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View Full Version : Shapiro delay-how is the distance to Mercury calculated for this experiment?



Frog march
2009-Feb-23, 06:27 AM
I was just reading about the Shapiro delay experiment, and it seems that a radar signal is bounced off mercury just before it goes behind the Sun.

A delay was found, which is supposed to show that light slows down while passing through a strong gravitational field.

To calculate the speed of light along its path, you would need, a-the time taken for the light to complete its course, and b-the distance to the object being bounced off(in the case of the experiment, Mercury).

What I want to know is, how was the distance to Mercury worked out?

Hornblower
2009-Feb-23, 12:57 PM
I was just reading about the Shapiro delay experiment, and it seems that a radar signal is bounced off mercury just before it goes behind the Sun.

A delay was found, which is supposed to show that light slows down while passing through a strong gravitational field.

To calculate the speed of light along its path, you would need, a-the time taken for the light to complete its course, and b-the distance to the object being bounced off(in the case of the experiment, Mercury).

What I want to know is, how was the distance to Mercury worked out?

We can measure the distance to Mercury with radar at other points around its orbit, and fit an ellipse to those points. That will tell us when the pulse at superior conjunction should be expected in the absence of the strong solar gravity, and the difference between that and the actual time is the Shapiro delay.

Canticle
2009-Jun-09, 07:12 PM
Just a correction. The Shapiro delay is due to the bent light path. There are some bizarre theories about gravity slowing light, and dilation, due to anomolies in delays at a cosmic level, but these are very inconsistent and rarely admitted (and are due to something else a bit technical)

undidly
2009-Jun-10, 01:11 AM
Just a correction. The Shapiro delay is due to the bent light path. There are some bizarre theories about gravity slowing light, and dilation, due to anomolies in delays at a cosmic level, but these are very inconsistent and rarely admitted (and are due to something else a bit technical)

Gravity slows TIME.(Why?That is for another thread.It is GR).
Light is slowed only to an outside observer.
Go into the G field and measure the speed.It is C.

Mass distorts space so distances increase.(rubber sheet).
Slow time and greater distance add to give the Shapiro delay.

RussT
2009-Jun-10, 01:26 AM
Gravity slows TIME.(Why?That is for another thread.It is GR).
Light is slowed only to an outside observer.
Go into the G field and measure the speed.It is C.

Mass distorts space so distances increase.(rubber sheet).
Slow time and greater distance add to give the Shapiro delay.

Sorry, but the Shapiro Effect has nothing to do with Time Dilation!

a1call
2009-Jun-10, 04:12 AM
Interesting topic.

We can measure the distance to Mercury with radar at other points around its orbit, and fit an ellipse to those points.

*- 3 points define a circle. I.e. One and only one circle can be drawn through any given 3 points.

*- Circles are subsets of ellipses

*- A circle can be fit through any arbitrary 3 points along a non-circular ellipse.

*- This would imply that infinite number of ellipses can be drawn through any given 3 points(thinking of the infinite number of ellipses morphing between the circle and the ellipse in the last pointer/note )

Question:

**- What is the minimum number of distinct points required to define one and only one ellipse?(my guess = 4)

Josh
2009-Jun-10, 04:27 AM
Minimum number of points needed to fit an ellipse is five.

a1call
2009-Jun-10, 04:33 AM
Thank you Josh,
Just one more thing to keep me up at night. :lol:

Josh
2009-Jun-10, 05:02 AM
Here's (http://users.cs.cf.ac.uk/Paul.Rosin/resources/papers/ellipse3.pdf) a paper backing that up. I don't know why I knew it was five points. Just one of those things we pick up along the way I guess.

And here's (http://www.absoluteastronomy.com/topics/Ellipse) an astronomy-centric site.