Just ask this scientician

[Melville]

I gather that at least a few of you on this site have at least a slight interest in physics. In this thread, I'll do my best to answer any physics-related question you might have, be it about black holes, dark energy, quantum mechanics, superconductivity, or whatever else. I'm an expert in only one small subject within the one small field of general relativity, but in principle I should be able to muster up a coherent answer to questions from other fields—and lovejuice can correct me if I'm wrong. Since I suck at explaining things to non-experts, perhaps this will help me prepare for my PhD thesis defense, which is (hopefully) fast-approaching.
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Yes, I expect this thread to die a swift death. But I can't be bothered to do anything else.

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[Spaceman Spiff]

Not really a question, but what's your favorite paradox?

[Mysterious Dude]

Is it physically possible for man to travel at (or close to) the speed of light (and survive the journey)?

[Melville]

Not really a question, but what's your favorite paradox?

In physics? I think I've only learned about two: the twin paradox and the EPR (Einstein-Podolsky-Rosen) paradox. In case you haven't heard of them, I'll summarize both.

The twin paradox is from special relativity. It's kind of boring. Two twins begin on Earth. Twin A stays there. Twin B leaves Earth, travels at some high speed for a while, and then returns. When they reunite, which twin is older? Because twin B has been moving relative to twin A, special relativity dictates that twin A sees twin B aging more slowly. But twin A has been moving relative to twin B, so twin B sees twin A aging more slowly. Therefore, each should be younger than the other when they reunite. However, the correct answer is that twin B, the space traveler, is younger. The resolution of the paradox is that the situation is asymmetric: twin A kept moving in the same direction, while twin B had to have turned around to come back to Earth. During his departing and returning trips, twin B sees twin A age more slowly, but in the instant that it takes him to turn around, he sees twin A ages a whole lot.

The EPR paradox is an experiment that Einstein proposed in order to show that quantum mechanics must be wrong. It starts with two particles that are entangled—meaning, basically, that the state of each of them is linked to the state of the other. Because of the way quantum mechanics works, we can prepare these two particles in a superposition (basically, a combination) of states, such that each particle has a 50% chance of being in state 1 and a 50% chance of being in state 2. Since the particles are entangled, we might have, for example, that if particle A is in state 1, then particle B must be as well. Now we send them off in opposite directions. After some time, someone measures the state of particle A. The two particles have not interacted in any way since they were prepared, so it seems that particle B should still have a 50% chance of being in state 1 and a 50% chance of being in state 2. But because the particles are entangled, as soon as we've measured the state of the one, the state of the other is instantaneously determined. Einstein called this a "spooky action-at-a-distance": the act of measuring particle A seems to instantaneously influence the state of the now far-away particle B. That seems to run counter to everything we intuit about the physical world. The resolution to this paradox is that quantum mechanics is some crazy shit.

[Melville]

Is it physically possible for man to travel at (or close to) the speed of light (and survive the journey)?

Only a massless particle can travel at the speed of light, so a man can't move that fast. However, as long as you're moving at a constant velocity, you don't feel the motion. In fact, velocity is purely relative; if you didn't have other objects to measure your velocity relative to, you'd never be able to define whether or not you're moving at all. So if you could get to a velocity close to the speed of light, you wouldn't notice anything unusual. However, you'd have to accelerate up to that speed, and you'd feel the acceleration. According to Wikipedia, humans can withstand accelerations of about 5-10 g. According to a calculation I just did, it would take about seven years to get to 99% the speed of light (relative to Earth, say) at a constant acceleration of 1 g. So you could survive that without trouble. However, you'd require so much energy on your spaceship that it would collapse into a black hole, killing you before you even started.

EDIT: I screwed up the calculation the first time.

[Watashi]

On the Itchy & Scratchy CD-ROM, is there a way to get out of the dungeon without using the wizard key?

[Melville]

On the Itchy & Scratchy CD-ROM, is there a way to get out of the dungeon without using the wizard key?

What the hell are you talking about?

[amberlita]

So basically you're saying my dream of having my great great great great grandson helm the Starship Enterprise to a distant galaxy at warp speed is pretty much impossible? Bummer.

[Mysterious Dude]

So basically you're saying my dream of having my great great great great grandson helm the Starship Enterprise to a distant galaxy at warp speed is pretty much impossible? Bummer.

My dream is a little more modest: I plan to establish a colony around the 40 Eridani system, which is only 16.5 light years away.

But that black hole is troublesome.

[amberlita]

Speaking of black holes, what would it be like to travel into one? Would you die immediately? Get crushed or flattened or something?

[Melville]

My dream is a little more modest: I plan to establish a colony around the 40 Eridani system, which is only 16.5 light years away.

But that black hole is troublesome.

Well, it really depends on your propulsion system. If you could somehow convert the entire mass of the fuel into the kinetic energy of your ship, the total mass of your fuel would only need to be seven times the mass of your ship (and everything in it). Although it's been so long since I've done any calculations involving numbers that now I'm starting to doubt everything I'm saying.

Speaking of black holes, what would it be like to travel into one? Would you die immediately? Get crushed or flattened or something?

It depends on the size of the black hole. If it's really big, like the supermassive black hole that's thought to reside at the center of the Milky Way, then you wouldn't notice anything at all as you entered "into" it (that is, as you passed through its event horizon, the point at which light cannot escape from it). Eventually, once you got deep inside the black hole, you'd get ripped apart by the tidal fields (the difference in strength between gravity at your head and at your feet). If the black hole were much smaller, you'd get ripped apart before you made it through the horizon. Eventually, your ripped-up self would get crushed into the geometric singularity at the center of the black hole. However, from the perspective of a distant observer, you'd never actually make it through the event horizon; you'd seem to approach it ever more slowly, becoming pancaked onto it. The reason is that the force of gravity just outside an event horizon makes light just barely escape, such that it takes a long time to get to the distant observer, so that what he sees occurring just outside the horizon seems to him to take forever.

In the Star Trek thread a while back, I answered a lot of questions about falling into a black hole. That conversation started here.

[Melville]

So basically you're saying my dream of having my great great great great grandson helm the Starship Enterprise to a distant galaxy at warp speed is pretty much impossible? Bummer.

Wait, isn't warp speed just the same as light speed? In that case, it would take 2.5 million years to get to another galaxy, even on the Starship Enterprise. I think the Star Trek stories are restricted to one part of our galaxy.

[amberlita]

Wait, isn't warp speed just the same as light speed? In that case, it would take 2.5 million years to get to another galaxy, even on the Starship Enterprise. I think the Star Trek stories are restricted to one part of our galaxy.

Well whatever. The point is still made. It's impossible.

Furthermore, I find it fairly depressing that there is that much out there we can never explore. We are one galaxy of billions that we'll never get to see except like this:

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I want to go to there.


Thanks for the above explanation by the way. And the link to the Star Trek thread. Clears it up. I had to give up when you got to the diagrams though. My brain can only get fucked for so long at one time.

[lovejuice]

is big bang a black hole in reverse?

[Melville]

Well whatever. The point is still made. It's impossible.

Furthermore, I find it fairly depressing that there is that much out there we can never explore. We are one galaxy of billions that we'll never get to see except like this:

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I want to go to there.

Wait, after doing a quick calculation, I think I've been spreading misinformation. The nearest galaxy is 2.5 million light years away as measured in the reference frame of Earth. So if you traveled there at, say, 99.99999999% the speed of light, people on Earth (or people on a planet in the other galaxy) would measure the length of your journey to be roughly 2.5 million years. However, the time for you, the traveler, would be much less: from your perspective, the journey would last only 35 years. (Even better, Antoine would age only 5 years on his comfortable 1g-journey to 40 Eridani.) If you could somehow convert mass into kinetic energy with 100% efficiency, the mass of your fuel would need to be 70000 times the mass of your ship and its contents. Using current fuel sources, your fuel would probably have the mass of the solar system or something; just traveling to the nearest star outside our solar system would require all the energy on Earth. You'd probably need a pretty sturdy ship too, since space dust and such would cause serious damage at those speeds. But I don't really know anything about fuel efficiency or structural integrity of spaceships' hulls.

Of course, there are still parts of the universe that you'd never be able to explore even if you got to those speeds, since, for example, while you're on your way to the most distant galaxies, they will experience billions of years of time's passage, so they'll probably have died by the time you get there. (The expansion of the universe might also make it impossible to get to some places, since they'll be receding away from you faster than you can move.) So really, there isn't that much of a difference between not being able to visit distant galaxies and not being able to visit the past of our own galaxy.

is big bang a black hole in reverse?

Kind of. The big bang is a geometrical singularity (a place of infinite spacetime curvature) in the past, while the black hole contains a geometrical singularity in the future. However, the singularities are somewhat different. The big bang singularity is a single point marking the beginning of spacetime. If you follow every particle back through its past history, they all end up at that same point. The black hole singularity, on the other hand, is actually a curve, because the black hole is moving through spacetime. For example, if a black hole is sitting at the center of our galaxy, then its external region is moving forward in time (the singularity at its center is more like a point in time moving through space, though, rather than a point in space moving forward in time, because the strength of gravity kind of switches the roles of space and time). So if you travel into a black hole, you'll eventually get crushed to a point, but that point can be at different locations.

Also, while everything started from the big bang's singularity, not everything ends up at the black hole's singularity: you can avoid the singularity simply by never passing through the black hole's event horizon.

[Kurosawa Fan]

How do you not have daily panic attacks?

[megladon8]

Can you teach me how to love?

[Melville]

How do you not have daily panic attacks?

I do. But they're unrelated to physics. When I started out in physics, I was interested by the immensity and bizarreness of it all, but I was never really struck with terrified awe or anything like that. And as I learned more, even the immensity and bizarreness started to disappear. The thing is that physics is basically a set of mathematical tools and concepts for describing physical situations. For the most part, when you do research in physics, you try to construct a useful mathematical model for a situation, and then you try to glean some information from that model. If you work at the most abstract end of physics, where you're trying to make new fundamental physical theories, then the "situation" you're trying to model is the fundamental form of physical reality. But either way, rather than trying to wrap your mind around the physical situation, you're wrapping this mathematical apparatus around it. To a large extent, you think of the situation in terms of the math. (Keep in mind, though, that it's not as reductive as it sounds, since there's a lot more to math than basic algebra, calculus, etc.) So, for example, the immense size of the universe ceases to be impressive, because you're not trying to somehow visualize it all in your mind's eye; instead, you just think of it in terms of numerical orders of magnitude.

Once you get to actually working with physics, the thing that saps your will to live isn't the strangeness of the universe, but the blasted difficulty and tedium of trying to come up with a model that makes sense and then working through the endless, harrying mathematics within that model.

Certainly some physicists do think more in terms of physical intuition and less in terms of math (though you need two kinds of that intuition: one for classical physics and another for quantum physics). And the division between them does become blurry. And some relatively small number of physicists actually do maintain a sense of awe about all this stuff. They're usually the ones writing popular science books. But I definitely fall amongst the more mathematically-minded.

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I don't really have daily panic attacks, but when I do have them, they're definitely unrelated to physics.

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[Melville]

Can you teach me how to love?

I think Jen would be the more relevant person to talk to...unless, in accord with the thread's title, you're making a veiled Simpsons reference, in which case I've been out-referenced.

[megladon8]

Here's a serious question:

What's the plausibility of "other dimensions"?

[Melville]

Here's a serious question:

What's the plausibility of "other dimensions"?

The idea of them is entirely speculative, but it's perfectly plausible. There's no evidence in favor of them, but neither is there any evidence against them (besides our personal perception of reality). Some theories such as string theory, which try to determine the fundamental physics at work in the universe, require extra spatial dimensions in order to work. (String theory requires seven such extra dimensions.) And while string theory certainly has no empirical evidence supporting it, or even concrete numerical predictions, it is a promising theory in that it unifies general relativity with quantum field theory. The existence of some of the requirements of the theory (e.g. supersymmetric particles) might be discovered at the Large Hadron Collider. Those requisite things can also exist in theories without higher dimensions, so discovering them wouldn't be direct evidence in favor of more dimensions, but it would lend some credence to string theory.

Other theories kind of just add on extra dimensions because they can, or in order to explain certain sets of data. For example, a popular theory these days is that the universe is five-dimensional, with our universe being a four-dimensional membrane (usually shortened to "brane") embedded in that higher-dimensional universe. All the fundamental interactions that give rise to forces are constrained to act within the brane, except for gravity, which is allowed to act in all five dimensions. This explains the weak strength of gravity compared to the other forces, and as I recall, it can be used to explain the value of the cosmological constant (which determines the rate of acceleration of the universe's expansion). It's my understanding that this theory is supposed to be tested at the LHC.

This kind of stuff is definitely outside my realm of expertise, though, so I could be wrong.

[megladon8]

Long division is outside my realm of expertise so believe me, your answer has more value than you think

[Qrazy]

What kind of new and interesting things has the Large Hadron Collider unearthed these days?

[megladon8]

What kind of new and interesting things has the Large Hadron Collider unearthed these days?

The impending end of the world?

I think we need to be spending more time exploring the bottom of the ocean to find Cthulhu's resting place, and less time on all this theoretical space-age malarchy.

[Melville]

What kind of new and interesting things has the Large Hadron Collider unearthed these days?

To the best of my knowledge, it hasn't found anything. I don't think it's operating at its full energy range yet.

EDIT: in case you didn't read about it in the news, the reason it's so far behind schedule is that it broke down almost immediately when they first turned it on. They're just now getting it operating again.

POST-EDIT EDIT: deep sea creatures are indeed cooler than most spacetime physics.