I just can’t put my head around the fact that gravity isn’t just a force
EDIT : I now get how it gets stretched and how it’s comparable to putting a ball on a stretchy piece of fabric and everything but why is *gravity* comparable to that. I guess my new question is what is gravity ? 🙂 and how can weight affect it ?
In: Physics
Time as we perceive it, is just a side effect of moving through 3D space. If something impacts movement through space, like gravity, it affects time.
I like to think of it (perhaps completely erroneously) as a parable to optics: Like diffraction isn’t a force, but curvature caused by differing density of media, gravity isn’t a force but curvature caused by differing density of spacetime.
The big difference is density of media usually has sharp defines, where spacetime density is made up of gradients.
It’s the other way around. You have a bunch of answers, so I’d assume there’s something decent in there, but it’s actually the fact that time is curved that makes you accelerate, causing gravity, and not the other way.
I recommend this General Relativity video (it’s available in French too):
[A new way to visualize General Relativity](https://www.youtube.com/watch?v=wrwgIjBUYVc)
It will explain why the “stretchy piece of fabric” visualization is actually pretty bad (as it misses the time stretching, and it explain gravity with… gravity) and propose a few different visualizations.
Unfortunately, I can’t really explain the content of the video with text, here, as this is about visualizations… The only thing is that in reality, when we “fall” we actually convert our time-speed into space-speed (if one were able to see space-time itself).
The easiest way is to think about this:
We *know* that no matter how you are moving everyone observes light to travel at c.
So person A sitting in the chair measures a photon at c. PersonB flying in a rocket at .99c STILL MEASURES a photon flying past them to travel at C. We know this because they discovered it trying to investigate something else.
Now consider that if light always moves at c regardless of frame of reference, how else can equations that deal in time or distance make sense?
Distance = velocity times time. If velocity MUST be constant at light speed, what else can change?
Very simple intuition:
Velocity = Distance / Time
(you measure your car speed in miles per hour).
The speed of light, C, is constant in all reference frames (hint: this is part of what let us discover space & time warp).
Since gravity curves space, it makes Distances longer (think windy road is longer than straight road between two points).
If C is fixed, and mass/gravity elongates space, the only other thing that can change to make it work is time changing.
C = ⬆️ Distance / ⬆️ Time
i.e. time slows down for light (and actually all things) in high gravity/warped space
Einstein’s brilliance was showing how a constant speed of light in ANY reference frame (moving rocket, in curved space time, etc…) implies that time and space themselves warp.
Note: the actual relationships involving mass are what general relativity specifies and are more complicated. This example only explains the intuition on how time and distance are fundamentally related.
I think about it like this: The speed of light is a universal constant. Nothing can travel faster, not even information, or reality. So in a very real sense, the speed of light is also the speed of time. The speed of reality.
Also, space and time are two facets of the same thing. They even call it space-time.
Gravity isn’t a field or a force at all. It is the effect that matter in the universe has on space-time. It curves space, and since they are the same thing, gravity also curves time. So the closer you get to a massive object, the more compressed time becomes. When you were floating in space (or orbiting a mass) you are experiencing uncompressed time.
Now for acceleration: When you roll down a hill you accelerate, right? In relativity calculations, gravity and acceleration are the same thing with different names. Areas of space-time get compressed by massive objects. As you approach a massive object, the compressed time feels normal to you, but an outside observer is experiencing uncompressed time, so your and their measurements of your speed would be different. As you approach the mass you experience time the same, but an observer in orbit would see you falling more slowly because of the compression effect. The more massive the object, the more pronounced the difference.
If someone saw you fall into a black hole, they would see you zip into it at enormous speed, but from your perspective, it would take much longer. The event horizon non-physical area around the black hole where the gravity is so strong that the speed of light is not fast enough to overcome the space-time compression. And remember that acceleration and gravity are the same thing. The effect is so pronounced that if you were in a spaceship that accelerated to near the speed of light, a trip to the nearest star might seem like 4 years to an outside observer, but would only seem like 8 months to you.
If it’s hard to grasp, don’t worry, even though we know that the universe works in this way, it’s very counterintuitive. We can prove it mathematically, and confirm it’s true with observations, but any physicist will admit that is very hard to understand because the scale that we live at, Newtonian physics rules. Our brains are meant to understand cause an effect of small objects. During our evolution we were never challenged with universal constants that affect black holes.
Here’s a video that explains it better than I can. https://youtu.be/QQRj78jOxWo
Here’s a video about why gravity is not a force: https://youtu.be/XRr1kaXKBsU
Hi. Isn’t gravity technically still a mystery?
Meaning the how not the fact that we have it etc. I think even Neil Degrassi Tyson said it once not too long ago either.
Gravity is what we call the math that explains how objects move. We can predict a lot stuff using that math. We can tell you exactly where Venus will be in the night sky eight years from now using that math, it’s really good math.
Very clever people noticed that there were predictions that weren’t so accurate using our gravity math, and they worked on the problem a long time until they discovered that they could make predictions more accurate by distorting time.
Since the new math is more accurate we can say with as much certainty as anything that space and time are both variables of our universe that can be measurably effected.
We evolved to pick ticks off one another and spot predators in long grass. Don’t put so much pressure on that brain of yours to wrap itself around this. You’re doing fine.
The fun part is being still, moving in a straight line at constant speed AND accelerating along your reference frame ARE ALL THREE THE SAME THING. Now, what does this “reference frame accelerating with you” thing means? If you accelerate in a car, you get pushed back. That’s obviously not the same thing as just sitting still at your living room. That happens because your reference frame (the car) gets accelerated while you don’t.
BUT if you are falling with your car then both you and the car are accelerating at the same time. Both of you are falling at 9.8m/s^2 . So that’s the same as being completely still at space! (That’s how those zero g planes work). Thanks, gravity!
BUT wait again! Orbiting (like the moon orbits the Earth or the Earth orbits the moon) IS falling! Orbiting is literally just falling constantly. So, orbiting must be the same as standing still and the same as moving in a straigth line at constant speed.
But how the hell could we say the moon is moving in a straigth line when we literally see it going in circles around the Earth every single day? That’s where deforming the spacetime comes into play. The moon IS moving in a straight line… is just that the space it’s moving through is curved around the Earth. It’s moving straigth in a curved space. In a curved spaceTIME, since they’re kinda the same thing. Thanks again, gravity!
Latest Answers