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ThaBizness
sage

Reged: 12/28/05

Loc: St. Louis, MO
Understanding speed of light and gravity.
#2347091 - 04/23/08 03:47 PM

Stupid question....

I was listening to a episode of astronomy cast (The General Relativity episode) and in that they were discussing how they could bounce a radar signal off Mars and depending on if the signal had to go past the sun it would take longer. They explained this as relative time speeds up or slows down depending on how close one is to a object with a lot of mass/gravity.

If this is the case then are the calculations we do to determine distance to deep space objects off because I assume the light would have to pass by multiple massive objects/empty space and slow down/speed up.

Or am I just reading too much into this?

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jcely7
super member

Reged: 10/21/07

Loc: Minnesota
Re: Understanding speed of light and gravity. [Re: ThaBizness]
#2347139 - 04/23/08 04:07 PM

my understanding is that the time it takes light to travel is the same, even as it passes through many gravitational wells, becuase the speed of light is constant no matter your frame of reference. The thing that changes is the wavelength of the light, it will lose energy and thus increase in wavelength as it goes through the wells. Thus, the redshift of far-off objects. anyone correct me if i'm off....?

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llanitedave
Humble Megalomaniac

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: Understanding speed of light and gravity. [Re: jcely7]
#2347396 - 04/23/08 06:05 PM

You don't have to worry about bouncing radar signals of deep space objects, because it isn't done.

What's being referred to here, I believe, is that if you have to send a radar signal past the sun to bounce off of Mars, then the sun's gravitational field will curve the light path, meaning the radar waves will have to travel farther to get to Mars and back. That's what takes longer.

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Dane B
super member

Reged: 02/23/08

Loc: Seattle, Wa
Re: Understanding speed of light and gravity. [Re: jcely7]
#2347404 - 04/23/08 06:10 PM

I'm pretty sure the extra distance added by the curvature of spacetime from massive objects is miniscule compared to the overall distance.

If you're talking distances to deep space objects, you're talking about multiple lightyears. As I'm sure everyone knows, a lightyear is so big it is hard to comprehend. The little bits of curvature from massive objects, found few and far between in the vast interstellar void, is surely negligible compared to the overall distance scale. It is also likely that the additional distance is within the uncertainty of the original distance measurement, essentially making it futile to try to account for curvature.

EDIT: Oops, you were talking about changes in time not distance. I would still imagine that the differences, if any, are negligible given the magnitude of the scale.

Edited by Dane B (04/23/08 06:17 PM)

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: Dane B]
#2347641 - 04/23/08 07:54 PM

Quote:

I'm pretty sure the extra distance added by the curvature of spacetime from massive objects is miniscule compared to the overall distance.

It is not necessarily miniscule. See Einstein's Cross http://en.wikipedia.org/wiki/Einstein_cross. Sometimes the curvature can be used as a super telescope, bending light from behind a massive object to focus and magnify a more distant object. Try to calculate the focal length on that.

Regards,
Charlie

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Fish
professor emeritus

Reged: 10/13/07

Loc: Norridgewock, ME
Re: Understanding speed of light and gravity. [Re: Charlie B]
#2347813 - 04/23/08 09:19 PM

Good evening to all,

See what happens? A simple (!) question opens up a flock of others! That's why I love physics!

Now, then, here's a couple of points on this discussion, hoping to clarify some but not all issues brought up. It's too late in the evening for me to go on ad nauseum . . .

On the radar-to-Mars issue, I agree with Dave that the increase in distance is due to gravitational bending. Any change from a straight line will increase distance traveled and time to do so. The change would be very slight but present. I do wonder if, though, the concept is valid since in the time it takes for a radar ping to travel to Mars (past the sun) and back a significant shift in positions of the relevant bodies would make it difficult to hear the echo!

Secondly, perceived redshift is not caused by gravitational bending but rather the discovery that the more distant an object is from us the faster it is receding from us. This rate of recession is defined by Hubble's Law. Yes, that Hubble.

Now if you will pardon me, I'm being pulled into my bed because I passed too close to it.

Marc

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jupiterzkool
Carpal Tunnel

Reged: 05/08/06

Re: Understanding speed of light and gravity. [Re: Fish]
#2347926 - 04/23/08 10:06 PM

In order for the Sun to have any gravitational effect on the time it would take for the signal to get by it, it would have to be extremely close. With each solar conjunction period experienced by Cassini, we downlink to the Earth when the Earth is within 3 degrees from the Sun (from Saturn's point of view). Relativistic effects are barely a blip on the overall light time. The Sun's plasma acts to drown out the signal within 1 degree of separation. RADAR would be affected in the same manner (there is not a large difference between RADAR wavelengths and the radio bands used by Cassini). As Dane B has stated, the bigger difference is due to the fact that Mars in on the other side of the Sun.

-S

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Dane B
super member

Reged: 02/23/08

Loc: Seattle, Wa
Re: Understanding speed of light and gravity. [Re: Charlie B]
#2348035 - 04/23/08 10:44 PM

Quote:

It is not necessarily miniscule. See Einstein's Cross http://en.wikipedia.org/wiki/Einstein_cross. Sometimes the curvature can be used as a super telescope, bending light from behind a massive object to focus and magnify a more distant object. Try to calculate the focal length on that.

I don't know much about gravitational lensing - so I guess I don't see how this example is relevant to a change in calculated distance. Please explain

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: Dane B]
#2349714 - 04/24/08 05:41 PM

Gravitational lensing results from a large mass in between the observer and the observed object. This is the same as the sun bending the light or radar return from mars. If you were measuring the star's distance from the image without consideration of the gravitational effects large mass, you would likely have a relatively large error. The cross is a great example because all 4 stars in the image are the same star with the light bent showing 4 images.

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: Dane B]
#2349728 - 04/24/08 05:44 PM

Apparently, the period should not be in the link. See http://en.wikipedia.org/wiki/Einstein_cross
Charlie

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Dane B
super member

Reged: 02/23/08

Loc: Seattle, Wa
Re: Understanding speed of light and gravity. [Re: Charlie B]
#2349794 - 04/24/08 06:21 PM

I still don't get it.

If I measured the distance from Earth to one of the four images of the lensed star, I can see how in theory I wouldn't be getting the highest precision possible.

But how far is the lensed image really off from it's true position? Orders of magnitude smaller than a single degree. I don't see that this would result in a large error, and it still seems likely that the error would be well within the uncertainty of the original distance measurement, making the difference between the two measurements negligible.

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: Dane B]
#2349895 - 04/24/08 06:59 PM

Quote:

If I measured the distance from Earth to one of the four images of the lensed star, I can see how in theory I wouldn't be getting the highest precision possible.

How would you measure such a distance? If geometrically, the image looks to be next to the forground galaxy (400 million light years), but the true distance is about 1 billion light years and the object is behind the forground galaxy and completely obscured. Of course, you can't measure such distances geometrically, so the redshift and spectra of the image tell you the true distance.

Charlie

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Dane B
super member

Reged: 02/23/08

Loc: Seattle, Wa
Re: Understanding speed of light and gravity. [Re: Charlie B]
#2349993 - 04/24/08 08:00 PM

I guess I figured a person measuring the distance to a gravitationally lensed object would know to account for the gravitational lensing

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: Understanding speed of light and gravity. [Re: Dane B]
#2350738 - 04/25/08 06:54 AM

In the case of Mars, we know the actual distance from geometric measurements (we know the orbits of earth and Mars precisely enough to get a probe from here to there).

If you draw on a piece of paper the earth, Sun, and Mars lined up, and draw a line through the sun to the earth, it will have a certain distance. Now draw a line that goes from Mars to just past the edge of the sun, then bends and goes to the earth. You now have a triangle (a very low triangle, but a triangle). The path the light travels (the two lines just past the edge of the sun) is longer than the single line distance from Earth to Mars. Not by a large percentage, but remember that we are talking a total distance of ~250,000,000 miles (earth's orbital radius plus Mars' orbital radius). We can use radar to time the speed of light travel to objects a few feet away, so it doesn't take much bending to change that total distance enough to detect it (a change of 1 mile would be easily detectable).

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: Jarad]
#2351653 - 04/25/08 03:38 PM

Good explanation! I found a link with pictures that can expand it. See http://imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html

Charlie

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jupiterzkool
Carpal Tunnel

Reged: 05/08/06

Re: Understanding speed of light and gravity. [Re: Charlie B]
#2351917 - 04/25/08 05:54 PM

This paper refers to Cassini's attempt to measure relativistic effects with the Sun.

-S

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wirenut
scholastic sledgehammer

Reged: 09/21/06

Loc: m'dale Pa
Re: Understanding speed of light and gravity. [Re: jupiterzkool]
#2352332 - 04/25/08 10:07 PM

my understanding of redshift is that it's due to the distance the light traveled not how long it took. remember a light year is a unit of distance not time. I dont know if they can figure out how much the light was bent with a gravitational lens to figure out it's true location. but I've been wondering if gravity can affact the direction of light can it affect it's speed too I wonder if anyone every checked the speed of light from a gravitational lens?

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Charlie B
scholastic sledgehammer

Reged: 03/22/08

Loc: Sterling, Virginia
Re: Understanding speed of light and gravity. [Re: wirenut]
#2352789 - 04/26/08 08:21 AM

Quote:

my understanding of redshift is that it's due to the distance the light traveled not how long it took

Almost true, but redshift is a ratio of wavelengths indicating a velocity. The Hubble redshift-distance relationship or Hubble's constant (~ 75 km/sec/Mpc) converts the redshift velocity to a distance.

Quote:

I dont know if they can figure out how much the light was bent with a gravitational lens to figure out it's true location.

It can be done if the foreground or bending object's mass is known (see the posted links).

Quote:

I've been wondering if gravity can affact the direction of light can it affect it's speed too I wonder if anyone every checked the speed of light from a gravitational lens?

The speed of light c is constant. There have been attempts to measure the speed of gravity (see http://www.nrao.edu/pr/2003/gravity/ )

Charlie

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