Can a Software has a Zero Defect?

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When I was preparing for engineering exams, in the physics book it is mentioning that an Apple and a Bird Feather if drop from the same height both will land to the ground at the same time. I got surprised, simply the answer is never possible. But  the law of motion in physics says it is absolutely possible. Reason the time being  same for the Apple and the Bird feather to reach ground is because of the physics formula  to calculate the time.It depends on the height and acceleration of gravity and it is independent of mass.For Apple and Bird Feather both are dropped from the same height and gravity acceleration g is also same for both. So the time taken is same.

But is this possible in realty no way ...

The important consideration for the above rule of physics is that it is Under Ideal Conditions, keeping the external factors away like the wind velocity , weather conditions or any unfavorable conditions and the number of such external factors are enormous and unknown .Only then the Apple and Bird Feather will land the ground at the same time.

Many time my laptops and desktops machines hangs . Many times I am unable to call my friends over mobile phones.Many times many software stops working without being the failure notice and many time the missiles and aeronautical software fails and tolls the human life's.

But why this happens. Do the manufacturers have any clue of the same?Answer is No, because the number of external factors are enormous and unknown like the dropping of Apple and Bird Feather.Most of the time manufacuring company want to know the problems when software stop working a pop up message appears stating that "DO you want to report this error?" and usually we click on NO button and close the program and again launch the program.Manufacturing company want to known all the unnotticed  external behaviours under which application failed so that they can fix it in next release.

Now come to topic of this post - Can a Software has a Zero Defect.
My answer is Yes under the ideal condition like the Apple and Bird Feather.