Exerting forces on massless objects

[holezch]

Hi, I'm pretty sure that you must have a mass inorder to exert a force on something (at least from what I know). But can you exert a force on something that is massless? :S You can right? Because when we talk about massless springs and cords, things are exerting forces on them.. but then if you exert a force on something, without any other forces, it may have an acceleration.. can massless objects accelerate? I don't think it would make much sense thanks

 

[radou]

The objects (springs etc.) you are referring too are not massless - they posess a mass, but it's negligible, since they don't affect the overall behaviour of the physical system considered.

 

[holezch]

thanks, so that's why I was so confused. So even hypothetically, we aren't saying that it doesn't have a mass, but just that its mass is so small we can ignore it. so you cannot exert a force on a massless object. I think that a massless object cannot exist, is that true? how can something exist with no mass? to exist means that there is some kind of entity representing an objects existence..

 

[radou]

Well, there are some massless elementary particles as far as I know, but that goes far beyond the scope of my knowledge.

 

[torquil]

The photon is a massless particle. Massless particles always move at the speed of light.

Massless particles can interact with itself and/or other particles, so it is possible to exert a force on a massless particle. Newtons second law doesn't apply though, since massless particles are only described correctly in relativity theory.

In a way, two photons can interact with each other, via production of temporary electron/positron particles:

http://superstruny.aspweb.cz/images/fyzika/spacetime/photon-photon.gif

Two of the outer perforated lines represent incoming photons. They transform into electron/positron pairs (the solid lines). These in turn combine to form two photons that are again represented as outgoing perforated lines.

This process results in a perceived force that acts between the two photons, changing their directions.

Torquil

 

[Andy Resnick]

Hi, I'm pretty sure that you must have a mass inorder to exert a force on something (at least from what I know). But can you exert a force on something that is massless? :S You can right? Because when we talk about massless springs and cords, things are exerting forces on them.. but then if you exert a force on something, without any other forces, it may have an acceleration.. can massless objects accelerate? I don't think it would make much sense thanks

Massless particles can indeed exert forces: radiation pressure. It's also possible to exert forces on massless particles: refraction.

But this is probably not what you mean, I suspect you are thinking about mechanical systems. When we use massless springs and massless ropes (or rods) in mechanical systems, usually they are used to *transmit* forces from one object to another, or used to constrain the motion of a test mass. It's an abstraction that's useful to simplify analysis, nothing more or less.

 

[holezch]

are the things you two mention (photons specifically) LITERALLY massless? Or is it just that their masses are so small they are virtually massless?

 

[Andy Resnick]

Photons are *predicted* to be massless, and measurements place an upper bound of their mass at somewhere between 10^-14 and 10^-18 eV.

 

[UgOOgU]

are the things you two mention (photons specifically) LITERALLY massless? Or is it just that their masses are so small they are virtually massless?

Yes, photons are massles. But the more fundamental concept in applying forces is not mass or inertia; on the contrary, is the momentum, because force changes the momentum $$\vec{F} = \frac{d\vec{p}}{dt}$$. Although photons are massless, they have momentum $$\vec{p} = \hbar\vec{k}$$, the necessary condition for the application of forces.

 

[holezch]

hm, but momentum defined in terms of mass, so of course, mass is fundamental to applying forces

 

[holezch]

Photons are *predicted* to be massless, and measurements place an upper bound of their mass at somewhere between 10^-14 and 10^-18 eV.

ah okay. so it wouldn't be *incorrect* to say that they do have a mass as much as it wouldn't be incorrect to say that they are massless? thanks

 

[torquil]

hm, but momentum defined in terms of mass, so of course, mass is fundamental to applying forces

No, momentum is not defined in terms of mass. In the standard model of particle physics, which is the current accepted model of fundamental particle physics, the photon is massless. It is important to separate between rest mass and energy here.

For a massive particle, momentum is related to its rest mass and velocity, but for a massless particle, momentum is also well-defined, even though it's rest mass is zero.

ah okay. so it wouldn't be *incorrect* to say that they do have a mass as much as it wouldn't be incorrect to say that they are massless? thanks

The most correct thing to say is that it has a mass that must be less than some bound.
In practice this means that the model can be simplified mathematically by assuming it to be massless. But this is nitpicking about a quantity that is extremely small/zero.

Torquil

 

[Andy Resnick]

ah okay. so it wouldn't be *incorrect* to say that they do have a mass as much as it wouldn't be incorrect to say that they are massless? thanks

It would be incorrect to claim that a photon has a mass- the experimental data is consistent with photons being massless, not massive. Any mass that a photon may have is less than can be measured at the current time.

 

[ZapperZ]

hm, but momentum defined in terms of mass, so of course, mass is fundamental to applying forces

Where is the "mass" in the definition of momentum as $p=\hbar k$?

Zz.