How close to a black hole (with 10 solar masses) must you be before time for you is dilated significantly, compared to your crew’s, 1 million km out?

A thought experiment: I was imagining a scenario where a spaceship tethered their compatriot and lowered him near the event horizon & then pulled him back towards the spaceship before being pulled into the black hole. Wouldn’t time go by quicker for him? Would there be a discrepancy in their ages?

Not very far away from the event-horizon. Here’s a scenario to contemplate:

They would lower him on a tether and his shuttle craft would appear to freeze in time. Pulling back would bring him back with great difficulty and very slowly. He would appear to be stuck. If they continued to pull, it would take decades to get him back.

Inside the shuttle craft, he gets lowered toward the event horizon, stays a few minutes, then gets pulled back up.

When he gets back his crew on the main space craft has grown very old, and as far as he was concerned, he was there only a few minutes. As far as they were concerned, he was down there for decades. This is the correct answer.

Can time travel happen, even if it’s a century from now?

I have written a few answers about this topic already and this one might end up TL;DR but what the heck, Here goes.

By Chris Craddock

I originally wrote: “No. Nobody can convince you that time travel can happen because it really can’t happen. Not in the sci-fi sense everybody seems to long for.

We are all “traveling” forwards into the future at an eye popping rate of 1 second per second. We can’t go forwards any faster than that and we can’t EVER go backwards in time. Bottom line, time travel isn’t possible.

Then David Chidakel asked “I would like to hear your reasoning”. I started to answer David in the comment section but my answer got quite long, so I decided to edit my original post instead. I have written a few answers about this topic already and this one might end up TL;DR but what the heck, Here goes.

Time’s arrow…
Some physics equations seem not to care about the sign of the time variable, or in many cases not to care about time at all. If you suspend disbelief for a moment, they seem to allow for “closed time-like curves” and quantum entanglements which is geek code for “time travel”. But those are only hypothetical quantum scale effects. They probably don’t work out when considered in conjunction with other known physical laws and certainly don’t work for massive objects like people and time machines.

First of all, let’s dispose with time travel into the past… that is ruled out by thermodynamics.

First (law): just for grins and giggles let’s assume a time lord has invented a blue box that somehow instantaneously leaps the traveler back to some particular time and place in the past (relative to the traveler’s rest frame yada yada) where/when the travelers step out of the box and start interacting with the universe and alien creatures as they were back then.

But atoms/subatomic particles and their constituent energy fields are pretty tough things. They might get smashed in an accelerator or reactor, or be fused inside a star or have their electrons torn off inside a 9v battery along the way, but for the most part they are eternal and indestructible. Certainly within your own lifetime your atoms belong to you and only to you.

The problem then is that  instantly upon arrival all of the atoms making up the traveler and his/her time machine would suddenly have to exist in two places at the same time: Inside the traveler (obviously) and ALSO inside  whatever those same atoms happened to be contained within at that exact moment in the past. That can’t happen. If it could the whole edifice of physical laws would fall apart. So strike one.

Second (law): a.k.a. “entropy always wins”. The universe is made up of atoms/energy fields each more or less randomly going about its business. At any given moment we can’t even know everything there is to know (position, momentum etc) about a single atom (according to Heisenberg and half a dozen other quantum laws) -AND EVEN WORSE- a moment later even that information is lost.

So no matter how much energy you’re willing to expend, there is simply NO way to rearrange all of the particles in the universe back to some prior state. The information about that state doesn’t I have written a few answers about this topic already and this one might end up TL;DR but what the heck, Here goes.I have written a few answers about this topic already and this one might end up TL;DR but what the heck, Here goes.exist. Fried eggs can’t become fresh unbroken eggs no matter what you do. So even if you can decide “when” to go back to, there is no “there” to go back to. Strike two.

Third (admittedly just a thought experiment): While we know that time and space are entangled from a relativistic point of view, they are not the same thing. If time was just another dimension that you could (somehow) independently move along like a bead on a wire, then perhaps you could just pop out of the blue box and the universe would be sitting there exactly as it was “before”.

But whose version of “before” are we talking about? There is no universal time reference. That line of reasoning leads to grandfather paradoxes and infinite regression. If we supposed for a moment that it could happen, how could you ever tell? Aside from the violent anti-matter annihilation of the travelers atoms 🙂 I imagine the rest of the universe would simply carry on exactly the way it did last time. So logically it can’t happen. Strike three.

Now let’s think about time travel to the future. This also fails the thought experiment above.

The Future
Now let’s think about time travel to the future. This also fails the thought experiment above. In gross details if we look at (say) a tennis ball in motion right now and we know its position and velocity and mass and angular momentum and air density etc, we can predict its future position quite accurately for a short time O(seconds). Beyond that, we just can’t predict its future. What happens to the ball next week, or in a hundred years from now? How about the position and momentum of every atom in a nearby star or the couple of hundred billion of its cousins in our galaxy?

Time travel to the future isn’t possible because the future has not happened yet. Which future would be traveling to? Our blue box can’t slide the travelers time bead along the wire of the time axis because we just proved (above) that there IS no wire. And the blue box can’t rearrange the entire universe’s atoms into some as yet unseen future state because it can’t predict what that state will be, even if it had the means to rearrange them to its desired state.