Can I use Google Glass without a phone?

Does glass require you to have a phone with it all the time? Can I replace my phone with it? If I do "Tether" it to my phone, how much data will this thing use? What if I leave my phone at home on runs, but wear Glass, will it still function without trouble (Minus notifications and such). Is there a glass with its own data plan? Can glass play music like my phone would? Thanks!