I understand that gravity is the curvature of space time. If that’s true, doesn’t it fully explain gravity? Why do we need gravity waves and/or gravitons?
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I am not a physicist, so please correct me if I’m wrong, but my understanding is that gravity waves are the result of the curvature of space time.
If we use the familiar metaphor of a bowling ball bending the fabric of a trampoline, then a gravity wave would be like a sudden disturbance (say dropping the bowling ball on the trampoline from a height) sending ripples out in the fabric in all directions, until the ball and trampoline settle.
Scientists have a theory called “general relativity” that explains how gravity works. According to this theory, gravity is caused by the way that objects bend something called “spacetime.” Spacetime is like a fabric that everything in the universe is made of. When an object with a lot of mass or energy is nearby, it can bend spacetime. This is kind of like what happens when you put a bowling ball on a trampoline. The bowling ball makes a dent in the trampoline, and if you roll a smaller ball towards the bowling ball, it will follow the curve of the trampoline and roll towards the bowling ball.
Gravity waves and gravitons are ideas that scientists have come up with to try to understand how gravity works on a very small scale. Gravity waves are like ripples in spacetime that are created when two massive objects collide or when a massive object changes shape or orientation very quickly. Gravitons are tiny particles that are thought to be responsible for carrying the force of gravity from one object to another. Scientists are still trying to figure out if these things really exist and how they might fit into our understanding of gravity.
Gravity waves are just changes in gravitational force due to changes in mass (well location of the mass)
You can’t really increase/decrease the mass of an object, but you can move it around, or maybe jettison a big chunk of it away. So if you take like two black holes orbiting each other very quickly, they move closer and farther from you increasing and decreasing the amount of gravitational force you feel as their distance from you increases and decreases, and that shows up as a wave which can be detected with sensitive enough instruments.
As for gravitons though gravitons are a particle that falls under the standard model of quantum physics. Curved space falls under relativity, a completely separate and independent theory. Both explain things well enough one on large scales and one in small scales, but both are not complete theories of everything. And unifying the two, is one of those things physicists as re trying really hard to do.
The idea with the graviton is that it’s behavior when properly described would reproduce the same behavior relativity describes, just the same way that relativity reproduces the same behavior newtons laws do given the right scale and wiggle room for measurement tolerances.
Nitpick: the term for those is “gravitational wave”. Gravity wave refers to something else, it refers to waves in a liquid that are driven by the weight of the water (as opposed to capillary waves, which are driven by the surface tension).
Gravitational waves are just a consequence of the curvature of space time. If you take some empty space time, and apply a small distortion to it, the spacetime wants to return to an ordinary flat state. This causes it to vibrate, just like how when you pluck a string on a guitar or what have you, it wants to return to where it started, and that makes the string vibrate. In spacetime, that vibration is a gravitational wave.
Gravity waves are a phenomenon, they were theorised but not observed until recently.
One of the goals of scientists at this moment is to complete the relationship between mass and space-time.
Space-time tells mass how to move and mass tells space-time how to curve/distort.
We already have a good understanding of the first, although the typical explanation is often badly explained
Gravity as we commonly observe is a pseudo-force, like the coriolis force or the centrifugal force, felt and usefull but not actually there.
The more correct explanation relates to general relativity and how the continuous distortion/movement/transformation/curving of spacetime effectively draggs the reality around you downwards and you experience the reaction force from the ground.
( I recommend the ‘scienceclic’ YouTube channel for a more intuitive explanation).
What we do not understand is how mass influences space-time. We know it happens, but not what mechanisms is involved.
Discoveries like the higgs boson are part of this search for an explanation.
This is probably way to dumbed down but this is how I like to think of it. Space is a medium much like air. When something has so much energy (speed) it creates shockwaves in the medium (sonic boom). Well spacetime is the same way. When something huge happens (colliding supermassive blackholes or supernovas) they release so much energy in spacetime it propagates out. What moves out through spacetime is the gravitational wave
The answer to your question of why that didn’t fully explain gravity is because of space. An object has mass and therefore has gravity and then therefore pulls on other objects with mass so that they move towards each other.
The problem is that because space is _empty_ what does the actual pulling? You need something that acts like that rubber band or sheet the pulls the objects together. If you imagine two bowling balls on a trampoline what is the thing acting like the net?
We know that light moves like a wave (light particles spread out over space) but acts on matter at a single point (individual particles can only measured/captured at exactly one spot). We think gravity works the same way. It spreads out over all the space around the object with mass, but can only be felt by other discrete objects with mass/energy (light is also affected by gravity). We have taken measurements that show that gravity moves at the speed of light (thanks black holes!) so we can observe “ripples” in spacetime. Something must be communicating that gravity at the speed of light similar to photons and we call it, colloquially, “gravity waves”
Imagine things like a really big trampoline. Big things bend the trampoline, so that other things tend to move towards them. This is a standard – if not great – explanation of gravity.
Now, if there’s a big enough thing moving fast enough, it not only bends the trampoline, it sends waves out. Technically, everything does it; but the waves are so small that you can’t detect them from most things – so only really big things moving really fast send out waves that are large enough you can detect them.
This is an answer to the second part of the question: why do we need gravitons?
We have two very good theories for describing how things work. For the very small, we have quantum mechanics, and the Standard Model of particle physics. This describes all the fundamental particles, how they interact, and what they can and can’t do. For very large things, we have general relativity, which describes the curvature of spacetime around massive objects. The problem is that these two models are incompatible. General relativity deals with exact positions and values, but quantum particles don’t have exact positions and values. According to quantum physics, every interaction is the result of a force-carrying particle, but quantum physics doesn’t describe a gravity-carrying particle. According to quantum physics, gravity shouldn’t exist. According to general relativity, gravity isn’t even a force.
So physicists made up a new particle, the graviton, to describe gravity. At first, it went pretty well. Then, it went less well. They discovered that for the graviton to exist, it needed to have properties never observed in an elementary particle before. We also can’t build a particle accelerator to detect a graviton, because gravity is such a weak force. If we built a particle detector the size of Jupiter and placed it next to a neutron star (the most dense non-black hole objects in the universe), we would expect to detect one graviton every ten years.
So physics is stuck. To understand how gravity is described by quantum physics, we need to measure the inside of a black hole. Which is impossible.
Space is so large it creates its own gravity field from literally just space itself. These fields can then interact with each other in interesting ways. Possibly leading to a big bang. I think they are called Quantum fields or something. A guy can come in and explain it like we all have physics degrees but honestly we just want the five year old basics right?
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