BeechTalk - BT

Username Protected · **Posted:** 29 Dec 2021, 21:14

Yes, the general relativity approach (which I like) is that mass bends space. Then objects travel in "straight" lines in that curve space.

Taking an example from Kip Thorne: Imagine an apple with 2 ants on the equator. Here we imagine time goes upward (increasing latitude) so all objects have to move up the apple as time passes. Space is longitude. So the two ants walk "north"on the apple and after some time find that some mysterious "force" has caused them to meet at the north pole.

As to "why" mass bends space - I don't know that there is an answer. It may be like asking "why" an electron has charge. Maybe there is some deeper "reason" but as far as I know its not known to science.

In any case you can think of gravity bending 4-dimensional space time into hither dimensions - which is of course hard to imaigine. So the best examples have one space dimension, and one time dimension - and then bend that 2 dimensional sheet into 3 d (like the example I gave).

Please ignore all of the rubber-sheet, and cone shaped descriptions of black holes. I know what they are trying to say, but they end up just confusing people. Its a terrible analogy and I wish educational materials would stop using that.

In any case in a black hole, space is bent so much that if you are at its surface, your "time" dimension points toward the black hole - in fact any possible future you can reach ends up bent into the center of the black hole.

Its almost the same as saying that the gravity is so strong light can't escape. The difference is that here, if you were on a spaceship orbiting just above the event horizon, and you tried to lower a rope intot the black hole, the force on the rope would become infinite as it reached the event horizon. (not just very strong).

Username Protected wrote:

I'm still trying to get my head around gravity bending light.

I’m not sure gravity bends light. Light travels in a straight line through space. Gravity seems to bend space itself!

It is mind-bending, regardless what else it bends.

Username Protected · **Posted:** 29 Dec 2021, 21:26

If you're interested in cosmology and the outer limits of what we understand about phyics, I recommend Sean Carroll's Mindscape Podcast.

He often discusses these topics with other experts, and he does a monthly Ask Me Anything session in which he answers questions about mind-bendy topics.

Carroll is (at present) a theoretical physicist at California Institute of Technology. He's written several popular books about quantum mechanics, cosmology, etc.

Username Protected · **Posted:** 01 Jan 2022, 12:54

More good news: Looks like the deployment of the sun shield is going well.

Quote:

...a critical milestone for Webb: all 107 membrane release devices associated with the sunshield deployment — every single one of which had to work in order for the sunshield to deploy — have now successfully released.

...As the mid-booms slowly pushed out horizontally from the spacecraft, each driven by a motor, they pulled the folded membranes of the sunshield with them. This extended the sunshield to its full 47-foot width all the way across the observatory.

Username Protected · **Posted:** 01 Jan 2022, 13:19

Username Protected wrote:

More good news: Looks like the deployment of the sun shield is going well.

Quote:

...a critical milestone for Webb: all 107 membrane release devices associated with the sunshield deployment — every single one of which had to work in order for the sunshield to deploy — have now successfully released.

...As the mid-booms slowly pushed out horizontally from the spacecraft, each driven by a motor, they pulled the folded membranes of the sunshield with them. This extended the sunshield to its full 47-foot width all the way across the observatory.

I wonder if the Hubble could be pointed at the JWST to view the deployment?

Username Protected · **Posted:** 01 Jan 2022, 15:02

This is exciting, can’t wait to see how it does. Hopefully they don’t find mirror defects like they had to fix on the Hubble.

Username Protected · **Posted:** 01 Jan 2022, 15:09

Yabbut, Hubble has likely has the wrong optics for something so close.

Username Protected · **Posted:** 01 Jan 2022, 15:17

Username Protected wrote:

This is exciting, can’t wait to see how it does. Hopefully they don’t find mirror defects like they had to fix on the Hubble.

The mirrors can be aligned in space. For details, see this explanation. Excerpt:

Quote:

Aligning The Mirrors On Earth And In Space
Once the telescope is in orbit, Engineers on Earth will need to make corrections to the positioning of the Webb telescope's primary mirror segments to bring them into alignment - ensuring they will produce sharp, focused images.

These corrections are made through a process called wavefront sensing and control, which aligns the mirrors to within tens of nanometers. During this process, a wavefront sensor (NIRCam in this case) measures any imperfections in the alignment of the mirror segments that prevent them from acting like a single, 6.5-meter (21.3-foot) mirror. Engineers will use NIRCam to take 18 out-of-focus images of a star - one from each mirror segment. The engineers then use computer algorithms to determine the overall shape of the primary mirror from those individual images, and to determine how they must move the mirrors to align them.

Username Protected · **Posted:** 01 Jan 2022, 16:59

Username Protected wrote:

I wonder if the Hubble could be pointed at the JWST to view the deployment?

I think it's too close, plus JWST doesn't view in visible light.

Username Protected · **Posted:** 01 Jan 2022, 18:02

Username Protected wrote:

I wonder if the Hubble could be pointed at the JWST to view the deployment?

I think it's too close, plus JWST doesn't view in visible light.

Username Protected · **Posted:** 01 Jan 2022, 18:09

I'm surprised with all the complexity of the mechanism that they didn't embed a small camera to view the spacecraft. Could possibly help debug the situation if there were some issue during deployment.

Username Protected · **Posted:** 01 Jan 2022, 18:15

Username Protected wrote:

I think the resolution isn't there in Hubble. So in that respect JWST is too far away

Yep, Hubble resolution at that distance is ~quarter mile.

Username Protected · **Posted:** 04 Jan 2022, 18:45

The sun shield is fully deployed and tensioned. Read more here.

Username Protected · **Posted:** 04 Jan 2022, 23:44

Shield seems to be working.

Sun side: 128 F

Shade side: -323 F

Man, it sure is cold in the shade in space!

Mike C.

Username Protected · **Posted:** 05 Jan 2022, 00:02

Username Protected wrote:

My comment was in jest, but your question is a good one. My space technology knowledge is very limited, but I would imagine that such operations are coordinated among the various space agencies around the world.

I don’t think that any coordination is required. I’m pretty sure that the JWST will be the first man made object to occupy a Lagrange point.