system
Given the following post, respond as if you're commenting on Reddit:
human
How does gravity effect time?

I once heard that if aliens were looking at the Earth due to how gravity bends time they would be seeing the Earth ruled by dinosaurs, is this true? If it is isnt any planet we view possibly at a totally different place in its own history?
gpt
This is combining two different ideas.

The simplest one is just light travel time. Light moves very fast, but it doesn't move instantly. It takes about a second for light to reach us from the Moon, so we see the Moon as it was about a second ago. If something is a light-year away, you're seeing what it looked like one year ago. So if aliens were a hundred million light-years away, they would see Earth as it was a hundred million light-years ago. This doesn't in itself mean they were actually are at different times. It just means it takes time for the "signal" to reach someone. These aliens would be able to work out approximately how far away they are, and everybody would agree that they are seeing what Earth looked like hundreds of millions of years ago.

The more complex one is time dilation due to gravity and speed. This is a real effect, and not just related to how you see things. Here, if you're in a strong gravitational field, time really does move slower for you. Speed also matters - if you're moving quickly compared to someone, then from their point of view, time moves slower for you. This works both ways, because if you're moving quickly from their point of view, they're moving quicker from your point of view. So you both observe time moving slower for the other person. And just to emphasise the point, this is a *real* effect - it's not just that the light is arriving in such a way to make someone look like they're moving slower. You actually are moving slower from their perspective, and vice versa.

As you might expect, this causes trouble. It means there is no universal "now" in the universe. However, it turns out this isn't much of an issue, unless you need to be really really accurate. The gravity of Earth is far too weak for its gravitational time dilation to be a large effect (except if you're designing a GPS system and really care about very fine levels accuracy), and planets and stars and galaxies move far too slowly for the time dilation from speed to really matter very much either. You might be off by hours or days across a hundred thousand light-years, which isn't enough for you to see the wrong period of history.

Overall, the bigger effect (by far!) is the simpler one. We see distant stars and planets as they looked like in the past, just because it takes time for the light to get here. But if we know the distance to the planet, then we know how far into the past we're looking.