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#109115 A little thought about general relativity

Posted by odc on 03 February 2018 - 02:47 PM in SCIENCE!

Hi all

 

we all have heard about general relativity. Something related to space-time curvature...

 

At the time of Einstein (around 1905), only two fundamental forces were known (Newtonan law of gravitation and Maxwell laws of Electromagnetism). Today, we know two others (weak and strong interactions). The special relativity had focused on electromagnetism and solved the problem of constant speed of light in every inertial referentials.

 

After his theory of special relativity, Einstein still had a problem. The Newtonian law of gravitation was not relativistic. He first emitted his principle of equivalence which will eventually lead to the theory of general relativity, the only theory at that times able to predict the existence of blach holes !

 

But let's just make a pause on the principle of equivalence. Basically, this principle states that it we are in a rocket without window to see the outside, it is impossible to guess with physical experiment (mecanics, electromagnetism) if the rocket is accelerated in outer space or if it is landed at the surface of a planet creating the equivalent gravitational acceleration. For exemple, at the surface of the earth this acceleration is about 9.8 meter per second square, ie a speed variation of 9.8 m/s per second.

Let's now go a little bit further, In outer space, we feel weightlessness if the rocket is inertial (engines are off). The principle of equivalence states that if we are in an imaginary lift dropping in the gravitational field (without air in the elevator shaft), we will feel the same a few seconds before the crash. So if we had enough time to make physical experiments in this dropping elevator, we would obtain the same results than people in the ISS. For exemple, if we fire a horizontal light in the elevator cage, we will see the endpoint exactly at the horizontal of the source "in the elevator". But during the time necessary for light to travel the distance from source to endpoint, the elevator has accelerated. That means that from the point of view of the earth surface, the light ray has been curved...

 

All this to say that the inertial referential is not the earth surface (it is accelerated upwards), but the dropping lift which goes at a constant speed in a curved space-time due to the presence of mass.

 

Amazing, isn't it ???

 

Hope you enjoyed this little trip in the world of theoretical physics

 




#109112 New to programming, whats a good first language?

Posted by odc on 03 February 2018 - 11:07 AM in Coding

Hi

 

compiled languages are useful for professionals or people caring about performance. Interpreted ones are easier to get into.

 

For you, I would recommend Python 3. This is a interpreted script language very user friendly, very portable, with lots of libraries (graphical, network, scientific calculation, etc.). With it, you can do quite everything you can imagine (desktop or mobile apps, even back-end web servers can use python).

 

however, is you wish to do your own website, you'd rather have to get into (in this order) HTML 5, CSS 3, javascript, jQuery and bootstrap for the front-end, and then, for the back-end, learn about databases (mySQL, etc.) and back-end language (php, Node.JS, etc.) to start with. When you have an idea of all these language can do, you will be able to have fun with your high quality website.




#109111 I'm having trouble with CSS

Posted by odc on 03 February 2018 - 10:37 AM in Coding

Hi,

 

you can try this playlist :

 

All his explanations are crystal clear. Short videos, easy to jump to your topic. The best css playlist I know...




#109110 Albert Einstein

Posted by odc on 03 February 2018 - 10:08 AM in SCIENCE!

E=mc2 is a formula figured out from the special relativity which is one of the most beautiful discovers of the 20th century. This theory is now accepted as the main framework for all phisics theories like mecanics, electromagnetism, particule physics, etc. Thus, nowadays, if a theory is not relativistic, no theoretical physicist will want to ever think about it.

 

E=mc2 is only the surface of the iceberg, it "just" tells that there is an equivalence between mass and energy, which means that from empty space, a particule can be spontaneusly created provided that enough energy is located in the "same space", leading to quantum fluctuation.

 

Regarding the nuclear bomb, I will just ask a simple question :

Do you think that the first guy who discovered how to make a fire should be ashamed because of flamethrowers




#109109 PLEASE HELP ME UNDERSTAND THIS VACUUM SCIENCE

Posted by odc on 03 February 2018 - 09:44 AM in SCIENCE!

Hi boba,

 

atmospheric pression at low altitude (sea level) is a huge force per area (~ 1 Kg / cm2). So, even if molecules have a big energy (thanks to ambiant temperature), they are not powerful enough to fight against this force. But when pression vanishes, ambiant temperature is large enought to make molecules leave the little pot and get vaporized. It is the same phenomenon that explains why water boils at less than 100°C (212 °F) in the mountains or in any low pressure spaces.

Some molecules keep connexions with their neighboors and finally encompass some air, creating bubbles. We can guess that gel molecules are something like "amphiphilic" like soap, which means that they have one side of the molecule which is hydrophilic and the other which is hydrophobic and thus being able to create a thin surface that will enclose air.