Ask a physicist anything. (6)

chris4243 · Aug 13, 2011

Cactus Jack said:
Define for me what gravity is, and how is it we can expect to have artificial gravity on such a small scale.

Per relativity gravity is indistinguishable from acceleration (unless you look out the window). For our future space habitats, we can expect big wheel-shaped habitation areas rotating to provide centrifugal force as a substitute for gravity. This is better than the silly artificial gravity of the movies because it uses (on average) no energy nor can it accidentally switch off or malfunction.

A more awesome artificial gravity would consist of a rocket ship continuously accelerating at 1 g. More awesome but very difficult.

chris4243 · Aug 13, 2011

Lillen said:
My theory is that we need another perception of nuclear science before we can simulate fusion... We need to adress particles with a masculine and a feminin attributes...

How about if we get some quarky particles and call them names like "charm", "strange", or "beauty"?

Chalnoth · Aug 13, 2011

chris4243 said:
Per relativity gravity is indistinguishable from acceleration (unless you look out the window). For our future space habitats, we can expect big wheel-shaped habitation areas rotating to provide centrifugal force as a substitute for gravity. This is better than the silly artificial gravity of the movies because it uses (on average) no energy nor can it accidentally switch off or malfunction.

A more awesome artificial gravity would consist of a rocket ship continuously accelerating at 1 g. More awesome but very difficult.

Actually, it's not quite indistinguishable. The difference is the tidal force: gravity is stronger nearer a massive object than further away. So you can actually measure the difference between acceleration and gravity by measuring the differences in acceleration at different places within the craft.

That said, there is still an overall level of acceleration that is arbitrary in General Relativity.

But yes, a rotating habitat is our best bet for "artificial" gravity. And yes, having a rocketship accelerating at 1g would be amazing. You'd have the whole observable universe at your fingertips, pretty much, because at that acceleration most trips only take a few years. However, the energy requirements are so enormous that it is essentially impossible. You can't take enough fuel on board to do it, for instance, because even taking anti-matter as fuel with 100% conversion of energy to propulsion isn't enough to accelerate a ship at 1g for long enough to make it to the nearest star.

chris4243 · Aug 13, 2011

Lillen said:
I got another question. If we read the bible we see that God sets forth a law that says that nothing that was made crocket can become streight again!

Can this law be falisified? Or is it true?

Is it even a law? Is it even literal? Can you give an example of something that God made crooked, so that we can try to make it straight to see if we can? If not, then we can't falsify it (for various reasons depending on which answer was the "no".)

Exiledoomsayer · Aug 14, 2011

What do we know about the evolution of dinosaurs? Particulary rise.

mzungu · Aug 14, 2011

Chalnoth said:
Actually, it's not quite indistinguishable. The difference is the tidal force: gravity is stronger nearer a massive object than further away. So you can actually measure the difference between acceleration and gravity by measuring the differences in acceleration at different places within the craft.

That said, there is still an overall level of acceleration that is arbitrary in General Relativity.

But yes, a rotating habitat is our best bet for "artificial" gravity. And yes, having a rocketship accelerating at 1g would be amazing. You'd have the whole observable universe at your fingertips, pretty much, because at that acceleration most trips only take a few years. However, the energy requirements are so enormous that it is essentially impossible. You can't take enough fuel on board to do it, for instance, because even taking anti-matter as fuel with 100% conversion of energy to propulsion isn't enough to accelerate a ship at 1g for long enough to make it to the nearest star.

I am sure that given time we will move away from conventional modes of space travel assuming we are able to bend spacetime and thus allowing interstellar travel to not take more than a foot step away. I suspect that we will need a couple of black holes for that but hey; Nothing wrong with dreaming eh?

ENJOY: http://www.youtube.com/watch?v=0-nP4yD1hkk

Chalnoth · Aug 14, 2011

mzungu said:
I am sure that given time we will move away from conventional modes of space travel assuming we are able to bend spacetime and thus allowing interstellar travel to not take more than a foot step away. I suspect that we will need a couple of black holes for that but hey; Nothing wrong with dreaming eh?

ENJOY: Alternative Reality Hyperspace Universe (part 1 of 5) - YouTube

Haha, that would be cool Not very likely, but still cool. My suspicion is that if we ever manage to travel to the stars (which I dearly hope we will), it will be performed via the use of ships that will take generations to reach their destination.

mzungu · Aug 14, 2011

Chalnoth said:
Haha, that would be cool Not very likely, but still cool. My suspicion is that if we ever manage to travel to the stars (which I dearly hope we will), it will be performed via the use of ships that will take generations to reach their destination.

I would gladly volunteer for a one way trip even if it meant certain death. Such is the attraction I feel for knowledge! Interstellar Spaceflight: Is It Possible?

Cactus Jack · Aug 15, 2011

Ok. Define magnetism and explain the potential it could, in theory, have in the medical and rehabilitative fields.

Chalnoth · Aug 15, 2011

Cactus Jack said:
Ok. Define magnetism and explain the potential it could, in theory, have in the medical and rehabilitative fields.

Well, magnetic fields are similar to electrical fields, but are caused by the motion of charged particles, whereas electrical fields are there even if nothing is moving. Magnetic fields impact how charged particles and other objects with magnetic fields move. (caveat: quantum mechanics changes this slightly because even single, unmoving particles can have magnetic fields that act as if those particles were spinning).

As far as medical uses go, there is almost zero effect of electromagnetic fields on the human body. The claims of therapeutic effects of magnetic fields are pure pseudoscience that have zero basis in reality whatsoever.

Where electromagnetic fields are useful in medicine is in terms of various tools that are used, such as MRI machines (MRI = magnetic resonance imaging). MRI's allow for extremely detailed images of the interior of human bodies with effectively zero risk of negative effects.

mzungu · Aug 15, 2011

Cactus Jack said:
Ok. Define magnetism and explain the potential it could, in theory, have in the medical and rehabilitative fields.

MRI has saved countless of lives!

Wiccan_Child · Aug 15, 2011

razeontherock said:
FALSE. When it's repeated, and consistently produces the same results, it counts.

Then it's not an anecdote, it's a testable claim with empirical evidence. The sheer utterance of a claim is not, in itself, evidence of anything, least of all itself.

razeontherock said:
In context here, something as simple as a 90 year old frail woman having different nutritional requirements from a 250 pound active male athlete, is basically true. Regardless of the fact the USRDA fails to recognize this, and is therefore pretty worthless.

Err... what? The RDA figures are approximate guidelines as to the nutritional intake of a healthy individual who performs an average amount of exercise per day/week. These figures are generally listed threefold, one for children, one for adult males, and one for adult females. A bodybuilder does not qualify as someone who has an average amount of exercise.

RDA figures are estimates, rough guidelines as to the nutritional intake of the average person. The more specific yours needs (such as if you're diabetic, suffer from Chron's disease, excessively exercising, pregnant, etc), the more your optimal nutritional intake will vary from the RDA.

But how any of that bolsters the empirical merit of anecdotal 'evidence' is beyond me.

Wiccan_Child · Aug 15, 2011

Exiledoomsayer said:
What do we know about the evolution of dinosaurs? Particulary rise.

The dinosaurs were (or are) a broad group of species that dominated the planet from about 260MYA to about 65MYA. As ever, Wikipedia has a graphic detailing the whole thing:

The dinosaur's relationship to as stars way back when a group called Amniota split into Sauropsida and Synapsida. Synapsida contains mammals - such as us - and Sauropside contains reptiles - such as birds and dinosaurs.

So what do we know about the evolution of dinosaurs? Quite a lot! We have, for instance, just discovered evidence that Plesiosaurs, a type of aquatic reptile, gave birth to live young.

Wiccan_Child · Aug 15, 2011

Cactus Jack said:
Ok. Define magnetism and explain the potential it could, in theory, have in the medical and rehabilitative fields.

As others have pointed out, magnetism has been an enormous boon to medical diagnostics, primarily in the form of magnetic resonance imaging (MRI) machines. I daresay there are innumerable other direct benefits, not to mention indirect ones - everything computer-based owes its existence to our understanding and manipulation of electromagnetic fields, from imaging and simulations to drug manufacture and state-of-the-art surgical equipment.

As for what magnetism is, Chalnoth said it best.

Tuddrussell · Aug 15, 2011

If you could rotate a magnet at near light speed would it do anything interesting?

I heard that there is no limit to how large a black hole can get, but is that true? I couldn't see how, I mean there has to be a hard limit. Can an infinitely large black hole exist? Would it do anything interesting?

What would happen if an infinite black hole tried to absorb another infinite black hole? Would it be doubly infinite, would the black holes be "full?"

What would happen if something totally indestructable, like say the Juggernaut, were to be sucked into a black hole?

chris4243 · Aug 15, 2011

Cactus Jack said:
Ok. Define magnetism

The magnetic field is what a moving electric field looks like.

and explain the potential it could, in theory, have in the medical and rehabilitative fields.

Others have mentioned Magnetic Resonance Imaging (MRI), a technique that has saved thousands of lives. There's also Nuclear Magnetic Resonance (NMR), an important technique for chemical analysis, important enough that 5 Nobel prizes were awarded for work concerning that technique. Given the importance of chemistry to the pharmaceutical industry, I'm quite certain that NMR has also saved thousands of lives. If you consider computer hard drives, and maybe also other computer components and the importance of computers in modern medicine, that's pretty significant too.

Maxwell511 · Aug 15, 2011

Chalnoth said:
As far as medical uses go, there is almost zero effect of electromagnetic fields on the human body. The claims of therapeutic effects of magnetic fields are pure pseudoscience that have zero basis in reality whatsoever.

Therapeutic uses of electromagnetic simulation have been used for millennia. Although back in ancient times they did not understand the mechanism. Basically they used marine animals (I forget which one) to shock the patient to provide anesthetic or pain relieving effects. This has been verified by modern science, through the application of electromagnetic fields correctly to the arm, for example, you can apply greater pressure on a subjects finger without them feeling pain.

Modern applications of electromagnetic fields for therapeutic reasons include regulation of heart beat (e.g. pacemakers) and deep brain simulation of parkinson's victims.

There is a lot of pseudoscience about electromagnetic radiation but the concept itself is sound.

chris4243 · Aug 15, 2011

Tuddrussell said:
If you could rotate a magnet at near light speed would it do anything interesting?

Well, rotating at the speed of light would be quite interesting in itself. If you rotate a magnet, you generate an electric field. I think if you get a strong enough electric field you get funny stuff like particle-antiparticle pairs being spontaneously generated.

I heard that there is no limit to how large a black hole can get, but is that true? I couldn't see how, I mean there has to be a hard limit. Can an infinitely large black hole exist? Would it do anything interesting?

What would happen if an infinite black hole tried to absorb another infinite black hole? Would it be doubly infinite, would the black holes be "full?"

A black hole in reality can only be expected to get so big, due to not having enough matter nearby to absorb. It will be limited in roughly the same way the size of galaxies is limited.

If you do the calculation for the Schwartzschild radius of a black hole with the mass of the observable universe, you get approximately the size of the observable universe. Though I'm told that formula is inappropriate for using of the universe since it doesn't account for some things. Still, you can't see past our observable universe and you can't see outside a black hole, so food for thought.

What would happen if something totally indestructable, like say the Juggernaut, were to be sucked into a black hole?

Obviously, there is no such thing as something totally indestructible. As I understand it, if you put on a space suit and jumped into a really really big black hole, you'd survive going through the event horizon into the black hole. But, there's no coming back out of it, I'm not entirely convinced you actually can fall in, and if you do fall in there's a pretty good chance that you'll be falling faster than the speed of light towards a singularity at the center of the black hole.

Chalnoth · Aug 15, 2011

chris4243 said:
Others have mentioned Magnetic Resonance Imaging (MRI), a technique that has saved thousands of lives. There's also Nuclear Magnetic Resonance (NMR), an important technique for chemical analysis, important enough that 5 Nobel prizes were awarded for work concerning that technique. Given the importance of chemistry to the pharmaceutical industry, I'm quite certain that NMR has also saved thousands of lives. If you consider computer hard drives, and maybe also other computer components and the importance of computers in modern medicine, that's pretty significant too.

NMR and MRI are the exact same thing, just with slightly different machines used

Chalnoth · Aug 15, 2011

Maxwell511 said:
Therapeutic uses of electromagnetic simulation have been used for millennia. Although back in ancient times they did not understand the mechanism. Basically they used marine animals (I forget which one) to shock the patient to provide anesthetic or pain relieving effects. This has been verified by modern science, through the application of electromagnetic fields correctly to the arm, for example, you can apply greater pressure on a subjects finger without them feeling pain.

Electric shock therapy? Well, yes, there is some benefit to this. But there is a big difference between using electromagnetic fields on the body and inducing a current through it.

There are only two possible effects of electromagnetic fields on biology:
1. Deposition of heat. Electromagnetic fields do carry energy, and can warm of cells. You have to use a huge amount of radiation to have any biological effect in a warm-blooded animal like ourselves, though, so that this is almost never a danger.
2. Chemical reactions. Chemical reactions can be caused when the energy per photon in the electromagnetic radiation is large enough to start kicking electrons out of their orbitals. This starts to happen in the ultraviolet range, which is why UV radiation from the Sun causes sunburns and can cause skin cancer. No radiation lower in frequency can do this, which means that any radiation in the visible, infrared, microwave, or radio range is perfectly safe, provided the total energy deposited isn't too high.

Magnets themselves fall under electromagnetic fields in terms of how they can impact biology. And in realistic situations, it is just not possible to use a magnet to produce anywhere near as strong of electromagnetic fields as we experience on a daily basis. So the use of magnets in therapy is pure woo.

Except as components in more complicated machinery, of course, such as MRI machines, which make use of radio-frequency waves to cause the nuclei in our atoms to sort of spin (which has almost zero effect on biology), and then powerful electromagnets to carefully measure how quickly those spins relax. Different materials tend to relax at different rates, giving us an image of the internal structure of the body.

Ask a physicist anything. (6)

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