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Cosmic Variance

Wednesday, January 25, 2006

Idiots at the other Cosmic Variance

Mark Troden at the other Cosmic Variance talked about a so called recent discovery of an earth like planet 25,000 light years away. The discovery was made using the gravitational lensing effect of the General Relativity.

I say it's totally nonsense. And too bad that cosmologists like Mark Troden et al spend a lifetime studying General Relativity, but could not carry out a little bit order of magnitude estimation and and see through it that the observed brightening effect actually has NOTHING to do with gravitational lensing.

I am not saying gravitational lensing does not exist. But what the research group observed could not be explained as gravitational lensing.

If there is a gravitational lensing, you can imagine the gravitational field acts like an optical lense. The diameter of that lense is about the distance where the gravity is significant, i.e., approximately some multiples of the diameter of the object that causes the gravitational lensing. What about the focus length? We know star lights are bended approximately 1 arcsecond when it skip over the surface of the sun. So the focal length would be approximately the diameter divided by the sine of that bending angle, i.e., about 2x10^5 times the diameter of the lense. The star is about the same size as the sun, so its diameter is 2x10^9 meter, multiply by let say 5 times, 1x10^10, that's about how big the diameter of the lense is. So the focal length of the gravitational lense would be about 2x10^15 meter, or, 0.2 light years distance at most.

The distance of the observed object is 25,000 light years away, more than 1x10^5 times longer than the possible focal length of the gravitational lense. Clearly, had it been a gravitational lense, you would only observe the star to be dimmer, not brighter, because the light is diverged by the lense. You would have to be at a distance of approximately the correct focal length, to be able to see the light more concentrated and hence see the star become brighter.

And not to meantion that the gravitational lense of that planet would have to be of much smaller size. Consider that the observer, the earth is moving at 30km per second speed around the sun. The time that this planet sits right in the path of the light would last a much shorter time period, probably a few minutes, instead of half a day as reported. Also, the chance that the planet happen to be sitting at exactly the right place to block the star light to earth, would be so small that you are unlikely to see the lensing effect.

I challenge Mark Troden or Sean to use his knowledge of GR, to do a more careful quantitative estimate of exactly how big the lensing effect would be, and what the focal length would be like, therefore to rule in or rule out that reported observation as gravitational lensing.

Cosmic Variance


At 8:56 PM, Anonymous Anonymous said...


At 2:51 AM, Anonymous Anonymous said...

Mark Troden and Sean Carroll are just pathetic teachers who tow the mainstream line and can't think outside the box, for fear of being called crackpot.

The same thing happens to people when they become religious ministers. They preach about something they don't understand, because they are paid to do it.

Can't you see?


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