Does gravity travel at the speed of light?

These threads make my stomach hurt. It hurts, it churns. The reply quoting Einstein gave me dry heaves
 
TheSpirit said:
gravity doesn't travel...

gravity is an effect on objects due to one mass being larger than another. There is a maximum acceleration level for the gravity on earth, which is roughly 9.8 m/s2, but... it doesn't travel, per se.
Click to expand...

"max acceleration"

that's hilarious

i am doubting this guy even knows what acceleration is
 
}x{ EViL JiM said:
And nothing exsists in the universe which can go faster than light or transmit data faster than light (if you will) because that violates causality (cause and effect)
Click to expand...


Mmmm Quantum Physics.
 
actually, this thread is answered quite nicely by the one about spacetime having a "ping" below it

"pinging" is a good metaphor for the cause/effect relationship of everything, and explains quite nicely how simultaneity is relative
 
^Edge^ said:
I seem to remember learning that gravity propagated instantaneously in highschool for some reason... I must be remembering wrong. A lot of physics phenomena like this are unknown to me... For instance, how photons travel at the speed of light (and are massless) but clearly transmit kinetic energy (in high school we had a mirrored paddlewheel hanging in a vacuum and shot a light at one side of it, and it spun... it's the same principle behind solar sails).
Click to expand...
It's kind of hard to imagine a massless particle. You have to think of light as waves - waves clearly have energy of hf. However, in some cases it is important to recognize that albeit waves, light do have particle-like properties which explain several phenomena. The energy in light is imparted in discrete quantities by, what we call, photons. This explains such things as a threshold frequency, as each photon individually needs to surpass the given energy barrier, and each photon's energy is dependent on its frequency as opposed to the wave's intensity, according to classical thinking.
 
actually I think I know what you meant now

if you don't go below the surface of the earth, 9.8 m/s^s is about the max acceleration that one can experience from the earth's gravity

so i guess you're right

of course if you ever explained that to someone learning physics they would say, "what"

what you should say is that the force of gravity is inversely proportional to the square of the distance between two objects, therefore if you assume that one can never go below the crust of a planet, the max acceleration of that planet would be experienced while on the crust

another great metaphor for gravity warping spacetime is a bowling ball on a rubber sheet suspended in the air

you have a nice rubber plane, now put a bowling ball on it. the rubber is distorted. If you drop a marble somewhere on the plane, the marble will roll towards the bowling ball due to the distortion

this is just like gravity. We're all rolling down towards earth because the curvature of spacetime is shaped in that way.

Newton's (1st?) law says that in the absence of outside forces, all things will travel in a straight line at a constant speed

now imagine that gravity is not a force and is just a warping of spacetime. An object who travels into the warping caused by the earth will appear to bend it's trajectory due to an unseen "force." In actuality though, it is moving in a perfectly straight line, and it's trajectory does not change. Only the makeup of spacetime changes

Imagine one of those biking tracks they use in the olympics that are curved inward. The bikers don't really need to turn their wheel or lean inward very much on the turn, because the ground they are driving on turns for them. They basically keep driving like normal, and turn due to a curvature of the space they are driving on.

That is like gravity.
 
SegaRob said:
It's kind of hard to imagine a massless particle. You have to think of light as waves - waves clearly have energy of hf. However, in some cases it is important to recognize that albeit waves, light do have particle-like properties which explain several phenomena. The energy in light is imparted in discrete quantities by, what we call, photons. This explains such things as a threshold frequency, as each photon individually needs to surpass the given energy barrier, and each photon's energy is dependent on its frequency as opposed to the wave's intensity, according to classical thinking.
Click to expand...

Yep, I'm familiar with the particle/wave duality that you have to consider when dealing with electromagnetic radiation. It just seems intuitively strange to me that the paddlewheel in the vacuum moves without any mass (or 'classical waves' since it's in a vacuum) pushing it. I guess I just like to hold on to the Newtonian paradigm. :shrug:

I wish I could take some physics courses now, but I have no room to fit them in to my schedule. :(
 
Last edited:
OT:

Loop quantum gravity is a really interesting topic. Though, every time I read an article about it, my head explodes.

Every time.
 
^Edge^ said:
Q: "In one section, you say that gravity has a finite propagation speed, which is the speed of light. However, gravity is caused by particles called gravitons, as you know, and these are massless. If they are massless, then they travel faster than light i.e. Tachyons, so which do you think they are? Massless or have mass?
thank you for your time"

Dave

UK



A: "Dave,
Yes, you are correct that the graviton is thought to be massless (we have never actually observed one, but we are pretty sure that they are there.) But this does not make it a 'tachyon.' There are other examples of massless particles, such as light itself (photons) which travel at the speed of light. In other words, just because something has no mass does not mean it will travel faster than light.

A tachyon (theoretically) would be a particle that travels faster than light. But no tachyons have ever been observed.

So I still hold that gravitons are massless and that they travel at the speed of light.
Thank you for your question."

---------------------------------------
Click to expand...

Tachyons don't exist :)

I seem to remember learning that gravity propagated instantaneously in highschool for some reason... I must be remembering wrong. A lot of physics phenomena like this are unknown to me... For instance, how photons travel at the speed of light (and are massless) but clearly transmit kinetic energy (in high school we had a mirrored paddlewheel hanging in a vacuum and shot a light at one side of it, and it spun... it's the same principle behind solar sails).
Click to expand...

Newton said that gravity propagates instantly - this is what you were taught in high school. It's not a suprise that Newton thought this, because c is really, really fast. It takes very very precise equipment and knowing exactly what to look for to see the effects of gravity propagating at c.

Classical physics and modern physics have different explanations for most things.

Classical physics says gravity is an instantaneous effect, modern says it travels at c. Classical physics says the mass of an object is constant, modern physics says..well that gets kind of hairy.

Einstein showed that E=mc^2, namely that mass and energy are related. Photons have energy and they impart this energy to the solar sail, and this energy is converted into kinetic energy through interatomic processes. They don't transmit "kinetic energy" in the sense with which you are familiar, it suffices to say that they really just transmit "energy" which can then be expressed in many different ways. (heat, light, motion, radiation, whateva)
 
so if the sun just disappeared we would contiune on a orbital path for 8 minutes before the gravity ran out? or would we go out on a tangent before we saw the sun disappear?
 
^Edge^ said:
Yep, I'm familiar with the particle/wave duality that you have to consider when dealing with electromagnetic radiation. It just seems intuitively strange to me that the paddlewheel in the vacuum moves without any mass (or 'classical waves' since it's in a vacuum) pushing it. I guess I just like to hold on to the Newtonian paradigm. :shrug:

I wish I could take some physics courses now, but I have no room to fit them in to my schedule. :(
Click to expand...
Waves also have momentum. In classical mechanics p=mv which like F=ma is only an approximation, so a particle with 0 mass can still have a momentum if it has sufficient velocity (c). Momentum for waves is given by p=hf/c. I could go into further detail about the relations between these different equations but I think you should understand this now.
 
xsMaster said:
so if the sun just disappeared we would contiune on a orbital path for 8 minutes before the gravity ran out? or would we go out on a tangent before we saw the sun disappear?
Click to expand...
now this, is an interesting question..
 
^Edge^ said:
Yep, I'm familiar with the particle/wave duality that you have to consider when dealing with electromagnetic radiation. It just seems intuitively strange to me that the paddlewheel in the vacuum moves without any mass (or 'classical waves' since it's in a vacuum) pushing it. I guess I just like to hold on to the Newtonian paradigm. :shrug:

I wish I could take some physics courses now, but I have no room to fit them in to my schedule. :(
Click to expand...

Physics courses won't make it any more intuitive :D

If you think about it enough, you eventually realise that "it MUST be this way, because this is the ONLY way it could possibly be for everything we've observed to work in the way it does"

You look at quantum effects long enough, and it doesn't become anymore "intuitive" but you realise that it all MUST work this way or the world wouldn't be as it is. It's reality as shown by observation.

It only really makes sense because there is no other explanation :D I mean, a particle having a frequency and wavelength is just dumb. How can a particle have a frequency and wavelength?

Truthfully, it can't. In English we've defined 'frequency' and 'wavelength' to be properties of waves. But it turns out that in reality we must use these concepts to explain behaviour which is at odds with everyday intuition. And after working through it enough, we realise that there is really no other option. So then it becomes a matter of accepting the facts.

Eventually our children will learn and observe quantum processes from an early age, they'll grow up and in highschool will learn quantum mechanics and be like "duh this is easy. Of course the photon's kinetic energy is given by Planck's constant times it's frequency! That is common sense! Fucking high school"
 
You must log in or register to reply here.
Back
Top