# Can we know that space is 'curved'

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##### Guest
Another pellet to my scattergun topics because I just can't tie something up. I know what I don't know but not what it is <img src="/images/icons/laugh.gif" /> (eat your heart out George Orwell)<br />We would know it we were travelling at C because the Universe would be just a point of light directly ahead of us but this obviously isn't the case.<br />Einstein said that the faster we go everything wraps around us & moves to the front, the dot above being the ultimate case.<br />As there is no reference frame we can't know at which speed we travel so by inference can't know by how much space is curved so how can we say anything is at a distance or angle from us.<br />This is not the same as gravitational bending.

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#### Leovinus

##### Guest
I think the question is: "Can we know that space is 'curved'" <div class="Discussion_UserSignature"> </div>

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#### killium

##### Guest
What is a straight line ? If the space is curved, what we consider to be a straight line would in fact been curved. The true straight line would be the line that we see curving in the other direction.<br /><br />So if space is curved, and we know the direction of the curve, we just have to throw 2 objects, one in the direction of one star, and one off-target. The one off-target has to actively describe a curve, in the other direction of the space curvature.<br /><br />If the space is curved, the object sent off-target with a curved trajectory would reach the star first. <div class="Discussion_UserSignature"> </div>

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#### killium

##### Guest
What is a straight line ? If the space is curved, what we consider to be a straight line would in fact been curved. The true straight line would be the line that we see curving in the other direction. <br /><br />So if space is curved, and we know the direction of the curve, we just have to throw 2 objects, one in the direction of one star, and one off-target. The one off-target has to actively describe a curve, in the other direction of the space curvature. <br /><br />If the space is curved, the object sent off-target with a curved trajectory would reach the star first. <br /><br /><br />PS Sorry if this is a double post, it doesn't add up in the list. The board crashed again, sql error.... <div class="Discussion_UserSignature"> </div>

S

##### Guest
OK can we know by <b>how much</b> space is curved?<br />At C the Universe is a dot in front & as we slow it wraps around us so is it possible to determine that curve. From this we may possibly know our reference frame.

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#### killium

##### Guest
In my example (theorical example), you'd have to know it before hands cause the object with a curved trajectory must curve at the right angle, or it would miss the target. Maybe it could be determined with several attempt and derived from the results.<br /><br />Btw: if the space is curved, following my reasonnment, the smallest distance between 2 points is a curved line ! This would be a short cut to any target. You shoot off-target and as you go, you're continuously turning a bit (you have to know of course in what direction is the curve and what is the amount of curvature). Maybe Alpha centaury really is "only" at 3 light years with a correctly curved trajectory.....<br /> <div class="Discussion_UserSignature"> </div>

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#### a_lost_packet_

##### Guest
If space was curved, you could take a set of parrallel lines and project their course. Follow this "parrallel" course. If your two flight paths converge or diverge, then space is "curved." <div class="Discussion_UserSignature"> <font size="1">I put on my robe and wizard hat...</font> </div>

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#### killium

##### Guest
What will you use to tell your lines are parallel ? If you use sight, well your parallel lines will follow the curvature of space because light follow it, and you wouldn't know that your lines are describing a curve <div class="Discussion_UserSignature"> </div>

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#### slayera

##### Guest
Well something little you may help me out that had wondering- then a straight line is curved and curve is straight?

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#### killium

##### Guest
Yes, in a curved space, the line that we describe as straight would be the one that we see straight. But seeing means that your using light for your measurement, if that light follows the curvature of space, your straight line is in fact curved.<br /><br />If you draw a real straight line in that space, it would "appear" (note again the use of light) curved ! So in a curved space, the smallest line we can draw between two point is a line that will appear curved.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> </div>

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#### bobw

##### Guest
They are trying to measure it right now with the Gravity Probe B satellite. This experiment will probably be as famous as Michelson & Morely. They fully expect to find the effects of curved space. You might have your answer if a few years.<br /><br />http://einstein.stanford.edu/<br /><br />"The experiment will check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 400-mile altitude directly over the poles. So free are the gyroscopes from disturbance that they will provide an almost perfect space-time reference system. They will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, have far-reaching implications for the nature of matter and the structure of the Universe. "<br /><br />They are hoping to get a million home computers to crunch some numbers for them starting about next February. <div class="Discussion_UserSignature"> </div>

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#### CalliArcale

##### Guest
I'm a bit out of my depth, but one example of curved space is an Einstein Ring. Einstein predicted that gravitational lensing would occur -- if space is curved, it will curve light, and that will allow astronomers to see things directly behind a very massive object, and often magnified as well. (Glass lenses just curve light. So a gravity well should be able to do the same thing.)<br /><br />Here's a Hubble picture of an Einstein ring. They're rarely this perfect; usually what you see is a distant galaxy (or two or three or so) appearing to the side of a supermassive foreground galaxy (and sometimes appearing more than once!), generally bent into a curve. An Einstein ring occurs when the background galaxy is directly behind the foreground galaxy, and is projected around the foreground galaxy in a ring.<br /><br />Interestingly, Einstein believed that this was unlikely to ever be observed, since the circumstances have to be just right, but I think he underestimated how much the technology was going to advance over the course of the 20th Century. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### rocketbodypart

##### Guest
I guess if you consider that on earth, while it appears we are walking a straight line, we are not.

S

##### Guest
Calli that's gravitational curving. What I'm on about is velocity curving.<br />Have you seen those TV pictures where all the buildings move in front & bend in towards you when going fast.<br />That's what I mean & as the <b>bend for speed</b> is known then by measuring it we could know our true speed not just local reference speed we now have.

S

##### Guest
So it's a <b>visual</b> effect only & space isn't curved except locally by gravity.

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#### jatslo

##### Guest
Space-time is not a fabric; observed curvature is defined by the presence of fabric. Without fabric, you can have no curvature.

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#### a_lost_packet_

##### Guest
<font color="yellow">Jatslo - Space-time is not a fabric; observed curvature is defined by the presence of fabric. Without fabric, you can have no curvature. </font><br /><br />?<br /><br /> <div class="Discussion_UserSignature"> <font size="1">I put on my robe and wizard hat...</font> </div>

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#### jatslo

##### Guest
Many people, including you, confuse space-time with gravitational curvature. Space-time and gravity are completely different animals.

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#### kmarinas86

##### Guest
Gravity is an attraction. It must be an emergent property.<br /><br />Spacetime, if that's what it is, must also be an emergent property (if my view of the universe is correct).<br /><br />We know that the falling of objects emerges from gravity, and that gravity emerges from "space-time" (if that's what it is). So what does "space-time" (if that's what it is) emerge from?<br /><br />Let's put it this way. Movement in space-time is an emergent property of energy (change). The fabric of space-time (again, if that's what it is), if it changes, must be an emergent property of change. If it is unchanging, then it might as well be the cosmological constant. If there is no cosmological constant, then space-time is subject to change, making it an emergent property of change (energy). Such a source of change must be smaller than the fabric of space-time itself (again, if that's what it is). <br /><br />http://en.wikipedia.org/wiki/Emergent_properties<br /><br /><font color="yellow">In some theories of particle physics, even such basic structures as mass, space, and time are viewed as emergent phenomena, arising from more fundamental concepts such as the Higgs boson or strings. In some interpretations of quantum mechanics, the perception of a deterministic reality, in which all objects have a definite position, momentum, and so forth, is actually an emergent phenomenon, with the true state of matter being described instead by a wavefunction which need not have a single position or momentum.</font>/safety_wrapper>

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#### kmarinas86

##### Guest
<font color="yellow">What is a straight line ? If the space is curved, what we consider to be a straight line would in fact been curved. The true straight line would be the line that we see curving in the other direction.</font><br /><br />Saying that a curved line is straight is a bogus intepretation.<br /><br />An abstract line "does not exist" in space time, however light beams do. Take two light beams, parallel to one another and let them go through space in the same direction. If space time is curved, these light beams can diverge and converge - depending on the gravitational sources. If there is only one gravitational source around (which is not the case) then the two light beams will converge towards the gravitational source, but they will not cross one another (at least during their first time around). In order for those two beams to cross one another, the must be more than one gravitational source - unless there is some way for those light beams to cross one another on an subsequent return.<br /><br /><font color="yellow">So if space is curved, and we know the direction of the curve, we just have to throw 2 objects, one in the direction of one star, and one off-target. The one off-target has to actively describe a curve, in the other direction of the space curvature. <br /><br />If the space is curved, the object sent off-target with a curved trajectory would reach the star first.</font><br /><br />bingoz

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#### jatslo

##### Guest
<font color="yellow">If it is unchanging, then it might as well be the cosmological constant.</font><br /><br />I think space-time is constant, and what these individuals are describing as curvature is simply the interaction of four known forces, gravity, electromagnetic, strong, and weak forces. Albert Einstein is misunderstood frequently.

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#### jatslo

##### Guest
<font color="yellow">Im goign to say you have no experience in General Relativity and say your wrong. Im also going to say your wrong because I just finished studying general relativity 2 weeks ago in school.</font><br /><br />Oh yeah, How is that spelling course coming along?

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#### kmarinas86

##### Guest
Spelling is irrelevant if the author can fix the mistakes. If the content is seen for what it is, and not the spelling, then we have clear focus of the actual meaning being expressed.

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#### kmarinas86

##### Guest
<font color="yellow">I think space-time is constant, and what these individuals are describing as curvature is simply the interaction of four known forces, gravity, electromagnetic, strong, and weak forces.</font><br /><br />If space-time was distorted enough, then we could be seeing double images of stars - that does not happen though. The only things that could affect the speed of light in a vacuum is changes in the "dark" energy density. According to my view of the universe "dark" energy density changes (i.e. quintessence). But currently, we know very little about the universe - that's true enough given that there is exists a "cosmological constant problem" link.<br /><br />http://www.google.com/search?num=100&hl=en&lr=&safe=off&q="not+so+constant"+"cosmological+constant"<br /><br />http://en.wikipedia.org/wiki/Expanding_universe#The_cosmological_constant<br /><font color="yellow"> In fact, most theories of particle physics predict vacuum fluctuations that would give the vacuum exactly this sort of energy.</font><br /><br />Future discoveries of smaller particles could prove the cosmological "constant" to be a different value (or prove it to be variable). Occam's Razor judges theories based on what we know now but not on what we will know tommorow. We don't "know" what will be discovered tomorrow, but we can imagine that certain discoveries will be made (especially in predictable fields such as nanotech and biotech).

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