Can two planets in an empty universe meet/be pulled together?
$begingroup$
For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless.
Only two identical earths without moon are left in the entire universe and they are 5 billion light years apart. From their starting position the earths do not spin or move.
Would these planets ever meet/pulled together?
Or are the their respective gravities too small to have any meaningful effect?
gravity universe observable-universe rogue-planet
asked 18 hours ago
tomtom
516
$endgroup$
|
show 2 more comments
$begingroup$
For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless.
Only two identical earths without moon are left in the entire universe and they are 5 billion light years apart. From their starting position the earths do not spin or move.
Would these planets ever meet/pulled together?
Or are the their respective gravities too small to have any meaningful effect?
gravity universe observable-universe rogue-planet
asked 18 hours ago
tomtom
516
$endgroup$
1
$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
2
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago
|
show 2 more comments
$begingroup$
For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless.
Only two identical earths without moon are left in the entire universe and they are 5 billion light years apart. From their starting position the earths do not spin or move.
Would these planets ever meet/pulled together?
Or are the their respective gravities too small to have any meaningful effect?
gravity universe observable-universe rogue-planet
asked 18 hours ago
tomtom
516
$endgroup$
For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless.
Only two identical earths without moon are left in the entire universe and they are 5 billion light years apart. From their starting position the earths do not spin or move.
Would these planets ever meet/pulled together?
Or are the their respective gravities too small to have any meaningful effect?
gravity universe observable-universe rogue-planet
gravity universe observable-universe rogue-planet
asked 18 hours ago
tomtom
516
asked 18 hours ago
tomtom
516
edited 11 hours ago
tom
asked 18 hours ago
tomtom
516
asked 18 hours ago
tomtom
516
asked 18 hours ago
tomtom
516
516
1
$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
2
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago
|
show 2 more comments
1
$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
2
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago
1
1
$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
2
2
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago
|
show 2 more comments
2 Answers
2
active
oldest
votes
$begingroup$
Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula is
$$sqrt{frac{d^3}{2G(m_1+m_2)}}$$
where $d$ is the distance, m1 and m2 are the masses of the planets, and G is the Gravitational constant. This gives a time of about $10^{23}$ years, much much longer than the universe has existed. This assumes Newtonian mechanics. Relativity would not change the conclusion much.
There is no known upper limit to gravity, and plenty of indirect evidence that it has no upper limit.
answered 18 hours ago
James KJames K
33.8k254116
$endgroup$
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
add a comment |
$begingroup$
I think Yes and No
Yes: Given that the universe isn't expanding at a faster rate than the speed of the gravity effect of those objects, then the gravity effect of them would be able to reach one another.
Even then it would take a very long time for the gravity effects of the objects to reach one another, so the objects would just stay there motionless for ages before starting to move towards eachother.
No: For the objects to reach one another it would mean that the universe would need to be expanding slower than the maximum speed that those objects can reach, any faster than that and they would never reach eachother even if they were moving towards eachother.
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
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oldest
votes
$begingroup$
Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula is
$$sqrt{frac{d^3}{2G(m_1+m_2)}}$$
where $d$ is the distance, m1 and m2 are the masses of the planets, and G is the Gravitational constant. This gives a time of about $10^{23}$ years, much much longer than the universe has existed. This assumes Newtonian mechanics. Relativity would not change the conclusion much.
There is no known upper limit to gravity, and plenty of indirect evidence that it has no upper limit.
answered 18 hours ago
James KJames K
33.8k254116
$endgroup$
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
add a comment |
$begingroup$
Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula is
$$sqrt{frac{d^3}{2G(m_1+m_2)}}$$
where $d$ is the distance, m1 and m2 are the masses of the planets, and G is the Gravitational constant. This gives a time of about $10^{23}$ years, much much longer than the universe has existed. This assumes Newtonian mechanics. Relativity would not change the conclusion much.
There is no known upper limit to gravity, and plenty of indirect evidence that it has no upper limit.
answered 18 hours ago
James KJames K
33.8k254116
$endgroup$
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
add a comment |
$begingroup$
Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula is
$$sqrt{frac{d^3}{2G(m_1+m_2)}}$$
where $d$ is the distance, m1 and m2 are the masses of the planets, and G is the Gravitational constant. This gives a time of about $10^{23}$ years, much much longer than the universe has existed. This assumes Newtonian mechanics. Relativity would not change the conclusion much.
There is no known upper limit to gravity, and plenty of indirect evidence that it has no upper limit.
answered 18 hours ago
James KJames K
33.8k254116
$endgroup$
Yes, they would experience gravitational attraction. It would take a long time for them to collide... the formula is
$$sqrt{frac{d^3}{2G(m_1+m_2)}}$$
where $d$ is the distance, m1 and m2 are the masses of the planets, and G is the Gravitational constant. This gives a time of about $10^{23}$ years, much much longer than the universe has existed. This assumes Newtonian mechanics. Relativity would not change the conclusion much.
There is no known upper limit to gravity, and plenty of indirect evidence that it has no upper limit.
answered 18 hours ago
James KJames K
33.8k254116
edited 10 hours ago
answered 18 hours ago
James KJames K
33.8k254116
answered 18 hours ago
James KJames K
33.8k254116
answered 18 hours ago
James KJames K
33.8k254116
33.8k254116
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
add a comment |
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
4
4
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
"the planets would (after billions of billions of years) actually be travelling at relativistic speeds." - Would they, though? If the planets were originally motionless, that means that when they encounter each other, they must be traveling slower than escape speed, right?
$endgroup$
– Tanner Swett
11 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
I think you are right. I shall edit.
$endgroup$
– James K
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
The collision speed of a test particle falling to Earth from infinity is equal to the escape velocity, about 11 km/s. For 2 Earth-mass bodies, the speed will be similar, I think you can just multiply it by $sqrt 2$, but I'm too sleepy to do the algebra right now. ;)
$endgroup$
– PM 2Ring
10 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
$begingroup$
Can you add a source for that equation? If I set $d$ to either the total distance between the two, or half that, I don't get the same answer as from numerical integration. There might need to be a slice or two of pie in there somewhere; physics.stackexchange.com/a/14702/83380 and also physics.stackexchange.com/a/90722/83380
$endgroup$
– uhoh
3 hours ago
add a comment |
$begingroup$
I think Yes and No
Yes: Given that the universe isn't expanding at a faster rate than the speed of the gravity effect of those objects, then the gravity effect of them would be able to reach one another.
Even then it would take a very long time for the gravity effects of the objects to reach one another, so the objects would just stay there motionless for ages before starting to move towards eachother.
No: For the objects to reach one another it would mean that the universe would need to be expanding slower than the maximum speed that those objects can reach, any faster than that and they would never reach eachother even if they were moving towards eachother.
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
$begingroup$
I think Yes and No
Yes: Given that the universe isn't expanding at a faster rate than the speed of the gravity effect of those objects, then the gravity effect of them would be able to reach one another.
Even then it would take a very long time for the gravity effects of the objects to reach one another, so the objects would just stay there motionless for ages before starting to move towards eachother.
No: For the objects to reach one another it would mean that the universe would need to be expanding slower than the maximum speed that those objects can reach, any faster than that and they would never reach eachother even if they were moving towards eachother.
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
add a comment |
$begingroup$
I think Yes and No
Yes: Given that the universe isn't expanding at a faster rate than the speed of the gravity effect of those objects, then the gravity effect of them would be able to reach one another.
Even then it would take a very long time for the gravity effects of the objects to reach one another, so the objects would just stay there motionless for ages before starting to move towards eachother.
No: For the objects to reach one another it would mean that the universe would need to be expanding slower than the maximum speed that those objects can reach, any faster than that and they would never reach eachother even if they were moving towards eachother.
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
$endgroup$
I think Yes and No
Yes: Given that the universe isn't expanding at a faster rate than the speed of the gravity effect of those objects, then the gravity effect of them would be able to reach one another.
Even then it would take a very long time for the gravity effects of the objects to reach one another, so the objects would just stay there motionless for ages before starting to move towards eachother.
No: For the objects to reach one another it would mean that the universe would need to be expanding slower than the maximum speed that those objects can reach, any faster than that and they would never reach eachother even if they were moving towards eachother.
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 9 hours ago
answered 10 hours ago
NoobNoob
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 10 hours ago
NoobNoob
112
answered 10 hours ago
NoobNoob
112
112
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Noob is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
add a comment |
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$begingroup$
For a good 5 billions ly they ignore each other.
$endgroup$
– Alchimista
18 hours ago
$begingroup$
I want to say that probably you consider the entire universe as to be the entire visible and/or observable universe. The accepted answer is indeed satisfactory only under several assumption. It is more to answer about two generic masses in a static enormous room. Else cosmology sets in
$endgroup$
– Alchimista
17 hours ago
$begingroup$
I am familiar with the differences, but for this question to work i needed to be sure there where no outside factors affecting the two planets.
$endgroup$
– tom
17 hours ago
2
$begingroup$
Is the universe expanding?
$endgroup$
– userLTK
15 hours ago
$begingroup$
It is not I'm sorry this was not mentioned.
$endgroup$
– tom
12 hours ago