# Four to Six times the speed of light?!?

Um, isn’t that against the law? Somebody call a cop! Seriously, how can this be?

I don’t know but I am pulling out two quotes from the article and speculating.

I believe that there is nothing moving at 4-6 times c. I would guess that the track of the particles as they get thousands of light years away from a very rapidly spinning object is what is moving 4-6 c.

Imagine taking a laser pointer and spinning around as fast as you can. A few thousand light years away from you, the track of the dot will be moving in excess of the speed of light, but no object, particle, waveform, or information is actually moving faster than c.

I am going to speculate that first of all, the journalist is not cosmologist (well, neither am I, and IANAL, but I know the law :)). Second, the headline is formulated to be sensational but that is not the important part of the event.

Makes sense.

What they describe is what it looks like - by observing the light from the jet. There is a difference in what the image we see - which is light - depicts, and what is actually happening. The jet is pointing nearly right at us, and the matter in the jet is itself moving at very close to the speed of light - right towards us. What we see is the image of that jet moving, and yet the light that conveys that image of movement is only a tiny bit faster than the jet itself. At this point we can get the apperance of things happening at a pace that isn’t at all real. Including things travelling faster then light. But they aren’t.

I wouldn’t put too much stock in the factual accuracy of an article that has a [citation needed] copied from wikipedia… :dubious:

ETA: In fact, the relevant quote is expanded upon in the wiki article: [

](http://en.wikipedia.org/wiki/Messier_87)

Spin a record. Put a quarter near the middle. Measure its speed. Now spin a record and put the quarter on the outer edge. Its going faster right? Now try this with something spinning at the speed of light.

Picture a lighthouse spinning at the speed of light. Now picture it reflecting its beam onto mountains 10 miles away. The light event you see on the mountain will look like a beam of light going faster than light, but it will really just be photons at the speed of light bouncing off. Your eyes are not sophisticated enough to see the difference so you can say something like “Thats going much faster than light.” Its not. It just looks that way. Its an illusion.

Perceived events can happen at faster than light speeds. That doenst mean c is violated.

I don’t understand. If the source is spinning, wouldn’t the whole product look spiraled, like a water sprinkler?

The posters above are correct about how this is only an apparent effect, not a real one. However, the analogs with laser pointers and lighthouse aren’t quite right. As noted by Francis Vaughan above, there actually is a physical object moving along the path we observe; however, the fact that it’s moving very quickly, and that light itself has a propagation speed, causes it to appear to be moving faster than light.

Here’s an example. Suppose I’m on Jupiter, and I have a particle gun that can shoot off beacons at a good fraction of the speed of light. You’re on Earth, and you can track the paths of these beacons. Also suppose, for the sake of argument, that you can’t easily track the distances of these objects; you can only see how far away they are from the line of sight from you to Jupiter.

At some time, I fire off a beacon at a good fraction of the speed of light, whose path makes a very small angle with the straight line between Earth and Jupiter. (I’m ignoring gravity here.) Let’s say that the object is moving along the line of sight at 9/10 the speed of light, and perpendicular to the line of sight at 1/5 the speed of light. This means that the object is moving at about 92% the speed of light, at an angle of 12° to the line of sight; there’s no violation of relativity here, since the object I fired is moving slower than light.

What does this look like from the Earth? Well, suppose I’ve carefully calibrated the beacons to send off light pulses once every ten seconds.[sup]1[/sup] Since the object is already moving towards you at 9/10 the speed of light, it will “catch up” somewhat with a given pulse before it emits the next one; in the second between one pulse and the next, the first light pulse will travel 10 light-second, but the beacon itself travels 9 light-seconds. The net effect is that these pulses will arrive on Earth spaced out by only 1 second.

At the same time, though, you can watch the motion of the object perpendicular to the line of sight between you and Jupiter. Since the object is travelling at 1/5 the speed of light in this direction, the beacon appears to you to be travelling 2 light-seconds between each pulse. The net effect is that you see the object moving 2 light-seconds in one second, and so it appears to be moving at twice the speed of light.

This phenomenon is called superluminal motion, and it’s observed in several galactic objects that emit gobs of particles at a sizeable fraction of the speed of light. However, there’s no violation of relativity here. If you carefully recorded the light pulses and (here’s the important part) extrapolated them back in time to figure out when and where they were actually emitted, rather than just when you received them, you would find that my beacon was moving slower than light. It’s only because the probe is moving towards you at a large fraction of the speed of light that the pulses sort of “pile up” like this, causing the illusion of faster-than-light motion in the direction perpendicular to the line of sight.

[sup]1[/sup] This careful calibration would also have to take into account the relativistic time dilation experienced by the beacon itself. However, hopefully you can see that this argument doesn’t depend on the intervals between pulses being exactly one second, but rather just that they’re evenly spaced.

Allow me to set up an example…

Imagine a spinning source. Viewed from a “pole”, the particle jet resembles the “seconds” sweep arm, moving clockwise. The source is spinning very fast, near the speed of light.

We are observing this jet from edge-on, at the 6-o’clock position, and many light years distant. Also assume this jet is powerfull enough for us to detect with our telescopes (or whatever) at almost all positions on the clock face.

The light (or xrays, or whatever) from the jet, when the jet was directed at the 2-o’clock postion should be delayed in reaching our telescope. It may be arriving after the sweep arm has since rotated towards us (moving clockwise towards the 6-o’clock position). It will be arriving before the light from the equally distant end of the sweep arm was pointed at the 1-o’clock position.

A rather odd view… A time delayed sprinkler.

MikeS has the correct answer, and about as clear an explanation as can be made of it without a diagram. Here’s a more detailed explanation, with diagrams.

The link from Chronos is a great explanation which I am still trying to absorb. I suspect my explanation was grossly simplistic :o But I still maintain that the journalism is even wronger than me.

Your explanation was fine, it was just an explanation for a different phenomenon than what the article in the OP was talking about.

“the motion of M87’s jet was measured at four to six times the speed of light”

This quote from the link sounds, at best, wrongly worded. CookingWithGas, if that’s what you mean, I agree.