I’ve been thinking about gravity and time. I get that percieved time changes depending on which planet you’re on, more gravity= faster time is. I also understand that the person on that planet will not notice any difference.
For an example, ben teleports to a super big planet, and hangs out there for 5 hours, teleports back to earth and boom, its been 5 months in earth time.
But, if we somehome could, with a telescope, see ben on that big planet, would we percieve him as going slomo, or almost not moving?
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Actually time slows down as gravity gets stronger.
As of your question, yes, you would “percieve” it going slower than you. But at this scale human perception is faaaaar from being fine enough to *actually* see it.
You’d need to plunge into a black hole to see a time dilation at the human scale.
But you can measure it around earth with the right instruments. In fact it’s a requirement for GPS to work, as satellites have faster running clocks in orbit than they would on the ground. It’s small, but significant when you’re doing GPS computations and measurements.
So it’s actually the reverse, more gravity slows the passage of time relative to an outside observer. This real life phenomenon is actually what inspired part of the movie Interstellar, where astronauts visited Miller’s planet which orbited just beyond the event horizon of a supermassive blackhole. The gravity of the black hole is immense and so this translated into a time difference of 7 Earth years for every 1 hour on Miller’s planet. The astronauts ended up stranded on Miller’s planet for several hours (from their perspective) but when they returned to their mothership over 20 years of Earth time had passed. The crazy part is that this phenomenon is actually a real, experimentally verified thing. Theoretically if such a planet did exist you could take a trip there and return younger then your own children.
To the outside observer time would be ticking considerably slower for the astronauts (or Ben in your example). They would appear to be moving very slowly and would appear dark red (if they’re even visible at all). This is because of a phenomenon know as [gravitational redshift](https://en.m.wikipedia.org/wiki/Gravitational_redshift). Light is a wave, and the color you see corresponds to it’s wavelength when it reaches your eye – with red being low frequency (long wavelengths stretched apart) and blue being high frequency (tiny wavelengths squished together). Because of time dilation the time you receive each wave of light from the astronauts is longer, which effectively stretches the wavelength like an accordion and turns it red from your perspective.
Another way to think about gravitational redshift is to imagine someone clapping their hands. The slower they clap their hands (lower frequency) the redder the light and the faster they clap their hands (higher frequency) the bluer the light. If the astronauts clap their hands once per second (as they measure it) then because of gravitational time dilation the outside observer sees the time it takes to finish one clap taking considerably longer, so the astronauts light appears more red to the outside observer.
If you could watch someone moving at relativistic speeds, such as on a very fast spaceship, then yes they would be moving in slow motion from your perspective. You would *not* see this from someone on another planet; planets don’t provide the gravity needed to make that kind of difference in time dilation (the planet in Interstellar was an unrealistically specific scenario involving a planet orbiting a supermassive black hole).
To add on to what a lot of people are saying- your initial comment amount Gravity is backwards, but your description of Ben is about right.
This is pretty much the twin Paradox- let’s say Ben has twin who stays behind on Earth as Ben travels at near light speeds to another planet. If they both watched each other, they would see similar things. Ben would see his twin on Earth also moving very slowly. Same thing for on his higher gravity planet- the light from Earth must pass through the gravity field to reach Ben. By the time it does, it’s at a lower frame rate, so to speak.
Now Ben is done and he gets back in his ship and heads back to Earth. As he accelerates towards Earth he watches his twin. At first his twin is the same age as him- but as he heads towards Earth, his twin starts speeding around Earth, aging super fast. By the time Ben gets there his twin is an old man.
In Relativity, acceleration is key. Ben is the twin who experienced Time dilation because he accelerated away from Earth and accelerated *back*. Otherwise, just looking at each other, neither Ben nor his twin could tell who was aging slower.
If you are deep in the gravity well of a planet then time for you passes slower than elsewhere, not faster.
Time dilation is a **tiny** effect for realistic scenarios. If Ben is on Jupiter then 1 year for Ben is 1 year plus half a second on Earth. If Ben is on the most massive planets we know of then the difference can be up to 10 seconds per year. So yes, in principle you would see Ben move around slower (and the light from him is redshifted), but the difference is too small to notice. You need atomic clocks to measure a difference.
It’s not just gravity, anything being accelerated will cause time to slow relative to anything outside the field of acceleration.
When I looked this up on Wikipedia, I see two objects travelling towards or away from each other at a fixed near-light velocity will also experience time dilation.
Although both frames of reference will experience equal dilation. You need acceleration, or gravity, for one reference frame to experience permanent time dilation relative to the other.
I can see how moving towards each other at near light speed will cause time dilation, as otherwise light may appear to travel faster than light speed.
However moving apart at near light speed will also cause this. So light cannot appear to travel slower than light speed either. It is always at a constant velocity, depending on the medium it is travelling through.
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