# Deciphering the monsters of our universe, aka black holes

in #steemstem4 years ago

In this era of gastrophysics (this is how we called gravitational wave related astrophysics), black holes have attracted more and more interest. Indeed, the observation of 5 or 6 gravitational waves by the LIGO and Virgo experiments have allowed us to conclude to the existence of black holes.

And for some reasons, I have never written anything about black holes on Steemit. Time to change this…

[image credits: NASA]

Another reason for doing so is that a triplet of Steemians (@wiplala87, @fproductions and @turksays) asked for a post on black holes, and that I promised @suesa to write it (see here).

And I am a man of words :)

And actually, as a surprize never comes alone, I will even write several posts on black holes.

In the second post (most probably next week), I will explain how black holes can actually be dark matter. But this will be for the post of next week (this one being already very long).

## THE FLATNESS OF THE UNIVERSE

In order to understand what a black hole is, one must first go back to the roots: general relativity. I will not enter into details here, but I will instead provide a sketch of what is going on.

First, one starts from the fact that the universe is a flat surface to a very good approximation. This does not come from nowhere. We have very strong measurements (i.e. data) pointing to this conclusion.

I know that this may sound weird. However, everything becomes clear once one defines flatness.

Let us take a planet as an example. For sure, a planet is not flat.

[image credits: Wikipedia]

Let us put a bunch of observers at the equator of the planet, each of them being well separated from the other. We then ask them to draw parallel lines towards the north pole of the planet. Their lines, even if they are parallel, will cross each other exactly at the north pole. Conclusion: a planet is not flat.

The definition of flatness then follows: a surface is flat if two parallel lines drawn on it never cross.

Now, to check what is going on in the universe, it is a bit more complicated. By using data on the cosmic microwave background (that travels within the universe from the early days), we can derive the fact that the universe is flat. Data is data after all!

But let us remember, the universe is flat in the sense of our definition of flatness. Fun fact: we have so far fixed the geometry of the universe, but not its topology. For instance, a cylinder is flat using our definition!

## SOME INTRODUCTION ON GENERAL RELATIVITY

It is now time to go back to general relativity.

[image credits: Wikipedia]

We have said above that the universe is flat. This is true on a large scale, but this statement is not entirely true on small scales.

On small scales, general relativity predicts that mass and energy deform the structure of spacetime and bend it.

In other words, one can for instance see the universe like a sheet of paper, and the masses (stars, galaxies, planets, etc.) can be seen as marbles to be put on the paper. The marbles will of course deform the paper.

This is illustrated on the picture above with spacetime (the grid) and Earth.

Now let’s push the comparison one step further.

Let us imagine that we put a glass marble on the paper, and a bit further, we put a lead marble on it. We can imagine the paper to be slightly deformed by the first marble, and largely deformed by the second one, as lead is much denser than glass.

[image credits: Wikimedia]

What we have there is actually pretty accurate, and we obtain our general relativity lesson number 2.

The more compact is the mass, the more it will deform spacetime. And if we take a sufficiently compact mass, it will form a black hole.

This is kind of illustrated on the figure on the right, where one compares the deformation that would be induced by the sun, by a very massive and compact neutron star, and by a black hole (to some extent as in principle, one should get a singularity here and not a finite well).

## MOTION IN SPACETIME

There is one extra thing with back holes. One usually says that black holes are monsters that eat everything, including light… This can be explained by studying (free) motion in our flat spacetime deformed by mass and energy.

[image credits: Wikipedia]

Objects in free motion, including light, follow what we call the geodesics. These geodesics correspond to what straight lines would be in our universe.

In all the pictures above, those geodesics consist of the grid overlaid with the pictures. And if we look closely to these grids, one can see that masses, through the associated spacetime deformations, define what the geodesic ares.

Close to a black hole, the geodesic points towards its center. But the black hole curvature grows infinite, so that light will just take forever to go out of it. In short, it will never go out of a black hole.

And since nothing can go faster than light…

There is thus a concept here, that stems naturally from the discussion: the concept of the event horizon. A kind of border in space time that once crossed by an object, the object will be absorbed by the black hole.

## SOME FURTHER DISCUSSIONS

Originally, I wanted to discuss here the fact that back holes could be dark matter. But I finally decided that it would be good to first describe what black holes are. So for the dark matter stuff, please stay tuned (and check my next post).

There are many many other things to tell about black holes, like for instance how they naturally arise as solution of Einstein equations. These equations are by the way the central equations of general relativity.

I could have also discussed how black holes have been observed for the first time through the recent observation of the gravitational waves emitted by the merging of two black holes orbiting around each other. I indeed mentioned it at the beginning of this post.

So for more information, maybe having a look here or there could be helpful :)

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"Let us take a planet as an example. For sure, a planet is not flat."

hidden message here I feel

[whistling...]

For a moment there, I thought we've lost him to the flat earthers :)

Don't worry with that :)

👍👍👍

Interesting and simple information This is what everyone is looking for... There are those who believe that this information is useless...
But NASA has plans for a spacecraft capable of travelling at unimaginable speeds above the speed of light, which is about 300,000 kilometers per second.

All of this came from these ideas in this wonderful topic

I agree, this was a simplistic approach and helps me to start understanding black holes. @lemouth, I am looking forward to additional posts. "Any fool can make things more complex. It takes a touch of genius to move in the opposite direction." Albert Einstein.

4 years ago (edited)

Well, there is a strong theoretical problem with this propulsion method (known as Alcubierre drive).

The principle is that you curve positively space time in front (say with a point of huge energy or mass in front of the ship), and negatively at the back... that would create a gravitational pulse that will propulsive the ship forwards (the ship surfs the wave).

But how do you curve space-time negatively? With negative energy / mass? Good luck with that...

How do we wrap into our head that the universe is flat when we can see stars around us like enclosed in ball?

Is the flatness geometric or cosmic? I don't know how to ask this question to be sincere :(

Like you take the example on earth, if two people draw a straight line, the lines will intersect at the north pole. So if two people at different part in the universe draw straight parallel line, will the lines meet or will they go on forever?

From the explanation that you gave, I feel that the universe has true flatness and lines will never meet.

How do we wrap into our head that the universe is flat when we can see stars around us like enclosed in ball?

Is the flatness geometric or cosmic? I don't know how to ask this question to be sincere :(

We are here only discussing geometry: two parallel lines stay parallel. This is what 'the universe is flat' means. Remember, a cylinder is flat. There are actually several acceptable topologies for which the universe is still flat.

So if two people at different part in the universe draw straight parallel line, will the lines meet or will they go on forever?

They will never cross. They can either go forever, or come back to their starting point. It depends whether the universe is finite or infinite.

4 years ago (edited)

Exactly, the "Flat" or Euclidean Universe doesn't mean two-dimensional either, because we obviously have depth in our universe. As @lemouth said we can theorize and measure this if we place two parallel lines in space; throughout all of space they should remain parallel.

What are your thoughts on the holographic principle? Really great lecture from Leonard Susskind.

Seems like information is stored in 2D, giving cred to this theory, i agree with this model.
Solving string theory equations in 11D results in combinations of 1´s and 0´s, self repairing code if i remember correctly!

But there are interesting philosophical aspects as well, if we lived in a simulation, the prof of it not being a simulation could simply be simulated :)
Proving it was a simulation on the other hand would work , leaving no other outcome. Its impossible to disprove :)

Applying the size of the universe and numbers of planets, gives great odds of a civilization existing, that have the technology required to able to simulate an existence with self experience.

I find it a credible idea at least and a interesting one.

4 years ago (edited)

I am not a string theorist. Therefore, I am afraid I won't comment on this (because I don't know more than what it is in a paragraph). We cannot know everything about everything ;)

I actually never knew that space is considered flat from a geometrical point of view but it makes pretty much perfect sense. I love how you differentiated between geometry and topology. I know you didnt come up with this youself, but the distinction really helped me a lot o understand the concept of space-time more.

What about that singularity... Does it actually make a rip in the fabric of space? Can we perscribe an elastic constant to space-time fabric like we can to different materials and then study the space-time as an actual material?

I know you didnt come up with this youself,

Of course I don't. This is much older than me. There were many people who wrote nice reports and lectures on this. I learned from there and now it is my turn to transmit :p

What about that singularity... Does it actually make a rip in the fabric of space?

There is no way to know about that. The theory stops to be valid there and we actually do not know how to treat that.

Always love your simplistic style in describing things, makes it accessible to almost anyone.
And happy holidays my friend, whichever planet you're on today ! :)

Thanks a lot for your nice message. I am on planet Belgium today (still traveling, but less exotic) :p

4 years ago (edited)

For the first time, I'm now aware that the universe is approximately flat. I guess I should just unlearn everything I've known in the past about the shape of the universe. :(

Not really unlearning, but more understanding better :D

Very well written and very easy to understand! You have truly outdone yourself @LeMouth, I am looking forward to part 2 that makes a connection between dark matter and black holes (something that I have never read of before)

I will hold my questions on this subject for subsequent dispatches (as they relate to dark matter and may be covered or not) and will instead start to look into this before attempting to ask something. Thank you for sharing!

Thanks for your comment. For the second part, you will have to wait 4-5 days. Will take an extended week-end offline followed by some traveling.

I am looking anyways for your future question :D

No worries, I won't be spending much time online right now since I am spending time with family. I hope you enjoy your extended weekend!

Hello @lemouth

First, one starts from the fact that the universe is a flat surface to a very good approximation. This does not come from nowhere. We have very strong measurements (i.e. data) pointing to this conclusion.

You're definitely right here, it sounds weird. I don't know much or anything about General relativity. But I was thinking since the universe was made of space and time, how is it a flat surface (where does the depth comes from, if it was a surface?).
Thoughts like; are we (our planets and stars) on the universe just like we (humans) are on earth.
What defines the universe? I mean if the universe was some surface, it should be contained in some other material that has a volume, since space itself is a quantity that has volume.
All these thoughts are just coming to my mind.
I'm not as smart as my username 😢.

@geniusvillain

Some hints below that maybe will help you (otherwise please come back to me).

You're definitely right here, it sounds weird. I don't know much or anything about General relativity. But I was thinking since the universe was made of space and time, how is it a flat surface (where does the depth comes from, if it was a surface?).

Everything likes in the definition of flatness (and of surface by the way). As I said in the post, a cylinder is a flat surface. It is all a matter of shooting parallel lasers and have them never crossing each other.

What defines the universe? I mean if the universe was some surface, it should be contained in some other material that has a volume, since space itself is a quantity that has volume.

We have the observable universe that is well defined. For the rest, we can speculate anything as it is impossible to test it and verify it.

4 years ago (edited)

These astrophysics always amuse me. I am a great physics fan and I find reading these posts very interesting. But btw ,light does not necessarily travel in straight lines. For example black holes, it's gravitational power is so huge that it even sucks in light towards it. The masses in the space bends the space time fabric and creates a cavity. If the cavity is too huge, even light cannot pass in straight lines and has to bend a little... So, light does not travel in straight line, it only travels in the lines what it thinks is straight....

This is a great post altogether.. I myself have posted an article about dark matter. As a physics enthusiast,i would request you to go through it by clicking the link below

3 years ago (edited)

Thanks for your comment.

These astrophysics always amuse me. I am a great physics fan and I find reading these posts very interesting. But btw ,light does not necessarily travel in straight lines. For example black holes, it's gravitational power is so huge that it even sucks in light towards it. The masses in the space bends the space time fabric and creates a cavity. If the cavity is too huge, even light cannot pass in straight lines and has to bend a little... So, light does not travel in straight line, it only travels in the lines what it thinks is straight....

You are right. The curvature of the universe (originating from the masses in it) defines what a straight line is. Geodesics would be the right word to use.

Yes.. that's the word... Keep posting mate .

Don't worry, I will ;)

Good to know mate. ;-) ;-)

This post is very interesting ! Thank you.
I want also the post on dark matter ;)

Congratulations for this post, you have explained pedagogically very good and also congratulate them for the comparison of gravity with the density of objects, when curving in space-time. I am anxious to read the following post. I will follow you and I hope you follow me...

Thanks for your nice comment. I am following (not directly however) everyone who is using the #steemstem and #science tags as I am reading most of the posts posted there :)

4 years ago (edited)

Great article @lemouth.
I am very curious on what you are going to write in the next episode when approaching the idea that black holes could be dark matter.

Thanks a lot! For the next article, you will have to wait for after Xmas, in the middle of next week :p

If the dark holes are the monsters, can I call the dark matter "the beast"?

BTW: I never knew the universe is flat. My physics lecturer has a lot of explanation to do

Dark matter may be the beast ;)

This is the simplest, almost ELI5, explanation of the black hole that anyone irrespective of inclination in education could read. You just made science great again :)

Thank you very much! :)

You are most welcome.

I totally agree with you, even high school students would understand this perfectly

Thanks Carlos :)

Good post thank

You are welcome! ^^

Very interesting article!

Thanks for passing by! :)

excellent post e taste a lot, greetings

Am not so sure about the taste of a black hole. Personally, I won't try ;)

hahaha I die of laughter, your answer was very good :P

:)

4 years ago (edited)

Amazing post. Its very interesting to read the way you are writing. I have just two question. As you said the more the compacted object is, the more it is going to bend the space, Lets say one star with 10000 times massive than sun but not compacted like like neutron star which has for example 3 times mass of the sun. who will bend the space more ? Is black whole the last stage of a star or even black hole dies?

Lets say one star with 10000 times massive than sun but not compacted like like neutron star which has for example 3 times mass of the sun. who will bend the space more

How can a star be at the same time 10000 times more massive than the sun and only three times more massive than the sun?

Is black whole the last stage of a star or even black hole dies?

This depends on the mass of the back holes. There are tow phenomena here: black holes aggregate matter and evaporate. According to its mass, one or the other dominate. The most massive is the black hole, the more it will have the tendency to grow bigger. Does it answer?

3 years ago (edited)

not the same star its two differnt stars i mean, one is a star 10000 times massive than sun ,another is a neutron star. Obviously the neutron star is more compacted than the other star. But the other star is more massive. who will bend space more? that was my question? thank you for the answer of the second question.

The neutron star will bend the space more as it is more compact. The size plays a role too.

4 years ago (edited)

Actually, this post very interesting, but my head already devide by zero (error), I am very appreciate for all of the story and every theoretical. Thanks very much prof.

You are welcome. Where does it start to be unclear? I will be happy to explain more :)

whistling."Let us take a planet as an example. For sure, a planet is not flat."

hidden message here I feel

Hi, I found some acronyms/abbreviations in this post. This is how they expand:

 Acronym Explanation LIGO Laser Interferometer Gravitational-wave Observatory
Please leave an up-vote if you find this comment usefull.

this is such a solid post. Very informal! I thoroughly enjoyed it. Keep up the good work.

Thanks you for reading it :)

I have never written anything about black holes on Steemit

I wouldnt expect anything less than excellent from a Professor. I obviously would look forward to learning some more on dark matters. Thank you for sharing this @lemouth.

It is my pleasure! For the dark matter stuff, it will be for after Xmas :)

my friend whose a physicist once told about black hole but the way he explained it made it sound like fiction...but thanks for the clear explanation

My pleasure! I am happy to read that you liked my post :)

You must be a successful person in steemit, I want to follow you and your success in steemit, I also hope you follow me and help me with my phos, thanks

I don't know. You tell me :)

3 years ago (edited)

Thank you very much for this excellent post, I especially like the spacetime deformation part, because it explains gravity in perfectly understandable way for us non-physicists :) For people like myself who always wanted to know more about these things I would also recommend an amazing documentary Fabric of the Cosmos by Brian Greene, where he explains what is space (part one) in a very interesting and easy to understand way:

Thanks for your comment. Brian Greene is indeed a very good choice for learning about physics :)

I really love the simplicity of your discussion Sir, very easy to understand. I remembered when I first write about blackholes as an entry to @suesa's contest haha. By the way, why is it that our universe encourages flatness? I mean the rotation of planets and other objects inside a solar system tends to rotate in flat manner rather than spherical way?

Well, data is what it is. The universe is flat, but not flat in the sense you may understand it. We are talking about large scales here. Much larger than the level of the solar system.

What is the most exact size of the initial state of the "Big Bang"? Asking for a friend, and I feel lazy to Google search this time. I want a humane response. Lol

I beg your pardon? I don't think I understand the question. I will try: As at the big bang, time and space are not really defined, there is no answer to that question :)

What is your, at least, approximate to that initial point before the Big Bang? Asking for a friend.

Then the answer is given above: there is no defined "before the big bang" moment as time is only defined from the big bang.

Okay then. What is the size of the universe at the beginning of the Big Bang itself?

Same answer as before: space is not only defined from the big-bang. The question cannot therefore be answered. In fact, the theory breaks down at the moment of he big bang (we go a singularity), which just means we should replace the theory by another one we have not found yet.

Again, I am afraid I can't give any answer.

That's so interesting about black hole.Sir I also write about dark matter pls also read it.I give You upvote .please read than suggest and upvote also to me.