# What causes space curves? Energy or rest mass?

1. Sep 28, 2004

### quasar987

If it's energy, a photon must curve space.

If it's rest mass a photon doesn't curve space and an object going at speed 0.99c doesn't curve space more than when it's not moving.

A friend of mine asked me this question after asking two of his profs at McGill University and getting two different answers! So.. tensors masters, which is it?

2. Sep 28, 2004

Staff Emeritus
What curves space according to Einstein's field equations is momentum-energy, and its stress. Recall that in relativity momentum and energy are combined into a single four-vector; this four-vector forms the first row and first columns of the 4x4 momentum energy tensor. The other components of the tensor are derivatives of the above wrt the spacetime variables.

3. Sep 28, 2004

### Garth

A photon curves space-time. But because it is trace-free the extra curvature created by adding electromagnetic energy is also trace free so
R = 0 where R is the Ricci or Curvature scalar due to that electromagnetic energy.

Mass and other forms of energy also curve space-time.

Garth

4. Sep 28, 2004

### DrWarezz

Hi, sorry to 'barge in on this convo', but could someone explain what is ment by "warped space-time"? \
I'm kinda new to all this, and I've read and heard of it all too many times. I just don't get it.
Anyone? )

[r.D]

5. Sep 28, 2004

### Severian596

At the risk of blowing your mind, we see "warped space-time" as gravity. Yay!

6. Sep 28, 2004

### Severian596

Okay, okay. A diagram is in order at least:

http://www.astronomynotes.com/evolutn/grwarp.gif

This illustrates warped space-time. Space-time is the "sheet" defined by the grid, and the recessed areas in this sheet signify that something of significant mass exists in space-time. The mass warps the surrounding space-time, creating a gravity well. A beam of light moving past a large object is bent according to this well's curvature.

7. Sep 28, 2004

### Gonzolo

Except I don't know what "trace-free" means.

8. Sep 29, 2004

### pervect

Staff Emeritus
Anything that has energy, or that has momentum, curves space-time. This includes electric fields, and electromagnetic radiation.

9. Sep 29, 2004

### Antonio Lao

My 2 cents idea is that the cause can be from the product of very strong infinitesimal orthogonal forces applied at a distance comparable to the Planck length. Applying twice give an expression for the square of energy. The square roots of square energy give two solutions as kinetic energy and potential energy. And these energies define a moving mass and a rest mass. Therefore, by simply ignoring the effect of vacuum polarization, curvature is affected by the square of energy derived from rest mass as well as moving mass (momentum).

10. Sep 29, 2004

### Garth

If you are not familiar with tensors then I wouldn't worry about it.

However if you do...
The (stress-[energy)-momentum] tensor Tuv describes in tensor form the mass, momentum and stress intensities of a system at any point in a field. That tensor for electro-magnetic radiation takes a particular form and when you perform a tensor operation on it, called 'taking the trace', you end up with zero. It is therefore described as being trace-free. This tensor then tells space-time how to curve through another tensor called the Einsteinian Guv, which in turn then tells matter and radiation 'how to move'. There you are GR in a nutshell!

Garth

11. Sep 29, 2004

### blue_sky

I'm really interested in your answer. There is experimental evidence of this?

blue

12. Sep 29, 2004

### pervect

Staff Emeritus

The best experimental evidence I can think of off the top of my head is the bending of light, with the additional assumption that momentum is conserved. If gravity bends light, there must be a reaction force on the body doing the bending to maintain the conservation of momentum. Also, I believe the advance of the perihelion of mercury's orbitpredicted by GR can be traced to this effect, but I'm not 100% positive about that.

In any event, the assumption that the stress-energy tensor causes the bending of space-time is the heart & soul of Einstein's general theory of relativity. So, if you prefer, add the words "according to General Relativity" to the statement I made.

Einstein's field equations are

$$\large G_{uv} = 8 \pi T_{uv}$$

The entity on the left, Tuv, is the stress energy tensor. (The entity on the right is a measure of the curvature of space-time.

The stress energy-tensor is nothing but the density of energy and momentum per unit volume. When you multiply the stress-energy tensor by a vector representing a volume, the result you get is the total energy and momentum in that volume as described by the energy-momentum 4-vector.

(Note, it may not be obvious how to represent a volume by a vector. The way it's done is to take the only vector that's orthogonal to the volume in 4d space-time, this is the "time vector" associated with that volume).

You can also understand the stress-energy tensor as describing the "flow" of energy-momentum, as per Baez's webpage on the stress-energy tensor

http://math.ucr.edu/home/baez/gr/stress.energy.html

Last edited: Sep 30, 2004
13. Sep 30, 2004

### pmb_phy

If you're asking "What is the source of gravity?" then the answer is, to quote MTW (page 404) "Mass is the source of gravity." (If this is wrong then someone should send the corrections to the authors explaining the errors of their ways. )

I say this so that people won't think that Wheeler was sloppy or ignorant when he said that "mass tells spacetime how to curve and spacetime tells mass how to move" which is a very famous saying and one that is 100% correct.

But this "mass" is not rest mass. Its "mass-energy" (aka inertial mass aka relativistic mass) i.e. the m in m = E/c2. This is like saying that charge is the source of an EM field. Charge in one frame is current stress/pressure in another. Just like current generating a magnetic field, the momentum effects the gravitational force on a partricle only when the particle is moving.

And yes - light generates a gravitational field. E.g. http://www.geocities.com/physics_world/grav_light.htm

The only reason people can say that "energy is the source of gravity" is by involking the fact that mass is proportional to energy. This is exactly what Einstein did.

I've yet to see a 'derivation' of Einstein's equation which didn't make this association. Had they assumed that this "mass" in m = E/c2 was only rest mass then they'd be making a serious error.

A tensor is the mathematical object which describes mass-energy. Recall that 4-current is the mathematical object that is the source of the EM field. Charge is the time-time component of the 4-current and mass-energy is the time-time component of the energy-momentum tensor.

Note: Mass can generate a gravitational field in the absence of spacetime curvature so its more appropriate to say that mass generates a gravitational field rather than mass generates spacetime curvature.

There will be a strong tendancy to think that since the time-time component of the stress-energy-momentum tensor does not have the units of mass density then it can't be mass that generates a gravitational field. But this is hardly a good claim. It'd be similar to claiming that because the time-component of 4-currerent has the units of current density (i.e. same units as spatial component) then charge is not the source of an EM-field.

Pete

ps - Surgery went fine and I'm a whole person again who is relaxing and getting better. I took a cleansing ride/walk downtown to read my e-mail and thought I'd poke my head in.

Last edited: Sep 30, 2004
14. Sep 30, 2004

### pervect

Staff Emeritus
Nitpick time.....

It's generally regarded as incorrect to call E/c^2 mass. Pete likes to call E/c^2 "relativistic mass", which I don't have a real problem with, though I would not urge anyone else to copy that practice. Calling E/c^2 mass, though, is wrong (at the very best, it's ambiguous).

As far as I'm aware Pete and I both agree on the physics, that it's the energy E (or if you prefer, the relativistic mass, or to use MTW's phraseology, mass-energy) that curves space-time.

As far as Wheeler goes, I belive that Wheeler's actual quote was not "mass tells space how to curve" but "matter tells space how to curve". But I haven't found a totally definitive source for the quote.

Saying that mass curves space-time is wrong, as "mass" means "invariant mass". To give the usual URL (which Pete has seen once or twice before)

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

15. Oct 3, 2004

### blue_sky

Pervect, my question was unclear sorry about that. I was asking if there are experimental evidence that also the electric fields, and electromagnetic radiation curve the space-time. I understand that it's true for mass and the exemplum u made are clear. But what about electric fields, and electromagnetic radiation? There are experimental evidence that they curve space-time?

blue

16. Oct 3, 2004

### quasar987

This is bugging me. Suppose your photon is initially traveling vertically (in the Oxy plane). When it passes the sun, it is deviated horizontally. One could say it gains an horizontal velocity component. But when it gains horizontal component, it must lose some vertical components because (Vx² + Vy²)^½ must be c.

So if you suppose that the photon does't attract the sun in it's turn then it's "partly OK", the total momentum of the photon in conserved (but not his parts). By conserving horizontal momentum, you violate conservation of vertical momentum.

But if you suppose it does attract the sun, the horizontal momentum of the sun-photon system is conserved but the vertical is not.

Something else that is funny: If the photon is coming directly at the sun, say vetically, then the initially at rest sun is attracted towards the photon and it gains a vertical component in its momentum. If you want momentum to be conserved, the vertical momentum of the photon must be altered in the same amount. But the vertical speed of the photon cannot be modified, it is already c. Since p = E/c = hf/c. => the frequency of the photon gets bigger.

Same thing for a photon getting away from the sun, its frequency gets smaller.

17. Oct 3, 2004

### pervect

Staff Emeritus
A photon gains and loses momentum without changing its speed. A photon going into or out of a gravity well changes it frequency. Falling down, it blue-shifts, climbing out of a gravity well, it red-shifts. However, measured by the rods and clocks of a local observer, it will always move at the same speed, 'c'.

18. Oct 3, 2004

### quasar987

Nice.

(I have to enter 10 characters :tongue2: )

19. Oct 7, 2004

### pmb_phy

Okay.
That is waaayyyyy wrong. pervect believes that its very rare for somone to define mass in such a way that E = mc2 = Kinetic Energy + Rest Energy.

Many people do this in GR. Please note that m is not defined as E/c2. In fact Alan Guth does this when he teaches his Early Universe course at MIT. E = m2 is an equality between E and m. It is NOT not a definition of m. Mass, m, is defined as p/v. In some circumstances (free/isolated particle/object) it can then be proved that E = mc2 for a particle or an isolated object. I.e. it can be proved that E/c2 = p/v. If an object has a finite extent and it is not isolated then E/c2 does not equal p/v.

Note -

For some reason pervect keeps mixing these up. Why? Do you have a very limited exposure to the relativity literature? To expand your horizons try reading something new, e.g. D'Inverno, Mould, Rindler, etc.

That is inaccurate. I don't like to call p/v "relativistic mass." I prefer to call it either simply "mass" or inertial mass. I use the term "relativistic mass" so people will be less likely to get it confused with "rest mass." I think it is not a good idea to call p/v anything but "mass." But that's another story.
Why would you urge someone to stop thinking along a particular line? It is not logical to assume that since you found it of no use that it is impossible for others to find it useful. There is simply no reason for such urging. Especially since the concept is widely used in the physics literature and is 100% precise and logical when it is used. Taking such a term out of one's language is highly unfruitful and can lead to igorance. If a student does not learn what the m in E = mc2 means in all places that he might come into contact with it (i.e. texts, journals etc.) then how do you expect them to understand such texts such as Cosmological Principles, John A. Peacock, Cambridge University Press, (1999) (This is the cosmology text that is used at MIT by the way).

See examples (Especially Peacock and MTW) at http://www.geocities.com/physics_world/relativistic_mass.htm
That is highly incorrect. It is completely inappropriate to call something wrong just because you have personally chosen to use a different definition. It is very clearly not wrong.

Why do you ignore their use of the term "mass" when it doesn't suit you? I've given you examples in the past if I recall correctly. There are clearly places in MTW in which they use the term "mass" to mean E/c2. I've shown you those places. If you forgot (or I recalled incorrectly) please see quotes from Peacock and MTW in - http://www.geocities.com/physics_world/relativistic_mass.htm
He has said it in different ways in different places. In fact in his and Taylor's latest book Exploring Black Holes he wrote in the front portion of the book. In - http://www.eftaylor.com/pub/front_matter.pdf the authors write

Spacetime tells mass how to move; mass tells spacetime how to curve ...

His co-author Edwin F. Taylor also quotes that in his acceptance speech for the 1998 Oersted medal presented by the American Association of Physics Teachers, 6 January 1998 - See http://www.eftaylor.com/oersted/

Last edited by a moderator: May 1, 2017
20. Oct 7, 2004

### pmb_phy

In some cases that is true. But in cases when the radiation is disordered for example the trace is not zero.[/QUOTE]Pete