# Why does mass warp spacetime?

1. Jan 11, 2013

### guitarphysics

I know that mass warps spacetime and that's basically the force of gravity (warped spacetime), but why does mass warp spacetime? Is it because mass implies energy and energy warps spacetime? If so, then why does energy warp spacetime?

2. Jan 11, 2013

### Whovian

Because it does. Unfortunately, any theories about why mass warps spacetime aren't going to result in any predictions we can test (in part because they'll be tailor-made to predict GR.)

3. Jan 11, 2013

### Staff: Mentor

It's turtles all the way down from there...

4. Jan 11, 2013

### guitarphysics

Oh, I love that Hawking book :D
But that's kind of disappointing though about not knowing why mass warps spacetime....

5. Jan 12, 2013

### Staff: Mentor

This is a fundamental limitation of the scientific method, it is rather poor at answering "why" questions. The problem is basically what kinds of answers the questioner would like to their "why" question.

Sometimes the asker of a "why" question wants a scientific answer, which means that the question must be in terms of one scientific theory and the answer must be in terms of another, more fundamental, scientific theory. This means that you can get answers to "why" questions about Newtonian gravity by explaining GR, since GR is a more fundamental theory than Newtonian gravity. However, this also means that it is inherently impossible to answer why questions about fundamental theories. We use our fundamental theories to explain other theories, we don't have an explanation for them other than they fit the data.

More often the asker of a "why" question wants some kind of philosophical answer, which doesn't belong here anyway.

6. Jan 12, 2013

### guitarphysics

I was actually looking for an answer that would say that a property of spacetime is that it reacts to mass (or something like that, I don't know), but yeah, what you're saying makes sense.

7. Jan 12, 2013

### pervect

Staff Emeritus
The only other time I can think of where you can answer a "why" question is when you have a theory that's more fundamental that underlies the original theory.

Basically, you can always ask "why", and when you get an answer you can ask "why" again.

At some point you have to stop because you've reached the most fundamental known reason "why".

However, there isn't (as of yet) any more fundamental theory that GR is based on, so there isn't any answer of this type.

8. Jan 12, 2013

### Staff: Mentor

To the great amusement of your average 3-year-old and the consternation of their parents

9. Jan 12, 2013

### pervect

Staff Emeritus
Yep, exactly :-).

10. Jan 12, 2013

### Rotating Frame

Okay I have no clue what I am talking about, but I am trying to understand this. I think each element only appears warped from the perspective of other elements of the superposition of the wavefunction of each structure within the system. So if you consider things from the perspective of the mass structures, the spacetime elements appear warped. I am sure if you looked at it from the other perspectives, mass would appear warped. Don't listen to me I have no idea what I am talking about. I am sure I am at least an order of magnitude or a dimension away from being able to grasp it. :)

Edit: I think a more accurate reason is that gravity and acceleration can be considered the same, in accordance to what we can measure and observe in spacetime, from the frames of reference of knowing that our theories have proven experimentally verifiable in those frames. I think the train of logic is something along that path. There are probably frames in which mass doesn't warp spacetime but something else would have to give and we probably never get to observe from those frames.

Last edited: Jan 12, 2013
11. Jan 12, 2013

### Staff: Mentor

I'm sorry, your universe is in another castle!

12. Jan 12, 2013

### haael

There is a speculation (not a theory) that the spacetime warping is the same thing as the quantum (de Broglie) waves. This hypothesis might get attention if we discover a graviton. With graviton, this is definitely true. For example, the frequency of a graviton's de Broglie wave calculated from QM and the frequency of tidal waves generated by the graviton calculated from GR are the same. This has yet to be confirmed by experiment, but if it turns out true, then it would be natural to generalize it to other particles.

13. Jan 12, 2013

### Rotating Frame

I think I have a better answer. Consider mass as one manifestation of the energy of a body in spacetime. A body at rest moves through time at the speed of light. An body in motion has some of it's energy diverted away from the time dimension, into the spatial dimensions, causing warps. I'm sure I'm on the right track..

14. Jan 12, 2013

Staff Emeritus
Sorry, but we don't allow posting of speculative or personal theories here.

If you want to ask questions about conventional physics, go ahead. However, "Here's my theory, what's wrong with it?" can't be one of them.

15. Jan 12, 2013

### Staff: Mentor

Nope. I suggest you read up on what energy is and what an inertial frame is.
And as Vanadium said, please don't try to come up with your own reasons for things happening. This forum is just for mainstream scientific theories.

16. Jan 13, 2013

### jk22

we could use the principle of equivalence saying gravitation can be reproduced by a local inertial frame and get hence a locally induced space contraction time dilation. But this doesnt explain why a mass create gravitation and in some sense the question remains

17. Jan 20, 2013

### Zmunkz

Although phrased as his own idea, this is not. I've heard Einstein attributed to this sort of interpretation on some of those science channel shows, like Through the Wormhole (or one of those). I've never been able to find a technical discussion of what this interpretation comes from, or if Einstein ever said anything like it, but I just wanted to throw it out there that this reasoning has worked it's way into the non-technical science mainstream. It was described more-or-less as above: every single object has a "total velocity" of c through space and time. Photons move entirely along the space axis, and everything else has a vector with components in both space and time, changing in proportion according to relativity, but always maintaining a magnitude of c.

Perhaps we can dismiss RotatingFrame's delivery and treat it as a question: is there any validity to that interpretation?

18. Jan 20, 2013

### Staff: Mentor

This is why the rules of the forum specify mainstream scientific references, not pop-sci references. Do a search for "Brian Greene" on this forum and you will get a taste of the headache that pop-sci treatments cause.

19. Jan 20, 2013

### Naty1

Although there is no real scientific answer to your question, string theory offers some interesting insights. [That mass and its equivalent, gravity, can affect space and as well the relative passage of time is one of most profound findings of all time. It's downright 'crazy' based on our everyday intuitions.]

In string theory, fundamental components of particles, strings, are vibrating energy modes. It turns out that these interact with the degrees of freedom, or geometrical dimensions, in which we all find ourselves. Different sizes and shapes of additional dimensions can be mathematically associated with different characteristics of strings: varying vibration patterns correspond to things like particle size, charge, spin that we observe macroscopically.

So strings and geometry, spacetime, interact analogous to mass/energy in general relativity. This offers some insights, perhaps, why not only mass, but also energy and momentum density warps spacetime.

20. Jan 20, 2013

### Naty1

I've got Brian Greene's book FABRIC OF THE COSMOS where he explains acceleration using the above concepts. I found it useful as ONE perspective, but it seems unpopular here among some. I found it helpful when approaching light cones for the first time....I don't see much difference...

I find Greene's above description along the lines of the 'rubber sheet' analogy for gravity...[which Greene discuss right after in his book] or the 'balloon analogy' for cosmology, useful as a perspective, but they all come with limitations.

21. Jan 20, 2013

### Whovian

Definitely. Depending on what this 4-velocity actually "means," if we manage to interpret it correctly, we're going to be able to predict Special Relativity.

Though this has nothing, or almost nothing, to do with gravity.

22. Jan 20, 2013

### Fredrik

Staff Emeritus
Brian Greene actually uses language like that, but (as you can see in his books) it doesn't have anything to do with "warps" (i.e. gravity). He uses it to explain time dilation and other SR phenomena. Here's a quote from "The fabric of the cosmos".

I think this way of looking at it was made popular by Brian Greene. He uses it in both "The elegant universe" and "The fabric of the cosmos". The technical explanation (in units such that c=1, and with a -++++ signature) is as follows.

In my own rest frame, my world line coincides with the time axis. So the tangent to my world line is in the 0 direction (axes numbered from 0 to 3, with "time" being 0). Every vector of the form
$$\begin{pmatrix}r\\ 0\\ 0\\ 0\end{pmatrix}$$ where r is a real number is a tangent vector to the world line. The tangent vector with Minkowski "norm" -1 (-c for those who don't set c=1) is called my four-velocity. I will denote its coordinate matrix in my own rest frame by u. We have
$$u=\begin{pmatrix}1\\ 0\\ 0\\ 0\end{pmatrix},\qquad u^2=u^T\eta u=\begin{pmatrix}1 & 0 & 0 & 0\end{pmatrix}\begin{pmatrix}-1 & 0 & 0 & 0\\ 0 & 1 & 0 & 0\\ 0 & 0 & 1 & 0\\ 0 & 0 & 0 & 1\end{pmatrix}\begin{pmatrix}1\\ 0\\ 0\\ 0\end{pmatrix}=-1.$$
Greene calls $\sqrt{-u^2}$ the speed through spacetime. We have $\sqrt{-u^2}=1$. If we restore factors of c, this is $\sqrt{-u^2}=c$. (Greene uses the metric signature +--- instead of -++++, so when he does this, he gets $u^2=c^2$, and is therefore able to write the speed through spacetime as $\sqrt{u^2}$. His $u^2$ is equal to my $-u^2$).

Let's boost my four velocity to the rest frame of an observer who has velocity -v in my coordinate system. I should have velocity v in his.
$$u'=\Lambda(-v)u=\gamma\begin{pmatrix}1 & v^1 & v^2 & v^3\\ v^1 & * & * & *\\ v^2 & * & * & *\\ v^3 & * & * & *\end{pmatrix}\begin{pmatrix}1\\ 0\\ 0\\ 0\end{pmatrix}=\gamma\begin{pmatrix}1\\ v^1\\ v^2\\ v^3\end{pmatrix}.$$ The asterisks denote matrix elements that are irrelevant to what we're doing here. If anyone cares, they are the components of the 3×3 matrix
$$\frac{1}{\gamma}I+\left(1-\frac 1 \gamma\right)\frac{vv^T}{v^Tv}.$$ The velocity components can be calculated like this:
$$\frac{dx^i}{dt^i}=\frac{u'^i}{u'^0}=\frac{\gamma v^i}{\gamma}=v^i.$$ As expected, my velocity in the new coordinate system is minus the velocity of the boost. This result is the reason why the normalized tangent vector is called the four-velocity.

The world line is the range of a curve $C:\mathbb R\to M$ where M is Minkowski spacetime. Its representation in a global coordinate system $x:M\to\mathbb R^4$ is the curve $x\circ C:\mathbb R\to\mathbb R^4$. The world line is said to be parametrized by proper time if the curve C that we use to represent it has the property that for each point p on the world line, the number $\tau$ such that $C(\tau)=p$, is the proper time along the curve from C(0) to p. Such a C has the advantage that the four-vector with components $(x\circ C)^\mu{}'(t)$ is automatically normalized. So if y is my rest frame, and x is the coordinate system we transformed to above, we have $u^\mu=(y\circ C)^\mu{}'(\tau)$ and $u'^\mu=(x\circ C)^\mu{}'(\tau)$. It's conventional to denote $(x\circ C)^\mu(\tau)$ by $dx^\mu/d\tau$, so we have
$$u'^\mu=\frac{dx^\mu}{d\tau}.$$ Now let's use the fact that $u^2$ is Lorentz invariant.
$$-1=u^2=u'^2 =-(u^0)^2+(u^1)^2+(u^2)^2+(u^3)^2 =-\left(\frac{dt}{d\tau}\right)^2+\sum_{i=1}^3 \left(\frac{dx^i}{d\tau}\right)^2.$$ Let's manipulate this result with some non-rigorous physicist mathematics. (These things can of course be made rigorous).
\begin{align}
&\frac{dt}{d\tau} =\sqrt{1+\sum_{i=1}^3\left(\frac{dx^i}{d\tau} \right)^2}\\
&\frac{d\tau}{dt} =\frac{1}{\sqrt{1 +\sum_{i=1}^3\left(\frac{dx^i}{d\tau} \right)^2}}\\
&1=\left(\frac{d\tau}{dt}\right)^2 \left(1+\sum_{i=1}^3\left(\frac{dx^i}{d\tau} \right)^2\right) =\left(\frac{d\tau}{dt}\right)^2+\sum_{i=1}^3 \left(\frac{dx^i}{dt}\right)^2.
\end{align} Greene calls the square root of the first term the speed through time and the square root of the second term the speed through space. (This is according to note 6 for chapter 2 (p. 392) of "The elegant universe"). This allows him to say that an increase of the speed through space must be accompanied by an decrease of the speed through space.

If we had been talking about the motion of a massless particle (i.e. light) instead of the motion of an observer, we would have had $u^2=0$ instead of $u^2=-1$. It's easy to see that what this does to the calculation above is to eliminate the first term on the right-hand side above. Since $\tau=0$ along the world line of a massless particle, this means that the result
$$\left(\frac{d\tau}{dt}\right)^2+\sum_{i=1}^3 \left(\frac{dx^i}{dt}\right)^2=1$$ holds for massless particles too.

Last edited: May 16, 2013
23. Jan 20, 2013

### Naty1

Fredrick:

yes,
it is A way to view the Lorentz transforms.....that space and time 'morph' into each other as a result of speed...are seen differently by different observers....in his explanation is the [unstated, I think] assumption that space and time remain a fixed background.

He also uses it to explain that acceleration is a curve in space-time, fixed velocity plots as a straight line, while rotational motion appears as a corkscrew. It is easy to picture yourself riding along in such situations where you 'see' space-time different from your neighbor, and they different from you.

He does NOT [and cannot] use it to explain the dynamical nature of space-time due to mass,energy or gravity.

24. Jan 21, 2013

### DimReg

I haven't thought about this for too long, so it might not be an air tight argument, but hopefully it's a good qualitative explanation.

Since you have the equivalence principle, you cannot distinguish acceleration from gravity (effect of spacetime) directly. However, you can distinguish indirectly, by observing matter around you, which is a subject of common confusion for students first learning GR (if you had a window in your accelerating elevator, you could see things accelerating outside of it which would be still if it is only gravitation). So the distribution of matter is important for probing the structure of spacetime. This suggests that one can write a relation between the stress-energy tensor (a tensor that contains the information about the matter distribution of the universe) and the metric (another tensor that contains the information about the structure of spacetime).

Note, we have not yet implied that matter warps spacetime, only that since you can use matter to measure spacetime, the two are related somehow, and we simply propose that there is a relation.

From here we follow with some mathematics, and one thing we know about matter, which is that 4-momentum is conserved. In the language of the stress-energy tensor, this means that the divergence of the stress-energy tenser is 0. So we write one side of the relation as the stress-energy tensor, and the other side we write the most general mathematically sensible tensor we can make out of the metric, but also make sure that this general tensor has the property that it's divergence is zero. If you do this you get einstein's equation, at which point you have GR and gravity warping spacetime etc.

So it's all an observation that the equivalence principle makes it so that the only way you can measure spacetime is through matter distributions.

Also, the idea that matter warps spacetime is a little misleading. The stress energy tensor typically depends on the metric, and thus depends on the spacetime itself. So spacetime and matter are determined simultaneously. It's better to think that two are closely related, but one does not cause the other. Quantum mechanically this might change though (for example, string theory is typically written as perturbations of a fixed background spacetime), but classically this is the most "complete" understanding of GR.

Last edited: Jan 21, 2013
25. Jan 22, 2013

### jnorman

hmmm - doesnt the "fact" that higgs bosons have been "discovered" now, which disconnects mass as an instrinsic aspect of matter, mean that there is some particle nature of gravity, or some connection between the action of the higgs boson and the gravitational field, or some other ridiculously confusing "explanation" for gravity now? i am unable to fathom how gravity is merely a warped field since we have now introduced the idea that the higgs boson is responsible for "mass" in some manner that is separate from the matter itself...