I just watched interstellar, and enjoyed the concept that 1 hour on a planet orbiting a massive black hole is equal to 7 years of time on earth.
So how does that work? I found this
But it mentions how after you cross the event horizon thousands of years would pass in what feels like a very short time to you.
Can you get time dilation by orbiting a black hole? What impact does the size have? What impact does the distance from the center of the black hole have (I forget all my college physics).
What impact would orbiting a black hole based on mass the size of the sun, or the size of VY Canis Majoris, or a supermassive black hole the size of the black hole at the center of the galaxy have?
In 10,000 years if humans were extremely advanced, and some human wanted to travel into the future could he just take a spaceship in a lap around the black hole in the center of our galaxy? Can you experience meaningful time dilation without crossing the event horizon?
The stronger gravity is, the slower time passes. So if you are in a area with stronger gravity time will pass slower than someone in a weaker gravity area.
This has been demonstrated here on earth, see link.
Note- I can’t give a detailed answer to this question, but here’s a link to an article by someone who can - the Bad Astronomer; (once a poster here, if I recall correctly).
Phil Plait has revised his opinion of some of Interstellar’s maths, and now thinks that bits of it are okay - and he hasn’t decided about the other parts yet. An important point is that the supermassive black hole in the film was rotating, which apparently makes all the difference.
Time dilation just slows your clock, it’s not time travel in the sense of going to a future that hasn’t happened yet. Even with time dilation, you can’t get to the future until I do.
In other words, if you set out for the year 2020, you won’t get there until 5 years have passed for me, even if it seems like you get there in 15 minutes. Your clock may only show a few minutes passing while my clock shows five years passing, but we both wind up at the same place at the same time.
I’m not sure what this is supposed to mean. If I get to the year 2115 in five minutes, when it takes someone else a hundred years, then I don’t see how that can be described as anything other than time travel.
(As for whether the future has “already happened” or not, that’s a deep philosophical question, to which the answer is almost certainly “yes”. But I digress.)
We’re all time traveling into the future anyway, all the time. Usually, we go at a rate of one hour per hour. You can adjust that rate, as measured relative to someone else, using time dilation.
But as Wesley Clark points out, that is time travel. The only problem is it’s not symmetrical – you can’t go back to the past!
BTW, Kip Thorne, who’s a great physicist and was a technical advisor for Interstellar, wrote a book about the science behind the movie, some of which is real and some quite speculative.
We’re all traveling through time right now, at a rate of just about one second per second. Keep that in mind, and thinking about time dilation becomes easier.
That’s just a poor choice of words. Once you’re past the event horizon there’s no way for you to signal a faraway observer for them to compare their clock to yours so time dilation becomes a moot concept.
The time dilation factor of an observer in a circular orbit around a non-rotating black hole (with units chosen that constants equal 1) is (1- 3M/r)^-0.5, where M is the mass of the black hole and r the radius of the orbit. This diverges (goes to infinity) at r = 3M (1.5 times the radius of the event horizon) as the minimum circular radius is at this distance (the photonsphere). The last stable orbit though is at r = 6M which means the maximum time dilation factor is sqrt(2), which is approximately 1.4.
For practical purposes then for someone orbiting a non-rotating black hole for every hour they experience the most a faraway observer can experience is 84 minutes.
The effect of the size of such a black hole has is merely one of scale. The smaller the mass of the black hole, the smaller the radius of the orbit needed to achieve the same time dilation factor. In addition though there are tidal forces, which are approximately inversely proportional to the radius cubed. This means that larger objects can orbit larger black holes without being torn apart.
As alluded to in eburacum45’s link When we allow the black hole to rotate though there is effectively no limit to the time dilation factor of the last stable orbit as long as you allow the rotation to be arbitrarily close to the extremal value beyond which the solution becomes degenerate and you have a naked singularity.
I strongly suspect though for a black hole to have enough rotation to allow an object the size of a planet in a stable orbit around it with the time dilation factor you quote, the rotation needed would be more than you could reasonably expect a black hole of the size needed to have.
My point is that the future doesn’t exist until it becomes the present.
You and I have clocks that both say 1:00pm and then you get into your time machine and when you come out, my clock says 2:00pm and yours says 1:05pm, but you’re still in my 2:00pm present no matter how long it took you to get there.
You can never get ahead of me. The only future you can get to is one where me or my dusty bones have already arrived.
It’s not a matter of what you can “reasonably expect”. There’s a maximum possible amount of rotation a black hole can have, and it’s expected that most real-world black holes are quite close (about 97%) to that maximum possible rate. Below that, and it’s quite reasonable to expect. Above that, it’s not just unexpected, but impossible.
Let me introduce you to the concept of a “frame of reference”. What you say is true only in your particular frame of reference, which the universe doesn’t care about because it has an infinite number of them, and they’re all different.
To put it another way, there’s no such thing as “the present” except at one particular point in space that you happen to be using as a reference. You seem to be under the impression that the universe as a whole has to churn through time in a synchronized fashion. That’s anything but true. Every point in space has a different “present”, and as soon as you start moving, you change the meaning of “the present”. Space-time is a very fluid thing. Apparently it’s all relative.
For what sort of (hypothetical) time travel wouldn’t that be true? If I take Doc Brown’s DeLorean 100 years into the future, your dusty bones will be there. Does that mean that the DeLorean is not really time-traveling into the future?
By the same way of thinking, I can’t really travel to a different place, because no matter where I go, I’ll be “here” and you’ll be “there” and those will both be true no matter how much you walk or how fast your car. I’ll always be “here” and you’ll always be “there”.
By this definition, the protagonist of H G Wells’ The Time Machine didn’t travel to the future either.
You are free to use the words however you like, but please realize that when you use them, you mean something rather different than what other people mean when they use those same words. That’s okay, and there is much precedent for it:
As I alluded to in the paragraph above there is an extremal value of the spin a black hole can have, beyond which it is a (almost certainly unphysical) naked singularity.
As for the real-world typical values for black holes, I must admit I’m not so au fait as to what they are, though I believe a = 0.97 is too high for a typical value. However the spin parameter must be much, much closer to 1 than 0.97 to get a time dilation factor corresponding to 1 hour equals 7 years for the lowest possible stable orbit.
In fact spacetime is so fluid, if you don’t enforce some basic standards about the spacetimes you use, you can get some bizarre situations. For example in a non-time-orientable spacetime you can have two observers starting at the same event A, initially agreeing on the the arrow of time, but meeting up at some other event B where the first observer sees the second observer as travelling backwards through time and vice versa! The spacetime can even be flat (i.e. no gravity)!
Right. I think what trips some people up is that “time travel” sounds like scifi. But time travel into the future is actually fairly mundane. Your GPS wouldn’t work without accounting for it.
Astronauts do a measurable amount of it. According to Wiki:
Time travel into the past, on the other hand, would turn you into your own grandpa in short order. That’s not just problematic, it’s almost certainly impossible.
Sure. It’s a valid frame of reference to say that the wheels of the car are pushing then entire observable universe around so that a different portion of it is directly around you.
Not intuitively sensible, but valid from a theoretical perspective.
