How much matter does the universe contain?

A single look at the night sky will reveal there's far more nothing than something out there. The tiny specks of light are stars, made of something, and the vast oceans of blackness surrounding them are nothing. The universe contains gigantic amounts of void, but how much, I wonder. To visualise this, I decided to picture all the matter in the universe clumped together. Not super-tightly, like in a neutron star, but still put together closely so that the density is 1 kilogram per litre: the density of water. The main reason I chose this density is, of course, that it makes it very easy to calculate. So if we pack all matter in the universe together like this, is the result larger or smaller than a single galaxy?

Before I answer this question, I should point out I will give large numbers first in the scientific notation, then give their name according to the long scale. I'm using the long scale because it's more sensible and because it's the scale I grew up with using. But for people who are more keen on the short scale (the one generally used in English-speaking countries) I will put the short scale name of large numbers in parentheses. A second thing I should point out is that I'll be using all the matter in the observable universe, not the entire universe.

First of all, we have to find out the total mass of matter in the universe. Wikipedia gives several estimates of the mass of the universe. I will be using the calculated mass based on the critical density, 1.53×1053 kg, as it seems to be the best corresponding to our observations of the universe. Basing the mass of the universe on stars alone is silly due to the presence of dark matter, and assuming a steady state-universe requires the pretty big assumption that our universe is a steady-state universe.

So, since we assumed the density of water for our clump of all matter, that means it's 1.53×1053 litres in volume. 153 octilliard (153 septendecillion) litres is unimaginably huge, so we'll have to use bigger units to make this more imaginable. Let's start with converting to cubic metres. Thanks to space being three-dimensional, a cubic metre contains not ten, but a thousand litres. So that means this clump of matter is  1.53×1050, 153 octillion (153 sexdecillion) cubic metres in volume.

We can get closer to imaginable numbers by converting to cubic kilometres. Similarly to how a cubic metre contains a thousand litres, a cubic kilometre contains a milliard (billion) cubic metres. That means our clump of all matter is 1.53×1041, 153 sextilliard (153 duodecillion) cubic kilometres.

To get closer to home, we'll convert to cubic megametres. The megametre is a pretty rarely used unit, but it's equal to a thousand kilometres. The clump of matter has a volume of 1.53×1032, 153 quintillion (153 nonillion) cubic megametres.

To take another step, I'll now convert to cubic gigametres. A gigametre is a million kilometres, a bit more than a journey to the moon and back. We divide by a milliard (billion) and get 1.53×1023,153 trilliard (153 sextillion) cubic gigametres.

We're still not using imaginable numbers, so we convert to cubic terametres. A terametre is a milliard (billion) kilometres, 1.5 times the distance from here to Jupiter. The clump of matter is 1.53×1014,153 billion (153 trillion) terametres. Hey, that's approaching numbers we use (on occasion) in our daily lives.

But let's take another step and convert to petametres, one billion (trillion) kilometres. I would like to give an example of how big this is, but there are few meaningful distances close to a petametre. It's 220 times the distance from here to Neptune, or a fiftieth of the distance from here to the nearest star (aside from the Sun). All the matter in the universe taken together is, if my calculations are right, 1.53×105, 153000 cubic petametres.

While a hundred-and-fifty-thousand is quite a low number, we can use cubic lightyears instead. A lightyear is approximately ten petametres. Actually, it's 9,5 petametres, but considering how rough our estimate of the universe's mass was I think we can get away with rounding it to ten, thus putting a thousand cubic petametres in a cubic lightyear. One final calculation, and we get 153 cubic lightyears. A cuboid of five by five by a little over six lightyears. That's all the matter in the universe. Not only would it fit inside a single galaxy, if its centre was at the Sun's position it wouldn't even reach the nearest star. That's all the matter in the universe, everything else is void.

But where is all this void exactly? You might think the gigantic gaps between galaxies are where it is, but these are actually relatively small. The Milky Way is a hundred thousand lightyears across, though only ten thousand lightyeard thick, and the nearest big galaxy is at two million lightyears, only twenty times the Milky Way's own diameter. If we count dwarf galaxies the nearest other galaxy is much closer even. If you imagined the big galaxies as coins, they would be less than a metre apart.

The real distances that are responsible for all this void are the distances between stars. When I tried to give an example of the size of a petametre and failed we already saw there is a huge gap between distances within a solar system and distances between stars. Our Sun is 1.4 million kilometres across, but the nearest other star is more than four lightyears away. If we imagine the Sun as a grape in the centre of Amsterdam, the nearest starsystem consists of two grapes and a grain of pepper located near Brussels. And that's a good comparison for interstellar distances: a piece of fruit in every European capital. Those distances are mainly responsible for the huge amounts of void in the universe.


Time travel guidelines

If you have recently acquired a time machine, then please, read through these guidelines carefully before using it.

The Timeline

If you're going to travel through time, the first thing you should try to find out is the shape of the timeline. This is very important to know as a time traveler, as it will affect how causality works and what happens if you try to change the past.There are three possibilities, the Linear timeline,the Changeable timeline, and the Branching timeline, which I will each explain and what the results of attempting to make a grandfather paradox (killing your grandfather before your father was born, thus preventing you from ever being born to kill him) would be.

In a Linear timeline, past, present, and future are all one line, and when you go back in the past, you will have already gone back there, and since whatever you will do in the past will have already happened, you can never change it. If you attempted to kill your own grandfather in a linear time, you would fail, as you have evidently been born, so your grandfather apparently survived your assassination attempt. Or the man you thought was your grandfather was not actually your real grandfather. Either way, whatever happens was part of the timeline all along, and therefore you can't create a paradox.

In a Changeable timeline, you can actually change the past by going back in time. As a result, whatever you do in the past will have an effect on your present self. This is the only timeline where you can actually cause the grandfather paradox, as killing your grandfather would actually have an effect on you. While this timeline may seem the most intuitive, the fact that paradoxes could be created here seems to indicate it is also the least likely.

In a Branching timeline, the universe splits into two universes whenever someting happens, and one of the possible outcomes becomes a reality in each of the universes. Thus, when you go back in time you will arrive in a new branch of the timeline. If you tried the grandfather paradox here, your grandfather would die in the new timeline, but you would be fine since the grandfather from your own timeline was alive and well. Paradoxes would be impossible in this one too.

If you built a time machine, presumably you discovered the shape of the timeline during this. If you didn't, try this: leave your house for about an hour. When you come back, look into an empty drawer and make sure it is actually empty. Then, go 30 minutes in the past and put a sock in the drawer. Go back to the future (about an hour would suffice so you don't run into yourself), and check the drawer. If the sock isn't there, you're probably in a Linear timeline. You just have to worry about the fact that apperantly someone broke into your house and took the sock while you were away. If the sock is there, you are probably in a Changeable or Branching timeline. Repeat this experiment a few times to be sure. If you keep finding you actually made small changes to the past, you're now certain you are not in a Linear timeline. Finding out whether you are in a Changeable or Branching timeline will be a bit trickier, but there is a way that you can use for it.

Go back in time 3000 years. Depart to the present immediately, to make sure you don't actually change anything. If you're in a Changeable timeline, your effect on the timeline should be negligible, and history will have played out just as you remembered it. However, if you're in a Branching timeline, your arrival in 989 BCE will have created a new alternate timeline where you have arrived in. At every small little happening in history, even the smallest, this universe split again, and you end up in the year 2011 CE in a random one of these universes. Thus, when you get out of your time machine in your new timeline's present, history will probably have happened vastly differently. By the way, this is probably a good moment to mention the fact that it will be impossible to return to our original timeline, though with many careful time trips you may manage to end up in a very similar one.

So now that you know the shape of the timeline, this tells you how careful you'll want to be in the past. If you're in a Linear timeline, you don't have to worry about changing the future, and only have to watch out for your own neck. If you're in a Changeable timeline, be very careful, as a single mistake can vastly change the whole world history, and you may find your own present gone if you are not very careful. Thanks to the butterfly effect, even a really small change could culminate into massive differences over a long period of time. If you are in a Branching timeline, changing the past means going back to a present similar to your own present will be difficult, but by traveling back even farther you can return to a present similar to the one you knew. 

Some things to watch out for

When you arrive, if your time machine does not have some way to tell you the date, try to find out the year. Don't go asking random strangers what year it is, though, as that question is considered weird. If newspapers are printed in this time, just look at the date on them. Or go into a store and ask to buy a calendar. If you're in the past, some careful references to historical events in casual conversation can help you pinpoint the date (And if an event turns out not to have happened yet and you're asked about it, just say it was just something minor in your home country).

Try to blend in as well as possible. When you're time traveling, try to wear something that will look not completely bizarre in most eras. When you arrive, try to switch it for era appropriate clothing as soon as possible. Looking at how most people around you dress can provide you with a good guideline.

Don't tell people you're a time traveler. If you're lucky, they'll think your nuts and ignore you. If you're only slightly lucky, they'll think you're nuts and lock you up. If you're unlucky, they'll believe you, kill you, and steal your time machine.

Don't act smug about your era when you're in another era. First of all, talking about your era strongly borders on telling them you're a time traveler. Also remember that you're a guest in their era. Acting condescendingly about their era is considered rude. If you're in the past, remember that change is gradual, and that to reach the situation of the world in your home era, it had to pass through other eras, and that your own home era may be viewed just as condescendingly in the future. If you're in the future, remember that they went through the present and went on from that. Presumably they have good reasons for changes made since your own home era.

Whether you're in the past, alternate present, or future, always wash your hands with ethanol or another bacteria killing substance. A squirtbottle of ethanol can be found in most labs and classrooms, and they presumably buy them from somewhere, so find out where and buy some to take on your journey. The past had many potent diseases that your lazy 21st century immune system will not be prepared for. In the future new diseases will have evolved that you will not have had yet, so you can easily be infected by them. There's also the danger of spreading your own germs around and causing diseases.

Interactions with Yourself

When you travel through time, meeting yourself should generally be avoided, especially in a Changeable timeline (Who knows how the timeline will change when your past self meets your present self). If you're in a Linear timeline and you can't remember ever having met yourself, then don't try it as you will not succeed and may in fact be run over or something before you reach your past self. Even in a Branching timeline it should be avoided, as meeting yourself is a great way to get into trouble. Meeting your future self can be considered somewhat safer, as both parties will be aware of the situation (you're in it and your future self remembers it unless your in a Branching timeline), and your future self will probably remember the encounter and thus be able to prepare for it, but I still discourage it.

To avoid meeting yourself, try keeping a log of where you were at any given time. This can also come in handy for many other reasons. Just make sure it doesn't fall into the wrong hands (Wouldn't want to make it too easy for Terminators targetting you).

If you HAVE to interact with yourself and can't hide your identity, there are some basic rules:

1: NEVER kill yourself. This one should go without saying, but aparrently it doesn't. Killing yourself is never a good idea, especially not in a Changeable timeline, where you will in fact cause a paradox. 

2: Avoid confusion between you. If you have a middle name, now is finally the time to use it. One of you (If it's the home era of either of you, that should be the guest) uses his/her middle name as a first name. Alternatively, you can try nicknames, numbers, or letters. If your in the home era of either of you, that person always gets first pick. If you both traveled to a year that's not your home era, just work it out like adults.

3: Talking about avoiding confusion: have some sort of password to your own mind, which you can use to prove you are really the same person. Never tell this password to anyone, and never write it down, and if someone knows your password, assume they are either yourself or someone deeply trusted by yourself. This one will be more effective if you think of that password even before you even consider time traveling. Seriously, choose a mind password right now. Your future self may thank you for it.

4: If you're in a Changeable timeline, watch out for paradoxes. During interaction with yourself, these are very easy to cause. In a Branching timeline, try not to get your other self killed and/or in big trouble and/or with a completely ruined life/world. While paradoxes may not occur, that person is still you and you don't want that kind of stuff to happen to you, do you? In a Linear timeline, be careful not to do anything you can't remember while interacting with your past self. If you tried it, evidently you were stopped by something, and that something may actually be very dangerous to you.

Three is a crowd

Don't visit the same moment more than twice. While avoiding one other you is quite doable, avoiding two or more other yous will quickly cause a mess. There's also the problem that the chance increases that people who know you see you at the same time in different locations when there's more you's running around. Watch Primer. They went back in time across the same 4 days multiple times, and made a real mess. That's the kind of thing you want to avoid.


Speaking of Primer: time travel can be hideously complicated. Try watching Primer and understanding it the first time through. That's the kind of complicated you'll be dealing with. Time travel is an inherently complex activity, and should not be attempted unless your mind can keep track of it all.

Time travel is also an inherently dangerous activity. Even discounting the possibility of flaws in your time machine, danger lurks everywhere. The past was quite often a dangerous time, full of violence and diseases. In the present, you have to always be watchful of running into yourself (or, if that is part of your plan, to prevent trouble if you do, see the section 'Interactions with yourself' for that). If you go into the future, you never know what you might find, and if you're really unlucky you could end up appearing straight in the middle of a nuclear detonation. In a Changeable timeline, even a slight change in the past could forever change the present, and quite possibly not for the better. Any change in the past in a Changeable or Branching timeline could make it very difficult to return to the version of the present you call home. Your time machine could get destroyed or stolen in a different era and you;d be stuck there forever.

P.S.: I just thought of another handy tip: If you don't need to get to a specific date, but just a year, try going at Carnival or a similar holiday featuring people dressing up and lots of alcohol. Even if you completely dick up your attempts to blendin with the time, you won't stand out.



2011 just started, so happy new year and all that. I kind of liked 2010 on a personal level, though it was pretty sucky on a global level. We had the Haïti earthquake, the eruption of that volcano on Iceland with the completely unpronounceable name and the ensuing mayhem, everyone going nuts about the football world cup (How can people find 22 millionaires running after a little ball so interesting?), Leslie Nielsen died, the Republicans won the American House of Representatives elections (I have no love for the Democrats, but at least they're better than the Republicans), the new Dutch cabinet is even more right-wing than the previous one, and the American plans for new manned missions to the Moon have been canceled (The one good thing Bush ever did).
Still, I had a lot of fun in 2010. I started playing Dungeons & Dragons, rediscovered the Pokémon games, read a lot of good books, went on vacation to Switzerland and saw mountains, and some more fun things. I just hope 2011 will be better.