## Can anybody tell me why...

How does science work? And what's all this about quantum mechanics?

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sally
Posts: 20
Joined: Wed Oct 17, 2007 7:29 pm

### Can anybody tell me why...

OK, well this isn't strictly philosophy I think, but there are certain things I've learnt about science which I cannot figure out. Can anybody help me?

...

1. Newton's 3rd Law: Every action has an equal and opposite reaction.

My problem: How on earth can anything move if any attempt to do so is met with an equal and opposite reaction? Is this rule only true for statics?

2. The steeper the slope, the faster things travel down it.

My problem: OK, this should be obvious, I realise, but I cannot figure out why this is the case... I guess that looking at it like vectors, for each set distance traveled on a steeper slope the ratio of gravity to friction is greater... Is that why? Je sais pas

3. Einstenin- time is relative because if you rode a beam of light away from earth it would look like time had stopped on earth.

My problem: Maybe this has just been badly explained to me, but how can you equate it LOOKING like time has stopped with time ACTUALLY having stopped. The people on earth are still living as normal, but you can't see them because you're escaping all the light they're giving off when travelling away at the speed of light.

4. Is it all really unpredicatable at the quantum level? Does this mean that cause and effect aren't really the necessqrily real features of the universe that we generally presume them to be?

Sorry that these aren't all specifically philosophical, but it is my philosophical leaning that I am trying to satisfy I'll be very grateful if anyone can help me out with these.

Merci beaucoup pour votre correspondance,

Sally

p.s. anyone with similar questions could post them here too, like a little adivce surgeory for the scientifically bemused?

miss lewis
Posts: 11
Joined: Wed Oct 17, 2007 2:21 am
Location: Pittsburgh, PA, USA
Try this link to see if it helps with your first. question.www.glenbrook.k12.il.us/ gbssci/phys/Class/newtlaws/u2l4a.html - 15k -

Richard Baron
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Hi Sally

1. Newton's third law.

A billiard ball rolls along and strikes a static billiard ball. The first one stops and the second one starts, by virtue of the action and reaction. So we get something moving, but at the cost of stopping something else.

Here is a different example in which we get something moving without stopping something else. A lead ball rolls along and strikes a static ping-pong ball. The lead ball keeps on rolling and the ping-pong ball gets moving too. The action of the lead ball on the ping-pong ball has been met with an equal and opposite reaction from the ping-pong ball on the lead ball, so the lead ball has slowed down a tiny bit. But the ping-pong ball gets moving quite fast. (Momentum, mv, gets conserved, and since m of the lead ball is a lot bigger than m of the ping-pong ball, v of the lead ball goes down by a lot less than v of the ping-pong ball goes up.) So you get your equal and opposite reaction, but the effects of the action and the reaction depend on the characteristics of the things being acted and reacted upon.

2. The steep slope.

The nearer the slope approaches to the vertical, the more gravity (which is the force producing the acceleration) is lined up with the direction of travel, getting the ball moving, and the less gravity is wasting its efforts by contributing to the force normal to the slope (the force which would pull the ball through the surface of the slope if it was not solid). So yes, think of it in terms of vectors.

3. Time

The catch is in the way you phrase the question: "looking like time has stopped" and "time has actually stopped". You are, like all normal human beings, thinking in terms which Einstein has told us not to think in. He does not allow us to think in terms of one time, which runs at different speeds (down to very close to stopped) for different people. There is no universal time. So my time as it appears to me, running as normal, does not have to match my time as it appears to you, whizzing off in your spaceship.

4. Quantum mechanics

The first question, is it all really unpredictable, is is the question of hidden variables. The observations which we make look genuinely indeterminate, but is there some deterministic mechanism which we are missing? The jury is out, but I believe that the current bias is against there being hidden variables.

The second question, about cause and effect, is a genuinely philosophical question. But the answer may not follow from the answer to the first question. There could be real causation which was only probabilistic. And even deterministic causation might not be real (whatever we mean by real).

Doug_Shaver
Posts: 2
Joined: Thu Oct 18, 2007 4:10 am

### Re: Can anybody tell me why...

sally wrote:My problem: How on earth can anything move if any attempt to do so is met with an equal and opposite reaction?
You have to understand what, precisely, "reaction" means in the context of the Third Law.

To a first approximation, you can think of "action" and "reaction" as equivalent to "force." The law thus is affirming that one object cannot exert a force on another object without experiencing a force itself, equal in magnitude but directed in the opposite direction.

An object moves if and only if the forces acting on that object are unbalanced. An object in the earth's gravitational field falls toward the earth if it is not supported by something that can exert a force equal and opposite to the gravitational force. At all times, though, the object is itself attracting the earth with a force exactly equal to its own weight. That force is the equal and opposite reaction between the earth and the object.

Once consequence of this is that for a set of objects forming a closed system, there will indeed be no movement of their center of mass. If no forces act on the system, then the system as a whole will be stationary, regardless of the movements of any of its components.

S G R
Posts: 38
Joined: Wed Oct 17, 2007 11:05 pm
Sally

1. Newton is credited with discovering gravity but what this gives rise to is that there can be no gravity. The key to understanding Newtonian physics is to imagine it happening in freefall, in this situation everything bounces around all the time. Conservation of momentum makes sense in space because things will never stop moving unless they get stuck to something by gravity, or glue.

2. Yes it's vectors but its not friction that makes the difference its just the length of the arrow along the direction of motion when you split the forces up into that vector triangle thingy.

3. I'm with you on this one, i've tried to read Einsteins Theory of Relativity but it made my head hurt but the idea of time slowing down depending on how fast you're moving and at the same time your mass increasing doesn't make sense to me because from your own perspective if you travel at the speed of light there is no reason why you should reach infinite mass and implode. Also the idea that time slows down as you fall into a black hole sounds like Zeno's Arrow and that was definitely wrong.

4. I believe that David Hume explored causation in his First Enquiry and came to the conclusion that cause and effect are concepts that human minds project onto the outside world in order to understand it. Quantum mechanics does not seem to abide by human rules of causation which would seem to prove his point.

Regards

SGR

Doug_Shaver
Posts: 2
Joined: Thu Oct 18, 2007 4:10 am

### Re: Can anybody tell me why...

sally wrote:2. The steeper the slope, the faster things travel down it.

My problem: OK, this should be obvious, I realise, but I cannot figure out why this is the case.
It is good to be skeptical of any "it's obvious" answers. Sometimes there is a good reason for it to be obvious, and sometimes not.

Anyway, I think this is a case where the only reason something is the way it is, is because it cannot logically be any other way. If it were not the case that things traveled faster down steeper slopes, then all sorts of contradiction would happen. But reality cannot contradict itself, or else there wouldn't be any reality, and so the way objects move on slopes is the only way they can move.
sally wrote:I guess that looking at it like vectors, for each set distance traveled on a steeper slope the ratio of gravity to friction is greater... Is that why? Je sais pas
The vectors just describe, in mathematical language, what does happen. They have nothing to do with why it happens.

sally
Posts: 20
Joined: Wed Oct 17, 2007 7:29 pm

1. So… When an objects exerts a force on another object, this force isn’t always equal to the total “forward force” it has, is it? Is the force the billiard ball exerts on the ping pong ball dependant on the mass etc of the ping pong ball? Otherwise, the whole forward force would be countered by the ping pong ball and the billiard ball would stop. But the ping pong ball can only exert so much force on the billiard ball, relative to its mass, and so in turn this limits the amount of force the billiard ball needs to exert on the ping pong ball. ???

2. I gotcha, I think!

“the vectors just describe, in mathematical language, what does happen. They have nothing to do with why it happens.” No, but they help me to visualise, and in turn understand why it happens.

“Yes it's vectors but its not friction that makes the difference its just the length of the arrow along the direction of motion when you split the forces up into that vector triangle thingy”. But isn’t friction one of the arrows, pointing backwards up the slope?

3. “my time as it appears to me, running as normal, does not have to match my time as it appears to you, whizzing off in your spaceship”

But isn’t time about more than just how things appear? Isn’t it like a standard measurement that keeps running, and that we measure other events against?... But I guess if its use is as a measurement then in the spaceship case we should treat it as if the earth-time is running at 0 in comparision to our time for any practical purposes?

4. “There could be real causation which was only probabilistic.”

How? Causation means that there is a specific reason for every action/ event. For the cause to be a decent explanation, mustn’t it make it necessary for the effect to occur from it? Because there is a reason that effect occurs after that cause?

........

Thank you. This is all helping me sleep easy at night

Richard Baron
Posts: 204
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Hi Sally

1. Yes, you've got it. You may find it useful to play around with descriptions which get closer to the story told by the equations you know and love (F = ma, kinetic energy = 0.5 x m x v^2, momentum = mv). The same force acting by the heavy ball on the light ball and the light ball on the heavy ball transfers a given amount of momentum from the heavy ball to the light ball, but because momentum = mv, that has very different effects on the velocities of the two balls.

2. Yes, friction will be an arrow pointing up the slope. And its size is determined partly by the steepness (because a steep slope will mean that only a small component of gravity will be pressing the moving object against the surface of the slope). But its size is also determined partly by other things, particularly the materials of which the moving object and the slope are made. Even if we had very low friction, eg an oily ball bearing moving down a smooth and oily piece of metal (yuk), we would still get a faster descent with a steep slope then with a shallow slope.

3. "Isn’t it [time] like a standard measurement that keeps running, and that we measure other events against?" Yes, you use your time to measure how fast things happen around you. I use my time to measure how fast things happen around me. But once we start moving relative to one another, we lose the right to match up each other's times. I know that it makes brain hurt, but that is how it is.

4. Probabilistic causation is indeed contentious, but it is not obvious nonsense. (It may be non-obviously nonsense.) If you really want to go into it, you could start here:

http://plato.stanford.edu/entries/causa ... abilistic/

sally
Posts: 20
Joined: Wed Oct 17, 2007 7:29 pm
"I use my time to measure how fast things happen around me. But once we start moving relative to one another, we lose the right to match up each other's times."

Oky doke, I gotcha know. That must cause quite a few problems for scientists ...

"you could start here" (*hands Sally a 7,000 page tome on probabilistic causation).

Thanks!

No really, I shall have a look at the web page. Thank you. Its helpful to know the name of what I'm trying to understand too (probabilistic causation).

Murky buckets

John W. Kelly
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Joined: Thu Oct 25, 2007 9:43 pm
Location: Gruithuisen's Lunar City
Imagine a large box or cube in outer space. At each corner of this cube is a clock. Your job is to synchronize all these clocks from your location in the center of the box. As you move back to Earth, you notice the "box of clocks" aren't in sync. You go back (the center of the cube) to re-sync them and discover that they are correct! It just means that light takes longer to travel toward you from the further clocks than it does from the nearer ones.

sally
Posts: 20
Joined: Wed Oct 17, 2007 7:29 pm
Thanks. I'm quite relieved to know that that's all there is to it. That's not mind-bogglingly wierd at all!

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