# Iggy

Senior Members

1607

1. ## How big is the universe?

Quite welcome. In any units there is more than one kind of cosmological distance. A galaxy moving away from you might send you a beacon of light when it's some distance away. By the time the light reaches you the galaxy is no longer where it was when the light was sent. How far is the galaxy really? The commoving distance is not the light travel time. Einstein is often quoted on the matter: The speed of light is not globally equal to c in gtr. And, to continue from the previous subject--if there is more than one way to define distance then shouldn’t it follow that there is more than one answer for the speed of a ray of light. Speed is distance / time. If two rays of light are traveling different speeds then the idea of exceeding the speed of light gets a little more complicated. Nothing in the distant galaxy exceeds its own local speed of light. The link I gave explains very well: http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD That is only paradoxical if you add velocities u+v=w. That’s not how you add velocity in special relativity so it’s not paradoxical. No, you’re just not doing it right. Read: http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html Yes. In the distant future the only visible galaxy will be the one you are on.
2. ## How big is the universe?

Different things are meant by distance, http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD and universe, http://en.wikipedia.org/wiki/Observable_universe#Size_of_the_observable_universe A center isn't an event, but yes things are moving faster than light relative to other things (namely, us). In special relativity no two things have a relative speed greater than c, but in the more general, general relativity, they can exceed a relative velocity of c. yes yes The radius of the visible universe is less than 13.7 billion lightyears measured in light travel time.
3. ## A black hole traveling at .99999999999 c

I agree that the hole would not transfer kinetic energy to the planet the way a normal object would do, but I think the effects would be very devastating because of how mass outside the horizon but near enough to get flung around would act. Considering a test particle 100 km from the point of entry the particle accelerates (where the hole is stellar sized, 2E30 kg): $a = \frac{GM}{r^2} = \frac{1.33 \times 10^{20}}{10^{10}} = 1.33 \times 10^{10} \ m/s^2$ [with a post-Newtonian correction it is (1-2GM/rc^2)^(-1/2)*GMm/r^2 = 1.36 * 10^10] Since the hole is moving near the speed of light the test particle would not feel that acceleration for long. Just for a rough estimate it takes something moving at the speed of light about 0.00033 s to travel 100 km making it safe to say the test particle is accelerated in the order of 10^10 m/s^2 for about 0.00001 s. Multiplying those, it gets a speed of about 1 million m/s. As the black hole passes through the earth it would make a clean hole which isn't very devastating. But, all the mass around the axis-shaped hole would be flung toward the center of the axis with devastating speeds. if a good portion of earth's mass were to collide with itself at hundreds of thousands of meters / second then the resulting energy would surely be enough to vaporize the planet. Unless something is wrong with my reasoning (which is entirely possible, I'm not a trained physicist), I believe the planet would be vaporized.
4. ## Question about space twins return

Not true. See post #9. Compensating for doppler shift the stationary twin calculates that his brother ages slower the entire trip. Is that correct? The 'accelerating' twin can't consider himself at rest while the rest of the universe relatively accelerates in general relativity? You sure?
5. ## the effect of gravity

In this case it is clear that distance does not affect the size of the two girls but only their perception of each other. Relative size difference is an effect of perspective or perception only. Relative time dilation is not. We know this because when Anna and Betty approach each other Betty no longer seems smaller--they both appear the same size again. When time dilated clocks are brought back together they are no longer synchronized. It's not just perception. On the issue of time being a sixth sense... it seems clear that things without senses are affected by time.
6. ## sodium bicarbonate

this isn't entirely accurate, on exposure to moisture it will decomposed to sodium carbonate which will also fizz in vinegar. What you say is exactly accurate which is why vinegar is a good test. Sodium carbonate is a cleaning agent (ie "washing soda"). True, but it's not hygroscopic. My point being, it's more stable than a 1 month expiration date.
7. ## sodium bicarbonate

One month is probably the expected limit to its deodorizing ability--which would also mean after a month of being open to the air it would no longer taste fresh. As a cleaning agent, that expiration date would be meaningless. Sodium bicarbonate is very stable. If you're in doubt then mix some with vinegar and see if it fizzes out carbon dioxide - if it does then its good for cleaning.
8. ## Electricity

Put the batteries in series and arrange the plates facing each other and almost touching, like this: Put something non-conductive between the plates (like paper) then sandwich them together. It won't work great, but it will work.
9. ## Electricity

No To store a negative charge on a plate you have to load it up with electrons. Simply placing the negative side of a battery on a metal plate won’t force electrons on to the plate. For one thing--a battery doesn’t have a bunch of extra electrons stored up in it to give to something. It needs to pull electrons in through the positive terminal in order to pass them out the negative side. So logically you would wire a plate to the positive side and another plate to the negative side. If you try this (and you should - it’s easy) you could attach the battery between the plates for awhile then disconnect it. Measure the difference in potential between the plates with a voltmeter. Unfortunately, you'll probably notice it didn't store a charge. Last thing you have to do is arrange the plates facing each other and nearly touching. Put a piece of paper between them would be easiest I suppose. Now you’ve got it. Hook the battery up between them... let it charge... remove the battery and replace it with a voltmeter or a light bulb. Voltmeter shows difference in voltage and bulb lights up. You’ve stored a charge. Actually, you’ve made a capacitor (which stores a charge) The reason you have to arrange the plates facing each other is to create an electric field between them. In simple terms, the negative charge on the ‘negative plate’ will reach out and push the electrons out of the ‘positive plate’. It’s really the field between the plates that stores the energy... but we won’t quibble.
10. ## New Student.

Hello LostLabyrinths, I'm also new here... Let's make introductions together I'm an avid armature and tinkerer in the sciences with a love for logic and reason. I've been pointed to this forum twice from google, so decided to dip my feet. For an explanation of relativity, LostL, have you seen wikipedia's wikibook on special relativity? I've read it top to bottom and it's a really thorough treatment. The regular wikipedia page on special relativity is also really good if you're looking for something less extensive. A simple description would be that the speed of light is always constant - no matter how fast you're moving. If two people both see the same light moving the same speed, but they are each moving relative to each other then something's gotta give way. It turns out that time and space themselves give way. If someone is moving relative to you then their clock will appear to run slow compared to your clock and their ruler will appear short compared to your ruler. The faster these two people go compared to each other... the more drastic these effects. That is a simple description, but a helpful start I hope. I look forward to having fun in the forums everybody
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