Why does mass/energy have inertia?

[elpidiovaldez5]

I have been googling this topic for some time, but I still don't know if this is still an unsolved mystery of physics (it's just so) or if there is a deeper underlying theory.

I get the idea that mass/energy distorts spacetime, justified by thought experiments with moving objects and photons bouncing between mirrors. I see how the distortion in spacetime produces the effects we call gravity. But where does inertia come in ? All mass/energy has inertia, but I don't see that inertia (or resistance to acceleration) comes naturally out of the arguments about spacetime and gravity. Why is inertial mass equal to gravitational mass, such that all free falling objects accelerate at the same rate in a gravitational field ?

I am really not qualified to come up with ideas at this level, but I thought about the problem and came up with this: An object comprising mass/energy creates a distortion in spacetime around it, its gravitational well, which extends to infinity. If the object is pushed so that it begins to move, the gravitational well around it will move too, so as to stay centered on the mass. However we know that the effect on the gravitational well cannot propagate faster than light, so some of it will remain as it was until a ripple in spacetime reaches it. Could inertia be the result of an object trying to climb out of its own 'lagged' gravitational well ? Climbing out of a graviational well requires input of energy, which requires work to be done on the object which could explain inertial forces...

 

[sophiecentaur]

@elpidiovaldez5 you are jumping into General Relativity with both feet here. You would need to quote a reasonable reference about this as PF doesn't discuss personal theories.
Are you sure that you have a solid knowledge of Newtonian Mechanics? I have to wonder, in the light of your question:-

Could inertia be the result of an object trying to climb out of its own 'lagged' gravitational well ?

Objects with mass make no conscious effort to do things. Anthropomorphism really has no place in Physics.

 

[anorlunda]

I'm not sure what kind of answer you are looking for.

To me, Noether's Theorem (https://en.wikipedia.org/wiki/Noether_theorem), and the fact that Newton's Laws of Motion can be derived from that (plus the symmetries of nature) is a very satisfying answer. It is also much simpler than General Relativity.

I can recommend Leonard Susskind's video course on Classical Mechanics. Spending the time to go through those 10 lectures is very entertaining and it will give you an excellent grasp of all those things including the use of Noether's Theorem.

 

[russ_watters]

I get the idea that mass/energy distorts spacetime, justified by thought experiments with moving objects and photons bouncing between mirrors. I see how the distortion in spacetime produces the effects we call gravity. But where does inertia come in ? All mass/energy has inertia, but I don't see that inertia (or resistance to acceleration) comes naturally out of the arguments about spacetime and gravity. Why is inertial mass equal to gravitational mass, such that all free falling objects accelerate at the same rate in a gravitational field ?

Inertia isn't really related to gravity except by the totally unsurprising coincidence that both are proportional to the amount of matter you have. So are specific heat capacity and chemical binding energy and other properties.

 

[paradisePhysicist]

An object, such as a photon, will continue its trajectory. We can use this as an axiom.

Using this axiom, we can say that other objects, such as a molecule, will continue its trajectory. Therefore a bunch of molecules, that form 1 object, will continue its trajectory.

As for objects bouncing off other objects, when a liquid object bounces off another object, it loses its form. Thus there is no guarantee objects will keep their form when bouncing off of another object. At low speeds, objects will not penetrate through other objects but that is also not a guarantee.

 

[weirdoguy]

An object, such as a photon, will continue its trajectory.

There is no meaningfull way to define a trajectory of a photon - and I guess it boils down to the fact that there is no position operator for massles spin-1 (and higher) fields. Thus, one should avoid using photons outside of quantum context. Unless by "photon" you mean something different, as unfortunately quite a lot of physicists do...

 

[paradisePhysicist]

There is no meaningfull way to define a trajectory of a photon - and I guess it boils down to the fact that there is no position operator for massles spin-1 (and higher) fields. Thus, one should avoid using photons outside of quantum context. Unless by "photon" you mean something different, as unfortunately quite a lot of physicists do...

I mean like how if you shoot a laser, or a star emits light, that light moves in a direction...

 

[weirdoguy]

Yes, but light is not a stream of photons, the way river is a stream of water molecules. Again, if context is non-quantum then there is no need to invoke photons.

 

[paradisePhysicist]

Yes, but light is not a stream of photons, the way river is a stream of water molecules. Again, if context is non-quantum then there is no need to invoke photons.

Should I use the term light-burst instead?

 

[weirdoguy]

Yes

 

[sophiecentaur]

Should I use the term light-burst instead?

There is a popular misconception that talking in terms of electrons and photons is 'posher' and also makes things easier to understand. The reverse is the case; wherever possible, Physicists steer clear of quantum descriptions when they can - even the guys who actually have a chance of knowing enough about QM. 'Posh' means working at the appropriate level and appropriate usually means sticking to classical.

 

[paradisePhysicist]

There is a popular misconception that talking in terms of electrons and photons is 'posher' and also makes things easier to understand. The reverse is the case; wherever possible, Physicists steer clear of quantum descriptions when they can - even the guys who actually have a chance of knowing enough about QM. 'Posh' means working at the appropriate level and appropriate usually means sticking to classical.

Hmm, is using light-burst okay, or is it better to just describe the trajectory of molecules for my post earlier?

 

[sophiecentaur]

or is it better to just describe the trajectory of molecules for my post earlier?

But molecules have mass and light has no mass. The 'path' near a massive object will be different.

 

[paradisePhysicist]

But molecules have mass and light has no mass. The 'path' near a massive object will be different.

Light is still attracted to gravitational bodies isn't it, what are the significant differences from say, a molecule? Yes the path will be different but also molecules will have very subtly different paths than heavier molecules.

 

[sophiecentaur]

Light is still attracted to gravity isn't it,

It doesn't behave in the same way as objects with mass. Have you ever heard of light in orbit?
Read Wiki about General Relativity before trying to make. your own conclusions.

 

[Ibix]

Have you ever heard of light in orbit?

Technically yes, but only at one radius and it isn't a stable orbit. As you say, this is rather different from the orbital behaviour of massive particles.

 

[paradisePhysicist]

It doesn't behave in the same way as objects with mass. Have you ever heard of light in orbit?
Read Wiki about General Relativity before trying to make. your own conclusions.

Had no particular conclusion on the matter was mostly looking for your opinion. From what I read online, light is pulled by gravity. And no I haven't heard of light in orbit, is that impossible or just doesn't occur because solar systems aren't wide enough for how large the orbit would be?

 

[256bits]

Had no particular conclusion on the matter was mostly looking for your opinion. From what I read online, light is pulled by gravity. And no I haven't heard of light in orbit, is that impossible or just doesn't occur because solar systems aren't wide enough for how large the orbit would be?

You mean, solar systems aren't small enough for how small the orbit would be.
At light speed, the orbital radius around the sun would be inside the sun, which wouldn't happen.

 

[Ibix]

At light speed, the orbital radius around the sun would be inside the sun, which wouldn't happen.

Not quite. There is no lightlike orbit inside the Sun. There would be an unstable 4.5km radius (9$\pi$km circumference, anyway) light orbit around a solar mass black hole.

 

[256bits]

Not quite. There is no lightlike orbit inside the Sun. There would be an unstable 4.5km radius (9$\pi$km circumference, anyway) light orbit around a solar mass black hole.

Which is why a light orbit around the present day sun wouldn't happen.

 

[paradisePhysicist]

You mean, solar systems aren't small enough for how small the orbit would be.
At light speed, the orbital radius around the sun would be inside the sun, which wouldn't happen.

sophiecentaur said "light has no mass" which gave me the notion that light would be less effected by gravity, are you saying that light actually gets more pull from gravity than planets around the sun do? Also here is a disclaimer, this is only a only notion not a conclusion, I don't have any particular opinion on this matter yet.

 

[berkeman]

sophiecentaur said "light has no mass" which gave me the notion that light would be less effected by gravity, are you saying that light actually gets more pull from gravity than planets around the sun do? Also here is a disclaimer, this is only a only notion not a conclusion, I don't have any particular opinion on this matter yet.

https://en.wikipedia.org/wiki/Einstein_ring

https://en.wikipedia.org/wiki/Photon_sphere

 

[256bits]

sophiecentaur said "light has no mass" which gave me the notion that light would be less effected by gravity, are you saying that light actually gets more pull from gravity than planets around the sun do? Also here is a disclaimer, this is only a only notion not a conclusion, I don't have any particular opinion on this matter yet.

If a planet traveled near light speed would you think an orbit would be of a lesser or a larger radius than light.
Orbital speed matters for both.
Pluto has a low orbital speed since it is so far out. Compare that to Mercury.