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Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create s

by _8j50 4y ago

Isn't gravity just a fundamental force? Could it be matter reacting to gravity? Light is energy but gravity is not energy and has no mass so how can it create something out of nothing? How is hawking radiation even possible unless something else was involved?

habibur 4y ago
Or gravity is just bending of space around a mass. And not a force.
- cronix 4y ago
  Not a physicist, but how do you get something to bend without a force exerted upon it?
  - habibur 4y ago
    They attribute it to the fact that space around the object is bent. Which is causing the other object to drop, accelerate or bent. Check General Relativity's explanation of Gravity. That might not answer all of your or my question. But maybe we didn't give enough time to ponder on that line.
  - scotty79 4y ago
    The cool thing about gravity is that it doesn't bend anything it just alters space so that straight things are bent.
    - _8j50 4y ago
      So without it planets are like cubes? Can't even imagine how deformed people would be
  - a_wild_dandan 4y ago
    It's just unfortunate nomenclature. Space(time) is a set of dimensions. It's not a substance, like a ruler, anymore than temperature is a substance in a thermometer. The word "bend" misleads us by equivocating "bending" a ruler (forcefully rearranging its atoms into a new shape) with "bending" spacetime (e.g. changing how fast someone's clock ticks).
  - LegitShady 4y ago
    the simplest way to describe it is that an object in freefall moves in geodesic in spacetime, which is a is not straight line in 3dimensional classical physics but produces gravitational motion. https://en.wikipedia.org/wiki/Geodesics_in_general_relativity https://en.wikipedia.org/wiki/Geodesics_in_general_relativit... The basic idea is that depending on your frame of reference, you might experience what is called a fictitious or psuedo force that disappears in a more simple frame of reference. Examples of these include centrifugal force, the Coriolis effect. https://en.wikipedia.org/wiki/Fictitious_force https://en.wikipedia.org/wiki/Fictitious_force Einstein's great revelation in the theory of general relativity was that there was no difference between gravity and free fall - which is called the equivalence principle. https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle In special relativity, einstein's theory had a preference for inertial frame observers where the observer essentially became the origin of coordinate system to map space by, but in general relativity einstein does away with inertial frames as privileged, and an observer is literally decided by what light it can see, and their coordinate system cannot be universal because of curved space. https://en.wikipedia.org/wiki/Inertial_frame_of_reference#Special_relativity https://en.wikipedia.org/wiki/Inertial_frame_of_reference#Sp... https://en.wikipedia.org/wiki/Observer_%28special_relativity%29 https://en.wikipedia.org/wiki/Observer_%28special_relativity...
  - antonvs 4y ago
    General relativity doesn't really say forces don't exist, it just says that they're a secondary, and relative, effect caused by a more fundamental phenomenon, i.e. the curvature of spacetime due to the presence of mass/energy. Just as centrifugal force is a relative effect that you only observe in a rotating (i.e. accelerated) reference frame, the force of gravity is also a relative effect that you only observe in an accelerated reference frame. Standing on Earth, we're accelerating towards the center at 9.8 m/s^2, and we feel that as weight due to a force of gravity that we experience in our reference frame. If you jump out of a plane, though, you feel weightless (especially if you can arrange to do so in a vacuum), because you're in "free fall" - you're following the curvature of spacetime. In your reference frame, you're not experiencing the force of gravity. This demonstrates that gravity, like centrifugal force, is a "fictitious" force - which doesn't really mean that it's not real, but rather that it's a secondary effect that depends on your reference frame.
- joadha 4y ago
  I used to share your confusion on this topic, but I think I got it: General Relativity posits that gravity embodies both of those qualities you mentioned. It is a force that bends space and dilates time.
- narag 4y ago
  https://www.youtube.com/watch?v=ZHxoM9lvzVA https://www.youtube.com/watch?v=ZHxoM9lvzVA
Maursault 4y ago
> Isn't gravity just a fundamental force? Not really. Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1] [1] https://en.wikipedia.org/wiki/Gravity https://en.wikipedia.org/wiki/Gravity
- _8j50 4y ago
  Ok, so how does curvature cause light and radiation on its own?
- jerf 4y ago
  You have to remember that General Relativity is wrong. We don't know exactly how it's wrong, but it's wrong. In light of that fact, it's not really a proper answer to say that "General Relativity says it isn't so it isn't." It is true that GR says gravity isn't a force, but the implication doesn't follow. We don't know whether a Grand Unified Theory would have gravity as a force or not. Given that a great deal of the answer to "Is gravity a force?" depends heavily on the definition of force being used, we don't even know what the final definition will be in light of a GUT. There may or may not be a meaningful distinction between gravity and other forces in the real universe. In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question. See also https://www.youtube.com/watch?v=EmrZ3ZaXmS4 https://www.youtube.com/watch?v=EmrZ3ZaXmS4 .
  - Maursault 4y ago
    > You have to remember that General Relativity is wrong. I really don't, because it really isn't. All I need remember is that you are wrong.
  - scotty79 4y ago
    We don't have a single test that shows that GR is wrong in any way and we have many tests that show that it is exactly right to any arbitrary precission we can dream of achieving.
_8j50 4y ago
Ok, why all the downvotes even after starting a healthy discussion thread? And can someone help me understand in layman terms how current physics posits that gravitation is generating light or even hawking radiation?
- pa7x1 4y ago
  Gravity couples with anything that carries energy (in more precise terms with the energy-momentum tensor). Photons carry energy. Therefore there is interaction between photons and gravity. In particular, you can write Feynman diagrams for the interaction between a quanta of gravity (a graviton) and a photon. Some of the simplest ones look like this: https://www.researchgate.net/figure/Tree-level-Feynman-diagrams-of-graviton-photon-transitions-3-Wavy-lines-represent_fig3_328933275 https://www.researchgate.net/figure/Tree-level-Feynman-diagr... There you can see different ways in which a pair of gravitons can interact leading to a pair of photons. If you want to dig deeper in the math behind it: https://physics.stackexchange.com/questions/335917/how-do-i-derive-the-feynman-rules-for-graviton-photon-coupling https://physics.stackexchange.com/questions/335917/how-do-i-... Regarding Hawking radiation, it's not gravity itself that causes it. It's a consequence of the existence of an event horizon. In fact, you can have a similar effect in flat spacetime (i.e. no gravity), an accelerating observer will see an event horizon and it will feel itself immersed in a thermal bath of particles at certain temperature that has to do with its acceleration. This is called the Unruh effect. Further reading: https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect
  - _8j50 4y ago
    Thank you so much, this was the explanation I was looking for.

by _8j50 4y ago

habibur 4y ago

Or gravity is just bending of space around a mass. And not a force.

cronix 4y ago
Not a physicist, but how do you get something to bend without a force exerted upon it?
- habibur 4y ago
  They attribute it to the fact that space around the object is bent. Which is causing the other object to drop, accelerate or bent. Check General Relativity's explanation of Gravity. That might not answer all of your or my question. But maybe we didn't give enough time to ponder on that line.
- scotty79 4y ago
  The cool thing about gravity is that it doesn't bend anything it just alters space so that straight things are bent.
  - _8j50 4y ago
    So without it planets are like cubes? Can't even imagine how deformed people would be
- a_wild_dandan 4y ago
  It's just unfortunate nomenclature. Space(time) is a set of dimensions. It's not a substance, like a ruler, anymore than temperature is a substance in a thermometer. The word "bend" misleads us by equivocating "bending" a ruler (forcefully rearranging its atoms into a new shape) with "bending" spacetime (e.g. changing how fast someone's clock ticks).
- LegitShady 4y ago
  the simplest way to describe it is that an object in freefall moves in geodesic in spacetime, which is a is not straight line in 3dimensional classical physics but produces gravitational motion. https://en.wikipedia.org/wiki/Geodesics_in_general_relativity https://en.wikipedia.org/wiki/Geodesics_in_general_relativit... The basic idea is that depending on your frame of reference, you might experience what is called a fictitious or psuedo force that disappears in a more simple frame of reference. Examples of these include centrifugal force, the Coriolis effect. https://en.wikipedia.org/wiki/Fictitious_force https://en.wikipedia.org/wiki/Fictitious_force Einstein's great revelation in the theory of general relativity was that there was no difference between gravity and free fall - which is called the equivalence principle. https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle In special relativity, einstein's theory had a preference for inertial frame observers where the observer essentially became the origin of coordinate system to map space by, but in general relativity einstein does away with inertial frames as privileged, and an observer is literally decided by what light it can see, and their coordinate system cannot be universal because of curved space. https://en.wikipedia.org/wiki/Inertial_frame_of_reference#Special_relativity https://en.wikipedia.org/wiki/Inertial_frame_of_reference#Sp... https://en.wikipedia.org/wiki/Observer_%28special_relativity%29 https://en.wikipedia.org/wiki/Observer_%28special_relativity...
- antonvs 4y ago
  General relativity doesn't really say forces don't exist, it just says that they're a secondary, and relative, effect caused by a more fundamental phenomenon, i.e. the curvature of spacetime due to the presence of mass/energy. Just as centrifugal force is a relative effect that you only observe in a rotating (i.e. accelerated) reference frame, the force of gravity is also a relative effect that you only observe in an accelerated reference frame. Standing on Earth, we're accelerating towards the center at 9.8 m/s^2, and we feel that as weight due to a force of gravity that we experience in our reference frame. If you jump out of a plane, though, you feel weightless (especially if you can arrange to do so in a vacuum), because you're in "free fall" - you're following the curvature of spacetime. In your reference frame, you're not experiencing the force of gravity. This demonstrates that gravity, like centrifugal force, is a "fictitious" force - which doesn't really mean that it's not real, but rather that it's a secondary effect that depends on your reference frame.
joadha 4y ago
I used to share your confusion on this topic, but I think I got it: General Relativity posits that gravity embodies both of those qualities you mentioned. It is a force that bends space and dilates time.
narag 4y ago
https://www.youtube.com/watch?v=ZHxoM9lvzVA https://www.youtube.com/watch?v=ZHxoM9lvzVA

Maursault 4y ago

> Isn't gravity just a fundamental force? Not really. Gravity is most accurately described by the general theory of relativity... which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines.[1] [1] https://en.wikipedia.org/wiki/Gravity https://en.wikipedia.org/wiki/Gravity

_8j50 4y ago
Ok, so how does curvature cause light and radiation on its own?
jerf 4y ago
You have to remember that General Relativity is wrong. We don't know exactly how it's wrong, but it's wrong. In light of that fact, it's not really a proper answer to say that "General Relativity says it isn't so it isn't." It is true that GR says gravity isn't a force, but the implication doesn't follow. We don't know whether a Grand Unified Theory would have gravity as a force or not. Given that a great deal of the answer to "Is gravity a force?" depends heavily on the definition of force being used, we don't even know what the final definition will be in light of a GUT. There may or may not be a meaningful distinction between gravity and other forces in the real universe. In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question. See also https://www.youtube.com/watch?v=EmrZ3ZaXmS4 https://www.youtube.com/watch?v=EmrZ3ZaXmS4 .
- Maursault 4y ago
  > You have to remember that General Relativity is wrong. I really don't, because it really isn't. All I need remember is that you are wrong.
- scotty79 4y ago
  We don't have a single test that shows that GR is wrong in any way and we have many tests that show that it is exactly right to any arbitrary precission we can dream of achieving.

_8j50 4y ago

Ok, why all the downvotes even after starting a healthy discussion thread? And can someone help me understand in layman terms how current physics posits that gravitation is generating light or even hawking radiation?

pa7x1 4y ago
Gravity couples with anything that carries energy (in more precise terms with the energy-momentum tensor). Photons carry energy. Therefore there is interaction between photons and gravity. In particular, you can write Feynman diagrams for the interaction between a quanta of gravity (a graviton) and a photon. Some of the simplest ones look like this: https://www.researchgate.net/figure/Tree-level-Feynman-diagrams-of-graviton-photon-transitions-3-Wavy-lines-represent_fig3_328933275 https://www.researchgate.net/figure/Tree-level-Feynman-diagr... There you can see different ways in which a pair of gravitons can interact leading to a pair of photons. If you want to dig deeper in the math behind it: https://physics.stackexchange.com/questions/335917/how-do-i-derive-the-feynman-rules-for-graviton-photon-coupling https://physics.stackexchange.com/questions/335917/how-do-i-... Regarding Hawking radiation, it's not gravity itself that causes it. It's a consequence of the existence of an event horizon. In fact, you can have a similar effect in flat spacetime (i.e. no gravity), an accelerating observer will see an event horizon and it will feel itself immersed in a thermal bath of particles at certain temperature that has to do with its acceleration. This is called the Unruh effect. Further reading: https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect
- _8j50 4y ago
  Thank you so much, this was the explanation I was looking for.