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# Astronomy/New question for you Prof Gort

Question
they are not homework, or term paper projects, these questions cannot be found searching on the internet
I want to now how possible is it to orbit a neutron star, or black hole safely for a human being to survive the trip without getting harmed, to time travel into the future with gravitational time dilation.
if you had a space shuttle could it be possible to orbit the neutron star safely, I asked if it was possible to orbit a neutron star online, and people said yes it was, but other people said the shuttle would get spaghettified.
So can you help with these questions.
Question 1. Can you orbit the neutron star, or is the gravity too strong like a black holes gravity, and the shuttle would get spaghettified.
Question 2. How fast does your space shuttle need to travel to be able to orbit the black hole, the fastest a space shuttle can travel is around 25,000 mph to 35,000 mph do you have to travel faster than this to orbit the neutron star.
To stop the strong neutron stars gravity from pulling you onto the neutron stars surface.
Because the gravity from the neutron star can accelerate something as soft as a marshmallow to it's surface from 0 to 100,000 mph in around less than a second.
And the something as soft as a marshmallow with have the same destructive power as a atomic bomb, if the marshmallow hits the surface of the neutron star.
Question 3. What does the shuttle need to be made from in order protect itself, and the people inside, from the strong magnetic forces, it cannot be made of metal right because metals magnetic.
Question 4.  Is there a way to keep shuttle, and the people inside safe from the strong magnetic forces.
If you made the shuttle out of the best material to keep it safe from the magnetic forces, and if the shuttle had really thick walls, like say two meters thick, or five meter thick walls in every direction up, down, left, right, forward, backwards, and the shuttle had no windows, just cameras outside of the space shuttle, that sent television signals to a television monitor inside the shuttle so the people inside could see where they were going.
So the people inside are completely encased inside the shuttle behind five meter thick walls in every direction, are the people inside now safe from the magnetic forces, because the difference in gravity from their head to their feet is different, and would pull them apart, is this if the shuttle is close to the neutron star where the gravity is stronger.
What if you orbited far away in gravitational time dilated field, just inside the point where the space-time curvature begins, would the people ob board be safe in this region of the neutron stars gravity.
is five meter thick walls is not enough to keep the shuttle safe from the magnetic forces what if the shuttle walls were thicker say twenty, or even fifty meters thick, in every direction.
would this be enough, or does it not matter how thick the shuttles walls are is it not going to offer any protection against the magnetic forces.
Question 5. How much will you time travel into the future orbiting the neutron star, does it depend how close you are to the neutron star, the closer you get to the neutron star where the gravity is more stronger, is this where you would get spaghettified.
You cannot land on the neutron star because its spinning too quickly, so how close could you orbit with your shuttle being thick as possible to protect it from the strong magnetism.
If you orbited for a month how much time would pass for outside observers watching you orbit the neutron star.
Question 6. Do you have to keep moving at all times while orbiting the neutron star, even if you orbited at the weakest point at the surface of the of the space-time curvature.
Also are there any other effects of a neutron star that are dangerous
that I have missed.
Question 7. while you are orbiting would you have to make sure a asteroid, comet, or meteor did not hit the neutron star while you are orbiting because of the gamma ray burst that would be caused by the explosion on the surface, again is the thick walls on the shuttle going to keep the shuttle,and the people safe from the gamma waves.
Thank you for your time, and help with these questions

Best Regards,

Nicholas Lee.

You ask some very interesting questions! I'll try and answer the best I can, and I'll explain the math, but I hope you'll check my work and use some of the equations to explore other questions you may have.

1)Yes, you can orbit a neutron star with no problem, as long as you're far enough away - like the distance the earth is from the sun (1.5 x 10^8 km, or one Astronomical Unit (AU)). Other than gamma ray bursts and some other radiations, a neutron star is just a slightly more massive star than the sun, but happens to be very, very small. For sake of simplicity, I'll consider a neutron star with a mass exactly twice that of the sun. Then, by Kepler's law (P1/P2)^2 = (r1/r2)^3. So, if you calculate that out, we can orbit our little neutron star with a period (length of the 'year') reduced by 1/sqrt(2) from our normal year. So our orbital period would be 258 days. Our orbital velocity would increase from 108,000 km/hr to 153,000 km/hr (again, just a factor of sqrt(2) difference). So far so good. Would we be pulled apart as we orbited the star? NO! Because there'd be no difference in orbiting the neutron star and orbiting a slightly more massive sun. The gravity (at one AU distance) is the same! The only problem comes about because we can now get very close to the center of the neutron star, if we wanted to. Since an average neutron star might be only 10 km radius, we could (in principle) orbit just above the surface - at 10 km. Then, the gravity is very strong and things get more interesting. So to recap:

No spaghettification at one AU. What about at 10 km?

If we use Newton's laws (I know things get relativistic, but we'll stay classical and not be too far wrong), then F = G*M1*M2 / r^2. Let's assume M1 = mass of shuttle = 10^4 kg. Use anything you'd like! M2 = 2 x 10^30 kg. r = 10^4 m. G = 6.67 x 10^-11. Then (calculate this yourself!), F = 1.334 x 10^16 N. That's the force that the shuttle feels in orbit. But remember - it's in orbit so it's in 'free fall' - the occupants would be weightless. But wait! This shuttle (and the occupants) have a certain size, so their fronts and backs would feel different forces. Let's assume the shuttle is 100 m across. Again, choose any size you'd like, and do the math yourself! If we use r = 10.1 x 10^4 m, then F (back of the shuttle) = 1.3077 x 10^16 N. So the difference between the front and back of the shuttle is 1.334 x 10^16 N - 1.3077 x 10^16 N = 2.6 x 10^14 N.

How does this compare to the weight of the spacecraft (at 1 'g')? F (on earth) = m*g = 10^4 kg x 9.8 m/s/s = 10^5 N. So the spacecraft is experiencing a force (keeping it in orbit) of 1.3 x 10^16 / 10^5 = 10^11 g's!! And it's experiencing a "delta force" (the difference between the front and back) of 10^9 g's!! That's a billion g's trying to rip apart the spacecraft. No material known to man can withstand that. The spacecraft would be torn apart.

What about the occupants? Say they're only one m instead of 100 m (top to bottom). Do the math! They'd also be torn apart! You see, neutron stars and black holes are only dangerous if we get very close to the surface (or event horizon). At far distances, they're just like any other mass. They DO NOT suck things in UNLESS we get VERY close - and then - watch out!! We only get spaghettification at very close distances (because the gravitation force is inversely proportional to distance squared, and the very small size of neutron stars (and black holes) means we can get very close to their centers).

2) As far as orbital velocities are concerned, it again depends on how far you are away from its center of mass. As calculated above, if we were the same distance from the neutron star's center as the earth is from the sun's center, then our spacecraft would have an orbital velocity of 153,000 km/hr. When you say a space shuttle has a maximum speed of 25,000 to 35,000 mph, you're referring to an orbit around earth. That's the speed it must have to remain in orbit, or 'free fall'. However, the sun is much more massive, and the spacecraft would have to move just as fast as earth moves in its orbit to remain in orbit about the sun.

So 153,000 km/hr is fast, but no problem for interstellar space ships. But what if we ventured closer to the neutron star? Say we wanted to orbit just above its surface, just 10 km from its center. Then, we just use Kepler's Law again: (P1/P2)^2 = (r1/r2)^3. Let P1 be the orbital period of the shuttle at earth's distance = 258 days = 2.23 x 10^7 s. P2 = Period of our shuttle at 10 km or 10^4 m. r1 = earth's distance from sun = 1.5 x 10^11 m. r2 = 10^4 m. Do the math! The period of our shuttle at 10 km is now 3.84 x 10^-4 s!! That's 0.4 of a millisecond! How fast must it travel to remain in orbit? Well, the circumference is just pi * (20 x 10^4 m). The velocity is v = circumference / period = 1.6 x 10^9 m/s. But that's greater than the speed of light (3 x 10^8 m/s). So we CANNOT orbit at 10 km above the star's center. We can't go fast enough. If we tried to get that close, we'd go faster and faster, get more massive (relativistic effects come in here), and spiral into the neutron star.

2) continued. And you're correct about the energy released by that falling marshmallow. The acceleration due to gravity is just a = G*M / r^2. So acceleration at the neutron star's surface is a = 6.67 x 10^-11 * 4 x 10^30 kg / (10^4 m)^2 = 2.6 x 10^12 m/s/s. So in less than a second, the marshmallow would approach the speed of light, and virtually all of its mass would be converted to energy when it hit the surface. Assume a 20 gram marshmallow. E = m c^2, so E = (.02 kg) (3 x 10^8)^2 = 1.8 x 10^15 J. That's just slightly less than the energy released by a one megaton atomic bomb.

3 and 4) Magnetic fields can be partially shielded by "mu metal", which has high magnetic permeability. Shields made of this material can reduce magnetic fields several thousand times. However, it wouldn't do much to shield the field surrounding some neutron stars. They could exceed 10^10 Tesla. By comparison, an MRI machine might have a field of 1 to 2 Tesla. According to one researcher at the University of Texas (http://solomon.as.utexas.edu/~duncan/magnetar.html), a field greater than 10^5 Tesla would be lethal. Therefore, the field around a neutron star would be lethal to occupants at least out to 1000 km from its surface, with normal shielding possibly reducing that to 500 km. Yes, if you made the walls thicker, you could get better shielding, but the expense (and mass) would make that impractical. But if you assume a 0.01 m thick wall reduces the field by a factor of 1000, then you'd need a 100 km thick wall to bring the field to a one Tesla level. However, if you could stand 100 or 1000 Tesla, then your walls would only need to be 100 m thick.

5) You will not travel into the future by orbiting a neutron star or black hole. Rather, as you speed up, time will dilate (slow down) for you, as seen by a distant observer. You wouldn't notice any difference. But your clock would run slower if it could be seen by someone else far away. If a month of your time passes, the amount of time which passed for a distant observer would depend on how close you were to the star and the speed of your craft in orbit. Just use Einstein's time dilation equation (look that up) and orbital speeds which I've described above.

I've given you the mathematical tools for you to compute when you'd become spaghettified. So I'll leave that for you. It's obviously a gradual process. You'd have to determine how much force your body can take before it's torn apart.

6) Yes, you must keep moving to remain in stable orbit. If your orbital speed falls, you'd spiral into the star.

7) Yes, many things can trigger a lethal gamma ray burst.

I hope now you can compute some of the answers to those challenging questions yourself. If you stay away from relativistic equations (which can be quite complex) and stick with Newton and Kepler, you'll be able to easily find most of the answers. Good luck!

Prof. James Gort

---------- FOLLOW-UP ----------

QUESTION: Dear Professor Gort,

I am trying to find the best way to time travel into the future, using gravitational time dilation, and other ways.
I was thinking it was safer to orbit a neutron star, rather than a black hole, but then I found out about all the magnetism, and radiation, plus you cannot land on the neutron star because its spinning around 700 times a second, and when you orbit to close to the surface your space shuttle would get crushed by the gravity, so its probably safer to orbit the black hole then, I'm guessing.
If something happens to the space shuttle while its orbiting, and say the rocket boosters fail, your going to get pulled on to neutron star by the gravity, and get crushed, and die, just as much your going to get swallowed by the black hole.
So based on the answers you have in terms of orbiting both the black hole, and the neutron star, to get time dilation, in the end its probably safer, and better to orbit a black hole.

1. Its possible to orbit a black hole to time travel, for every sixteen minute orbit your shuttle takes, observed by people outside, and far away from the black hole, you would only experience eight minutes.
I do not know if this time dilation factor is for a small, or supermassive black holes.
So long as the black hole is not consuming anything, and giving off gamma waves, or other types of radiation, is it safe to orbit.
It has to safer than orbiting the neutron star right, because the neutron star has magnetism, and radiation you have to make your space shuttle prepared for, to protect the people on board.
So is just orbiting a non-consuming black hole safe, does the black hole give off radiation, or anything else dangerous to a space shuttle with people on board.
So is the black hole just dangerous to the people on the space shuttle if the black hole is consuming matter, stars and gasses.

Question 2. you just get 30% time dilation orbiting a neutron star, but is this for regular neutron stars, or magnetars which of them have stronger gravity.

Question 3. If you orbit a black hole for five years, then leave the black hole, ten years will have past when you returned to earth.
But smaller black holes have the tidal forces outside of the event horizon, on a supermassive black hole the tidal forces are inside the event horizon.
So which type of black hole would give you the longest time dilation, for example does orbiting the smaller black hole with the gravity being outside of the event horizon, slow you down more.
Or does it not make any difference what type of black hole you orbit are you going to get the same amount of time dilation.
Because there is a point in distance your shuttle cannot pass, because if it gets too close to the black hole it will get spaghettfied.
So as long as it orbits far away from the black hole it is safe, so you can orbit in a black holes spacetime curvature, as long as the space shuttle keeps moving.
But can you tell me the exact point in distance from where the black holes spacetime curvature begins where it's safe to orbit, to the exact point where your space shuttle would spaghettified, in meters, or miles.

Question 4.  To increase the rate of time passing, while you orbit the black hole, if you could travel at the speed of light while orbiting a black hole how much is this going to increase the time dilation factor, so time is slowing for you with gravitational time dilation, and with lightspeed time dilation.
Also can you think if a third way to increase the time dilation amount.

Question 5. This is probably a physics question, but this is a idea I had to double the time dilation rate.
We cannot travel at the speed of light, but if you had a space shuttle that could travel at the speed of light, and the shuttle was traveling in a north direction
Now what if you were traveling at the speed of light inside the space shuttle that was already traveling at the speed of light, but in  the west direction
So if you can use your imagination, you have a space shuttle traveling at the speed of light, in a north direction.
Let's say this space shuttle is big, big enough for another smaller space shuttle to travel at the speed of light inside of it, but in a different direction, so it's traveling in the west direction inside of it.
So this smaller space shuttle travels at the speed of light inside the bigger space shuttle in a west direction, it would reach the end of the ship when it came to a wall, then turn around, and travel in the east direction, so basically the space shuttle travels around in a circle, at the speed of light, inside the bigger space shuttle.
So if you travel at the speed of light for a year, seventy years would pass on the earth.
So if there were three twins the same age, say 20 years old, a twin stayed on earth, the other twin was on the big shuttle traveling at the speed of light, the third twin was in the small space shuttle traveling at the speed of light in the west direction in circles inside the big space shuttle.
After a year when the twin on the shuttle completed traveling at the speed of light for a year, the twin on the shuttle would be 21, and his brother on earth would be 90 years old.
If he looked at his other brothers clock, who is traveling in a circle at the speed of light, he will see 5 weeks have past on his clock.
Remember both brothers on the big shuttle started traveling at the speed if light at the same time.
So the third brother on the small shuttle knows after 5 weeks from his perspective, his brother on the big shuttle going to be 353.5 days older than he was, and his brother on earth is going to be around 70 years old.
So the brother traveling in the smaller space shuttle continues traveling at the speed of light in a circle, in the big space shuttle for a year, then as he completed traveling for a year, from his perspective, after a year has passed, wouldn't the brother who is on the big space shuttle with him be 90.7 years old, and when he visits his brother on earth he is 141.4 years old, well he would have died, but you get the point right.
So the brother traveling at the speed of light, while traveling in the big space shuttle at the speed of light in a circle has doubled the amount of time dilation he would have gotten.
To put this experiment more simply, look at the time for the brother who is traveling in the big space shuttle in circles, from his perspective he has traveled a year and 141.4 years have passed on the earth, rather than 70.7 years, because he  is moving at the speed of light, inside a big space shuttle already traveling at the speed of light, so the time dilation he receives gets doubled.
Can you think about, and calculate this experiment, and do you find it correct.
We cannot travel at the speed of light, but this is just a thought experiment.

Best regards,

Nicholas Lee.

Hello Nicholas,

1) Yes, if you can orbit a "quiet" neutron star or black hole which is not emitting dangerous radiation and if hot gases are not being accelerated in (where they would emit gamma rays and x-rays) and if magnetic fields were not too great (some stars have weaker magnetic fields), then it would be safe to do so. If we were in the right spot, your 3 second orbit (you'd have to be moving very fast, since you'd need very strong gravitational fields to get enough relativistic effects) as seen by an outside observer might be only 1.5 seconds as measured on your clock.

2) 30% time dilation (or 50%) can be achieved by orbiting most neutron  stars. Magnetars don't necessarily have stronger gravity - the gravity is only related to the star's mass. Magnetars are a particular type which have extremely strong magnetic fields (I'd avoid them for reasons already mentioned).

3) The only consideration is the gravitational strength at your point in orbit. While it's true that very massive black holes have a lower gravitational gradient (so you won't get spaghettified so easily in the strong field), the actual value of the gravitational field is what matters for time dilation. So in a large black hole, even though you won't get spaghettified until you're well inside the event horizon, I wouldn't advise orbiting there, because you can't! No orbit inside the event horizon will be stable - it would require an infinite amount of energy. You would immediately accelerate to the center and become part of the singularity. As far as when you'd become spaghettified, I gave you the tools (Question #1) last time. Do the calculations yourself.

4) Yes, the faster your ship orbits, the more time dilation you experience. If you could orbit at the velocity of light (at 1.5 the Schwarzschild radius around a black hole, photons (and your ship at v=c) can orbit - see photon sphere). At that velocity, time would stop (as seen by a distant observer). In other words, you'd disappear from their telescopes. There's no third way to increase time dilation (that we know of).

5) When speeds get close to the speed of light, they don't add the same way lower speeds add. So if one brother travels at the speed of light and the other brother orbits at the speed of light, both brothers would look (to the distant third brother) like they were traveling at the speed of light. No time (relative to the third brother) would have passed for either of the other two brothers. If you travel at the speed of light, NO time passes for a distant observer. If you travel at 80% the speed of light, then your time is only 60% of the time passed by a distant observer.

By the way, when you return to earth, you are, of course, younger than your twin brother. Say 10 years pass for you and 20 years pass from your "stationary" brother. But is that really traveling into the future? I don't think so. You simply missed 10 years of your brother's life! It's almost like you stayed on earth and you were in a coma for 10 years - in suspended animation. You can't see your brother's future and he can't see yours.

But if you're interested in this subject, I'd recommend you get a copy of "Black Holes and Time Warps" by Kip Thorne. Dr. Thorne was one of the inventors of wormholes, which are the best candidates (in my opinion) for real time travel.

Prof. James Gort

Astronomy

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#### James Gort

##### Expertise

Questions on observational astronomy, optics, and astrophysics. Specializing in the evolution of stars, variable stars, supernovae, neuton stars/pulsars, black holes, quasars, and cosmology.

##### Experience

I was a professional astronomer (University of Texas, McDonald Observatory), lecturer at the Adler Planetarium, professor of astrophysics, and amateur astronomer for 42 years. I have made numerous telescopes, and I am currently building one of the largest private observatories in Canada.

Publications
StarDate, University of Texas, numerous Journal Publications