# Gravity & warping of spacetime

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1. Sep 5, 2014

### DOCK1088

So magnetism has definite lines of force. Does gravity have an equivalent device. References would be good just looking to learn up.

P.S.
I am aware that mass warps space time. I am having difficulty visualizing this in three dimensions.

2. Sep 5, 2014

### Matterwave

Mass warps space-time in 4 dimensions, so that's going to be even harder to visualize!

General relativity is the current state of the art on gravity, but Newtonian gravity is still very much valid for weaker gravitational fields and speeds <<c. In the Newtonian framework gravity is a force field, just like electro-magnetism. So you can draw field lines for gravity as well, they just all point inwards, there are no "positive charges" in gravity, so to speak.

3. Sep 5, 2014

### DOCK1088

Okay that takes care of one problem. Are there any good books on the subject that you can recommend?

4. Sep 5, 2014

### A.T.

Space time is four dimensions, not three. And that's hard enough to visualize without the wrapping. We can visualize flat 3D and wrapped 2D, so you have to reduce the visualization to 2 of the total 4 dimensions. You can choose different 2 dimensions (space-time or space-space) to show different aspects of gravity:

http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html

The one mentioned in the link above.

5. Sep 5, 2014

### vociferous

If you do GR, you can get the analog of Maxwell's Field Equations for gravity (usually taught at the graduate level)

http://en.wikipedia.org/wiki/Gravitoelectromagnetism

But more mathematically down to Earth is Gauss's law for Newtonian gravity (usually taught at the upper division/ introductory graduate level).

http://en.wikipedia.org/wiki/Gauss's_law_for_gravity

Books on GR and Classical Mechanics would discuss these field equations for gravity respectively.

6. Sep 5, 2014

### wwoollyyhheeaa

space curvature

Could someone please explain the reason why we asume that space which has no massive objects nearby is regarded as not being curved, or at least only very slightly curved in that location? What does "curved" mean and where does the non curvature of empty space originate, ie what makes space so intrinsically flat in the first place? I know the maths describes it as such but that's just a description of how it is, not a reason for it being flat.

Last edited: Sep 5, 2014
7. Sep 5, 2014

### fobos

My reflexion would be :if mass warp space time a very small surface with a big mass, sharp enough, should be able to flex the space time till it eventually breaks creating a hole to another dimension. That is why we cannot see a black hole because it belongs to a dimension inaccessible to our 3d senses

8. Sep 5, 2014

### Matterwave

You still refer to "space" being curved. In GR, we care about "space-time", not just space.
Space-time is not intrinsically flat. The FLRW metric is not a flat metric (we live in a spatially flat universe, but that's a different can of worms). Space-time is intrinsically curved, and the expansion of the universe is one manifestation of this curvature!

9. Sep 5, 2014

### Matterwave

This would depend on your physical background. For a layman, the books that I would recommend would be far different than for a physicist.

10. Sep 5, 2014

### Damo ET

You may need to do a little research on event horizons, even the wiki has enough info to correct some of your misconceptions: http://en.wikipedia.org/wiki/Event_horizon
The reason we can't see a black hole has nothing to do with breaking space time creating a portal to another dimension, it is purely a function of photons (of any description) being unable to leave the horizon once inside. All the directions that can be traveled inside the event horizon lead toward the singularity of the black hole, ie all straight lines in space-time whilst inside the event horizon end at the singularity! There is no need for extra dimensions or alternate realities.
If 'a hole to another dimension' was being created, all the mass of the infalling matter isn't going with it, because the mass of a black hole is increasing with the added matter, not staying the same or shrinking.

Damo

11. Sep 5, 2014

### fobos

Damo thank you for your reply. I don't appreciate though your tone which is rather critic not to say arrogant. You talk like you would be knowing the absolute true about black holes and the "dark stuff" of physics. We are here sharing reflexions...and every idea should be considered. I remind you that somebody not long ago had the "misconception" that the earth is not in the center of the solar system. Physics proved to be wrong many times in the past and still has troubles explaining lots of things ( particle entanglement, dark matter, dark energy) we need to be open to all new ideas and reflexions and I am not saying that what I say it was right it was just a reflexion on something that physics doesn't understand yet and neither you.

12. Sep 6, 2014

### vociferous

One thing you should never say to a scientist is, "science has been wrong in the past so you could be wrong now," for the following reasons:

1) It is illogical. Specifically, it is a non sequitur fallacy. Unless someone is claiming that a scientific conclusion cannot possibly be disproved (which no credible scientist ever would claim), then it is not even relevant to the conversation.

2) The philosophy of science and the actual conditions of the practice of science, more than most professions, recognize the limits of their field. They recognize that any theory, no matter how cherished or useful, is subject to disproof. Scientists adopt theories because they are useful and have not been disproved. Scientists do not adopt theories as commandments from the almighty to be worshiped and held true even when faced with overwhelming contradictory evidence.

3) Like in all other logical enterprises, the burden is always on the person making the affirmative claim to provide evidence, not on the skeptic to disprove it. We have a pretty good understanding of why event horizons form around black holes. It is based on well-established physics. As Carl Sagan wrote, extraordinary claims require extraordinary evidence. If you want to propose that event horizons form for some other reason, you need to provide some pretty compelling evidence. You cannot simply say, "well, our current understanding of physics could be wrong." That's not a valid argument. We all know our current understanding of physics is almost certainly wrong in many respects. The onus is upon you to convince us it is wrong using empirical evidence.

13. Sep 6, 2014

### wwoollyyhheeaa

Why is space intrinsically curved?

Matterwave, thanks for your reply. I'm reading those references now and finding it a most interesting subject. Thanks again.

14. Sep 6, 2014

### A.T.

Because it fits observation. That is true for all physics, which is just a bunch of such assumptions.

15. Sep 6, 2014

### Staff: Mentor

Vociferous posted an excellent reply to your post, so all I'll add is that non-mainstream ideas (of which yours is) are not allowed on the forum per the rules, as it leads to pointless discussions and arguing. Also, I don't find Damo's post to be anything but helpful. I advise you to not take things personally here on the forums, as text-based communication inherently lacks the verbal and non-verbal cues that we depend on to help facilitate communication and it is very easy to misunderstand someone's attitude.

16. Sep 7, 2014

### Chronos

The one thing every modern scientist would admit is science is always wrong. It is not, however, wrong to the extent it was in the past.

17. Sep 7, 2014

### Staff: Mentor

I agree, but I find it more useful to think in terms of accuracy instead of right and wrong. Newtonian gravity is *wrong*, but is still accurate in many situations.