whY is sPACE 3D

 SPACE; 3D !  why

In a new paper published in EPL, researchers have proposed that the second law of thermodynamics may explain why space is 3d.

"A number of researchers in the fields of science and philosophy have addressed the problem of the (3+1)-dimensional nature of space-time by justifying the suitable choice of its dimensionality in order to maintain life, stability and complexity," coauthor Julian Gonzalez-Ayala, at the National Polytechnic Institute in Mexico and the University of Salamanca in Spain.

"The greatest significance of our work is that we present a deduction based on a physical model of the universe dimensionality with a suitable and reasonable scenario of space-time. This is the first time that the number 'three' of the space dimensions arises as the optimization of a physical quantity."

The scientists propose that space is 3D because of a thermodynamic quantity called the Helmholtz free energy density. In a universe filled with radiation, this density can be thought of as a kind of pressure on all of space, which depends on the universe's temperature and its number of spatial dimensions. 

The key idea is that 3D space was "frozen in" at this point when the Helmholtz density reached its first maximum value, prohibiting 3D space from transitioning to other dimensions.

This is because the second law allows transitions to higher dimensions only when the temperature is above this critical value, not below it. Since the universe is continuously cooling down, the current temperature is far below the critical temperature needed to transition from 3D space to a higher-dimensional space. "In the cooling process of the early universe and after the first critical temperature, the entropy increment principle for closed systems could have forbidden certain changes of dimensionality," the researchers explained.

In the future, the researchers plan to improve their model to include additional quantum effects that may have occurred during the first fraction of a second after the big bang, the so-called "Planck epoch." In addition, the results from a more complete model may also provide guidance for researchers working on such as quantum gravity.

Source; phys.org

More information: Julian Gonzalez-Ayala, Rubén Cordero and F. Angulo-Brown. "Is the (3 + 1)-d nature of the universe a thermodynamic necessity?" EPL. DOI: 10.1209/0295-5075/113/40006
Also (early version) at arXiv:1502.01843

Journal reference: Europhysics Letters (EPL)