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The Question

(Submitted December 28, 1998)

I am somewhat acquainted with astronomy and relativity. My question is this: Quasar 3C273 has a jet moving at 9 times light speed as seen from earth. This has been explained away by science by saying that the angle of the jet towards earth is just small enough to actually only give an illusion of faster than light speeds relative to earth.

However there must be a counter-jet and I have been told by an astronomer from Flagstaff, AZ that this is true. The existence of a counter-jet showing the same relative speeds would surely obviate the 'small angle' explanation and would then prove that faster-than-light velocities do exist in the universe relative to earth. This, then would indicate that relativity is wrong in its assertion that FTL relative to earth cannot be. Could you please tell me where I'm wrong. It is a major stumbling block in my acceptance of relativity. Thank you very much and could you, if possible, indicate who is answering if I should have a follow-up question. Again, thank you.

The Answer

I don't know offhand whether the counterjet for 3C273 has been seen. http://www.cfht.hawaii.edu/Reference/Bulletin/Bull36/bulleti3.htm#764973 implies that it hasn't. It would be expected to be dimmer (since radiation tends to be beamed in the direction of motion) and not show apparent superluminal motion.

You are right that IF a superluminal counterjet were seen, that would mean that something is very wrong with our understanding of the system or of relativity.

I do know that the the 'microquasar' in our galaxy 1915+105, shows both jets, and the one pointed towards us is apparently-superluminal while the one pointed away is not.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

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