RELATIVITY QUESTIONS! (and other common queries)

[Sir_Elderberry]

Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

While I am not sure whether it qualifies as information, I do believe that a wave function collapses instantaneously after an observation, as well as all wave functions entangled with the particle.

You can't use it as communication, though.

[Tchebu]

It, indeed, does not qualify as information, because "Gee Willikers, look, the wave-function has collapsed" is not something that you can do. "Collapsed" and "not collapsed" aren't states that you can observe and therefore cannot be carriers of information.

[thoughtfully]

Free-falling objects experience the same acceleration no matter their mass.

Neglecting air friction.

Just to be perfectly correct...

Actually, when he stipulated "freely falling" (just to be gramnmatically correct), he removed any forces acting on the body, other than gravity.

So we need another factoid/definition:

Free-fall is the state a body is in when it has no forces acting upon it, other than gravity.

[free]

Havent read the every entry due to being at work n stuff..
but for the "information doesnt travel faster than light".

Well .. depends what "information" is.
If you have a lightray that goes from the center of a HUUUUUUUUUUUUUUUUGE sphere to the inner side of the construct, and you start to spin the center fast enoug, the lightdots movement should be able to be faster than light i think ^^

Just trying to get my ass kicked OR maybe being right ; P Thats something totally random that popped up in my head so..dont kill me..crippling is ok as long as you leave my hands alone (gotta play the guitar no matter what <3 ^^)

[gmalivuk]

That example has been brought up, and it doesn't count as information traveling faster than light, because it's just information traveling from the center to the sphere at exactly the speed of light.

Nothing is traveling faster than light, just like if I sent a signal off in two opposite directions at the same time, and people a lightyear away in both directions received my message at the same time (from my frame of reference), it wouldn't mean any information had traveled instantaneously between the two of them.

[Bluggo]

The laws of thermodynamics are very, very true. You cannot get free energy, no matter how nice it would be, and no matter how much science fiction you read.

Nitpick: the laws of thermodynamics are very, very, very true - for macroscopic systems, and in a statistical sense.

A ten-molecules system, for example, would not even have a well-defined temperature to begin with, and could spontaneously evolve from a disordered configuration to an ordered one.

As for the first law, a single electron does not have an energy in the classical sense: the closest match, if I am not mistaken, would be the expected value of the Hamiltonian - which is conserved, of course, since the Hamiltonian commutes with itself; but it's only an expected value, after all, and an actual measurement of the energy of the electron could give a different result.

Not that any of this implies that you can get free energy, of course...

I hope I did not make mistakes, it's been a while since I studied this stuff - by the way, what is a good graduate-level book on thermodynamics? I have been working quite a bit on information theory lately, and I would like to better understand the physics behind Landauer's principle.

Plus, thermodynamics is fun

[iop]

A few things about evolution

1) Natural selection has nothing to do with how natural the environment is. The natural distinguishes it from selective breeding. Natural selection is the evolutionary process by which a selective advantage of a specific allele leads to its spreading in the population.
2) Aside from natural selection, there are three other drivers of evolution: Mutation, genetic drift and recombination. Natural selection is only important in large populations and/or when the selective advantage is very strong.
3) Humans are still evolving. Human evolution due to natural selection has been accelerating over the last 40'000 years because of the population increase (see 2).
4) A genetic condition that would have reduced reproductive success for hunter/gatherers, but that can be treated thanks to modern medicine such that it leads to no disadvantage at all may become increasingly prevalent due to genetic drift. This does not mean that there is no more evolution today (genetic drift is also an evolutionary process, and see 3). This also doesn't mean that our gene pool is getting "worse", except if you think that humans should be highly adapted to an environment of hunter/gatherers (see also 1).

[Sir_Elderberry]

A few things about evolution
2) Aside from natural selection, there are three other drivers of evolution: Mutation, genetic drift and gene duplication. Natural selection is only important in large populations and/or when the selective advantage is very strong.

Something about this statement sits wrong with me. The three things you list are ways variance occurs, surely natural selection is in its own category, as it does not directly cause physical changes, but rather only "selects" from among the different organisms presented to it?

I'm not a biologist, I'll admit. I haven't even take high school biology yet, so, be aware that the above statement may be wrong. (Bio was scheduled for us this year, but I skipped out on it for AP Physics. I'll get around to bio...eventually...need it to graduate.)

[iop]

. The three things you list are ways variance occurs, surely natural selection is in its own category

If you look for drivers of evolution, i.e. mechanisms that lead to changes of allele frequencies in the gene pool, then selection is just one of the mechanisms.

It is in a category with genetic drift in that in itself, it doesn't cause the physical changes (like mutations and recombination). It is in a category on its own in that it is the only adaptive mechanism.

[Scigatt]

On the Quantum thing: Whether there are actual non-local effects depends on how you interpret QM. For example, if you look through the RQM interpretation(you can look it up in Wikipedia), there is no non-locality at all.

[K^2]

Another common misconception: The idea that "quantum physics and relativity are completely irreconcilable" is not correct. Quantum mechanics and special relativity have been combined to a high degree of success: QED, one resulting major theory, is the most accurately-validated physical theory in existence.

I'm sorry, but as someone doing a degree in QFT, I'm going to have to disagree with you. Dirac's equation is a correction to the QM for the relativistic effects. It does not fix the problem. Dirac's equation, for example, does not work in curved space-time. As soon as you introduce gravity into the system, you get errors. It goes as far as violations of gravitational-inertial mass equality. I don't know about you, but when solutions to QM equations violate the first postulate of General Relativity, I would say that at least one of the two is wrong.

[Eps]

K-squared, did you even read what I wrote, or did you just skim the post? Pertinent words bolded:

Another common misconception: The idea that "quantum physics and relativity are completely irreconcilable" is not correct. Quantum mechanics and special relativity have been combined to a high degree of success: QED, one resulting major theory, is the most accurately-validated physical theory in existence.

I'd have expected someone "doing a degree in QFT" (what does that mean?) to know the difference between referrring to the Dirac equation and referring to full-blown gauge theories, and also to know that explicitly referring to the particular case of special relativity is not the same as referring to GR. I think most people on here are aware that GR and the Standard Model have not yet been fully reconciled.

[TizzyFoe]

ill fight the urge to make this its own thread.

I was thinking during the winter the energy efficiency of every appliance (if you use electric heating) is 100%, because the the energy of all appliances is eventually converted into heat which just means your heater has to run that much less. Also the energy efficiency of every electric heater is 100% (although this isn't true for the duct systems). There are a few exception i can think of like light bulbs which might emit light out windows.

am i wrong?

its interesting that energy efficient appliances might be a scam for people living in Alaska.

[evilbeanfiend]

well it assumes a few things

1) the house is heated via resistive heating
2) none of the energy escapes the house as light or sounds
3) you would require your heating to be on when your appliances are anyway but at a lower rate i.e. there isn't a point at which the house get too hot even with the conventional heating off
4) it ignores how you get hot water

[alterant]

Here is one thing I've always been curious about. Please tell me if I'm correct about this.

If you're in an airplane and you start jumping up and down, jumping up and hitting the roof, throwing yourself against the sides, etc., you do affect the plane's motion instantaneously, but not over the long term (unless you break a window or land on the flight controls or something). One way of seeing the reason for this is that any momentum you might gain this way, you got from the plane in the first place. When you hit the wall or whatever, you just give it back again. So the plane jerks slightly, but returns to where it started in the end.

This is not true for, say, a bus. This is because the bus' wheels connect you to the earth's momentum and allow you to gain momentum from the earth, then transfer it to the bus. In this case, the earth taken as a system has zero change in momentum.

....Right....?

[evilbeanfiend]

it is true for the plane and the bus, however it wouldn't be very noticeable because the bus/plane is so much more massive than you, it will barely accelerate at all. on smaller cars its pretty easy to bounce the suspension by jumping up and down inside (the best method of doing this is left as an exercise for the reader )

[M.qrius]

Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

Could you explain why that wouldn't send information faster than light? Entanglement is instantaneous, right?


As for the bus story: You're right. That's why we can't make the world spin faster by running east (in the long run (pun not intended)).

[Owehn]

Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

Could you explain why that wouldn't send information faster than light? Entanglement is instantaneous, right?

How would you plan to use entanglement to send information? It turns out that:
1. Once a measurement on either of two entangled particles is made, the pair is no longer entangled.
2. With access to only one of the particles, there's no way to determine whether it's still entangled to the other.

So if your plan involves selectively measuring your photon and hoping I get the message, it won't work.

[alterant]

This question is directed to anybody here who knows anything about cosmology (I'm interested but rather clueless).

Does the fact space itself is expanding outward (as opposed to the visible universe expanding into space) have anything to do with the speed of light?

Because I can just picture how the light from the universe's matter can only have gone so far since the big bang, which means that in a sense nothing outside that radius can have been affected by events here or elsewhere in the visible universe, since information travels at the speed of light or less. Does that provide a sort of operational definition of the bounds of the universe? Or is my little speculation neither here nor there?

[gmalivuk]

I removed a couple posts from this thread. The reason this topic is stickied is so we can concisely address some of the more commonly asked science-related questions. NOT so people can have silly logic arguments that aren't actually related to any of those questions.

I rescued the one legitimate question in the middle - SV

[evilbeanfiend]

perhaps it would be better to maintain a single post with common questions, and links to threads that answer them, then? a thread isn't the best model for a FAQ really.

[psykx]

there is a wiki being developed (by me) as an add on to phpBB3 it's mostly beta at the moment although the user permissions code is still only alpha at the moment it's called bbiwiki and it's hosted on source forge.

[Waylah]

Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

Really?????
Show show I'm interested. What about the double slit experimet where the observation took place after the photon traveled through, and that observation changed what it had done? That sounds like information travelling pretty fast to me! And what about the entangled photon experiments too? I thought they could send information with blatant disregard for c?

[Master Gunner]

See any one of the countless numbers of threads on this forum about those topics.
Simply put, when you measure an entangled photon, it collapses in a way that cannot be predicted. an it becomes un-entangled with it's "twin". So when you measure your photon, you'll see that it has been collapsed, no matter what. Because you can't predict how it will collapse, you have no way of knowing if it collapsed just now when you measured it, or 5 minutes ago when your partner a light-year away collapsed his. So there's no way to transfer information, because the results will always be the same whether you collapse the photon or someone else does.

I'll let someone else who knows more about the double-slit experiment answer that one, however I can assure you, no information is been transfered.

[mattie]

Your explanation of why material things cannot move faster than c doesn't sit right with me. You (Meteroswarm, in post #1) said, "The reason is that as you approach the speed of light, you gain kinetic energy at a much faster rate than you do at low speeds." True as that may be, it skirts the core issue: the value of your kinetic energy is clearly dependent on how and where the observer is situated who is assessing you. There's no such thing as a body's definitive rapidity "through space", so no constraints based on such a surmised absolute travel speed can be rightly cited as grounds for the inability to attain light speed. You cannot move at or above light speed relative to a material observation platform, because every time you apply thrust and increment your speed, the aforementioned observer (behind you) assesses less of a speed increment from his vantage than you yourself rightly attribute. It's a simple case of diminishing returns due to the way velocities add. But you yourself, and the craft you're riding in, don't actually get heavier -- not in your own native frame certainly; so it clearly will NOT become more and more laborious for you to accelerate, as most folks believe, since your thrusters are part-and-parcel of that native frame of your's.

Okay, that wasn't exactly the verbiage you applied (things get heavier near light speed), but that's the common misconception and you were clearly bolstering it.

[slightlymadscience]

Free-falling objects experience the same acceleration no matter their mass.

Neglecting air friction.

Just to be perfectly correct...

Actually, when he stipulated "freely falling" (just to be gramnmatically correct), he removed any forces acting on the body, other than gravity.

So we need another factoid/definition:

Free-fall is the state a body is in when it has no forces acting upon it, other than gravity.

Indeed.

I went to a Protestant private school for one year (6th grade) - it was my parents attempt to get me out of the bullying I used to have to deal with.

This school seemed to have the policy of "You have a pulse and are Christian? Throw a dart at this board, and see what subject you get to teach."

The "science teacher" told us that there's no gravity in space, that planets are held there by God's will. That was how she explained weightlessness. I asked for clarification, as that made no sense. She said "if you know so much, YOU come up here and teach." SO I did. I got up and explained the physics of the situation, which I learned by watching Public Broadcasting and other self-education. I talked about people in falling elevators, the idea of falling "past" the earth, all objects "falling" at the same speed, etc. She humored me while smirking at first, but got steadily angry and started trying to poke holes in my argument, which failed. So after I was done, she took me to the principals office to have me paddled for "insubordination." The principal wasn't sure if I was right, either, but gave me the chance to explain my point. So, using the CHRISTIAN science book (heavily edited), and books from their library, I found information to back my "claim," thus I had simply done as the teacher asked - no insubordination involved.
Boy, was that place glad to see me go back to public school. I decided that sacrificing my education wasn't avoiding dealing with some bullies - which I learned how to do properly later.

The topic of weightlessness in space still gives me a chuckle to this day.

Actually, one of my favorite podcasters just did a video about the difficulty of getting that simple concept across.

[alterant]

One thing I've wondered for a long time about vision:
I notice that, especially in the dark, what I'm seeing is, for lack of a better word, pixelated. I see tiny, tiny dots flashing on a blackish background. I'd estimate that a 10 degree angle slices through 200-300 of them, although it is very hard to "pin them down" so to speak. I can think of a few explanations:

1) It's simply "noise" in the signals getting to the brain;
2) It has something physically to do with the individual rods and cones.

The first seems more likely, but the second not necessarily impossible. Does anyone know anything about this?
-Ian

[evilbeanfiend]

i'm pretty sure it's noise, but there will be contributions from everything you can see and from everything involved in seeing, not just from the brain.

[nevskey1]

Here's one that I expect is pretty simple and not worthy of a thread.

c=3x10⁸ m/s. A person is launched at 2.5x10⁸ m/s. An asteroid is coming straight at him in the opposite direction at 2.5x10⁸ m/s. From the frame of reference of the person, is the asteroid moving at 5x10⁸ m/s? Does it seem to move faster than the speed of light?

[mattie]

Here's one that I expect is pretty simple and not worthy of a thread.
c=3x10⁸ m/s. A person is launched at 2.5x10⁸ m/s. An asteroid is coming straight at him in the opposite direction at 2.5x10⁸ m/s. From the frame of reference of the person, is the asteroid moving at 5x10⁸ m/s? Does it seem to move faster than the speed of light?

Velocities are not additive in the usual manner. You cannot assume an accurate velocity value by simply adopting it directly from a frame of reference that's alien to one's own. This leads to the need for a special (relativistic) formulation. The launched person will see the asteroid approach him at about 2.95x10⁸ m/s. Refer to the "Addition of Velocities" entry at Wikipedia.

[Arancaytar]

For everything we know about, including the matter you're made of, we cannot travel faster than, or even at the speed of light. The reason is that as you approach the speed of light, you gain kinetic energy at a much faster rate than you do at low speeds. This is why small particles going REALLY close to c, the speed of light, can pack a lot of energy.

Hm... I guess you could put it that way, but it is more comprehensible when you put it as "as you approach the speed of light, it takes much more kinetic energy to gain the same increase in velocity". Or "as you gain kinetic energy at a constant rate, the your velocity will approach the speed of light asymptotically".

[alterant]

Okay, another question. I know that if a radioactive substance has a half-life of time T, then if we take any large sample of it and wait for T, there will be half the original amount left.

But what does "sample" mean? If I have say 2 kg of a radioactive substance, and put 1 kg in one box, 1 kg in the other, will the amount of substance in each box halve itself over the course of the half-life T, or will the full 2 kg halve itself? What defines the boundary between one sample and another? Physical connection?

Why do I have the uneasy feeling this is going to lead to another eminently incomprehensible, quantum explanation?

[nevskey1]

Note: I'm far from a specialist. If you don't believe me, see my question three or so posts ago.

Shouldn't the math work out the same either way. The half-life formula is N(t_1/2)=.5N₀. So, if you start with 4 grams, then after half-life time (t_1/2), you end with 2g.

Now, split that 4g in half between 2 containers. You start with 2g, and end with 1g in the first container. The same goes for the other container. Together they give you 2g of substance, the same as if you had just started with 4g, as above.

If I'm right then this is the first time I've actually been helpful here.

[Master Gunner]

I believe you are correct. The "sample" is just however much you're looking at. Either way, you'll end up with the same results.

[btarlinian]

But what does "sample" mean? If I have say 2 kg of a radioactive substance, and put 1 kg in one box, 1 kg in the other, will the amount of substance in each box halve itself over the course of the half-life T, or will the full 2 kg halve itself? What defines the boundary between one sample and another? Physical connection?

Nothing really defines the boundary between the two samples. Take the original 2 kg substance, E (your radioactive element). After time T, there will only be 1 kg of E left. However, we can safely assume that the remaining 2 kg of E will be evenly distributed throughout the sample. Therefore, if we split the decayed sample in half, each half will contain 1 kg of E, giving you the same result as if you had split the sample before time T.

The question you are really asking is why the remaining 2 kg of E is distributed evenly throughout the sample. This is just a consequence of statistics. Each atom has an equal probability of decaying in a given period of time. It is extremely unlikely, that all the atoms in sample one will decay in time T and none in sample two will. By extremely unlikely, I mean "all of us magically disappearing: unlikely. The sample which you speak of is just defined as however many atoms of E that you decide to look at.

Edit: Atrocious grammar is a little better now.

[alterant]

This is why I shouldn't be allowed to talk when tired.
I guess half-life seemed odd to me because of this probabilistic aspect of it. Thanks, I'll shut up and go wear the dunce cap now.

[nevskey1]

You can take mine. *feels relieved*

[Øsse]

Can photons be forced to travel at speeds lower than the speed of light in that particular medium?

[Master Gunner]

Yes. Photons only travel at C (299 792 458 m/s) in a perfect vacuum. In anything else, even air, light/photons slow down, this is how you get things like refraction and Cherenkov radiation.

[Øsse]

Yes, but as far as I know they still travel at the speed of light in that particular medium, don't they?

Let me put it this way: Can photons have different velocities in the same medium?