Gravitational time dilation is incredibly weak on Earth, since Earth is not very massive on cosmological scales.
Yeah even if it were stronger we probably wouldn’t be able to notice anything on the surface
Here, smoke this. Time dilation gets funky.
Matter always moves through time at the speed of light. Everything experiences their personal time the same way, regardless of gravity or acceleration.
Gravity and acceleration change the relative time between different objects that are experiencing different gravity or acceleration from the observer.
Actually, to answer your question as written… gravity does not bend time. Mass bends spacetime. Also, the acceleration of mass bends spacetime. Gravity is the result of mass moving through bent spacetime. What you feel as gravity is actually the force of the ground stopping you from falling further into the ‘gravity well’ of the planet. It’s the same as the centripetal force of a ball on a string spinning over your head.
It starts with a paradox - if light has no rest mass, how can it be affected by gravity? Gravity does not exert a force on massless objects.
The answer is that mass warps the fabric of spacetime around it. As the fabric is warped/bent by massive objects, light travels along these bends. This is why gravitational lensing occurs for example - distant galaxies warp spacetime, so light traveling around them gets bent back toward itself. We can use this to see distant objects in the cosmos that would otherwise be too faint to detect.
We don’t feel the effect because it’s very small, however we can detect it. Some atomic clocks are so incredibly sensitive that they can detect just a few meters’ difference in altitude from the surface of the earth.
Additionally, time dilations due to height differences of less than one metre have been experimentally verified in the laboratory.[14]
https://en.wikipedia.org/wiki/Gravitational_time_dilation?useskin=vector https://www.science.org/doi/10.1126/science.1192720
Gravity can affect light. If gravity didn’t affect light, it would be possible to violate the conservation of energy.
A high energy photon can turn into an electron/positron pair. A massless particle can convert into massive particles. Imagine I got really good at managing the back-and-forth transition between high energy photons and electron/positron pairs. Enough that I could control it with very high efficiency and control. (Unlikely, but we’re talking physics violations here. Assuming difficult engineering is fine.)
So let’s say I’m really good at converting back and forth between photons and electron/positron pairs. Good enough that I can build a machine that can do this in bulk with high efficiency. (We’ll also assume mirrors of unreasonable reflective efficiency.)
If gravity did not affect light, I could use such a conversion machine as a free energy device.
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Convert into photons and store in a mirrored box.
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Lift the mirorred box to a great height.
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Once at a height, convert the photons back to electrons/positrons.
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Lower the box down, using the falling weight to spin an electric generator.
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Once on the ground, convert back to photons and repeat.
Conservation of energy demands that gravity be able to affect light.
Is this why light cannot pass through a black hole? Probably a dumb question but doesn’t the light go into a singularity? And is that singularity like a fixed point in time?
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We do actually, just at a very small scale compared to what you may see around black holes.
Satellites in LEO need their clocks adjusted by a few dozen picoseconds I believe. Very cool.
We naturally move through spacetime on geodesics, which could be defined as “the trajectory along which you don’t feel the effects of gravity or acceleration”.
This kind of makes sense, i guess. We don’t feel it much because a) earth is smoll and b) we mostly stay on its surface and keep the distance. Is that about right?
When we’re on the surface of the earth we do feel the effect of gravity, because the surface is preventing us from moving on a natural geodesic (which would be a free-fall/orbit).
Could you elaborate?
I’m not an expert, but from my understanding it’s kind of something like this:
So a geodesic is a line of shortest length between two points on a curved surface, like two points on a ball, the geodesic would be the shortest line that connects the two.
If you look at flight paths compared to a flat representation of Earth, you’ll see that aircraft seem to follow a very strange arc, that’s because they’re actually flying the geodesic of their route, which looks funny when presented flat.
If you imagine Earth as the aircraft and spacetime as the globe, that’s essentially what is meant. However do keep in mind that aircraft follow a two-dimensional geodesic, whereas we would follow a three-dimensional one due to spacetime being four-dimensional; due to how reductions work. (3D object casts 2D shadow, 4D object casts 3D shadow).
My knowledge is rather vague past this point, but essentially orbits are geodesics against 4D spacetime, and gravity affects that temporal component.
Very nice answer. Thanks. I get the geodesic part. It’s the “don’t feel gravity” bit I’m missing. Do we mean don’t feel because it’s not a force acting on an object it’s a curvature in space-time making the geodesic?
That turn of phrase sounds off to me either way, because gravity defines it, so how x Can it be not felt? Do you just mean balanced between the geodesic gravitational “pull” and “push” of velocity’s vector? (Orbital velocity)
Your first part was exactly correct by my understanding, gravity is not a force, it doesn’t act on matter, so matter doesn’t feel it, because in a over-simplification it essentially doesn’t exist as far as matter is concerned.
Matter, the Earth, you and I, we occupy the three spatial dimensions of our universe where gravity does not act; we occupy the space part of spacetime. Space itself doesn’t occupy time, but it is intertwined with it, and thus consequence is shared bidirectionally.
In my mind I treat it somewhat like a train, space is the carriage with things inside, and time is the track and locomotive. They are separate, but one can’t be without the other (technically however in this analogy yes they can be, so it’s not perfect).
Gravity can bend the track, which will result in space going in a different way, because it has to follow the locomotive (time).
Now, relative to inside the carriage (space), things will move, anything that is loose will likely drift around, especially if it’s a tight bend; that’s why gravity does manifest spatial forces such as weight, inertia, G-force, etc.
Orbits, say our moon orbiting us, are actually straight lines relative to the orbiting body; Earth doesn’t “know” to curve around the sun, it has no “lateral” force, it is propagating through space in a straight line with velocity given to it billions of years ago. However, again, Earth is matter, which only occupies space, so while Earth is going along a straight line in space, inside that train carriage, spacetime, the locomotive and track, is bent into an ellipse due to gravity. Because matter travels in a straight line relative to itself that intrinsically resolves to a spacetime geodesic (again, to go back to the aircraft, they follow a straight line relative to themselves).
Again, not an expert, just a young adult with a fascination about space and time. :)
spacetime event interval compression and expansion with entanglement entropy of sub causets seems like a fun idea to build GR. the denser the spacetime atoms the denser stochastic sprinkling in that volume. causal set theory is neat to think about
People have answered the ‘how come we don’t feel it or realize it?’. The ‘How does gravity bend time’ part we mostly don’t know (yet). Mathematics that matches what we observe we have, it’s called General Relativity, it works very, very well. But how (and perhaps why) gravity bends time, what makes mass bend spacetime, we are relatively clueless about…
There are hopes quantum gravity will give more of a mechanism, one interesting approach currently gaining momentum, binds quantum information and computation into spacetime and starts to pull General Relativity out as a consequence. Here is a good primer.
Kepler: "I have conclusively proven that the planets orbit the Sun in elliptical paths!
“But why do they orbit in ellipses?”
Kepler: “I have no idea!”
Newton: “I have explained Kepler’s findings! There is an invisible force called “gravity” that attracts all masses in the universe to each other, at a magnitude proportional to mass and inversely proportional to the square of distance!”
“But what is this force, what causes it?”
Newton: “I have no idea!”
Einstein: “I have explained Newton at last! What we call gravity is actually the warping of space-time caused by massive objects!”
“But why do massive objects warp space-time?”
“I have no idea!”
There is always another why. For any mechanism explaining any phenomenon, there will always be another “why” you can ask. Science is ultimately about explaining our observations. And as any child knows, there is always another why.
Heh, fair cop, doesn’t really answer the question though, and quantum gravity, whenever it arrives is the next why. The interesting thing about the above theory is that instead of only reformulating the mathematics, it changes the underlying physical / philosophic paradigm.
Physics is reverse engineering the universe’s game engine.
Whe can only ever observe what’s happening, piece by piece, not why.
You feel it by being stuck to the ground.
