Can Black Holes Increase in Mass?

[Nova]

It is confirmed that Black Holes have variations of masses. But these masses are said to be gained from the base mass from the Supernova it's born from. Another way a Black hole gained mass was that the collisions of multiple Black Holes caused it to add on to the mass of each other, such as when two super massive Black Holes collide to form an elliptical galaxy. But if these black holes can gain mass from each other, then it should gain mass from the matter it devours, right?

 

[Simon Bridge]

Depends on where you are looking from and what you count as the mass of the black hole.
In the sense that black holes can "devour matter", yes, they gain mass as a result.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[Dale]

A thread derailment has been removed and the thread reopened. I remind participants to limit responses to topics about which you have some mainstream scientific knowledge, and to report nonsense and speculation rather than further promulgating it.

 

[mfb]

It is confirmed that Black Holes have variations of masses. But these masses are said to be gained from the base mass from the Supernova it's born from. Another way a Black hole gained mass was that the collisions of multiple Black Holes caused it to add on to the mass of each other, such as when two super massive Black Holes collide to form an elliptical galaxy. But if these black holes can gain mass from each other, then it should gain mass from the matter it devours, right?

Right. The usual way to define "mass" here would be the Komar mass before and after matter falls in.[/size]

 

[Simon Bridge]

I think the following discussion is relevant - particularly the last post.
https://www.physicsforums.com/showthread.php?t=355960
... the usual issue about BHs gaining mass is the observation that it takes an object an infinite amount of time, from the POV of a distant observer, to cross the event horizon.
The actual math is trickier than that.