Can someone explain how a jet of matter can be measured at a speed C and still not violate Einstein's rule?

Question

Most superluminal jets are outside our galaxy. But, right in our neighborhood in the trifid nebula is a jet that has angular movement > 500,000 km/second. How can that be?

See:
http://oposite.stsci.edu/pubinfo/pr/1999/42/content/9942y.jpg

for a hubble picture of the jet. How can such a thing be possible and not violate the maxim that the speed of light is the maximum speed? Also, when you look at the picture, notice there is a foretrail that shows this jet of matter is led by a jet of some transparent substance that actully makes the jet twice as long as the original jet's measured speed. Can someone explain please?

Anonym · Accepted Answer

Superluminal motion is an illusion, nothing more. It is caused by a high velocity jet which is pointed nearly (but not quite) face on with you. The material in the jet is traveling at nearly c, the speed of the light that it emits. So imagine a blob of gas in such a jet. it emits light towards you at c, but its own speed is nearly c itself. Suppose at time t=0, it emits a light pulse towards you. Then later, at time T, it emits another light pulse so that the time between emission of the two light pulses is T.

However, on reception, the time between receiving the two light pulses is much, much less than T because between the two emissions, the blob approached you so that the two pulses were not emitted from the same place.

In time T, the first light pulse traveled a distance cT. The blob itself, at velocity v, traveled a distance vT. So when the blob emits the second pulse, the distance between it and the first pulse is (c-v)T, and that must be the amount by which the second flash trails the first as they travel to you. So when you observe, the two flashes are separated by a time (1-v/c)T. If v is very close to c, then the time difference in receiving the two flashes is extremely small. But in that time, the blob has apparently moved a great distance. The actual speed was v, but the apparent (illusory) speed was v' = (distance traveled)/(reception time) = (vT) / [(1-v/c)T] = v/(1-v/c), which could be very large indeed.

In other words, the blob is moving so fast that it nearly catches up with its own light signals towards you. As a result, you see its behavior very speeded up, because all the light signals it sends are compressed into a tiny space.

Edit: I'm not talking about blueshift.  I'm talking about an optical effect that speeds up the apparent action when something is moving towards you at nearly c. There is no reason to believe the jet in the image is at nearly right angles to the line of sight. Indeed, it must be aimed pretty closely towards you to see the illusion.

Can someone explain how a jet of matter can be measured at a speed > C and still not violate Einstein's rule?

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