Do photons have inertia?

The scientific method is what told you that all energy is electromagnetic?

 

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Which doesn't say anything about your claims.

 

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Chiller, it's not hard to spot the fault in your statements and your reasoning.

Here's one simple way to see it all clearly: Take a look at the Periodic Table. Beginning with hydrogen, energy is released when the nuclei fuse to form helium. (and you should note that the conversion is NOT complete.) You can continue the fusion process on down the Table until you reach iron (if memory serves me correctly). At that point, though, iron cannot be fused without providing additional energy - it's the dead-end point for fusion.

You can continue with the table, of course, until you reach the point where FISSION starts to take place. But that conversion is not complete either - meaning that parts of the atom do NOT convert to energy.

So, think that over for a little while... Once you've grasped the full implications you'll no longer claim that "energy and mass are equal."

 

Some comments:
Mass is a form of energy. Suppose I have an object with mass m. Simply because it has that mass, and supposing it is at rest, it's energy due to it's mass is \(mc^2\). It's exactly the same as having an object of mass m in motion with a speed v. The energy due to it's motion is \(\frac{1}{2}mv^2\)

AlexG: momentum is not a "measure of energy." It's an entirely separated conserved quantity (actually, energy, momentum and angular momentum form the stress energy tensor which is the underlying conserved quantity. Energy, momentum and ang mom are conserved because of the invariance of physics under time translations, spatial translations and rotations respectively.)

 

Strictly speaking, the Standard Model predicts a zero mass for the photon. A large number of experiments has been dedicated for setting ever diminishing limits on the photon mass.
Having said that, there is still some controversy relative to the above experiments, an issue that, as of 2003, hasn't been settled yet. See this paper in Phys.Rev.Lett.
There are perfectly consistent formulations (google "N. Proca theory of electromagnetism") that allow for a non-zero mass of the photon. The Proca theory is the foundation of all experiments that attempt to put ever narrower limits on the photon mass. The paper mentioned above, while accepting some limits, rejects the second, more stringent set.

 

I haven't claimed ER is the only energy there are kinetic energy, potential energy and mass is a form of energy as well.

I haven't claimed that mass and other energy are equal they are equivalent because mass is also an energy form.

 

Kinetic energy and frame dependence are another thing. There is certainly something strange at the very basic level going on between mass and energy, but energy is basically a very complicated accounting system for entropy. Mass and energy are related but a photon has no mass and yet it has momentum. If you work out the photoelectric effect or Maxwells equations this actually makes sense even if it doesn't translate into any sort of common experience, meaning we can't point to something it is 'like'.

ETA: Massive things have inertia. These are definitions, not derivations.

 

What is the formula for inertia?

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Photon momentum is similar to that for energy but divided by c.
Momentum = frequency times Planck's constant/speed of light

From Wikipedia on Inertia:
"In common usage the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context."

So is inertia just mass? Does a photon have inertia? It certainly doesn't like changing velocity, so one might say yes.

 

Well, F=ma. That's the convention anyway.

 

"a" is acceleration or the change in velocity /unit time
"m" is the mass
force is the measure of the mass times delta Velocity / Delta Time

If mass was inertia we would make it clear. It is like attribute to mass, without a formula.

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damn you.

 

http://www.physicsclassroom.com/class/newtlaws/u2l1b.cfm

You are tricksy.

But I learned my Newton once and don't think I forgot it that badly. Inertia is a property of mass. Photons have no mass.

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Big Chiller:

Another way to look at what the other folks are explaining is as follows. I will preface this by saying that matter and mass can be considered the same thing for purposes of comparing matter and energy.

Consider thinking of this in units. Energy can be expressed in Joules. A Joule can be expressed as an amount of force exerted over distance: 1 J = 1 N·m. Force (Newtons) is an amount of mass (kg) subjected to an acceleration (m·sˉ²): 1 N = (1 kg)×(1 m·sˉ²). Now if you go back and insert this in the above definition of the Joule: 1 J = 1 kg·m²·sˉ².

So the thing we call matter or mass (1 kg) and the thing we call energy (1 kg·m²·sˉ²) are not exactly the same, since there is a difference of (1 m²·sˉ²) in the units.

You can also get this directly from E = mc² (m here is mass in kg), by noting that the units of c are m·sˉ¹ so the units of c² must be m²·sˉ².

In any case matter and energy cannot be the same merely by the presence of the c² term.

It would be correct to say matter can be converted to energy in an amount that is equal to its mass times c², but you can't drop the c² without talking about something different, which is not energy, but rather E ÷ c², which can't be the same as E. In fact, it's mass.

I think the confusion arises from the idea that matter and energy can be converted back and forth. In this sense you may feel they are the same. But the conversion changes them from one to another in a way that they cannot possibly be the same. For example, for energy to be the same as matter, then all of the spacetime that is represented by the the m²·sˉ² term would have to be folded up into nothingness. This would seem to be equivalent to somehow plucking an amount of energy out of spacetime only to be left with its mass component. I'm not sure exactly what that means other than to say it lies outside of reality.

 

Photons have Energy and momentum they might not have mass but could they have inertia? Momentum implies inertia??? What else has momentum but no inertia
OK it is very hard to make light increase in velocity what would you use to speed it up. You can slow light down (putting it into some other medium) is there a sign of inertia as it slows and speeds up again (refraction angle changes)
Differing energy levels different amounts of slowing so suggestive of inertia (similarity). [Refraction - New field for me]

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actually, I don't think light can be made to technically slow down. It's been awhile since I reviewed the optics in question but I think what happens is a frame dependent thing. I'll go see if I can't find a reference for that.

 

Those neutrinos travel at the speed of light (?? not much doubt about that now) and have very small mass, so why is it so vital that light doesn't have even a very very small amount of mass?

http://en.wikipedia.org/wiki/Neutrino#Mass

 

Also, light speed has never been measured accurately. Those decimals are expensive.

 

It is so convenient just to round it up to 3E+10 m/s in any case. Close enough is OK.

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I think first of all it boils down to definitions. Commonly we think of light as something that illuminates our world, and mass is what we intuitively ascribe to an object's weight.

Now if you were to define light in terms of photons, then it's a little easier to cope with the question of mass, since mass is intuitively ascribed to particles and we usually think of photons as particles.

So if you're asking why can't photons have mass, then (notwithstanding Tach's comments about the search for this answer) the first thing that jumps out at you is the problem that photons move at lightspeed so the question of their mass is made problematic. Lightspeed is a boundary condition for objects moving in the real world, and we don't normally encounter things happening right on top of a boundary. Yet there the photon is, existing solely in its "out of bounds" world.

It's one thing to intuit that a real world object has mass because of its weight but what does it mean to apply this idea to the lightspeed particle living in its boundary world?

One idea that comes to mind is what happens when the photon crashes into a real world object. Is it like a collision? Is there recoil or energy exchange? This takes us into another kind of question. It seems that this photon will find its way to some electron in the material, and rather than crashing into it the way real world objects crash together, instead the photon apparently vanishes, giving the electron its energy. Assuming the electron was stable to begin with, it will jump and fall back to its initial state, generating a photon "out of thin air" - the thin air from whence the previous photon vanished. Under particular circumstances the new photon exits at the Brewster angle (or at least the ray of light does) and then we experience illumination, a glint off the surface of that object.

The original question of a photon's inertia seems to be trapped by the fact of this kind of interaction. In other words, our common use of the word inertia only applies to the real world, where objects recoil when they crash. We don't expect them to vanish into or appear out of thin air. Apparently when this particle appears it's already at lightspeed, so it doesn't need to be accelerated which would pose some other dilemmas for us to ponder.

My thinking is that real-world inertia doesn't really apply to the photon if it's a quality that has no relevance to its mode of existence. If it can't be accelerated or decelerated, and simply vanishes when something gets in its way, then what good is it it ascribe this quality to it after all?

Having said that, there is apparently a lot going on with all kinds of sub-particle interactions in which things get annihilated and created, preserving momentum, converting energy and mass back and forth, simply because the photon lives in that boundary world where the rules are so different than in our world. So momentum is a quality that clearly seems to intersect both worlds. However, unlike our common sense of momentum as something that arises from a weighty object in motion, in the photon world its momentum is instead tied to its frequency. Of course we don't have a very good real world sense of how an object can have frequency until we cause it to vibrate, which doesn't make sense to us if we try to use this as a substitute for being weighty. Somehow in the photon's world that is what apparently has happened. The thing that would be its mass, if it could have some, is instead transposed into another quality, the quality of a wave of a a specific frequency, which is akin to saying a particle of a specific mass.

It is this inability to trace common forms of existence between the real world and the photon's world that challenge the intuition. The two worlds intersect in some ways, and in other ways they are disjoint, transposed or transformed versions of each other. Consequently we are required to blend a certain amount of intuition with ideas that are entirely counter-intuitive. One of them is that this particle either has no mass, or nearly no mass, and the other is that it has frequency. And it evidently blinks in and out of existence. And it evidently does nothing except move, and when it moves it's at that one impossible rate that boggles the mind - lightspeed.