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If light photons have no mass, then how can gravity affect them? i.e., being pulled into a black hole.

Perhaps light, from its source, follows the curvature of space-time as created by gravitational objects such as stars and black holes.

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    Dec 19 2012: We should figure out how gravity and light actually work first. Doesnt that make more sense? But ultimately science is based on assumptions. Look up for your self how we measure light, you might find it interesting that it is based on one of the worlds largest assumptions
    • Dec 19 2012: Maybe you'll enjoy listening to this guy
      http://www.youtube.com/watch?v=9R2dNwih-1s
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        Dec 19 2012: you know it based on the assumption that everything rotates in a circle and not an eclipse right? Light takes the same amount of time to travel no matter what distance it travels

        check out this video
        http://www.ted.com/talks/sean_carroll_on_the_arrow_of_time.html

        I can provide scientific proof that most of science is based on assumptions
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          Dec 19 2012: Re: "I can provide scientific proof that most of science is based on assumptions."

          You assume you can...
        • Dec 19 2012: Thanks for the links but i am currently on a very slow connection and having difficulties with downloading the videos. I'll watch them later.
          Re : most of science is based on assumptions
          I have no doubt :), but it would be very interesting to hear the scientific proof .
          Thanks !
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        Dec 19 2012: And the best thing about science is even scientist will admit that everything is a theory and can always be proven wrong....
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          Dec 19 2012: Perhaps, you didn't say it right. I have learned from experience to avoid phrases like "everything... can always...". The first thing that can be proven wrong is a statement like this.

          Carl Popper said that unfalsifiable theories (i.e. such that cannot be proven wrong in principle) are bad theories and have no place in science. In this sense, you are right. I wouldn't say, however, that any theory can always be proven wrong. Not any and not always. Only if we are lucky to have evidence and technology to discover it.

          "We can't all, and some of us don't" -- Eeyore.
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          Dec 19 2012: RE: the Buddha comment: Mr. Christofaris, the contents of the link Ms. Niikulina provided is the subject of my comment. It was addressed to her.
        • Dec 19 2012: Arkady,
          what is 'wrong' with science is its doctrine of objectivity , which until recently has been holy in this enterprise.
          And objectivity can be viewed as assumption on previously achieved assumptions.
          I am very interested in what Casey is/was about to say.
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        Dec 19 2012: Far out man! So, did Buddha understand, and explain to us, light and time? Someone should have stumbled upon that gem long ago.
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        Dec 19 2012: Oddly enough, everything the hippie in the video says are scientific facts, not religious or philosophical blibber-blubber. Forget about the question, why photons are not affected by gravity. One can say that they simply move in a space warped by mass. But how can we say that photons "move" if from photon's perspective, time does not exist?! Photon's clock does not change! And if photon's clock does not change, a photon cannot change its position in space with time too... Which means MBR radiation does not come from "deep space" as an echo of big bang that happened 13.7bln years ago. From photon's perspective, MBR radiation is here and now. And the big bang is not something that happened somewhere 13.7 bln years ago. Big bang is also here and now. It's not even "happening" which implies some development in time. It, simply, *IS* here and now, along with everything else that ever was and will be...

        Oh, and, by the way, if space and time do not exist in photon's world, what exactly is photon frequency and wave length?

        Scientists seem to have hard time explaining what time is
        http://discovermagazine.com/2007/jun/in-no-time#.UNIKvKKmc5A
        http://arxiv.org/pdf/0805.1947.pdf

        When scientists say that religion does not make sense, it reminds me of that joke about a kid peeping into his parent's bedroom and saying "And these are the people who won't let me pick my nose!".
        • Dec 19 2012: Congratulations, Arkady ! :)
          You sound like Einstein who ask the child -like question :
          How does the time look from the crest of a light wave ?
          The answer is that , at the speed of light, time stops.
          So... non-material consciousness riding a light wave would see the whole of the past simultaneously with the whole of the future.
          To such a mind there would be no before and no after , only an undivided now. If it is not what can be called God, i don't know what it is, but it has little to do with religions OMHO : )
        • Dec 20 2012: Edgar Casey said, " Time is an illusion. Everything happens at once."
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        Dec 19 2012: OK. But, again, did Buddha or I Ching explain light and time?
        • Dec 19 2012: In the video there is a quote :" nothing has independent existence from everything else " ( sounds quite quantum mechanical :) )
          It is the answer to your question; Buddhism doesn't dissect reality , which is the illusion of the human mind into parts like time space light ..etc.
          As far as i understand Buddhism doesn't care to explain anything , it's scientific 'faustinian' claim :) It teaches how to deal with illusion ( Maya ) And language, the very tool we are using now is the weaver of illusion.
          I Ching ' studied ' time as if consists of elements and achieved much more accurate and applicable understanding of time (or dynamics within time ) as we have now.
          But i am not an expert, really , almighty Google help you , if you are interested : )
          Best to you !
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        Dec 20 2012: Re: "Arkady, what is 'wrong' with science is its doctrine of objectivity , which until recently has been holy in this enterprise."

        It's wrong for me to say it, but I've also learned that it's wrong to think of things as "wrong" or "right". "Do not judge..." If you know what I mean... :-)... But I agree with you.
    • Dec 20 2012: Hey Casey,

      We possibly found a new issue to disagree upon!

      The ONLY assumption science makes is that "We can percieve reality". All of science is based upon "describing the reality". Then this leads to a theory which "describes reality as closely as possible".
      Then there are a few options from there.

      1) We discover a new place where the theory should apply. But it fails. Then the theory is falsified. Which will result in scientists trying to come up with a different theory which is accurate in more cases.
      (this happened with Newton gravity laws when we realised that our solar system revolves around the sun and that some 'stars' are actually planets)

      2) We discover a new place where the theory should apply and it does! The theory gains probability of being correct.
      (This happened in 1919 when Einstein's theory correctfully predicted how much light would be bent by... and many more times after that as well)

      3) Some other scientist comes up with another theory for the same phenomenon and both seem to hold in the same cases where the rule is correct. (This somewhat happens with String theory... but I must confess that I am unsure how accurately this statement is... it is however also what Einstein did to Newton)

      So basically scientific theory describes all of reality as far as we know it! (So the assumption is that our perception of reality is accurate)
      To say however that theory is true is stupid because we can never(!) know if we know (or can even percieve) all of reality.
      Also to say that you can prove a theory to be false is stupid because you would have to find an occurence in nature where the theory does not hold up. And science only accepts a theory as valid when ALL KNOWN occurances are accurately described.
      So you HAVE to find an, up till now, unknown occurance of where the theory should apply AND on top of that it has to be one where the theory fails!
      Good luck!
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    Dec 19 2012: I'm not sure why people use the word time as if time is a universal element. Time is merely the measurement of progression or developments in the universe. Time has no meaning outside of these measurements. No mass in the universe uses time to regulate itself or influence anything around it.

    You can either say time or measurement of development. I don't know how a black hole has anything to do with time but it seems scientists love speaking about time. When something moves in the universe is the result of consequence. Matter will only react to other matter and specified laws of the universe. Therefore, time, or the process in which matter moves, changes forms, or is somehow manipulated, is merely a way for humans to monitor development and relate that development to an understandable scale.

    Light is a wave so it may be manipulated by objects in its path. However, to say that time is warped really just means your perception is warped. Light, matter, and everything else in the universe follows laws. When object a comes into contact with object b both objects combine their laws and the results can be calculated to a T every time.

    The only random aspects of the universe artist ones we don't understand. I don't think time plays a part like many believe. I think the use of time is just a misunderstanding of how things really work.
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      Dec 19 2012: For every action requires an equal but opposite reaction...... to create its negative
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      Dec 19 2012: BINGO! I thought I was the only one who thinks Time is simply a unit of measure of Change. :-)
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    Dec 19 2012: When I went to school many years ago, I learned it like Lejan states. But since that time there has been much work on space-time. I am no theoretical physicist and they can probably explain this better but this is how I understand it in my rudimentary way.

    Gravity has a exertional force on space-time. It can bend it. So when light goes past a super massive object, it appears to bend its course. But from the beam of lights perspective, it has been going in a straight line the whole time. Think about this like a person walking in a straight line until they walk all the way around the earth. They completed a circle but by their perspective they walked a straight line. Gravity kept them on earth and they completed a circle.

    So gravity bends space-time but the light has been going straight the entire time. What happens in a black hole is that the gravity is so great, that space-time has been bent completely around it. Like a cocoon if you will. So when something that has mass that gives off light falls into a black hole, it still has mass and is giving off light in a different configuration of space-time than the one we live in, effectively causing us not to see it any more once a star passes the event horizon.

    That is how I understand it. But there are other ways of looking at it...
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      Dec 19 2012: Hi Linda,

      what you describe is what Albert Einstein proposed in 1915 in his theory of general relativity, and as much as I know, this theory has withstood every 'test' ever since. Physicist have not yet measured the mass of a photon, but they calculate its upper limit it can possibly have, which is precise enough to calculate the existing 'radiation pressure' in our good old Newtonian universe... :o)

      I have no current update how string theory is doing on its way to combine general relativity with quantum mechanics and just recall it got stuck somewhere on its way of just another view years to have them finally united. So if a string theorists out there could help us out in EASY explanations and WITHOUT any formulas, it would be very helpful ... :o)
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      Dec 19 2012: Is the definition of a straight line still; "the shortest distance between two points"?
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        Dec 19 2012: The shortest distance between two points as a straight line only works in 2-D. On earth, the shortest distance between two points is a curved line. That's why flight plans look curved on a map. Pilots can explain that better than I can.
        • Dec 19 2012: no it's not... the lines are curved on a flight plan because they are straight lines drawn on a round object (earth). Which is 'flattened out' to make a map.
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          Dec 19 2012: Using an X-Y-Z coordinate system to define locations for two points is a 3-D exercise. If the two points have dissimilar Z coordinates then the shortest line that can be drawn between them is, by definition, a straight line. The shortest distance between Moscow and Albuquerque is through the Earth. I think the definition of a straight line applies universally. A photon traveling from a star to Earth either will, or will not, be affected by the gravity of masses it passes (cool rhyme). If it is unaffected by those masses it will travel in a straight line. If it is affected then its trajactory will not be a straight line. Am I wrong? Thanks!
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        Dec 19 2012: Sorry, Richard, Ed,
        In Euclidean geometry, a straight line between two distant points on earth would be THROUGH the earth.
        Look it up. Do the math.
        http://en.wikipedia.org/wiki/Great-circle_distance
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          Dec 19 2012: Through the Earth is what I said 4 hours ago. We are not talking about the best possible air route, which is called a GEODETIC. We are talking about the definition of a STRAIGHT LINE, which is the shortest distance between two points. Thank you!
        • Dec 19 2012: Hello Linda,

          Ehm you are somewhat right on that but it's not what I said.

          I said a flightplans are "straight lines drawn on a round object". So they can't go through the earth as they are 'on the surface'.

          Also they are not straight in the mathematical sense of being straight in all dimensions. They are straight in the linguistic sense of the word that you can get from point A to point B while only going in "a straight line" nobody talks in x,y,z coordinates.

          But you seem confused about something. Namely: "The shortest distance between two points as a straight line only works in 2-D."
          This is false.
          The shortest distance from Amsterdam to New York would be through the earth! And there is no distance shorter than the straight line (where in this discussion I should actually place an if but that will make it too hard for you to see which point I'm trying to make).
          So in 3d the shortest way to get somewhere is still a straight line. But if you place an object within that space where you have to "go around" then obviously you can't go through it.
          The "great-circle distance" just describes how you can best move across a sphere which is not the same as in 3D (as there are no given objects in a "3D space").
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        Dec 19 2012: Yes Ed. But the shortest distance between two points on a sphere is a curved line. Which was what I was saying. The distance through the earth has no practical application unless you are a neutrino.
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          Dec 19 2012: No, I am a Baptist! Anyway, does a photon travel on the surface of a sphere while hurtling through space? I don't think so. I think it travels in a straight line IF not acted upon by an external force.
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        Dec 19 2012: Hmm. Interesting question. Since we know that light waves can be bent and they can bounce off stuff maybe there should be an answer to your question.

        We use mass to change trajectory of spacecraft which also has the effect of acceleration. So if we are effecting both the mass and the acceleration of the particles, perhaps it holds true that the shortest distance between two points is a curved line.

        Kinda like taking the express way home vs. the direct route through the neighborhoods.
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    Dec 20 2012: Gravity doesn't directly affect light. Strong gravity curves space-time so from our perspective the light travels a curved path, but if we were in the same location as the light it would appear that the path was straight, If you paddle a canoe past a whirlpool your path is curved by the whirlpool even though you are paddling in a straight line. The whirlpool has no direct effect on your canoe but it curves the water through which you are moving
    • Dec 20 2012: If you look at it from GR, gravity doesn't directly affect anything, but make no mistake: photons are nothing special to the force of gravity, since photons have energy and that's all that gravity cares about. To further illustrate this point a photon will actually exert a gravitational pull on other matter, including other photons.
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        Dec 20 2012: I was trying to keep it simple but from a standard model-quantum mechanics stand point photons have no mass or they wouldn't be able to travel at "c". I've never seen any experimental results that indicate two photons interacting due to gravity.
        • Dec 20 2012: "I've never seen any experimental results that indicate two photons interacting due to gravity."

          Well, they do: it's used in models of the very early universe.
  • Dec 19 2012: "If light photons have no mass, then how can gravity affect them? i.e., being pulled into a black hole."

    Gravity couples to energy, that energy can be "bottled up" in mass, but it doesn't have to be, it can also be "bottled up" in momentum ( E=sqrt[M^2c^4 + p^2c^2], where M is the mass, c is the speed of light and p is the momentum). Photons have energy and thus they react to gravity. All elementary particles have energy that allows them to heat or move something they hit, but not all of them have the additional form of energy that we call mass and is caused by the Higgs mechanism (you can easily distinguish the two kinds because particles without mass always travel at the speed of light while particles with mass can never reach the speed of light). Gravity does not distinguish between forms of energy so it acts on all particles, even the ones that don't have some of their energy bottled up in mass.

    Alternatively you can look at it from the viewpoint of general relativity where gravity is a warping of a patch of space-time and everything that exists within that patch of space-time will feel the effects of the warping.
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      Dec 19 2012: c - could not possibly be the measurement of what the speed of light is
    • Dec 20 2012: I get it. That's how light can push a solar sail . Maybe light is helping to push the galaxies apart from every direction. The ones not linked by gravity.
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    Dec 19 2012: The official answer (strictly following Einstein's General Relativity (GR)):
    Gravity warps the shape of space-time. That is, it 'pulls in' both the shape of space, AND it 'pulls in' the shape of time. Mass or no mass, EVERYTHING is affected by TIME.

    Newtonian physics can be used to predict light curvature under gravity where a = GM/r^2 (a = acceleration)
    This effect can be understood to be the warping of time around a massive object, thus providing an acceleration.

    When Einstein came up with his GR theory it was used to predict the curvature of distant starlight around the sun during an eclipse in 1919 (I think). Everyone expected the sunlight to curve (even Newtonian physicists), but the question was 'by how much will the light curve?'. In fact the light curved by almost exactly twice the amount predicted by standard (at the time Newtonian) physics, showing that (according to GR) its curvature was affected by a curvature in both the shape of time (Newtonian) AND the shape of space (GR).

    My version:
    It is not space that is warped at all, space, or nothingness, cannot change its non-existent shape. If we say that space is shaped, then all we can really be saying is that our rulers are measuring an effect which LOOKS LIKE space is shaped.

    On the other hand, time exists, everyone experiences it all the time, it is a real thing. So it is time that is warped by twice as much as Newtonian physics predicted. Half of this warping of time is interpreted as the warping of space in Einstein's GR.
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      Dec 19 2012: Light = Time?
      • Dec 19 2012: Nope, light is light.
        Which is something we're unsure of what it is (best guess is an energetic wave which when interacted with could turn into a particle).

        We know however that (whatever it is) when emitted travels 299 792 458 meter away from it's source in 1 second.

        There might be a correlation to time which we do not know yet though.
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          Dec 19 2012: Based on the vocabulary--"unsure"; "guess"; "could"; "might be"; "we do not know"-- It looks like the answer to the OP is some years away. I'm hoping Mr. Shaw answers my question. Thanks!
      • Dec 19 2012: it's fine with me that you want an answe from shaw, but the "We know however that (whatever it is) when emitted travels 299 792 458 meter away from it's source in 1 second." is not a line which is questionable."
        This already means that light is not equal to time. For instance putting more lights on the same area won't influence the time there.

        The rest of my post is though.. as that is the current scientific view (at least up till the 2000's and I haven't read any "ground breaking quantum mechanics" newsreports so I have reason to believe the rest stilll is the current scientific view.)
    • Dec 19 2012: affected by a curvature in both the shape of time (Newtonian) AND the shape of space (GR).

      Ehhh isn't Einstein saying that "how fast time flows is related to the shape of space"?
      He has the "space-time continuum" right? Which means that you cannot actually seperate time from space as they are somehow dependant upon eachother (or both are similarly related to something else).

      If I'm wrong then please correct me :)
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    Dec 19 2012: The definition of light photons states, that their 'rest mass' is zero, not their mass in motion, which, preferably to them, is light speed.

    And because there are no 'resting' photons, their moving mass differs slightly from zero due to their tremendous speed, which is just another scale of their energy, according to E=mc^2.

    300 million photons, depending on their color and therefore frequency, have approximately the same mass as a single proton or neutron. The overall mass of photons which reach earth adds up to about 168 metric tons on a SINGLE day, yet it dissipates as energy and does not add to the mass of our planet itself.

    This tiny mass of a single and moving photon is enough to be influenced by intensive gravity fields which black holes and massive stars are capable of, and that's why it gets either bend or finally 'trapped' by it, depending on its trajectory and approaching distance and the intensity of the gravitational force.

    This property of light has already been put into practice for trajectory corrections of the Messenger probes which reached Mercury in 2011. This and similar applications are called 'solar, light or photon sails' in space technology as they use the 'radiation pressure' of photons such as a regular sail uses the current flow of the wind.

    You can find more details about it on Wikipedia:

    http://en.wikipedia.org/wiki/Solar_sail#Satellites
    • Dec 19 2012: E=Mc^2 only applies to particles with mass and at rest, E=sqrt[M^2c^4 + p^2c^2] applies to moving particles with mass and E=pc apples to photons. Photons do not have any mass whatsoever, not even a tiny bit. The difference between a photon and the neutrino, a particle with a tiny mass is stark: photons always travel at the speed of light, neutrinos can never reach this speed (nothing with mass can).
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    Dec 19 2012: There are two key words in your query: "If", and "how". Indeed, IF gravity acts upon light (waves or particles) then it does not matter (no pun intended) if light has mass, unless we knew for a fact that gravity cannot exert force on energy, which we don't. So, because there are two variables I can't answer your question. Either, 1) light has mass and gravity affects it; or, 2) it has no mass and gravity affects it; or, 3) it has mass and gravity does not affect it; or, 4) it does not have mass and gravity does not affect it. Let us know if you find a unifying theory for quanta and gravity.
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      Dec 19 2012: Go for 1) and you are fine... ;o)
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        Dec 19 2012: Have 2, 3, and 4 been scientifically falsified?
        • Dec 19 2012: 3 and 4 kinda have been. As light is affected by gravitational forces.
          Ofcourse here it becomes somewhat debatable as to if you should still call the force gravity or something else... and we ofcourse cannot be sure if it's gravity that 'pulls it in' or a force which is linearly related to gravity. (aka gravity creates (or is created by) an effect which causes light to bend)
    • Dec 19 2012: Grvity couples to all forms of energy, this is why a massless particle is still influenced by gravity.