# Does a magnet slow time?

## Main Question or Discussion Point

Nobel prize winner Richard Feynman explains in his sum over histories interpretation of Quantum Mechanics that photons go where time is least. If we consider that EMF are a lot stronger (in force magnitude) than gravity and
if a magnet is emitting massive numbers of virtual photons, do we know if then, the magnet is loosing mass somehow in the process (a small but significant amount of mass being accelerated to the speed of light) ? If yes, where is that mass going or converting to ? Energy ?

Finally, can the magnetic field around a magnet be seen as a curvature of space time ?

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HallsofIvy
Homework Helper
Where did you get the idea that virtual photons have mass?

Where did you get the idea that virtual photons have mass?
E=MC2

Theoretical physicist and Nobel laureate Sheldon Glashow explains for PBS NOVA online (Einstein's Big Idea, October 11, 2005) that, "When an object emits light, say, a flashlight, it gets lighter."

"
Stick a magnet to the bottom of a metal shelf. Watch it defy gravity. If it is defying gravity, it must be expending energy.

By E=mc2 it must be losing mass."

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Doc Al
Mentor
Stick a magnet to the bottom of a metal shelf. Watch it defy gravity. If it is defying gravity, it must be expending energy.
Really? So, by this same logic: Put a book on a shelf. Watch it defy gravity! Does it expend energy in sitting there?

Really? So, by this same logic: Put a book on a shelf. Watch it defy gravity! Does it expend energy in sitting there?
You can say it is the same logic, one will answer that the book is not defying the gravity in your description because it does sit on a shelf, anyway, i think we'll never know since i got infraction for asking, let's close this thread.

PS: my post have been edited (censored?), probably by the person who sent me the infraction, ZappersZ, so the link i posted about this "magnetism issue" that i wanted to discuss here has been deleted... i have no problem about my post being edited if it is against the PF rules (that i have accepted), but i want to state that it is not a personal "claiming" at it could look like now, since the link has been removed from my post, just to avoid any confusion.

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Energy is expended when an object moves some distance against a force. So in Newtonian thinking: if an object moves upwards (i.e. away from the centre of a gravitational field) then energy is expended. Similarly, if it moves downwards (towards the centre) then energy is gained.

This is formalised in the expression for work done (which is the change in energy of an object):

$$W=\int\vec{F}\cdot\vec{ds}$$

This integral is zero if the displacement and force field are orthogonal to each other.

Finally, can the magnetic field around a magnet be seen as a curvature of space time ?
Very interesting question! This is the "dream" of Albert Einstein!

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pervect
Staff Emeritus
Not only magnets, but even electrons (or any sort of charge) emit virtual photons all the time.

However, neither electrons, charges, or magnets lose energy just sitting there, as should be obvious.

Magnetic fields (and electric fields, for that matter) do contribute to the stress-energy tensor and hence do affect the gravitational field of an object.

For example, an uncharged black hole has a Schwarzschild metric, while a charged black hole will have a Reissner-Nordstrom metric. The difference in the metrics means that an uncharged test particle will follow different orbits, i.e. the "gravitational field" of an uncharged black hole will be different than that of a charged black hole of the same mass.

Magnetic fields contribute to the stress energy tensor in much the same way as electric fields do.

Note that in GR it is the stress-energy tensor which causes gravity, not "mass". This comes directly from Einstein's equations :: G_uv = 8 pi T_uv, where T_uv is the stress-energy tensor, which can be interpreted as the "source" of gravity in GR.

The idea that mass causes gravity is basically a carryover from Newtonian theory which is not really correct in the context of GR.

A closer look at the stress energy tensor shows that for a static field, energy and pressure both contribute to gravity.

It's a bit hard to find elementary discussions of the stress-energy tensor, but see for instance http://www.black-holes.org/numrel1.html [Broken] and http://math.ucr.edu/home/baez/einstein/node3.html. It can be seen from the above URL's that components of the stress-energy tensor include both "energy density" and "pressure" (along with other components such as momentum, etc.)

There are significant pressure terms in the stress-energy tensor of an electric or a magnetic field, so any attempt to look at just the energy terms will not give a correct answer, a full GR analysis must include the pressure terms as well.

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However, neither electrons, charges, or magnets lose energy just sitting there, as should be obvious.
What is puzzling me is that magnets have abilities to "defy the gravity", not when "sitting there" of course, but they can counterbalance gravity force on many situation, very simple example is to make a magnet attracted or repulsed by another magnets on a table (even with strong diamagnetic material that repulse). When a permanent magnet litterally "fly" to join the opposite pole of another permanent magnet, how do we explain this ? Is that magnet doing this without any need of "Energy" ?

Magnetic fields (and electric fields, for that matter) do contribute to the stress-energy tensor and hence do affect the gravitational field of an object.

Note that in GR it is the stress-energy tensor which causes gravity, not "mass". This comes directly from Einstein's equations :: G_uv = 8 pi T_uv, where T_uv is the stress-energy tensor, which can be interpreted as the "source" of gravity in GR.

The idea that mass causes gravity is basically a carryover from Newtonian theory which is not really correct in the context of GR.

A closer look at the stress energy tensor shows that for a static field, energy and pressure both contribute to gravity.
So, if Magnetic fields do contribute to affect a gravitational field of an object , and if gravity affects time, what do we know about "Magnets" affecting time, has this been demonstrated ? Can someone point me on "verified" website(s) on this topic (i'll review those you posted) ?

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pervect
Staff Emeritus
The closest I could come to finding a peer reviewed article was

http://www.iop.org/EJ/abstract/0143-0807/28/2/006

Which talks mainly about the electric field, not the magnetic.

I could probably look some stuff up in my textbooks when I unpack, but I'm not sure it would do much good (unless you happen to have "Gravitation" by Misner, Thorne, Wheeler - and I would also expect it to be indirect, talking about the electromagnetic stress-energy tensor, for instance.

Note that there is nothing more mysterious about a magnet holding up another magnet than there is a table holding up a book. This isn't really related to the rather small effect that electric and magnetic fields would have on gravity.

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Thank you, i'll have a review of your link and will try to found myself some reference on the subject.

russ_watters
Mentor
What is puzzling me is that magnets have abilities to "defy the gravity", not when "sitting there" of course, but they can counterbalance gravity force on many situation, very simple example is to make a magnet attracted or repulsed by another magnets on a table (even with strong diamagnetic material that repulse). When a permanent magnet litterally "fly" to join the opposite pole of another permanent magnet, how do we explain this ? Is that magnet doing this without any need of "Energy" ?
You need to understand that energy is conserved in the motion of magnets. A magnet pulls a piece of metal to it, converting potential to kinetic energy. Pulling the piece of metal away returns the system to its initial state. It is no different from the movement of a weight against a spring. There is no continuous expenditure of energy as you implied in a previous post (how can there be - there is only a one-time movement?).

If you don't first understand the Newtonian basis of energy conservation, this talk about relativity is only going to confuse you more.

E=MC2

Theoretical physicist and Nobel laureate Sheldon Glashow explains for PBS NOVA online (Einstein's Big Idea, October 11, 2005) that, "When an object emits light, say, a flashlight, it gets lighter."

"
Stick a magnet to the bottom of a metal shelf. Watch it defy gravity. If it is defying gravity, it must be expending energy.

By E=mc2 it must be losing mass."
The magnetic force is goverened by the arrangment of atoms. As far as I know the magnetic force is constant for each atom. So the only way I see this force being altered is by actually rearraging the atoms themselves. But I don't think the force of gravity can alter the arrangment of atoms. If it did you could find the link between the electromagnetic force and gravity! But if you put a magnet ON TOP OF A WOODEN TABLE, does it not also couteract the force of gravity? And what forces should be interchanged by something sitting on a table?

Oh ya, and light has no mass....

Does a magnet slow time?

Well I tried it. I put my wristwatch on top of a big magnet and it stopped completely!

Oh ya, and light has no mass....
Why is it affected by gravity then?
So it goes on lightspeed so its time slows down, and he is slowed by gravity aswell. So its going backward then? :p