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Stability of Matter  
  
1484   02:10 صباحاً   date: 14-12-2020
Author : E. R. Huggins
Book or Source : Physics 2000
Page and Part : 542


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Date: 15-10-2020 2178
Date: 2-1-2017 2829
Date: 4-1-2017 1876

Stability of Matter

The conservation of electric charge may be related to the stability of matter. The decay of elementary particles is not an exceptional occurrence, it is the general rule. Of the hundreds of particles that have been observed, only four are stable, the proton, the electron, the photon and the neutrino. (Neutrons are also stable if buried inside a nucleus, for reasons we will discuss in the next chapter.) All the other particles eventually, and often very quickly, decay into these four.
The question we should ask is not why particles decay, but instead why these four particles do not. We know the answer in the case of two of them. Photons, and perhaps, neutrinos, have zero rest mass. As a result they travel at the speed of light, and time does not pass for them. If a photon had a half life, that half life would become infinite due to time dilation.
Why is the electron stable? It appears that the stability of the electron is due to the conservation of energy and electric charge. The electron is the least massive charged particle. There is nothing for it to decay into and still conserve charge and energy.
That leaves the proton. Why is it stable? We do not know for sure. There are a couple of possibilities which are currently under study. One is that perhaps the proton has some property beyond electric charge that is conserved, and that the proton is the least massive particle with this property. This was the firm belief back in the 1950s.
In the 1960s, with the discovery of quarks and the combining of the electric and weak interaction theories, it was no longer obvious that the proton was stable. Several theories were proposed, theories that attempted to unify the electric, weak, and nuclear force. These so– called Grand Unified Theories or GUT for short, predicted that protons should eventually decay, with a half life of about 1031 years. Since the universe is only 1010 years old, that is an incredibly long time.
It is not impossible to measure a half life of 1031 years. You do not have to wait that long. Instead you look at 10 31 or 10 32 particles, and see if a few decay in one year. Since a mole of particles is 6× 1023 particles, you need about a billion moles of protons for such an 
experiment. A mole of protons (hydrogen) weighs one gram, a billion moles is a million kilograms or a thousand metric tons. You get that much mass in a cube of water 10 meters on a side, or in a large swimming pool. For this reason, experiments designed to detect the decay of the proton had to be able to distinguish a few proton decays per year in a swimming pool sized container of water.
So far none of these detectors has yet succeeded in detecting a proton decay (but they did detect the neutrinos from the 1987 supernova explosion). We now know that the proton half-life is in excess of 1032 years, and as a result the Grand Unified Theories are in trouble. We still do not know whether the proton is stable, or just very long lived.




هو مجموعة نظريات فيزيائية ظهرت في القرن العشرين، الهدف منها تفسير عدة ظواهر تختص بالجسيمات والذرة ، وقد قامت هذه النظريات بدمج الخاصية الموجية بالخاصية الجسيمية، مكونة ما يعرف بازدواجية الموجة والجسيم. ونظرا لأهميّة الكم في بناء ميكانيكا الكم ، يعود سبب تسميتها ، وهو ما يعرف بأنه مصطلح فيزيائي ، استخدم لوصف الكمية الأصغر من الطاقة التي يمكن أن يتم تبادلها فيما بين الجسيمات.



جاءت تسمية كلمة ليزر LASER من الأحرف الأولى لفكرة عمل الليزر والمتمثلة في الجملة التالية: Light Amplification by Stimulated Emission of Radiation وتعني تضخيم الضوء Light Amplification بواسطة الانبعاث المحفز Stimulated Emission للإشعاع الكهرومغناطيسي.Radiation وقد تنبأ بوجود الليزر العالم البرت انشتاين في 1917 حيث وضع الأساس النظري لعملية الانبعاث المحفز .stimulated emission



الفيزياء النووية هي أحد أقسام علم الفيزياء الذي يهتم بدراسة نواة الذرة التي تحوي البروتونات والنيوترونات والترابط فيما بينهما, بالإضافة إلى تفسير وتصنيف خصائص النواة.يظن الكثير أن الفيزياء النووية ظهرت مع بداية الفيزياء الحديثة ولكن في الحقيقة أنها ظهرت منذ اكتشاف الذرة و لكنها بدأت تتضح أكثر مع بداية ظهور عصر الفيزياء الحديثة. أصبحت الفيزياء النووية في هذه الأيام ضرورة من ضروريات العالم المتطور.