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Messages - Da6onet

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31
Off Topic / Re: randomness (music)
« on: June 04, 2014, 10:18:31 pm »
Work is cray cray right now, I'll get to the astronomy stuff this Saturday. In the mean time, a brief musical interlude.

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32
Off Topic / Re: randomness
« on: June 03, 2014, 06:08:21 am »
Reminds me of Caprica and "apotheosis"

33
General / Re: Superluminal (a question)
« on: May 31, 2014, 09:15:18 am »
Thanks! Lots of food for thought.

Two things I'm not quite clear on:

1. ...what difference would it make in determining the velocity of an object relative of another object?

2. If the dancer were spinning at same rate as the photons (speed of light), they would see Pluto nearly stationary with a very small angular displacement.

Uh ... why? Uh ... Is the distance between one photon and the next similar or greater than the circumference of the spin? Or what?

1. Light from an object can be look redder or bluer depending on it's radial velocity. Light from Andromeda is blue shifted because it's head straight for us! Of course we're talking nanometer range shifts, but they are measurable to a high degree of accuracy.

2.
Sorry I was trying to pick an extreme example to show how the dancer and Pluto are observing the same event. Light behaves as both a particle and a wave. The waveform of light has a period (meaning there is a delta t between photons). If the dancer were spinning at the speed of light her period wouldn't match all the incoming light, so she wouldn't see a continuous rotating sky anymore, she could only see light when the multiple of her period matched up with the multiple of a period of incoming light. I was implying the assumption that the dancer's period exactly matches the light coming off Pluto, then all she would see (for the most part) is darkness and a blinking Pluto -- she couldn't perceive any rotation. So from her point of view Pluto is the only object in the sky and appears not to move much each time it is observed. Just as the observer on Pluto only sees the light from the dancer's eyes each period. Slowing this back down to normal speeds, this matching still works, just now the dancer perceives continuous light from all around her (because it's so much faster than her rotational period) but should only measure Pluto's apparent position each time she sees it (pretend she's closing her eyes between each full rotation) -- measuring the change in position compared to difference in time between each observation (she's counting seconds between each observation). delta x/delta t = Pluto's actual speed

34
General / Re: Superluminal (a question)
« on: May 31, 2014, 03:54:51 am »
Okay, so you're saying that nothing can move faster than light WITHIN an inertial frame of reference, and by "inertial frame of reference" you mean a section of space where ... hmm ... the curvature of space is negligible, while gravity is not - if I understand correctly.

You've made a great point about looking to the west, and then to the east. Dancers make Pluto exceed the speed of light every time they do a pirouette - but that "doesn't count," because the force of gravity between them and Pluto is negligible.

So - correct me if I'm wrong - Relativity is similar to Euclid's geometry in that it only works within certain parameters and does not apply to the entire Universe or even galaxy.

More like, within what we would consider normal or 3D space, like an ant moving across graph paper, special relativity works like a charm (length contraction, time dilation). In that sense the rules are the same everywhere in the observable universe including galaxies (at least outside of the Schwarzschild radius of black holes). General relativity steps in to explain situations such as gravitational lensing - which was used to prove that space isn't really Euclidean :)

Another way to think of special relativity is that the two observers must agree on an event happening. In the dancer scenario, an observer on Pluto would observe a spinning dancer with a very small angular displacement. The dancer's view of Pluto rapidly passing him/her by every turn doesn't match the same angular displacement, so they aren't observing the same event. Remember the dancer is actually just viewing streams of photons. If the dancer were spinning at same rate as the photons (speed of light), they would see Pluto nearly stationary with a very small angular displacement.

35
General / Re: Superluminal (a question)
« on: May 30, 2014, 11:29:39 pm »
I'm rather slow at times, so please be patient.

The Theory of Relativity states, in no uncertain terms, that:

1. Nothing can travel faster than light
2. ANYTHING AT ALL can be viewed as a motionless reference point.

The Earth rotates and revolves, so let's set it aside.

Let's select a point anywhere on Earth's orbit except where the Earth actually is. Let's elect it The Reference Point of the Day.

Alpha Proxima (and all the rest of the stars in the galaxy except the Sun) do actually rotate around this point at speeds that exceed the speed of light.

What gives?

Simplified postulates of special relativity:
1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light in free space has the same value c in all inertial frames of reference.

The stars in the galaxy are not actually rotating about this point's inertial frame of reference and have very small velocities compared to the speed of light. The inertial frame you're describing is analogous to putting a stick in a stream and measuring how fast the current flows past. What you would see is the stars in the plane of the galaxy (either looking toward the center or toward the outer edge) passing by you at around 200-300km/s.

The confusion may be because you're thinking of what the point "sees" as it would orbit. However, you have to remember that there is literally a constant stream of photons that the observer on Earth, or in orbit, or anywhere, is running into. Every photon you're seeing was emitted  from their star 4.3 years ago (alpha centauri a/b and proxima) to ~100,000 years ago (rough diameter of galactic disk) but obviously you can't perceive the time difference now. If I were unaware that I was turning, I could look to the west, then look to the east, then say, half the universe just rotated around me!

The real question you should ask is why the speed is roughly constant throughout the disk of the galaxy regardless of radius from the center!

36
Off Topic / Re: randomness
« on: May 26, 2014, 11:52:27 pm »
I posted that on my FB a couple of days ago. I'm glad it is make the rounds.


Yeah to be honest, I thought there was no way in hell they'd ever make it through the bureaucracy/establishment (grand ideas require agile government), so I brushed it off at the time. Since I saw people posting it on Facebook, I thought about it some more and decided to be a little less cynical this time.

You can read more about the project here --> http://www.solarroadways.com/intro.shtml

37
Off Topic / Re: randomness
« on: May 26, 2014, 11:42:03 pm »
I met a materials engineer last year at an academic talk who was a big proponent of this and his research is in coming up with more cost effective solar panels (either through new laminate layers/types and/or the manufacturing process). It's actually a really elegant solution to current problems as well as those 50-100 years out.

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38
Off Topic / Re: Astronomy Question of the Week
« on: May 25, 2014, 11:05:13 am »

-Rise in average temperatures caused the Larsen B ice shelf to break up in early 2002. This ice shelf, which is about the size of Rhode Island, is thought to have been part of the Antarctic coast for the past 12,000 years.

Week of 5/25/14

Antarctica has an area of 13 million square kilometers and is covered by an icecap that varies in thickness from 300 meters near the coast to 1800 meters in the interior. Assuming that water and ice have roughly the same density, estimate the amount by which the water level of the world’s oceans would rise if Antarctica’s ice were to melt completely (you must show your maths!).

Bonus: Look up coastal cities like New York (think Manhattan) and compare their altitudes above mean sea level to the amount you calculated!

39
Off Topic / Re: Astronomy Question of the Week
« on: May 22, 2014, 11:58:52 am »
Ah yeah, I should have specified actual information propagation rather apparent effects of us running into a stream photons or in the case of AGN blobs, a trigonometry trick of the eye. I think I'm going to post a math based question next just so Lithium can show you all what's up. <3 engineers.

40
Off Topic / Re: Astronomy Question of the Week
« on: May 22, 2014, 07:56:25 am »
i still think comet was correct;)

Ah right I forgot about your answer. So most comets come from the Kuiper Belt and Oort Cloud and are on elliptical orbits. Some comets come from outside the solar system and have hyperbolic trajectories. Both types can be modeled by

v^2 = G*(Msun + mobject)*(2/r - 1/a)

Where G is the universal gravitational constant, r distance of the object from the sun and a is it's orbit semi-major axis. Since the mass of the sun >> than the mass of a comet, this equation can be simplified to

v = 29.8*(2/r - 1/a)^0.5 where r and a are measured in AU

(G = ~6.67E-11 N*(m/kg)^2, Msun = ~1.99E30 kg, 1 AU = ~149.6 million km for those that wish to keep the equation in SI).

Let's assume the comet is from so far away that a --> infinity and that term drops out. Then lets estimate such a comet at perihelion for a couple ranges.

r = 0.9 v = 44 km/s (this is about Hale Bopp's distance/speed)
r = 0.1 v = 133 km/s
r = 0.01 v = 421 km/s (this was ISON's closest approach and it was utterly destroyed)

For comparison, our Voyager spacecraft plod along at 17km/s and LEO objects like the ISS are as low as 8km/s.

The escape velocity from the galaxy is right around 500km/s (from 8000 parsecs out that is), but our Sun isn't massive enough to fling anything that fast without it passing too close and getting destroyed.

Fun fact, based on this equation, and given that Earth is moving at 30km/s at 1 AU, the speed limit of asteroids and comets impacting Earth is ~70km/s.

I digress, these velocities are nice, but light in a vacuum travels at 300,000 km/s!

41
Off Topic / Re: Astronomy Question of the Week
« on: May 21, 2014, 09:09:31 pm »
Broin's phrase "universal expansion" and subsequent wording correctly explains the natural example of superluminal separation. Your phrasing that spacetime can expand faster than the speed of light is just another way of saying it using the term spacetime, which is why I included your quote, but Broin posted first and you had some slightly mis-worded information in your post.
It is worth clarifying a few things.
-normal energy/matter as we know it doesn't move outside spacetime, they move with spacetime. Outside of spacetime would indicate moving in some extra dimension (in sci fi this commonly referred to as things like subspace, hyperspace, etc.) String theory theorizes this but no observational data backs it up.
-dark energy is the currently accepted name for the force expanding the universe (that is to say it fits mathematically but we don't understand what it is exactly).
-while the universe did expand rapidly during the inflationary period, it is still expanding today and is actually acceleratng it's rate of expansion-- Hubble's constant is not actually constant.

I would also argue that Einstein's theory of general relativity explains a great deal of how the universe works and there is a large quantity of evidence--binary pulsar period changes, satellites being framed dragged by Earth, and more recently gravity wave detection in the cosmic microwave background. These are all observed effects of spacetime behavior that fit predicted models.
I will admit the two ways of human spaceflight I mentioned are very far-fetched, but again, predicted by the same mathematical models. Real life physicists are trying to prove existence of these ideas (like string theory and WIMPS) and it's neat when they do (like the higgs boson, like negative energy). A theory is only thin until it's verified :-)

42
Off Topic / Re: Astronomy Question of the Week
« on: May 21, 2014, 03:18:49 pm »
Quote from: Longboard
Isn't the theory that light travels faster then itself at a black hole or event horizon. I think it is something close to that.

Light as we are defining it here are photons (that behave like waves/rays or particles depending on the situation). While some interesting things happen at the event horizon (such as Hawking radiation), the important idea there is escape velocity is exactly c (where c is the speed of light in a vacuum). Beyond the event horizon no information escapes, so it is fair to say that physics as we define it for our universe does not apply for anything inside this radius. That said, outside the black hole, all mass and energy moving through space is governed by laws of physics and thus cannot exceed the speed of light.

Quote from: Tbone
I believe a black hole is where the gravity is so strong that light is unable to escape the pull. The event horizon is simply the point where that pull becomes so strong. I don't think that gravity speeds up light. It's not faster, but stronger. I could be wrong.

As far as I knew, any attempt at going faster than the speed of light would result in everything relative slowing down. For example, if the speed of light was 60 MPH and you were on a bus going that speed and tried to throw a ball, relative to the ball, the bus would slow down below 60 MPH since the ball cannot exceed that.

Correct. An interesting effect of this is that if we threw Longboard into a blackhole, we would see him appear to slow down as he approached the event horizon, then stop at the event horizon. The photons that allow us to see him would be continuously red shifted and fade from view. Doesn't answer the question though! :-)

Quote from: Broin
Universal expansion shows that it is possible that some galaxies are moving away from us so rapidly that the light from them may me never reach us. Kind of like two cars traveling down the road in the same direction. One car has a head start on the other and is moving faster so that the car behind will never catch up and see the car in front of it. It's possible I suppose with universal expansion  that we'll all be traveling faster than light someday

Quote from: Lithium
If an object is outside of space-time and it is being pushed by the expansion of space-time, the space-time in theory can expand faster than light. This likely happened in the first trillionth of a trillionth of a second after the big bang when the universe expanded faster than light.

ANSWER
Objects that are within spacetime are limited to the speed of light. Spacetime itself could give two shits about speed "limits" and expands (or contracts) at whatever rate it wants, like when it grew in size by 10^50 around between 10^-40 to 10^-30 seconds after the big bang started. Today we can observe that there are galaxies with a redshift of z>1.4 (where z = 1.4 is a redshift due to moving away at c). An analogy would be two slugs on an expanding balloon (where the ballon is spacetime and the slugs are galaxies. The slugs are moving less than the speed of light in their frame of reference, but the balloon can expand very quickly, thus the slugs are moving apart faster than c.

What's troubling to some cosmologists is that we observe that all galaxies outside our local cluster (where gravity trumps dark energy), are actually accelerating away from us (and the further away, the faster the acceleration). Thus at some point in the far future we will not be able to observe anything beyond our local cluster, the galaxies will appear to freeze in space, red shift and fade from view. It possible/likely that this has already happened and why our observable universe isn't necessarily the whole thing.

There are two leading theories of moving through the universe that don't violate special relativity (moving faster than c through space) and in fact are predicted by general relativity.

1. Connect two points in space via a worm hole. This is hypothosized in general relativity as one possible way supermassive black holes form and gain so much mass so quickly (10^6-10^9 solar masses in a period of 10^5 years or so). It's not technically moving outside space time since the tunnel is just two highly warped regions of space time connecting.

2. Make use of the flexibility of spacetime (expansion/contraction). An Alcubierre warp drive, utilizing negative energy could contract the spacetime immediately in front of it while expanding it immediately behind. The ship itself would not actually be moving, rather it would be "surfing" on a spacetime wavefront.


Broin has the most correct answer so he is this week's smart ass!

43
Off Topic / Re: randomness
« on: May 20, 2014, 12:21:08 pm »
This is how I feel after my thermodynamics exam.


44
Off Topic / Re: Astronomy Question of the Week
« on: May 18, 2014, 06:20:06 pm »
If you were on a space ship travelling the speed of light and then walked forward, you would be moving faster than the speed of light. Not sure about examples in nature...


Einstein's special relativity says that to an observer outside the light speed train, time proceeds normally but you and the train are both infinitely thin. Since you traverse no extra distance, you have no additive velocity. Don't worry, in your frame of reference you appear to be walking normally across the train car, but the outside world appears to be stopped.



1st hint General Relativity

45
Off Topic / Re: Astronomy Question of the Week
« on: May 18, 2014, 05:43:44 pm »
ANSWER
Because the Earth's synodic day (3) is 4 minutes longer than it's sidereal day (2), stars rise about 4 minutes earlier each night. Thus, a week later, Bellatrix will rise at about 8:02pm (8:00 pm is close enough) on November 8.




NEW QUESTION
So I just saw a trailer for a new Chris Nolan movie - Interstellar.

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This brings up a good question for this week.

Under what circumstances can you travel faster than the speed of light? Are there examples of this in nature?

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