Casey

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Does a bullet speed up before slowing down?
« on: December 11, 2008, 01:27:35 am »
Recently I was telling someone how Dr. Craig uses the Standard Model of the Big Bang to argue that the universe must have had a beginning, and from there to argue that God is the cause of the universe's existence.

   

   But the person I told this to said the following: "Although we can know that the Big Bang happened, we cannot know that there was not a prior period of expansion and collapse."  Basically he was defending the possibility of an oscillating model of the universe, I believe; I know there are a few problems with such a model, but I only brought up one.  (My knowledge of cosmology is so rudimentary.)

   

   I said, "Well, my understanding is that at present the rate of the universe's expansion is in fact increasing, whereas if the universe were to ever collapse its rate of expansion would now have to be decreasing."  In hindsight I think I would have had to have added something like the following for what I said to be to the point: "Maybe you could think that every prior cycle of the universe has expanded and collapsed until our own, which is special in that it will only expand and never collapse in on itself."  (I do not know what the argument against accepting this view would be, except perhaps to say that there is no evidence for it, which I do not know to be the case.)

   

   But my real question has to do with what the person I was speaking to said next after I noted the increasing rate of the universe's expansion.  He said, "That doesn't prove anything.  A bullet moves faster before it starts to slow down."  He was implying that the universe's rate of expansion could be increasing right now and yet in the future that rate could slow down until finally it reversed and the universe collapsed in on itself.

   

   I was somewhat skeptical about the bullet example, specifically about whether a bullet DOES in fact begin to speed up before slowing down (assuming normal circumstances) as opposed to just beginning to slow down immediately after it is fired.  If anyone can answer this question for me, I would appreciate it.  I know this is a softball.

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Bill Clute

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Does a bullet speed up before slowing down?
« Reply #1 on: December 11, 2008, 07:43:01 am »

I believe that once the bullet clears the barrel of the gun and the propulsion from the burst of the powder it would begin to slow down right then due to friction from air (i.e. drag).  That drag might be light since the bullet has a small surface area so it may maintain a speed for a very short period of time but I don't see any way it would be accelerating without any propulsion.

You could use an airplane as another example.  While the engine is running it can accelerate.  Once you shut the engine off you can maintain speed for just a brief time but you can't accelerate unless you use gravity as a propulsion mechanism and accelerate by diving the plane but eventually you run out of altitude that you can't gain back - kind of like the 2nd law of thermodynamics.

One other thing to point out, which I'm sure you are aware of, but when comparing the bullet to the "oscillating universe", the bullet may slow down and may stop but it won't go backwards under normal circumstances.  It won't get sucked back to the gun.

Truth is sacred and if you tell the truth too often nobody will believe it.  - G.K. Chesterton
If you tell a lie big enough and keep repeating it, people will eventually come to believe it. - attributed to Joseph Goebbels
To give truth to him who loves it not is only to give him more plentiful mat

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LNC

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Does a bullet speed up before slowing down?
« Reply #2 on: December 11, 2008, 02:11:11 pm »
Casey1981 wrote: Recently I was telling someone how Dr. Craig uses the Standard Model of the Big Bang to argue that the universe must have had a beginning, and from there to argue that God is the cause of the universe's existence.

But the person I told this to said the following: "Although we can know that the Big Bang happened, we cannot know that there was not a prior period of expansion and collapse."  Basically he was defending the possibility of an oscillating model of the universe, I believe; I know there are a few problems with such a model, but I only brought up one.  (My knowledge of cosmology is so rudimentary.)

I said, "Well, my understanding is that at present the rate of the universe's expansion is in fact increasing, whereas if the universe were to ever collapse its rate of expansion would now have to be decreasing."  In hindsight I think I would have had to have added something like the following for what I said to be to the point: "Maybe you could think that every prior cycle of the universe has expanded and collapsed until our own, which is special in that it will only expand and never collapse in on itself."  (I do not know what the argument against accepting this view would be, except perhaps to say that there is no evidence for it, which I do not know to be the case.)

But my real question has to do with what the person I was speaking to said next after I noted the increasing rate of the universe's expansion.  He said, "That doesn't prove anything.  A bullet moves faster before it starts to slow down."  He was implying that the universe's rate of expansion could be increasing right now and yet in the future that rate could slow down until finally it reversed and the universe collapsed in on itself.

I was somewhat skeptical about the bullet example, specifically about whether a bullet DOES in fact begin to speed up before slowing down (assuming normal circumstances) as opposed to just beginning to slow down immediately after it is fired.  If anyone can answer this question for me, I would appreciate it.  I know this is a softball.


Here is the exception regarding the universe.  The universe went through an explosive expansion during the inflationary period, followed by a slowing and is now expanding at increasing velocity again.  So, your friend would have to explain why the universe inflated explosively, slowed, and is now again expanding at increasing velocity.

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DouglasMoore

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Does a bullet speed up before slowing down?
« Reply #3 on: December 15, 2008, 02:40:49 pm »
What LNC mentions is typically explained by an alternate force which causes expansion; the cosmological constant, correct?

If there is such a force we wouldn't expect it to disappear. Meanwhile, the forces of gravity, which would act against this expanding force, are being weakened by the expansion itself (that is to say, as distances get greater, the influence of gravity diminishes).

Ultimately you must keep in mind that the cosmological argument has a probabilistic conclusion. Thus, there will be vaguely possible alternatives, unknowns, etc. The conclusion isn't undeniable, only, we claim, more probable than known alternatives, and unknowns remain unknowns.


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Casey

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Does a bullet speed up before slowing down?
« Reply #4 on: December 28, 2008, 10:09:37 pm »

Thank you all very much!