficulties in the "natural" formation of stars, also.

PS I was away to Boston for a week (my son works for Bose there) and got to jump into hyperspace on the Falcon at the Science Museum. Also saw the travelling StarWars exhibit. Don't miss either. They are awesome.

 

[ThePhy]

From bob b:

… the point of BobE's argument, which was simply that a swirling cloud of gas which then condensed to form the Sun and planets should consistently form them spinning in the same direction.

Your paraphrase is substantially different than what Enyart actually said. And your paraphrase is no more true than BobE’s claim, if by “consistently” you mean “always”.

This is not the only time Enyart has made this specific claim about Venus.

1) Did or did not BobE specifically point to the retrograde rotation of Venus as evidence of a violation of the Law of the Conservation of Angular Momentum?

2) Do you agree with BobE that Venus’s spin is an impossible result if the planets in the solar system are the result of contraction of a cloud of matter?

BobE recently initiated another thread in this forum, so it is very likely that he knows of this thread. Let's see if he is willing to stand behind the specifics of his claim.

 

[bob b]

More obfuscation from an expert in the art. My previous posting said all that needs to be said about this topic and he knows it.

 

[ThePhy]

More obfuscation from an expert in the art. My previous posting said all that needs to be said about this topic and he knows it.

Good dodge, Bob. You have found It's best to avoid answering specific questions where your generalities must be backed by specifics. I understand.

 

[sentientsynth]

Well, Phy, I dug out the old physics textbook, looked up how to calculate the moment of inertia of the Earth and drew out some free body diagrams. It's been tough trying to work this out. Honestly, I'm not fully satisfied with your answer. Don't get me wrong. You cleared a few things right up (like why a planet wouldn't fall into the Sun, and the like.)

I'm going to need more powerful simulation software.

Because what I want to see is a rotating cloud of dust condense into individual, unique planets plus a Sun. I want to program it, hit RUN and watch cosmos creation. And then I want to watch a soup of chemicals form DNA and all the little machines that replicate it and translate it into proteins. And then what will the proteins do once they're assembled? You'll have to wait until I create the program to find out!!

But seriously. I learned about evolution and the big bang in public high school, during which I was an atheist. And I have a pretty curious mind, my wife tells me, so I went to try to figure out how chemicals could evolve into cells, and cells into multicellular organisms, and ultimately to us. You know what I figured out? IT CAN'T!!! It takes a lot more than a phospholipid bilayer to make a metabolizing, self-replicating cell. And that's just the first step in a nearly infinite series.



Anyway, I guess the point is that Venus's retrograde rotation does not falsify an old universe, to the best of my understanding. I would say that the trial is suspended until further witnessess can be brought forth to testify (bob b?). As Venus's retrograde rotation does not falsify a young universe, I will continue to maintain a young universe until such is falsified beyond a reasonable doubt.

Dismissed.

 

[Balder]

...stretch credulity beyond the breaking point...

Come in, Houston. This is Hank H...

Roger on Abp. Initiating BAM.

 

[sentientsynth]

Come in, Houston. This is Hank H...

Roger on Abp. Initiating BAM.

:LoJo: That's hilarious. Yep, I'm busted. I've listened to Hank for a while now. Ya know, you hear a man say something over and over, it sticks in your head.

How funny. Balder, how are you familiar with Hank's ministry?

SS

 

[Balder]

:LoJo: That's hilarious. Yep, I'm busted. I've listened to Hank for a while now. Ya know, you hear a man say something over and over, it sticks in your head.

How funny. Balder, how are you familiar with Hank's ministry?

SS

Yes, I'm pretty familiar with him. Recently I haven't been able to listen to him, but until my schedule changed, I was listening to him on my commute home every day for a couple years.