Subatomic vs Gravitational forces

1. Aug 26, 2011

zeffur7

It has been said that electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravity. A fundamental property of matter is that matter attracts other matter to itself. If this is true, then why do we consider the electromagnetic force and gravitational force to be so different?
I understand the internal energies are hugely different between atomic bodies & celestial bodies, but they are both composed of matter. Any thoughts are welcome.

Last edited: Aug 26, 2011
2. Aug 26, 2011

Ryan_m_b

Staff Emeritus
There are "en.wikipedia.org/wiki/Fundamental_interaction"[/URL] (of which electromagnetic and gravity are just two) and they have very different properties.

Last edited by a moderator: Apr 26, 2017
3. Aug 26, 2011

zeffur7

The other 2 are the nuclear forces, but your response isn't exactly a specific answer to my question. Both forces (subatomic & gravity) are attractive. What exactly are the differences? One difference that I can think of is that subatomic particles don't usually collide with each other unless extreme forces drive them together, whereas planetary bodies would collide if close enough to each other.

Last edited: Aug 26, 2011
4. Aug 26, 2011

Pengwuino

Electromagnetic charges can also repel. There is a huge difference.

5. Aug 26, 2011

Ryan_m_b

Staff Emeritus
I'm not an expert, I'd suggest you read through the relevant wiki articles. What I can think of is that electromagnetism is mediated by the photon whereas we do not know what mediates gravity, gravity is far weaker and electromagnetism forms in the familiar magnetic field whereas gravity is homogeneous.
EDIT: Can't believe I forgot that one.

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6. Aug 26, 2011

Couchyam

Another major difference comes from the fact that gravitational fields are responsible for the geometry (distances between points) of the universe, while electromagnetic fields and the particles that mediate them are accurately described acting on relatively flat (where the pythagorean theorem holds) backgrounds with Quantum Field Theory. The same approaches that were used to describe interactions between particles in QFT fail pretty miserably when applied to gravitational fields. As far as I know, string theory tries to establish the most symmetry between the fundamental forces by expressing them as emergent properties of more general interactions between dimension<=10 "branes", so you might try to read about it there.
However, you are correct in indicating that there are lots of similarities between the two different kinds of fields in classical physics: both systems radiate energy from accelerating particles, and the governing equations are similar in form (although those for gravity are far more complicated in the GR representation than those for E&M).

7. Aug 26, 2011

zeffur7

"E&M" = energy & matter, electrical & magnetic, or something else?

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8. Aug 26, 2011

zeffur7

On a side/related tangent...

Suppose we built a planetary-sized object in the middle of no where (e.g. at the edge of the known universe or within the interstellar medium/space) by starting with one atom & continuously adding a stream of atoms to it until it had the same mass as our sun.

1. Do you think the body gradually curves space-time as its mass increases or it reaches a certain density & then whap! space-time curves--kind of like breaking the sound barrier (i.e. a sudden event occurs, rather than a gradual deformation of space-time)?

2. If we continued to increase the mass of the body, what happens to the whole space of the universe? Does it stretch like a sheet of latex, like when we place a heavy mass in the center of the sheet (i.e. just stretching in the middle)? Or, is there a universal stretching of space as the mass of the body continues to increase? If the latter, is there any theory that describes the limits of spacial stretching/distortion/deformation? I'm guess that the space-time deformation is like a sheet of latex (i.e. when the mass is small, it affects only the latex in the center of the sheet, but as the mass increases, more of the latex sheet gets stretched). I'm also wondering if a very large mass in space would eventually punch through space somehow (perhaps a black hole does this) like a large enough mass would eventually cause a thin latex sheet to break. Then the next question is... if that break in space occurs, where does all the matter-energy go beyond the break point? I've wondered if it may go into another dimension or into an adjacent universal bubble or perhaps it creates a new universal bubble.

3. If all mass is moving away from the center of the universe (where the big bang occurred), does the increase of mass in our hypothetical location in space affect the other bodies that are accelerating away from the center of the universe? If yes, in which way/s & how much? And by the way, into what medium are those accelerating bodies going? Is there any known or theoretical limit to the space in outer space?

Curious to know your thoughts... sorry if this seems to be leading to what should be different thread.

Last edited: Aug 26, 2011
9. Aug 26, 2011

WannabeNewton

Where does the extra mass come from? It can't just be added in from nowhere, that clearly violates physical law. If the masses are coming in from r = infinity, and the gravitational field they generate is weak, then you can linearly superpose them to first order and say the field is getting stronger and this is basically Newtonian in principle. If the masses coming generate a strong field then the interaction is too complicated to put in words. One thing you should know is that the universe is not a sheet and there is no absolute center of the universe.

10. Aug 26, 2011

Pengwuino

1) It's a continuous process; the deformation starts when the first mass is there.

2) What do you mean the "whole space" of the universe? It sounds like you should just learn about general relativity

3) There is no "center of the universe" and there is no medium that all the mass in the universe is expanding into.

11. Aug 26, 2011

Drakkith

Staff Emeritus
Yes, all mass curves space time, so the curvature of the area would gradually increase as you bring more mass.

The curvature falls off at a distance, so the effects are negligible beyond a certain distance for the amount of mass you are talking about. While the image of a large object sitting on a sheet of rubber isn't exactly correct, it is a good way to visualize the effect. As far as I know there is no limit to how much spacetime can be curved. (The effects is curvature of spacetime, not stretching of, which is one downfall to the 2d rubber sheet picture)

There is no center of the universe. The big bang occured everwhere in the universe all at once, it was not an explosion in space. Spacetime itself did not exist before the big bang, so there was nothing for the universe to expand into. The view of the expansion of the universe does NOT require that it be expanding INTO something, it is simply that all points in space get further apart as time goes on.

12. Aug 26, 2011

zeffur7

By "whole space of the universe", I was referring to all of the space within the universe.

It seems to me that for all visible celestial bodies to be moving away from each other, there has to be a starting point--like the center of a bomb that causes all parts of the bomb to move away from the center of the explosion.

13. Aug 26, 2011

Drakkith

Staff Emeritus
Nope. Understand that if you were 1 billion light years away from Earth on another planet or whatever you would ALSO see everything receding from you. The effect is identical no matter where you are at in the universe. The distance between any point in space and any other point in space is always increasing.

14. Aug 26, 2011

WannabeNewton

There is no absolute center. One condition we take into account when modeling the universe is isotropy: there cannot be, within the universe, a preferred direction. Any observer has to be able to call his point the center such that the universe looks roughly the same in all directions around that point and he/she can say that everything is moving away radially from him. If there was an absolute center then obviously there is a preferred direction which does not satisfy the condition of isotropy.

15. Aug 26, 2011

zeffur7

Our planet revolves around our sun, our sun revolves the center of our galaxy, doesn't our galaxy revolve around something... or is it just stuck in one place within the universe?

Where did the big bang occur and into what did it expand? Did the energy & matter in that explosion not move outwards in all directions from the origin of that bang?

16. Aug 26, 2011

zeffur7

Suppose you could put a box around the universe. If every body is moving away from every other body within that system, then there must be an origin from which those bodies originated.

Also, if all bodies are moving away from each other as you have indicated, then how can collections of bodies collide with other collections of bodies (e.g. galaxies colliding with other galaxies or galaxies (or parts of galaxies) being pulled into black holes?

17. Aug 26, 2011

Drakkith

Staff Emeritus
From the point of view of a galaxy 10 billion light years away we are receding from it. AND from another galaxy directly opposite of the 1st from Earth's view we are also receding from it as well. One cannot define a position in space without referring to another position.

The big bang was not an explosion in space. While we cannot really know for sure what happened AT the big bang, we can make logical conclusions about what happened directly afterwards. The universe expanded and cooled, and that expansion does not mean that the universe is expanding INTO something. It merely means that all points in space are receding from all other points.

Per here: http://en.wikipedia.org/wiki/Big_bang

18. Aug 26, 2011

zeffur7

"According to the theory, the universe was once in an extremely hot and dense state that expanded rapidly (a "Big Bang"). As there is little consensus among physicists about the origins of the universe, the Big Bang theory explains only that such a rapid expansion caused the young universe to cool and resulted in its present continuously expanding state." http://en.wikipedia.org/wiki/Big_bang

As you can see above, according to the BBT, the universe expanded rapidly & continues to expand. To expand, it has to expand into some thing (i.e. space).

According to the same wiki article:
"Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating." http://en.wikipedia.org/wiki/Big_bang

The expansion of the universe appears to be accelerating. What force is causing it to accelerate? The only way for something to continue to accelerate is if the source force is continuing to affect it or if the resistance to its expansion is reduced as it cools and condenses.

How would you rationalize the above quotes with respect to your statement that the universe has no origin/center and it doesn't expand into anything?

19. Aug 26, 2011

Drakkith

Staff Emeritus
The proposed force is usually referred to as Dark Energy. We don't know why the universe is accelerating, or why the rate of acceleration is increasing, only that it is. (Althought I believe there are several models trying to explain it) And all of that still fits exactly with the view that there is no center and the universe isn't expanding into anything.

20. Aug 26, 2011

WannabeNewton

I see nothing from those quotes nor what you said that contradicts the isotropy of the observable universe. In fact, the positive rate of change of expansion is easily seen from matter - dominated scale factor solutions to Friedmann's equations and these equations correspond to a metric that assumes isotropy.