M87's Jet is 5,000 LY long. In a story today, HST watched it brighten, dim and brighten again in only 7 years?

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"Among the consequences of these near light-speed or relativistic jets are flashlightlike beams of high-energy x-rays and gamma rays as well as the illusion of superluminal (faster-than-light) speeds when viewed straight on..."
http://www.sciam.com/article.cfm?id=black-hole-plasma-jets-trace-corkscrew-path

"Sequence of Hubble images showing apparent motion at six times the speed of light in the galaxy M87" (in 1997 photos)
http://www.stsci.edu/ftp/science/m87/m87.html...Show more

I think that's a really good question. In that picture, the area of brightening looks pretty small, though. I don't know where you get that the area of brightening is 100 LY across--the article doesn't say anything about the size of the brightened area. It's a pretty fuzzy photo, so I'm guessing that the actual bright event is much smaller and it is just blurred out.

It is possible, however, for certain events to appear faster than the speed of light without anything breaking any laws of physics. For example, imagine that I shine a very bright light at a disk of dust that is 100LY wide. if things are oriented right, the entire disc may light up at approximately the same time--within a few years, and much faster than 100 years, since the light is hitting the whole surface at about the same time. That could be what's happening in a cloud outside of M87. As another example, I could train a big line of fireflies to all light up at exactly the same time, hundreds of years from now. I set them all out in a line 100 light years long (which will take me at least 100 years to set up), and then I travel far away and look at it--depending on how I programmed the fireflies ahead of time, you could have the whole thing light up at once, or go from one side to the other much faster than the speed of light. The fireflies and the light are not going faster, just the phenomenon is moving faster than light....Show more

I'm not even going to pretend that I can shed any light on what is happening in the jet. But a mechanism to produce these kinds of effects are far from impossible.

Imagine an immense 2-D sheet of water. light years in size. Now let an equally immense plane wave burst of electrons travelling at .99c impinge on the slab along the normal of the sheet. If you are very far away, you will see the entire slab illuminate uniformly from Cerenkov radiation, then become dark, regardless of how far apart the pieces of it are.

In contrast, if you were actually on the sheet, you would see the immediate area around you light up, then the sheet's illumination would move away from you at c. Something more similar to what you were expecting?...Show more

Ok, from reading the article, it is clear that what is brightening and dimming is "a clump of matter embedded in the jet", not the entire jet.

I can think of a few things that might explain that (the jet is running into something, like some interstellar gas, for example) but the article seems to make it clear to me that it is *not* the entire jet that is flaring, but rather a clump of matter in a specific part of it....Show more

I'm guessin that it could be an "optical illusion"...

1) The flare doesn't has to come from a single-point-event (like a supernova event), so it could actually be happening in a large area at te same time...

2) ... But we're observing it from far enough to percieve it as almost a single-point-event.

I think it's an excellent question anyway. Cheers....Show more