# Does matter follow the curvature of space-time?

How is matter affected by the warping of space-time? Does it expand and contract, follow the curvature of space? What happens the shape/volume/density of matter when it enters a gravity well, or washed over by gravitational waves?

Note: I dont have a background in physics. Just want a basic layman's explanation, so I can visualize.

• Clarification - I am not asking about "mass tells space how to curve, and space tells mass how to move" - I want to know how an individual object is affected by curved space. - Thank you. Aug 14 '20 at 4:25

If the matter consists of a loose collection of falling objects, say raindrops, or a group of rocks, then each individual piece of matter will not expand or contract owing to curvature of spacetime as it falls (except a tiny bit as I will explain in a moment), but the distances between the objects will expand or contract, depending on what spacetime is locally doing. Notice that this prediction is the same as the one made by the more familiar Newtonian picture of gravity: particles falling to Earth are each attracted to the centre of the Earth so two particles at the same height but separated by some horizontal distance will get gradually closer together as they fall. Two particles starting out from different heights get further apart because the lower one accelerates a bit more than the upper one.

The curvature of spacetime is also inviting each individual raindrop or rock to be squeezed or stretched in the same way, but the internal electromagnetic forces between the atoms are resisting this, so the individual raindrops or rocks remain of almost constant size.

When a gravitational wave passes by, the story is similar: separate freely falling rocks will be moved closer or further apart, but the size and shape of each individual rock is hardly affected because the gravitational effect is small compared to the internal electromagnetic forces. However, in an extreme case such as near a neutron star or a black hole then the gravitational effects will overwhelm everything else so that even rocks are crushed in one direction and pulled apart in the other.

• This was a very lucid and beautiful explanation. Much appreciated. Here, it seems we are describing movement of objects (or collection of objects) in terms of gravitational force and acceleration vs. electromagnetic forces binding matter. This has a familiar "tug-of-war" aspect. But I wonder what happens to 2 points in space as spacetime is distorted, and if this has any effect on the shape of matter, regardless of acceleration. Maybe I am thinking of this in the wrong way. Maybe space and matter are two independent "fields" that dont interact like 2 dots drawn on stretchy fabric. Aug 15 '20 at 14:02
• @spaceface It is not possible to define "two points in space" in any physical way except by putting something at the those two points, such as a pair of labelled pins or spots of ink or just single atoms. But now you have to also say whether any forces are acting on those things to stop them moving. Moving relative to what? etc. If you say "no forces except gravity" then they are in freefall and my answer applies. Aug 15 '20 at 15:04
• I understand there are no absolute values (relativity), but the speed of light. But what is warped space? Thought experiment: How would a giant block of cheese (1000 miles long) register the effects of gravitational waves passing thru its length? Assume this block of cheese is rock solid, its constituent parts immune from forces of gravity. Assume wave frequency is less than 1K miles such that one could imagine those waves rippling through. What does warped space actually do to matter? Aug 15 '20 at 17:46
• Clarification - going under the assumption that gravity is warping space (spacetime). So, ignoring the effects of gravity itself on an object in space, what effect does "warped space" have on that object, If that makes sense.. Aug 15 '20 at 17:53
• "Rock-solid" cheese (say a huge diamond crystal) would experience an oscillating squeezing force, but since it has a high Young's modulus it would not move noticeably in response. So in this case the gravity wave produces pressure and tension forces, but the object in question does not move (if we idealize it to the case of a huge Young's modulus). Aug 15 '20 at 19:50

How is matter affected by the warping of space-time?

To start with, if there is not matter in the universe, there is no warping. Everything is flat.

If the only matter in the universe is your "test matter" , it will be the one warping spacetime around it, as three dimensional space goes, it will just be the newtonian gravitational field, unless it is a huge mass moving with relativistic velocities when one has to learn the mathematics of general relativity to plot the way a single mass warps space-time.

If we are in the present universe where for large masses and large velocities general relativity has to be used , there is spacetime curvature defined by all those masses in the universe, in the (x,y,z,t) point where the center of mass of your test "mass" is.

Does it expand and contract,

Not for low values of energy and mass of your test particle. In the regime where general relativity has to be used and the curvature is strong, there are various effective forces acting on the masses .

follow the curvature of space?

If it is a test point mass, it will follow the curvature of spacetime. Other wise its own mass will also contribute to the curvature.

What happens the shape/volume/density of matter when it enters a gravity well,

a gravity well belongs to Newtonian gravitation. When at relativistic sizes and speeds you can see what happens in the presence of large masses in the LIGO experiment.

or washed over by gravitational waves?

again you can get an idea from the LIGO obsrvation of the merging of black holes.

• Believe or not, but I was actually able to follow along most of your explanation.. somewhat. I dont know the maths, but I am familiar with the terms. LIGO tested changes in spacetime indirectly by calculating minute changes in distance based on constant velocity of light (laser). So if distances change in gravity well or waves, does that affect the shape of objects (from any frame of reference)? Aug 15 '20 at 14:09
• Really nice answer. Aug 23 '20 at 16:11

Questions of this sort in relativity are quite dodgy, because the notion of "size" is inherently subjective--literally, it is dependent on the observer in spacetime. Even in special relativity, we know the phenomenon of length contraction. A fast-moving box will appear to a stationary observer to be shorter, but in its frame it will be the same dimensions. This is slightly altered in general relativity, but the observer-dependent nature of the question remains.

You can think of spacetime curvature as a change in your spacetime yardstick as you move to different points in spacetime. Say, per your question, we have a box in the presence of a black hole. We drop the box, and it falls in. Like the apocryphal Alice or Bob, from our perspective the box will spaghettify--it will appear to be stretched, contorted, due to the effect of gravity on the light it sends our direction. In its frame, it sits in freefall. Indeed, as it approaches the black hole, the gravity at its bottom will be greater than the gravity at the top. This will induce some stress within the box. If the box is conventionally sized by our standards, in almost no observable astrophysical black hole will this become an issue for the box--the tidal disparity due to gravity is too small outside the event horizon. Similarly, a box in a rotating spacetime may experience some tidal shear force resulting from "frame dragging" of the gravitational field. All these effects would be miniscule, except in a region of extremely high spacetime curvature.

But in principle, an object with any finite extent will be subject to the changing metric on spacetime in the presence of a gravitational field. The notion of "length" simply ceases to be very helpful when your measurement of such a quantity depends on a dynamical field.

• Tidal forces are small outside the event horizon of a supermassive black hole, but they are large for stellar mass BHs. From en.wikipedia.org/wiki/Spaghettification a rod of mass 1 kg, length 1 m, and tensile strength 10000 N, falling lengthways towards a 10 solar mass BH (Schwarzschild radius = 30 km) will break at 320 km. Also see vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator Aug 14 '20 at 13:14
• Thank you for including Relativity in your response. Asked another way - will the shape or size of an solid object change from an external frame of reference? i.e. viewing said Box near the surface of a Neutron star (strong gravity) from the vantage point of someone outside its gravitational influence. Will someone inside the box measure size of box differently than someone outside gravity well? Thank you. Aug 15 '20 at 14:17
• in strong fields there is en.wikipedia.org/wiki/Spaghettification . i. Note it is a newtonian gravity effect too, and will be felt by inside and outside observers. In gravitational interactions at our level the effect will be minimal Aug 15 '20 at 14:37