Miscellaneous Science Questions

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Mon Apr 25, 2011 11:04 pm UTC

Yakk wrote:I had an amusing thought.

Given that you have a perfectly focused laser sail that is pumping out X power at a frequency of lambda (does this matter?), and you have a perfectly efficient, perfectly reflective light sail, and your ship has mass M, how fast can you get going? Is there a limit?

Now, if you managed constant acceleration, clearly there would be a limit on how much power you'd get (the even horizon would form behind you). But as your power input falls, so does your acceleration, making the event horizon fall back further and further.

If your laser is perfectly focused, your power won't decrease from distance, but it might due to redshift. Even so, each photon would give you a little bump, so you'd continue accelerating without end (obviously staying under c) if there wasn't anything slowing you down. At high accelerations, you run into a heat bath from the Unruh effect, but that shouldn't come into play here, as the accelerations are low. However, even discounting inter-planetary/stellar/galactic gas and dust, you still have to contend with the cosmic background radiation, which isn't in the microwave spectrum anymore now that you've blueshifted it with your enormous velocity. How big that effect is will depend on how long you've been accelerating while the Universe expands and lowers the energy of the background photons. Come to think of it, GR redshifting is going to affect your laser too, if your thought experiment goes on long enough.

Eventually you'll be outside the observable universe of the laser source due to the expansion of the Universe. A constantly accelerating body in a static universe will have a horizon too, but your acceleration is decreasing, and that makes it complicated/contingent on details/etc.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby collegestudent22 » Wed Apr 27, 2011 8:45 am UTC

thoughtfully wrote:However, even discounting inter-planetary/stellar/galactic gas and dust, you still have to contend with the cosmic background radiation, which isn't in the microwave spectrum anymore now that you've blueshifted it with your enormous velocity.


I'm not quite sure I understand what the linked article has to do with blue-shifted microwave background radiation.

doogly wrote:
collegestudent22 wrote:Time dilation (and length contraction) does NOT affect measured velocity or acceleration (in either reference frame), which are based on unit time, so in point of fact, there must exist an "absolute" time and relativistic "time" must be something else entirely (perhaps some sort of "Doppler" effect on measurement of time?).

There is no such thing as absolute time, this is one of the most very basic facts of relativity. Tone it down and pay attention here.


I understand that it is a basic fact of relativity. I am trying to reconcile what my mind perceives as a conflict between the concept of time dilation and a constant velocity, which depends on how time is defined, especially as one approaches the limit of c.

Here's a thought experiment for you. Say you are an observer on a gravitationally balanced, stationary space station (thus, no general relativity factors in). You observe a spacecraft approaching at constant velocity 0.9c from exactly 0.9 light-years away (to simplify calculation, setting tA = 1 yr). Using the equations of special relativity, you calculate that the pilot of that spacecraft will experience 0.435 years before he passes. However, using the same calculation, the pilot observes that you are moving at 0.9c towards his stationary spacecraft, and he will experience one year, while you will experience the 0.435 years. So, assuming that before the journey you were the same age (say twins), which then would be older when the spacecraft passes the station? (Also assume that the accelerations involved to get out from the space station, turn and accelerate to the 0.9c is accounted for, or whatever else - the point is it doesn't apply to the thought experiment.)

The point being that when one examines the standard twin paradox in both reference frames, one realizes that the Earth must be a "preferred" reference frame. Examining the situation while considering the spacecraft at rest, in addition to the "Earth at rest" frame) results in a true paradox, where both the participants are simultaneously older than the other by the same amount.

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Time dilation would virtually cancel out any possible effects, as those provide change over time, and almost no time is appearing to unfold. In fact, it would not be able to move (change position over time). If an object is moving so fast as to be undergoing nearly complete time dilation, this would be the observed effect. Right?

If you're talking about the space/time-warping effect in a photon's reference frame, yes; the photon travels zero distance in no time at all. However, this "frame" is only really the limit of what happens when you get increasingly close to c, so the velocity (normally 0/0) can still be treated as c. Special Relativity proper doesn't work with a frame travelling at c.


This is the problem I am arising at, however. If it doesn't work, doesn't that seem like a glaring omission - shouldn't we seek to find a way to make it work? And if it doesn't work at c, then can you really say it works at v ~= c, where you have accelerated until you are so close to c it is undetectable with even modern equipment?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby phlip » Wed Apr 27, 2011 9:28 am UTC

collegestudent22 wrote:So, assuming that before the journey you were the same age (say twins)

The same age according to who? See also: relativity of simultaneity.

If you see the two of you as being the same age when the ship leaves its 0.9c-away departure point, then your twin will not see you as being the same age as the ship leaves its departure point. The only time you'll agree on your relative ages is when you're in the same place at the same time.

Considering the usual phrasing of the twin paradox, where you start at the same place and the same time, one twin leaves and then returns, and the other twin stays still, then the situation isn't symmetrical as one twin has a rather large acceleration in the middle. If you actually run the calculations, then even from the point of view of the travelling twin, the stationary twin will be older when the travelling twin returns home, even though while they're travelling at a constant speed, the stationary twin appears to be aging slower... because of that frame shift in the middle, when the travelling twin turns around and returns home, which complicates the equations.

Note that you don't need a privileged frame for this. You can consider, say, person A stays still, and person B travels off at 0.5c. Then, a time later, person A accelerates to 0.8c and catches up with person B. This is equivalent to simply having person B be stationary, and having person A leave at 0.5c for a while, then turn around and return at 0.5c. And both will have person B be older than person A when they meet again.

collegestudent22 wrote:This is the problem I am arising at, however. If it doesn't work, doesn't that seem like a glaring omission - shouldn't we seek to find a way to make it work? And if it doesn't work at c, then can you really say it works at v ~= c, where you have accelerated until you are so close to c it is undetectable with even modern equipment?

No, travelling at near-c is still very different to travelling at-c. Just look at, say, the LHC - the particles there are travelling very very close to c, but we can still tell that they're travelling below c, and by how much. The speed of light is a critical point in the whole system... we expect things travelling below c to behave differently to things travelling at c to behave differently than things travelling above c.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Wed Apr 27, 2011 9:06 pm UTC

phlip wrote:
Robert'); DROP TABLE *; wrote:This is the problem I am arising at, however. If it doesn't work, doesn't that seem like a glaring omission - shouldn't we seek to find a way to make it work? And if it doesn't work at c, then can you really say it works at v ~= c, where you have accelerated until you are so close to c it is undetectable with even modern equipment?

That was collegestudent responding to me.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby phlip » Thu Apr 28, 2011 12:12 am UTC

Robert'); DROP TABLE *; wrote:That was collegestudent responding to me.

It may have appeared to be, in your frame of reference.

(Oops, sorry. Post edited.)

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby nickjacksonD » Tue May 03, 2011 2:58 am UTC

Hi there all. . . I'm new so I hop I'm posting this in the correct place. I read that this is the place to post simple questions regarding light so I'll give it a try:
(I'm still in my first physics classes in college, so if this comes off as stupid, I apologize)

I understand that light moves at a constant veloicty through a given medium, but I couldn't get a strait answer from my proffessor when I asked about what happens when light changes mediums: Does it undergo an accelleration? Or does it simply change speeds?

I can only think of position of an electron changing due to excitement as an example. . . like how you can only go up on stair at a time, not a half a stair, so is that the same thing with the velocity of light? It is one speed then it is just another speed?

Hopefully I wasn't too confusing here.

Thanks forum!

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Tue May 03, 2011 12:55 pm UTC

My very vague understanding of this is that it appears because light behaves like a wave. What happens is that one end of the wave enters the medium before the other end, and diffraction happens so that momentum can be conserved.

However, it's also important to realize that light always travels at c. What happens in mediums is that the photon-traveling-at-c gets absorbed by atoms, and isn't released instantly. The delays add up as the light travels through the medium, and so it takes longer than [imath]d/c[/imath] to travel [imath]d[/imath] distance. Since distance/time is how speed is defined, it's speed appears slower.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Spambot5546 » Tue May 03, 2011 2:47 pm UTC

Whoa, you can stop light temporarily?

Anyway, does that mean that the speed of light through a vacuum is just a trend as the amount individual particles are slowed down will average out? Is it possible for some photons on the far end of that bell curve to to get significantly far ahead of the majority?

Also how does cherenkov radiation work, while we're on this topic?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Tue May 03, 2011 2:53 pm UTC

They can indeed get ahead of the pack. This is why in a wave packet you can get the phase velocity to exceed c, but the group velocity must always be \leq.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Tue May 03, 2011 2:59 pm UTC

Spambot5546 wrote:Anyway, does that mean that the speed of light through a vacuum is just a trend as the amount individual particles are slowed down will average out?
They're not slowed down in a vacuum, though. That's why light in a vacuum is the fundamental constant, rather than, say, light through glass.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Tue May 03, 2011 3:28 pm UTC

Spambot5546 wrote:how does cherenkov radiation work, while we're on this topic?

Cherenkov radiation happens when a massive object travels faster than the speed of light through a medium. That is, it passes through the medium faster than light would move through that same medium. (It's impossible to travel faster than light in a vacuum, in any medium)
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Tue May 03, 2011 3:34 pm UTC

Right. So for example, light through water travels at about 0.75c, and thus anything going more than 75% of light speed through water would give off Cherenkov radiation.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Spambot5546 » Tue May 03, 2011 3:37 pm UTC

gmalivuk wrote:
Spambot5546 wrote:Anyway, does that mean that the speed of light through a vacuum is just a trend as the amount individual particles are slowed down will average out?
They're not slowed down in a vacuum, though. That's why light in a vacuum is the fundamental constant, rather than, say, light through glass.

Right, I meant just in the medium. In other words I was looking for exactly the answer Doogly gave. :)
Robert'); DROP TABLE *; wrote:
Spambot5546 wrote:how does cherenkov radiation work, while we're on this topic?

Cherenkov radiation happens when a massive object travels faster than the speed of light through a medium. That is, it passes through the medium faster than light would move through that same medium. (It's impossible to travel faster than light in a vacuum, in any medium)

I got that part from Wikipedia, I was more curious about why that happens.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Tue May 03, 2011 4:34 pm UTC

Spambot5546 wrote:I got that part from Wikipedia, I was more curious about why that happens.

The wiki article is pretty complete. Is there any particular part that is still fuzzy?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Spambot5546 » Tue May 03, 2011 4:36 pm UTC

Yeah, good point. If I didn't grasp it from reading wikipedia I'm not likely to have a breakthrough reading it here.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Quantum Potatoid » Sat May 14, 2011 6:23 pm UTC

So, there are big things I'm obsessed with: Abstraction and Evolution.
Whenever I learn things the first thing I do is abstractify it as much as possible to apply as broadly as possible. (And I attempt to do this to literally everything.)
So over the years starting with knowledge of biological evolution I ended up abstractifying evolution to apply to all interacting systems. (Like a good science nerd should?)
I notice the similarities between entropic process and evolution. (Affectionately, I can refer to them as twin brothers. :3 )
Would it be fair to call them one and the same, or two aspects of the same process? Or maybe evolution being a subset to entropy?

Am I crazy? :P
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby dontthrowpooh » Sat May 14, 2011 6:50 pm UTC

Ok so I've been debating this for so many years in my head and have yet to care enough to build a working theory to support my hypothesis. I don't believe that time should work into the equations of physics. Time has been shown to be relevant in its use in physics (not including quantum) as a constant although it is relative. I was hoping some physicist might be out here on the inter-webs who can see that time is really just a manipulation of gravity and distance. I don't know if that is a fact but something I've been rolling around in my head. Gravity she is such a mystery to us men.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Sun May 15, 2011 4:56 am UTC

What do you mean? Something like what Julian Barbour is doing? Something else?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby dontthrowpooh » Wed May 18, 2011 5:37 am UTC

doogly wrote:What do you mean? Something like what Julian Barbour is doing? Something else?

He's on the exact track I'm talking about, thank you.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Wed May 18, 2011 5:42 am UTC

http://www.fqxi.org/community/essay/winners/2008.1

Worth a look! Compare Barbour with Carroll's and Fotini's, they're quite excellent..
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby legopelle » Sat May 28, 2011 6:30 pm UTC

Quantum Potatoid wrote:So, there are big things I'm obsessed with: Abstraction and Evolution.
Whenever I learn things the first thing I do is abstractify it as much as possible to apply as broadly as possible. (And I attempt to do this to literally everything.)
So over the years starting with knowledge of biological evolution I ended up abstractifying evolution to apply to all interacting systems. (Like a good science nerd should?)
I notice the similarities between entropic process and evolution. (Affectionately, I can refer to them as twin brothers. :3 )
Would it be fair to call them one and the same, or two aspects of the same process? Or maybe evolution being a subset to entropy?

Am I crazy? :P

Well, evolution is just what happens when you have a system that satisfies three requirements: variation, inheritance and some rules that makes the variation significant. The application on culture is called memetics. It might be relevant to your thoughts.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby nehpest » Wed Jun 01, 2011 12:20 am UTC

In my Googling, I encountered this. Two questions:
  • Is measuring the gravitational field of a relativistic object as challenging as implied?
  • Is this
    Above a certain critical velocity, Felber believes, any mass will gravitationally repel other masses, an effect that is twice as strong in the forward direction of motion, but also works in the backward direction.

    as ridiculous as it sounds?

My own knowledge of GR is only conceptual, as I lack the math to understand the equations (I'm self-studying tensor calculus this summer, though, so I'll be one step closer then), but what I know makes it hard for me to believe these claims.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Wed Jun 01, 2011 12:37 am UTC

100% crackpot.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby nehpest » Wed Jun 01, 2011 1:02 am UTC

doogly wrote:100% crackpot.

Is the rest of the Tau Zero Foundation equally reputable?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Wed Jun 01, 2011 1:25 am UTC

A cursory glance suggests they just don't do a lot of discriminating, but will happily talk about anything exciting or cool. Which includes some incorrect things, unfortunately. But hey, if wrong things can get us to the stars, then off we go, right?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby ahippo » Mon Jun 06, 2011 4:24 am UTC

I'm not really interested in reading this entire thread searching for an answer to my question, but my dad and I were having a discussion about relativity and time travel (well sort of. It's nothing crazy or stupid). So anyway my brother my dad and I were talking about the distance to Alpha Centauri. If I remember correctly that is the closest system to Sol. They said it was 4.3 light years from Sol. Not a figure I would know of the top of my head but I'll take their word for it.

So the dispute came here. We were thinking about traveling at speeds very close to light to Alpha Centauri. Now both of us agreed on the fact that perception of time changes as you approach the speed of light while traveling in a straight line. Particularly that any person traveling to Alpha Centauri would experience less time passing than those on Earth. However we disagreed about this:

So it is 4.3 light years from Sol to Alpha Centauri. His position was that those traveling to Alpha Centauri would experience 4.3 years of travel assuming they traveled at speeds approaching the speed of light, and those on the outside would experience some greater passage of time. I don't really know how much but the point is still made. So therefore the transport to Alpha Centauri would actually appear to be going slower than almost light speed. Considerably more and more so as it approached the speed of light. This is what I had a problem with

My position was that if the journey was made by the above conditions the folks back on Earth would experience 4.3 years going by for the transport to get to Alpha Centauri, and the folks on said transport would experience only weeks, or even days, of travel. This was my understanding about relativistic travel.

So who's right?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Mon Jun 06, 2011 4:42 am UTC

You. 4.3 light years means light, from our point of view, takes 4.3 years to make the journey. Time dilation is--without getting into issues like proper time--proportional to a factor called the Lorentz factor, usually represented by the Greek letter gamma.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby eSOANEM » Mon Jun 06, 2011 4:40 pm UTC

He's wrong. If A is 4.3 light years away from B in some reference frame then an observer stationary in the frame will see anyone travelling from A to B taking more than 4.3 years (and only massless things (like light) taking 4.3 years). The person travelling will experience less time as they experience length contraction so the 4.3 light years looks shorter to them and, when they decelerate back into the reference frame mentioned above, they perceive themselves as having experienced time dilation where 1 second on the ship's clock whilst they were moving is longer than 1 second on their now "stationary" ship.

Both of these things should tell you that the crew experienced less time than they now perceive the journey to have taken because time is now faster/the distance was less (than it is now).

I think I explained that correctly. I could be wrong though.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Robert'); DROP TABLE *; » Mon Jun 06, 2011 6:34 pm UTC

The statement he made is right, I think, although he's got the dilation factors the wrong way around. There is some speed where the travelers experience time dilation such that they arrive at Alpha Centauri in 4.3 years of subjective time. This speed would be less than c, and so a stationary observer would see the journey taking more than 4.3 years.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby eSOANEM » Mon Jun 06, 2011 7:53 pm UTC

*facepalm* I forgot this (what little bobby drop tables said). Below a certain speed they'll still experience more than 4.3 years, above it they'll experience less and on it they'll experience exactly 4.3 years depending on exactly how the time dilation/length contraction plays out.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby phlip » Tue Jun 07, 2011 12:24 am UTC

The magic speed is 1/sqrt(2) c. Say there's two places, stationary relative to each other and an observer, which observes the distance between them as x light years, and sees you travelling from one to the other at 1/sqrt(2) v... they'll see you take x*sqrt(2) years to make the journey, but with a time dilation factor of sqrt(2), so for you, x years have passed. Alternatively, in your point of view, length contraction means the distance between the two endpoints is only x/sqrt(2) light years, and the end is coming towards you at 1/sqrt(2) c, so it'll take x years to arrive.

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Light as a universal constant

Postby caldercleavelin » Wed Jun 22, 2011 3:49 am UTC

If light is the upper limit of speed in the universe, how can a black hole have an event horizon with a force greater than the speed of light?

Would the fact that there is an equation, calculating the force of the event horizon, that accounts for a speed greater than the speed of light imply that light is not the upper limit of potential velocity?

In short: if you can practically calculate something that implicates speeds faster than light, then doesn't that imply light is not the fastest possible thing in the universe?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Wed Jun 22, 2011 10:03 pm UTC

caldercleavelin wrote:a force greater than the speed of light
This doesn't make any sense. Forces and speeds are not the same thing.
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Re: Light as a universal constant

Postby Eebster the Great » Thu Jun 23, 2011 12:10 am UTC

caldercleavelin wrote:In short: if you can practically calculate something that implicates speeds faster than light, then doesn't that imply light is not the fastest possible thing in the universe?

No.

I can practically calculate the speed required to reach the Sun in five seconds, but that doesn't mean I can make a spaceship (or anything else) that flies that fast.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Thu Jun 23, 2011 5:05 am UTC

I don't suppose it would be useful here to mention something about proper time and chunky salsa?
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Proper time is time as observed by the reference frame in motion. At relativistic speeds, your personal clock runs more slowly than that of an observer in the reference frame you are moving relative to. Also, space is contracted in the direction of your motion. There is a speed at which you can travel 1 AU (measured by someone at rest with respect to the Earth and Sun) and only experience five seconds of elapsed time. However, your personal reckoning of the distance will be less, always by more than enough to make your speed subluminal.

Achieving this speed starting from rest on the Earth will require some absurdly stupendous acceleration, that will render you into a red smear on the trailing edge of the craft, a fate traditionally referred to as becoming "chunky salsa". This condition can be avoided easily by accelerating up to the desired speed from a vastly greater distance.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Mr Scrotier » Thu Jun 23, 2011 11:02 am UTC

Hey all, I've never done any physics (biochem ftw!), so over the years I have encountered questions...

1. Everyone says information cannot travel faster than the speed of light. What would happen if you had a tube from here to the moon, filled it full of ball bearings. Then, as you would, shoved another ball bearing in from the earth end and welded the tube shut before one popped out from the moon end? I assume it would shatter or the like, so what if I was really clever and made all the equipment out of indestructible material X?

2. A documentary told me that a millionth of a second after the big bang, the universe was about the size of our solar system. This is faster than the speed of light, no?

3. Someone told me that photons and gluons are massless. My very poor understanding of moving at the speed of light tells me that the movers mass increases exponentially so that the energy required to move faster also increases exponentially towards infinity (or some shit). Could we theoretically shoot a gluon at the speed of light?

I refuse to apologise for asking dumb, or potentially dumb questions. That is a running theme of my life.

Thanks all.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Thu Jun 23, 2011 11:52 am UTC

1) Information would travel at the speed of sound through whatever material you made your tube with. Positing an indestructible material (by which I assume for this question you actually mean perfectly rigid) breaks relativity precisely because it would allow instantaneous information transfer. So positing such a material is equivalent to just going all the way and saying "What if we *could* send information instantaneously?"

2) Space itself is allowed to expand at whatever speed it wants. It's just that nothing can move *through* it faster than light.

3) A massive particle has its energy increase asymptotically (which is faster than exponential) as its velocity approaches light speed. A massless particle, on the other hand, *always* travels at the speed of light.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Yakk » Thu Jun 23, 2011 1:11 pm UTC

gmalivuk wrote:3) A massive particle has its energy increase asymptotically (which is faster than exponential) as its velocity approaches light speed. A massless particle, on the other hand, *always* travels at the speed of light.

Don't be so hyperbolic.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Spambot5546 » Thu Jun 23, 2011 1:31 pm UTC

Ow. That one was so lame hurt a little bit.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Sir_Elderberry » Thu Jun 23, 2011 1:35 pm UTC

gmalivuk wrote:3) A massive particle has its energy increase asymptotically (which is faster than exponential) as its velocity approaches light speed. A massless particle, on the other hand, *always* travels at the speed of light.

So not only can we shoot a gluon at the speed of light, but we can't shoot it at any other speed. Same with photons, although for them at least we should say "a" speed of light because of refractive indices and the like. (Gluons may have some similar effect but I don't know of it and I imagine it'd be very very hard to study.)
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