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SaganRules
2009-Dec-07, 08:50 AM
Hi all, bear with me I'm new to this. I just read an article about the M87 supermassive black hole in which it said that the supermassive black hole at the centre of M87 is ejecting material at 4 to 6 times the speed of light. How can anything go faster than light???:confused: I thought that light speed was the speed limit of the universe. It was an article in the Daily Galaxy.

hhEb09'1
2009-Dec-07, 10:58 AM
Welcome to BAUT, SaganRules. Here's the The Daily Galaxy article:

Mystery Jet from M87's Supermassive Black Hole Ejecting at 4 to 6 Times Speed of Light (http://www.dailygalaxy.com/my_weblog/2009/12/mystery-jet-from-m87s-supermassive-black-hole-ejecting-at-4-to-6-times-speed-of-light.html)

That measurement from Hubble was done in 1999.

ngc3314
2009-Dec-07, 01:24 PM
This phenomenon - motions projected across the line of sight ("transverse motions") exceeding c (or simply superluminal motion) has been known in the radio regime for several decades. It is well explained in most of the cases by relativistic motion (sometimes as fast as gamma>10 or v>0.99c) in jets that we see near our line of sight, where the location of radiation nearly catches up with the wave fronts of previously emitted radiation. The idea has been confirmed for so-called microquasars in our own Galaxy, where we can see the motions of individual ejected blobs and compare their brightnesses (plus, for the lone case of SS 433, Doppler shifts). This wold mean that M87's jet is pointed within about 15 degrees of us; a relativistic effect known as Penrose rotation means that internal structures such as shock fronts (which may be seen in M87) appear to be more perpendicular than they "really" are, since light that we see from the far side had to leave earlier when it was closer to the core. (I can't believe the Daily Galaxy article did not include this bit of background, but it skipped quickly from the apparent velocity to the variability of knot HST-1).

StupendousMan
2009-Dec-07, 01:34 PM
Here's an explanation of apparent superluminal
motions of material in a jet:

http://spiff.rit.edu/classes/phys200/lectures/superlum/superlum.html

SaganRules
2009-Dec-09, 08:36 AM
Thanks all for clearing that up. Cosmology and physics are really mind bending but incredibly fascinating. I wish I had been good enough at maths at school to be involved in these areas.
Cheers,
SaganRules::)