# Thread: Heavy light, is there such a thing?

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## Heavy light, is there such a thing?

Do all forms of light weigh the same? If we wanted to make "heavy light" what would/could we change?

2. Well my first thought is in basic layman terms - "weight" as the general accepted definition is the measurement of mass in relation to a gravity field from another mass, for example I "weigh" here on the surface of Earth (1G) around 170lbs, where as on the surface of the moon this would be much less, in the vacuum of space nothing at all, even though my "rest mass" remains constant.

Since light is considered mass less I can't see how its "weight" (in this context) can be measured to be greater than zero?

Someone with more knowledge on the subject may educate me different, which I greatly welcome!

3. Light is massless and weightless.

Are you possibly having fun with us?

Light/heavy?

4. Originally Posted by DaveC426913
... Are you possibly having fun with us?

Light/heavy?
Right! He might be looking for an Oxymoron.

The question in the OP "what would/could we change?" seems odd. We can't change anything about light.

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Originally Posted by DaveC426913
Light is massless and weightless.

Are you possibly having fun with us?

Light/heavy?
LOL, it's kind of a childish question but I was serious about an answer.

Originally Posted by antoniseb
Right! He might be looking for an Oxymoron.

The question in the OP "what would/could we change?" seems odd. We can't change anything about light.
What about frequency, color, mass???. Are all photons created equal? No matter the source.

6. Originally Posted by DaCaptain
The higher the frequency, the greater its energy. Since e=mc2, thus m = e/c2, then in this sense they have mass, but not rest mass. Momentum is a better way to think of the mass effect, at least for me.

Photons each are both particles and waves of electric and magnetic energy. They don't have color since color is just our brains responding to stimuli from the retina. The different frequencies of photons trigger different color cones that signal the brain, which then decides what color to assign that combination of signals.

Are all photons created equal? No matter the source.
There are about 1089 number of photons in the observable universe at a huge range of frequencies. However, their em nature and their speed are the same.

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Originally Posted by George
The higher the frequency, the greater its energy. Since e=mc2, thus m = e/c2, then in this sense they have mass, but not rest mass. Momentum is a better way to think of the mass effect, at least for me.
The m in E=mc2 is explicitly rest mass because the equivalence is only true when the momentum is zero. The equation is actually E2 = (mc2)2 + pc2.

Photons don't have rest mass and relativistic mass is a horrible concept. It is more correct to say that gravity depends on the stress-energy-momentum tensor and thus higher energy photons contribute more to a gravitational field.

8. Originally Posted by Shaula
The m in E=mc2 is explicitly rest mass because the equivalence is only true when the momentum is zero. The equation is actually E2 = (mc2)2 + pc2.

Photons don't have rest mass and relativistic mass is a horrible concept. It is more correct to say that gravity depends on the stress-energy-momentum tensor and thus higher energy photons contribute more to a gravitational field.
Yes, that's important and even the momentum isn't p=mv but p = h/wavelength (h is Planck's constant). It's not all that easy, however, to say which description is less "horrible". Tensor Aversion Syndrome may be more real than not, at least for folks like me.

It is a little tricky to answer this kind of OP question because it's easy to either oversimplify or overcomplicate the answer, especially if we don't know what level of answer is desired. The Bohr atom, for example, is a great start for atoms even if it's more complicated than this simple model. Since photons behave as if they are particles of mass, such as a laser-power shining upon a sail in space, it helps me, at least, to associate the only thing I understand them to have (ie energy) with mass-like behavior and e=mc2 helps me even if it's an imagined handle to try and hold.

For comparison, would it not also be fair to consider neutrinos, though they don't have zero rest mass, to be more correctly described by saying that "gravity depends on the stress-energy-momentum tensor"? [I honestly don't know.]

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Originally Posted by George
For comparison, would it not also be fair to consider neutrinos, though they don't have zero rest mass, to be more correctly described by saying that "gravity depends on the stress-energy-momentum tensor"? [I honestly don't know.]
Any time the momentum component of the energy is of similar or greater magnitude to the rest mass you should really be using GR terms.

I'd actually have no issue with relativistic mass if it were caveated correctly. If up front there was a disclaimer saying "This is not mass. This is something we do to try to avoid actually using General Relativity when we are in a situation where we really should. Don't treat it like mass outside the very narrow range of applications, don't try to reason with it as if it were mass. And whatever you do don't confuse it with rest mass. If you want to do anything other than some relatively low energy kinematic approximations step away from the Relativistic Mass and pick up the textbook."

Edit to add: Why this is relevant to the OP is that it touches on what is meant by heavy light. If you mean has a larger rest mass - no, it cannot. If you want to use the concept of relativistic mass then it can but you have to be very careful what you take away from this.
Last edited by Shaula; 2018-Jun-13 at 04:38 PM.

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Originally Posted by Shaula
Why this is relevant to the OP is that it touches on what is meant by heavy light. If you mean has a larger rest mass - no, it cannot. If you want to use the concept of relativistic mass then it can but you have to be very careful what you take away from this.
Hmm, I guess initially I was thinking rest mass. It seems odd that all photons are identical no matter the source. The photons I see coming from the tiny led on my computer is the same as a photon coming from the sun.

11. Originally Posted by DaCaptain
Hmm, I guess initially I was thinking rest mass. It seems odd that all photons are identical no matter the source. The photons I see coming from the tiny led on my computer is the same as a photon coming from the sun.
They are the same species of objects might be one way to see it, and no two are exactly the same if you include their direction. Otherwise, their difference in wavelength gives each character, else we wouldn't have a colorful universe.

12. Originally Posted by Shaula
Any time the momentum component of the energy is of similar or greater magnitude to the rest mass you should really be using GR terms.
That makes sense. Yet I thought GR and particle physics (ie quantum mechanics) were like oil and water. Is this unification something fairly new or is it just highly limited in QM? It seems ironic that the relativity in e=mc^2 can be problematic, but GR is not.

I'd actually have no issue with relativistic mass if it were caveated correctly. If up front there was a disclaimer saying "This is not mass. This is something we do to try to avoid actually using General Relativity when we are in a situation where we really should. Don't treat it like mass outside the very narrow range of applications, don't try to reason with it as if it were mass. And whatever you do don't confuse it with rest mass. If you want to do anything other than some relatively low energy kinematic approximations step away from the Relativistic Mass and pick up the textbook."
Do you recommend a general velocity range that should place emphasis on the GR approach? Radioactive decay, for instance, I would assume would be fine with the relativistic mass approach. But the 7% conversion of mass to energy in the Sun's core requires very high velocities, but don't they simply use e=mc^2 for this as well? Is there an example of the GR approach for something like this that will help my eyes get its daily doze of glazing?

13. Originally Posted by DaCaptain
Hmm, I guess initially I was thinking rest mass. It seems odd that all photons are identical no matter the source. The photons I see coming from the tiny led on my computer is the same as a photon coming from the sun.
That's true of fundamental particles in general, though. The electrons carrying current in a wire are indistinguishable from the electrons emitted by beta decay from Carbon-14. The high energy protons we observe as cosmic rays are fundamentally the same as the protons that are the nuclei of ordinary hydrogen (and that are part of the nuclei of all other atoms). That's actually one of the phenomenal successes of the Standard Model: that we can explain all the amazingly different things we see in the universe with only 17 distinct types of thing.

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Originally Posted by George
That makes sense. Yet I thought GR and particle physics (ie quantum mechanics) were like oil and water. Is this unification something fairly new or is it just highly limited in QM? It seems ironic that the relativity in e=mc^2 can be problematic, but GR is not.
It is just limited. But hardly new - the first results relating to relativistic wave equations were from the 30s, I believe.

Originally Posted by George
Do you recommend a general velocity range that should place emphasis on the GR approach? Radioactive decay, for instance, I would assume would be fine with the relativistic mass approach. But the 7% conversion of mass to energy in the Sun's core requires very high velocities, but don't they simply use e=mc^2 for this as well? Is there an example of the GR approach for something like this that will help my eyes get its daily doze of glazing?
In fusion and fission calculations using E=mc^2 as it is commonly used is fine because what you are interested in is the change in the rest mass between the parent system and the daughter products to understand the amount of energy released. If you wanted to dive into the kinematics of the reaction you may need to consider relativistic effects. I can't give you a fixed velocity range for when it becomes important - you need to look at the system and what you want to understand about it. For example we got along just fine with classical optics for a long time and still use it despite the main component of the theory travelling at c. But you can't understand, for example, why gold is so unreactive and that particular colour without considering relativistic effects.

15. Originally Posted by Shaula
In fusion and fission calculations using E=mc^2 as it is commonly used is fine because what you are interested in is the change in the rest mass between the parent system and the daughter products to understand the amount of energy released. If you wanted to dive into the kinematics of the reaction you may need to consider relativistic effects. I can't give you a fixed velocity range for when it becomes important - you need to look at the system and what you want to understand about it. For example we got along just fine with classical optics for a long time and still use it despite the main component of the theory travelling at c. But you can't understand, for example, why gold is so unreactive and that particular colour without considering relativistic effects.