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Author Topic: Does light have inertia?  (Read 17846 times)

Offline kaukcz

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Does light have inertia?
« on: 27/05/2008 03:18:07 »
since light displays particle properties does it also display the effects of inertia                                             

[MOD: modified subject to make it a question. CS]
« Last Edit: 27/05/2008 09:13:55 by chris »


 

lyner

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Does light have inertia?
« Reply #1 on: 27/05/2008 12:50:51 »
The word 'inertia' is not really defined but light does display properties of momentum.
It can transfer this momentum to a particle (dust, etc.) during a collision and produce 'light pressure'. One effect of light pressure is the fact that radiation  'pushes' dust away from the Sun.
 

Offline lightarrow

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Does light have inertia?
« Reply #2 on: 27/05/2008 12:55:39 »
Usually "inertia" refers to "inertial mass" and a beam of light has zero mass.
 

lyner

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Does light have inertia?
« Reply #3 on: 27/05/2008 13:36:45 »
The trouble is that, although it has no 'inertial mass', it still shows a property, momentum' which makes it behave as if it had some.
I think people find this confusing because momentum doesn't get its proper status; it is treated as a second class quantity. Whereas the conservation of momentum is a more rigorous 'law' than the conservation of energy, in many ways.
 

Offline lightarrow

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Does light have inertia?
« Reply #4 on: 27/05/2008 16:27:23 »
The trouble is that, although it has no 'inertial mass', it still shows a property, momentum' which makes it behave as if it had some.
I think people find this confusing because momentum doesn't get its proper status; it is treated as a second class quantity. Whereas the conservation of momentum is a more rigorous 'law' than the conservation of energy, in many ways.
I agree with your concern, but I think it's probably better to answer the specific question. If then the OP wants to talk about the connection between inertia and momentum we should let this decision to him.
 

Offline kaukcz

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Does light have inertia?
« Reply #5 on: 28/05/2008 04:22:59 »
With that said how exactly does light display momentum and not inertial mass, and what is the difference?  Also why is this?
 

Offline Soul Surfer

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Does light have inertia?
« Reply #6 on: 28/05/2008 09:37:51 »
To measure inertial mass you need to stop something and then see what force you need to accelerate it. You cannot stop light or accelerate it because it is always travelling at the speed of light so the concept of inertia just does not apply.  however if you shine a beam of light on a reflective surface and it is reflected it generates a peressure by virtue of the momentum energy that the light beam carries.  This can be measured and corresponds to theory.
 

Offline lightarrow

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Does light have inertia?
« Reply #7 on: 28/05/2008 12:57:46 »
With that said how exactly does light display momentum and not inertial mass, and what is the difference?  Also why is this?
This question has been recently discussed in this thread. Hope it helps you.
http://www.thenakedscientists.com/forum/index.php?topic=14606.msg174225#msg174225
 

lyner

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Does light have inertia?
« Reply #8 on: 28/05/2008 22:19:31 »
Quote
however if you shine a beam of light on a reflective surface
Or an absorbing surface - only in that case, there is only half the amount of momentum transferred to the surface. Crookes Radiometer shows this when there is a 'proper' vacuum in the envelope.
 

Offline Mr. Scientist

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Does light have inertia?
« Reply #9 on: 19/11/2009 12:28:57 »
since light displays particle properties does it also display the effects of inertia                                             

[MOD: modified subject to make it a question. CS]

It has a finite inertia. The weak-equivalence principle unifies not only the gravitationally-radiating mass-body, but anything which creates a gravitational field as well;

Viktor
 

Offline Pmb

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Does light have inertia?
« Reply #10 on: 22/11/2009 06:31:03 »
since light displays particle properties does it also display the effects of inertia.
Yes. Light has inertial masss which means, by defition, that light can give momentum to another body.
 

Offline Farsight

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Does light have inertia?
« Reply #11 on: 22/11/2009 12:07:37 »
This is a slightly tricky question. The answer isn't in your textbooks yet, but it will be eventually. A photon "has no mass" and exhibits no inertia because you can't speed it up or slow it down. Instead it exhibits energy E=hf and momentum p=hf/c, these being distance-based and time-based measures of energy/momentum. The speed of light c is distance over time, so you convert from one measure to another by dividing by c.   

However when you look at pair production you see that a +1022keV gamma photon is employed to create an electron and a positron. These are particles with spin and angular momentum, and mass - they do display the effects of inertia. Then when you look at annihilation you see that the electron and the positron can be combined to release two 511keV photons. So, what is an electron "made" of? Pair production quite literally tells us that it's made of light, and annihilation backs this up. See this peer-reviewed paper for details: The Nature of the Electron, but in a simple sense the electron is a 511keV photon going round and round, and you can deflect this photon via what is in essence the Compton effect. When you do this the electron moves. It exhibits inertial mass, and since it's made of light, this is light displaying the effects of inertia.

The underlying reason is that there's a symmetry between momentum and inertia. The difference between them depends on who you say is moving. A fast-moving cannonball is hard to stop because it has considerable momentum. But if it was you moving instead of the cannonball, you'd say it was hard to accelerate it to your own velocity because it has considerable inertia. 
 

Offline yor_on

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Does light have inertia?
« Reply #12 on: 22/11/2009 16:50:49 »
That's a real tricky one isn't it. Inertia is a kind of resistance all matter shows. And depending on momentum and invariant mass it will change. and we know that light have what we call a momentum relative any other 'object' it meets.

Let us assume (love that word:) two parallel lightbeams (lasers) traveling side by side, unhindered in a perfect vacuum. Will they bend toward each other?

Let us then assume :) several pairs traveling of different energies.
Will that make a difference to how fast they 'bend' if they 'bend?

And to up the quality of this question, let us furthermore assume that there is no 'distortion' to the vacuum they travel in due to 'gravitational forces' 'bending space'.

If they attract each other then, they should have a 'mass', right, or the concept of 'mass' is not thought through by us.

momentum is after all a property not existent until measured by something. As inertia.
 

Offline Farsight

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Does light have inertia?
« Reply #13 on: 22/11/2009 18:12:01 »
The answer is: yes, with a caveat. Matter causes gravity, but E=mc2 tells us that matter is "made" of energy, hence energy causes gravity. You can understand this when you put a photon in a mirrored box. The additional photon energy means the mass of the box+photon system is increased, and that increased mass will cause more gravity. Hence a photon causes gravity, hence two light beams will attract one another gravitationally. However a huge mass like the earth deflects light by only a tiny amount, so the very small energy of a photon will deflect another photon by such a small amount it won't be measurable any time soon.
 

Offline Mr. Scientist

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Does light have inertia?
« Reply #14 on: 22/11/2009 22:41:53 »
The answer is: yes, with a caveat. Matter causes gravity, but E=mc2 tells us that matter is "made" of energy, hence energy causes gravity. You can understand this when you put a photon in a mirrored box. The additional photon energy means the mass of the box+photon system is increased, and that increased mass will cause more gravity. Hence a photon causes gravity, hence two light beams will attract one another gravitationally. However a huge mass like the earth deflects light by only a tiny amount, so the very small energy of a photon will deflect another photon by such a small amount it won't be measurable any time soon.

It has a finite inertia related to its momentum. So we are all agreed with this?
 

Offline yor_on

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Does light have inertia?
« Reply #15 on: 23/11/2009 00:04:00 »
No, I don't think Light have inertia.
Inertia is something belonging to matter (invariant mass)
Light has a momentum but that's a different property.

If we could do my thought example I believe light would bend, but it would be to light influencing space around it, which then should mean that momentum can do much the same as mass does. That is warping space. And I also expect a higher 'energy' to warp space more. Which then should mean that a sun warps space further out that an object of the same invariant mass does as we have light 'traveling' from it outwards.
 

Offline Mr. Scientist

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Does light have inertia?
« Reply #16 on: 23/11/2009 00:08:22 »
Inertial effects of matter may have a different model to that of a photons. A photon may have an infinitessimally-small inertia to about the scale of 10^50kg.
 

Offline Farsight

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Does light have inertia?
« Reply #17 on: 23/11/2009 00:42:07 »
It has a finite inertia related to its momentum. So we are all agreed with this?
Hmmn. Looking at what soulsearcher and pmb said, it seems to be a matter of definition. If you look at Inertial mass and say you can't make a photon go faster or slower, the answer is no. But you can use the Compton effect to change the direction of that photon, and that's an acceleration. So then you say the answer is yes.



But most people associate inertia with rest mass, so I'm saying no, but then momentum and inertia are like two sides of the same coin depending on who's moving, and I'm saying yes again. And looking at the original post afresh, this isn't anything to do with light displaying particle properties. A wave conveys energy-momentum, but if you said it was you moving but not the wave, it wouldn't deliver a bump, it would be a bump and you'd say it had mass instead of momentum.

Interesting. But when it comes to mass, I think the important point is that you can make an electron out of a massless photon, and then it's got mass because it isn't moving at c any more. And that means the photon is boson enough. The mass isn't there because the electron is interacting with the Higgs field. The only thing "in there" going round and round is a 511keV photon, so the only thing it can be interacting with is itself.
 

ScientificBoyZClub

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Does light have inertia?
« Reply #18 on: 23/11/2009 01:33:54 »
I love this question.
 

Offline Mr. Scientist

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Does light have inertia?
« Reply #19 on: 23/11/2009 02:18:49 »
It has a finite inertia related to its momentum. So we are all agreed with this?
Hmmn. Looking at what soulsearcher and pmb said, it seems to be a matter of definition. If you look at Inertial mass and say you can't make a photon go faster or slower, the answer is no. But you can use the Compton effect to change the direction of that photon, and that's an acceleration. So then you say the answer is yes.



But most people associate inertia with rest mass, so I'm saying no, but then momentum and inertia are like two sides of the same coin depending on who's moving, and I'm saying yes again. And looking at the original post afresh, this isn't anything to do with light displaying particle properties. A wave conveys energy-momentum, but if you said it was you moving but not the wave, it wouldn't deliver a bump, it would be a bump and you'd say it had mass instead of momentum.

Interesting. But when it comes to mass, I think the important point is that you can make an electron out of a massless photon, and then it's got mass because it isn't moving at c any more. And that means the photon is boson enough. The mass isn't there because the electron is interacting with the Higgs field. The only thing "in there" going round and round is a 511keV photon, so the only thing it can be interacting with is itself.

Yes, this is greately more correct. I couldn't have said it better myself. It does have an inertia, but that is based upon the equations of general relativity linking acceleration and energy-momentum as being equivilent. Much the same way it relates inertia to mass and to curvature and distortions.
 

Offline yor_on

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Does light have inertia?
« Reply #20 on: 23/11/2009 03:16:03 »
Farsight? "But you can use the Compton effect to change the direction of that photon, and that's an acceleration." You're sure about that?

What 'acceleration' a photon might have will only be expressed in a different wavelength as far as I know? Like a 'higher energy' relative the observer.

And that is also a very 'relative' expression defined by you observing.
 

Offline variationz

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Does light have inertia?
« Reply #21 on: 26/11/2009 04:30:12 »

Here are the answers for your questions...

http://www.thenakedscientists.com/forum/index.php?topic=26911.0

Light has intrinsic(inherent)/relativistic mass and its velocity is relative to change in gravity.
 

Offline Farsight

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Does light have inertia?
« Reply #22 on: 28/11/2009 21:45:44 »
Thanks Mr Scientist.

Farsight? "But you can use the Compton effect to change the direction of that photon, and that's an acceleration." You're sure about that?
I don't like it, but acceleration is a change of speed or direction. So yes.

What 'acceleration' a photon might have will only be expressed in a different wavelength as far as I know? Like a 'higher energy' relative the observer.
Yes, but it changes direction too. And when you accelerate something you're typically giving it more energy. Or less, though we call that deceleration. 

And that is also a very 'relative' expression defined by you observing.
I know. If you look at this thread it's clear that issues revolve around definition, and what things mean. And when you think about it, that's pretty much the whole issue when it comes to physics. But hey, we're getting there. 
 

Offline lightarrow

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Does light have inertia?
« Reply #23 on: 29/11/2009 12:31:51 »
... But you can use the Compton effect to change the direction of that photon,...
Couldn't you do it simply with a mirror?
« Last Edit: 29/11/2009 13:10:37 by lightarrow »
 

Offline PhysBang

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Does light have inertia?
« Reply #24 on: 29/11/2009 14:04:54 »
I know. If you look at this thread it's clear that issues revolve around definition, and what things mean. And when you think about it, that's pretty much the whole issue when it comes to physics. But hey, we're getting there. 
Well, as always you forget over 99% of physics: accurate description and prediction. You are trying to determine what things mean without any understanding of how things are. How things are in physics is given using very detailed measurements and very detailed mathematical relationships between types of events and objects built up with painstaking attention to detail. You ignore all that work and all that detail. This is what gets you banned from serious forums again and again.
 

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Does light have inertia?
« Reply #24 on: 29/11/2009 14:04:54 »

 

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