Why does Space-Time curve and more importantly, why and how does Space and Time come together to form a “fabric”?
In: Physics
Reading about these things makes me sad because I used to dream about being an astrophysicist. Then I found out most of them spend all day doing math, not brainstorming about the nature of the universe. Idk why I didn’t consider that this all comes from math and that I would need to be an incredible mathematician to just be an average physicist capable of partaking in the discussion. I’m about to go to school for IT because I like computers and there’s practical job opportunities in that, but I hope I never stop thinking about the big questions and I hope those of you smart enough to do this will answer them some day.
Lots of technical stuff in this thread, which is great, but doesn’t explain like you’re 5. Simplest explanation of space-time is to imagine cutting time into increments, then laying the 3D space on top of itself so it forms a block. As if you take all the individual frames of a film and stacking them, so you get a big pile of individual spatial snapshots that move forward and backward through time as you look up and down the pile. As the aforementioned Einstein Nova said, we are all spaghetti strands moving through both space and time simultaneously.
Curvature is just mass warping this field. Our inability to differentiate between inertia and gravity (is the ground moving us up or is gravity holding us down?) breaks our lizard brains so leave this to the theoretical physisists lol.
FWIW Hawking’s Brief History of Time is surprisingly understandable (at least some of it) to the non-genius out there, recommended.
Note: Sorry if this is already posted, didn’t feel like reading “billions and billions” of posts (RIP Sagan).
The Space-Time Fabric is a **metaphor** used to explain to the space-time curvature and why things get funky around around super-massive objects like black holes.
Imagine space as a 2d plane, in this case a well made checker quilt. You and your friends each grab a corner and stretch it taut. Another friend rolls some marbles across the quilt’s surface. The marble are fairly small and don’t distort the surface as they move across in a straight lines. Now, the friend drops a bowling ball onto the quilt. It’s big and heavy enough that it does distort the surface of the fabric, causing it to bend and curve under the weight of the bowling ball. Some more marbles are rolled across and instead of moving in straight lines, they curve following the shape of the fabric underneath them. You can physically see the distortions in the surface as the straight lines of the checker’s grid are pulled down and twisted.
At this point, things still make sense, as the gravity of super-massive objects like black holes as the same effect on space-time as the bowling ball has on the fabric of the quilt. Where the metaphor sheers is that since space is 3d, the curvature doesn’t appear in a spatial dimension, it appears it time. What that physically means, I am not entirely sure, but it makes the math work out and what little observations we have match the math. I’m not a physicist, so I am out of depth, but hopeful this answers your question.
Time is more of an illusion. All it is, is the process of things travelling from point A to B… If there was no universal speed limit, essentially the universe would happen all at once in an instant. But since there is mysteriously a speed limit (the speed of light) in which information can move through space, it has created the illusion of time.
This is why space and time are tied together, because time requires space to exist.
Two questions. I’ll answer the second one first.
You always move through time, but if you move through space then you move through time more slowly. If you move as fast as you can (the speed of light) then you stop moving through time.
Photons don’t age.
Time is different for different things because they move through space differently.
Space is curved because a straight line isn’t what you would think of as straight if there’s an object big enough to mess up the curvature of space in your path.
Let’s say you shoot two beams of light in the same direction, but a meter apart. If space were flat they’d stay a meter apart. But space isn’t flat, so as the beams of light travel, they get closer to each other, or farther away.
Why does it work this way? We don’t really know. But it does.
Everything you can touch is called matter. That matter stuff has mass, which creates gravity. The more mass the more gravity. Big massive things can pull on other things with gravity, and hold them in orbit like the moon orbits the earth. The moon follows a curved path in space. If the Earth were replaced instantaneously with the Sun, the moon would curve even more and crash into the Sun. So gravity curves space.
Time is only realized and measured when things move, like a pendulum clock or atomic orbitals.
If you and a friend set your watches to the exact same time, and you travelled really fast away from them and came back, your watches would have different readings. Time is something you take with you when you move in space. So when you move in space and in time, you are moving in spacetime.
How the fabric is woven is determined by relativity and the speed limit the universe has set upon matter.
Why ? You would have to ask Steven Hawking.
There is an awesome Netflix special on Einstein that explains a lot of his theories and how he arrived at them in eli5 terms. I think they go over this in the first episode….
I’m going to have to go with “it really doesn’t”, but that is about the best analogy we can come up with so we understand it. It is a mental model, not really what it is, just something we do understand that is sort of close to it.
All multi-dimensional systems have a structure of sorts, but all we humans have to use for imagining such things is how this 3-d world looks to us. So we think about other dimensions and other systems as if they were something that looks like what we actually can relate to. This does not make them like we imagine, it makes it so we can imagine something that at least starts to approach what the unknown thing actually is like.
Light has mass. Mass is effected by gravity. So light bends because it has mass? Gravity effects time. The stronger the gravity the faster time? Weaker gravity slower time?
God I’m a dense individual. I wish I could grasp this!!
Oh, that’s all.
The notion of a fabric and curvature are mostly just tools to help us wrap our heads around it. These physical theories are basically just mathematical models of how things actually work. In the case of general relativity (GR) the models predict pretty accurately.
The concept of curvature comes from an extrapolation of something we can understand well. If you take a piece of fabric and pretend it’s infinitely thin, it becomes a 2 dimensional surface embedded in a 3 dimensional space. You can bend and deform it. But if you were a 2 dimensional creature on that surface, it would just appear to be a flat plane, because you have no way to observe a 3rd dimension. An object moving along that fabric would twist and turn in ways you couldn’t understand. But to us 3 dimensional creatures looking at the fabric we can see the bends and deformations. To us it’s obvious why the object is moving in the way it is.
Gravity works similarly. Objects in space bend in the presence of a gravitational object – the moon orbiting the earth. But how does it do that? There’s nothing ‘pulling’ the moon to the earth – no particle we can see, no string, etc. Well, if we consider that our 3 dimensional space may be bending and deforming in 4 dimensional frame, in ways we can’t see and understand, we can visualize how that might work – that just like putting a weight in our 2d fabric distorts it and therefore distorts the path of objects traveling on it, without any obvious interaction of particles, string tying them together, etc. Massive objects do the same to 3d space. The earth distorts 3d space causing the moon to orbit it.
Is it an actual distortion? Don’t know. Doesn’t really matter, either. What matters is that the model works well enough that we can predict things we previously couldn’t. Further, and this is a sign this is a good model, it predicts things we’ve never seen. When we discover one of those things, it serves as good evidence that the model is valid. Distortion of light around the sun, gravitational lensing, time dilation in a gravitational field, etc. are all things the theory predicted that weren’t observed until later.
So, mainly it’s a way of conceptualizing a physical effect in a way that allows us to understand the interaction between these objects without seeing an exchange of information between them (particles, etc)
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