# light = energy, energy = mass, so light = mass ?

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#### killium

##### Guest
There is that little concept i think i don't get. If light has energy and energy is equal to mass (e=mc2), how can we affirm that light has no mass ?

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#### origin

##### Guest
killium":3vh6em3n said:
There is that little concept i think i don't get. If light has energy and energy is equal to mass (e=mc2), how can we affirm that light has no mass ?

Here's the problem energy does NOT equal mass. What you are quoting is the mass to energy equivilancy. It is rather subtle I guess but mass is not energy and energy is not mass. Mass can be converted to energy. Energy can be converted to mass. But they are definitely not the same.

It is kind of like this. Remember I said KIND OF like this. Steam is the gas phase of water ice. You can convert ice to steam but ice is not steam. Try to run an power plant with ice instead of steam.

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#### drwayne

##### Guest
I think of

e = mc^2

as an exchange rate, much like a currency exchange rate. You can convert a dollar to a euro, but that does
not mean that a dollar is a euro.

My little heuristic view of the world...

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#### killium

##### Guest
i see. This would mean that the mecanism thru which matter is converted into energy and vice-versa is assymetric in regards to gravity...

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#### Couerl

##### Guest
killium":1igzf3ir said:
how can we affirm that light has no mass ?

Well the short answer is that you can't. :lol:

http://www.desy.de/user/projects/Physic ... _mass.html

There's a longer answer, "It is almost certainly impossible to do any experiment that would establish the photon rest mass to be exactly zero. The best we can hope to do is place limits on it."

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#### origin

##### Guest
killium":ajnd4z1c said:
i see. This would mean that the mecanism thru which matter is converted into energy and vice-versa is assymetric in regards to gravity...

Huh?

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#### ramparts

##### Guest
There's a little more to this. The mass-energy equivalence (which, as said above, does not mean they're the same thing) manifests itself in an interesting way in Einstein's theory of gravity, general relativity. In regular Newtonian gravity, you need both objects to have mass in order to have gravity between them. In Einstein's modification to it, gravity is an effect of spacetime curvature, and both mass and energy can curve spacetime. So in theory a field of pure radiation actually has an effect on the spacetime of the universe, as if it had a mass. In fact, for the first few tens of thousands of years of the universe, it was radiation dominated: that is, all the light left over from the Big Bang actually had more of a gravitational impact on the universe's expansion than did matter. Cool beans!

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#### kelvinzero

##### Guest
Im pretty sure mass and energy are meant to be equivalent, or more accurately a certain amount of energy always implies a proportional amount of mass, regardless of the form of the energy. Dont confuse matter with mass.

Light has mass, just not rest mass. If it had any finite rest mass at all, it would gain infinite mass and require infinte energy to get it to light speed.

I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed. however since they have infinitely small rest mass, they are totally inconsequential until their mass is increased by an infinite factor, by accelerating them to light speed.

Another example,
You know how a positron and an electron can annihilate to form two photons? I reckon they have to have the same total mass before and afterwards. Otherwise you can sort of imagine an unobtanium bottle containing either matter/antimatter, or the equivalent energy in photons, and the weight of the bottle can be switched from one value to another depending what form the energy is inside it.. even though nothing goes in or out.

Another example,
If you shine a light into a black hole, you would make that black hole more massive. How could it be otherwise? If energy could just vanish from the universe and not have any effect, such as increasing the mass of the black hole, then energy would not be conserved.

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#### ramparts

##### Guest
Kelvin - No. Light does not have mass.

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#### origin

##### Guest
kelvinzero":3u211z2k said:
I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.

Photons cannot move slower than light speed. That does not make any sense.

Another example,
You know how a positron and an electron can annihilate to form two photons? I reckon they have to have the same total mass before and afterwards.

No, this is where E=mc^2 comes in, the the mass is completely converted to energy there is no mass in the resulting photons.

Otherwise you can sort of imagine an unobtanium bottle containing either matter/antimatter, or the equivalent energy in photons, and the weight of the bottle can be switched from one value to another depending what form the energy is inside it.. even though nothing goes in or out.

That is actually sort of correct. If there was a magic bottle that could hold positrons and electrons and was also able to hold the resulting photons that result from anihilation, then that is just what would happen, you would have the mass from the particle pairs and after the anihilation you would have a bottle full of energy and the weight would decrease due to the disapearence of all of the particles

Another example,
If you shine a light into a black hole, you would make that black hole more massive. How could it be otherwise? If energy could just vanish from the universe and not have any effect, such as increasing the mass of the black hole, then energy would not be conserved.

The mass of the black hole would increase due to the mass energy equivilancy E=mc^2

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#### ramparts

##### Guest
Exactly. Good explanation, origin. The key confusion here is that E=mc^2 and the mass-energy equivalence doesn't mean they're the same thing, just like dollars aren't the same as euros, even though they do similar things and you can convert one into the other.

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#### origin

##### Guest
ramparts":3mmolcmj said:
Exactly. Good explanation, origin. The key confusion here is that E=mc^2 and the mass-energy equivalence doesn't mean they're the same thing, just like dollars aren't the same as euros, even though they do similar things and you can convert one into the other.

Thanks ramparts, you know this stuff alot better than me. When I first saw that you responded I thought, "oh, oh; musta screwed something up in the explanation".

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#### darkmatter4brains

##### Guest
Probably good to point out - despite the excellent responses given - that nobody really knows what energy, light or even mass truly is.

It's like Richard Feynman said (at the time he was one of the leading guys in Quantum Mechanics and Quantum Field Thoery, and making huge strides as far as progress in the field) "I think I can safely say, NOBODY understands quantum mechanics". We still don't really :lol:

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#### kelvinzero

##### Guest
me":3s130hx5 said:
Light has mass, just not rest mass

ramparts":3s130hx5 said:
Kelvin - No. Light does not have mass.

Look.. things would proceed much more rapidly if people just assumed I was always right.

There is a bit of room for confusion over terminology, but I think in context it is pretty clear that I am talking about relativistic mass.

A photon most definitely does have relativistic mass. It most definitely does warp spacetime, the warping is most definitely simply a function of the total energy, and you really do not need to worry about whether the energy is in the form of matter, photons, or even sound waves. A bell that is ringing is slightly heavier than a bell that is not ringing. If you put a lump of matter and antimatter into a bottle, their gravity will not vanish or change in any way when the matter/antimatter converts to photons, given the bottle is impermeable enough to contain them.

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#### ramparts

##### Guest
That really wasn't clear from your posts, and if you want to be clear you should consider using terms in the way that they're actually used by scientists today. Light doesn't have mass in any meaningful sense. This relativistic mass concept in the way you're using it as far as I can tell hasn't been used since the first half of the 20th century. Mass in the way that everybody uses it - as a property of a particle's internal structure - doesn't change with a particle's speed, and it's something that for light is almost certainly zero. There's some decent info on this Wiki page (this section and the next, on modern views): http://en.wikipedia.org/wiki/Relativist ... c_concepts

Meanwhile you have yet to respond to any of origin's well-thought out objections to the other arguments in your last post.

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#### kelvinzero

##### Guest
origin":1qlaavlx said:
kelvinzero":1qlaavlx said:
I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.

Photons cannot move slower than light speed. That does not make any sense.

Most of the confusion in the rest of your message seems to be about a detail about the terminology of 'mass'. When I was taught relativity (admittedly only to a pretty basic graduate level) it was using the concept of total mass being equal to rest mass and relativistic mass. Apparently this terminology has gone out of fashion.

Given this, my statements are not 'sort of correct'. They are absolutely correct.

..with one exception, which I qualified. I personally like the notion of a sea of massless, energyless particles that switch from being effectively nothing to being something when accelerated to light speed. it does work in a mathematical sense as the limit of a simple progression but I didn't claim it was what really happens.

I resent the dismissal that it does not make sense, but if understanding it does not interest you then explaining myself does not interest me.

Now having vented some steam, I will storm off in a huff.

cya

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#### kelvinzero

##### Guest
ramparts":3032uvuy said:
That really wasn't clear from your posts, and if you want to be clear you should consider using terms in the way that they're actually used by scientists today. Light doesn't have mass in any meaningful sense. This relativistic mass concept in the way you're using it as far as I can tell hasn't been used since the first half of the 20th century. Mass in the way that everybody uses it - as a property of a particle's internal structure - doesn't change with a particle's speed, and it's something that for light is almost certainly zero. There's some decent info on this Wiki page (this section and the next, on modern views): http://en.wikipedia.org/wiki/Relativist ... c_concepts

Meanwhile you have yet to respond to any of origin's well-thought out objections to the other arguments in your last post.

(storms back in)

I honestly think it was clear from my post, but it isnt worth continuing any further.

(edit)
However everyone here does at least agree that even though photons do not have rest-mass, If you trapped photons going in oposite directions (ie zero net momentum) in a massless bottle, that bottle would behave exactly as if it had a rest mass. If you accelerated the bottle, photons hitting one side would be redshifted, and those hitting the other would be blueshifted providing resistance to acceleration, ie inertia. You can say that is not rest mass.. but it has to quack exactly like rest mass or we could all be building inertialess drives, and physics would be screwed.

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#### Tritium

##### Guest

If everything is a vibration,if energy,matter,mass,gravity are all variations of frequency over time,if everything is a wavelength with different properties,just as water exists as ice,fluid,and steam,depending upon the level of molecular vibration,which is caused by temperature,how far does the Law of Thermodynamics go?Cannot the question to the topic be rephrased to read "light =energy=matter=mass=gravity=mass=energy=light"And what we are on the verge of discovering is that everything consists of frequencies on an infinite scale.That it is possible for there to be wavelengths so long and so low in amplitude that they are infinite as far as we are concerned because they are outside of our ability to detect and measure.The same applies to high frequencies beyond our ability to detect and measure.

We are still children in this universe.As much as we think we know,we still know very,very little about the actual truth.

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#### Tritium

##### Guest
origin":vnfcz189 said:
kelvinzero":vnfcz189 said:
I dont know if this is valid, but I like to think of photons as always existing, even when moving below light speed.

Photons cannot move slower than light speed. That does not make any sense.

Another example,
You know how a positron and an electron can annihilate to form two photons? I reckon they have to have the same total mass before and afterwards.

No, this is where E=mc^2 comes in, the the mass is completely converted to energy there is no mass in the resulting photons.

Otherwise you can sort of imagine an unobtanium bottle containing either matter/antimatter, or the equivalent energy in photons, and the weight of the bottle can be switched from one value to another depending what form the energy is inside it.. even though nothing goes in or out.

And what if the black holes are spewing out everything they take in,in another part of the universe? :?:

That is actually sort of correct. If there was a magic bottle that could hold positrons and electrons and was also able to hold the resulting photons that result from anihilation, then that is just what would happen, you would have the mass from the particle pairs and after the anihilation you would have a bottle full of energy and the weight would decrease due to the disapearence of all of the particles

Another example,
If you shine a light into a black hole, you would make that black hole more massive. How could it be otherwise? If energy could just vanish from the universe and not have any effect, such as increasing the mass of the black hole, then energy would not be conserved.

The mass of the black hole would increase due to the mass energy equivilancy E=mc^2

O

#### origin

##### Guest
Tritium":3dpmddvy said:

It needs some experimental verification. String theory is interesting but it has a ways to go to.

What was the point of your last post, I saw my name and kelvinzero's name, but there was no comment from you?

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#### Astro_Robert

##### Guest
An interesting aside to this is that although photons are massless (no massive particles can reach lightspeed, yada yada), photons do carry MOMENTUM, hence solar sails work. If I recall correctly, momentum is written as:

momentum = mass * velocity. so zero mass times infinite velocity = finite momentum (heehee)

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#### Astro_Robert

##### Guest
Ramparts,

I have a related question to your comment that light exerts gravitational influences.

I was once thinking about the expansion of the universe (I detest Dark Energy) and pondered if the cummulative photons emmited by the universe could be supplying the expansionist effect we perceive as dark energy. Ie, over time, most of the photons from the history of the universe are beyond the observable mass of the universe so their gravitational influence would be to 'pull' the universe towards the outer reaches and cause expansion we summon Dark Energy to solve.

At the time I decided that the distant photons would have such negligible gravitational influence due to the inverse r-squared nature of gravity that they could not have such an effect, even after accumulating for 13 Billion years. I am wondering if you know if any physicists have discussed this and if they concluded the same thing?

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#### ramparts

##### Guest
Astro_Robert":1xvgbx6h said:
momentum = mass * velocity. so zero mass times infinite velocity = finite momentum (heehee)

Mmmm not exactly After all, photons don't have infinite velocity, right?

As with most basic physics equations, p=mv is just a non-relativistic approximation. The full relativistic version is:

E^2 = (mc^2)^2 + (pc)^2

This equation encompasses a lot of things. For example, if you take an object at rest (p=0), then you get E=mc^2, the object's rest energy (just take the square root of both sides). p=mv is also hidden in there (for v much smaller than c) though it takes a little bit of work to get there. But if you set m=0, then you find E=pc. Since photons definitely have energy, they also must have momentum, equal to E/c.

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#### ramparts

##### Guest
Astro_Robert":1dea5j3d said:
Ramparts,

I have a related question to your comment that light exerts gravitational influences.

I was once thinking about the expansion of the universe (I detest Dark Energy) and pondered if the cummulative photons emmited by the universe could be supplying the expansionist effect we perceive as dark energy. Ie, over time, most of the photons from the history of the universe are beyond the observable mass of the universe so their gravitational influence would be to 'pull' the universe towards the outer reaches and cause expansion we summon Dark Energy to solve.

At the time I decided that the distant photons would have such negligible gravitational influence due to the inverse r-squared nature of gravity that they could not have such an effect, even after accumulating for 13 Billion years. I am wondering if you know if any physicists have discussed this and if they concluded the same thing?

Photons were the dominant gravitational influence on the expansion of the universe for the first 50,000 years or so after the Big Bang, at which point the matter density overtook the photon density, and matter was then the dominant force on the expansion for another few billion years until the dark energy - or whatever is causing the acceleration - took over because the matter became too diffuse.

That's for photons in the observable universe. The biggest problem with any idea which, like yours, claims to talk about forces from "outside" is that those should be perfectly balanced by forces from "inside," assuming the universe is (on large scales) the same everywhere. This is an assumption which has been very well tested and any theory which violates it loses some serious aesthetic points - even if you don't like ad hoc dark energies, theories which claim we're in a special place in the universe are worse.

It gets worse. Given that we see the universe accelerating uniformly in all directions, we can conclude one of two things: either the acceleration is happening in all parts of the universe (in which case your idea doesn't hold), or we are exactly in the center of the acceleration, literally the only place in the observable universe where the acceleration is the same in all directions. How's that for a coincidence.

The other problem is that photons which are beyond the observable universe won't be able to exert a pull on us in the first place - if they're outside the observable universe, it means we can't see them, with light or with gravity. The exception to that is for objects which were in our observable universe before inflation, but the photons in your scenario wouldn't qualify since you claim they left the observable universe later.

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#### Tritium

##### Guest
My cerebral cortex begins to ache when I try to conceive the image of nothingness,which begat the Big Bang,and the universe expanding into this infinite nothingness,only to reach some outward point where the energy expanding it can no longer sustain the outward motion,and the whole thing begins to collapse upon itself and condense until it reaches critical mass and explodes again,as it has done throughout infinite time.Simple human.Simple brain. :|

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