# What attraction/repulsion?

1. Oct 1, 2007

2. Oct 1, 2007

### malawi_glenn

what do you mean "how" ?

3. Oct 2, 2007

### Staff: Mentor

The "exchange particles" transfer momentum and energy from one particle to another. Simplistically speaking, you can think of one particle as "emitting" an exchange particle, which carries off some of the first particle's energy and momentum. Then the exchange particle is "absorbed" by the second particle, whose energy and momentum changes as a result. The energy and momentum gained by the second particle equals the energy and momentum lost by the first particle.

4. Oct 2, 2007

### Skhandelwal

But all of these particles are massless, so how can they possibly CARRY energy?

5. Oct 2, 2007

### malawi_glenn

They are not massless?

And there is the Heisenbergs uncertainty priciple:

$$\Delta E \Delta t \geq \hbar /2$$

6. Oct 2, 2007

### Staff: Mentor

In relativity, the general relationship among mass, energy and momentum of a particle is

$$E^2 = (pc)^2 + (m_0 c^2)^2$$

When $m_0 = 0$, $E = pc$. Note that classical electromagnetic waves also have this relationship between the energy and momentum that they carry, so it fits that photons have the same relationship.

7. Oct 3, 2007

### Skhandelwal

In english?(I am sorry but for once, I want to understand the whole thing, I look at math as a tool, not as a conceptual aid.) After I understand it completely, then I'll be interested in the math.

8. Oct 3, 2007

### malawi_glenn

Okay, but the language of modern(quantum) physics is math, that's just the way it is. Langragians, hamiltonians, propagators etc..

But these are quite easy ones:

$$\Delta$$ means uncertaninty,
E is energy, t is time.
So the Energy conservation can be violated, for as long as this equation is fulfilled.

And the equation jtbell wrote just shows the equivalence of mass and energy and momentum. Massless particles DO carry energy, if they are in motion.

9. Oct 3, 2007

### arivero

10. Oct 3, 2007

### Skhandelwal

Although, this time, I understood what you told me..it just made me more confused. How can massless particles carry energy?

11. Oct 3, 2007

### malawi_glenn

Thay can because there is "different types" of energy.

Look at the photon:

$$E_{\gamma} = hf = \dfrac{hc}{\lambda}$$

No mass in the expression for energy

Have you somewere motivated WHY you think that massless particles cant carry energy?

12. Oct 4, 2007

### arivero

$$F . l$$

If you can push an object with a force F along a distance l, you are carrying energy. Force is the quantity of momentum you can transfer by unit of time. The photon can transfer momentum, thus it can carry energy.

Of course this leaves the real fundamental question: how is it that a massless particle can transfer momentum? Here you can start to discuss the foundations of special relativity and quantum mechanics. But do not worry about Force and Energy, they are mathematical concepts with big names.

Last edited: Oct 4, 2007
13. Oct 4, 2007

### mjsd

How can massive particles, in your opinion, carry energy then? What makes you think that mass is a necessary criterion to define/carry energy? are you thinking about $$E=\frac{1}{2} mv^2$$ ?
look at the sun's radiation, it is full of energy right? but do you actually question whether it is the radiation that carries the energy or something else? Is the radiation massive or massless?

14. Oct 4, 2007

### Skhandelwal

I guess you are right, I have a mental block here. But then how do we define matter? I used to think anything that has mass or energy is matter. Anything else is vacuum. But now, I have been introduced to new concepts like this.

How are massless particles created?

Since anything that has a weight needs infinite energy to travel to the speed of light....and since energy is mass...how can photons carry energy and still travel at the speed of light? Won't energy slow them down?

15. Oct 4, 2007

### arivero

If you want to think in dual terms (matter/vacuum) probably the best framework is to call "matter" to the fermions of the standard model and "fields" to the gauge bosons.

Another alternative is to call "matter" to any distribution of mass or energy and "space" to the space-time of general relativity.

16. Oct 6, 2007

There are some, rare, few, who don't believe that particles play catch... I believe it was Newton who first suggested that forces at a distance, through empty space, was ridiculous-and the first to admit that he had no idea. What makes any of us think that we are smarter than Newton? In other words, Newton vocated a medium. How particles passing through space, therein causing two bodies to come closer together, could have ever been proposed- is clear evidence that our intellect is decaying.

17. Oct 6, 2007

### malawi_glenn

It is a difference having great knowledge and beeing smart/talented. We have much more knowledge today and we have made very large progresses in both theoretical and experimental physics. Its not beeing smart or not, the only thing that matter is what is true. You can't compare Newton with todays science.

18. Oct 11, 2007

### Skhandelwal

Question still sparks...how can massless particles carry energy? Since energy is mass in a converted form...won't it slow it down? Since it takes infinite energy to accelerate mass to the speed of light.....I think you are getting the point.

And why do they transfer energy? What makes them do so? How are these massless particles created? How do they die?

19. Oct 11, 2007

### malawi_glenn

you are confusing rest mass and energy. Photon has indeen zero rest mass.

20. Oct 11, 2007