# Does binding energy mean energy has a mass?

1. Oct 14, 2009

### brandy

Is the Mass Defect in the atom due to the Binding Energy and does this account for that small amount of mass?
does this mean that it contributes to the atom's mass and has a mass itself?

2. Oct 14, 2009

### HallsofIvy

Staff Emeritus
Yes, of course. $E= mc^2$.

3. Oct 14, 2009

### brandy

how? energy=mass * speed of light ^2
big woop. i dont get it.
mass is proportionately equal to energy but this doesn’t mean energy has mass.

4. Oct 14, 2009

### tuoni

One could say that mass is nothing but an incredibly compact bundle of energy. Mass is how we deal with energy in the frame of reference of gravity. Energy IS mass, mass IS energy.

5. Oct 14, 2009

### ZacharyFino

how do you not get it, for small amounts of mass the energy is large so it takes equally large amounts of energy to have enough mass to notice yes energy has mass.

6. Oct 14, 2009

### HallsofIvy

Staff Emeritus
Yes, you are right. You don't get it. So try learning some relativity. That equation is NOT just a statement that two things are proportional. Wether an equation simply states a proportion or not depends on the physical meaning of the equation, not just the equation itself,

7. Oct 14, 2009

### Staff: Mentor

Hi brandy, maybe this can help. The general form of the equation is:
$$E^2 = m^2 c^4 + p^2 c^2$$
From this equation you can see that if energy is added and the momentum does not increase then the mass must increase. In that sense energy does have mass.

8. Oct 16, 2009

### DrZoidberg

Of course energy has mass. In fact energy and mass are just 2 different words for one and the same thing.
e.g. gravity is the tendency of energy to attract other forms of energy. Therefore since matter is a form of energy it attracts other matter.

9. Oct 16, 2009

### colin456

this also has the consequence that photons attract each other gravitationally

10. Oct 16, 2009

### Topher925

I think of it as the binding energy is due to the mass defect.

binding energy = mass defect x c2

11. Oct 16, 2009

### rcgldr

If energy is mass, then why does gravity only accelerate a photon perpendicular to the direction of travel, never changing it's speed?

12. Oct 16, 2009

### Redbelly98

Staff Emeritus
Not sure what that has to do with the thread topic, but:

Since a photon is massless it must have speed c, therefore nothing will change that.

If the gravitational force is not acting perpendicular to the photon's direction, |p| will change for the photon, even though the speed does not change. I.e., its frequency will shift.

13. Oct 16, 2009

### Redbelly98

Staff Emeritus
Agreed, though it might be easier for the OP to think of it this way:
Δm = ΔE / c2
where ΔE is the binding energy, so this gives you the mass defect. And yes, it contributes to the total mass of the atom.