What is String Theory?

Question: What is String Theory?
I've heard the term string theory, but don't really know what it means. How does it relate to quantum physics?

Answer: String theory is a mathematical theory that tries to explain certain phenomena which is not currently explainable under the standard model of quantum physics.
The Basics of String Theory
At its core, string theory uses a model of one-dimensional strings in place of the particles of quantum physics. These strings, the size of the Planck length (i.e. 10-35 m) vibrate at specific resonant frequencies. (NOTE: Some recent versions of string theory have predicted that the strings could have a longer length, up to nearly a millimeter in size, which would mean they're in the realm that experiments could detect them.) The formulas that result from string theory predict more than four dimensions (10 or 11 in the most common variants, though on version requires 26 dimensions), but the extra dimensions are "curled up" within the Planck length.
In addition to the strings, string theory contains another type of fundamental object called a brane, which can have many more dimensions. In some "braneworld scenarios," our universe is actually "stuck" inside of a 3-dimensional brane (called a 3-brane).

String theory was initially developed in the 1970s in an attempt to explain some inconsistencies with the energy behavior of hadrons and other fundamental particles of physics.

As with much of quantum physics, the mathematics that applies to string theory cannot be uniquely solved. Physicists must apply perturbation theory to obtain a series of approximated solutions. Such solutions, of course, include assumptions which may or may not be true.

The driving hope behind this work is that it will result in a "theory of everything," including a solution to the problem of quantum gravity, to reconcile quantum physics with general relativity, thus reconciling the fundamental forces of physics.

Variants of String Theory
Bosonic String Theory: The first string theory, which focused only on bosons.
Superstring Theory: This variant of string theory (short for "supersymmetric string theory") incorporates fermions and supersymmetry. There are five independent superstring theories:

•Type 1
•Type IIA
•Type IIB
•Type HO
•Type HE
M-Theory: A superstring theory, proposed in 1995, which attempts to consolidate the Type I, Type IIA, Type IIB, Type HO, and Type HE models as variants of the same fundamental physical model.
Research in String Theory
At present, string theory has not successfully made any prediction which is not also explained through an alternative theory. It is neither specifically proven nor falsified, though it has mathematical features which give it great appeal to many physicists.
A number of proposed experiments might have the possibility of displaying "string effects." The energy required for many such experiments is not currently obtainable, although some are in the realm of possibility in the near future, such as possible observations from black holes.

Only time will tell if string theory will be able to take a dominant place in science, beyond inspiring the hearts and minds of many physicists.

From:
http://physics.about.com/b/2009/11/04/alternate-universe-countdown.htm

آدرسهای مورد درخواست

در جلسه این هفته که در مورد ترمودینامیک صحبت کردیم، منابعی در اینترنت معرفی شد که بنا به درخواست همکاران آدرسهای آنها در اینجا ذکر می شود.

http://www.physics.ucsd.edu/~tmurphy/phys12/lectures/07_heat.ppt

http://www.blogger.com/www.physics.ucsd.edu/~tmurphy/phys12/lectures/07_heat.ppt

http://www.physics.ucsd.edu/~tmurphy/phys12/lectures/07_heat.ppt

http://me.kaist.ac.kr/upload/course/MAE554_2009/L7_Stirling_2009.ppt

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=23

http://www.cs.sbcc.net/~physics/flash/heatengines/Carnot%20cycle.html

http://galileo.phys.virginia.edu/classes/109N/more_stuff/flashlets/carnot.htm

http://www.shermanlab.com/xmwang/myGUI/CarnotG.html

http://www.k-wz.de/vmotor/v_omotore.html

http://www.educypedia.be/education/carjava.htm

مسئله ای در مورد فشار

Scuba diving
In scuba diving, a greater water pressure acts on a diver at greater depths. The air pressure inside the body cavities (e.g., lungs, sinuses) must be maintained at the same pressure as that of the surrounding water; otherwise they would collapse. A special value automatically adjusts the pressure of the air breathed from a scuba tank to ensure that the air pressure equals the water pressure at all times. The scuba gear in Figure consists of a 0.0150-m3 tank filled with compressed air at an absolute pressure of 2.02 × 107 Pa. Assuming that the air is consumed at a rate of 0.0300 m3 per minute and that the temperature is the same at all depths., determine how long the diver can stay under seawater at a depth of (a) 10.0 m and (b) 30.0 m.
Reasoning The time (in minutes ) that a scuba diver can remain under water is equal to the volume of air that is available divided by the volume per minute consumed by the diver. The available volume is the volume of air at the pressure P2 breathed by the diver. This pressure is determined by the depth h beneath the surface, according to P2= P1 + ρgh , where P1= 1.01 × 105 Pa is the atmospheric pressure at the surface. Since we know the pressure and volume of air in the scuba tank, and since the temperature is constant, we can use Boyle’s law to find the volume of air available at the pressure P2 .
Solution
Using ρ=1025 kg/m3 for the density of seawater, we find that the absolute pressure P2 at the depth of h= 10.0 m is:
P2 = P1 + ρgh = 1.01 × 105 Pa + (1025 kg/m3)(9.8 m/s2)(10.0 m)
= 2.01 × 105 Pa
The pressure and volume of the air in the tank are Pi =2.02 × 107 Pa and Vi = 0.0150-m3 , respectively. According to Boyle’s law, the volume of air Vf available at a pressure of Pf = 2.01 × 105 Pa is
V_f=((2.01 × 〖10〗^7 Pa)(0.0150 m^3))/(2.01 × 〖10〗^5 Pa)=1.51 m^3
Of this volume, only 1.51 m3 – 0.0150 m3 = 1.50 m3 is available for breathing, because 0.0150 m3 of air always remains in the tank. At a consumption rate of 0.0300 m3/min, the compressed air will last for
t=(1.50 m^3)/(0.0300 m^3/min)=50.0 min
The calculation here is like that in part (a). Equation P2 = P1 + ρgh indicate that at a depth of 30.0 m, the absolute water pressure is 4.02 × 105 Pa. Because this pressure is twice that at the 10.0 m depth. Boyle’s law reveals that the volume of air provided by tank is now only Vf= 0.754 m3. The air available for use is 0.754 m3-0.0150 m3=0.739 m3. At a consumption rate of 0.0300 m3/min, the air will last for t= 24.6 min , so the deeper dive must have a shorter duration.
From:
Physics (6 Edition) Cutnell & Johnson - Wiley International Edition- Page 398-399

Misconceptions

یکی از موضوعات جالبی که با آن در اینترنت برخورد کردم، بدفهمی یا کج فهمی موضوعات فیزیک است، که اگر رخ دهد، بزحمت می توان آنرا یا اثراتش را زدود. بنابراین آدرس زیر را معرفی می کنم، که می توانید با رجوع به آن مطالبی را در این

زمینه بخوانید. البته این موضوع جلسات آینده ما نیز خواهد بود.

http://www.lhup.edu/~dsimanek/scenario/miscon.htm