# If you hover just above the event horizon of a black hole, would you see the future of the universe?

Let's do this thought experiment. You have an insanely powerful rocket and it can accelerate to 0.999999c. Now you fly to a supermassive black hole and hover just above its event horizon, where the inner gravitational pull towards the singularity matches the outward thrust of your rocket.

Consider: 1. The black hole is non spinning, and doesn't distort space-time into swirling around it.

1. The black hole is so massive that above the event horizon, you are not sphagettified.

2. The rocket can shield the energy of extreme blueshifted photons that bombard you.

At this point over the event horizon, you would see like 1 million or even 1 billion years of the universe in 1 second of your life, right? Can you witness the end of the universe if you hung around for long enough?

And what if the black hole evaporates by Hawking Radiation before that? Will you immediately return to real time?

PS: This is quite different from falling into a black hole. In this thought experiment, we are not crossing the Event Horizon of the black hole in a split second, but we are physically hanging in there for a long time by Counter Force.

Edit: I'm not sure why this question has been marked as a duplicate, but this differs vastly from an observer falling into a black hole, and nowhere else has it been answered so perfectly and succinctly, as done by Mr. Bob. Many thanks to him.

• @AccidentalFourierTransform: In that question, the observer was freely falling and hits the singularity in finite proper time; whereas in this question the observer is using their handwavium drive to "hover" above the event horizon indefinitely. This makes a substantial difference to how long the observer survives, and (I think) changes the answer to the question as well. – Michael Seifert Mar 26 '17 at 20:20
• @Bagho the other post is very nice and informative though, so give it a read, you might find it useful :-) – AccidentalFourierTransform Mar 26 '17 at 20:25
• To be pedantic, you could witness the end of the Universe if you hovered at any radius above a black hole, not just at points near the event horizon. It would just take a bit longer. – Michael Seifert Mar 27 '17 at 16:18
• Possible duplicate of Practical time travel: time dilation above the event horizon – stafusa Jul 19 '18 at 11:10
• Note that the proposed duplicate question has now been closed as a duplicate of this one. – rob Jul 20 '18 at 21:06

A short answer to your question is yes. If you were to hover a short distance above the event horizon for short time, a very long time would pass for asymptotically flat observers.

Let's do some calculations! If you were to hover say distance $h\ll r_s$ above the event horizon, then the time dilation factor $\gamma$ that you would feel would be of the form

$$\gamma=\frac{1}{\sqrt{1-r_s/(r_s+h)}}\sim\sqrt{\frac{r_s}{h}}$$

And thus if you spend a small time $t$ above the event horizon, a time of $t\sqrt{r_s/h}$ would have passed for the rest of the universe. In particular, say you are $1$ meter above a stellar mass black hole (with three times the mass of the sun -- the minimal stellar black hole mass), the dilation factor is $\gamma\approx 100$. If you were to hover just one centimeter above, the dilation factor would be $\gamma\approx 1000$. If you want one million years to go by in one second, you would need to hover around 0.1 angstroms from the event horizon. On this range, the actual description of the event horizon most likely breaks down due to quantum mechanical effects.

Now let's consider a supermassive black hole of one million times the mass of the sun. Then to get one million years to pass in one second would require $h\approx 3\,\mu\text{m}$, which is much more realistic.

Now, your second question was about Hawking radiation. The short answer to your question is yes. A black hole will evaporate after a certain time (very large times for very large black holes). This timescale puts an effective limit to your travels to the future and thus means, since black holes don't seem to be able to live forever, you cannot continue this for an indefinite period.

Black holes are cool. Keep learning and and I hope this helped!

• Wow! What an amazing answer! Thank you, million times, Bob! 😊 – Bagho Mar 27 '17 at 0:26
• As to whether the loss of "fast forwarding through time" would happen instantly when the black hole evaporates, the answer is no. The rate of passage of time depends on γ and γ changes gradually as the black hole evaporates. – Paul B Mar 27 '17 at 16:54
• This is an interesting phenomena, if we are to take the calculations as realistic. Because it also says, that if you want to use blackhole-like horizons to travel across the universe we better not try to accelerate or hover to make time pass, or we will never be able to return to our buddies. Basically, in order to travel through worm holes, we better go with the flow. – not2qubit Apr 25 at 8:34

I once learned that humans can survive up to 10 G of acceleration. A black hole has to be much more massive in order for you to survive hovering at 2 Schwarzchild radii than it does for you to survive freefalling past the event horizon. I'm giving a rough approximation of what would happen so I'll omit numerical constants that are close to 1. To accelerate at only 10 G while hovering at 2 Schwarzchild radii, the black hole must have a mass of about $1.25 \times 10^9$ solar masses. For larger black holes, the distance you can hover from the event horizon while accelerating at 10 G in your frame of reference varies as the reciprocal of the mass of the black hole and the rate you perceive time pass there varies linearly with the mass of the black hole. If the black hole is massive enough, you can hover at 10 G close enough to the event horizon that your time is dilated a lot and the incoming light is heavily blue shifted. You actually have to move through the space that's getting continuously dragged into the black hole and the closer to the event horizon you are, the closer to the speed of light you have to move through that space so the more aberration of light you will see and the narrow the area almost all the light seems to be coming from. For a black hole to be massive enough that you can hover at 10 G close enough to the event horizon that you see 1 million years go by in a second, the black hole would be bigger than the current size of the universe.

• I'm surprised that the previous answer & comments didn't mention that hovering near black holes involves squishy acceleration. BTW, you should break your answer up into paragraphs to improve its readability. – PM 2Ring Jul 2 '18 at 1:12
• They mentioned: physics.stackexchange.com/q/238343 – sanmai Jul 20 '18 at 7:56