Scales of the Universe

Most of the essays in this section deal with different ‘perspectives’ in one way or another, but generally on the scale of human perception and opinion. However, there is possibly some value in considering this perspective in a wider context, in fact much wider, such that we might consider human activity on the ‘scales’ of the universe, both microscopically small and macroscopically large. As a generalisation, it might be accepted that most of us are often preoccupied with the day-to-day events that surround us, such that we often have little time, or even inclination, to consider the wider world, let alone the universe beyond.

But, why worry about the universe?

In many respects, this discussion is going to try to highlight that humanity only occupies a very small space on the scale of the universe. The diagram right is scaled in metres, such that a 1.8 metre (6ft) person exists at the [100] scale. The average human eye can see an object at the [10-4] scale and while the eye can see light from the stars, in more practical terms, the horizon on Earth, for a 1.8 metre person, would be about 5*103 metres, assuming that they were prepared to take their eye of their smartphone, held at less than 1*100 metres. Therefore, having some wider perspective of the scale of the universe may highlight the potentially precarious nature of our existence and the limited options for most of humanity beyond the relative safety of planet Earth. We might start with the following sobering thought about the scale of humanity.

If you could take away all the empty space in the subatomic particles of the 7.8 billion people on planet Earth, they might be compressed into a volume only a little larger than a grain of rice.

We might pursue this comparative, and possibly humbling, assessment of humanity on the microscopic scale. While people do not usually think of themselves as a collection of atoms, it is estimated that the average person is made up of about 7*1027 atoms, which if multiplied by 7.8*109 people would equate humanity to about 5.46*1037 atoms in total. While this might seem to be a very large number, everything is relative.

For example, an average planet like the Earth is estimated to have about 1050 atoms, such that people only account for about 0.000000000055% of this total. Just to clarify, as it is not always obvious, the exponential sum (1050-1037) approximates to an almost negligible difference from [1050], such that in terms of atoms, humanity hardly registers.

So, while humanity may have some justification for its own importance in the world, it might still benefit from a better understanding of the relative scale of our existence. If so, we might continue with the comparison of atoms, but now on the much wider scale of the universe. Typically, the planets of a solar system may only represent about 1% of the mass of the central star, e.g. the Sun, such that we might estimate the number of atoms in a star being in the order of 1052. We might then expand this number by assuming that a galaxy consists of 100 trillion (1014) stars, which would give us an estimated figure of 1066 atoms. Finally, we might extend this figure to a ballpark estimate to account for the possibility of a 100 trillion (1014) galaxies in the visible universe, which gives us a grand total of 1080 atoms. While this estimate should not be taken too seriously, we might attempt to crosscheck this estimate with a cosmological assessment of the scale of the universe.

Note: The size of the visible universe corresponds to its estimated age, which when based on the assumptions of the Big Bang model is estimated at 13.7 billion years. If we multiplied this age by the speed of light [c], we arrive at an estimated radius of the visible universe in the order of 13.7 billion light-years. By way of comparison, an Astronomical Unit (AU) is the distance of planet Earth from the Sun, such that the radius of the visible universe would equate to about a million, billion (1015) AU’s. From this figure we might estimate a volume of 1080 cubic metres. While this is a volume of space that the human mind may struggle to comprehend, most of interstellar space may have as few as 10 particles per cubic metre. If so, the number of particles in the visible universe would be in the region of 1081.

Again, while these estimates should not be taken too seriously, they possibly provide a comparative measure of the scale of the universe. However, as one final comparative step, the scale of the 'emptiness' of space might also be described in terms of the cosmological density model - see the cosmic calculator for further details. Within the limitations of this cosmological model, the present-day energy density is estimated to be 8.53*10-10 joules/m3. However, only 4% of this total can be attributed to any known mass particle, i.e. 3.41*10-11 joules/m3. However, in order to equate this energy-density to the concept of a particle mass, in kilograms [kg], we need to divide by [c2=9*1016], which gives a figure of ~10-28kg/m3.

Note: A mass-density of 10-28kg/m3 suggests that only 0.00000000000000000000000001% of the universe has any substance that might be quantified in terms of the standard particle model. While this is a limited perspective of the nature of space, as there is more to the universe than mass particles, our material existence in terms of atoms is virtually zero in comparison to the totality of the universe.

In part, the purpose of the previous outline was simply to highlight that on the scale of the universe, space is both very big and apparently very empty. However, the scale of this reality has not stopped humanity from assuming that it will simply ‘conquer space’ at some point in the future. We might characterise this idea in terms of the opening words of the science-fiction series, Star Trek, dating back to 1960’s.

Space: The final frontier. These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds. To seek out new life and new civilizations. To boldly go where no man has gone before!

While acknowledging this to be a noble ambition, we might also attempt to put our progress in achieving this ambition into some wider perspective based on the reality of the physical size of the universe. However, we might first consider the relative confines of our local solar system.

Planet   Kilometres Miles AU   Seconds Minutes Hours   Voyager
Mercury   5.79E+07 3.60E+07 0.39   193.0 3.2 0.05   979
Venus   1.08E+08 6.72E+07 0.72   360.7 6.0 0.10   1,829
Earth   1.50E+08 9.30E+07 1.00   498.7 8.3 0.14   2,529
Moon   3.84E+05 2.39E+05 0.00   1.28 0.02 0.0004   6
Mars   2.28E+08 1.42E+08 1.52   759.7 12.7 0.21   3,853
Jupiter   7.78E+08 4.84E+08 5.20   2,594.3 43.2 0.72   13,158
Saturn   1.43E+09 8.87E+08 9.54   4,756.7 79.3 1.32   24,126
Uranus   2.87E+09 1.78E+09 19.18   9,566.7 159.4 2.66   48,522
Neptune   4.48E+09 2.78E+09 29.94   14,930.0 248.8 4.15   75,725
Pluto   5.90E+09 3.67E+09 39.44   19,666.7 327.8 5.46   99,749

This table is separated into four sections, first we see the names of the planets in our local solar system with the exception of Earth’s moon. Second, are the distances from the Sun, except for the Moon, given in kilometres, miles and Astronomical Units (AU). Third, are times in seconds, minutes and hours for light to travel from the Sun to the planets. Again, the Moon is the exception, where the times shown are from the Moon to the Earth. Finally, on the right, is the equivalent estimated travel time of the spacecraft Voyager-1 to reach each of the planets in hours.

Note: Voyager-1 was launched in September 1977 and has continued to operate for 42 years and 10 months. During this time, it has reached a distance of 148.61 AU from Earth, i.e. 13.8 billion miles or 22.2 billion kilometres, such that an average velocity of 36,753 miles or 59,149 kilometres per hour can be calculated. However, for comparison with the speed of light [c=3*108 m/s], Voyager’s velocity is only 16,430 m/s or 0.0055% of [c].

While humanity’s ambitions have achieved reaching the Moon, we might recognise that they have fallen short of the earlier expectations of science fiction, e.g. 2001:A Space Odyssey. As such, we have yet to reach our nearest planetary neighbours and despite the times inferred in the table above, it is estimated that a one-way journey to Mars will probably take about 260 days – see Path to Mars and Space Developments or more details of the difficulties yet to be overcome.

But is this simply a defeatist perspective?

Well, while not reflecting the optimism of science-fiction, it is possibly more reflective of reality, at least, in the near-term of reaching another planet, even within the confines of our own solar system. Of course, the problems are not simply related to the limitations of current propulsion systems, but also involve the many and considerable problems of long-term survival outside the ecosystem of planet Earth – see Life Support Systems and Future in Space for more details.

Note: In this context, humanity might possibly want to reprioritise the importance of planet Earth to the survival of most of its human population, as well as all other species. For reality is telling us that we are a very little fish in a very big pond. In addition, we might also realise that as ‘little fishes’ we really have only evolved to survive in a very specific ‘little pond’.

While the previous table provided an initial perception of distance, and the travel times, we might recognise that the solar system is but one star, the Sun, in the Milky Way galaxy, where even the nearest star, Proxima Centauri, is over 4.25 lightyears away, such that it takes light 4.25 years to travel from our Sun to its nearest neighbour. However, this star is only one of an estimated 100+ billion stars in the Milky Way galaxy, which has a diameter in the order of 100,000 lightyears, where the nearest comparable neighbour galaxy, Andromeda, is separated by 2.5 million lightyears of space.

Object Kilometres Miles AU Seconds Minutes Lightyears Voyager
Proxima Centauri 4.02E+13 2.50E+13 2.69E+05 1.34E+08 2.23E+06 4.25E+00 77,601
Milky Way 9.46E+17 5.88E+17 6.32E+09 3.15E+12 5.26E+10 1.00E+05 1.83E+09
Andromeda galaxy 2.37E+19 1.47E+19 1.58E+11 7.88E+13 1.31E+12 2.50E+06 4.56E+10
Visible Universe 1.30E+23 8.05E+22 8.66E+14 4.32E+17 7.20E+15 1.37E+10 2.50E+14

So, while science-fiction may simply be able to engage its warp-engines and achieve velocities far in excess of the speed of light [c], the physical laws of the universe may actually impose far more restrictive limits on interstellar travel. Based on the previous estimate of the average velocity of Voyager-1, it would take 77,601 years to reach the nearest star. Even if we ignore all the technical problems of achieving relativistic velocities in the order of 0.1c, the journey time to the nearest star would still take over 40 years with no guarantee of a habitable planet that remotely approximates Earth, other than the possibility of having the raw materials to produce air and water. Again, science fiction may hold out the hope of terraforming another planet, such that it might approximate the atmosphere, temperature and ecology of Earth, but we might also need to reflect on the scale of  this ambition by an honest evaluation of our current ability to simply sustain the current ecosystem of Earth into the future.

Note: The goal of this short essay was not to be pessimistic about humanity’s future, only to inject some realism into the idea that we can simply ‘go where no man has gone before’ and survive. For the scales of the universe suggest that space is not only very big and very empty, but also very hostile to human life. If so, we should possibly put more priority on the problems of sustaining planet Earth, where most problems appear to have been self-inflicted - see Population & Resources plus Brave New Worlds for wider discussions.

This latter section has been added to this discussion by way of an addendum after seeing a video by Richard Dawkins, which can be referenced via the link in the question below.

Who Was the First Human?

While this addendum acknowledges that this video is the source of the idea, it attempts to elaborate on the timeline, such that it might be contextualise on the scales of the universe in respect to time, as well as distance. The idea of the video is based on a conceptual series of photographs of each generation stretching back in time to when the first ‘human’ might have appeared. However, the key insight of the idea is that while each generation must give birth to its own species, somewhere along this timeline a multitude of different species have emerged.

Note: Simply as a point of reference for this discussion, a species is defined as a group of organisms in which any two individuals can produce fertile offspring. However, over multiple generations, one species might evolve and diverge away from its origins, such that it becomes a separate species.

The timeline of planet Earth is estimated to have started some 4.5 billion years ago. At this point in time, and given the hostile environment, it might reasonably be assumed that no life existed. How life initially evolved remains a matter of debate, however, there is evidence that the very first life from which we might assume that everything evolved might have begun as a simple prokaryote cell some 3.8 billion years ago. Life, in this simple form is believed to have existed for some 2 billion years until the first, more complex, eukaryote cell evolved some 1.8 billion years ago. While this type of cell is normally considered as the evolutionary root of most of life on Earth, larger multi-cellular lifeforms only started to appear in an era described as the Cambrian Explosion some 541 million years ago. We might therefore try to summarise the evolution of life on Earth in terms of both years and generations, where a generation in human terms is simply approximated as 20 years.

Event Years Generations
Earth Formation 4,500,000,000  
Prokaryote Cell 3,800,000,000 190,000,000
Eukaryote Cell 1,800,000,000 90,000,000
Cambrian Explosion 541,000,000 27,050,000
Australopithecus 4,000,000 200,000
Homo Sapiens 300,000 15,000
Recorded History 10,000 500

Note: Over the period of some 541 million years, it is estimated that 99% of all species that previously existed, approximately 5 billion, are now extinct. Initially, it was estimated that the current number of species range between 10 to 14 million, where only about 1.2 million were documented, such that 86% were essentially unknown. However, in 2016, another scientific estimate suggested that there may be closer to 1 trillion species on Earth, if so, only 0.001% have been described.

So, as Dawkins points out, the series of photographs might be extended to 190 million generations, at least, on a human scale. However, we might reduce the photographs to the genus of hominids that existed in Africa from around 4.2 to 1.9 million years ago to Australopithecus from which the genus Homo, which includes Homo Sapiens, to a much smaller number of generations, i.e. 15,000. However, all of human history only amounts to some 500 generations on this scale. On this basis, it might be suggested that human existence only appears as an insignificant blip on the scales of the universe, when measured in terms of either space or time. So, while humanity is allowed to dream of its potential, but not guaranteed future, it possibly should do so with some humility of its overall position in the universe.