# I 10 questions about science

#### pac0master

Hi everyone,
I wasn't too sure where to post this, so I would like to apologies in advance in case it isn't the right place.

So I've made a little video a while ago and thought I could share it with out.
It's called 10 Questions for scientists and it relate to my curiosity about General science, (mostly physics)

I might not be able to understand the maths behind some of the subject, so if it's possible to explain them in details that would be appreciated

Here's the video, (escuse my broken English)

Here's the questions in text format.
This is the first in a possible Series of Questions dedicated in Scientific enlightenment and education.
1: Black holes
For the sake of the argument, let’s say that the Universe only has one gigantic black hole with a fairly large radius 0.5 AU and a Mass of 1 Billion suns, what would be the speed of the object orbiting 1 km over the Event Horizon?

(if the size and mass isn’t right, feel free to use a different value)

My goal was to try to create an orbit very close to the Black hole but require a FTL speed to achieve a circular orbit.

According to the theory of Relativity it is not possible to get to the speed of light. which obviously, this orbit won’t be possible.

My question is what would happen to an object crossing that orbit?

would it accelerate to a point where it simply stop accelerating and don’t have enough speed to make the full orbit thus make it fall into the black hole?

Or is it by definition the point we call the Event horizon?

If so, Imagine small body such as as a spaceship orbiting at the Event horizon of a large black hole (big enough to prevent the spaghettification at this distance)

If that spaceship is at the exact point of the Event horizon. Let’s say it is halfway in the black hole. (Left side is in. Right side is out)

How would the crew experience it?

2: the speed of light.
This is a follow up to the question above.

Once again, in theory (and even in practice) Matter can’t reach the Speed of light nor go faster

My question relate more to the concept of acceleration.

If we have a space ship with infinite massless fuel and a powerful engine that is 100% efficient capable of giving a constant acceleration

What happens when it get near the speed of light?

If I remember right, the reason why we can’t get faster is due to the relativistic mass increasing which require more energy to push which then require an infinite amount of energy to pass the speed of light.

So if we have that space ship described above. how would the acceleration looks like (relative to the crew)?

Will slow down until it reach a critical point and completely stop accelerating?

or something else I just can’t figure out?

But then, when it reach it maximum speed.

What happen if you walk forward into that spaceship? would you go to a speed closer to the speed of light or you would it be physically impossible for you to walk?

3: The Big Bang
It is often said that the Universe began at the Big bang. I wanted to make sure that my understanding is right.

Is the Big Bang the known beginning of the Universe as a Whole or is it for the Observable Universe?

I’ve seen many people using it as if the Universe as a Whole was compressed into a single point. but it defy the idea that the Universe is infinite.

So when I look at it I see it in a slightly different way.

Basically, the Big bang would be the known beginning of the Universe as a Whole however it is by itself infinite in size so what we describe as the very compressed point is only the Observable universe. 13.82 Billion years ago.

I wanted to know if I made a mistake,If so, I wish to have more information about it as it can be confusing sometime.

4: Entropy and the Universe
This is more of a follow up for my previous question.

I wonder how the laws of thermodynamics (especially entropy) apply when we deal with a Universe that is Infinite in size.

If the Universe wasn’t infinite then it would be easier to deal with the “border” and define a specific size so we can deal with it as being a Closed system.

However if the Universe is really Infinite, then how does Entropy works?

If the Universe is infinite, can it has an infinite amount of useable energy?

If so, how can it be affected by Entropy?

5: Vacuumseption

My question has more to do with the efficiency and structure of the vacuum itself.

If I remember right, the limit of most vacuum is caused by it composition.

Usually the cheap materials end up being crushed by the Earth atmosphere when we evacuate too much air.

So my question is:

What if we build a Vacuum inside a bigger vacuum?

The concept is to reduce the stress load to the vacuum inside in order to push its limit further. by creating a small vacuum around it.

For instance, let’s say I could reduce the pressure by 90% in the big Vacuum. Then the small inside could be used to reach an even lower pressure which would be far beyond it structural limit since it’s already in a vacuum thus it’s not subject to the stress.

I understand that It might not have any scientific value but can it be done for cheap?

6: Planck length & Maths *Paradox
I’m very interested in mathematics and physics, however i noticed something very intriguing and I never had any answer.
If the Planck length is real and the physical limit (to our current understanding) What shall we do with the repeating decimal/periodic/irrational numbers? One example is Pi, Since the Planck length is about 1.6 x 10-35 metres, would there be any good reasons to keep counting down the decimal of Pi? or the Square root of 2?

What is the point to know the 100th digit of pi if the physical limit is the planck length?

*I called it a Paradox because it seems to be a contradiction between maths and physics.

It might not be the proper term to use though.*

There is an interesting hypothesis saying that our Universe might be entirely simulated. Some great example would be things such as the movie: The Matrix. However, from what I have seen, many people tries to find ways to prove that idea. However my problem is that it might be completely impossible.

If we are within a Simulated Universe, then I believe that Everything, including our Laws of Physics are made up by this simulation. Therefore, The “computer” or whatever technology is used to create the Simulation we live in could run in a different world and have most likely a Different set of laws of physics.

If so, then we can not use our understanding of physics in order to prove the idea valid or even possible.

My best example is how we can run a simulation and change the value of different Physical constants such as gravity, which would result in an entirely different type of world.

Many people tries to prove this hypothesis by using our laws of physics as a stand point. For instance, If we were to find the “last” digit of Pi or the square root of 2, then it would be evidence in favour of the Simulated Universe as no computer can have infinite memory thus have a limit of informations thus creating that last digit.

But that only apply if that said Computer runs within our laws of physics.

Who knows?

Then we end up with the possibility that the World or Universe in which the Computer simulate our Universe could also be part of a Simulation…. that concept can go infinitely deep and there is no way to escape it.

*Once again, I called this a Paradox, however it might not be the proper use of the term. I used it because it reflect the paradoxical idea behind it.

8: Dark Energy
This is a very interesting subject, unfortunately there is so little information about it.

I understand that it is the force that drive the rapid expansion of the Universe. What I wonder is, does this Energy carry information? I understand that we can’t directly observe it. We can only approximate it effect via mathematical models of the Expansion of the Universe. But I’m curious about it.

9: The Expansion of the Universe
It seems to be a pretty well known subject when we speak of the Universe and it “shape”

However I always wondered about it.

How did we calculated the expansion and the fact that it’s accelerating?

I understand that we have the Red Shift of the galaxies that shows that they are moving away, However, Since most of the things we see are billion years ago, does it mean that this expansion is about the same age? Could it have slowed down since then?

We barely watched this for the last 60 years or so, This is why I ask, Was it enough? and how much of an acceleration could we have measured within that time span? What is the maths behind it and how does it work?

To me, I feel like that we are still in a very young Universe where the expansion is still going on fast but will eventually slow down and collapse on itself.

Imagine the expansion of Gas a tenth of a second (or less) after the explosion of a kilo of TNT

It would surely look like it’s expanding and accelerating,

10: Nuke Jupiter
We are all familiar with the concept of Nuclear Fusion, I wonder what would happen if we were to drop a Super strong Nuclear Fusion Bomb in Jupiter’s atmosphere.

Since Jupiter is mainly composed of Hydrogen, What kind of reaction should we expect if it was to happen?

Some people have described Jupiter as being a “Failed star”

Would it be possible to maintain a nuclear fusion for a period of time on jupiter by Blowing up nukes in it atmosphere?

I ask this question because the Bombs we have created reach the required temperature for fusion (obviously), I wonder what kind of reaction it would create when such thing happen in a dense Hydrogen atmosphere.

I wonder if the atmosphere itself will catch fire and maybe even create it own Storm.

If it can’t happen due to our lack of firepower, What kind of bomb would be required to create a visible mark in the atmosphere?

Then, what would it looks like if a Planet was to fall into Jupiter?

The energy would surely be incredible but would it be enough to achieve fusion?

(I do not mean to turn the whole planet into a small Star, but leave a mark big enough to be visible)

Related Other Physics Topics News on Phys.org

#### Drakkith

Staff Emeritus
2018 Award
I've given my understanding of the various topics below. As always, someone correct me if I'm wrong.

According to the theory of Relativity it is not possible to get to the speed of light. which obviously, this orbit won’t be possible.

My question is what would happen to an object crossing that orbit?
It would fall into the black hole.

If that spaceship is at the exact point of the Event horizon. Let’s say it is halfway in the black hole. (Left side is in. Right side is out)

How would the crew experience it?
Assuming a very large black hole, so that we don't get spaghettification, the crew experiences nothing unusual at all. They just accelerate under their engines like normal, or they free fall if their engines are off.

Note that notions of where the event horizon is at relative to the rest of the black hole is... problematic. Spacetime is so warped in and around a black hole that distances are very tricky to talk about. Keep in mind that once you go beyond the event horizon, there are literally no paths through spacetime that take you out. If my understanding is correct, it's not simply a matter of "you're X distance from the event horizon but can't reach it". It's a matter of "we're seeing the outside universe still, but the event horizon is not in the direction we're looking".

If we have a space ship with infinite massless fuel and a powerful engine that is 100% efficient capable of giving a constant acceleration

What happens when it get near the speed of light?
Nothing. The crew onboard feel no different. If you were to sustain a 1g acceleration, then that 1g acceleration remains 1g for as long as you fire the engines. Note that you're still thinking in terms of velocity being absolute and not relative. For all you know, you've been decelerating the whole time. That's why the crew new experiences anything different. Their velocity is only near the speed of light when compared to some other object, not some absolute background.

Is the Big Bang the known beginning of the Universe as a Whole or is it for the Observable Universe?
The Big Bang encompasses a couple of different ideas. One is that there was an 'event' that occurred that created the entirety of the universe. Meaning that the universe did not exist prior to this event. This is viewed as unlikely by most cosmologists.

The second idea is that the Big Bang simply describes the extremely hot and extremely dense state of the very early universe. In this idea there is no 'event' that created the universe. The universe may be infinite in age, or it may not. What the universe was like prior to this state is unknown, but may have included things like inflation or other speculative ideas.

These are the two main ideas of what the Big Bang was.

If the Universe is infinite, can it has an infinite amount of useable energy?
No. Any particular volume of the universe has only a finite amount of usable energy. All of these volumes are likely using up their energy at roughly the same rate (given the volumes are large enough in scale), leaving no extra for any other particular volume to use.

Imagine a hotel with an infinite number of rooms, each one filled with a guest. Each guest has access to a refrigerator containing 10 bottles of water. If each guest drinks one bottle of water every day, then after ten days all of the water has run out, despite the fact that there was an infinite number of water bottles as a whole to begin with.

What if we build a Vacuum inside a bigger vacuum?
You quickly run into diminishing returns and your vacuums become useless. If you want a very, very 'deep' vacuum, you need to buy specialized vacuum equipment.

If the Planck length is real and the physical limit (to our current understanding) What shall we do with the repeating decimal/periodic/irrational numbers? One example is Pi, Since the Planck length is about 1.6 x 10-35 metres, would there be any good reasons to keep counting down the decimal of Pi? or the Square root of 2?
There is absolutely no relationship between the digits of any number and the Planck length. It's as simple as that.

How did we calculated the expansion and the fact that it’s accelerating?
The amount of redshift gives us an indication of the recession velocity of objects. By using type-1a supernova, which all explode at roughly the same brightness, we can also tell if the expansion is accelerating or not by checking to see if the brightness matches the redshift. If there is a discrepancy, then either the supernovae are not as uniform in brightness as we thought, or the rate of expansion has changed over time. Current evidence indicates the latter.
We are all familiar with the concept of Nuclear Fusion, I wonder what would happen if we were to drop a Super strong Nuclear Fusion Bomb in Jupiter’s atmosphere.

Since Jupiter is mainly composed of Hydrogen, What kind of reaction should we expect if it was to happen?
Nothing out of the ordinary. Nuclear fusion mustn't just be initiated, it must be sustained. The only thing capable of doing both is having enough mass that a stellar object both heats up enough to start fusion, and is massive enough to keep its core at an immense pressure and density. Jupiter can do neither. A nuclear explosion in its atmosphere, or even in its core, would do nothing.

#### jbriggs444

Homework Helper
For the sake of the argument, let’s say that the Universe only has one gigantic black hole with a fairly large radius 0.5 AU and a Mass of 1 Billion suns, what would be the speed of the object orbiting 1 km over the Event Horizon?

My goal was to try to create an orbit very close to the Black hole but require a FTL speed to achieve a circular orbit.
Circular orbits at close range around the event horizon of a black hole are not possible. It is not Newtonian physics down there. Light can trace out a circular orbit at 1.5 times the "radius" of the event horizon.

Note that the "radius" of a black hole is not really a radius. It is not Euclidean geometry down there. The surface area of the event horizon is well defined. The "radius" is obtained by taking the surface area applying the A=4 pi r2 formula in reverse.

If so, Imagine small body such as as a spaceship orbiting at the Event horizon of a large black hole (big enough to prevent the spaghettification at this distance)

If that spaceship is at the exact point of the Event horizon. Let’s say it is halfway in the black hole. (Left side is in. Right side is out)

How would the crew experience it?
If the engines are on, attempting to hover, then infinite acceleration will be required to keep the part of the craft that has not yet crossed the horizon from doing so. Nothing can keep the part that has crossed from falling away. So spaghettification is guaranteed.

But that is only if the engines are turned on in a heroic attempt to save the right side of the craft. Those engines had better be mounted on the right side. And they need to be pushing the craft radially outward. Tangential velocity does not buy you anything.

If the engines are turned off, the whole craft falls into the hole intact. It'll be shredded later as it approaches the singularity. This will happen in finite time by the ship's clock. Turning the engines on, regardless of direction, will only hasten the event.

Locally, the event horizon is an imaginary surface that is moving outward at the speed of light.

If you start out ahead of it (outside the horizon), the geometry of space time is your friend. It allows you to get away with a slightly sub-light speed as long as you keep accelerating hard enough.

If you start out behind it (inside the horizon), the geometry of space time is your enemy. You cannot catch up to the surface with any sub-light speed and acceleration only makes the problem worse.

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#### mfb

Mentor
What if we build a Vacuum inside a bigger vacuum?
That is the usual approach if you need an extremely low pressure. A single pump system can't provide that, so you use a big pump that can pump out most of the air and create a rough vacuum, and then use a specialized smaller pump that evacuates the volume better - its "high pressure" side is kept at a low pressure by the other pump.
For the walls layers make no sense: If you can build a wall that can withstand 99.9% of the air pressure you can also make one that can withstand 100% of it.

There is an interesting special case of a vacuum in a vacuum solution: LHCb, one of the detectors at the LHC at CERN, tries to measure tracks of particles as precisely as possible. The collision point of the beams is in an extremely high vacuum (to avoid particles hitting gas in the accelerator). Normally you just put a regular beam pipe around it that separates this vacuum from the detector which is at atmospheric pressure. That needs a relatively thick pipe, however, which scatters particles going through it.

LHCb avoids this by putting the first sensors in a vacuum, getting rid of the thick beam pipe. The accelerator vacuum is really good, however - it would be really difficult to maintain it over the whole detector component, especially as the sensors would release a bit of gas over time. LHCb found a compromise: The sensors are in an intermediate vacuum (2*10-10 times atmospheric pressure). A thick wall takes care of the large pressure difference between sensors and the outside world. Between sensors and beam there is only a thin foil - it can easily handle the remaining pressure difference from 2*10-10 times to 10-12 times atmospheric pressure.

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