"Thus, matter can be created out of two photons." Hello, and thank you for opening my thread! I have read over the FAQ on this forum, looked over a few similair topics, and still feel that I lack understanding with this concept. To me, the wiki stated below sounds like it is saying light + light can = mass. On this Wiki page, it states "Thus, matter can be created out of two photons." http://en.wikipedia.org/wiki/Matter_creation On this page as well, they describe a collision with electrons and photons to produce matter from light itself. But I do not know whether the mass of the new particle created was greater than the mass of the electron. In other words, does this electron to proton collision produce a mass greater than the electron initially used? http://www.sciencedaily.com/releases/1997/09/970918045841.htm So by this, even though photons do not have mass, they can be collided to make something with mass? Another concept I am having trouble with is the following: "she slowed light, which travels in free space at a speed of 186,000 miles a second, to just 38 miles per hour in a cloud of ultracold atoms. Einstein and others have theorized that the speed of light in free space can't be changed" From: http://www.photonics.com/Article.aspx?AID=28520 I thought the speed of light is to be considered a constant? How can they slow this down, or even stop it? And when stopped, what does this light become? Thank you all for your help, it is much appreciated.