Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: kaukcz on 27/05/2008 03:18:07
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since light displays particle properties does it also display the effects of inertia
[MOD: modified subject to make it a question. CS]
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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.
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Usually "inertia" refers to "inertial mass" and a beam of light has zero mass.
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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.
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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.
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With that said how exactly does light display momentum and not inertial mass, and what is the difference? Also why is this?
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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.
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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
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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.
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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
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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.
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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 (http://en.wikipedia.org/wiki/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 (http://en.wikipedia.org/wiki/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 (http://arxiv.org/abs/physics/0512265), 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 (http://en.wikipedia.org/wiki/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.
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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.
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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.
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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?
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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.
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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.
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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 (http://en.wikipedia.org/wiki/Mass#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.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2FHbase%2Fquantum%2Fimgqua%2Fcompton.gif&hash=72061dada118f5b7f78421007c6b2986)
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.
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I love this question.
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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 (http://en.wikipedia.org/wiki/Mass#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.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fhyperphysics.phy-astr.gsu.edu%2FHbase%2Fquantum%2Fimgqua%2Fcompton.gif&hash=72061dada118f5b7f78421007c6b2986)
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.
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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.
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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.
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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.
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... But you can use the Compton effect to change the direction of that photon,...
Couldn't you do it simply with a mirror?
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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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Lightarrow: a mirror isn't so clear cut, because the wavelength isn't reduced.
I don't ignore all that detail PhysBang, not at all. You do, just as you ignore what people say and contribute only spoiler bile. And as for getting banned from "serious forums" time and time again, it isn't true. Care to provide an example to back up your claim? Oh, and I don't think anybody will be impressed by your insinuation that this isn't a serious forum.
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One of the joys of mossbauer spectroscopy is that it relies on the momentum of (gamma ray) photons.
As far as I can tell the difference between inertia and momentum is simply a point of view.
If a car hits me is it my inertia that causes the damage, or the car's momentum?
photons definitely carry momentum as witnessed by spectroscopy, the compton effect and photon pressure.
I don't see how it can avoid having inertia.
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Lightarrow: a mirror isn't so clear cut, because the wavelength isn't reduced.
Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
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As for photons accelerating :)
That's a 'nono'
As for direction changes? Well, what we see after that direction change is in fact not our 'original photon', ah, mainstream seen (if 'photons' now are 'traveling' at all?)
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I don't ignore all that detail PhysBang, not at all. You do, just as you ignore what people say and contribute only spoiler bile.
Yes, how awful for people that I warn them that you tell them your own pet theories as if they were scientific gospel, theories that even you admit are rejected by mainstream scientists. And indeed, you admit that you ignore the important details, because you admit that you have done none of the relevant mathematics!
And as for getting banned from "serious forums" time and time again, it isn't true. Care to provide an example to back up your claim?
http://forum.richarddawkins.net/viewtopic.php?f=9&t=91298&p=2261402#p2261402
http://www.bautforum.com/forum-rules-faqs-information/30979-baut-banned-suspended-posters-log-23.html#post1628425
http://www.bautforum.com/forum-rules-faqs-information/30979-baut-banned-suspended-posters-log-23.html#post1632224
Oh, and I don't think anybody will be impressed by your insinuation that this isn't a serious forum.
I didn't say that this wasn't a serious forum. They may ban you, too, but banning you is not a pre-requisite for being a serious forum.
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Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
Sorry, I meant frequency. I prefer Compton scattering because it's just a free electron and a photon, it's cleaner.
As for photons accelerating, That's a 'nono'. As for direction changes? Well, what we see after that direction change is in fact not our 'original photon'
I don't like photons accelerating either. But what can you do? Re original photons, if all you've got is a photon and a free electron, there's no bond there to absorb and re-emit the photon.
Thin gruel, Physbang. A partial bar on Dawkins for mentioning a book (the irony!), and a temporary suspension on Baut for not answering questions, despite answering about two hundred. I contribute to physics discussions. Might be an idea if you did too, instead of being a stalker and a troll intent on spoiling them.
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Did you mean the wavelenght or the frequency? Anyway, if you also want to reduce the frequency it's very simple, you make the mirror recede at speed v with respect to the EM wave:
f' = f(c-v)/(c+v).
Sorry, I meant frequency. I prefer Compton scattering because it's just a free electron and a photon, it's cleaner.
As for photons accelerating, That's a 'nono'. As for direction changes? Well, what we see after that direction change is in fact not our 'original photon'
I don't like photons accelerating either. But what can you do? Re original photons, if all you've got is a photon and a free electron, there's no bond there to absorb and re-emit the photon.
Thin gruel, Physbang. A partial bar on Dawkins for mentioning a book (the irony!), and a temporary suspension on Baut for not answering questions, despite answering about two hundred. I contribute to physics discussions. Might be an idea if you did too, instead of being a stalker and a troll intent on spoiling them.
Bolded by me.
Why farsight? The idea of a photon accelerating works very well with the equivalances set by relativity... I think it's noval.
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Because if you contrive multiple Inverse Comptons (http://www.cv.nrao.edu/course/astr534/InverseCompton.html) so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!
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Because if you contrive multiple Inverse Comptons (http://www.cv.nrao.edu/course/astr534/InverseCompton.html) so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!
Because if you contrive multiple Inverse Comptons (http://www.cv.nrao.edu/course/astr534/InverseCompton.html) so that the photon ends up travelling in its original direction albeit it with a increased frequency, that means you've accelerated it. But it's still going at the same speed. It's acceleration Jim, but not as we know it!
There is always the mathematical relative speed law of motion. Acceleration of a photon is no more incomplete than having a vacuum accelerate at speeds faster than c.
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As for photons accelerating :)
That's a 'nono'
As for direction changes? Well, what we see after that direction change is in fact not our 'original photon', ah, mainstream seen (if 'photons' now are 'traveling' at all?)
That is incorrect in general. A photon can accelerate when its moving in a gravitational field. This is due to an modified coordinate speed and a change to the time it takes the photon to move.
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Maybe I was unclear.
A photon will not accelerate inside a perfect vacuum.
But you can use a laser in another medium, for example a plasma, and then witness a laser frequency upshifting that is equivalent to photon acceleration.
But the main thing here is that its speed won't change no matter what you do to it, and no matter what 'medium' you do it in as far as I know, the only thing you will change is its frequency/energy relative your frame of reference.
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Usually "inertia" refers to "inertial mass" and a beam of light has zero mass.
Reply from LukeDaniel
For me to understand this question better, I would need to know this; when someone fires a high power laser beam, does a force push back on the device that fired the laser beam, in a direction opposite of the direction of the laser beam.
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when someone fires a high power laser beam, does a force push back on the device that fired the laser beam, in a direction opposite of the direction of the laser beam.
Yes.
This was recently answered on the podcast: http://www.thenakedscientists.com/HTML/podcasts/qotw/show/20161024-1/