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I'm not sure what that comment is in reference to, but our observations impacting Relativity span the entire observable universe.john-of-the-divine said:
Light & Space: Investigating the Unknown
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The post that @bahamagreen edited used to have reference to the experiments using the retroreflectors on the moon.russ_watters said:
john-of-the-divine
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how the heck came we see 46.5 billion light years when, as far as we can tell, the universe is only 14 billion years old. lol. guess the better guestiin to ask is what makes the force of gravity work? I see the model, I understand the model, what is the fabric of space time made of? it can't be nothing, because something can't bend nothing. 1×0=0
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I'm saying that compared to the size of the universe, the distance to the moon is like an inch compared to 100 miles. I don't know the actual scale, but I'm saying we can't know the true effects on light until we can go a light year away, shoot a laser back to earth, and study that light.russ_watters said:
We've basically done that. We can look at the light from certain light sources (different kinds of stars and related bodies) at very different distances and compare them. What do you think the main difference is in the light coming from a nearby star and one a few billion light years (LY) away?john-of-the-divine said:
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It may not give quite the same information, but studying light that has traversed much of or all of the universe is plenty helpful.john-of-the-divine said:
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ah, but all that is based on the belief that in space, the light takes x amount of time to travel x amount of miles, when we really don't know what the "fabric" of space time is made out of? I don't know, maybe I'm thinking too high for my tiny mindberkeman said:We've basically done that. We can look at the light from certain light sources (different kinds of stars and related bodies) at very different distances and compare them. What do you think the main difference is in the light coming from a nearby star and one a few billion light years (LY) away?
If you're really asking what the "fabric" of spacetime is, I think I'll move this thread to the Relativity forum, where you can get more knowledgeable replies. I'll mark it with a "B" prefix to let others know that you'd like responses on a Basic level.john-of-the-divine said:
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guess my guestiin can't be answered till we go out and see. not just
lol. thanks guys.berkeman said:If you're really asking what the "fabric" of spacetime is, I think I'll move this thread to the Relativity forum, where you can get more knowledgeable replies. I'll mark it with a "B" prefix to let others know that you'd like responses on a Basic level.
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There isn't an answer to "what is spacetime" in relativity. Relativity just says that spacetime has non-Euclidean geometry and predicts the rules of that geometry based on the mass and energy present. It doesn't say what spacetime is or how mass and energy modify geometry, only that they do.
"We don't know, we're working on it" is probably the high level answer. I don't know enough about quantum gravity theories to comment authoritatively, but I think explaining at least some of "what is spacetime" is part of what they are supposed to do. But we don't have a complete quantum gravity theory yet, let alone a complete and tested one.
"We don't know, we're working on it" is probably the high level answer. I don't know enough about quantum gravity theories to comment authoritatively, but I think explaining at least some of "what is spacetime" is part of what they are supposed to do. But we don't have a complete quantum gravity theory yet, let alone a complete and tested one.
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ah, see, now that's what I'm trying to understand. I see the model of how gravity works in space-time, and I understand it, somewhat. so my original question is, how do we know that the "fabric" doesn't, over time, absorb, break down, destroy or even slowdown a photon of light? the math might work within our solar system but, what about the vast distance of space. if light is traveling through space-time, how can we know that there is no effect on the photon that is unseen, as of yet?
rootone
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No they don't.john-of-the-divine said:
Aside from which the Earth is observed to be a spheroid, unless you think satellites are fake or something.
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hence the reason why I said "or so it seems"rootone said:
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There is no fabric in GR. That is a very misleading pop-sci trope and is utterly useless for actually doing GR.john-of-the-divine said:
Spacetime is the geometry of physics. It consists of distances and angles. Since spacetime includes time the spacetime notion of distance includes duration and the notion of angles includes relative velocity. The math of spacetime is simply the math of a specific type of curved surfaces.
john-of-the-divine
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empty space really is just empty space?
rootone
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No, Quantum field theory suggests that everywhere in space has some or more or less of the same field stuff,
though the field is not itself a physical object, it is a distribution of probabilities.
If you think that's weird, a lot of people agree.
though the field is not itself a physical object, it is a distribution of probabilities.
If you think that's weird, a lot of people agree.
There's some serious good news here: Thanks to the internet you don't have to go to a ridiculously expensive top-flight school to learn this stuff. Textbooks and lecturers' notes from very strong academic programs are available online; for example, Sean Carroll and Gerard t'Hooft have both placed first-rate general relativity textbooks on the web, and there are many more good online resources.john-of-the-divine said:
On the other hand, you do have to work at it. Plan on spending several years of the same sort of effort that you'd put into learning anything else worthwhile, and resist the impulse to skip learning the math that's needed to deal with all of physics since Newton (who more than three hundred years ago invented calculus because he needed it to solve the open physics problems of his era). Nonetheless... A few years of concentrated effort will give you what some of the smartest people who ever lived spent centuries developing... That's good news.
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I know there has to be something that mass effects to create gravity. I used the term "fabric" as a reflection of that something knowing it's really not a fabric, hahaha. But, this field, do you think it's the "aether", just different then what was once thought to exist?
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Yes, tautologically. X really is just X for any X.john-of-the-divine said:
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The Michelson Morley experiment ruled out a rigid aether and ring interferometer experiments ruled out a dragged aether. The only aether that remains compatible with experiments is experimentally indistinguishable from no aether.john-of-the-divine said:
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I've gotten most of my knowledge from science, discovery, ext. can see and I understand the theories, until I asked the great "what if" question. now I want to know what "it" is that mass effects to make gravity. guess I need to understand the mathNugatory said:
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if that were true then you wouldn't be asking about fabric and aether. Pop sci sources are entertainment, not education. They make you feel good, but they don't teach you the theory.john-of-the-divine said:
It is good that they motivated you to ask more, but you need to recognize that if you are interested in the answers then you are going to have to unlearn a bunch of pop-sci baggage
"It" is the geometry of physicsjohn-of-the-divine said:
As @berkeman says, wikipedia is generally OK for well-understood stuff that's not near the cutting edge. A pretty good rule of thumb is that wikipedia coverage at the undergraduate level in math and science (but emphatically not the social sciences!) is probably reasonably good. Anything beyond that... Not so much. It is prudent to read the "Talk" section of any wikipedia article before you decide how trustworthy it is.john-of-the-divine said:
The Physics Forums policy is that wikipedia is not in general an acceptable source, but we routinely allow and provide links to wikipedia articles that the mentors have found to be good.
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that's why i put it in quotations.Dale said:
that's why I'm here. and I ment I might not know the math, but I can invision it. beyond what their cg shows usDale said:
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Excellent!john-of-the-divine said:
What is your math background? If you know algebra and vectors you can start with special relativity and spacetime.
john-of-the-divine
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see, that's my biggest problem, I've never been able to make myself do math. I've always had a great imagionation, and that's good, till it takes me away from what I'm trying to consentrait on.Dale said:
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Then realistically I think that you have to pick a different goal. At least I don't know of a way to learn physics without doing math.john-of-the-divine said:
rootone
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You were asking earlier about knowing the math.
I am not any kind of math expert, but I think if you can grasp Newton's Principia, then you are doing OK.
If you want to build a bridge over a river it works just fine.
I am not any kind of math expert, but I think if you can grasp Newton's Principia, then you are doing OK.
If you want to build a bridge over a river it works just fine.
john-of-the-divine
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honestly, I'm trying to figure out why you think light coming from billions of light years away hasn't been altered by the space it's traveled through. if you can explain, without math, exactly what causes the effect of gravity and how it effects light, I will understand. if you can't explain without numbers, then what makes the math so infallible? but I guess that's why it's all only theoretical, which makes none of it fact. I just wanted to know how do you know %100 that the light from billions of light years away, when in fact, it's nothing more than a best guess? even though I'm no math expert, it really isn't that far fetched to believe the math could have been forced to fit a model. just because the math works doesn't make it anything less then a best guess. but, like I said, we won't know for sure till we go out there and look.
rootone
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https://en.wikipedia.org/wiki/Tired_light
Probably not.
Probably not.