# Distance between 2 orbits of a black hole

• ArnSpin
In summary, the distance between 2 orbits of a black hole is 3.05M, the distance between 3 orbits is 1.54M, and the distance between 4 and 6 orbits is 2.60M.
ArnSpin
[SOLVED] Distance between 2 orbits of a black hole

## Homework Statement

Hello.
I am currently working on the black holes in the Schwarzschild metric, an exercise course asks me to calculate the physical distance between different orbits.

$$d_{1}$$ = distance betwenn 2M and 3M
$$d_{2}$$ = distance betwenn 3M and 4M
$$d_{3}$$ = distance betwenn 4M and 6M

Where (x)M are the black hole orbit's.

How to calculate this physical distance ?

I know that the distance between 0 and 2M is 3km M/Msun but, I think that this distance isn't really physical because it's expressed as a function of considered mass object normalized by the mass of sun.
I thought integrate the Schwarzschild metric on the spatial term, but isn't trivial...

Exercise may be simple, I do not know, but I do not know the answer, so I thank you in advance for your possible assistance.

Sincerely
ArnSpin

Last edited:
ArnSpin said:

## Homework Statement

Hello.
I am currently working on the black holes in the Schwarzschild metric, an exercise course asks me to calculate the physical distance between different orbits.

$$d_{1}$$ = distance betwenn 2M and 3M
$$d_{2}$$ = distance betwenn 3M and 4M
$$d_{3}$$ = distance betwenn 4M and 6M

Where (x)M are the black hole orbit's.

How to calculate this physical distance ?

I know that the distance between 0 and 2M is 3km M/Msun but, I think that this distance isn't really physical because it's expressed as a function of considered mass object normalized by the mass of sun.
I thought integrate the Schwarzschild metric on the spatial term, but isn't trivial...

Exercise may be simple, I do not know, but I do not know the answer, so I thank you in advance for your possible assistance.

Sincerely
ArnSpin

In general, to find a radial distance what you must do is set $$d\phi = \d\theta = dt = 0$$ in the metric. Then ds is the physical distance between two points along a radius. You then have a relation between ds and dr (the coordinate displacement). Can you write that relationship down?

The total physical distance between two points is the integral of both sides of this relation from the initial to the final coordinate.

PS: You are using geometrized units to write positions in terms of masses?

Yes it's geometrized units, I don't use the technical terme sorry.

I understand that my distance is (for d1)

$$d_{1} = \int_{2M}^{3M} (1 - \frac{2M}{r})^{-1/2} dr$$

with no intergration of $$dt$$ and $$d\Omega$$

But the integration is difficult not?
I've no idea to resolve this...

ArnSpin said:
Yes it's geometrized units, I don't use the technical terme sorry.

I understand that my distance is (for d1)

$$d_{1} = \int_{2M}^{3M} (1 - \frac{2M}{r})^{-1/2} dr$$

with no intergration of $$dt$$ and $$d\Omega$$

But the integration is difficult not?
I've no idea to resolve this...

if you have to do it by hand it's tough. But you can look it up, for example using http://integrals.wolfram.com/index.jsp. It's simply a log (or may be written as an inverse sinh) and a bunch of square roots. It's not that bad

Ok, thank you very much, indeed I hoped a solution more elegant that the integration beast and nasty, but if it's impossible, I will do that. Thanks for all Kdv :)

ArnSpin said:
Ok, thank you very much, indeed I hoped a solution more elegant that the integration beast and nasty, but if it's impossible, I will do that. Thanks for all Kdv :)

You are welcome. if the points were very close to each other, you could Taylor expand and the integral simplifies considerably. But you can't do that in your case.

While you had right, it was necessary to integrate it numerically, using mathematica or something like that.
The distance between two orbits, 2M to 6M is7.2M!

$$x=r/M$$

$$x_1= 2$$ $$x_2= 3$$

$$d_1 = M\int_2^3{(1-x/M)}^{-1/2}dx$$

$$d_1= 3.05M$$ $$d_2= 1.54M$$ $$d_3= 2.60M$$

$$d_{tot}= 7.2M$$

We can see the curvature of space-time ...

Again, thank you for your help.

ps: Sorry for my english, I'm french.

ArnSpin said:
While you had right, it was necessary to integrate it numerically, using mathematica or something like that.
The distance between two orbits, 2M to 6M is7.2M!

$$x=r/M$$

$$x_1= 2$$ $$x_2= 3$$

$$d_1 = M\int_2^3{(1-x/M)}^{-1/2}dx$$

$$d_1= 3.05M$$ $$d_2= 1.54M$$ $$d_3= 2.60M$$

$$d_{tot}= 7.2M$$

We can see the curvature of space-time ...

Again, thank you for your help.

ps: Sorry for my english, I'm french.

I really thought it could be done analytically. I will check on this later when I have a few minutes.

Et ne vous en faites pas pour votre anglais, il est en fait tres bon!

## 1. What is the distance between two orbits of a black hole?

The distance between two orbits of a black hole can vary greatly depending on the size and mass of the black hole, as well as the speed and trajectory of the objects in orbit. Generally, the distance can range from a few hundred kilometers to several million kilometers.

## 2. How is the distance between two orbits of a black hole measured?

The distance between two orbits of a black hole is typically measured using astronomical units (AU) or light-years (ly). These units are used to describe distances in space and are based on the distance between the Earth and the Sun.

## 3. Can the distance between two orbits of a black hole change over time?

Yes, the distance between two orbits of a black hole can change over time. This is due to the fact that black holes can grow in size and mass as they consume matter from their surroundings, causing the gravitational pull to increase and possibly altering the orbits of objects around them.

## 4. How does the distance between two orbits of a black hole affect the objects in orbit?

The distance between two orbits of a black hole can have a significant impact on the objects in orbit. The closer an object is to the black hole, the stronger the gravitational pull it experiences, which can cause the object to accelerate or change its trajectory.

## 5. Is there a limit to the distance between two orbits of a black hole?

There is no specific limit to the distance between two orbits of a black hole. However, the farther away an object is from the black hole, the weaker the gravitational pull it experiences and the more stable its orbit will be. Objects that are too far away may not be able to maintain a stable orbit and may eventually be pulled out of orbit altogether.

• Special and General Relativity
Replies
13
Views
669
• Special and General Relativity
Replies
57
Views
2K
• Astronomy and Astrophysics
Replies
13
Views
776
• Special and General Relativity
Replies
32
Views
2K
Replies
8
Views
1K
Replies
7
Views
2K
• Special and General Relativity
Replies
1
Views
349
Replies
1
Views
2K
• Astronomy and Astrophysics
Replies
9
Views
1K