Gravity the result of space-time curvature?

[benhur]

Can someone explain this in simple words (no, don't give me the rubber sheet example, lol) without giving a link which contains complicated terminology? How can anything curve space and time, and if it did, what exactly constitutes space and time, they appear so non-physical, it's hard to imagine. Is gravity then an illusion?

[Rosa Lichtenstein]

Gravity isn't, but the explanation is pure Platonism, unless, that is, it is treated as metaphorical, once more.

[KC]

It doesn't curve space and time, it curves spacetime (http://en.wikipedia.org/wiki/Spacetime). As for how gravity works with respect to space time, refer to this image:

http://upload.wikimedia.org/wikipedia/commons/2/22/Spacetime_curvature.png

The planet in this image is curving space time.

There's more info on the general relativity theory of gravity and what this all means on the wikipedia page (http://en.wikipedia.org/wiki/Gravity#General_relativity). Also, this was verified by Einstein and his contemporaries when it was shown that light curves around the sun.

[Woland]

Gravity also curves the whole universe, one reason why you can never reach the ''edge'', because you will eventually just come back to your starting point.

[benhur]

KC,

Thanks for the info.

Behemouththecat,

Gravity is caused by spacetime curvature. So I am not sure how gravity can curve the universe. It's the effect rather than the cause.

Rosa:

How is this platonism, when Einstein came up with this? He didn't view gravity as a force, as Newtonians do.

[Woland]

Erm..are you sure about that? seems a bit unlikely. I would say its the opposite.

[black magick hustla]

Gravity also curves the whole universe, one reason why you can never reach the ''edge'', because you will eventually just come back to your starting point.

This is not true. The Universe is planar, if you fly with a spaceship you will travel for ever. You will be creating spacetime as you do this btw.

[Woland]

Maybe that would be possible, but if you fly -straight- gravity will curve you in a complete circle after some time, considering the commonly accepted version of the universe. And creating -spacetime-? In what way should this be understood?

[Rosa Lichtenstein]

BenHur:


How is this platonism, when Einstein came up with this?

Einstein, like the vast majority of mathematicians, was a Platonist, who actually believed, as many of those posting in this thread seem to do, too, that the mathematical objects and rules he used actually existed in the real world, or at least in an 'abstract' world which somehow made things happen in this world.

[black magick hustla]

Maybe that would be possible, but if you fly -straight- gravity will curve you in a complete circle after some time, considering the commonly accepted version of the universe. And creating -spacetime-? In what way should this be understood?

Well, I will be able to answer this more in depth in a semester lol but:

The accepted theory is that the Universe is flat. I actually asked this to my astro professor. google it, and most pages accept this.

Spacetime is formed as the stuff of the big bang expands. There is no use of talking outside this expansion or before this expansion. So if you managed to travel faster than this expansion, you would be creating spacetime around you.

[black magick hustla]

BenHur:


Einstein, like the vast majority of mathematicians, was a Platonist, who actually believed, as many of those posting in this thread seem to do, too, that the mathematical objects and rules he used actually existed in the real world, or at least in an 'abstract' world which somehow made things happen in this world.

I agree with you that a lot of physicsts are platonists. I think they dont become christians because it seems more unscientific. However, the concept of space and time is treated very precisely in the semantic context of physics. Space are the coordinates x, y, and z given in a mathematical frame. t is well, t. This concepts are useful for doing calculations and predicting phenomena. So when physicsts talk about space and time, it might not be in the same context as everyday folks do, but in scientific context, they have very precise definitions and are treated as dimensions. It is true that these are just mathematical models to predict phenomena, nothing more. It is like saying a computational model of the universe is the universe, it is not. The value of G is used because it was a "random number" that fitted with GmM/r^2 not because it actually exists in nature.

[Woland]

Hmm, interesting. I'd better be deserting this thread before I get hit on the head with my dusty book of physics, but anyway, thanks for the insight ^ ^

I always considered the theory that the Universe, with the big bang expanding, is an inflating bubble sort of thing. Maybe I'm ages behind XD

Ofcourse then, the last part makes sense.

[Kwisatz Haderach]

When discussing highly abstract scientific topics, it's important to remember that science is all about models. Models are mathematical constructs that serve to explain the way the universe works and - most importantly - can be used to make predictions about the way the universe will behave in the future. The goal of science is to answer two questions:

1. What will happen naturally in the future?
2. What are the consequences of my actions? (or in other words, what will happen if I do X?)

The models we use to answer these questions don't have to actually be "real" in a physical or a Platonic sense. They're just a bunch of equations on a sheet of paper. As long as we can use those equations to accurately predict the way the universe actually behaves, they are considered to be good models of reality. Sometimes our models are obviously untrue or at least incomplete (for example, Quantum Mechanics and General Relativity blatantly contradict each other, yet as far as we can tell they are both excellent models of the universe). But the purpose of science is to help us design better technology. As long as our technology works the way we expect it to work, we don't really care if the science we used to design it is "true" in some deeper philosophical sense.

So what is the curvature of spacetime? It's a bunch of equations on a sheet of paper that we can use to make accurate predictions about weird gravitational effects that we could not explain before Einstein.

[Rosa Lichtenstein]

Marmot:


However, the concept of space and time is treated very precisely in the semantic context of physics. Space are the coordinates x, y, and z given in a mathematical frame. t is well, t. This concepts are useful for doing calculations and predicting phenomena. So when physicsts talk about space and time, it might not be in the same context as everyday folks do, but in scientific context, they have very precise definitions and are treated as dimensions. It is true that these are just mathematical models to predict phenomena, nothing more. It is like saying a computational model of the universe is the universe, it is not. The value of G is used because it was a "random number" that fitted with GmM/r^2 not because it actually exists in nature.

I agree, but it is when they begin to project these abstract ideas onto nature that the problems arise, for there is no way that the mathematical objects and rules they use can possibly have a physical nature.

[Rosa Lichtenstein]

KH, I think what you say is basically right. However, what you are advocating is known as operationalism (or possibly instrumentalism), but I am not too sure, since you give too few details.

The problem is that in their attempt to make their ideas comprehensible, scientists (in this field) have to talk as if their models are literally true.

http://en.wikipedia.org/wiki/Operationalization

http://en.wikipedia.org/wiki/Instrumentalism

[mikelepore]

Here's an example that I think addresses the original question is. The "principle of least time" shows that a ray of light has to travel in a straight line. However, a ray bends as it travels past a star, as shown from photographs taken during solar eclipses. The light doesn't bend because of gravitational attraction by the star. That would imply that the light has mass, which it can't have, because it wouldn't be able to travel at the speed of light if it had mass. Instead, the bending is caused by the fact that the light is still traveling in a straight line, and the presence of a nearby mass has modified the space in which the straight line path occurs. It's still the shortest possible distance between any two points.

[mikelepore]

Space shouldn't be thought of as a non-physical abstraction. It must exist objectively, because the universe has a finite radius even though it's unbounded. If you were to travel for a long distance, in any direction, in a perfectly straight line, you would return to your starting point. This the only way we can make sense of the observation that the background microwave radiation left over from the big bang arrives at the earth from all directions in the sky simultaneously.

[Rosa Lichtenstein]

Mike, sure space exists, but for physicists, its either a Hilbert Space or a Banach space, or a ...

These can't physically exist since they are composed of mathematical objects and knitted together by mathematical rules.

[benhur]

Space shouldn't be thought of as a non-physical abstraction. It must exist objectively, because the universe has a finite radius even though it's unbounded. If you were to travel for a long distance, in any direction, in a perfectly straight line, you would return to your starting point. This the only way we can make sense of the observation that the background microwave radiation left over from the big bang arrives at the earth from all directions in the sky simultaneously.

But the problem is, how does one define space, its properties and such, when it can't be perceived by senses, or by finer instruments?:( It applies to time, as well. Both time and space are not like other physical objects which have clear attributes like weight, shape, color etc. So what exactly are space and time?

[Rosa Lichtenstein]

BenHur:


But the problem is, how does one define space, its properties and such, when it can't be perceived by senses, or by finer instruments?

Space in its ordinary sense is very easy to perceive; the objects around you, unless there are no gaps between them, frame space for you. Difficulties arise, however, when we try to incorporate scientific ideas of space (particularly 'empty space') into our ordinary understanding. You can sort of experience empty space if you leave the surface of the earth, but few of us have done this, and it's not totally empty anyway, as I am sure you know.

So, the further we move away from the ordinary sense of this word, the harder it is for us to make physical sense of it.

But, we do not have to experience something to make sense of it; I am sure you understand that 1,000,000,001 is one less than 1,000,000,002, but you are never going to experience it.

Theoretical notions of space are somewhat similar; we can make some sort of sense of them (by our use of them), but there is no way that they can be made physical sense of.

Time is somewhat different, and more complex. In ordinary language we have many different ways of speaking about time, not all of which express the same concept, nor even the same concept as that which physicists study.

So, asking someone what time it is arguably not the same as saying you had a great time at the beach, or that its time to get a move on, or that you have no time to lose.

The modern physics of time was modelled on the real numbers at first, and as such it inherited the latter's properties. In that case, time as physicists refer to it, cannot physically exist.

Many of the 'paradoxes' of time travel, for example, depend on running together these different senses of time.

There is an illuminating article on this here:

http://www.uea.ac.uk/%7Ej339/dummettreply.htm

[Junius]

Hang on Rosa, wasn't the lesson of Einstein's special relativity that by relying on the ordinary sense of a word (i.e. time, simultaneity) can actually hinder science? Hence, Bridgman emphazised scientific concepts solely in terms of the operations required to measure instances of the concept (i.e, defining length as an operation of measuring from 'A' to 'B' rather than the property of taking up space.) Do you object to this?

Somewhat off-topic question, but what do you think of the British empiricists, like Locke, Berkeley, Hume? Have you written about them?

[Rosa Lichtenstein]

LC: Sure, Einstein certainly began with these notions, but very soon moved away from them.

[I do not think much of Locke, but Hume had important things to say about religion, and Berkeley about mathematics and abstract objects in general. I have only written about them in passing.]

In practice, I think that most physicists are operationalists or instrumentalists, but when it comes to interpreting their ideas to one another and to the rest of us, they soon become Platonists.

[Junius]

Fair enough. I don't know what Hume had to say about religion, nor Berkeley on mathematics. But wasn't Berkeley an immaterialist - he denied matter altogether - God doesn't use matter to cause our experiences but that God produces ideas in us directly. God has set up patterns which science develops to reach formulate predictions. Just strikes me as odd that these empiricists are some hardcore idealists. What value do you see in this?

[Rosa Lichtenstein]

You are right about Berkeley, but that did not stop him writing an excellent critique of Newton's ideas on the calculus (in The Analyst), available here:

http://www.maths.tcd.ie/pub/HistMath/People/Berkeley/ETexts.html#Analyst

I do not see any value at all in his idealism, however.

[black magick hustla]

Space shouldn't be thought of as a non-physical abstraction. It must exist objectively, because the universe has a finite radius even though it's unbounded. If you were to travel for a long distance, in any direction, in a perfectly straight line, you would return to your starting point. This the only way we can make sense of the observation that the background microwave radiation left over from the big bang arrives at the earth from all directions in the sky simultaneously.

The spherical model of the universe is not accepted by everyone. Most scientists think the universe is flat.

[Rosa Lichtenstein]

Marmot:


The spherical model of the universe is not accepted by everyone. Most scientists think the universe is flat

These are, of course, technical terms. If they are interpreted as statements about physical reality, they can only be metaphorical.

The problem is, wtf do these metaphors mean?

[DesertShark]

How can anything curve space and time, and if it did, what exactly constitutes space and time, they appear so non-physical, it's hard to imagine. Is gravity then an illusion?
I think the best answer to your question was given by mikelepore:

Here's an example that I think addresses the original question is. The "principle of least time" shows that a ray of light has to travel in a straight line. However, a ray bends as it travels past a star, as shown from photographs taken during solar eclipses. The light doesn't bend because of gravitational attraction by the star. That would imply that the light has mass, which it can't have, because it wouldn't be able to travel at the speed of light if it had mass. Instead, the bending is caused by the fact that the light is still traveling in a straight line, and the presence of a nearby mass has modified the space in which the straight line path occurs. It's still the shortest possible distance between any two points.

Gravity is not an illusion but "a natural phenomenon by which objects with mass attract one another." Einstein's claim was that matter curves spacetime (as shown in the example above given by mikelepore). How Stuff Works has an extremely simple breakdown of gravity: http://science.howstuffworks.com/question232.htm. It also talks about the equation Marmot described [Gravitational force = (G*m1*m2) / (d^2); where G is the gravitational constant (more on that below), m1 is the mass of the first object, m2 is the mass of the second, and d is the distance between the objects].


However, the concept of space and time is treated very precisely in the semantic context of physics. Space are the coordinates x, y, and z given in a mathematical frame. t is well, t. (...) The value of G is used because it was a "random number" that fitted with GmM/r^2 not because it actually exists in nature.
I'm pretty sure 't' is not just 't', but time in almost every use in scientific equations. Also, your claim that G is a "random number" and 'doesn't exist in nature' is not true. From the link above:
"G has the value of 6.67 x 10E-8 dyne * cm2/gm2. That means that if you put two 1-gram objects 1 centimeter apart from one another, they will attract each other with the force of 6.67 x 10E-8 dyne. A dyne is equal to about 0.001 gram weight, meaning that if you have a dyne of force available, it can lift 0.001 grams in Earth's gravitational field. So 6.67 x 10E-8 dyne is a miniscule force. When you deal with massive bodies like the Earth, however, which has a mass of 6E+24 kilograms, it adds up to a rather powerful force. It is also interesting to think about the fact that every atom attracts every other atom in the universe in some small way!"

-DesertShark

[DesertShark]

The goal of science is to answer two questions:

1. What will happen naturally in the future?
2. What are the consequences of my actions? (or in other words, what will happen if I do X?)

(...)

So what is the curvature of spacetime? It's a bunch of equations on a sheet of paper that we can use to make accurate predictions about weird gravitational effects that we could not explain before Einstein.
I always thought the goal of science was to understand/discover how things work and increase human understanding of these things; not figure out the future, but understand what's going on now. The curvature of spacetime or gravity is not solely "a bunch of equations on a sheet of paper" but an observable phenomenon, which is why knew about it before Einstein.


The spherical model of the universe is not accepted by everyone. Most scientists think the universe is flat.
Cool video from How Stuff Works and the Discovery Channel about the shape of the universe, it doesn't give any definite answers but it talks about the possibilities (which are pretty cool): http://videos.howstuffworks.com/discovery/4864-shape-of-the-universe-video.htm

-DesertShark

[black magick hustla]

I'm pretty sure 't' is not just 't', but time in almost every use in scientific equations. Also, your claim that G is a "random number" and 'doesn't exist in nature' is not true. From the link above:
"G has the value of 6.67 x 10E-8 dyne * cm2/gm2. That means that if you put two 1-gram objects 1 centimeter apart from one another, they will attract each other with the force of 6.67 x 10E-8 dyne. A dyne is equal to about 0.001 gram weight, meaning that if you have a dyne of force available, it can lift 0.001 grams in Earth's gravitational field. So 6.67 x 10E-8 dyne is a miniscule force. When you deal with massive bodies like the Earth, however, which has a mass of 6E+24 kilograms, it adds up to a rather powerful force. It is also interesting to think about the fact that every atom attracts every other atom in the universe in some small way!"

-DesertShark

The physical phenomena these equations explain are true. My point is that the equation itself is not nature, its a mathematical tool used by people to predict phenomena to a certain degree of success. When I meant "random number" I implied that when Newton was probably formulating the equation, he found that G fitted to the formula quite nicely, hence he just mutliplied it by certain number that seemed to fit. However, these are just equations, nothing more.

[DesertShark]

The physical phenomena these equations explain are true. My point is that the equation itself is not nature, its a mathematical tool used by people to predict phenomena to a certain degree of success. When I meant "random number" I implied that when Newton was probably formulating the equation, he found that G fitted to the formula quite nicely, hence he just mutliplied it by certain number that seemed to fit. However, these are just equations, nothing more.
That wasn't clear in your original post. I thought that Newton only came up with the law, not the equation or if he did come up with the equation it didn't contain the constant (http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html and http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3d.html). About the gravitational constant (from wikipedia, cause its easier):
"The value of the constant G was first accurately determined from the results of the Cavendish experiment (http://en.wikipedia.org/wiki/Cavendish_experiment) conducted by the British (http://en.wikipedia.org/wiki/United_Kingdom) scientist Henry Cavendish (http://en.wikipedia.org/wiki/Henry_Cavendish) in 1798 (though Cavendish did not himself calculate a numerical value for G[1] (http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation#cite_note-0)). This experiment was also the first test of Newton's theory of gravitation between masses in the laboratory. It took place 111 years after the publication of Newton's Principia and 71 years after Newton's death, so none of Newton's calculations could use the value of G; instead he could only calculate a force relative to another force."

[mikelepore]

But the problem is, how does one define space, its properties and such, when it can't be perceived by senses, or by finer instruments?:( It applies to time, as well. Both time and space are not like other physical objects which have clear attributes like weight, shape, color etc. So what exactly are space and time?

The attributes seem definite to me. The speed of light in a vacuum is measured by independent labs, so that must be real. A second is defined as the time required for a certain number of cycles of the electromagnetic wave emitted by a certain transition of a certain kind of atom. Once you have time and speed, then you also have the product of the two, which is distance.