# Perception of time, relative and distant.

## Main Question or Discussion Point

Hello!

This is my first post, so I should make a small introduction. I am not a science expert in anyway shape or form, but I am a keen enthusiast who regularly keeps up to date with science news through popular websites such as IFLS, Space, BBC etc.

I have been wondering about this particular thing for some time, maybe somebody is able to shed some light.

If time, or the perception of time is affected due to the size of objects in space, then does it affect also how we see objects in space from either direction (i.e observing from here to there, or vice versa)?

So if there were, hypothetically, two planets inhabited by humans, far away from each other. One much larger than the other, where time moves slower. Say we could monitor each other, up close, through telescopes... would we from one side, see our fellow humans going about daily life in super slow motion, and they would see us as the opposite, in a sort of super speedy mode?

Any insights you can provide in order to help me understand the relationship here would be greatly appreciated :)

Thanks!

Mark C

## Answers and Replies

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Bandersnatch
Hi Mark, welcome to PF!

You're probably talking about the gravitational time dilation - time slows down near a massive body when compared to the passage of time far away from that body. Note that this is not about size, but about mass and the distance from it.

Yes, you could in principle observe the difference like you described, but it would be minuscule for any mass difference of the planets that would still allow humans to live and not be immediately crushed. In the order of extra milliseconds per day. Completely unnoticeable without very precise equipment.

The recent SF film 'Interstellar' sidesteps the above problem at one time by having a sensibly-massive planet orbit a very, very massive object in an extremely close orbit - it's the gravity of the larger object that is causing the time difference. If in the film they ever looked at the people on the planet (I don't think they do), they'd see what you described.

A real-life scenario could be looking at the astronauts on the Moon (caveat: we don't have such high-resolution telescopes), or those in orbit. Their time is passing at a different rate than for us on Earth. One needs to additionally account for an opposite effect due to relative velocities, but it's there. Again, milliseconds per day.

Those milliseconds do count for our GPS satellites - their clocks need to correct for the gravitational and velocity-related effects on time passage, else they'd accumulate as much as metres of error per day and that'd be no good for their intended purpose.

Thank you sir, for your answer and warm welcome :)

I assume then that measurements taken of the planet travelling through its orbit would be different if taken from both locations? but would the difference be in terms of time or distance travelled?
Does that make sense?

So if we were in direct communication with the other planet, both parties would be able to agree at what point the planet had travelled through a particular mile stone, say 100,000 km, but would not be able to agree on how long it took?

I guess I am trying to understand is whether time dialation affect the object from a space travel perspective, or does it only affect perception of time at the source?

So essentially distance travelled through space is more consistant than time? So in some sense if we wanted to arrange a meeting with our friends on the other planet, at a 3rd point in the universe, we should best use a moving object visible to both parties in order to agree on a point in time to meet, say for example be there by the time the agreed planet or body has travelled 3 times round the sun?

Thanks again!

P.S I did enjoy interstellar, though not sure about the ending haha

Bandersnatch
I assume then that measurements taken of the planet travelling through its orbit would be different if taken from both locations? but would the difference be in terms of time or distance travelled?
Using the same type of a clock, a person on a massive body and a person away from it would disagree on the duration of processes - like the time it takes to orbit a star or the age of the universe.

If one of the observers were travelling at high speed with respect to the measured distance, then the two observers would also disagree on the length of that distance - but that's due to a different effect (special relativity) than gravitational time dilation (general relativity). What I'm saying, neither time nor distance should be treated as cast in stone, even though in most cases - even in astronomy - they can be approximated as such.

So in some sense if we wanted to arrange a meeting with our friends on the other planet, at a 3rd point in the universe, we should best use a moving object visible to both parties in order to agree on a point in time to meet, say for example be there by the time the agreed planet or body has travelled 3 times round the sun?
Yes, this can be always done. As long as we agree to count the same instance of a process, and not the same kind of a process. Meaning, if we were sitting in different gravity wells, rather than count how many times mine and yours cessium clocks tick, we should count how many times a particular clock (orbit, pulsar, Big Ben, only mine or only yours clock, etc.) ticks. While we might disagree on how long it took between ticks, we'll both agree on when* it ticks.

*that's actually not really true, as the concept of simultaneity is also relative. But (I think - I'm myself a bit over my head here), as long as we're comparing events on two stationary bodies of different masses only, it should work.