[math-fun] speed of our galaxy : 630 km per second but how can this be possible ?
Hello, A very interesting news came out these days about a group of astrophysicists found a way to make a path of direction of many known galaxies in the area. By area I mean a large cube of 500 million light years wide called Laniakea which has nothing to do with Ikea. They used intense calculations, (Hélène Courtois), http://www.francetvinfo.fr/sciences/espace/qu-est-ce-que-laniakea-le-superco... The result is astonishing and the word is weak, it is mind blowing in my opinion. See the short film (in french). So according to those calculations, one thing we already knew is that our own galaxy is travelling at a speed of 630 km per second, this is impressive if we consider the mass of the galaxy, it takes 476 years to travel 1 light year and since the beginning of the known birth of earth, we travelled about 10 million light years. BUT : there is a simple question : how can this be possible if we consider the pure energy to accelerate such an astronomic mass (the weight of the galaxy), and in more general terms : how can such a mass can acquire any speed at all ?? Can anyone has an explanation for this ? best regards, Simon Plouffe
On 9/8/2014 11:53 AM, Simon Plouffe wrote:
Hello,
A very interesting news came out these days about a group of astrophysicists found a way to make a path of direction of many known galaxies in the area. By area I mean a large cube of 500 million light years wide called Laniakea which has nothing to do with Ikea.
They used intense calculations, (Hélène Courtois),
http://www.francetvinfo.fr/sciences/espace/qu-est-ce-que-laniakea-le-superco...
The result is astonishing and the word is weak, it is mind blowing in my opinion. See the short film (in french).
So according to those calculations, one thing we already knew is that our own galaxy is travelling at a speed of 630 km per second, this is impressive if we consider the mass of the galaxy, it takes 476 years to travel 1 light year and since the beginning of the known birth of earth, we travelled about 10 million light years.
BUT : there is a simple question : how can this be possible if we consider the pure energy to accelerate such an astronomic mass (the weight of the galaxy), and in more general terms : how can such a mass can acquire any speed at all ??
The obvious question is, speed relative to what? And why would be suppose it was initially stationary (relative to whatever) and got accelerated. In the Big Bang everything was moving at the speed of light, so the better question might be, how did it get so slow - but maybe it didn't since it's still moving at the speed of light relative to galaxies 40 billion light years away. Brent
Can anyone has an explanation for this ?
best regards, Simon Plouffe
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Hello, let me rephrase the question, I know of course the concept of relative speed, even if we take into account that, at the moment of the big bang, the whole thing was going at the speed of light. Just take 1 galaxy and if we calculate the amount of kinetic energy to move a few kilometres per second , the amount of energy necessary is astronomical, how can we explain that ?, Where is the energy coming from ? Best regards, Simon Plouffe
On 9/8/2014 9:05 PM, Dave Dyer wrote:
Where is the energy coming from ? The amount of energy to accelerate the mass of a galaxy is trivial compared to the amount of energy tied up in the mass itself.
The kinetic energy of the galaxy depends on what you compare it to. It didn't start at zero and get accelerated, it started at the speed of light. So initially almost all it's energy was kinetic and the mass-energy was trivial. Only now it is 40Glyr from that stuff relative to which it's moving at the speed of light and it's among other stuff moving the same way, so it seems to be moving slowly compared to the speed-of-light. But, as Albert said, it's all relative. Brent Meeker
The bang was really big.
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There are roughly two answers, probably neither one is entirely satisfying. If you forget general relativity for the moment then one possible answer is that the net energy is zero. The galaxies and other stuff have just enough energy to climb out of the gravitational potential well and will reach zero average relative speed when they are infinitely far apart. So the energy didn't have to come from anywhere, it was just zero and got divided up into negative gravitational energy and kinetic energy. But what, you will ask, about the mass energy to the matter which will still exists even though infinitely dilute? Well it's almost negligble but it's still a few percent of the total mass energy, so maybe the universe will recollapse (although it doesn't look like it). The second answer is, in a curved spacetime, like our universe, if there's no time-like Killing field then there's no way to define a conserved quantity you could call "the energy of the universe". John Baez has a good write up on this. Brent Meeker On 9/8/2014 8:54 PM, Simon Plouffe wrote:
Hello,
let me rephrase the question, I know of course the concept of relative speed, even if we take into account that, at the moment of the big bang, the whole thing was going at the speed of light. Just take 1 galaxy and if we calculate the amount of kinetic energy to move a few kilometres per second , the amount of energy necessary is astronomical, how can we explain that ?, Where is the energy coming from ?
Best regards, Simon Plouffe
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participants (3)
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Dave Dyer -
meekerdb -
Simon Plouffe