# How Big Is Our Solar System?

Defining the diameter of the Solar System is a matter of perspective and characterization. You can look at the Solar System’s diameter as ending at the aphelion of the orbit of the farthest planet, the edge of the heliosphere, or ending at the farthest observable object.

To cover all of the objective bases, we will look at all three. Looking at the aphelion(according to NASA figures) of the orbit of the farthest acknowledged planet, Neptune, the Solar System would have a radius of 4.545 billion km and a 9.09 billion km diameter. This diameter could change if the dwarf planet Eris is promoted after further study.

Sedna is three times farther away from Earth than Pluto, making it the most distant observable object known in the solar system. It is 143.73 billion km from the Sun, thus giving the Solar System a diameter of 287.46 billion km. Now, that is a lot of zeros, so let’s simplify it into astronomical units.1 AU(distance from the Earth to the Sun) equals 149,597,870.691 km.

Based on that figure, Sedna is nearly 960.78 AU from the Sun and the Solar System is 1,921.56 AU in diameter. A third way to look at the diameter of the Solar System is to assume that it ends at the edge of the heliosphere. The heliosphere is often described as a bubble where the solar wind pushes against the interstellar medium and edge of where the Sun’s gravitational forces are stronger than those of other stars.

The heliopause is the term given as the edge of that influence, where the solar wind is stopped and the gravitational force of our Sun fades. That occurs at about 90 AU, giving the Solar System a diameter of 180 AU. If the Sun’s influence ends here, how could Sedna be considered part of the Solar System, you may wonder.

1. While it is beyond the heliopause at aphelion, it falls back within it at perihelion(around 76 AU).
2. Those determinations of the diameter of the Solar System may seem about as clear as mud, but they give you an idea of what scientists are trying to place a definitive value on.
3. The distances involved are mind boggling and there are too many unknowns to place a absolute figure.

Perhaps, an exact number will be determinable as the Voyager probes continue their outward journey. Here’s an article on Universe Today about the closest star to Earth, and another about how long it would take to travel to the closest star, Here’s an article from the Physics Factbook about the diameter of the Solar System, and a cool way to visualize it using the Earth as a peppercorn.

### What is the size of the solar system in light years?

Now that you know how far a light year is, consider the fact that the diameter of our Solar System is approximately Page 2 7,440,000,000 miles, 80 AU, or about.00127 light years.

### How big is our solar system compared to the universe?

Did you Know? –

• Around 550 people have been into Space, and only three of them have died in accidents.
• The smallest thing in the Universe that we currently know of is the atom.
• The biggest thing we have discovered so far in our Universe is the Hercules-Corona Borealis Great Wall. It is a supercluster which has a diameter of around 10 billion light-years.
• Many believe that our Universe is only one of a set of separate universes, collectively known as the multiverse.
• The word cosmos, rather than Universe, implies viewing the Universe as a complex and orderly system or deity – the opposite of chaos.
• The observable Universe is 93 billion light-years, yet, our galaxy, the Milky Way, is just 100,000 light-years in diameter. It would take us endless generations just to explore our galaxy, let alone the Universe.
• Another ancient structure is the galaxy supercluster known as the Hyperion Supercluster. This celestial object has more than four quadrillion solar masses, and many estimate that it formed just 2 billion years after the Big Bang.
• Universe means “whole,” and it comes from the Latin word “universus.”

## How long would it take to leave our solar system?

How Long Would It Take to Cruise the Solar System? – While we may be just a speck in the Milky Way, and while the Milky Way may be just a speck on the landscape of the universe, our solar system is still really, really big. As we’re sure you remember from grade school, the solar system is the group of local planets, asteroids, and other small objects that orbit our sun.

Traveling outwards from the sun, we have Mercury, Venus, Earth, and Mars. After Mars comes the asteroid belt, a chaotic cluster comprised mostly of rock and metal. Then come the gas giants, Jupiter and Saturn, followed by the icy outer planets Uranus and Neptune. Beyond Neptune lie “trans-Neptunian objects” – including things like comets, dust clouds, natural satellites, and dwarf planets like Eris, and Pluto.

Finally, and far beyond everything else, there’s the Oort cloud – a theoretical cloud consisting of space dust and debris that marks the solar system’s end. While experts firmly believe that this cloud exists, no direct observations have been made of it.

It’s just too far away. But just how far does our solar system spread? What would you have to do to reach its outer edges? And how long would it take? Luckily, we have a nifty little device called Voyager 1 offering up a point of reference, traversing the outer reaches of our solar system as we speak.

The Voyager 1 space probe was launched by NASA on September 5th1977, and is currently still traveling at around 35,000 miles per hour. It reached Saturn, its primary target, in November 1980. But on August 25th2012, nearly 35 years after launch, it made even greater history by becoming the first spacecraft to enter the interstellar medium – a fancy term for the space between star systems in a galaxy.

• More specifically, Voyager 1 broke from the reach of the sun’s solar winds and entered into deep space.
• It’s currently at a distance of 21.2 billion kilometers from the sun, and is so far away from us that it takes a radio signal, traveling at the speed of light, roughly 17 hours to beam between the spacecraft and our home planet.

Sounds like a lot, right? Well, while the monumental achievements of Voyager 1 should never be underestimated, we’re still very far from making an even semi-significant dent in the solar system as a whole. It would take Voyager 1 another 300 years to reach just the inner edge of the interstellar Oort cloud, and up to 40,000 years to breach the cloud and finally break free from our solar system completely.

In truth, the solar system, and space in general, is just way too big for Earthly measurements like miles and meters. To measure distance in space, astronomers use something called an astronomical unit – with one unit equaling the average distance from Earth to the sun, or 150 million kilometers. If you were to somehow drive a car to the sun at highway speeds of 100 km/hr, you’d eventually reach your destination in 1,500,000 hours – or 171 years.

So no one’s surviving that! But let’s say you splashed the cash, pulled some strings, and took the fastest airbreathing, manned aircraft in the world – the Lockheed SR-71 Blackbird. Traveling at constant top speed, it would take you just 42,492 hours to reach the sun, or just under five years.

In contrast, definitely doable! But let’s be serious. The distance from the Earth to the sun is infinitesimal when compared to the size of the entire solar system. Way, way out there is everyone’s favorite dwarf planet – Pluto. Pluto ranges from 30 to 49 astronomical units away from the sun. So, at its average distance, it’s almost six billion kilometers away.

Light traveling from the sun takes about five and a half hours to reach Pluto. Driving a car at highway speeds, it would take you 6,849 years. But as we’ve established, the solar system extends far beyond Pluto. The outer edges of the Oort cloud are about 100,000 astronomical units away, or about 1.87 light years, or seventeen trillion kilometers – give or take.

Amazingly, the sun’s gravity can capture objects as far out as two light years away, meaning that the outer part of the Oort cloud is still theoretically shaped by the sun’s gravity. Just beyond the cloud’s outer edges is the halfway point between our sun and the next nearest star, Proxima Centauri. Beyond that, and you’re swapping systems.

So, in boring astrological terms, it’d take you nearly two years to reach the outer boundary of our solar system if you were traveling at the speed of light. But we can’t do that, can we? Let’s hop back into our hypothetical space car and go for the ultimate cruise.

Hell, you know what? Let’s do one better. Let’s forgo the standard highway driving and up the stakes. Let’s pretend that we’re in the fastest street-legal car in the world – the Bugatti Veyron. And let’s imagine we can travel at its max speed of 431 km/hr. Heading for the Oort cloud, traveling at the top speed of the fastest car in the world, it’d be four-and-a-half million years before you finished your trip.

And that’s without stopping for fuel or snacks. Now, let’s again pilot the fastest ever aircraft, the Lockheed SR-71 Blackbird. The Blackbird’s eight times faster than the Veyron at top speed, so the trip does shorten. But it’d still take an eye-watering 550,000 years to reach the fabled finish line.

1. Finally, let’s say we hitched a ride on NASA’s New Horizons probe, which left Earth at a staggering – and record-breaking – 58,536 km/hr.
2. It took this probe nine years to flyby Pluto, so even if it continues its unprecedented pace, it won’t breach the Oort cloud for another 30,000 years, or more.
3. How long it takes to cruise the solar system depends entirely on what you want to cruise it in.

In just your standard supercar? Four-point-five million years. In a high-spec aircraft? 550,000 years. On a NASA probe, and the fastest object ever launched from Earth? 30,000. No matter what you travel in, you’d be long dead and space dust way before you’d even considered the Oort cloud.

1. Of course, if you chose to travel at light speed, you’re looking at only around 1,000 days.
2. But A) that’s impossible, and B) that’s cheating.
3. So, yeah, the solar system is pretty darned big.
4. And remember, it’s just one small speck in the Milky Way, which is but a tiny grain in the grand scheme of the entire universe.

So next time you’re complaining about your daily commute, remember this video and relax.

## How many galaxies are in the universe?

How many galaxies are in the Universe? — March 8, 2022 Artist’s logarithmic scale conception of the observable universe. The Solar System gives way to the Milky Way, which gives way to nearby galaxies which then give way to the large-scale structure and the hot, dense plasma of the Big Bang at the outskirts.

1. Each line-of-sight that we can observe contains all of these epochs, but the quest for the most distant observed object will not be complete until we’ve mapped out the entire Universe.
2. Pablo Carlos Budassi; Unmismoobjetivo/Wikimedia Commons) When you gaze up at the night sky, through the veil of stars and the plane of the Milky Way close by, you can’t help but feel small before the grand abyss of the Universe that lies beyond.

Even though nearly all of them are invisible to our eyes, our observable Universe, extending tens of billions of light years in all directions, contains a fantastically large number of galaxies within it. The exact number of galaxies out there has been a mystery, with estimates rising from the thousands to the millions to the billions, all as telescope technology improved. Our deepest galaxy surveys can reveal objects tens of billions of light years away, but even with ideal technology, there will be a large distance gap between the farthest galaxy and the Big Bang. At some point, our instrumentation simply cannot reveal them all.

(: Sloan Digital Sky Survey). In an ideal world, we’d simply count them all. We’d point our telescopes at the sky, cover the entire thing, collect every photon emitted our way, and detect every object that was out there, no matter how faint. With arbitrarily good technology and an infinite amount of resources, we’d simply measure everything in the Universe, and that would teach us how many galaxies are out there.

But in practice, that won’t work. Our telescopes are limited in size, which in turn limits how many photons they can collect and the resolutions they can achieve. There’s a trade-off between the faintness of an object you can see and how much of the sky you can take in at once. The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to perfectly smooth the Universe gets, but there is a limit to the smoothness it could’ve achieved, otherwise we wouldn’t have any structure at all today.

To explain it all, we need a modification to the Big Bang: cosmological inflation. (: NASA/ESA/A. Feild (STScI)) So what we can do, instead, is to view a clear portion of the Universe without intervening matter, stars, or galaxies as deeply as possible. The longer you stare at a single patch of sky, the more light you’ll collect and the more you’ll reveal about it.

We first did this in the mid-1990s with the Hubble Space Telescope, pointing at a patch of sky that was known to have practically nothing in it, and simply sit on that spot and let the Universe reveal what was present. The blank region of sky, shown in the yellow L-shaped box, was the region chosen to be the observing location of the original Hubble Deep Field image.

With no known stars or galaxies within it, in a region devoid of gas, dust, or known matter of any type, this was the ideal location to stare into the abyss of the empty Universe. (: NASA/Digitized Sky Survey; STScI) It was one of the riskiest strategies of all-time. If it failed, it would have been a waste of over a week of observing time on the newly-corrected Hubble Space Telescope, the most sought-after data collection observatory.

But if it succeeded, it promised to reveal a glimpse of the Universe in a way we had never seen before. We collected data for hundreds of orbits, across a multitude of different wavelengths, hoping to reveal galaxies that were fainter, more distant, and harder to see than any we had detected before. The original Hubble Deep Field image, for the first time, revealed some of the faintest, most distant galaxies ever seen. Only with a multiwavelength, long-exposure view of the ultra-distant Universe could we hope to reveal these never-before-seen objects.

(: R. Williams (STScI), Hubble Deep Field Team/NASA) Everywhere we looked, in all directions, there were galaxies. Not just a few, but thousands upon thousands of them. The Universe wasn’t empty and it wasn’t dark; it was full of light-emitting sources. As far as we were capable of seeing, stars and galaxies were clumped and clustered everywhere.

But there were other limits. The most distant galaxies are caught up in the expansion of the Universe, causing distant galaxies to redshift past the point where our optical and near-infrared telescopes (like Hubble) could detect them. Finite sizes and observing times meant that only the galaxies above a certain brightness threshold could be seen. Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it.

(: Marla Geha/Keck Observatory) So we could push past our technological limits from that mid-1990s image, but even so, we could never record all the galaxies. The best attempt we ever made was the Hubble eXtreme Deep Field (XDF), which represented a composite image of ultraviolet, optical, and infrared data.

By observing just a tiny patch of sky so small it would take 32 million of them to cover all the possible directions we could look, we accumulated a total of 23 days worth of data. Stacking everything together into a single image revealed something never-before seen: a total of approximately 5,500 galaxies. Various long-exposure campaigns, like the Hubble eXtreme Deep Field (XDF) shown here, have revealed thousands of galaxies in a volume of the Universe that represents a fraction of a millionth of the sky. But even with all the power of Hubble, and all the magnification of gravitational lensing, there are still galaxies out there beyond what we are capable of seeing.

(: NASA/ESA/H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), and Z. Levay (STScI)) You might think, therefore, that we could estimate the number of galaxies in the Universe by taking the number we observed in this image and multiplying it by the number of such images it would take to cover the entire sky.

In fact, you can get a spectacular number by doing so: 5500 multiplied by 32 million comes out to an incredible 176 billion galaxies. But that’s not an estimate; that’s a lower limit. Nowhere in that estimate do the too-faint, too-small, or too-close-to-another galaxies show up. Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this ought to be taken to the extreme, and remains valid as far back as we’ve ever seen.

the ingredients that make up the Universe,the right initial conditions that reflect our reality,and the correct laws of physics that describe nature,

we can simulate how such a Universe evolves. We can simulate when stars form, when gravity pulls matter into large enough collections to create galaxies, and to compare what our simulations predict with the Universe, both near-and-far, that we actually observe.

Perhaps surprisingly, there were more galaxies in the early Universe than there are today. But unsurprisingly, they’re smaller, less massive, and are destined to merge together into the old spirals and ellipticals that dominate the Universe we inhabit at present. The simulations that match best with reality contain dark matter, dark energy, and small, seed fluctuations that will grow, over time, into stars, galaxies, and clusters of galaxies.

Most remarkably, when we look at the simulations that match the observed data the best, we can extract, based on our most advanced understanding, which clumps of structure should equate to a galaxy within our Universe. A simulation of the large-scale structure of the Universe. Identifying which regions are dense and massive enough to correspond to galaxies, including the number of galaxies that exist, is a challenge that cosmologists are only now just rising to. (: Zarija Lukic/Berkeley Lab) When we do exactly that, we get a number that’s not a lower-limit, but rather an estimate for the true number of galaxies contained within our observable Universe. Two nearby galaxies as seen in the ultraviolet view of the GOODS-South field, one of which is actively forming new stars (blue) and the other which is just a normal galaxy. In the background, distant galaxies can be seen with their stellar populations as well.

Even though they’re rarer, there are still late-time galaxies actively forming massive amounts of new stars. (: NASA, ESA, P. Oesch (University of Geneva), and M. Montes (University of New South Wales)) Over time, galaxies merged together and grew, but small, faint galaxies still remain today. Even in our own Local Group, we’re still discovering galaxies that contain mere thousands of stars, and the number of galaxies we know of have increased to more than 70.

The faintest, smallest, most distant galaxies of all are continuing to go undiscovered, but we know they must be there. For the first time, we can scientifically estimate how many galaxies are out there in the Universe. The next step in the great cosmic puzzle is to find and characterize as many of them as possible, and understand how the Universe grew up.

## How long would it take to travel 100 light-years?

It’ll take about 1.6 million years to travel 100 light years.

#### Is the universe infinite?

Tanya Hill, Astronomer – yes – There’s a limit to how much of the universe we can see. The observable universe is finite in that it hasn’t existed forever. It extends 46 billion light years in every direction from us. (While our universe is 13.8 billion years old, the observable universe reaches further since the universe is expanding).

• The observable universe is centred on us.
• An alien in a galaxy far away would have its own observable universe.
• While there may be some overlap, they would inevitably see regions we can’t see.
• Therefore, it’s not possible to see if the universe is finite, because we can’t see it all.
• Instead, we can tackle this question by exploring the universe’s shape.

While we don’t know the shape of all space, we do know our part of space is flat. This means two rockets flying parallel on cruise control will always remain parallel. Because space isn’t curved they will never meet or drift away from each other. A flat universe could be infinite: imagine a 2D piece of paper that stretches out forever.

• But it could also be finite: imagine taking a piece of paper, making a cylinder and joining the ends to make a torus (doughnut) shape.
• Therein lies the problem.
• Additionally, there are many ways the universe could have been curved, but instead we live in a region of flat space.
• This is a very specific condition and we use a theory called “inflation” to explain it.

Inflation is the idea that very early on the universe rapidly expanded for a brief moment, smoothing out all the kinks and curvatures in our part of space. After inflation, the universe grew to what we see today. But it’s possible inflation didn’t just seed our universe.

### What is at the edge of the universe?

More like this – The observable Universe is bounded by a ‘cosmic horizon’, much like the horizon at sea. Just as we know there’s more ocean over the horizon, we know there are more galaxies (possibly an infinite number) beyond the cosmic horizon. Their light simply hasn’t had time to reach us yet. Read more about the science of the Universe:

Inside the simple computer program that could explain why the Universe exists at all What if the Big Bang was not the beginning? Ghost stars: The radical theory that could solve the mystery of dark matter

#### Will humans ever reach another galaxy?

Intergalactic travel is the hypothetical crewed or uncrewed travel between galaxies, Due to the enormous distances between the Milky Way and even its closest neighbors —tens of thousands to millions of light-years —any such venture would be far more technologically and financially demanding than even interstellar travel,

Intergalactic distances are roughly a hundred-thousandfold (five orders of magnitude) greater than their interstellar counterparts. The technology required to travel between galaxies is far beyond humanity’s present capabilities, and currently only the subject of speculation, hypothesis, and science fiction,

However, theoretically speaking, there is nothing to conclusively indicate that intergalactic travel is impossible. There are several hypothesized methods of carrying out such a journey, and to date several academics have studied intergalactic travel in a serious manner.

#### Will humans travel to other galaxies?

Intelligent life probably exists on distant planets — even if we can’t make contact, astrophysicist says Recently released Navy videos of what the U.S. government now classifies as “unidentified aerial phenomena” have set off another round of speculative musings on the possibility of aliens visiting our planet.

Like other astrophysicists who have weighed in on these sightings, I’m skeptical of their extraterrestrial origins. I am confident, however, that intelligent life-forms inhabit planets elsewhere in the universe. Math and physics point to this likely conclusion. But I think we’re unlikely to be able to communicate or interact with them — at least in our lifetimes.

Wanting to understand what’s “out there” is a timeless human drive, one that I understand well. Growing up in poorer and rougher neighborhoods of Watts, Houston’s Third Ward and the Ninth Ward of New Orleans, I was always intrigued by the night sky even if I couldn’t see it very easily given big-city lights and smog.

And for the sake of my survival, I didn’t want to be caught staring off into space. Celestial navigation wasn’t going to help me find my way home without getting beaten up or shaken down. From early childhood, I compulsively and continuously counted the objects in my environment — partly to soothe my anxieties and partly to unlock the mysteries inside things by enumerating them.

This habit earned me nothing but taunts and bullying in my hood where, as a bookish kid, I was already a soft target. But whenever I looked up at a moonless night sky, I wondered how I might one day count the stars. By age 10, I’d become fascinated, even obsessed, with Einstein’s theory of relativity and the quantum possibilities for the multiple dimensions of the universe it opened up in my mind.

By high school, I was winning statewide science fairs by plotting the effects of special relativity on a first-generation desktop computer. So perhaps it’s not surprising that I have gone on to spend much of my career working with other astrophysicists to develop telescopes and detectors that peer into the remote reaches of space and measure the structure and evolution of our universe.

The international collaboration has been mapping hundreds of millions of galaxies, detecting thousands of supernovae, and finding patterns of cosmic structure that reveal the nature of dark energy that is accelerating the expansion of our universe. Meanwhile, the will make trillions of observations of 20 billion stars in the Milky Way.

1. What we’re discovering is that the cosmos is much vaster than we ever imagined.
2. According to our best estimate, the universe is home to a hundred billion trillion stars — most of which have planets revolving around them.
3. This newly revealed trove of orbiting exoplanets greatly improves the odds of our discovering advanced extraterrestrial life.

Scientific evidence from astrobiology suggests that simple life — composed of individual cells, or small multicellular organisms — is ubiquitous in the universe. It has probably occurred multiple times in our own solar system. But the presence of humanlike, technologically advanced life-forms is a much tougher proposition to prove.

1. It’s all a matter of solar energy.
2. The first simple life on Earth probably began underwater and in the absence of oxygen and light — conditions that are not that difficult to achieve.
3. But what enabled the evolution of advanced, complex life on Earth was its adaptation to the energy of the sun’s light for photosynthesis.

Photosynthesis created the abundant oxygen on which high life-forms rely. It helps that Earth’s atmosphere is transparent to visible light. On most planets, atmospheres are thick, absorbing light before it reaches the surface — like on Venus. Or, like Mercury, they have no atmosphere at all.

• Earth maintains its thin atmosphere because it spins quickly and has a liquid iron core, conditions that lead to our strong and protective magnetic field.
• This magnetosphere, in the region above the ionosphere, shields all life on Earth, and its atmosphere, from damaging solar winds and the corrosive effects of solar radiation.

That combination of planetary conditions is difficult to replicate. Still, I’m optimistic that there have been Cambrian explosions of life on other planets similar to what occurred on Earth some 541 million years ago, spawning a cornucopia of biodiversity that is preserved in the fossil record.

The more expert we become in observing and calculating the outer reaches of the cosmos, and the more we understand about how many galaxies, stars and exoplanets exist, the greater the possibility of there being intelligent life on one of those planets. For millennia, humans have gazed in wonder at the stars, trying to understand their nature and import.

We developed telescopes only a few hundred years ago, and since then the dimensions of our observable universe have expanded exponentially with technological advances and the insights of quantum physics and relativity. Beginning in the early 1960s, scientists have tried to calculate the odds of advanced extraterrestrial life.

In 1961, researchers at the NASA-funded search for extraterrestrial intelligence (SETI) developed the “Drake Equation” to estimate how many civilizations in the Milky Way might evolve to develop the technology to emit detectable radio waves. Those estimates have been updated over the decades, most recently by Sara Seager’s group at MIT, based on observations of exoplanets outside our solar system by successive generations of advanced space-based telescopes — such as the Kepler Space Telescope, launched in 2009, and NASA’s MIT-led, launched in 2018.

Detecting the presence of life on exoplanets requires large telescopes outfitted with advanced spectroscopy instruments, which is what the will deliver when it launches in November. In 1995 the first exoplanet was discovered orbiting Pegasus 51, 50 light-years distant from Earth.

Since then, there have been more than 4,000 confirmed discoveries of exoplanets in our galaxy. More important, astronomers agree that almost all stars have planets, which radically improves the odds of our discovering intelligent life in the universe. At the low end of consensus estimates among astrophysicists, there may be only one or two planets hospitable to the evolution of technologically advanced civilizations in a typical galaxy of hundreds of billions of stars.

But with 2 trillion galaxies in the observable universe, that adds up to a lot of possible intelligent, although distant, neighbors. If only one in a hundred billion stars can support advanced life, that means that our own Milky Way galaxy — home to 400 billion stars — would have four likely candidates.

Of course, the likelihood of intelligent life in the universe is much greater if you multiply by the 2 trillion galaxies beyond the Milky Way. Unfortunately, we’re unlikely to ever make contact with life in other galaxies. Travel by spaceship to our closest intergalactic neighbor, the Canis Major Dwarf, would take almost 750,000,000 years with current technology.

Even a radio signal, which moves at close to the speed of light, would take 25,000 years. The enormity of the cosmos confronts us with an existential dilemma: There’s a high statistical likelihood of intelligent life-forms having evolved elsewhere in the universe, but a very low probability that we’ll be able to communicate or interact with them.

1. Regardless of the odds, the existence of intelligent life in the universe matters deeply to me, and to most other humans on this planet.
2. Why? I believe it’s because we humans are fundamentally social creatures who thrive on connection and wither in isolation.
3. In the past year, many of us felt the hardship of isolation as deeply as the threat of a potentially fatal infectious disease.

Enforced seclusion during the pandemic tested the limits of our tolerance for separation and made us acutely aware of our interdependence with all life on Earth. So, it’s no wonder that the idea of a trackless universe devoid of intelligent life fills us with the dread of cosmic solitary confinement.

For a hundred years, we’ve been emitting radio signals into space. For the past 60 years, we’ve been listening — and so far, in vain — for the beginning of a celestial conversation. The prospect of life on other planets remains a profound one, regardless of our ability to contact and interact with them.

As we await evidence of extraterrestrial intelligence, I draw comfort from the knowledge that there are many powerful forces in the universe more abstract than the idea of alien intelligence. Love, friendship and faith, for example, are impossible to measure or calculate, yet they remain central to our fulfillment and sense of purpose.

As I head into my mid-50s, I look forward with an infinity of hope to the moment when humans will finally make contact with extraterrestrial intelligence — in whatever far-flung star system they may live, and in whatever century or millennium moment that momentous meeting may occur. Until that day, I have no doubt that generations of young humans around the globe will continue to stand watch, looking skyward with the same sense of amazement and wonder that intoxicated me as a young boy.

Hakeem Oluseyi, president-elect of the National Society of Black Physicists, has taught and conducted research at MIT, University of California at Berkeley and the University of Cape Town. His memoir, “” co-written with Joshua Horwitz, was published last week.

#### How heavy is the universe?

The Mass of the Universe I FIND certain difficulties in connexion with the mass of the universe considered as a finite sphere of radius 4.9 × 10 23 miles and of volume 5.2 × 10 71 cubic miles. Eddington gives the mass of the universe as 10 22 stars averaging our sun in weight.

1. Taking 2.0 × 10 27 tons as the sun’s weight, then the mass of the universe would be 2.0 × 10 49 tons.
2. There are, says Eddington, 1.575 × 10 79 electrons and an equal number of protons in the universe.
3. Assuming the mass of these units to be respectively 9.038 × 10 -28 gm.
4. And 1.65 × 10 -24 gm., then weight of the electrons must be 1.4235 × 10 52 gm.

and that of the protons 2.598 × 10 55 gm.; their combined masses would amount to 2.599 × 10 55 gm. or 2.55 × 10 49 tons, which is a fairly close approximation to the weight of the universe calculated on the basis of stars. : The Mass of the Universe

### How far in space can we see?

How Far Away is the Edge of the Universe? | Museum of Science, Boston We ask Museum educator Janine all your questions about how far away things are, from the Moon to the end of the universe, during this Pulsar podcast brought to you by #MOSatHome. We ask questions submitted by listeners, so if you have a question you’d like us to ask an expert, send it to us at [email protected]

ERIC: At the Museum of Science, we’re often asked how far away things are in space. The simple answer is, really, really far away. Today on Pulsar, we’ll get some more exact answers, starting with the closest things to our home planet and making our way out to the edge of the universe. And along the way, we’ll find out: how do we know how far away these things are? Thanks to Facebook Boston for supporting this episode of Pulsar.

I’m your host, Eric, And my guest today is Janine from our forums department. Janine, thanks so much for going on this journey through the universe with me. JANINE: Yeah, absolutely, happy to be here. ERIC: So let’s start with the closest natural object to us here on the Earth.

1. How far away is the moon? JANINE: OK, so I’ll use a unit of measurement that you’re probably pretty familiar with.
2. It’s about 238,855 miles on average, and I say on average, because the distance does change.
3. The moon does not orbit the Earth in a perfect circle, but that’s kind of an abstract thing, and it doesn’t really mean anything to you, right? So if the Earth was the size of a basketball, the moon would be about the size of a tennis ball.
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They would be about 23 feet, 9 inches apart, which is about 30 earths, which is crazy to me. ERIC: It’s further away than you would think. JANINE: It really is. I always think everything in space has more space than we expect it to, so even our closest neighbor is 30 times away the size we are.

• Everything that you put on a mission to go into space costs fuel, so the more fuel you have, so to go faster, would actually make you weigh more, so there’s this balance of power and efficiency, and you’re always trying to make it as light as possible.
• It was kind of more of a circle around the Earth and then a couple of circles around the moon and then a landing rather than a straight shot.
• ERIC: So we could have got there a little bit quicker than four days, but not too much quicker.
• JANINE: Yeah, I think they say, on average, over the course of all of the missions is about three days to get from Earth to the moon.

ERIC: So we haven’t sent any astronauts to the moon in nearly 50 years. Lately, they spend their time on the International Space Station. How far away from Earth’s surface is that? JANINE: So that’s actually a lot closer. It’s only about 254 miles away, and I was trying to figure out what cities on the Earth are at least in the US are close to that distance, and I figured out it’s about the distance if you were to fly from LA to Las Vegas.

ERIC: And the next object on our list at the center of the solar system, the sun. How far away is that? JANINE: So sun is our closest star, and it’s 92 million miles away, which is crazy, and now we’re starting to get to these distances in space where talking about them in miles really doesn’t mean anything.

So actually, the average distance from the Earth to the sun is a unit that astronomers used called an astronomical unit, so we’ve just decided that, for math, it’s a lot easier to figure out, we’ll just say that the distance from the Earth to the sun is 1, and then all of our math can be easier.

If you could travel at the speed of light, which you can’t because you’re made of mass, but if you could, it would take 8.3 minutes. The thing that blows my mind away about this is, since it takes eight minutes for light to travel, the sun could go out suddenly, and we wouldn’t know about it for eight minutes.

ERIC: Because it would take eight minutes for light to stop showing up on Earth. JANINE: Yeah, it’s crazy. ERIC: So jumping right out to the edge of our neighborhood, we often get asked how big the solar system is. So how far away is the edge of the solar system? Does it even have an edge? JANINE: OK, so it’s hard to talk about the solar system and what does it mean to be part of the solar system.

1. We’re considering the things in the solar system to be the things that are most pulled on by the sun, and so that’s at the edge of the Oort cloud, and to go back to that unit of the astronomical unit, that’s about 100,000 astronomical units away.
2. ERIC: So start on Earth, head past the sun, then go 100,000 times further than that before you leave the solar system.
3. JANINE: Yeah, isn’t that nuts?

ERIC: It is. That’s already so far, and speaking of that, when we mentioned the outer part of the solar system, we get asked about the robots that we’ve sent deep into space. So how far away is the furthest spacecraft that we’ve launched from the earth? JANINE: OK, so I looked this up yesterday.

So it’s a little bit further out now, but since we’re talking about astronomy, everything in astronomy has a big error range anyway, so that’s fine. Voyager 1, which was launched in 1977 is about 150 astronomical units away from the Earth. ERIC: So that’s wicked far, but it’s not anywhere close to leaving behind the effect of the sun’s gravity.

OK, so leaving the solar system behind, what’s the next closest star to us and how far away is it? And since this question comes up a lot how long would it take a rocket to get there? JANINE: So the closest star to us is actually part of a three star system.

The closest one of those three stars is Proxima Centauri, which is 4.22 light years away, and so if you could travel at the speed of light, it would take you 4.22 years to get there, but we can’t travel at the speed of light, so how long would it take Voyager 1 to get there? It would take over 73,000 years.

ERIC: So using current rocket technology, we’re just not going to get there any time soon. JANINE: No. No, space, as I think we’re going to establish in this podcast, is very big. ERIC: Now, before we continue our journey, this would be a good place to bring up a question we got from Sophie.

• The planets are pretty easy to measure, we’ve been to them all, we can see them moving, how can we measure the distance to stars and galaxies?
• JANINE: Yeah, so astronomers actually use a bunch of different tools, and we call it the distance ladder, although I like to think about it as if you had a bunch of yardsticks and you tried to tape them together and that first yardstick is really strong and by the end it’s bending over and not super great, because our error of knowing what is correct and how accurate something is increases as we use different steps on this ladder.
• But the first step that you can use is called parallax, and you can actually do an experiment with this right now if you want to.

You can hold a finger in front of your face and close your left eye and then close your right eye and look at what happens behind it. And you’ll notice that, with respect to the things behind it, it moves in front, just because there’s a little bit of distance between each eye.

1. And so we can do that with stars, but not with our eyes, because that’s too small of a distance with respect to how far away stars are.
2. ERIC: Yeah, stars don’t seem to move too much if you just go outside and wink at them back and forth a bunch of times.
3. JANINE: Yeah, so what we can actually do is use the Earth in its orbit as that kind of blinking, and so if we go out and measure in June and then we go out and measure in December, now we’ve got six months apart so we’re halfway around the sun.

So we’ve got that entire distance, which is 2 AU, going back to that astronomical unit is the longest baseline we can get while we’re on Earth. And we can look at stars and see how they change with respect to the things behind them, and that’s how we can get a direct distance.

ERIC: So parallax seems pretty good for stars that are fairly close, but you mentioned other methods too. So what’s next? JANINE: Yeah, so the next step is something called a standard candle, and actually the first standard candle was discovered not too far from the Museum of Science by Henrietta Swann Levitt at the Harvard College Observatory back in the early 1900s.

She was a computer there. If you’re interested in this at all, there’s a really good book called The Glass Universe that talks about all of these computers who worked at the Harvard College Observatory, including Annie Jump Cannon, who’s very famous for figuring out the brightness of stars, a relationship about that.

1. Henrietta Swan Levitt determined this first standard candle.
2. So she was working at the Harvard College Observatory, examining photographic plates from telescopes.
3. So these telescopes were taking all these images and they needed people to reduce the data, which is something that a lot of physical computers do now, but people did back then.

And she was looking at a particular type of star called a Cepheid variable, and she realized that there was some sort of a relationship between how fast they dimmed and brightened and what their brightness was. These Cepheid variables are very consistent, so she had this idea that, because luminosity and period are the same, maybe they could be used to figure out how far away something is.

So the standard candle idea is that a candle has an intrinsic brightness that we know. We can determine it because of some sort of physical relationship or just studying physics in general. This star, if we know this other thing about it, we know how bright it is if you were standing at a certain distance from it.

OK, so if we know how bright it should be and we know how bright we’re observing it, we can actually figure out the distance based on that, right? If you know how bright your flashlight is and you know how bright you’re seeing it, you can figure out how far away it is.

ERIC: So the further away something is, the dimmer it appears to us, and if we know its true brightness, it’s pretty easy math to calculate how far away it must be to appear how we see it. JANINE: Yeah, exactly. So they figured out that these Cepheid variables could be used in this way as a standard candle.

Although, my personal favorite standard candle is a type 1A supernova.

• And that’s entirely because, when I was in college, I worked on a project on SS Cygni, which is a very well known cataclysmic variable.
• And what a cataclysmic variable is is it’s a red giant star, and it has a partner a star, a binary star companion, called a white dwarf, and actually, most stars in the galaxy are in multiple star systems, so it’s pretty normal to find a binary star system.
• So in a cataclysmic variable, you have this red giant and you had this white dwarf, and the white dwarf is close enough to the red giant that it steals mass from the red giant.
• It doesn’t know what that mass belongs to and it takes it on and it turns into this disk that goes around the white dwarf and there is a point at which there’s too much mass in the disk, it becomes unstable, it all falls on to the white dwarf and the white dwarf brightness suddenly.
• And because we know what that mass is, there’s a mathematical physical relationship between how much mass is in that disk.

You then know how bright it is. You’ve got your E equals mc squared, so you know how much mass is going to turn into an energy, and then you can figure out how far away is. ERIC: And this takes us even further out on the distance ladder, because these things are so bright, we can see them from really far away and we can measure larger distances.

JANINE: Yeah. Yeah, and actually, that’s how we got our first distance to the Andromeda galaxy was Edwin Hubble, who you may have heard of because of a certain telescope. There was a person that that’s named after. So Edwin Hubble in 1924 used Cepheid variables that, as Henrietta Swan Levitt had posited you could, to figure out how far away the Andromeda nebula was, because at that point they didn’t know that galaxies were galaxies.

But he used it to prove that it wasn’t inside of the Milky Way, and his number was about 900,000 light years. He used 12 Cepheids to figure that out. We now think it’s about 2.537 million light years, but. ERIC: So in the ballpark, not too bad for telescopes from 100 years ago.

JANINE: It’s astronomy, right? So it’s pretty close. ERIC: All right, we can use these methods to estimate distances to other galaxies that make up the universe, and now, we’re at the end of our journey. How far away is the edge of the universe? JANINE: This one’s harder. There isn’t an edge to the universe, at least not one that we know of, and people who are trying to figure out this are actually called cosmologists.

So there are people who study what the shape of the universe is, how big it is, how it formed, all of these kinds of things. But we can talk about the edge of the visible universe or actually how far back in time, we can see. We talked about that time limit and how long it would take light from the sun to get to the earth and how we wouldn’t know for eight minutes.

Well, that applies to everything that we see in space, which means looking out into space is basically a time machine, right? We’re looking back in time the further out we go because it takes time for light to travel to us. So the furthest out we can see is about 46.5 billion light years away, which is crazy, but it also means you can look back into the past and try to figure out how the universe formed, which again, is what cosmologists do.

ERIC: Well, Janine, thanks so much for telling us how far away everything in the universe is. JANINE: You are so, so welcome. ERIC: You can find out more about the structure of the universe by tuning in to one of our virtual planetarium shows from the comfort of your own home.

## What is a real black hole?

A black hole is a region of spacetime where gravity is so strong that nothing – no particles or even electromagnetic radiation such as light – can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole.

The boundary of no escape is called the event horizon, Although it has a great effect on the fate and circumstances of an object crossing it, it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass.

This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly. Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace,

1. In 1916, Karl Schwarzschild found the first modern solution of general relativity that would characterize a black hole.
2. David Finkelstein, in 1958, first published the interpretation of “black hole” as a region of space from which nothing can escape.
3. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity.

The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. The first black hole known was Cygnus X-1, identified by several researchers independently in 1971.

1. Black holes of stellar mass form when massive stars collapse at the end of their life cycle.
2. After a black hole has formed, it can grow by absorbing mass from its surroundings.
3. Supermassive black holes of millions of solar masses ( M ☉ ) may form by absorbing other stars and merging with other black holes.

There is consensus that supermassive black holes exist in the centres of most galaxies, The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Any matter that falls onto a black hole can form an external accretion disk heated by friction, forming quasars, some of the brightest objects in the universe.

1. Stars passing too close to a supermassive black hole can be shredded into streamers that shine very brightly before being “swallowed.” If other stars are orbiting a black hole, their orbits can determine the black hole’s mass and location.
2. Such observations can be used to exclude possible alternatives such as neutron stars.

In this way, astronomers have identified numerous stellar black hole candidates in binary systems and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

On 11 February 2016, the LIGO Scientific Collaboration and the Virgo collaboration announced the first direct detection of gravitational waves, representing the first observation of a black hole merger. On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope (EHT) in 2017 of the supermassive black hole in Messier 87 ‘s galactic centre,

As of 2021, the nearest known body thought to be a black hole is around 1,500 light-years (460 parsecs ) away (see list of nearest black holes ). Though only a couple dozen black holes have been found so far in the Milky Way, there are thought to be hundreds of millions, most of which are solitary and do not cause emission of radiation.

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#### Is the solar system bigger than a light year?

Physics When we make a measurement, we use a number and a name called the unit of the measurement. We weigh 150 pounds; we buy 20 oz of a liquid; it is 150 km to the next town; we are going 50 mph; and so forth. There is always a number and some unit (a group of words).

Long ago the human body was used as a reference. A cubit was the distance from the elbow to the tip of the index finger; the foot was the length of a person’s foot; a fathom was the length of a man’s outstretched arms. Obviously this varied from person to person and there was little standardization. In 1670 a Frenchman, Gabriel Mouton, proposed standardizing the length to relate to a physical measurement of the Earth.

Thus began the decimal metric system. We have continued to refine and change our units, seeking to use those with more permanence. In 1875 the “Treaty of the Meter” was signed by the major countries of the world (including the United States), making the meter the official world standard for length.

• Whenever we make a measurement, whether in science or everyday life, we choose a unit appropriate to the scale of the object being measured.
• You would measure the length of a bug in inches, or the distance to Memphis in miles.
• You could measure insects in miles and highway distances in inches, and be technically correct.

But it’s awfully cumbersome to say that a beetle is 0.000016 miles long or that Memphis is 5 069 000 inches from Jackson. As we measure larger and larger sizes, different units are applied. The solar system is extremely large and the mile is just too small of a unit to use for measuring interplanetary distances.

Instead scientists devised another unit, called the astronomical unit (AU), which is convenient to use for making measurements within our solar system. One astronomical unit is defined as the average distance from the Earth to the Sun, approximately 93,000,000 miles. Then if we say an object is 35 AU from the Earth we mean it is 35 times as far from the Earth as the Earth is from the Sun or 35 times 93 million miles.

In reading astronomy people encounter the unit called a Light Year, which is the distance light travels in one year (about 6 trillion miles). You might wonder why we don’t use this unit when discussing the solar system. The Light Year is about 64,500 times larger than the Astronomical Unit, too large to be appropriate for an object the size of our solar system.

1. The Light Year is fine for measuring distances to stars or other galaxies but not for measuring distances within our own solar system.
2. An object 35 AU from Earth would only be a few thousandths of a Light Year away.
3. Thus the Light Year is just not a practical unit for our solar system.
4. Astronomers use another distance unit, the parsec, which represents 3.26 light years or about 20 trillion miles.

It is more difficult to understand and is linked to the measurement of angles and geometry. Nearby stars shift their position relative to those that are much farther away. That shift is measured as a tiny angle, and is correlated to the distance from the earth to the star.

#### How big is the Oort Cloud in light-years?

The Oort Cloud Copyright © 1999 by Rosanna L. Hamilton. All rights reserved. The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun, This vast distance is considered the edge of the Sun’s orb of physical, gravitational, or dynamical influence.

Within the cloud, comets are typically tens of millions of kilometers apart. They are weakly bound to the sun, and passing stars and other forces can readily change their orbits, sending them into the inner solar system or out to interstellar space. This is especially true of comets on the outer edges of the Oort cloud.

The structure of the cloud is believed to consist of a relatively dense core that lies near the ecliptic plane and gradually replenishes the outer boundaries, creating a steady state. One sixth of an estimated six trillion icy objects or comets are in the outer region with the remainder in the relatively dense core.

In addition to stellar perturbations where another star’s Oort cloud passes through or close to the Sun’s Oort cloud, are the influences of giant molecular clouds and tidal forces. A giant molecular-cloud is by far more massive than the Sun. It is an accumulation of cold hydrogen that is the birthplace of stars and solar systems,

These are infrequently encountered, about every 300-500 million years, but when they are encountered, they can violently redistribute comets within the Oort cloud. Tidal forces affecting the Oort cloud come from stars in the Milky Way’s galactic disk with some pull from the galactic core.

The tide results from the sun and comets being different distances from these massive amounts of matter. The force on the comets from these tides is greater than the perturbations of passing stars, and comets beyond 200,000 AU are easily lost to interstellar space. This pull contributes to the steady state which replenishes the outer comets that are randomly distributed away from the ecliptic plane.

The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth. This matter is believed to have originated at different distances and therefore temperatures from the sun, which explains the compositional diversity observed in comets.

1. Typical noontime temperatures are four degrees Celsius above absolute zero.
2. As temperatures move toward absolute zero, the kinetic energy of the molecules approach a finite value.
3. Absolute zero should not be considered a state of zero energy without motion.
4. There still remains some molecular energy, although it is at a minimum, at absolute zero.

The Oort cloud is the source of long-period comets and possibly higher-inclination intermediate comets that were pulled into shorter period orbits by the planets, such as Halley and Swift-Tuttle. Comets can also shift their orbits due to jets of gas and dust that rocket from their icy surface as they approach the sun.

Although they get off course, comets do have initial orbits with widely different ranges, from 200 years to once every million years or more. Comets entering the planetary region for the first time, come from an average distance of 44,000 astronomical units. Long period comets can appear at any time and come from any direction.

Bright comets can usually be seen every 5-10 years. Two recent Oort cloud comets were Hyakutake and Hale-Bopp. Hyakutake was average in size, but came to 0.10 AU (15,000,000 km) from Earth, which made it appear especially spectacular. Hale-Bopp, on the other hand, was an unusually large and dynamic comet, ten times that of Halley at comparable distances from the sun, making it appear quite bright, even though it did not approach closer than 1.32 AU (197,000,000 km) to the Earth.

• Recognition of the Oort cloud gave explanation to the age old questions: “What are comets, and where do they come from?” In 1950, Jan H.
• Oort inferred the existence of the Oort cloud from the physical evidence of long-period comets entering the planetary system.
• This Dutch astronomer, who determined the rotation of the Milky Way galaxy in the 1920’s, interpreted comet orbital distribution with only 19 well-measured orbits to study and successfully recognized where these comets came from.

Additional gathered data has since confirmed his studies, establishing and expanding our knowledge of the Oort cloud.

### How many light-years is the universe?

Defining and measuring the observable and the whole universe Know about the concept of the observable universe and on measuring the observable universe within the whole universe Learn about defining and measuring the observable universe within the “whole” universe.

1. © MinutePhysics () The universe- how big is it? Does it have a center? Does it have an edge? Is it getting bigger, and if so, why? Well, we know that there are two different meanings for universe.
2. First, the observable universe is everything that we’ve been able to see or observe thus far.
3. And second, the universe, or the whole universe, means everything that exists, or has existed, or will exist.

More specifically, the observable universe is the region of space visible to us from Earth. And since the universe is only about 13.8 billion years old and light takes time to travel through space, then regardless of what direction we look, we see light that’s been traveling, at most, 13.8 billion years.

So it’s logical to think that the observable universe must then be 2 times 13.77 equals 27.5 billion light years across, but it’s not. That’s because over time, space has been expanding, so the distant objects that gave off that light 13.8 billion years ago have since moved even farther away from us.

Today, those distant objects are a bit more than 46 billion light years away. Multiply times 2, and you get 93 billion light years, the diameter of the observable universe. To give you a sense of scale, the size of the Earth within the observable universe is roughly equivalent to the size of a virus within the solar system, although that doesn’t help much because we can’t really appreciate the incomprehensible smallness of a virus, nor the bewildering bigness of our solar system either.

So let’s just say that the observable universe is stupendously big, but the whole universe, as far as we can tell, is a lot bigger. Space is most likely infinite, or at least it doesn’t have an edge, though the difference between those is another story unto itself. Now, what about the center of the universe? Well, the observable universe has a center, us.

### How BIG Is Our Solar System? | Earth Lab

We are at the center of the observable universe because the observable universe is just the region of space visible from Earth. And kind of like how the view from a very tall tower is a circle centered on the tower, the piece of space we can see from here is naturally centered here.

1. In fact, if you want to be more precise, each one of us is the center of our own observable universe, but that doesn’t mean we’re at the center of the whole universe, just like the tower isn’t the center of the world.
2. It’s the center of the piece of the world that it can see, up to the horizon.
3. But just because you can’t see beyond the horizon doesn’t mean there’s nothing there.

And so it is with the observable universe. Looking up at the sky, we see light that’s at most 13.8 billion years old and coming from stuff that’s now 46 billion light years away. Anything farther is beyond the horizon, but each second, we see new, even older light coming from slightly farther away, three light seconds farther, to be precise.

1. And so our view of the cosmos is literally getting bigger all the time.
2. All we have to do is wait and watch as the universe ages and light from more distant places has the time to get to us.
3. So here we are, sitting at the center of our observable piece of the whole universe.
4. How big is the universe? Well, the observable universe is currently 93 billion light years across.

The whole universe is probably infinite. Does the universe have an edge? The observable universe does. It’s 46 billion light years away in any direction, and the whole universe has a temporal edge, or what we call a beginning, but almost certainly not a spatial one.

• Does the universe have a center? Again, the observable universe does, you.
• The universe as a whole, almost certainly not.
• And is the universe getting bigger? Yes.
• Space is expanding, which makes both the observable universe and the whole universe bigger.
• Plus, over time, we see older and older light coming from farther and farther away, so our observable universe gets bigger that way too.

And that, in a nutshell, is our view from the tower. You are the center of the universe, and so am I, and so is everyone else, and so is no one. : Defining and measuring the observable and the whole universe

#### How long would it take to travel 4 light-years?

If humans are ever to colonize the galaxy, we will need to make the trip to a nearby star with a habitable planet. Last year, astronomers raised the possibility that our nearest neighbor, Proxima Centauri, has several potentially habitable exoplanets that could fit the bill.

Proxima Centauri is 4.2 light-years from Earth, a distance that would take about 6,300 years to travel using current technology. Such a trip would take many generations. Indeed, most of the humans involved would never see Earth or its exoplanet counterpart. These humans would need to reproduce with each other throughout the journey in a way that guarantees arrival of a healthy crew at Proxima Centauri.

And that raises an interesting question. What is the smallest crew that could maintain a genetically healthy population over that time frame? Today, we get an answer thanks to the work of Frédéric Marin at the University of Strasbourg and Camille Beluffi at the research company Casc4de, both in France.

• They have calculated the likelihood of survival for various-sized missions and the breeding rules that will be required to achieve success.
• First, some background.
• Space scientists and engineers have studied various ways of reaching nearby stars.
• The problem, of course, is the vast distances involved and the comparatively sedate speeds that human spacecraft can manage.

Apollo 11 travelled at around 40,000 kilometers per hour, a speed that would take it to Proxima Centauri in over 100,000 years. But spacecraft have since become faster. The Parker Solar Probe, to be launched this year, will travel at more than 700,000 kilometers per hour, about 0.067 percent the seed of light.

1. So Marin and Beluffi use this as the speed achievable with state-of-the-art space technology today.
2. At this speed, an interstellar journey would still take about 6,300 years to reach Proxima Centauri b,” they say.
3. Selecting a crew for such a multigenerational space journey would be no easy feat.
4. Important parameters include the initial number of men and women in the crew, their age and life expectancy, infertility rates, the maximum capacity of the ship, and so on.

It also requires rules about the age at which procreation is permitted, how closely related parents can be, how many children they can have, and so on. Once these parameters are determined, they can be plugged into an algorithm called Heritage, which simulates a multigenerational mission.

• First, the algorithm creates a crew with the selected qualities.
• It then runs through the mission, allowing for natural and accidental deaths each year and checking to see which crew members are within the allowed procreational window.
• Next, it randomly associates two crew members of different sexes and evaluates whether they can have a child based on infertility rates, pregnancy chances, and inbreeding limitations.

If the pregnancy is deemed viable, the algorithm creates a new crew member and then repeats this loop until the crew either dies out or reaches Proxima Centauri after 6,300 years. Each mission also includes a catastrophe of some kind—a plague, collision, or other accident—that reduces the crew by a third.

The algorithm then repeats each mission 100 times to determine the likelihood of this size of crew reaching its destination. A key question is what degree of inbreeding can be allowed. Marin and Beluffi measure this using a scale in which breeding between identical twins registers as 100 percent; brother/sister, father/daughter, or mother/son is 25 percent; uncle/niece or aunt/nephew is 12.5 percent; and first cousins is 6.25 percent.

One option is to limit inbreeding to less than 5 percent, so partners have to be more distantly related than first cousins. Another option is to stipulate that partners cannot be related at all, so that inbreeding is 0. Marin and Beluffi use this second scenario in their simulation.

The algorithm then determines the likelihood of success over 100 missions for different initial crew sizes. The results make for interesting reading. The Heritage algorithm predicts that an initial crew of 14 breeding pairs has zero chance of reaching Proxima Centauri. Such a small group does not have enough genetic diversity to survive.

Researchers have observed with animals that the genetic diversity of an initial population of 25 pairs can be sustained indefinitely with careful breeding. But when the Heritage algorithm uses this as the starting crew—25 men and 25 women—it predicts a 50 percent chance of dying out before reaching the destination.

That’s largely because of random events that can influence such a mission. The chances of success, according to Heritage, do not reach 100 percent until the initial crew has 98 settlers, or 49 breeding pairs. “We can then conclude that, under the parameters used for those simulations, a minimum crew of 98 settlers is needed for a 6,300-year multi-generational space journey towards Proxima Centauri b,” say Marin and Beluffi.

That’s interesting work that sets the stage for more detailed simulations. For example, fertility rates in deep space may turn out to be quite different from those on Earth. And the chances of a healthy child resulting from a successful pregnancy may also be much lower because of higher mutation rates due to radiation.

The chances of catastrophe because of accidents or plagues may turn out to be much smaller than the chances of catastrophe caused by social factors such as conflict. All this could be programmed into a more advanced version of Heritage. Indeed, these issues have already been explored by science fiction writers.

For example, in the book Seveneves, the author Neal Stephenson imagines a future in which humanity passes through a population bottleneck and all individuals are descended from seven women. Given Marin and Beluffi’s work, Stephenson’s imagined future looks highly unlikely.