And just think, that light traveled billions of years unimpeded, only to be stopped by your thumb. Or, if visible, by the retina of your eye.
So if we want a back of the napkin kind of calculation, we could do something like this:
In a sphere, there are 129,600 degrees (360x360) (square degrees?).
There are 2 trillion galaxies in the universe. (2,000,000,000,000),
2,000,000,000,000 / 129,600 = 15,432,098.76 galaxies covered by each degree.
According to this site:
your thumb takes up about 2 degrees in the sky. For estimation’s sake, let’s say your thumbnail is 2 degrees wide and 2 degrees tall, giving us 4 square degrees.
4 x 15,432,098.76 = 61,728,395 galaxies covered up by your thumbnail.
You also have to take into account that some of those distant galaxies may have long since ceased to exist.
That picture gets me every time. How many people of some kind are in those galaxies? How many are looking back at our galaxy? How many are wondering if there is anybody out there?
And we’ll NEVER know!
They are receding from us faster than the speed of light, but not because we are moving apart, but because new space is constantly forming all over the universe. At the smallest level (the Planck level) space itself can no longer be divided. It’s like the pixels of reality itself. But at all times, new pixels appear in between existing pixels, shoving them aside.
This is happening in the subatomic space inside your body RIGHT NOW! but the forces that keep the molecules that your body is formed out of together are stronger than this expansion. So your body doesn’t fly apart into its component quarks. Even the very distant stars in our galaxy are close enough together that their gravity is stronger than this expansion.
Outside our galaxy, we are part of a supergalactic cluster-- a collection of galaxies that are bound together by gravity. Eventually, all the galaxies in our cluster will be pulled together by their collective gravity, and we will form a merged supergalaxy (billions of years from now).
But outside our cluster, things are SO far away that this expanding space overpowers gravity. We are moving away from galaxies outside our cluster, and the farther apart we get, the more space is springing into being between us and them. So the speed at which a galaxy recedes increases as the distance between us increases. And since we aren’t actually moving relative to space itself, that pesky Speed of Light speed limit doesn’t apply.
Keep in mind that the further you go, the older what you see is. From the very outer reaches of the observable universe, we detect light from the time just after the big bang (though this light is so stretched out by its journey through an ever expanding spacetime that we detect it as radio waves). What this means is that even though the observable universe stretches away for billions of light years, most of these distant galaxies appear to us as they did very early in their development, long before the second generation stars like our sun that we think are likely to harbor life ever formed.
This seems to take a very short sighted view of humanity (in the grand scheme of things). While we could send a probe to Alpha Centauri (and in fact, had we gotten started when we first got that capability, that probe would be there by now), it would be enormously expensive and a very significant investment for any nation to undertake. But three hundred years ago, building a skyscraper was far beyond the capability of any of the nation states that existed at the time.
We have about a billion years left before the sun engulfs our planet. That is a mind-bogglingly long period of time. One billion years ago, the first eukaryotes to leave the sea had just reached land. If we or our descendants are still around on that time scale, it’s hard to imagine that we WOULDN’T expand into the universe.
I find that claim more than a bit far-fetched. Cite?
Project Orion (1958) and Project Longshot (1987) are both proposed Nuclear Pulse Propulsion rockets (essentially you have a probe, behind it is a shield, and you detonate small nuclear blasts behind this shield to push the rocket forward). Both had multiple proposals, at different scales; the smallest would take up to 150 years to reach Alpha Centauri while the largest would reach it in around 30 years. Note that none of this is particularly hard to accomplish. If we had enough of a motivation, we could build a nuclear pulse rocket today, or 50 years ago (and it’s not as crazy as it sounds, because you’d put it in orbit with conventional rockets, you don’t have to blow up any nukes anywhere near Earth).
Here’s the anti-cite. To get “just” to Proxima Centauri on the 62 years since Sputnik, you need to average about 2.5 million meters/sec. The fastest probe relative to the Sun has been the Parker Solar Probe launced last year which attained a speed of 68600 meters/sec.
That’s just a tad bit too slow.
No, you can’t calculate the “are” (solid angle) of a sphere as width x height. There are actually 41,253 square degrees in a sphere.
But this is just an order-of-magnitude estimate anyway. The answer is somewhere on the order of 100 million. (And any answer in this thread that provides more than 1 significant digit is making unfounded assumptions.)
That’s all true, but keep in mind, we’ve never sent a probe that was actually intended to be interstellar. The ones that HAVE “left” the solar system (they’re still in the Oort cloud I believe) did so at the end of an outer system mission, with a little residual velocity allowing them to just barely escape the sun’s gravity. Achieving speeds at the sun’s escape velocity is pretty counterproductive if the science your probe is doing is all here in the solar system.
Sure, we could do it if money was no object. But I don’t think we have the technology to do it within the cost constraints of a typical space project (JWST was supposed to cost $5 billion, it’s crept up to $10 billion). Or even within the cost of a science mega-project like the ITER ($20 billion) or SSC (canceled in 1993 after cost estimates went up to $12 billion, about $21 billion in today’s dollars).
My understanding is with our current technological capabilities, we could develop nuclear propulsion to explore our solar system in style, but an interstellar mission within reasonable time frames is a whole other kettle of fish. I’m not even sure if there is enough fissionable material that could be mined with current tech to make enough bombs.
This was a really interesting OP. Simple, yet it triggered such an interesting discussion.
Fair enough… but why is it not 129,600? If you could yaw a spaceship 360 degrees and then for each of those yaw degrees, also pitch it through 360 degrees, is that not 129,600 possible points you could point the craft at?
Stand in the middle of a sphere big enough that one degree of arc from where you are looking is one foot in length. That means that the sphere has a circumference of 360 feet and thus a radius of 360/(2 pi) feet. The area of a sphere is 4 pi radius-squared, which equals (for this case) 360*360/pi square feet - or 41,252 square feet. Since one degree across is one foot, the sphere covers a solid angle of 41,2521 square degrees.
Why? Because degrees near the poles of the sphere are smaller than degrees near the equator (one foot near the pole may cover dozens or hundreds of degrees).
This is not something that anyone knows. Anyone who claims to know anything about how physics operates at the Planck scale can be safely ignored.
bump, you could put a whole bunch of points on a sphere that way, but they won’t be evenly spaced. Think of the points on the Earth whose latitude and longitude are integer numbers of degrees: One degree away from the North Pole, you’ll have 360 such points, all clustered tightly together in a very small circle. Measured fairly, that circle should only contain pi square degrees, not 360.
Fine – think of it as an analogy, then. Our universe, as best as we can tell, behaves in a way consistent with space expanding in this way. We may at some point discover inconsistencies with this theory, in which case we would have to come up with something better.