"Curvature of space"... "compression" is a better word

[matt_mcbride]

I think the problem with understanding spacetime is the use of the word "curvature." It makes it confusing because when you try to explain the way objects move, the use of the word "curve" is tautalogical... objects fall because of gravity, gravity exists because of a "curve" in space, and we understand that objects can't fall in the first place along that curve unless there's gravity to make it fall. Because our concept of "falling" is one that's linked to gravity, we're essentially trying to define gravity in terms of itself.<br /><br />Gravity is more accurately understood if you think about space "compressing," not "curving." The essential concept remains the same - space can bend. <br /><br />When the spacetime is spaced out equally across an object, that object is in equilibrium with its surroundings, and is floating. <br /><br />(In reality the object is almost always moving towards something, because the compression of space is rarely equal in all directions.) <br /><br />When spacetime is more compressed over one end of the object, while not as compressed over the other end of the object, the object situates itself in such a way that the mass of the object is in equilibrium with spacetime. This move towards equilibrium is what we call gravity, and we perceive the object to be falling.

 

[bonzelite]

from my understanding, when a satellite orbits anything, it is actuallly falling around the body and not orbiting it. in this regard, it is falling. but most would not see it in this manner, but is technically what is really happening.<br /><br />the circularity of the movements defy the commonly held assumption that falling is only a straight line, as if you were to jump off a bridge. <br /><br />curvature of spacetime. from mathematically abstract to physical, this implies that any object possesses gravity as it possesses mass. then space must have mass in order to wrap around the object that is displacing and attracting/bending the spacetime. ie, the spacetime is falling around the object, too, just as a moon falls around a planet. <br /><br />so what is spacetime made of?

 

[jatslo]

Stuff? <img src="/images/icons/wink.gif" />

 

[bonzelite]

LOL! yes. stuff, indeed. "spacetime-atrons."

 

[pale_blue_dave]

I agree that "curvature" can be a little misleading for some people. You always see people explain gravity with the "bowling ball on the rubber sheet" scenario. That's fine to begin, but to truly understand you have to take that to the next dimension. (And then possibly the next, and the next, etc....)

 

[bonzelite]

as i suggested, if the mass of the object with a moon keeps it tidally locked and "falling around" its gravity envelope, then any object with mass, then, has gravity. true? and if this so-called "spacetime" substance is effectively bent, warped, moved, altered, impinged upon, by gravity, then it must, too, have some embodiment of mass, as a massless object cannot be influenced in this manner.<br /><br />except for -------- /> electrons. they are massless. yet matter is composed of them. so matter itself has a massless quality to it. yet it is massive. and gravitationally influenced. <br /><br />so we have a dilemma and do not fully understand the nature of matter. of mass. of gravity. what we have today is an approximation, at best, of what matter really is --what it "really" is is not known.

 

[drwayne]

"electrons. they are massless."<br /><br />What makes you say that? Electrons do have a rest mass. 511 KeV of it as a matter of fact.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

Determination of e/m was one of the more interesting labs we did as undergrads....<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

Not as good at the old ballistic pendulum, but a good one.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[jatslo]

..."<font color="yellow">old ballistic pendulum</font>... <--- That reminds me of gravity detection experiment or when someone used a pendulum during an eclipse of the Sun and observed a annomoly. The pendulum moved or something like that.

 

[drwayne]

In an undergraduate mechanics course, I had to build a Foucalt pendulum. It used a very heavy ball, and piano wire to secure it.<br /><br />Its an interesting demonstration of the Earth's rotation.<br /><br />The ballistic pendulum lab I mentioned used a 22 caliber bullet fired into a wooden block secured inside a pendulum. In addition, we had a strobe and an instant camera set up such that the motion of the bullet was captured. It was a real ... issue to set up, as the strobe was controlled by a power supply built sometime in the 60's - but the students loved it, the got to capture the motion of a real bullet on film, and could calculate its velocity in a couple of different ways and compare the results.<br /><br />They went to a different setup in my later years in graduate school, that was safer, and easier to set up - but I think they lost something there....<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[jatslo]

Here is a good link Foucalt Pendulum Yeah, I often think of utilizing strobes or something like them to make visible what is invisible or not often, but I have on occasion. It would be cool to see something undiscovered with a strobe. The strobes they have now are far more advanced, I am sure.

 

[yevaud]

Thank you, Harold "Doc" Edgerton, wherever you may be... <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>

 

[drwayne]

One night, Majid and I were trying to get that cranky strobe to work. It was tripped by the bullet breaking a set of contacts connected by aluminum foil.<br /><br />We were so focused on getting the strobe to work, we kept firing bullets into the block without rotating the block, meaning they went into the same hole in the block. After not a few bullets, we blew the back of the pendulum off, and the bullet went bouncing around the lab. It was an...interesting experience to have a little after midnight.<br /><br />Wayne<br /><br />p.s. The power supply for that strobe was as big as a small refrigerator...a tribute to tubes....<br /><br /> <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[jatslo]

How did you hit the same spot on a moving target? The pendulum moves X amount and the shuter was timed at a particular speed, which started when the bullet tripped a wire. Yada, Yada, Yada... Well I guess you could hit the same spot on a moving target, if you timed it right. You are using a strobe and cameraman, right?

 

[igorsboss]

In ballistiscs testing, one starts with a stationary pendulum. It simplifies the problem quite a bit...<br /><br />Why do bagpipers keep marching? Because a moving target is harder to hit.

 

[drwayne]

Nothing was moving.<br /><br />The bullet is fired from a fixed "gun", it passes through the gates, gets its picture taken 3 or 4 times while the shutter is open and the high speed stobe is firing.<br /><br />It then goes a little further downrange, and it comes upon a stationary pendulum with a cylindrical wooden block contained in it. The bullet is caught in the woden block, and the pendulum, which is free to recoil along a fixed axis recoils. It goes back and up a certain distance, which is a function of the speed of the bullet, and is measured by a sparker and spark tape.<br /><br />The calculation of the bullet speed from the pendulum interaction involves conserving momentum during the bullet/block collision (which is obviously inelastic), and consering energy up to the top of the pendulums motion, it is an excellent problem, both for its fun nature - and for the fact that the student needs to understand what is conserved when.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

Here is a picture which kind of gives you an idea of what I am mumbling about with respect to the pendulum. In our case, the pendulum was probably 10 - 15 feet downrange.<br /><br />http://www.physics.niu.edu/~scienceed/labs150/p150lab7.html<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

Interesting animation of the pendulum end of things:<br /><br />http://physics.bu.edu/~duffy/semester1/c12_ballistic.html<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[jatslo]

Oh, you said the gun kept firing successive blows while you were working the strobe, and I was wondering how the bullets kept hitting the same spot. So I am guessing that someone kept stopping the pendulum or you figured out how to successfully hit a moving target, which is possible.

 

[drwayne]

The pendulum came to a rest between firings. (While we tried to figure out what was going wrong).<br /><br />After firing, we were supposed to rotate the cylindrical wood block that caught the bullets (the did not go in the center) - so that each bullet hit the wood in a virgin spot.<br /><br />Its a case of focusing on one problem to the exclusion of everything else....it is a frequent cause of accidents.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[bonzelite]

i looked up the rest mass for an electron to be .511 MeV when its speed is "zero relative to an observer." <br /><br />but electrons are never at rest. for mathematical computation, it is necessary to assign them a "mass." <br /><br />but isn't rest mass just a form of measuring energy, thus "eV" --electron volts? i don't think electrons actually have an observable mass, ie, radius and structrue. they are energy states measured in volts. they're not really particulate matter as we know it. <br /><br />for an atom, the mass of the whole thing is not just the sum of the masses of its particles, but is really the sum of their energies, including kinetic, potential, and mass energy.<br /><br />i found this: if you solve E=mc^2 for m, you get m=E/c^2. in essence, mass = energy.<br /><br />energy is a wave or a field. electrons are, in a way of looking at them, massless waves of energy. they have properties of particles and waves. yet they lack physical dimension, individually. this is the paradox of "matter." it is not really matter. but it is. and is measurable. <br /><br /><br />

 

[drwayne]

"but electrons are never at rest."<br /><br />You are confusing the properties of an electron with those of a photon. They do in fact have an observable mass. If you want it in kg, its<br /><br />9.1093897e-31 ±; 5.4e-37 Kg<br /><br />That is in fact, an observable, and measurable mass.<br /><br />Wayne<br /><br /> <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

As I mentioned in a much earlier post, there are in fact experiments that one can do to determine the mass of an electron. It is in fact just about the smallest measurable mass that can be measured.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[drwayne]

By the way, I know I shouldn't find such things as cool as I do, but here is a web-based set of instructions, including a data sheet for an e/m experiment. This is just, well....cool - at least in contrast to the way we recorded lab data and analysis when I was going through school.<br /><br />http://phoenix.phys.clemson.edu/labs/cupol/eoverm/<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>