The relativistic mass of light?, It would be directional!

The argument is that light has no rest mass but has
mass by virtue of its motion.

But its motion isn't relative. If its mass comes from
its kinetic energy or speed then all light particles would
have the same mass.
Not a valid argument you see.
If you are to argue light's relativistic mass then you
must argue it comes from its wave and would be
quantified/measured by its wavelength.

The relativity of the energy of a light wave though is
directional. The Doppler shift on light can make the
wavelength grow(lower energy) if you are moving
away from the light source; or it can make the
wavelength shrink (higher energy) by moving toward
it.

This is only energy. Its not necessarily mass in
my opinion.

It is interesting to note that for accelerated matter
relativistic mass goes up regardless of direction of
motion. It is only relative to speed not direction.

So you can see relativistic mass for light is a totally
different beast than relativistic mass for matter in
motion!

Yes and no. In both cases, the concept of relativistic mass is not strictly necessary. It can simplify certain approaches to certain problems. But as we shall see later on, there is a difference in how it is conceptualized, how it is used, and possibly in how it behaves (I'm not sure!).

In the case of light, the relativistic (aka fictional) mass can be useful in visualizing interactions with particles, because of our everyday understanding of momentum. We know from everyday life that it is more difficult to significantly affect the momentum of heavy objects than of light objects. The same is true of high-frequency light as opposed to low-frequency light. So in certain cases, it can be "easier" to think of high-frequency light as being "heavier" than low-frequency light.

In the case of massive objects, relativistic mass is again useful in visualizing interactions between particles. So that thing they have in common. But there is a difference, as will become obvious in a moment. We know that relativity says that as the velocity of an object increases, it becomes exponentially more difficult (i.e. it takes more force) to continue to accelerate it at the same rate. What this means is that to increase the speed of something moving at .95c by 1m/s takes more force than it does to increase the speed of something moving at .90c by 1m/s. What other phenomenon causes it to be more difficult to accelerate one thing than another? Mass! So it can be convenient, in the case of high-speed massy objects, to think of the faster one as being heavier, because it takes more force to accelerate it by a given amount than for a slower object.

But in neither case is it strictly necessary to take these views. In the case of matter, the increase in force necessary for acceleration is built into relativity and the asymptotic velocity limit of c. And in the case of light, the momentum can be calculated by way of the frequency or wavelength, without involving any sort of mass at all.

I think the real test comes into play with gravity. We know that the bending of light by gravity is "achromatic" as someone else on the boards here put it. Meaning that the wavelength or energy of the photon does not affect the deflection angle at all. Only the mass of the body and the photon's distance from it. This would indicate that the relativistic mass of the photon is indeed fictional.

Again, with a massy object, a good question would be how gravity works with relativistic mass. If we can truly think of a particle travelling at 0.99c as being more massive than an identical particle travelling at 0.6c, then the faster one should be more affected by a gravitational body than the slower one. I confess I do not know if this is the case. But I have a sneaking suspicion it is not. Can anyone confirm or refute that?

If, in all cases, the relativistic mass of a massy object interacts in the same fashion as its rest mass, then maybe it could be just as valid to consider the relativistic mass as being "true mass". But not so for a photon, as noted above.

All I am saying is that its mass cannot come from its speed.
Its motion is not relativistic. Its speed is a constant.
Since this is true if you tried to say its mass comes
from its motion you would have to say all photons
have the same mass.

I know you can use frequency or wavelength in the
momentum equations for light but this proves nothing
since the mass(energy) of light would be in the wave
in the first place!

The relativistic mass depends on the wave and the wave
depends upon whether or not you are moving away from
or toward its source. Or not moving at all.

To repeat: The wave would be larger(lesser energy/mass)
if the source is moving away.
And the wave would be smaller(greater energy/mass) if
the source was moving toward the observer.

Well Nick you are keeping me from reading old novels.

Let us think em fields for a moment.

How many out there have encarta? raise your hands.

Encarta has one really messed up description of a photon(one wonders where the editors are).

Think three componants, frequency, speed, mass.

If the speed is set at C, then any mass componant will be either 0 or infinity.
(Well, the math is not the territory, but it is all we have.)

It was speculated a few years back that there were different types of infinity, that the infinity of whole numbers (for instance) is not the same as the infinity of fractional numbers.

So, what is the infinity of zero.

(see, done went all zen on your assets.)

But, that is the problem. Darn photon is doing C and has to have infinite mass, but has no mass at all.

Actually, I think you have to make the assumtion that the photon has no mass in order to do any calcuations.

*
For indiviual photons the energy levels involved depend only on frequency (blue has more energy than red).
*

As for as your (Doppler) moving to and from measurement, yes, you are correct, if it is moving away from you you will see a frequency shift toward the red *(less energy) and if it is comming at you (duck) then the frequency will be shifted towards the blue*.

Icy, the relativistic mass of light can't come from its motion.
Since C is a constant all mass of light would be the same.

There is really no rest mass for light because you can't
bring light to a rest ever. It is not that light has zero
rest mass;it is that rest mass doesn't apply to light.

We have to remember what is meant by the word frequency to understand what is implied by relativistic mass when it comes to light.

Frequency means how often the repeat state passes through a point.

Frequency does not talk about how far it is from one repeat state to the next (wavelength for waves) and it does not talk about how fast that distance is travelling; it only tells us how often.

That is why frequency is measured as number of repeats that occur per second. ie.:
frequency = velocity / wavelength
= (m/s) / m
= /s
(also know as a hertz)

The frequency will tell you nothing about the wavelength. The frequency will tell you nothing about how fast that length is travelling. (It is only because they assume that the speed of light is constant in all frames that they think they know the wavelengths of different frequencies of light).

This idea of frequency is in important distinction.

Nick is right in the notion that the relativistic mass can't come from the movement of the light as a whole. Instead it is directly proportional to its frequency or how often it cycles through a point in a given time. I even believe that this may even be the current accepted theory. Yipee!

I have my own ideas on other things that Nick is saying but I agree there.

The problem becomes how does light, which is said to be a wave, how can it also be said to act like a particle? A particle delivers an amount of momentum based upon its speed but we are told that light doesn't. This relates to what Nick was saying I believe.

This is where the irreconcilliable issue of light wave/particle duality remains even to today. In some experiments you have to say it is a wave. In other experiments you have to say it is a particle.

The doppler red/blue shift issue is a case in point where frequency through a point becomes relevant. If the point is moving (our eye for example) then this will change the frequency through that point. However according to Einstein the light as a whole will still be travelling at light speed relative to and through that moving point.

I know - as strange as that is to me - that nobody else seems to have problem with this notion.

What does this mean for the relativistic mass of light? What it means to me is different frequencies of light will bend different amounts around gravitational bodies.

I do wish they would use frequency instead of wavelength in their formulas as frequency is the relevant thing as far as I'm concerned.

WaterBreath,

It would indeed. Except that I don't agree that the bending of light by gravity is achromatic. I don't believe we 'know' this but are assuming it and are eliminating evidence that suggests otherwise - the evidenced aberration that is currently being assumed to be because of refraction in the chronosphere.

I agree. But, I believe at the moment that there is more to that idea. I believe it is increasingly difficult to increase the speed due to the external affects surrounding the mass. In otherwords the mass is affected by its surroundings. It experiences resistance as it generates a larger relative force due to its super fast relative movement in relation to its surroundings. How are the particles accelerated? By magnets.

It is actually the other way round. Faster objects will describe a smaller deflection than slower objects.

Does someone want to tell me what the legitimacy of this site is please?
The Mass of Light

Icecycle,

They treat the light as having momentum instead of mass. This momentum is treated as inversely proportional to its wavelength.
But they destinct matter from energy by saying that matter has momentum and weighs something and light has momentum but weighs nothing.
Go figure.

I do wish that instead of wavelength that they would use frequency which it is what I believe the momentum is actually proportional to. Mathematically that should be perfectly legitimate.

Gonegahgah,

I'm going to leave your argument alone for now. We've discussed this before. We differ on certain things that haven't been tested yet. I trust relativity will pull through those tests someday. You don't. That's fine, we can coexist peacefully until then . But where Nick is coming from seems to be from within the realm of relativity, so that's where I will answer him from.

To use an analogy, he's digging holes in a field looking for buried treasure that he knows is in a field somewhere. I think the field he's in has the buried treasure, but he hasn't checked the whole thing yet. Rather than let him wander off to look at different fields I'm going to encourage him to finish scoping out the one he's in. It would really suck to leave the field without finishing only to find out later you were about 10 minutes away from finding it, wouldn't it?

Nick,

Yes, this is precisely true, if you are considering a photon as having relativistic mass. You don't have a problem with this do you? It's the velocity thing you're not liking right?

If so, that's perfectly fine too. But I think it's equally valid to say the same thing for a massy object. (Dun-dun-DUN!) We tend to think of velocity as a fairly natural and fundamental thing, but this is often misleading. I think it is helpful rather to consider the momentum of an object as more fundamental than the velocity. For a massy object, yes momentum is related to mass and velocity, but that's "incidental". For a photon, momentum is related to frequency/wavelength. But the momentum of the massy object and the momentom of the photon are still the same type of momentum. It is still conserved in an interaction. It is still exchanged the same, despite the fact that we consider its "origin" to be different.

But origin is the key word here. I think it might help to consider velocity changes, and frequency/wavelength changes, (and relatvistic mass changes) and all that stuff to arise from changes in momentum, rather than vice versa. In short, the momentu is the "origin" of the velocity (or frequency/wavelength for light), rather than vice versa. IMHO this really cleans up the picture. But I can hear everyone screaming "What about the kinetic energy? We need velocity to figure that out for matter!" Well, not strictly. If you know the momentum, then kinetic energy is easy:

p is a known value.
p = m*v
p^2 = m^2*v^2
p^2 / m = m*v^2
(1/2) * p^2 / m = (1/2) * m*v^2
(1/2) * p^2 / m = KE

Similarly for a photon, it's fundamentally defined that:
KE = p*c

So there you go. Now there is an unfortunate "issue" in that the best way we have of measuring momentum of an object of unknown momentum, is by way of mass, or velocity, or frequency/wavelength. This makes it tough to think of momentum as fundamental. We tend to think of calculated things as functions of fundamental things, rather than vice versa. But we (as scientists/geeks/what-have-you) learned that perception is not ultimately reliable way back when quantum theory was introduced. So, the idea of changing our thought process on things like this shouldn't shake too many people to the core.

Anyway, if you think of "relativistic mass" as arising from a change in momentum, rather than as a side effect of moving, then suddenly it is very easy to think of it as "the same" between light and massy objects. However, if you can think of all these measureables as arising from change in momentum, then it might just be a more logical step to consider momentum as "the important thing" in relativity, rather than simply velocity, and then there's no need for conceptions of relativistic mass at all. Then you can apply it to things like light and massy objects and all that with equal ease. If you know the "relative momentum" between one thing and another, then you can calculate all the rest, even including relativistic effects. If it's a photon, you just take the relative momentum and calculate the frequency, and boom, now you know how blue or red of a light the other guy is seeing. If the thing is a particle, then you just take the relative momentum and calculate the velocity, knowing what the mass of such a particle should be. If it's something like, say, a meteor, well, it's tough to measure mass from a distance, but there are many indirect ways of doing it, including observing the effect of a nearby gravitational body, or mass-spectrometry to figure out its composition, etc.

Anyway, again I think that helps clear the picture a bit. It does for me, anyway. Helps homogenize a certain portion of the thought process to apply similarly to both light and matter. Now the big difference arises only in how the momentum manifests itself, depending on whether the entity is light or matter. But since that's usually pretty easy to determine, it doesn't bother me much.... for now.

Hope that all makes sense!

Hi WaterBreath,

Cool. Yep. It is good that you do so.

makes sense, but, i'd rather be watching tv folks!!

u bunch of novices, whence will you ever learn, the fact is light has weight, in the simplest sense. infact, its one hundred billionth of a gram. the fact is, light is quite elusive as specimens cause they behave like both particles and waves. they're electromagnetic, a flux capacitor would show you what i mean

That and a bolt of lightning (or any other 1.21 Gigawatt power source) will get you to back to the future.