Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: katieHaylor on 14/05/2018 10:46:37
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Raf says:
Anyone ever consider the weight of light?
What do you think?
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Raf says:
Anyone ever consider the weight of light?
What do you think?
Is weight defined as mass times acceleration?Then light would have gravitational acceleration but no mass.
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Anyone ever consider the weight of light?
Generally light is considered to have inertia and no mass, however that is not the only view.
Peter Brown posted the following document on "On the concept of relativistic mass" thread, giving some of the different definitions of mass.
http://arxiv.org/abs/0709.0687
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Pete's contributions are always worth reading. My trouble is I tend to get lost in the maths. He's always willing to explain, but I often get more lost in the explanation. On one occasion, I had to consult Chris Baird; http://wtamu.edu/~cbaird/sq/category/physics/ to untangle my mental knots. :)
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Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.
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https://en.wikipedia.org/wiki/Weight
In science and engineering, the weight of an object, either due to gravity or to a reaction force that holds it in place.[1][2][3]
Some standard textbooks[4] define weight as a vector quantity, the gravitational force acting on the object. Others[5][6] define weight as a scalar quantity, the magnitude of the gravitational force. Others[7] define it as the magnitude of the reaction force exerted on a body by mechanisms that keep it in place:
It looks as though weight has a few definitions; chances are there is one somewhere that would fit the powering of a solar sail.
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my apologies for the rookie looking mistake of using weight when referring to mass, but the impact imparted on a solar sail is exactly the weight I'm inquiring about.
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it may be my subconscious desire to coin the phrase ' lightweight'.....
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my apologies for the rookie looking mistake of using weight when referring to mass, but the impact imparted on a solar sail is exactly the weight I'm inquiring about.
What you are looking for is not weight but momentum.
Newtons original equations of motion used change of momentum rather than F=ma. Although light does not have mass it can, like many waves, transfer momentum and hence apply a force to eg a solar sail.
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my apologies for the rookie looking mistake of using weight when referring to mass, but the impact imparted on a solar sail is exactly the weight I'm inquiring about.
What you are looking for is not weight but momentum.
Newtons original equations of motion used change of momentum rather than F=ma. Although light does not have mass it can, like many waves, transfer momentum and hence apply a force to eg a solar sail.
Is there a relationship between momentum and mass?
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Momentum is mass times velocity. Mass is an inherent part of momentum. This is not true for photons, where it is energy that is important. Effectively, E/c is the momentum of the photon. Where E is the energy of the photon.
https://en.m.wikipedia.org/wiki/Photon_energy
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perhaps my rudimentary understanding of physics is not allowing me to see the "light"...... but is not mass and energy the same thing?
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e=mc squared, therefore mc squared=e...........
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Mass is measured in kilograms and energy is measured in joules. They are not the same thing. You have rest energy, which is based on mass, and equals m times c^2. This is not a relativistic term. It gets more complicated when relativity is involved.
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So e=mc squared is not correct? I thought the equation meant that matter is just cold energy. ....
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x amount of joules= 10bs iron times speed of light in a vacuum squared
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...... but is not mass and energy the same thing?
No, they are clearly not the same thing. A lump of rock is very different to the kinetic energy created by a moving object.
They are however equivalent as described by E=mc2
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This is another case of things that are equivalent not necessarily being identical. A bit like gravitational force and spacetime curvature, perhaps.
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then the flaw is the equal sign, an equivalent sign should be used instead. .....?
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You cannot state that E = m, where E is energy and m is mass. The units are not equivalent. That is why kinetic energy is the product of mass and the square of the velocity the object is travelling at. Rest energy is slightly different. Instead of a velocity we use the speed of light. This is not a vector quantity. Squaring a vector gives the square of a magnitude which is a scalar. The speed of light is already a scalar. It has no directional component. Therefore the square of the speed of light is defining an energy at rest.
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then the flaw is the equal sign, an equivalent sign should be used instead. .....?
No, there is no flaw.
Energy is a measure of the ability of something to do work, it is not a thing in itself. So kinetic energy = 1/2 mv2. In the case of E=mc2, E can be thought of as the amount of energy tied up in the creation of the mass.
The = sign is used because E is the measure of the energy contained in the mass. A liquid can be measured in litres, but the volume is not the liquid, it is just one of the measurements you can take.
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I'm just a poor country boy struggling with my limited intelligence and education, looking up at the stars and wondering about things.
I'm going to have to assume that I do not comprehend e=mc^2.
if supposed rock, whether it is falling or at rest, were to undergo total mass to energy conversion, would the energy output not justify the equation? like when two hydrogen atoms fuse to form helium. ..... 1 plus 1 does not quite equal 2, or am I misinformed that there is a small amount of mass that is ' lost ' and the only thing on other side of the equal sign are photons? surely a total conversion would have far more output. ...I appreciate y'all taking the time too help remedy my remedial understanding of what little I grasp.
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Raf says:
Anyone ever consider the weight of light?
What do you think?
Yes. Since photons have energy they have an equivalent amount of mass. Actually its by the photons momentum that it has inertial mass. Others here are referring to a photon's proper mass, not their inertial mass, when they claim its zero.
For a good discussion on this by an authority in particle physics and cosmology please watch:
http://www.newenglandphysics.org/common_misconceptions/Alan_Guth_01.mp4
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perhaps my rudimentary understanding of physics is not allowing me to see the "light"...... but is not mass and energy the same thing?
No.
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no to which question or all? and how does inertial mass differ from proper mass? do they react in any similar fashions?
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nevermind, I misread your response, I thought you were answering no to a different set of questions.
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I'm worried I will create a braching point in my questioning. .... but what the heck, full stream ahead. ..... first, if some mass is "lost" during a stars fusion process, does this mean that the universe is losing weight? would this "weight loss" explain why models of the early universe do not work correctly without the addition of more matter then we can account for?
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and the tip of the second question. ....... how much volume is in a.....say 4.24 light year radius sphere? how much volume of photons is that? if they have even the most miniscule amount of mass. ........................
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even at the surface of said sphere, and assuming light is massless, it can still impart inertia, giving everthing at said surface a gentle little push. .....
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if supposed rock, whether it is falling or at rest, were to undergo total mass to energy conversion, would the energy output not justify the equation? like when two hydrogen atoms fuse to form helium. ..... 1 plus 1 does not quite equal 2, or am I misinformed that there is a small amount of mass that is ' lost ' and the only thing on other side of the equal sign are photons? surely a total conversion would have far more output.
Fortunatly the processes in an atomic bomb are very inefficient. For the fusion bomb dropped on Hiroshima it is estimated that the amount of matter converted to energy was about 700 milligrams, less than one-third the mass of a U.S. dime. If the total mass of the bomb had been converted the result would have been earth shattering - literally.
This energy comes from the binding energy that has to be put in if you want to push the components (protons, neutrons) together to make Uranium, split those components apart and you release energy; however the energy you add also adds to the mass so the uranium atom has more mass than the individual components that make it up. The energy released is mainly heat, shockwave, and some radiation, so it’s really a case of energy changing the form it is held in.
if some mass is "lost" during a stars fusion process, does this mean that the universe is losing weight? would this "weight loss" explain why models of the early universe do not work correctly without the addition of more matter then we can account for?
The light doesn’t leave the universe, we see some of it, it just gets redistributed.
how much volume is in a.....say 4.24 light year radius sphere? how much volume of photons is that? if they have even the most miniscule amount of mass. ........................
The ‘volume’ of photons will depend on the intensity of the source and how long it keeps emitting. As I said earlier and as @PmbPhy says, they don’t have mass in the classical sense but have momentum .
It’s worth looking at the link he gave you. On the same site is a very good article on energy which you should read, you’ll find it helpful.
even at the surface of said sphere, and assuming light is massless, it can still impart inertia, giving everthing at said surface a gentle little push. .....
Yes. The voyager spacecraft had to have its course adjusted to allow for the push of photons from the sun. Remember though that the sun is close and big, intensity drops off as inverse square law so at the edge of your 4.24 ly sphere there won’t be any noticable push.
Just a point of forum etiquette. If you post a reply and you want to add to it, but no one else has replied in the meantime, can you modify your post rather than post a series of separate posts. Thanks.
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Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.
There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.
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my apologies for my lack of etiquette, I'm completely lacking in any experience using this forum type of thing, this was my first go ast something like this. many apologies for any unintended rudeness.
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Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.
There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.
And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?
I am wondering whether all these "things" such as energy ,momentum ,mass (what have I missed?) are all self referential and define each other..
I know the Higgs Field is supposed to give mass to a particular class of particles. Is that the best definition?
Mass is defined as resistance to momentum change isn't it?
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As I said earlier and as @PmbPhy says, they don’t have mass in the classical sense but have momentum .
I didn't say that. Photons do have mass in the classical sense since classically, m i.e. as Newton defined mass, m = p/v which is a classical expression. You'll note that all texts which use relativistic mass define it that way. The classically defined mass is jus another name for relativistic mass.
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so...... mass as defined as defined as resistance to momentum change would apply to light striking a surface and applying a minuscule push? then in the instance of my supposed 4,24 light year radius sphere, (distance to nearest star other then sol ), however hard to measure, would exist. multiply that by the number of stars in our galaxy times the number of known galaxies, and this number might add up to a tremendous amount of "light weight" slowly pushing things apart. I have no idea how photons react when headed in opposite directions, so I'll not speculate that that might add too it.
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I didn't say that. Photons do have mass in the classical sense since classically, m i.e. as Newton defined mass, m = p/v which is a classical expression.
You’re right Pete, me misusing the word classical.
...multiply that by the number of stars in our galaxy times the number of known galaxies, and this number might add up to a tremendous amount of "light weight" slowly pushing things apart.
You can answer your own question if you look at the night sky. How bright is it? Can you bear to look at the stars?
Now try looking at the sun in daylight. Actually, don’t because it will damage your eyes which gives you an idea of the amount of power reaching the earth. But, is the sun pushing the solar system apart? No, so what chance for weak starlight?
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Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.
There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.
And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?
I am wondering whether all these "things" such as energy ,momentum ,mass (what have I missed?) are all self referential and define each other..
I know the Higgs Field is supposed to give mass to a particular class of particles. Is that the best definition?
Mass is defined as resistance to momentum change isn't it?
Mass can be defined in terms of the Planck values. I worked this through a long time back. The equations are posted somewhere in new theories I think. This can ultimately lead to a mass number. This would be an integer value, and when used with the Planck action h, would lead to quantised mass.
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in response to Colin's last statement, (I still don't have this forum thing down yet, ain't figured how to copy quotes, please forgive), please add time to that equation, I overlooked that. a small hammer can drive a big nail given enough time to peck at it. as far as our sun being brighter then starlight, perhaps being caught in the suns gravity well can account for us not being pushed away. ....although the sunlight was recently used too flip over some sample vessel Japan sent to an asteroid somewhere, and this occurred inside our solar system. btw, all you people rock, I really appreciate the time y'all have invested discussing these things. they are probably all very trivial to y'all, but provide a lot of cool things to think about. or hot I reckon, if the subject wanders towards stars. ...
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And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?
I already said it. Mass is defined so that momentum us conserved (At least in SR). To be precise we sat that we define m so that the vector quantity mv is conserved. We then call the vector p = mv the momentum of the particle. If you try to go beyond this then I'm sure you can but then its metaphysics, not physics.
Mass is defined as resistance to momentum change isn't it?
Yes.
Mass can be defined in terms of the Planck values. I worked this through a long time back. The equations are posted somewhere in new theories I think. This can ultimately lead to a mass number. This would be an integer value, and when used with the Planck action h, would lead to quantised mass.
Please show me. Expressing one term in terms of another cannot be used as a definition.
Please watch the train of thought here. Hoping from the classical domain of SR to quantum mechanics you just did can lead to nonsense talk, For example, I can speak of velocity and acceleration as well as a trajectory but when switching to QM all that is meaningless.
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as far as our sun being brighter then starlight, perhaps being caught in the suns gravity well can account for us not being pushed away. ....
Good to think about that and it could be true if light intensity and gravitational force varied in different ways, however, they both reduce with distance as the inverse square law so the effects of both diminish together at the same rate.
There are some questions as to whether gravity varies to a different model at extreme distances or at extreme intensities, but in the case we are considering that is unlikely to be a factor, but we should keep an open mind.
PS don’t worry too much about the posting protocols, the moderators and other members will help you through and you’ll soon get the hang of it. The protocols are there to help you, for example if you post a series of ideas or questions as separate posts it’s likely only the last one will be read, so folks can miss what you are asking.
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I think i understand the inverse square law thing. sorta like shooting a shot.... further away you are from said target, the wider your shot pattern gets, i.e. less pellets per square inch. .....your comments have made me think of another question pertaining to light and gravity. does gravity"travel" at the same speed as light? I'm not referring to its effect on mass (9.7 meters per second per second or whatever that acceleration is), but the actual speed it operates at. there's not a physical rubber band connecting us to the sun, it's some kind of force. were the sun to be removed, would it take 8 minutes for the effect of gravity (or lack of) to be observed? or longer? surly it cannot be shorter? like magnets, gravity really trips me out, where does all that energy come from to make such things operate apparently indefinitely? gee, I haven't even finished my second cup of coffee. well, time to get to work, I'll check in on break.
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Hitch-hiker level understanding.
does gravity "travel" at the same speed as light?
Slightly misleading wording, as it suggests that gravity travels. Obviously you are aware of this, as you later refer to the effects of gravity travelling.
If the sun were removed, its light and gravitational attraction would vanish at the same time.
where does all that energy come from to make such things operate apparently indefinitely?
I struggled with this for a long time. Reading Mark McCutcheon’s “The Final Theory” compounded the problem, but it’s an interesting read. 😊
I think I got there in the end, and would be happy to share some thoughts, but there are others who would probably do a better job.
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Hitch-hiker level understanding.
does gravity "travel" at the same speed as light?
Slightly misleading wording, as it suggests that gravity travels. Obviously you are aware of this, as you later refer to the effects of gravity travelling.
If the sun were removed, its light and gravitational attraction would vanish at the same time.
where does all that energy come from to make such things operate apparently indefinitely?
I struggled with this for a long time. Reading Mark McCutcheon’s “The Final Theory” compounded the problem, but it’s an interesting read. 😊
I think I got there in the end, and would be happy to share some thoughts, but there are others who would probably do a better job.
Changes in the gravitational field propagate at the speed of light, This was proved experientially recently at LIOGS. Among others. Kip Thorne won the Nobel Prize because of it.
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See:
Apparent weight of photons by Pound and Rebka, Physical Review Letters. 4 (7): 337–341.
You might be able to download if from here
http://booksc.org/book/21582115/7c47c0
The experiment is described here. It's a very famous one
https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
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Pbm Phy, I'm obviously aware of very little. I have zero education in physics, and can only remember a little from algebra about speeds and acceleration formulas, high school was a long time ago. I'm just a hack observer that has finally found a place to ask questions that people don't laugh at. most of this stuff I ask is probably mundane extremis to everyone here. I am curious of the difference in definition between propagate and travel, I will look this up next break if I remember.
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Pbm Phy, I'm obviously aware of very little. I have zero education in physics, and can only remember a little from algebra about speeds and acceleration formulas, high school was a long time ago. I'm just a hack observer that has finally found a place to ask questions that people don't laugh at. most of this stuff I ask is probably mundane extremis to everyone here. I am curious of the difference in definition between propagate and travel, I will look this up next break if I remember.
When it comes to things like this my physics advisor/mentor made me look in a dictionary. Start there.
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like a physics dictionary? I'm not sure I could afford something like that right now but I'll keep my eyes open at goodwill.... is there an online one possibly? not that I think I could understand something like that, I barely grasp half of what's said here and the large amount may as well be Greek to me. .....very cool that gravity operates at the same speed as light, for some reason it seemed to me that they had to, almost like gravity was some weird anti electromagnetic radiation. ...does a magnetic field have the same property?
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sadly, I cannot seem to download the "apparent weight of photons" and I can't figure out how to edit my last post or do the quote thing y'all do.
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Changes in the gravitational field propagate at the speed of light, This was proved experientially recently at LIOGS. Among others. Kip Thorne won the Nobel Prize because of it.
hey, think I got the quote thing
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dictionary didn't help with propagate, talked about seeds and ideas. ....
ok, please correct me if I'm wrong and forgive me if I use simple language. ... light and gravity share a common quality. .... for lack of a better term, they both "manifest" themselves in a similar fashion, propagating at the same velocity. ....
gravity can affect light, whether by curving space or just tugging on it. ....does light have a similar interaction with gravity? the whole "for every action there is an equal and opposite reaction" bit. in a black hole, light is supposedly affected, it gets "stuck".......oh I have a bunch of stuff going oin my head now
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I would submit to you that you didn't quite get the "quote thing" right yet. Try to eliminate the "/" in the first use of the word quote.
ok, please correct me if I'm wrong and forgive me if I use simple language. ... light and gravity share a common quality. .... for lack of a better term, they both "manifest" themselves in a similar fashion, propagating at the same velocity. ....
That seems to be the case. I suppose that speculating about why that is the case would be on the boarder line for the type of content permitted in this particular sub-forum, but lets speculate about a possible explanation to why light and gravity could traverse space at the same velocity.
If the photon is a particle with mass, and if all particles with mass emit gravitational waves, then the out flowing gravitation wave energy from all particles and objects, including photons, would be called gravitational wave energy, and would traverse space at the same velocity, the speed of light. That would make the light of the electromagnetic spectrum the outflowing gravitational wave energy of the photon.
gravity can affect light, whether by curving space or just tugging on it. ....does light have a similar interaction with gravity? the whole "for every action there is an equal and opposite reaction" bit. in a black hole, light is supposedly affected, it gets "stuck".......oh I have a bunch of stuff going oin my head now
Hmm, many things to ponder.
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bogie smiles,
yes, epic fail trying to quote.
I will not even pretend to understand what you just said, perhaps I should change my name to "star struck Neanderthal with a phone".
is what your are saying implying that light has both mass and gravity? if either or both are true, could I mention my 4.24 light year radius sphere, and again ask what effect that volume of photons has on its surrounding space over billions of years? most see a star as a pin point of light, but I think otherwise. I might be wrong, but I don't think you can see light from the side, only the photons headed your way. I speculate the photons headed at right angles to ones observation point are "unseen". but nonetheless, very much there. but again, I'm not well educated and might be missing something. ...
but if correct, this its a huge volume of unobserved photons, and if they have mass (does mass indicate that they would have gravity? ) then regardless of the inverse square law, this is a tremendously large amount of "material" affecting its environment. However minutely, over millions of years surely this must amount to something, either with the push imparted by a photons velocity, or a pull from the photons mass. multiplied by the amount of estimated stars, makes for quite a wrestling match. .....unless they just canceled each other out. ........ugh. ......
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…
is what your are saying implying that light has both mass and gravity?
In case this discussion begins to get too “new theories-like”, it may not be appropriate for this sub-forum, where the consensus view of science is the first order, and discussions that are speculative can be subject to close scrutiny, so be aware of that.
What I said was presented as a speculation that could explain how both light and gravity travel at the speed of light. To answer your next question, “is what you are saying/implying is that light has both mass and gravity?”, then that would be a logical conclusion, if the given the circumstances were true, i.e., that if photons were particles with mass, and if particles and objects with mass all emitted gravitational waves, then light would be gravitational wave energy emitted by photons, and would have frequencies in the range of the electromagnetic spectrum.
A soft ball would have a higher frequency than a gamma ray. That is why they don’t let softballs travel at the speed of light; too dangerous. But yes, given the scenario I speculated about, light would have weight, just like a softball has weight, if we define “weight” as the mass of an object being accelerated in a gravitational field.
if either or both are true, could I mention my 4.24 light year radius sphere, and again ask what effect that volume of photons has on its surrounding space over billions of years? most see a star as a pin point of light, but I think otherwise. I might be wrong, but I don't think you can see light from the side, only the photons headed your way. I speculate the photons headed at right angles to ones observation point are "unseen". but nonetheless, very much there. but again, I'm not well educated and might be missing something. ...
but if correct, this its a huge volume of unobserved photons, and if they have mass (does mass indicate that they would have gravity? ) then regardless of the inverse square law, this is a tremendously large amount of "material" affecting its environment. However minutely, over millions of years surely this must amount to something, either with the push imparted by a photons velocity, or a pull from the photons mass. multiplied by the amount of estimated stars, makes for quite a wrestling match. .....unless they just canceled each other out. ........ugh. ......
I’m not going to try to get into all of that with you, but there are answers to that in the consensus theories, and someone more qualified than I am in those areas may jump in and help you.
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please speculate away if that is within the guidelines of this forum, as all I'm doing with my limited intelligence and education is speculating. ..... but if I'm inn contradiction to any rules regarding my primitive attempt at gaining knowledge, then perhaps you could redirect me to amore appropriate place to speculate. that would make me a bit sad, I've thoroughly enjoyed this whole experience here, and learned tremendous things, like gravity propagating at light speed. .....my apologies if I've caused disruption.
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I think you are fine here. It is just that I am a frequent poster in the New Theories sub-forum, and it is out of respect for the moderators and administrators that I try to be clear about what is appropriate in this sub-forum, vs what is appropriate in the "lighter side" sub-forums.
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dictionary didn't help with propagate, talked about seeds and ideas. ....
See https://en.oxforddictionaries.com/definition/propagate
[with adverbial of direction] (with reference to motion, light, sound, etc.) transmit or be transmitted in a particular direction or through a medium.
[with object] ‘electromagnetic effects can be propagated at a finite velocity only through material substances’
Try googling "<word> definition oxford"
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I am curious of the difference in definition between propagate and travel, I will look this up next break if I remember.
Although I agree with @PmbPhy sometimes I find dictionaries can be less than clear.
Consider a car, you can say it travels, but clearly it does not propagate ie make a new copy, or object.
However, waves do propagate. When you see waves approaching the shore it looks as though the water is moving, this is an illusion. The water does not flow as in a current, but the height of the wave is passed on (propagated, created anew) from one point to another. This is also true of sound and light waves.
sadly, I cannot seem to download the "apparent weight of photons" and I can't figure out how to edit my last post or do the quote thing y'all do.
The paper is rather technical, start with wiki.
I’ll send you a pm on editing
.....if all particles with mass emit gravitational waves
But, they don’t. Only accelerating masses emit gravitational waves and photons are not accelerating.
Also gravitational waves do not behave like em waves, very different properties, very different phenomena.
I might be wrong, but I don't think you can see light from the side, only the photons headed your way. I speculate the photons headed at right angles to ones observation point are "unseen". but nonetheless, very much there. but again, I'm not well educated and might be missing something. ...
You are right, only light reaching your retina, or a sensor are detected. However, if you move around a star you can check that light is emitted in all directions.
but if correct, this its a huge volume of unobserved photons, and if they have mass (does mass indicate that they would have gravity? ) then regardless of the inverse square law, this is a tremendously large amount of "material" affecting its environment. However minutely, over millions of years surely this must amount to something, either with the push imparted by a photons velocity, or a pull from the photons mass. multiplied by the amount of estimated stars, makes for quite a wrestling match. .....unless they just canceled each other out. ........ugh. ......
No they don’t cancel out, but the critical point is where you say “regardless of the inverse square law”. I’m sorry but you can’t disregard it, it is key to understanding why the force at a distance becomes negligible regardless of how much you start off with.
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like a physics dictionary?
Do such things exist. I use oxford. But in all cases I first look in a physics text. They don't always define things though such as these two terms since they're commonly used terms.
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perhaps I worded that poorly. .... I didn't say disregard the inverse square law, I said regardless, and intended the use of regardless to acknolage the law exists, and possible effects could co exist.....
.neg·li·gi·ble
ˈneɡləjəb(ə)l/Submit
adjective
so small or unimportant as to be not worth considering; insignificant.
taking this word as classically defined, I'm surprised it's used in science. if the amount of stuff 4.24 light years away becomes so insignificant it should be regarded as nothing, then we shouldn't be able to see other stars, the photons would be negligible. ...no?
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.....if all particles with mass emit gravitational waves
But, they don’t. Only accelerating masses emit gravitational waves and photons are not accelerating.
Can we say that any two objects in relative motion are accelerating relative to each other?
Also gravitational waves do not behave like em waves, very different properties, very different phenomena.
Can we say that photons are emitted by electrons, and are traveling at the speed of light when emitted?
Agreed, that certainly puts them in a different class from other particles with very different properties, and evidences that they are very different phenomena. But can we say that there is gravitational wave energy traversing space to and from all directions at all points in space?
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Can we say that any two objects in relative motion are accelerating relative to each other?
My understanding is that acceleration requires a change of speed or direction. One or both of these would be necessary in order to say that "two objects in relative motion are accelerating relative to each other".
Presumably, two objects that were circling each other at constant speed would qualify. (?)
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I'm surprised it's used in science. if the amount of stuff 4.24 light years away becomes so insignificant it should be regarded as nothing, then we shouldn't be able to see other stars, the photons would be negligible. ...no?
Here we are talking about 2 different things, the visibility of photons and the transfer of momentum. The eye can see even a few photons, but the momentum they transfer is negligible. They won’t push you over.
Negligible is an important word in physics, no use spending time calculating something you can ignore. Take a car, you know it’s weight (hence mass) and speed so you can calculate its momentum. If a fly lands on the car the effect on the momentum is negligible, no need to recalculate. If an elephant lands on the car .........
Key rule of science, recognise what is significant.
Can we say that any two objects in relative motion are accelerating relative to each other?
Not any, only some. The relative motion has to be an acceleration.
Can we say that photons are emitted by electrons, and are traveling at the speed of light when emitted?
We can
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Not any, only some. The relative motion has to be an acceleration.
Just semantics to say that all motion is acceleration to a greater or lesser degree and that unaccelerated motion is an idealization ,a mathematical limit?
Like your fly on the car it may be true but is of no consequence?
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I somewhat like the car analogy. ....perhaps I could pose a few stipulations to said scenario. ....
1)said car is driving in a vacuum
2)car has a set of alien technology frictionless tires.
3)the fly is immortal, is able to fly inn a vacuum, and lands oon car with same velocity or impact every time.
4)fly is very determined and lands oon car one million times over the next 10000 years.
5) fly weight is .01 grams.
does the accumulating impacts add up to the same impact a 22lb turkey hitting a vehicle one time?
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does the accumulating impacts add up to the same impact a 22lb turkey hitting a vehicle one time?
I can't speak for geordief, but I will try to test my understanding of his point: the fly has a tiny mass relative to the mass of the car. There is a mutual gravitational attraction between the fly and the car, but the effect of the presence of the fly on the motion of the car is inconsequential.
I'm not sure of the physics involved with the fly leaving and returning multiple times, but if the departure is not a push off, and if the return is a soft landing, hmm, maybe someone can explain the accumulative effect on the path of the car in space as a result of multiple tiny gravitational attractions from the repeated close proximity of the fly, in comparison to the collision with the turkey.
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assume fly falls off, and flies a long loop Aaron's to land on car in had on collision approach
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Can we say that any two objects in relative motion are accelerating relative to each other?
Not any, only some. The relative motion has to be an acceleration.
There is always a gravitational potential between two objects, though often it is negligible? Can we say that the gravitational potential between two bodies would be the mutual effect they have on each other’s spacetime curvature, i.e., their geodesics?
If so, am I right to think that their mutual gravitational attraction qualifies as an acceleration, and is included in the calculation of the geodesics that they will follow?
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does the accumulating impacts add up to the same impact a 22lb turkey hitting a vehicle one time?
Is the turkey frozen?
Apparently, one aircraft manufacturer, using chickens for impact tests, forgot to thaw the birds before firing them at the aircraft. Their results were quite different from anyone else's. OK, that's off topic, but I like it.
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i had not considered if the turkey was frozen. .... I would think that would be a slight, but, "negligible" density change. Although definitely a hardness change. I'm not sure how off subject that could be considered, I'm the one talking about flies and turkeys flying around in a vacuum hitting alien cars.
I understand that the single fly impacting the car would have thus said negligible effect. .... what can
drop of water do? nothing (negligible) ..... yet over time things like the grand canyon, and probably even our oceans them selves are created. I have no interest in what one or 2 photons do to an object far away. I'm going with the notion that millions of years of an endless barrage of photons, however inversely squared, must amount to something. I'm sure the math could be done by someone much smarter then me. I'll attempt to explain, but practically guarantee I'm probably not going too use the correct terms, so I'll take the liberty to try to think like someone smarter then i and create terms i think might be close enough.
example, the light sail, theoretically able to use light to get pushed, need a relatively high "photon density " striking it to operate in a usable manner. I'll make up a number, let's say out needs 1 million photons per second per centimeter striking its surface to created 1g acceleration (I know those numbers are probably way off )
each photon has same impact energy, correct? then could it not be said that 1 photon per second per centimeter would create 1 millionth of a g of thrust? would the inverse square law balance the gravity of the two different light sails from a single light source so the both will accelerate, one just taking a million times longer to accelerate?
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on a cosmic scale, a million of anything is probably a negligible number
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I'll make up a number, let's say out needs 1 million photons per second per centimeter striking its surface to created 1g acceleration (I know those numbers are probably way off )
each photon has same impact energy, correct? then could it not be said that 1 photon per second per centimeter would create 1 millionth of a g of thrust?
Yes, assuming you meant "square centimeter".
would the inverse square law balance the gravity of the two different light sails from a single light source so the both will accelerate, one just taking a million times longer to accelerate?
I'm not quite sure what you mean here. Are you asking if the gravitational attraction between the two solar sails would significantly impact how much acceleration the light gives the sails? That would depend on the mass and distance between the sails, as well as the energy of the light hitting them.
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my apologies for my vagueness, yes, I meant square centimeter. also, I didn't describe sails and conditions very well. assume sails are identical in all aspects except distance from light source. assume the further sail is so far away it only receives 1 photon per square centimeter. I realize these numbers only work for an instant, as soon as the sail moves any distance, acceleration will decrease, and probably proportionally according to inverse square law. .....and I'm assuming that gravitational attraction will behave the same. but I'm am just a primitive guessing at this.
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my apologies for my vagueness, yes, I meant square centimeter. also, I didn't describe sails and conditions very well. assume sails are identical in all aspects except distance from light source. assume the further sail is so far away it only receives 1 photon per square centimeter. I realize these numbers only work for an instant, as soon as the sail moves any distance, acceleration will decrease, and probably proportionally according to inverse square law. .....and I'm assuming that gravitational attraction will behave the same. but I'm am just a primitive guessing at this.
If you are considering the light source to be a star and the gravity is coming from that same star, then yes, the force of gravity and the acceleration caused by the light pressure will fall off at the same rate. They both follow the inverse-square law.
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then am I way off base asking what effect light has 4.24 light years away from an object, over millions of years, multiplied by trillions of stars, combined with the process repeating for billions of years with new stars replacing dying stars? 4.24 ly is distance to nearest star. granted, each photons effect is negligible, but the accumulated effect must amount to something. if we can see a photon, then it must affect an object, however miniscule. surely this must add up to a phenomenal amount of "light pressure"........ and if light has mass as some theorize, could this be the missing mass needed to make the models of the universe work correctly? ..........perhaps this material is not so dark after all. .....
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then am I way off base asking what effect light has 4.24 light years away from an object, over millions of years, multiplied by trillions of stars, combined with the process repeating for billions of years with new stars replacing dying stars? 4.24 ly is distance to nearest star. granted, each photons effect is negligible, but the accumulated effect must amount to something. if we can see a photon, then it must affect an object, however miniscule. surely this must add up to a phenomenal amount of "light pressure"........ and if light has mass as some theorize, could this be the missing mass needed to make the models of the universe work correctly? ..........perhaps this material is not so dark after all. .....
I'll see what I can come up with calculation-wise, but first I'd like to point out that (as shown by E=mc2) that energy already has an associated mass and therefore a gravitational field all its own. I'm sure scientists would have taken into consideration this fact when calculating how galaxy rotation curves should look. The anomaly remains.
Just for fun, let's see how much radiation pressure Proxima Centauri exerts on Earth. The power flux of the Sun at 1 AU is 1,360.8 watts per square meter. Proxima Centauri has a bolometric (total) luminosity of only 0.0017 times that of the Sun, resulting in a power flux of 2.31336 watts per square meter 1 AU away from it. Proxima Centauri is 4.246 light-years away, which is equal to 268,521.61 AU. Using the inverse-square law:
1/(268,521.61)2 = 1/(72,103,855,037) = 1.3868884 x 10-11
Multiply this by the power flux:
(1.3868884 x 10-11)(2.31336) = 3.2083721 x 10-11 watts per square meter
There are two different pressure equations that could be used now: one that assumes all the light is absorbed and one that assumes all the light is reflected. Reflecting all of the light doubles pressure. Earth, however, neither absorbs nor reflects all electromagnetic radiation. So instead, I will assume that the Earth reflects 30.6% of all radiation back into space (based on its bond albedo of 0.306). So to put that into equation form:
P = ((1.306)(I))/c
P = ((1.306)(3.2083721 x 10-11))/299,792,458
P = 4.190134 x 10-11/299,792,458
P = 1.3976783 x 10-19 pascals (a very small pressure)
Since the Earth's mean radius is 6,371,000 meters, that would make its total cross-sectional area:
A = π(6,371,000)2
A = π(4.0589641 x 1013)
A = 1.2751612 x 1014 square meters
Multiply that by the pressure to get the total force on the Earth:
F = (1.2751612 x 1014)(1.3976783 x 10-19)
F = 0.00001782265 newtons
To find the acceleration this force gives the Earth, we divide this force by the mass of the Earth:
Acceleration = 0.00001782265/(5.97237 x 1024)
Acceleration = 2.984184 x 10-30 meters per second squared
To get the total increase in velocity that this would have given the Earth over its entire 4.5 billion-year (1.42 x 1017 second) lifespan, we multiply the acceleration by the age:
Velocity = (1.42 x 1017)(2.984184 x 10-30)
Velocity = 4.2378158 x 10-13 meters per second (extremely slow)
So even after 4.5 billion years of shining on the Earth, Proxima Centaur's light still has a negligible effect on Earth's speed.
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There is always a gravitational potential between two objects, though often it is negligible? Can we say that the gravitational potential between two bodies would be the mutual effect they have on each other’s spacetime curvature, i.e., their geodesics?
If so, am I right to think that their mutual gravitational attraction qualifies as an acceleration, and is included in the calculation of the geodesics that they will follow?
Depends what you are considering.
In SR the assumption is flat spacetime far away from gravitational influence and small objects far enough apart for the gravitational attraction to be negligible. So non-accelerating motion is possible.
GR considers spacetime under the influence of gravitational forces including bodies where there is significant mutual attraction, so as you say, there will be relative acceleration included.
then am I way off base asking what effect light has 4.24 light years away from an object, over millions of years, multiplied by trillions of stars, combined with the process repeating for billions of years with new stars replacing dying stars? 4.24 ly is distance to nearest star. granted, each photons effect is negligible, but the accumulated effect must amount to something. if we can see a photon, then it must affect an object, however miniscule. surely this must add up to a phenomenal amount of "light pressure"........ and if light has mass as some theorize, could this be the missing mass needed to make the models of the universe work correctly? ..........perhaps this material is not so dark after all. .....
EDIT: while I was typing @Kryptid posted the one above, so this is to be read with what he wrote.
There are 2 things to consider, the ability of an object to move and the ability of something to move it.
Elsewhere there is a question about whether a rocket taking off from the moon can change the moon’s orbit and the same principles apply if we consider some examples.
- Starlight falling on the earth is of very low intensity due to inverse square law. However, the earth has a large surface area and so can collect a lot of starlight, but each photon of starlight carries an unbelievably small amount of momentum and the earth’s inertia is phenomenal - even large asteroids won’t kick it off orbit. So the earth does not move.
- a solar sail has a very large surface area, so can collect a lot of photons, it also has a very low mass (inertia) so near a large source of light eg the sun, it will move. As it gets further away the effect diminishes and the drive will stop working, also there comes a point where the sail is equidistant from 2 stars and the pressure from both would cancel out.
- take interstellar dust and gas. Very low mass, easy to move, but small cross sectional area so it won’t collect many photons and even then their momentum is minuscule compared to the inertia of the particles. Yes, over time they might move, but remember, wherever you are in the universe you are surrounded by stars so the net pressure on this interstellar material is zero. Question: would that, over time, lead to any form of gas/dust clumping and can it be detected? Would that clumping force be greater than the ability of gravity from nearby stars to attract the dust/gas, I suspect not.
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The thought that came to mind when reading #68 has probably been adequately covered, but I’ll add it to the mix.
If you assume that the number of photons per sec, per sq cm required to overcome the inertia of the sail is 1,000,000; then 1 per sq cm per sec will not move the sail. Presumably the energy of the photon will be dissipated as heat.
Therefore there is a reciprocity failure between 1,000,000 photons per sec, and 1 photon per sec x 1,000.000 secs.
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wow, very informative. .....I'm way out of my league here, obviously.
after reading the responses to my post, I did wonder about something else. do photons traveling in opposite directions in the same space affect each other? can they collide? it doesn't seem like it should, but I'm confident I've already proven my limited understanding of things. one other thing. .... if two photons travel next to each other for millions of years, will they remain on a parallel path? or could the tiny mass draw them closer to each other? or does a photon carry a charge, and being identical, repel each other?
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wow, very informative. .....I'm way out of my league here, obviously.
after reading the responses to my post, I did wonder about something else. do photons traveling in opposite directions in the same space affect each other? can they collide? it doesn't seem like it should, but I'm confident I've already proven my limited understanding of things.
In most cases, they will pass through each other and keep going on their merry way. Every once in a long while, though, they can scatter off each other: https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.111.080405 (https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.111.080405)
one other thing. .... if two photons travel next to each other for millions of years, will they remain on a parallel path? or could the tiny mass draw them closer to each other? or does a photon carry a charge, and being identical, repel each other?
The photons will slowly approach each other over time, since their energy produces a tiny gravitational field. They do not have any charge.
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interesting that they attract each other. ... should I assume the effect has negligible impact on the inverse squire law?
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interesting that they attract each other. ... should I assume the effect has negligible impact on the inverse squire law?
It would have no impact on the inverse square law. Any given photon being emitted from a point source will have other photons on all sides of it, meaning there is no preferential gravitational pull in any given direction.
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The photons will slowly approach each other over time, since their energy produces a tiny gravitational field.
It would be interesting to know how that impacts on the argument that photons "experience" neither time nor distance.
Two photons that had travelled together for 1k years, in our FR, isolated from other photons, would be closer together at the end of that time than at the start. If the time and distance are relevant only in our RF, what does that say about any measurements of the separation might be taken.
I realise that taking measurements tends to have a destructive influence on photons, but just thinking.
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could this attraction of photons be tested with lasers? fire one laser at a target, then fire another laser across the top very close at a 90 angle? then shutting one laser off and seeing if the light "bends " back? perhaps 90 degrees creates to short an interaction window, perhaps almost parallel, say cross the beams at an extremely acute angle, like a couple moa or even seconds of degree. ...
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It would be interesting to know how that impacts on the argument that photons "experience" neither time nor distance.
Two photons that had travelled together for 1k years, in our FR, isolated from other photons, would be closer together at the end of that time than at the start. If the time and distance are relevant only in our RF, what does that say about any measurements of the separation might be taken.
I realise that taking measurements tends to have a destructive influence on photons, but just thinking.
I'm not sure how a curved path for a photon in our reference frame would look in the photon's frame. We know it has to work somehow, since gravitational lensing is a confirmed phenomenon.
could this attraction of photons be tested with lasers? fire one laser at a target, then fire another laser across the top very close at a 90 angle? then shutting one laser off and seeing if the light "bends " back? perhaps 90 degrees creates to short an interaction window, perhaps almost parallel, say cross the beams at an extremely acute angle, like a couple moa or even seconds of degree. ...
I don't know if we have the needed sensitivity to perform such an experiment on Earth. If it was up to me, I'd prefer to perform the experiment in space where two laser beams fired parallel to each other could travel for millions of miles before hitting the detector.
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I had wondered if it would be able to be detected in the small space in earth available to play with, and that led me to thinking the experiment would possibly work better with two different strength lasers, themain laser having a high photon density, and measurements taken from a weaker laser fired "across the bow" so to speak. ....
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I had wondered if it would be able to be detected in the small space in earth available to play with, and that led me to thinking the experiment would possibly work better with two different strength lasers, themain laser having a high photon density, and measurements taken from a weaker laser fired "across the bow" so to speak. ....
Interestingly enough, the rate of deflection of the weak laser you speak of wouldn't change even if you powered up the weak laser to arbitrarily high levels. All objects fall at the same rate in the same gravitational field (recall Galileo's experiment). Increasing the strength of the "strong" laser would increase the rate of deflection of the weak laser, however.
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but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?
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and to go back to photons having "head on collisions"...... how rare is this occurrence? it must not be very prevalent as the universe appears black, and if it were to be common, the whole universe would glow somewhat. ....?
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but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?
The rate of deflection of the weak laser would depend upon the strength of the strong laser. The rate of deflection of the strong laser would depend upon the strength of the weak laser. The rate at which the gap between the two laser beams closes depends upon the strength of both of them.
and to go back to photons having "head on collisions"...... how rare is this occurrence? it must not be very prevalent as the universe appears black, and if it were to be common, the whole universe would glow somewhat. ....?
I don't know what the actual numbers are, but it's a very rare phenomenon under most circumstances. The more energy that the photons have, the more likely that this scattering will occur. It's important to recognize that it isn't actually the photons themselves that are scattering off of each other. Instead, photons are capable of transforming, temporarily, into other particles (such as electron-positron pairs). If both photons that are approaching each other transform into such a particle pair at the same time, those particles are capable of interacting with each other and then scattering before returning to their previous photon-selves.
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but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?
Let us not forget relativity. What's a strong laser beam in one frame of reference is a weak beam in another. So if you had two laser beams headed head on symmetry states that they'd collide. However if they two beams were parallel and not head on then a strong deflection in frame is a weak deflection in another frame.
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As far as I understand light can't have a 'rest mass', it's 'propagating', and never 'at rest'. When it comes to a box containing 'excited photons' it will be the box that has a increased 'rest mass', not the photons per se.
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A more interesting point here, and one that follows naturally, is what the heck we mean by 'propagating'.
That's worth thinking of
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This is a Nice general terms and definitions conversational thread.
It's very clear and informative. 👌
Helps to wash out confusion n put things into perspective, relatively.
Please keep this OP going! ✌
P.S. - Thanks to raf21 and all other contributing members. 👍
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I do not deserve the thanks, I've done nothing, I'm the monkey not understanding why I can't grab the hologram banana. bogie, pmb, Bill, Colin and kryptid, among others, deserve the thanks. I'm tempted to ask a couple black hole questions, I think they relate too this discussion. anyone object?
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I'm tempted to ask a couple black hole questions, I think they relate too this discussion. anyone object?
I would say go for it, but again you pick a topic is about science that is still on the drawing board, lol.
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am I understanding things correctly when they say a black hole is black due to the event horizion? event horizon (being an effect, not an object?) is caused by light being unable to escape the singularities gravitational field. .. correct? and I've heard that as an object approaches an event horizion it appears to approach the horizon at a slower and slower rate and you'll never observe it passing though the horizon, it appears to "freeze" at the surface.
I cannot make this work in my mind. if object appears to freeze at the surface, then black holes would not be black, but very red, a Doppler shifted image of everything the black hole has swallowed for it's entire existence.
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I cannot rationalize that scenario with my limited intelligence. how could an image still exist on the surface of the EH after said object has passed through? where does the energy come from to continue sending the light from said object back to us?
and more important, if light cannot escape due to gravity, does this violate the light speed law that light has to move at light speed? gravity can bend light, does gravity slow light like density can? compress space itself? it's space being compressed in a black hole? if space can expand, surly it can compress. .....how does light, traveling in space that is contacting or expanding appear to act to an observer in normal space? I'm sorry, that's a bunch of questions, I've been busy lately ast work and my mind sorta just unloaded.
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John Wheeler coined the term black hole due to the fact that anything that goes into it can never come out of it, including light. The term "black hole" sounded cool and it was accurate so it stuck.
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As far as I know a Black Hole has never been seen from a telescope or naked eyes. (I might be wrong)
I suppose AstroPhysicists collect radio signals or em waves or x or gamma rays n put the bunch through a computer program which processes all the information n then projects a cgi image on screen for a printout to be made possible. (I might be wrong again)
I certainly don't understand why it's still named Black, cause maybe Hawkings radiation could make it one of, or the brightest objects in the universe. (possibly wrong again)
N why add the Hole to it, when it's a bit clear now they are balls of dense matter. Whatever goes inside it isn't falling of into another parallel multiverse but adds on to its existing mass making it slightly more dense n larger than before orelse they would never grow in size. (Yeah right, u get it)duuh!
Nothing personal against Mr John Wheeler but if Pluto can be laid off from planetary status, then why can't Black Holes be renamed to something more precisely defined. (Yup I already know, not gonna happen)pfft!
P.S. - Isn't light made out of Photons, and don't Photons have incredible velocity but Zero Mass?
Doesn't that make Photons untouchables, as Gravity cannot act on massless particles?
Why then do they generally say that Light cannot escape a Black Hole, isn't light just following a straight path like a spaghetti n isn't the incredible dense gravity well of the blackhole creating a meatball by twisting the mere fabric of spacetime?
How could a train leave station (A) and arrive to another station (B) when the rail track itself is twisted n looped around station (A) and why would anyone blame the engine/train driver for not being able to escape station (A)
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I certainly don't understand why it's still named Black, cause maybe Hawkings radiation could make it one of, or the brightest objects in the universe. (possibly wrong again)
Names don't get changed when new theories come out. As I already explained, its called a black hole because whatever falls in can never fall out. What makes you think Hawking Radiation is so strong for all black holes? By the way, Hawking Radiation does not come from inside the event horizon but just outside of it. The name is based solely on analogy as so many things are. You don't object to the term "wormhole" do you? After all the person who coined the term didn't think that spaceworms are going through it.
N why add the Hole to it, when it's a bit clear now they are balls of dense matter.
Take a guess from what I already posted. They used to be called frozen stars. Note: No ice was thought to be used in their makeup.
If you want to know why an object has a name read what the man who coined the term says.
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Well now that u have brought it to my notice, I most certainly object to the term ' WormHole ' .
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Makes little sense as to how someone could dig the spacetime fabric n create a hole in it.
A Tunnel seems more appropriate.
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N don't even get me started on that ' worm ' thingy. PFFT!
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P.S. - OK Mr John Wheeler I accept n agree to the term BlackHole, now stop makin dat face n plz put d gun down & R.I.P.
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1. Electromagnetic waves consist of discrete, massless units called photons. A photon travels in vacuum at the speed of light.
2. Each photon has energy where is the frequency of the wave and h is a universal constant called Planck’s constant. The value of Planck’s constant is In other words, the electromagnetic waves come in discrete “chunks” of energy.
3. The superposition of a sufficiently large number of photons has the characteristics of a continuous electromagnetic wave.
Photon energy is E=h*f=p*c (where p - is momentum).
Can be derived from E=m*c^2 (photon m=0). In the same time m=m0*y (m0 - rest mass).
In another direction.
y*m0=(E/c^2)*c=E/c
Photon energy E=p*c.=h*f
Hence, photon momentum. p=h*f/c
The "weight"of light is a form of energy and not pounds or kilograms.
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The "weight"of light is a form of energy and not pounds or kilograms.
You could weigh it that way if you wanted to, though. If you had a container that was perfectly reflective on the inside and filled it with light, it would weigh more than if the container was empty. That's due to E=mc2.
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You could weigh it that way if you wanted to, though. If you had a container that was perfectly reflective on the inside and filled it with light, it would weigh more than if the container was empty. That's due to E=mc2.
What would be the measurement way and value as an instance?
I'd just like to mislead with confusions further.
Thank you.
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What would be the measurement way and value as an instance?
Putting it on a (sufficiently sensitive) scale would be enough. The weight would depend on how much energy the light has.
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Putting it on a (sufficiently sensitive) scale would be enough. The weight would depend on how much energy the light has.
It could be possible to direct enough directed flux a sufficiently sensitive scale, right.