Question regarding gravity

[observer]

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Well, we're all moving through space-time...towards the future and at the same pace. Future becomes present, present becomes past. We're all doing this even while we're asleep and motionless, because we're riding on the same spacecraft called "Earth"...which is rotating around its axis at a speed of about 1,000 miles per hour! On top of that, Earth is also moving around the Sun at about 67,000 miles per hour! It doesn't make sense, right? Do you feel the earth moving at those ridiculous speeds? Welcome to the world of science! Nothing in here is common sense.

Anyway, back to your question on relative ageing and if it's considered time traveling. Answer: Yes. Even a glimpse of a star that's thousands of lightyears away, through a telescope, would be considered looking into the past...i.e., what you're seeing are images that's traveled several years to reach your eyes; that star you're looking at may have already burnt out its hydrogen and turned into a white dwarf, but what you're seeing is a red giant.

All this goes without saying: I could be wrong.

[hexo]

mao jud ni ang ako gi libugan.
Using the pictures presented, If we think of space time as a rubber sheet.
Does that mean that space is "flat"?

Unsa man diay ang spaces above and below this rubber sheet?

[observer]

Ever since Einstein, our view of space-time and gravity have changed. We went from the Newton's absolute space and time and gravity as a force to Einstein's space-time (actually, a guy named Minkowski coined that term first) and relativity and gravity as a consequence of the curvature of space-time due to the presence of mass. John Archibald Wheeler (American physicist who helped develop the theory of nuclear fission) said it best: "Matter tells space how to curve, and space tells matter how to move."

Is space-time flat? Actually, the shape of the universe is still a matter of debate, although WMAP has confirmed that the universe is flat "with only 0.5% margin of error". But you have to leave that conventional understanding of the word "flat" behind. Cosmologists use that word differently when using it to describe the geometry of the universe. And the geometry of the universe depends on its density. Below is a copy-and-paste from this link) and it describes what's meant by "flat" or "curved":

1) If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere.

2) If the density of the universe is less than the critical density, then the geometry of space is open (infinite), and negatively curved like the surface of a saddle.

3) If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper, and infinite in extent.

On space-time as a rubber sheet. I guess this is our best representation of that idea. The analogy is probably meant as an explanatory tool or just a way to think about space-time. In any case, how would you describe space when an object travels in a straight line without gravity to curve its path? We would say that space is flat (okay, maybe not that straight...but almost), right? Get my point? I suppose if you graphed Einstein's equation to describe how a star of a particular mass and density would warp the space-time around it, it might look like the image on the left. For a more massive and compact star like a neutron star, it might look like the one on the right.



* I could be wrong about that graph thing...I hope my intuition's right about this. I wish I had the time to really understand those equations. Maybe one day.

Anyway, what good is a graph from some mathematical equation without some good ol' evidence, right? Here's the deal: What's the best method to produce a straight line? Answer: Laser beam. If light cannot go straight, what else is there? Therefore, the straightest possible path is...the path of a light beam. But what would a curved light beam due to the warping of space-time look like?



* A & B are the same star but with their apparent positions (not actual positions) shifted due to the space-time warping.

A guy named Arthur Eddington actually did just that: test Einstein's theory. Below is a very good image of what they should expect if a star's light curved due to the presence of the Sun.



*actually, the above image is a bit misleading...because the night time view of the star doesn't have the sun getting in the way of the star's light. But still, I think it drives home the point better this way.

Eddington used the Hyades as his test subject. In comparing individual star positions made during the eclipse with earlier observations of the cluster, you can see if they've shifted away. To make a long story short, when the eclipse and night images were compared, there was some considerable shifting (I guess they used some techniques to make sure that they made the comparisons as accurately as possible). Thus, it confirmed Einstein’s prediction.

As you know, with advances in technology, further tests only strengthened Einstein's theory (I think it's part of the scientific method to continually test a theory...it's the one that doesn't fit that's exciting, as they'd like to say). Latest of these experiments was the satellite-based experiment, Gravity Probe B, which was launched in 2004. It's aim was to measure the warping and twisting of spacetime with gyroscopes. Click here to view an animation of what that Probe was all about.

Hope that helps...coz writing this down also helped me understand it enough to explain it. It works both ways.

[s3thk]

hagbong ko ani pag exam