# Why does mass curve space-time?

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According to general relativity mass curves space-time giving the effect of gravity...

I have a challenge for you. Think of (a) reason(s) why mass causes space-time to curve. I have tried...You people are more qualified than I. So you should try too.

There is no prize.

Why am I making this thread? ...Because I haven't seen anyone attempt to answer this question of why.

If you think of a reason you like I encourage you to make our own thread in the speculations section.

I don't like equations...Try to not use them. You can add them to your post in the speculations section later.

...And the challenge begins in...1...2...3...Now...Enjoy! I will be interested by your attempts to think of why mass = space-time curvature.

Edited by ProcuratorIncendia

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I hope you realize that this really isn't physics. In doing physics, we observe that mass warps space, and attempt to describe what that warping is and what the ramifications are. The question of why is metaphysics, and not particularly relevant to the investigation.

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So this should be in philosophy? - I don't see why...Science should answer what, how and why...not just what and how...

Edited by ProcuratorIncendia
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Because I haven't seen anyone attempt to answer this question of why.

Because 'Why?' is a silly question to ask here. 'Why?' is a question regarding the intention of a causal agent with respect to the effect said agent caused. Mass alone lacks the necessary requisite ability of intent which would render the question meaningful.

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PI,

Sometimes the why comes out and sometimes it doesn't. Is there a why $\pi$ is the value it is? Maybe, but that doesn't hamper its usefulness. The why can be an interesting question, but it doesn't ruin or invalidate the physics or math. The why isn't really the answer that physicist are looking for; physics is about making mathematical predictions of what nature will do. If I drop this block weighing 1 kg from the height of my head, what speed will it hit the ground? If a sun has a certain composition of elements, what temperature is its corona? On what date will the next solar eclipse be? Etc.

Physics doesn't try to answer the why behind these questions, so the why is often out of scope. It may be that we never find out the why space-time gets warped, but we should still nonetheless be able to make predictions about what effects is has on objects. If the predictions are accurate, that is all it needs to be a success in the eyes of physics.

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The idea of space/time being curved is strange. If light follows a curved path as the shortest possible path between two points, how can it possibly be recognized as curved except when it noticeably diverges from another path? However, since mass seems to be transparently detectable, we assume that we can know which areas of space are relatively mass-free and therefore prone to straighter light-trajectories. But why should it be assumed that light follows curved trajectories but not gravitation? So to sum up, if light appears direct no matter how curved its path might be, how do you know when and how much space is curved? And why would you assumed light's path follows curved spacetime, but gravity proceeds in straight lines away from matter/mass instead of itself radiating according to curved space/time?

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Well, my guess would be that matter is made of compressed spacetime, so there's less spacetime to go around nearby matter. Of course that's just random gibberish until someone finds an equation that would describe such and checks that the predictions are correct. Is that the sort of "why?" that you were looking for? Because if you were looking for a "why?" that would not involve new equations, the sort of question that Newton thought irrelevant to his idea of gravity, then science isn't really the place to look for such answers.

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Because Gravity, Time and Space are the one and same thing.

Is that the kind of answer you are looking for?

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"Why" space-time is curved can be understood an inevitability (under some assumptions, like the equivalence principle) of wanting to include accelerating frames of reference in a relativistic theory.

This can be thought of as quite independent of wanting to combine a Newtonian-like gravity theory with special relativity.

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"Why" space-time is curved can be understood an inevitability (under some assumptions, like the equivalence principle) of wanting to include accelerating frames of reference in a relativistic theory.

(...)

I don't understand.

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A very physical argument proceeds as follows:

Consider a freely falling lift here on Earth. Lets say that the occupant has a torch and emits a flash at a right angle to the motion.

According to the equivalence principle the flash as seen from inside the lift travels in a straight line. However, relative to the Earth the flash travels along parabolic curve. The assumption is that this is not something peculiar to light but all things that propagate with a finite speed would show such behaviour.

This suggest that space-time itself in the presence of mass (recall the equivalence principle) is curved. All "straight phenomena" will follow a curved path defined by the curvature of space-time.

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A very physical argument proceeds as follows:

Consider a freely falling lift here on Earth. Lets say that the occupant has a torch and emits a flash at a right angle to the motion.

According to the equivalence principle the flash as seen from inside the lift travels in a straight line. However, relative to the Earth the flash travels along parabolic curve. The assumption is that this is not something peculiar to light but all things that propagate with a finite speed would show such behaviour.

This suggest that space-time itself in the presence of mass (recall the equivalence principle) is curved. All "straight phenomena" will follow a curved path defined by the curvature of space-time.

If instead of a flash, I throw a stone from the elevator, the same phenomena will occur. That means that observation is relative, not that space-time is curved.

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I should have stressed the finite constant speed as measured in an inertial frame, even then I am not sure it is of great consequence to the argument.

The point is that one cannot distinguish (in a small enough region) between gravitational free fall and acceleration. Applying this to a light beam suggests that space-time is curved.

Edited by ajb
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Lots of responses... Some telling me that there is no need for a why...Sounds relevant to me but you're the physicists here...

Others asking me what I mean by why and some answering the question...

I'm pretty sure I was looking for a why that would involve new equations...

[Think of it this way: Looking for this why will tell you whether mass really does cause space-time curvature. If it does then yay...more evidence for Einstein...If the conclusions don't match then either the conclusion of why is wrong or mass doesn't curve space-time and we need to re-think things. Therefore- even if this isn't physics...the answer will affect physics...so you should be trying to answer it anyway. Are you interested now?]

I just want to ask something...where do gravitons fit in this whole general relativity thing?

michel123456: That answer isn't really an answer. Yes gravity is [according to current understanding] caused by space-time curvature....meaning they are the same thing. The reason they are the same is because space-time curvature = gravity. And Mass causes Space-time curvature...my question is why mass causes space-time curvature.

ajb: I don't understand...

Mr Skeptic: Interesting idea...Doesn't sound correct to me though.

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ajb: I don't understand...

Basically, the curvature of space-time is due to the equivalence principle, in one form or another. Mathematically we can think of the equivalence principle as allowing a choice of very special coordinates at every point on space-time. This led Einstein to think that curved manifolds are the answer to including gravity and more general frames of reference in a relativistic way.

This is about the best answer one has as to "why" space-time is curved.

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My question is not why space-time is curved but why mass causes such curvature...

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because if it didn't then it wouldn't be mass.

I think you really want to know "how mass curves space time?" rather than why.

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So the meaning of mass is something that curves space-time...that doesn't sound right...I'm quite sure that [and wikipedia backs me up] that mass=weight.

Maybe I am...perhaps how is the right word...maybe it isn't though...I think I would like both answers...

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Because mass eat spacetime all around it

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mass=weight.

No. That's like saying charge=Felectric. Both forces vary by position whereas the property causing the force does not.

$F_{weight}=mg=mG\frac{M}{r^2}$, so your weight varies by the mass of the other object and your distance to its centre. Your weight here and on Jupiter are very different, but your mass will be the same.

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My question is not why space-time is curved but why mass causes such curvature...

The two things are just about the same. The equivalence principle, is best "physical" explanation to offer. I don't think anyone can really offer anything else.

The details of how are to be found in the field equations of general relativity.

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When you talk about "mass causing spacetime curvature," it seems to imply that spacetime exists in an un-curved form prior to or relative to its become curved due to mass as a cause. I don't think that assumption is tenable since spacetime never exists except in the presence of mass/energy, so mass doesn't cause its curvature so much as that curvature is co-present with mass.

I may be overextending this reasoning, but it seems analogous to the relationship between electrons and electromagnetic fields and radiation. The electrons do not necessarily "cause" the fields/radiation. Instead, the field and radiation may just be extensions of the electron itself.

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lemur...what you are saying sounds wrong...

What he said is still wrong: Mass is a property of matter equal to the measure of an object's resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from but proportional to its weight. Not something that curves space-time.

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I found Lemur's post quite sensitive.

One could say that the definition of mass is gravity.

Edited by michel123456
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What he said is still wrong: Mass is a property of matter equal to the measure of an object's resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from but proportional to its weight. Not something that curves space-time.

I know that weight is a measure of force (lbs or newtons) whereas mass is acceleration-independent. I wasn't claiming otherwise and I don't know where you got that I was. It sounds like you're talking about inertia regarding resistance to changes in motion. I am interested in what relationship could be possible between inertia and mass because they don't seem the same to me, since mass causes matter to push down on the ground even when it is not in motion.

One could say that the definition of mass is gravity.

I wouldn't have thought to put it that simply, but is that possible? Is it possible that particles of matter are themselves elementary units of gravitational field-force?

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