27 October 2023

Visualizing the Relative Sizes of Everything in the Universe

The Universe is mind-boggingly huge. It's filled with everything from the tiniest of subatomic particles to superclusters of galaxies, not to mention even larger voids between them all.

Needless to say, the wildly different scales of all these things makes it very hard to fit them all onto a single chart for the purpose of comparing their relative sizes. But thanks in part to the power of logarithms to compare things of vastly different sizes, Charles Lineweaver and Vihan Patel have done that for a wide range of selected objects. Here's the "Plot of All Objects", the second figure from their 2023 paper "All objects and some questions":

Lineweaver, C.H. and Patel, V.M. All objects and some questions. Figure 2: Masses, sizes, and relative densities of objects in our Universe.

The online version of the paper features a brief video discussing the mysterious "forbidden by gravity" and "quantum uncertainty" which we cannot embed, we do recommend clicking through to it for the valuable introduction it provides.

Lineweaver and Patel explained why they collected and presented the information as they have in the conclusion of their purposefully thought-provoking paper:

Here, we provide an overview of the history of the Universe and the sequence of composite objects (e.g., protons, planets, galaxies) that condensed out of the background as the Universe expanded and cooled. We describe the role of the effective number of relativistic degrees of freedom (⁠g∗⁠) needed to understand the thermal history of the Universe during the first few minutes after the big bang. We compute and plot the background density and temperature of the Universe (Fig. 1). To extrapolate into the first billionth of a second, we make some common, explicit, but speculative assumptions.

We then make the most comprehensive pedagogical plot of the masses and sizes of all the objects in the Universe (Fig. 2). This plot draws attention to the unphysical regions forbidden by general relativity and quantum uncertainty—regions bounded by black holes and the Compton limit. The Compton limit creates an ambiguous region beyond which object size and position are conflated by quantum uncertainty, thus undermining the classical notion that the size of an object can be arbitrary small. Figure 2 also helps navigate the relationship between gravity and quantum mechanics and helps formulate some fundamental questions about the limits of physics: How can we interpret the regions forbidden by general relativity and quantum uncertainty? How should we interpret the fact that the two boundaries of the forbidden regions intersect at the instanton (Planck-mass black holes)? Are instantons the smallest possible objects? Do their size, density and temperature make them the best candidates for the initial conditions of the Universe (Fig. 1)? Is the Schwarzschild radius the minimum size for an object of a given mass? Or might the non-singular cores of black holes be objects with the Planck density?

Those are the kinds of questions whose answers might win their answerers Nobel prizes in physics.

References

Lineweaver, Charles H. and Patel, Vihan M. All objects and some questions. American Journal of Physics, 91, 819-825 (2023). DOI: 10.1119/5.0150209. [PDF document].