# Tag Info

1

Given the context of the question, the fact that it seems to be about $E=mc^2$ specifically, and that the OP says he's having a hard time understanding it, i'm going to try and give a simple answer in plain english without a load more complicated formulae. I am no physicist, and although the concept may not be that easy, the formula is pretty simple, maybe ...

2

I will try to answer this question with my basic understanding of special relativity: Is matter condensed energy? It kind of is, but a better way to phrase it would be that everything that has energy, (behaves like it) has mass. Imagine you have a hollow box with the insides covered with perfect mirrors and you put it on a scale. If you shone a light ...

16

The famous equation $E = mc^2$ is actually just a special case of the relativistic equation for the total energy: $$E^2 = p^2c^2 + m^2c^4 \tag{1}$$ where $p$ is the relativistic momentum and $m$ is the (constant) rest mass: $$p = \frac{mv}{\sqrt{1 - v^2/c^2}}$$ For an object that isn't moving $p=0$ and equation (1) becomes: $$E = mc^2$$ which is ...

11

does this equation mean masses are just condensed energy? No, it means that mass is just another form of energy, just like heat, motion, electric attraction, etc. For example, the energy of a charged sphere is $$E=\frac{3}{5}\frac{Q^2}{R}$$ This equation doesn't mean that charge is just condensed energy; it means that charged objects have energy. ...

6

The two are indeed related. The relativistic expression for the total energy is: $$E^2 = p^2c^2 + m^2c^4 \tag{1}$$ where $p$ is the relativistic momentum: $$p = \frac{mv}{\sqrt{1 - v^2/c^2}}$$ and $m$ is the rest mass. If the object isn't moving then $p=0$ and equation (1) becomes: $$E = mc^2$$ which is of course the well known expression for the ...

4

In relativity we only use the rest mass, also known as the invariant mass, of an object. In days past the concept of a relativistic mass was used, but this is now strongly deprecated as it has caused endless confusion. For example an obvious question is whether the increase of relativistic mass with speed can cause an object to become a black hole (tl;dr it ...

0

Yes it will do. You can take the potential energy to be zero when the spring is neither compressed or stretched. In special relativity the total invariant mass of the system would then include a contribution from the potential energy / $c^2$. The concept of mass in general relativity is quite subtle, but for weak gravitational fields, the Newtonian limit for ...

1

Do we need to account for these? Yes: if the speed of an object is not small compared to the speed of light, the remaining terms become non-negligible and you have to use the exact expression for the energy, $$E=\frac{mc^2}{\sqrt{1-(v/c)^2}}$$ Sometimes, instead of using the exact expression it is convenient to keep the Newtonian term ...

3

Minkowski spacetime has the symmetries of the Poincaré group, which include the four spacetime translations. Noether's theorem then says that there are four conserved quantities, $p_0, p_1, p_2, p_3$, associated with these four symmetries. Typically $p_0$ is denoted by $E$. The structure of the Poincare group implies that these four quantities are related ...

1

Does any object needs force to move? For an object to have a velocity , dx/dt , no force is required, by Newtons fist law: If 'yes', does the matter needs mass to form a force? As the answer to the first part is no, the answer to the second one is no, too. It can just have momentum, since mathematically force is defined as dp/dt. Now in ...

2

Is there any possible way to extract all the mass of a quark?probably under extreme Gs or heat or pressure? Not with classical means, like Gs and heat and pressure, because the quarks are elementary particles held together within hadrons because of QCD . Quarks can give up all their energy when quark meets antiquark, as happens in proton antiproton ...

0

There are two questions here which it is easy to entangle: what motivated Einstein to assume $c$ was constant; what should motivate us to assume it is a constant. If the answer to the first is not the same as the answer to the second then it's not really a question about physics. This does not mean it is not an interesting question if you want to ...

2

The speed of light was first measured by Ole Christensen Rømer (Danish pronunciation: [ˈo(ː)lə ˈʁœːˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light. When Maxwell formed what is the classical electromagnetic theory it was evident that the speed of light would ...

0

I think it was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by Maxwell's equations. What would happen if you could travel along with a plane wave at the speed c? You would observe fields that would be fixed in space and ...

1

Your statement is not true. First, note Sofia's point that $J = Ma$, where $a$ is the angular momentum parameter inserted into the standard Kerr solution. Then to see that the claims in the OP are wrong, simply note that as $a\rightarrow 0$, the angular momentum goes to zero, but the mass does not. Meanwhile, the radius of the black hole horizon (I ...

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