Gravity's Effects on Light: Mass & Black Holes

[neh4pres]

if light can be affected by gravity then why is it said to have no mass? and science light is affected by gravity.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole?? this light would never be seen because it will never escape or plummet but would probably be intence enough to vaporize anything that passes through it...

 

[granpa]

if light can be affected by gravity then why is it said to have no mass?

1) gravitational time dilation should slow light (think refractive index).
2) it isn't said to be massless

 

[Fredrik]

Photons are massless. Massless particles always move with speed c on the straightest possible path through space-time, but the geometry of space-time around near a star is such that the corresponding path through space is actually a curved path in space.

I think there are circular paths around a black hole that light can actually follow (in principle), but those orbits are unstable. If the photon isn't going in exactly the right direction, it would either fall in or move away from the black hole.

 

[GRB 080319B]

if light can be affected by gravity then why is it said to have no mass?

Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. The rest mass of such an object is zero, and the only mass which the object has is relativistic mass-- a quantity which depends on the observer."

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole??

I am not sure what you mean by 'perfectly fixed gravity', but light can get trapped in a circular orbit by the gravity of a black hole based on the trajectory of the light entering it. See this: en.wikipedia.org/wiki/Blackhole#Photon_sphere

 

[Fredrik]

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by Newtonian[/color] gravity in the framework of special relativity[/color].

The red stuff should be included too. This is the reason I've never liked this explanation.

 

[blackwing1]

actually, you're also wrong about the energy not escaping. energy is slowly leaked out of black holes, and the black holes themselves destingerate after immensly long period of time.
An, the light affected by gravity is explained in the local frame/closed rocket acceleration thought expiriment done by einstein. it also explains y planets close to the sum have time differences in their orbits which can't be explained unless it's the theory of general relativity.

 

[DrGreg]

if light can be affected by gravity then why is it said to have no mass?

Remember the old experiment attributed to Galileo where he dropped heavy and light objects off the Tower of Pisa and they all fell at the same rate? The effect of gravity is independent of the mass of the object (for tiny objects near huge planets) so even something with zero mass should (and does) accelerate the same way.

and science light is affected by gravity.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole?? this light would never be seen because it will never escape or plummet but would probably be intence enough to vaporize anything that passes through it...

It is possible for light to orbit a black hole, but how would it get there? Any light approaching the hole would be on the wrong trajectory to orbit -- it would either fall in or else pass by and escape. Something (e.g. a collision) would have to move it onto a different trajectory. Only a small number of photons would get captured this way -- and after capture could later escape by the same mechanism -- so there wouldn't be an intense "belt" round the hole.

 

[neh4pres]

Remember the old experiment attributed to Galileo where he dropped heavy and light objects off the Tower of Pisa and they all fell at the same rate? The effect of gravity is independent of the mass of the object (for tiny objects near huge planets) so even something with zero mass should (and does) accelerate the same way.

It is possible for light to orbit a black hole, but how would it get there? Any light approaching the hole would be on the wrong trajectory to orbit -- it would either fall in or else pass by and escape. Something (e.g. a collision) would have to move it onto a different trajectory. Only a small number of photons would get captured this way -- and after capture could later escape by the same mechanism -- so there wouldn't be an intense "belt" round the hole.

Think of it this way. If an observer went to the point outside a black hole where the gravity was at the right amount that photons could orbit. Can you see light. Yes. and if you move along this sphere so as to position a specific star at they proper angle so that its photons are being caught you would see them enter their orbit. the preceding was only for thought. Light comes from every star at every conceivable angle, so of course you should have a steady flow filling this sphere of light. Read about the photon sphere on this page http://en.wikipedia.org/wiki/Black_hole

 

[DrGreg]

Think of it this way. If an observer went to the point outside a black hole where the gravity was at the right amount that photons could orbit. Can you see light. Yes. and if you move along this sphere so as to position a specific star at they proper angle so that its photons are being caught you would see them enter their orbit. the preceding was only for thought. Light comes from every star at every conceivable angle, so of course you should have a steady flow filling this sphere of light. Read about the photon sphere on this page http://en.wikipedia.org/wiki/Black_hole#Photon_sphere

Light hitting the photon sphere from outside wouldn't normally get captured, it would pass straight through it, spiralling into the black hole. Only those photons that are already in orbit within the photon sphere are at the right angle to remain in orbit. For a photon to be captured it would have to change direction, perhaps by collision, or by the influence of another massive object passing by. More likely, a photon would be produced as a result of particles colliding just as they pass through the photon sphere. This would be only a slow trickle of photons being added.

More importantly the orbits are "unstable" which means only those photons traveling in exactly the right direction can orbit indefinitely (and even then, only if they don't collide or interact with any other particle in the Universe). The tiniest error results in a slow spiral, inwards or outwards, so almost all the photons which appear to have been added will in fact gradually escape orbit some time later.

 

[kahoomann]

if light can be affected by gravity then why is it said to have no mass?

Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. The rest mass of such an object is zero, and the only mass which the object has is relativistic mass-- a quantity which depends on the observer."

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

This is incorrect. Light is affected by gravity, not because it has relativistic mass.
The reason is space-time is curved and light has to follow the curved path, called geodesic

 

[GRB 080319B]

Does this mean that http://uk.answers.yahoo.com/question/index?qid=20080418035618AACWgjD&show=7" is wrong?

 

[kahoomann]

Does this mean that http://uk.answers.yahoo.com/question/index?qid=20080418035618AACWgjD&show=7" is wrong?

If you agree with her/him, you will get only half of the bending, which Einstein did for his first try.
You will get the same answer using Newton's theory of gravity.
After Einstein developed GR, he use GR to calculate the deflection of light, he got the right answer which is twice as large. This is due to curved space

 

[Adonis]

everybody has mass
we can calculate his total energy from
the equation:
E=mc^2
if light has mass
we can calculate his Energy from: E=mc^2
and his speed is v=c
when a mass is moving a speed of light it will be infinited
that means E=infinite energy (by Joule) and that impossible of course
so...we calculate Energy of light from E=hv
h=planck constant
v=frequency of light electromagnetic waves
so the light has body with no mass! (Specifically the photon)

 

[MeJennifer]

if light can be affected by gravity then why is it said to have no mass?

Why do you think that is a contradiction?

Mass affects the curvature of spacetime. The worldline of unaccelerated obects follow the geodesics of spacetime. It is not the case that if they have no mass they can ignore those geodesics.

 

[Adonis]

the light (photons) has body with no mass
photons belong to the bosons
the bosons have spin=0,1,2...
like gluons (weak nuclear force)
i don't know how photons affect by gravitational fields...
i didnt read the General Relativity to answer this...

 

[pmb_phy]

This is incorrect. Light is affected by gravity, not because it has relativistic mass.

Actually it is quite correct. Both Einstein and Feynman stated this explicitly themselves.

The reason is space-time is curved and light has to follow the curved path, called geodesic

That is not a reason. That is a description. And even that description is in accurate since its possible to have a gravitational field in the absense of spacetime curvature (e.g. a uniform gravitational field has zero spacetime curvature). It is important to understand that light has gravitational mass because without that knowledge one has no reason to believe that light is deflected in a gravitational field. The fact that light has gravitational mass is the reason Einstein was led to postulate the equivalence between a uniform gravitational field and a uniformly accelerating frame of reference.

I recommend that you look this up in Eintein's book "The Evolution of Physics" by Einstein and Infeld. I'm out of town for the next week or two so I can't tell you where in that book Einstein says that. When I get back home I can give you the precise reference and quote.

Best wishes

Pete

 

[Fredrik]

Pete, thinking about it that way only made sense before GR was discovered. The reasoning would have to go something like this:

1. We can prove that a massive particle at rest has an energy mc².
2. We can define a velocity-dependent "relativistic mass" such that the energy of a massive particle is always mc².
3. This suggests that maybe it's E/c² that should go into Newton's law of gravity, not the rest mass.
4. A photon must have some energy, so if we were right in step 3, then light should be affected by Newtonian gravity too.

This doesn't really have anything to do with GR. Saying that the world line of a ray of light is a null geodesic is definitely a much better "explanation" of why the path of light is bent by a heavy object than the idea that it's the energy/c² that should go into Newton's law of gravity instead of the mass.

I'm also surprised that you're characterizing the GR explanation as a "description" and the SR+Newton explanation as the "reason" why gravity bends light.

If the relativistic mass of a photon is the reason why it's affected by gravity (as defined by GR), then its world line would be a time-like geodesic, not a null geodesic.

 

[pmb_phy]

Pete, thinking about it that way only made sense before GR was discovered.

I disagree of course. The assertion "only made sense" implies that the argument for it can be proven to be wrong and I certainly don't believe that is true. In fact I know of no valid argument which could possibly prove it wrong. Especially since it makes perfect sense. And just because that reasoning is what led to GR it does't mean that it can or should be dismissed after its discovery. Each of us is different and as such we all think differently. We all arrive at ideas along different paths. To say that one way of thinking is right and another, equally valid line of thinking, is wrong is a very wrong statement. In fact it is for that reason that both Feynman and Einstein presented those arguements.

Let me ask you this: If it indeed makes no sense then why do you think Einstein and Feynman explained the defection of light using the mass-energy equivalence arguement? Let me quote them

The Evolution of Physics - from Early Concepts to Relativity and Quanta, Albert Einstein and Leopold Infeld, Simone & Schuster (1938), page 221 - Einstein commented on an observation made by an observer inside an accelerating elevator that light is ‘weightless’ Einstein wrote

But there is, fortunately, a grave fault in the reasoning of the inside observer, which saves our previous conclusion. He said: “A beam of light is weightless and, therefore, it will not be affected by the gravitational field.” This cannot be right! A beam of light carries energy and energy has mass.

This same sentiment was expressed by Feynman in Feynman Lectures Vol - I, page 7-11. Section entitled Gravitation and Relativity

One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energy has mass -- mass in the sense that it is attracted gravitationaly. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.

The reasoning would have to go something like this:

1. We can prove that a massive particle at rest has an energy mc².
2. We can define a velocity-dependent "relativistic mass" such that the energy of a massive particle is always mc².
3. This suggests that maybe it's E/c² that should go into Newton's law of gravity, not the rest mass.
4. A photon must have some energy, so if we were right in step 3, then light should be affected by Newtonian gravity too.

This doesn't really have anything to do with GR.

Of course it does. What possible reason could you have for saying otherwise?

By the way, Einstein never proved that a body at rest has energy. When Einstein first derived the expression E = mc² he started the derivation with the assumption that a body which was at rest had energy. As such the body was able to emit radiation and this requires that the body had energy, E, to begin with. What Einstein showed was, not that a body at rest has energy, but that the energy of a body at rest is related to its proper mass, m, by E = mc².

Saying that the world line of a ray of light is a null geodesic is definitely a much better "explanation" of why the path of light is bent by a heavy object than the idea that it's the energy/c² that should go into Newton's law of gravity instead of the mass.

Why do you consider that a "better" explanation??

In any case it can't be taken as an explanation since it is a description of the phenomena and not an explanation. Its quite wrong to consider things like this as "explanations" since laws of physics are formulated, not to explain the phenomena in nature that we observe, but to describe the phenomena. This is a fundamental fact of all the sciences. Unfortunately many people miss this very important fact. If you ever have the chance I highly recommend reading the first chapter of Fritz Rohrlich's book "Classical Charged Particles" The author is very good and very well known physicist. He's top notch as a matter of fact, a first rate physicist. He certainly knows what he's talking about and that chapter does an excellant job at explaining the philosophy of physics. You'd do yourself a great service by studying that chapter and knowing the material by heart. I know it did me a world of good .. although I knew the material anyway. The author just does a wonderful job at it.

I'm also surprised that you're characterizing the GR explanation as a "description" and the SR+Newton explanation as the "reason" why gravity bends light.

Me? I wish I could take the credit but this is due to Einstein and not myself. Eddington himself explained that GR is not an explanation of gravity but a description of it. This must be kept in mind so that one has a good understanding of the relationship between mass and gravity in both its active and passive aspects.

If the relativistic mass of a photon is the reason why it's affected by gravity (as defined by GR), then its world line would be a time-like geodesic, not a null geodesic.

Because timelike geodesics are for particles which have a non-zero proper mass (aka rest mass).

I hope you're not confusing the notion of rest mass with the gravitational mass of light?

By the way. The definition of relativist mass is as follows: If "m" is the inertial mass (aka relativistic mass) of a body, v its velocity and p its momentum then p = mv. In otherwords inertial mass m is defined as the m such that the quantity mv is conserved in elastic collisions. This quantity is then given the name (i.e. defined) as the relativistic momentum of the object.

Best wishes

Pete

ps - By the way. One can go on and on about this and never get past semantics and opinions. If this thread gets to that point then I'll bow out.

 

[DrGreg]

I think some of the contributions to this thread are missing the point. If we are talking about a tiny mass near a huge mass (say a 1 kg mass near a planet) then the mass of the small object is irrelevant. A 1 kg mass, a 1 g mass, or a 1 nanogram mass all behave identically -- they all follow the same geodesics generated by the large object and we can ignore any minuscule spacetime curvature introduced by the small object. By that logic, a zero-proper-mass object should also behave identically (the only difference being a difference in velocity). It doesn't matter whether that object has some other sort of "mass" or not. For example, a high-energy high-frequency photon follows the same path as a low-energy low-frequency photon (in vacuum), energy being a frame-dependent concept.

The mass of the "small" object only becomes relevant when it is much larger, e.g. another planet.

 

[MeJennifer]

I think some of the contributions to this thread are missing the point. If we are talking about a tiny mass near a huge mass (say a 1 kg mass near a planet) then the mass of the small object is irrelevant. A 1 kg mass, a 1 g mass, or a 1 nanogram mass all behave identically -- they all follow the same geodesics generated by the large object and we can ignore any minuscule spacetime curvature introduced by the small object. By that logic, a zero-proper-mass object should also behave identically (the only difference being a difference in velocity). It doesn't matter whether that object has some other sort of "mass" or not.

Exactly!

 

[kahoomann]

If light has mass, its speed would slow down when it climb out gravitation field
This is the essential difference between Newton theory and GR

 

[yuiop]

If light has mass, its speed would slow down when it climb out gravitation field
This is the essential difference between Newton theory and GR

To a local observer the speed of light is constant everywhere in the gravitational filed and to a coordinate observer who has the "big picture" the speed of light speeds up as it climbs out of a gravitational field. I think Newton thought of a photon as a "corpuscle" (with mass?) so presumably Newton would have predicted the opposite (the light would slow down as it climbs out of a gravitational field).

 

[Fredrik]

The assertion "only made sense" implies that the argument for it can be proven to be wrong and I certainly don't believe that is true. In fact I know of no valid argument which could possibly prove it wrong.

The assertion is that gravity bends light because of its relativistic mass. I have thought about this some more and I think I have to agree. I haven't seen a way to prove that E/² should go into Newton's theory of gravity instead of mass, but it doesn't matter. Even if that was just a guess, the result is a valid theory (which I will call "SR+Newton" for the rest of this post) that says that gravity bends light. So saying that the relativistic mass is the cause is no different in principle than saying that the gravity of the moon causes tides on Earth.

And just because that reasoning is what led to GR...

I have a hard time believing that it did.

Of course it does. What possible reason could you have for saying otherwise?

The reason is of course that we're obviously talking about Newtonian gravity in the framework of special relativity. This doesn't even suggest that space-time might be curved. There certainly is no obvious connection to GR. The insight that's needed to take the step from SR to GR is that objects in free fall follow geodesics, and that there might be a way for this to remain true even when gravity is included. I really don't see how the idea that maybe E/c² should go into Newton's law of gravity instead of mass takes us even a little bit closer to that insight.

Why do you consider that a "better" explanation??

Do you realize that you're asking why I consider GR a better theory than SR+Newton? (And that the double question marks suggests that you think it's absurd to do so). GR is a better theory than SR+Newton for the same reasons that "the Earth is round" is a better theory than "the Earth is flat". (It agrees with experiments to a higher degree).

That's not the only reason why I consider GR better, but it's the only one that I'm sure is objectively true. I think these may be valid reasons too, but there is some subjectivity in all of them: GR is simple and beautiful. It's just one theory, and not two theories glued together like SR+Newton. It doesn't require action at a distance.

...not to explain the phenomena in nature that we observe, but to describe the phenomena...

This is junior high school semantics, and I'm very familiar with it. What you're saying is certainly true in the sense that any explanation leads to more questions, so nothing can really be explained. "Description" may be a more appropriate word than explanation, but if we're going to be that pedantic we might as well eliminate the word "explanation" altogether, since there are no better explanations around for anything than scientific theories supported by vast amounts of evidence.

Anyway, you seem to have misunderstood which one of your statements in #16 I objected to. I don't mind so much that you described GR as a "description" rather than an "explanation". That's just a matter of semantics. The thing is, you did this immediately after saying that light is affected by gravity because it has relativistic mass! So you dismissed GR as a "description", but you didn't do the same to SR+Newton. It's that last part I have a problem with. SR+Newton is clearly inferior to GR in every way that matters, but you said that SR+Newton is the real reason why gravity bends light while GR is not a reason at all! So you're implying that SR+Newton is superior to GR, and even has a property that you claim that no theory can have (the property of being an explanation).

I'm not saying that you don't understand these things well enough. I'm just saying that you chose your words very poorly.

 

[pmb_phy]

If light has mass, its speed would slow down when it climb out gravitation field
This is the essential difference between Newton theory and GR

I don't see how that would be neccesary. Seems to me that you're using Newtonian intuition. In any case the speed of light does change as it moves through a gravitational field.

I have a hard time believing that it did.

For now I'd like to retract that since I only saw Einstein state that explicitly in his book which was written much later. He did however base the equivalence principle on the fact that all matteer falls at the same rate regardless of its mass. If Einstein believed that light didn't have mass then I see no reason for him to state the equivalence principle. In Einstein's 1907 paper in which he announced the equivalence principle he argued for the deflection of light based on the variation of speed with gravitational potential. I believe that Einstein derived the fact that matter moves on geodesics by using the equivalence principle.

The reason is of course that we're obviously talking about Newtonian gravity in the framework of special relativity. This doesn't even suggest that space-time might be curved.

There is nothing in any comment that I've read or posted that would indicate that we're talking about Netonian gravity. And the presence of tidal forces is only required when there are tidal gradients. In, e.g., a uniform gravitational field there are no tidal forces and thus no spacetime curvature. Yet there is a gravitational field in which light and all other matter is affected by gravity.

This is junior high school semantics, and I'm very familiar with it. ...
Anyway, you seem to have misunderstood which one of your statements in #16 I objected to. ...
I'm not saying that you don't understand these things well enough. I'm just saying that you chose your words very poorly.

Then I guess there is no reason for me to continue.

Pete

 

[bockerse]

Pete:

You mentioned spacetime isn't curved in a uniform gravitational field. What's that? Seems to me like anything producing a gravitational field inherently causes a 'gravitational gradient' with distance. The only circumstance under which I can imagine a uniform field is at the center of a symmetrically oriented and moving group of bodies, like for instance a tetrahedron shape of 4 equal-mass bodies all slowly traveling, spinning and co-orbiting together, where the uniform field would be at the very center, equidistant from each, although for 4 bodies to be oriented that way, much less to have equal mass, would be a long damn shot. Also the 'center' would be a 'snapshot' and not really legitimate. Is there any other way to get a uniform field and thus 'flat' spacetime?

-Gerrit

 

[pmb_phy]

You mentioned spacetime isn't curved in a uniform gravitational field. What's that?

A uniform gravitational field is a gravitational field in which there are no tidal forces. In Newtonian gravity a uniform gravitational field is defined as a field in which the gravitational accelerations g is a constant. This gives a gravitational potential of \Phi = gz. The tidal force tensor for such a field is zero. The GR version of this means that the Riemann tensor (also known as the tidal force tensor) is zero. A zero Riemann tensor means that spacetime is flat.

To simulate a uniform gravitational field use the equivalence principle which states that a uniform gravitational field is equivalent to a uniformly accelerating frame of reference in flat spacetime.

Seems to me like anything producing a gravitational field inherently causes a 'gravitational gradient' with distance.

Why does it seem like that to you?

The only circumstance under which I can imagine a uniform field is at the center of a symmetrically oriented and moving group of bodies, like for instance a tetrahedron shape of 4 equal-mass bodies all slowly traveling, spinning and co-orbiting together, where the uniform field would be at the very center, equidistant from each, although for 4 bodies to be oriented that way, much less to have equal mass, would be a long damn shot. Also the 'center' would be a 'snapshot' and not really legitimate. Is there any other way to get a uniform field and thus 'flat' spacetime?

I know of case which would produce a very uniform field. In Newtonian mechanics it would be exactly uniform. In the weak field limit in GR it will also be uniform. This is generated in a spherical body which has a spherical cavity cut out of it. The mass density is uniform. See the diagram I created at

http://www.geocities.com/physics_world/gr/image_gif/gr10-img-01.gif

Check your PM. I'll send you a link to the proof.

A uniform field is also generated by a vacuum domain wall (something you'll find mentioned in cosmology). It is an infinite sheet of matter which has a constant energy density and is under tension.

 

[Antenna Guy]

... the speed of light does change as it moves through a gravitational field.

Pardon my interuption, but with respect to what does the velocity of light change as it moves through a gravitational field?

Regards,

Bill

 

[bockerse]

Big Pete

Well as I understand, gravity and curvature each contribute 1/2 of the bending in the case of light bending 'around' the sun as seen from Earth. How is a tidal force exerted on light as it passes by a massive body, since light photons are 'points'? Why is each contribution precisely 1/2 (or is that just approximate), and are they always 1/2 and 1/2 independent of the massive body's density?

 

[pmb_phy]

Pardon my interuption, but with respect to what does the velocity of light change as it moves through a gravitational field?

With respect to an observers coordinate system. The speed of light is always c when measured locally. This means that if the light is passing through the observers location (or through a particular region of space to be defined below) then the observer will measure the speed of light to be c. However when the light passes the observer and moves to a location which has a different gravitational potential then the coordinate speed of light will no longer be c. Regarding the "region of space" I mentioned above; If, after passing the observer, the light moves through space to points which have the same gravitational potential then the speed of light measured by that observer will still be c.

The Schwarzschild metric represents the metric in Schwarzschild coordinates. Those coordinates are (very loosley) defined as the spherical coordinates used by a distant observer. If you were to calculate the speed of light as measured using that coordinate system then you'd get a speed of light which varies with the gravitational potential.

Well as I understand, gravity and curvature each contribute 1/2 of the bending in the case of light bending 'around' the sun as seen from Earth. How is a tidal force exerted on light as it passes by a massive body, since light photons are 'points'?

Tidal forces represent differences in the gravitational force throughout the gravitational field. A point particle need not experience tidal stress in order for it to move through a field with tidal gradients

Why is each contribution precisely 1/2 (or is that just approximate), and are they always 1/2 and 1/2 independent of the massive body's density?

Yeah. It is weird that its exactly 1/2. As to why all I can say is that is what the math says. If there is another answer then I'm unaware of it. Regarding "always"; This will be true for all bodies which have a spherically symmetric distribution of matter because the gravitational field for such bodies will always be the same with the only difference being M, the mass of the body.

I'd like to clarify something regarding tidal force and spacetime curvature; The notion of tidal forces in GR is different than that in Newtonian gravity. In the later there are tidal forces present when there are differences in the gravitational acceleration throughout space. In the former case its a bit different in that tidal forces are more than that, i.e. tidal forces are said to exist when two nearby geodesics deviate. If there were no spatial contraction then the factor of two regarding the deflection of light would not be required, even though there is a variation of gravitational acceleration throughout space. In the case of Einstein's first calculation of the deflection of light he did use a curved spacetime in that he took into account the variation of the potential with position. What he didn't take into account was spatial contraction. It is the absence, not of spacetime curvature, but the absense of spatial contraction that is the cause of the lack of that factor of 2.

Pete

 

[Fast77]

OK I think light has mass because it's affected by gravity which means it has weight. So if light had no mass then it would be 0x9.8=0 and no weight, so gravity does not affect it. Which means it must have some mass (0.00000000000000000000000000000000000000000001kg).
By the way, Einstine was wrong and right. "That an atom would get more massive then the universe if it reached near speed of light". A particlee that has no mass or heat does not exist. But what if we could find a particlee that has mass, but no affected by gravity and make our space ships with that substance in the solar system(elevators). I came up with this theory, but since i have not learned calculas yet I don't know the math behind it. I have found this substance/ particlee and would like to share it with the world, if anyone is interested please message me. I also have many more theories to share.
Masih,13

 

[neh4pres]

OK I think light has mass because it's affected by gravity which means it has weight. So if light had no mass then it would be 0x9.8=0 and no weight, so gravity does not affect it. Which means it must have some mass (0.00000000000000000000000000000000000000000001kg).
By the way, Einstine was wrong and right. "That an atom would get more massive then the universe if it reached near speed of light". A particlee that has no mass or heat does not exist. But what if we could find a particlee that has mass, but no affected by gravity and make our space ships with that substance in the solar system(elevators). I came up with this theory, but since i have not learned calculas yet I don't know the math behind it. I have found this substance/ particlee and would like to share it with the world, if anyone is interested please message me. I also have many more theories to share.
Masih,13

I like the way you think.. I also don't have the proper education.. But imagination is the importent thing.. I would love to discuss theories with you..

 

[DaveC426913]

This thread is two years old.

OK I think light has mass because it's affected by gravity which means it has weight.

No. Gravity curves spacetime. Light, like mass, follows the geodeisc of this curved space. It is not necessary for light to have mass in order to do this.

By the way, Einstine was wrong and right. "That an atom would get more massive then the universe if it reached near speed of light". A particlee that has no mass or heat does not exist. But what if we could find a particlee that has mass, but no affected by gravity and make our space ships with that substance in the solar system(elevators). I came up with this theory, but since i have not learned calculas yet I don't know the math behind it. I have found this substance/ particlee and would like to share it with the world,

Stay in school. Learn.

 

[DaveC426913]

imagination is the importent thing..

Imagination is good. But you will have to choose between doing something practical with it and merely fantasizing.

If you want to do something practical, stay in school. You will learn why these ideas are naive, and you will learn to develop more robust ideas.

 

[atyy]

The Weight of Light
http://focus.aps.org/story/v16/st1

 

[Dale]

OK I think light has mass because it's affected by gravity

Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

 

[K^2]

No. Gravity curves spacetime. Light, like mass, follows the geodeisc of this curved space. It is not necessary for light to have mass in order to do this.

I don't know why so many people here are allergic to specifying which mass they are talking about, which causes all kinds of disagreements over terms and formulae where there should be none.

Light has no rest mass. It has gravitational mass. In a gravitational field, it has a weight. If you take lights dp/dt in gravitational field, you get a non-zero value. That's a force. That's weight. It's an inertial force, yes, but so is the weight of any other object.

 

[Naty1]

OK I think light has mass because it's affected by gravity

Not likely in the sense you mean it...but light does have relativistic mass if that's what you mean. See post # 4 in this thread.

From E = mc² we know energy and mass are two sides of the same entity...Einstein's work showed that in either form, mass or energy, gravity has an influence. It turns out pressure also has gravitational effects.

 

[Mueiz]

Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

No.. Newtonian gravity requires an object to have mass in order to be affected by gravity
that is clear from the law . if we put m=0 then f=0 so acceleration = o according to Newton's first law.
What is independent of mass is the magnitude of acceleration and not the existence of acceleration
In GR yes .. because acceleration is a result of space-time curvature (caused by M) which is independent of m
This is one of the differences between GR and Newtonian theory

 

[Dale]

if we put m=0 then f=0 so acceleration = o according to Newton's first law.

How much force does it take to produce a finite acceleration of a particle of 0 mass?

 

[granpa]

acceleration = 0/0

http://en.wikipedia.org/wiki/L'Hôpital's_rule

 

[Fast77]

Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

Oh Yes. I never accually thought of the acceleration and it's true, but it's still affected from something. I think if a substance is not affected by gravity then it would have no problem passing the light barrier. 1000 years ago people never dreamed of cars or planes, but we can now beak the sound barrier. Why not the light barrier, if we found a substance that is not affected by time, space or gravity.

Thanks for pointing the acceleration.

Masih,13

 

[Fast77]

Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

This thread is two years old.

No. Gravity curves spacetime. Light, like mass, follows the geodeisc of this curved space. It is not necessary for light to have mass in order to do this.




Stay in school. Learn.

Please I thought you were man of logics. Why is it that humans are so stupid, especially adults. They kill, they waste and all they want is power, but kids are smarter and yet because of jealousy and power all adults do to kids it make them have a bad life. By the way I can't imagine you being a 13 year and thinking the way I have and done and what have you discovered all your life with your stupid education. Yes I know education is good but what's more important is the way you think.

Masih,13

 

[Dale]

if we found a substance that is not affected by time, space or gravity.

That's a pretty big "if". But yes, something like that would clearly not behave according to current physical laws.

Thanks for pointing the acceleration.

You are very welcome!

 

[Fast77]

That's a pretty big "if". But yes, something like that would clearly not behave according to current physical laws.

You are very welcome!

Non Newtonian Fluids behave differntly then regular fluids. Even antimatter, so there is a possibility that just mabey Dark Matter might be the key to breaking the light barrier or reach that speed. That is what I believe and think. Another soloution might be to make a spaceship from space itself. I know it sounds stupid and insane but our universe is believed to be living in a ball of space.

Masih,13

 

[Dale]

there is a possibility that just mabey Dark Matter might be the key to breaking the light barrier or reach that speed. That is what I believe and think. ...

Masih,13

Look Masih, I hope when my kids are 13 that they will be as excited about physics as you, but you need to be careful. The rule against overly speculative posts is strongly enforced here.

 

[Fast77]

Look Masih, I hope when my kids are 13 that they will be as excited about physics as you, but you need to be careful. The rule against overly speculative posts is strongly enforced here.

Yes I understand and I shall take your advice, but all I am doing is including the possiblities that are not being presented. Thanks for the that.

By the way is that a rule in this website?



Masih

 

[Dale]

Yes I understand and I shall take your advice, but all I am doing is including the possiblities that are not being presented. Thanks for the that.

No problem, I hope you will continue to enjoy physics in general and PF in particular. It can be a great resource.

By the way is that a rule in this website?

Yes, there is a link to the rules at the top of each page, rught under the Physics Forums banner. It is under the bold heading "Overly Speculative Posts". I do realize that speculating can be a lot of fun, but that it the purpose of friendly chats and science fiction novels, not this site. There is plenty of enjoyable stuff to discuss that is well understood and not speculative.