# Thread: silly question? time slows down near a black hole...

1. ## silly question? time slows down near a black hole...

if time slows down as gravity increases..
then would the orbiting velocity of masses around a high gravity mass like a super massive black hole be observed to lose velocity or orbit slower the closer the mass is to the center mass?

could this time dilation explain the observation that stars near the center of a galaxy orbit the center mass at the same velocity as the stars on the edge?

2. I am not 100% sure but I think that the orbit would be the same around any mass regardless of if it were a BH or a star as long as the masses were equal.
Like earth would revolve in a similar manner around a 1 solar mass BH as it would around the sun.

The rate that an external observer would observer would depend on the amount of time dilation relative to the external observer. But unless the planet is very close to the EH the time dilation would probably not be much.

could this time dilation explain the observation that stars near the center of a galaxy orbit the center mass at the same velocity as the stars on the edge?
I wouldnt think so.

3. Originally Posted by tommac
I am not 100% sure but I think that the orbit would be the same around any mass regardless of if it were a BH or a star as long as the masses were equal.
Like earth would revolve in a similar manner around a 1 solar mass BH as it would around the sun.

The rate that an external observer would observer would depend on the amount of time dilation relative to the external observer. But unless the planet is very close to the EH the time dilation would probably not be much.

I wouldnt think so.
He got it on his own.

To those the difference in time dilation between a Earth sized/mass object orbiting a 1 solar mass black hole at 1 AU and a same sized object orbiting our sun at 1 AU, ie Earth, is negligible. The maths isn't hard and if you look in past posts I'm pretty sure I've given tommac the actual figures before and again we are talking like 7th decimal place difference here from memory. Note the post I'm thinking of is like 4 years old so I'm 4 years more senile then before so may be off by a decimal place or 2.

4. Originally Posted by sabianq
if time slows down as gravity increases..
then would the orbiting velocity of masses around a high gravity mass like a super massive black hole be observed to lose velocity or orbit slower the closer the mass is to the center mass?

could this time dilation explain the observation that stars near the center of a galaxy orbit the center mass at the same velocity as the stars on the edge?
No.

Now for the explanation for the lurkers (tommac has me on ignore)

Time dilation is only significant very close to the EH of a black hole. For a non rotating black hole there is no stable orbit within 1.5x the radius of the event horizon. This is what we call the photon sphere, ie only photons can orbit at this distance.

Time dilation from a few light years away from a SMBH at the centre of a galaxy to the edge of a galaxy is 5th-8th decimal place change. IE at worst .001% difference yet the radial velocity should fall off much quicker then this.

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