Is light considered to have kinetic energy even though is has no rest mass?

From what I hear if you trapped light within a perfect box that it would add its mass to the box. If the universe was closed like the box, does that mean all the light in the universe would add to its overall mass?

I am not sure if I am getting this right at all. But is static energy just a bunch of or a type of momentum-energy that can't pass a certain point? Could a closed univserse be thought of as a particle of static energy being that all the momentum-energy within the closed universe cannot leave its boundries? Is that what a particle is? Just a bunch of momentum-energy that isn't getting anywhere outside the particle itself? Is static/rest energy really still? Or it just can't move far?Meaning that light is static energy of the universe and a... quark is static energy of a proton or something like that.

Saith,

Kinetic energy is just an imaginary property conjured up by physicists.

If an object requires energy to accelerate, and the same object requires energy to decelerate, how can you say that a faster moving object has more "kinetic energy"??

Tom

Saith

Is light considered to have kinetic energy even though is has no rest mass?

Correct. That is how solar sails work.

From what I hear if you trapped light within a perfect box that it would add its mass to the box. If the universe was closed like the box, does that mean all the light in the universe would add to its overall mass?

The amount of mass or energy in the universe doesn't really change. Both are conserved and cannot be created or destroyed.

I am not sure if I am getting this right at all. But is static energy just a bunch of or a type of momentum-energy that can't pass a certain point?

Static energy is usually energy that has been lost in a system.

Static energy is usually energy that has been lost in a system.

This is how I understand it.

Light is pure momentum-energy.
Matter has both rest energy and momentum-energy.
Matter at 0 Kelvin would be pure rest energy.

So are you saying that rest energy is matter which has lost its ability to do work for the time being? So if I bumped into some pure rest energy, it would heat up and some of the rest would turn into momentum-energy. Or would I add momentum-energy to it, since the faster something moves the more mass it has, as if you transfered something to it. (now that i think about it, is there really a difference between momentum-energy (momenergy) and kinetic energy?)

Also I was wondering... kinetic energy has to do with both mass and speed. But what about light? Does it matter how fast it is going? It travels at different speeds through different mediums, but does the kinetic energy of it still stay the same no matter the speed? And if it doesn't have rest mass, what kind of mass does it have? I am pretty sure light doesn't have any kind of gravity right?

Saith

Not really. Static energy is the energy which may have been used in a system but was somehow lost due to a defect or inefficiency of the system.

I've also read somewhere that static energy could also be a form of binding energy for metal at the molecular level.

Also I was wondering... kinetic energy has to do with both mass and speed. But what about light? Does it matter how fast it is going?

Light travels at a constant 300,000 kilometers per second. Light speed is denoted as 'c'

It travels at different speeds through different mediums,

Correct, but the photons are absorbed and re-emitted from atom to atom within the medium. This is what actually causes the delay. The photons will still travel at c between the atoms.

but does the kinetic energy of it still stay the same no matter the speed?

The energy of light will differ depending on the source of the light. The speed will remain constant.

And if it doesn't have rest mass, what kind of mass does it have?

It has zero mass. Physicists use the term invariant mass when talking about mass at rest and relativistic mass for mass at high velocities. It's a rather confusing subject. This link may help:

http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

Thanks for the answers that really helps.

Light travels at c only when the medium is a vacuum. But otherwise, it's all good.

Considering there is no place for mass in the equation for light's kinetic energy, light has kinetic energy that only depends on its wavelength.

And I never got how light could have kinetic energy or be affected by gravity when it has no mass. Perhaps someone who's knowledgeable in relativity (I only learned about tensors a few days before, I'm building up the math ability to experience relativity for myself so maybe it'll take a while) will enlighten me on this?

Tom,

Kinetic energy only depends on speed (for an object with fixed mass). Zero speed means zero kinetic energy. The faster something goes the more kinetic energy is has.

Acceleration and deceleration is irrelevant to kinetic energy, except as it affects speed. Decelerating decreases kinetic energy.

James,

Kinetic energy only depends on speed (for an object with fixed mass). Zero speed means zero kinetic energy. The faster something goes the more kinetic energy is has.

How do you know that an object going 100 km/h has more kinetic energy than an object going 10 km/h???

You might say that the proof is the energy released when an object going 100 km/h hits a wall. But I would say that it just takes more energy to stop an object going 100 km/h than it takes to stop an object going 10 km/h because the difference in speed is greater, and not because the 100 km/h object has more "kinetic energy".

Tom

Tom,

When something hits a wall, energy is released (as heat.) Since energy has to be conserved, then this heat must come from somewhere. The only place it can come from is the relative velocity between the wall and the "something" hitting it. Hence, the notion of "kinetic energy" is not spurious at all but quite necessary to keep the tally of all energy in a system.

Q (or anyone else who knows the answer),

Perhaps you could answer a question I've had for a while concerning absorption/re-emission of light. Namely, light seems to travel through homogeneous matter in more or less straight lines (there's some scattering, but how much depends on the optical qualities of the material; high-quality optical media almost don't scatter light at all.) However, if a photon is absorbed by an atom, what makes the atom re-emit the photon in the exact same direction as the photon was originally going?? Wouldn't re-emission tend to happen in random directions, destroying the "rays" of light into a photonic fog?

TIA

Overdoze,

When something hits a wall, energy is released (as heat.) Since energy has to be conserved, then this heat must come from somewhere.

The heat comes from the compression of the material caused by the collision, not by the "kinetic energy" of the moving object.

Newton's Law claims that all objects moving in a straight line and at a constant speed will continue to move in a straight line and at a constant speed unless acted by an outside force. Newton's Law implies that energy is required to change the direction or speed of an object regardless of whether you are accelerating or decellerating it. If you must apply energy to an object to slow it down, how can the remaining object contain less energy after its speed was decreased?? Where did the extra energy go??

Example: Let's say I throw a baseball. Energy would be required to accelerate the baseball to a certain speed. However, energy would also be required to slow down the baseball based on Newton's Law. If it takes energy to increase the speed of the baseball, and it takes energy to decrease the speed of the same baseball, how can you say that a moving baseball has more "energy" than a stationairy one???

Tom

I think I get what you're saying. Potential energy is just the position of mass and Kinetic energy is just the motion of mass. But neither Potential or Kinetic energy is a real substance within the mass, just the condition of the mass right?

If you fold a piece of paper into a paper airplane to give it the ability to fly, you didn't actually add anything to it to give it the ability to fly, you just changed the condition of it. And the same goes with throwing the paper airplane. You didn't add anything to it to make it fly or to give it the ability to compress objects and create heat, you just changed the condition of it.

overdoze

However, if a photon is absorbed by an atom, what makes the atom re-emit the photon in the exact same direction as the photon was originally going?? Wouldn't re-emission tend to happen in random directions, destroying the "rays" of light into a photonic fog?

The re-emission is random. You're view of this phenomena is that of the random photons traveling straight to your eye. It is the same if the photons were propagating from a source. The source would propagate photons in random directions and in all directions. It is simply your line of sight towards the random photons which traveled in your direction.

Saith

I think I get what you're saying. Potential energy is just the position of mass and Kinetic energy is just the motion of mass. But neither Potential or Kinetic energy is a real substance within the mass, just the condition of the mass right?

Potential energy is energy stored in a mass where the mass's position is within a field of force. An object not in a field of force like a gravitational field for example, would not possess potential energy. The 'potential' has no relation.

Kinetic energy is energy of a mass in motion with velocity. All objects in the universe are in motion therefore, they all possess kinetic energy.

You may apply this to your paper airplane example.

Saith,

First, I have to tell you that what I'm proposing is not the theory accepted by the scientific community. Your questions led me to develop a theory, and explanation, of energy that will explain why the principle of conservation of energy is not always valid.

Although I don't have my theory completely worked out (considering that it's less than an hour old), here are the basics:

The scientific community has a very simplified concept of energy: they assume it's one dimensional. For example, an object can only have more or less energy (this model can be illustrated with only one line, or one dimension). However, I believe that you need to look at things in a multidimensional way to understand energy completely.

I have, so far, developed a two dimensional model describing energy. In reality, the actual model would contain over 8 dimensions, but I will try to explain it using the two dimensional model for simplification.

Imagine the universe as a two dimensional graph. One dimension indicates how much active matter an object has, while the other dimension indicates the state an object is in. By active matter I mean matter that, as a result of potential difference, can result in energy (exchange of active matter). By state I mean the fact that an object can be in two different states, even though the energy of the two states are identical.

Any object in the universe would be at a specific point on this graph. For example, one object may be at state:0, active matter:1, while another object would be at state:0, active matter:2. In this case, energy would be transferred as these two objects interact, because they are at different active matter locations. In this example, there would be a potential energy between the two objects, and active matter(energy) would be transferred until the two objects reach an equalibrium. This is the traditional view that the scientific community accepts. A good example of this would be how a hot object transfers active matter (energy) to a cold object.

Now, lets assume you had two other objects. These objects would be at the same active energy level but they would be at different states. If these two objects came in contact, active matter would be transferred, but their energy would remain the same. A good example of this are moving objects. You have two objects moving at different speeds, but their energies are the same. When they come in contact, their speeds will "average out" while their energies will still remain the same.

Another example is a photon. If a photon is spinning in a clockwise direction it has a certain amount of energy. However, if you apply energy to the photon to "force" it to spin counter-clockwise, the energy will be used but the photons energy didn't increase. Even though the photon's energy didn't change, as its spin changed from clockwise to counterclockwise, its state changed.

Conclusion: An object can have multiple states of existance which all have identical energies. However, energy is still required to change that object from one state to another.

That's all I have for now. I'm sure there are some errors in my theory since I haven't completely worked it out yet. I'm looking forward to any posts that point out the flaws in my theory. :)

Tom

Q,

The re-emission is random. You're view of this phenomena is that of the random photons traveling straight to your eye. It is the same if the photons were propagating from a source. The source would propagate photons in random directions and in all directions. It is simply your line of sight towards the random photons which traveled in your direction.

I believe that a laser is proof that the probability of emitted light being emitted in the same direction as the absorbed light, is greater than it being emitted in the opposite direction.

Tom

Prosoothus

Another example is a photon. If a photon is spinning in a clockwise direction it has a certain amount of energy. However, if you apply energy to the photon to "force" it to spin counter-clockwise, the energy will be used but the photons energy didn't increase. Even though the photon's energy didn't change, as its spin changed from clockwise to counterclockwise, its state changed.

The quantum spin of a photon is either left-handed or right-handed when absorbed by an object, and since the photon always travels at the speed of light, the spin is always parallel to the direction of travel, pointing either forward or back. In transit, photons can exhibit both states of spin.

Tom,

<i>How do you know that an object going 100 km/h has more kinetic energy than an object going 10 km/h???</i>

You plug the speed into the formula K = (1/2)mv², which defines kinetic energy. A bigger v gives a bigger K. It's really very simple.


overdoze:

In a transparent medium, light passes straight through. It interacts only with "virtual" energy levels of the medium, so it is not "really" absorbed and re-emitted by atoms in the medium. Actual absorption and re-emission from real energy levels causes emission of light in random directions, but virtual interactions preserve the direction of travel of the light.

James,

"How do you know that an object going 100 km/h has more kinetic energy than an object going 10 km/h???"

You plug the speed into the formula K = (1/2)mv2, which defines kinetic energy. A bigger v gives a bigger K. It's really very simple.


I don't think you understand what I'm saying. I'm questioning whether the formula Ek=(1/2)mv^2 is valid at all.

How would you prove that an object going 100 km/h has more kinetic energy than the same object going 10 km/h?

Tom

Originally posted by Prosoothus
The heat comes from the compression of the material caused by the collision, not by the "kinetic energy" of the moving object.


And before the actual collision occurred, where was that energy then?


If you must apply energy to an object to slow it down, how can the remaining object contain less energy after its speed was decreased?? Where did the extra energy go??


You answered your own question right there inside the question. The remaining object contains less energy precisely because its speed was decreased. The extra energy went into providing resistance to attempts at slowing it down.


Example: Let's say I throw a baseball. Energy would be required to accelerate the baseball to a certain speed.


Yes, in effect converting that propulsive energy into the baseball's kinetic energy. Remember, net energy of the system is conserved so if you spend energy accelerating the baseball then the baseball must be carrying that energy balance.


However, energy would also be required to slow down the baseball based on Newton's Law.


Because it has kinetic energy.


If it takes energy to increase the speed of the baseball, and it takes energy to decrease the speed of the same baseball, how can you say that a moving baseball has more "energy" than a stationairy one???


Because it takes energy to decelerate it.

Overdoze,

I don't think you understand what I'm saying. :(

When you throw a ball, the ball creates a force against your hand. This force is the result of the inertia of the ball. If you consider this force as a form of energy, does that mean that the ball had more energy at rest than when it's moving??

You indicated that when something hits a wall it produces heat, and that means that it has "energy". However if the wall accelerated a stationairy object, in the same amount of time that the wall decelerated the moving object, the wall would experience the same heat as if the moving object hit it.

Let me put it another way so that you can understand what I'm saying:

Let's say that a baseball traveling 100 km/h hits a wall. Lets also say that the ball decelerated, as a result of hitting the wall, from 100 km/h to 0 km/h in 0.1 seconds. As a result of the impact, let's say that the temperature of the wall, where the collision occured, increased by 10 degrees Celsius.

After the collision you feel the warm wall and conclude that the baseball had "kinetic energy". You would be wrong.

The fact is if the wall was to accelerate the baseball from 0 km/h to 100 km/h in 0.1 seconds, the temperature of the wall would also increase 10 degrees Celsius, just as if the wall stopped the moving ball.

Conclusion: There is no evidence that a decelerating object creates more energy than an accelerating object. In both cases, the object creates a force towards an object that is trying to change the object's speed or direction. This force is identical whether the object is being accelerated or decelerated (assuming that the acceleration is equal to the deceleration).

Because of these facts, I've come to the conclusion that kinetic energy DOES NOT exist.

Tom

I just have some general comments.

First, energy does not exist anywhere in this universe. It has no reality apart from its functional dependence on state variables that do have reality. Energy was defined because it is useful, and for no other reason. Its usefulness, obviously, is that it is conserved for closed systems.

Second, energy formulae (such as (1/2)mv^2) are logical deductions from the basic differential equations of motion of a system, and are therefore just as valid as the more intuitive systems to which they are equivalent. That is, nonrelativistic energetics is just as valid as Newton's laws because the former is equivalent to the latter.

Third, all of the above comments also apply to momentum and angular momentum. This brings me to...

Fourth, as for "multi-dimensional" energy, we have that theory already: Special Relativity. The fact that the 4-momentum of a particle is not only conserved, but is a 4-vector under the full Lorentz group, indicates that energy and momentum are just different aspects of the same thing.

That's my $0.02.

Tom

Tom,

How can you question whether K = (1/2)mv² is valid? Your questioning doesn't make any sense, since that equation is a definition of what we mean by the term "kinetic energy".

It's like questioning whether a lemon is really a yellow citrus fruit.

Yesterday i've read Hawking's Universe in a Nuttshell, and in the time travel chapter he said that time travel isn't possible without negative energie. Can anyone explain what is negative energie?
Thanks!