## RELATIVITY QUESTIONS! (and other common queries)

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

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Sir_Elderberry
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### RELATIVITY QUESTIONS! (and other common queries)

Mod edit: Due to some unfortunate deletions, the first post of this thread is now missing. It is reproduced in the quote box below. Sir Elderberry's post follows it.

Meteorswarm wrote:It seems the most common topic we get repeated questions on is relativity, general and special. Seriously, this thread is stickied and has relativity right in the title. Now it's even in all caps and the first word. So please ask such questions here. Or maybe read some of this thread to see if your question has already been asked and answered. - gmalivuk

Since we seem to see a lot of repeated things in this forum, it might be a good thing to compile simple, fast answers here. If you have a simple "how does this work?" question, this is the place. It is NOT the place to answer questions at length; any question worthy of lengthy response deserves its own thread. If your question involves light, please see the relativity statement below. I'll get the list started with a few things:

Gravity travels at the speed of light, 299792458 meters/second. This has been closely validated with an experiment, but it mostly comes from theory since it isn't easy to measure.

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.

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.

Light does not have (rest) mass. Photons only travel at c through a vacuum.

Light acts as both a particle and a wave.

Incidentally, everything acts as both a particle and a wave. You, me, cars, George Bush.

The big bang was an expansion of space itself, not just stuff within space. This is often hard to visualize.

If your question involves traveling at the speed of light, a light source and an apparent paradox, please read about special relativity, or consider that the speed of light is constant to everybody and derive it yourself. Also, see Sir Elderberry's excellent post on the subject.

Edit 2009.04.10 beefed up SR response with the aid of Sir Elderberry, changed topic statement to reflect changed nature of thread - it is actually a place to ask questions you think have a short answer

Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.
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ArmonSore
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### Re: Common Questions

The only change that I'd make (that didn't come at the cost of brevity) is that everything is both a particle and a wave. Electrons, protons, neutrinos, etc. Not just photons.
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Eps
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### Re: Common Questions

Change "roughly validated" to "validated to a high degree of accuracy" for the speed of gravity.

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.

OneLess
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### Re: Common Questions

{Edited by OneLess to remove idiocy.}
Last edited by OneLess on Sat Jan 19, 2008 8:37 am UTC, edited 2 times in total.
“Observation: Couldn’t see a thing. Conclusion: Dinosaurs.” –Carl Sagan

Last edited by OneLess on Sat Dec 17, 3003 10:35 am, edited 0 time in total.

Hawknc
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### Re: Common Questions

OneLess wrote:-(Conventional) airplanes cannot take off from a treadmill

Oh, someone's looking for a banning.

Minerva
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### Re: Common Questions

I believe it was Feynman who said something to the effect of... (I can't remember the exact quote) "Light doesn't behave like a wave on Tuesdays and like a particle on Thursdays."

Light - and everything else for that matter - is not "both a particle and a wave". It is neither - but it displays some of the characteristics of both.
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UmbralRaptor
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### Re: Common Questions

OneLess wrote:-(Conventional) airplanes cannot take off from a treadmill

From a conventional treadmill, anyway. A sufficiently large and strong one...
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Indon
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### Re: Common Questions

UmbralRaptor wrote:
OneLess wrote:-(Conventional) airplanes cannot take off from a treadmill

From a conventional treadmill, anyway. A sufficiently large and strong one...

Except for Harrier Jets.
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Tchebu
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### Re: Common Questions

Light - and everything else for that matter - is not "both a particle and a wave". It is neither - but it displays some of the characteristics of both.

You mean all the properties of both. Unless you include negatives like "particles *don't* diffract"...

Unless i'm misinformed...
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Luppoewagan
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### Re: Common Questions

Please explain how space itself can expand if expanding means taking up more space.

Tchebu
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### Re: Common Questions

Please explain how space itself can expand if expanding means taking up more space.

Well... the first thing you should conclude... is that "taking up more space" is not what is meant by "expanding" when the word comes out of the mouth of a physicist.

Basically it means that an object on which no force acts, actually moves away from you, rather than being immobile (unlike what newton's first law may say). And the further away it is, the faster it will move away.

The analogy commonly used is the surface of a baloon that's being blown up. Now just... you know... imagine that happening in more dimentions...
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

genewitch
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### Re: Common Questions

Yummy in my tummy! Microwave background radiation is also not common knowledge: http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation

And re: plane on a treadmill, mythbusters january 30th are gunna put it to bed
see preview of the two tests: http://dsc.discovery.com/video/?playerId=203711706&categoryId=210013704&lineupId=229524134&titleId=1344511100

anyone wanna take some bets on bragster?
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LoopQuantumGravity
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### Re: Common Questions

genewitch wrote:And re: plane on a treadmill, mythbusters january 30th are gunna put it to bed
see preview of the two tests: http://dsc.discovery.com/video/?playerId=203711706&categoryId=210013704&lineupId=229524134&titleId=1344511100

If they screw this one up, I'm going to be really pissed, because then I'll never be able to explain to people the right answer...
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Hawknc
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### Re: Common Questions

They'll screw it up because it is physically impossible to set up. That video isn't working for me unfortunately, but it's fair to say that they'll pick one interpretation of the puzzle and test that, when there's multiple variants of it. How they set up the treadmill will be the key to it - will it be freely rotating, matching the speed of the wheels, matching the speed that the aircraft would be at on a flat surface? How are they going to deal with friction?

Durandal
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### Re: Common Questions

.
Last edited by Durandal on Wed Jul 08, 2009 4:40 am UTC, edited 1 time in total.

hyperion
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### Re: Common Questions

Durandal wrote:I've never understood why exactly an airplane would take off on a treadmill... I mean, it's not the airplane moving fast in relation to the ground that allows the wings to displace a sufficient amount of air, but the airplane moving fast in relation to the surrounding atmosphere...

Peshmerga wrote:A blow job would probably get you a LOT of cheeseburgers.
But I digress.

OneLess
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### Re: Common Questions

OneLess wrote:-(Conventional) airplanes cannot take off from a treadmill

Okay, better scientific fact: I'm an idiot.
“Observation: Couldn’t see a thing. Conclusion: Dinosaurs.” –Carl Sagan

Last edited by OneLess on Sat Dec 17, 3003 10:35 am, edited 0 time in total.

Eps
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### Re: Common Questions

Last I heard was they found it to be .8c +-.2c or something of that order.

Wiki or Google for more on this. The short answer is that the study you're referring to is somewhat controversial and its results have not been widely accepted, particularly when a more accurate figure (approximately equal to c plus or minus about 0.5%) has been obtained via long-term measurements of PSR 1913+16 in its binary pulsar system. Disclaimer: This is outside my subfield.

Herman
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### Re: Common Questions

Quantum mechanics works for macroscopic ("everyday") stuff too. In fact it makes more accurate predictions than classical mechanics. We use classical mechanics because it's a good approximation and the math is easier. There is no sharply defined "quantum realm." The border between quantum and macroscopic phenomena is a convenient fiction and is subject to context, like the border between physics and chemistry.

Peripatetic
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### Re: Common Questions

Eps wrote:
Last I heard was they found it to be .8c +-.2c or something of that order.
Wiki or Google for more on this. The short answer is that the study you're referring to is somewhat controversial and its results have not been widely accepted, particularly when a more accurate figure (approximately equal to c plus or minus about 0.5%) has been obtained via long-term measurements of PSR 1913+16 in its binary pulsar system. Disclaimer: This is outside my subfield.

That binary star system has tested a lot of predictions of General Relativity (time dilation, space curvature, gravity radiation, etc.), but no one has unambiguously measured the speed of a gravity wave. GR predicts these travel at c, and experimental confirmation of just about every other aspect of GR makes this very likely, but we're not there yet. I think LIGO should go a long ways towards this goal.

Rook
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### Re: Common Questions

HYPERiON wrote:
Durandal wrote:I've never understood why exactly an airplane would take off on a treadmill... I mean, it's not the airplane moving fast in relation to the ground that allows the wings to displace a sufficient amount of air, but the airplane moving fast in relation to the surrounding atmosphere...

So to make it clear (which is the supposed point of this thread): conventional airplanes can take off from a treadmill.

Right? If that's not what you mean, I'll edit it back, but I think that's right.

It's just one of those annoying common sense brain teasers, like that bitchy maths one about 'the missing penny'. F***ing hate them.
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Peripatetic
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### Re: Common Questions

Meteorswarm wrote:Gravity travels at the speed of light, 299792458 meters/second. This has been closely validated with an experiment, but it mostly comes from theory since it isn't easy to measure.

Just to clarify, the results referred to by Eps assume General Relativity to be true (a safe assumption to be sure) and use other observations in calculating the speed of gravity. No one has measured the speed of gravity directly, independently of any theory. This is different than meaurements of the speed of light (see: http://en.wikipedia.org/wiki/Speed_of_l ... d_of_light), which are direct and do not rely upon any theory.

LoopQuantumGravity
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### Re: Common Questions

Peripatetic wrote:
Meteorswarm wrote:Gravity travels at the speed of light, 299792458 meters/second. This has been closely validated with an experiment, but it mostly comes from theory since it isn't easy to measure.

Just to clarify, the results referred to by Eps assume General Relativity to be true (a safe assumption to be sure) and use other observations in calculating the speed of gravity. No one has measured the speed of gravity directly, independently of any theory. This is different than meaurements of the speed of light (see: http://en.wikipedia.org/wiki/Speed_of_l ... d_of_light), which are direct and do not rely upon any theory.

That's not really true. The measurements of the speed of light are very dependent on what your theory says about space and time! So those measurements are not entirely model independent (although they come from a class of models larger than SR).
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Eps
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### Re: Common Questions

I'd just like to point out that I had a "This is Not My Field" disclaimer. If you want to discuss experimental particle/nuclear physics, however: let's dance*.

On topic: Physics might in principle be able to predict everything, but in practice the other scientfic fields are absolutely necessary, and likely always will be unless you happen to have a unified field theory, a computer the size of a galactic supercluster and some really good code.

* - I like the Macarena. What? What?!

Solt
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### Re: Common Questions

There is no such thing as a perfectly rigid material and thus any conclusions you can draw from that assumption are false.
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null
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### Re: Common Questions

Indon wrote:
UmbralRaptor wrote:
OneLess wrote:-(Conventional) airplanes cannot take off from a treadmill

From a conventional treadmill, anyway. A sufficiently large and strong one...

Except for Harrier Jets.

And V-22 Ospreys, F-35s etc etc...actually http://en.wikipedia.org/wiki/List_of_VTOL_aircraft

evilbeanfiend
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### Re: Common Questions

Eps wrote: Physics might in principle be able to predict everything, but in practice the other scientfic fields are absolutely necessary, and likely always will be unless you happen to have a unified field theory, a computer the size of a galactic supercluster and some really good code.

well that is just because different scientific fields are a somewhat arbitrary human categorisation. the universe just is, it doesn't care whether you are chemist a material scientist a physicist or even a lowly engineer.
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eternauta3k
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### Re: Common Questions

evilbeanfiend wrote:or even a lowly engineer.
Oh me yarm! Take that back!
Free-falling objects experience the same acceleration no matter their mass.
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Fat Tony
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### Re: Common Questions

The answer to the ultimate question of life, the universe, and everything is 42.
The question coresponding with the answer to the ultimate question of life, the univer, and everything is, "What do you get when you multiply six by nine?"
And yes, six times nine is 42 in base 13.
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Tchebu
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### Re: Common Questions

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

Neglecting air friction.

Just to be perfectly correct...
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

Fat Tony
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### Re: Common Questions

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

Neglecting air friction.

Just to be perfectly correct...

I've never understood this. Is it because the extra inertia an object has due to having more mass cancels out the increased pull of gravity?
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### Re: Common Questions

Fat Tony wrote:
Tchebu wrote:
Free-falling objects experience the same acceleration no matter their mass.

Neglecting air friction.

Just to be perfectly correct...

I've never understood this. Is it because the extra inertia an object has due to having more mass cancels out the increased pull of gravity?

Exactly so. The gravitational force is proportional to mass.

Weight = Mass * G (on earth, 9.8m/s2)
Acceleration = Force/mass
A = (GM) / (M)
A = G.
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### Re: Common Questions

1. "Datum" is the singular form of "Data." Not "Anecdotal Evidence."

2. Psychology is a science. Psychoanalysis and therapy are not. There is a difference between a research psychologist and a shrink. They have different degrees.

3. Your subjective experience of life is not adequate to explain behavior. Just like any other science, the behavioral and cognitive sciences require training before you can figure something out. See #1.

4. Any forum is a horrible sample of the general population for just about anything you could imagine. See #1.
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dosboot
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### Re: Common Questions

Am I missing something? The thread about a plane on a treadmill did not reach a consensus.

Tchebu
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### Re: Common Questions

Yes it did, the consensus was that there are two ways of intepreting it. Either the treadmill moves at the speed of the wheel which is physically impossible, or it it equal to the speed that the plane would have been going if it were on solid ground and then the plane takes off notmally with the wheels spinning twice as fast as they normally would have on solid ground, which is what has been said earlier.
Our universe is most certainly unique... it's the only one that string theory doesn't describe.

tendays
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### Re: Common Questions

(Stolen from the thedailywtf.com fora, where it was probably stolen from somewhere else)
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hyperion
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### Re: Common Questions

tendays wrote:(Stolen from the thedailywtf.com fora, where it was probably stolen from somewhere else)

That's pretty much the internet-wide standard for *facepalm.jpg*
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But I digress.

genewitch
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### Re: Common Questions

To say that mythbusters can't set up the treadmill scenario SMACKS of WOO <--link to how WOOs operate!

In fact, all the arguments about the stupid treadmill thing reek of WOOness.

It's like saying that a dowser can't perform dowsing when it's being accurately and scientifically judged (or psychics, or astrologers, or homeopaths, or mercury-autism linkers). I know that the episode won't put the argument to bed. Because it's a thing of faith, at this point. that's why it's irrelevant to post "there was a consensus reached" or "It is most certainly THIS ANSWER"; because it isn't. One side screams and calls people names, the other side says "you're not understanding all the principles: here's X and Y reason"; to which the other side goes "you're stupid, any idiot can see Z proves you wrong".

anytime you have to back up your theories with insults, being mean, or other general asshat moves, you should probably rethink your approach to life.
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gmalivuk
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### Re: Common Questions

Meteorswarm wrote:Look, as entertaining as the airplane-on-a-treadmill situation may be, this is really not the place for discussing it

And yet there you go, bringing it up again.

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Generic Goon
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### Re: Common Questions

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.