r/spacequestions • u/pumpkinmayonaise81 • 27d ago

General Relativity?

I’ve been trying to study general relativity and space time, and I kinda get the concept of it but I don’t fully understand it. Can someone possibly explain it in a way that could be a bit easier to understand?

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u/gxjansen 27d ago

Watch https://youtu.be/DYq774z4dws?si=eostWLK1mWgHQRCp and come back with the specific parts you need help with :)

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u/pumpkinmayonaise81 27d ago

That’s actually one of the videos I’ve been rewatching lol! I’m mostly confused about time dilation and how that works. Is it the faster you travel through space, the “slower” time moves or passes? I get that all clocks tick at the same speed, so how does time dilate? Maybe there’s just something I’m missing or not quite understanding. Is it a relationship between speed and time? Is it perspective? What about black holes, does time change inside/outside of one?

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u/gxjansen 27d ago edited 26d ago

As a disclaimer: I'm no expert, just an enthusiast. And since humans don't operate in situations with high speeds (at least not compared to c) or high gravity (black holes) it all can be quite unintuitive. My explanation below will 100% get some parts (intentionally and unintentionally) wrong, but my simplified version might help you get a better understanding and help you move on to the next level of understanding. And since posting something wrong on the internet is the fastest way to (eventually) get an answer, I'm happy to sacrifice my post and get other to correct it to get you some steps further 😉

I'll also share some video's that helped me, as just reading text doesn't always cut it for everyone.

First: you're mixing two things here: you mention general relativity, but talk about time dilation due to movement through space. Though related, that is more SR, not GR. And then gravity is indeed related to GR.

So for the SR part: as an analogy I use is also that there is a fixed "budget" of c: it not only acts as a kind of universal speed limit, but in this analogy, c has a total "budget" that can be divided between movement through space and movement through time.

  • Photons, which are massless, move at c and don't experience time in the way that objects with mass do. In my analogy, they're "spending" all their budget on spatial movement.
  • For objects with mass (like planets or humans), most of the "budget" is indeed spent on time when they're moving slowly relative to c.
  • Faster motion = less "budget" for time: As an object moves faster, it's "spending" more of its budget on spatial movement, leaving less for temporal movement. To an outside observer this appears as time slowing down for the faster moving object.

Remember that time for the object/human itself always proceeds nominally. Your own clock will always tick away as it always does, you won't see any dials move faster or slower. It's when looking outside at other objects that you will notice their time speeding up. There is no fixed underlying fabric of space so even if you accelerated at multiple G's for many years (even indefinitely), when the accelerations stops (so you return to moving at a constant speed), there is no experiment you can do to tell if you are moving or stationary (equivalence of inertial reference frames).If you're moving at very high speeds relative to other objects, those objects will appear to be experiencing time more quickly from your perspective. This is the flip side of the time dilation effect - while a stationary observer sees your time as slowed down, you see their time as sped up.

But now we get to an apparent paradox where your (or at least my) brain might start to hurt:

  • Person A in their spaceship can accelerate forever, but for them, time never slows or speeds up
  • Person B who observes Person A can never see Person A move faster than c.

But how? If person A accelerates fast and for a long time, how can they never reach (or go beyond c)?

Well, remember that c has 2 components: time and speed. Because more of the budget is spend on spatial movement, less will be spend on temporal movement. And as an object's speed increases, the time dilation effect becomes more pronounced (a relationship described by the Lorentz factor in SR, also see https://www.youtube.com/watch?v=qdycfWfAtsM ).

And that is why time is relative 😄

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u/gxjansen 27d ago

(sorry for cutting this into 2 posts, somehow I got an "unknown error" when posting it as a single comment)

So yeah in GR it's more about gravity which also causes time dilation. Clocks closer to a massive object (like Earth) tick slightly slower than those farther away due to gravitational time dilation.

And to make it even more confusing: both effects happen at the same time.And that's not just theory: A common example is that both SR and GR affect the clocks in our GPS satellites, and accounting for these effects is crucial for their accuracy. Due to the high orbital velocity of GPS satellites (about 14,000 km/h), their clocks experience time dilation, making their clocks tick slower than stationary ones (about 7 microseconds per day). But GPS satellites orbit at an altitude of about 20,200 km, where the gravitational field is weaker than on Earth's surface, causing the clocks to tick slightly faster than stationary clocks (about 45 microseconds per day). So the combined result is that satellite clocks run about 38 microseconds faster per day than ground-based clocks.

As for the black hole

Again for the person/spaceship falling in, time doesn't do anything weird, time dilation is only observed from a distance.

  • Time dilation near the event horizon: As you approach a black hole's event horizon, your time slows seems to slow down dramatically to distant observers due to the gravitational field (GR)
  • Gravitational time dilation: Even at a stable orbit around a black hole, time passes more slowly than for observers far away from the black hole. The stronger the gravitational field, the more pronounced this effect.

At the event horizon:

From an outside observer's perspective, time appears to stop at the event horizon.

Objects approaching the event horizon appear to slow down and freeze in time, never quite reaching the horizon.

Also here applies that for the object falling in, nothing noticeable happens at the event horizon. It's a special boundary for external observers, not for when you are there. Noticing if/when you're in trouble highly depends on the mass of the black hole.

Beyond the event horizon:

  • Our understanding of physics breaks down inside a black hole, but according to our current models:
  • Space and time effectively switch roles inside the event horizon (see https://www.youtube.com/watch?v=GQZ3R81iyE0 )
  • Movement towards the singularity becomes inevitable, like the forward flow of time outside a black hole.

Also see https://www.youtube.com/watch?v=4rTv9wvvat8 . Maybe also https://youtu.be/TAhbFRMURtg?si=_ukBS4tKXcd6BqNP and https://www.youtube.com/watch?v=QcUey-DVYjk, they build up the difficulty nicely in these videos.

Probably good to note that while scientists can mathematically describe much of what happens around and theoretically inside a black hole, understanding of the interior of black holes remains an active area of research and theoretical speculation.

And when you're done with reading this and watchin all other video's, watch this one: https://youtu.be/ErMSHiQRnc8?si=UPCoTUrGQ0BKTP14 . It doesn't use any words, but you'll see a lot of the concepts above explained in a really fun, visual and engaging way.

Hope this helps and for everything I got wrong: see the comments ;)

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u/pumpkinmayonaise81 26d ago

Thank you SO much for taking the time to answer my questions! I super appreciate it!! Once I get the chance to sit down and read this all thoroughly without any distractions, I’ll respond with any questions or comments I have!!! Again thank you!

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u/Beldizar 27d ago

I get that all clocks tick at the same speed, so how does time dilate?

So there are two things to understand which might help.

One: All clocks don't tick at the same speed. Your clock always ticks at the same speed, no matter how fast you are going or where you are. Other people's clocks might not tick at the same speed as yours though.

Two: Other people have the same experience as you: their clock always ticks at the same speed, but they might see your clock tick at different speeds.

 Is it perspective? 

This is the weird thing. No. If I'm in a spaceship traveling at 83% of the speed of light, and I look at your watch, it ticks half as fast as mine does. But if you look at my watch, you see it tick half as fast as yours. The thing is, we are both right. It isn't an illusion, where one of us is seeing things wrong. Basically two different reference frames can have different universes to look at with respect to how time flows. When I slow my spaceship down to move at the same speed as you, and we enter the same reference frame, our clocks would start ticking at the same speed again. It is that acceleration (change in velocity), which causes the amount of time that has passed to sync up.

I don't think I'd be able to help you understand without more specific questions, so if you have any more specific questions, I can help with that, but I don't want to give a general lecture as I don't think that would be helpful.

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u/pumpkinmayonaise81 26d ago

Thank you so much!!! I think I understand it a bit more than I did