# Matter and energy short question

1. ### Whitefire

35
Is it correct to define matter as a force moving primarily in time dimension, and energy as the same force moving primarily in space dimension of spacetime?

2. Physics news on Phys.org
3. ### dauto

Nope. Matter isn't a particularly useful concept and force and mass are separate concepts.

1,533
Dauto is true.Moreover,matter can also move in the space dimension.

### Staff: Mentor

Energy: The ability for one system to perform work on another.

Force (From Wiki): In physics, a force is any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or a flexible object to deform, or both.

There is no rigorous definition of matter.

1,533
Matter can be defined as things which has mass and volume.

7. ### Whitefire

35
I used the word 'force' to avoid saying that 'energy is energy', which is not really saying anything, and to avoid saying 'matter is energy' and 'energy is matter', which still means describing IT with two different words.

I asked the question because I am trying to define the difference between energy and matter. Well, I know that the difference is c^2, but it is just a number, I want to understand what this number really does with... IT.

9. ### dauto

You're confusing mass with matter. The m in the equation E=mc2 stands for mass, not matter.

### Staff: Mentor

The M in E=MC2 stands for mass, not matter. Be aware that a great many people say that mass is a form of energy. Whether this is true or not I don't know. The fact remains that the two are intimately linked together. If you remove energy from a system, it will also lose mass.

For example, let's say we lose 10 joules of energy in the form of light from a system (The form of the energy is irrelevant for our discussion). Putting 10 joules in the equation for E and solving for M gives us about 1x10-16 kg's lost from the system. So not much mass in this particular example.

Now, instead of measuring the energy removed from a system, we can also measure the mass before and after. Let's say our object radiates energy away and the mass afterwards is 1 kg less than before. Putting 1 in for M and solving for E yields about 9x1016 joules of energy lost as radiation. That is a LOT of energy, on par with a nuclear explosion.

Note that there is an extreme difference between the amount of energy and mass. This is simply a result of the units we choose to represent energy and mass.

11. ### Whitefire

35
sorry, my bad, I am not a native English speaker and I never learned physics in English :D

@ Nugatory: thanks, this seems to be exactly what I wanted - a discussion about the nature of energy and mass - even if they only appear to be different, it is still worth understanding why.

Last edited: Nov 19, 2013
12. ### dauto

In non-relativistic theory, energy momentum and mass are related to each other by the formula
$$E=\frac{p^2}{2m},$$
so if you set the momentum to zero (p=0) you get E=0. That means that an object at rest has no energy.

In relativistic theory, energy momentum and mass are related to each other by the formula
$$E^2=(cp)^2+(mc^2)^2,$$
so if you set the momentum to zero (p=0) you get E=mc^2. That means that an object at rest has some rest energy.

13. ### Whitefire

35
An object at rest not only has some rest energy, I daresay (E=mc^2) that it has a LOT of energy. You say that it is 'rest' energy and 'rest' object, but isn't there one process that happens even to the object with zero momentum? Time passage? We usually say 'time passes for it' as if it were something distant and not related, but we don't really understand why time passes, do we.

So this happens for objects.

Now on the other hand, if you imagine a basic form of energy - light... It moves through space, but does it really move through time? Aren't there assumptions that at the speed of light, time freezes? At least relatively, but from the same point of view that we considered the object a moment earlier. We don't really even say that 'time passes' for a wave of light in vacuum.

So basically this is the correlation between time, space, energy and mass (ok, not matter) that I wanted to talk about; that's why I said "primarily in time dimension" and "primarily in space dimension". Primarily, because of course objects usually have some momentum and even light can be slowed, perhaps stopped. I have no idea how this fits with the rest. Maybe what is mass and what energy is also only relative. Now that would be something.

14. ### dauto

light and energy are not synonyms. Light is a physical object. Having energy is one of its properties but it also has other properties such as momentum and angular momentum. There are many other forms of energy and different physical objects may have different forms of energy. The popular view of light as being "pure energy" doesn't really make any sense.

15. ### gabriel.dac

50
Matter can move in the space dimension as well, so no

### Staff: Mentor

Also, the claim that light does not experience the passage of time because it travels at c is a faulty use of the lorentz transformation. It only applies to objects whose frame can be transformed to a rest frame, where the object is at rest. But light can never be at rest and as such you cannot apply the lorentz transformation to it, so the whole idea just doesn't make sense. Also, consider that if light, an EM wave, has no passage of time, then why do the fields oscillate?