# NowThatWeKnow

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## Posts posted by NowThatWeKnow

### Sharing the same "now"

But an infinite number of cameras won't result the output of a single photograph (to use the definition you provided from einstein online).

You don't need a single photograph to know that every location will share a point in time with any other location. Einstein Online just shows that events at any two locations can be simultaneous and they call that "now".

### Most Distant Object

Gly is billion light years (see legend on calculator site). "Angular size distance D" is the distance the galaxy was away from us when the light was emitted. z at the top right is red shift and z + 1 is the expansion factor. "comoving radial distance" is the galaxies current location.

### Most Distant Object

If the light took 13 billion light years to get here, then you are seeing light that came from a galaxy when it was only 3.3465 Gly away. That galaxy is now 29.701 Gly away from us. Space had an expansion rate of 8.875 times while the light traveled.

http://www.astro.ucla.edu/~wright/DlttCalc.html

If you put 13 in the left box that says "light travel time in Gyr" and click flat, You will see where the #'s came from. The expansion of space is z + 1.

### Sharing the same "now"

How does that reconcile with "a universal absolute "now" is possible." ?

An infinite number of cameras would show any location would share a point in time with any other location.

Maybe my thoughts are not coming out clear but I do not disagree with what you are saying or have been saying. Actually, I am not sure how or why I got back into this thread.

### what is the universe expanding into?

But seriously, why did people invent ether, what good did it ever do?...

Einstein considered it more then once and your friend Frank Wilczek will probably define it soon. From a layman's point of view it would solve many problems. For me it is hard to grasp the uniformity of the expansion of the universe without an ether. Or, you can only break the speed of light with the help of space (metric of nothing) expanding.

### Sharing the same "now"

What you are saying is two people on different floors of a building can not share the same "now". Einstein Online defines "now" and I interpret it to say two events in two different frames can be simultaneous. Isn't that what you understand from the quote below?

As posted earlier in this thread.

The definition of "now"

http://www.einstein-online.info/en/spotlights/Now/index.htmll

"two events are simultaneous if and only if they can be seen on the same photograph, taken with a double camera placed exactly in the middle between their locations."

### Sharing the same "now"

I see "now" as a single point in time if I could be everywhere in the universe at once. The clocks on Earth and the clocks on the GPS satellites may run at different speeds but they do share a simultaneous point in time. With enough information you could compare your "now" to any locations "now" in the universe. Therefore a universal absolute "now" is possible. What is the flaw in my logic?

### what is the universe expanding into?

Martin, usually at least in special relativity we use the fundamental construction that regardless of relatively speeding frames of motion, physics is experienced the same in any frame. This is a curious turn where we find the CMB is a blackbody radiation field, which actually refers to equilibrium it once had with the average distribution of masses. Does this present a challenge to relativity?

It almost sounds like Martin is a closet ether person, waiting for the right definition of ether.

### Holocene Sixth Mass Extinction Event In Progress

...What do you suppose the effects will be if billions of people are permitted to undergo change in the same fashion and at today's rate of resource use? ...

At this point, who has the authority to permit or stop another country from developing? We will just sit here and watch the coal go into the furnace.

### Holocene Sixth Mass Extinction Event In Progress

Please note that I did not say we could continue with 'business as usual'...

No you didn't but your reply to me was in defense of a more optimistic view and did not offer any additional measures that might be considered for a healthy planet. I do not consider myself an environmental alarmist but do see a potential for future problems when it comes to population, pollution and consuming natural resources. I also believe a balance is good but if we do error one way or the other, not giving enough credit to the alarmist could be capable of more damage then following their advise.

### Holocene Sixth Mass Extinction Event In Progress

What you said is very much a matter of opinion and there is a more optimistic view.

So you are saying it is business as usual and no change is needed. Jump in the SUV and head to the boat on the lake. This planet can take any thing we can throw at it. The forest getting smaller and the deserts getting larger are no big deal. The scientist saying we are using up our planet faster then it can replenish itself are not only working with a bad time table, they are flat wrong. Hmmm, I am not so sure "optimistic" is the right word.

### Holocene Sixth Mass Extinction Event In Progress

While there may be some argument left on how quickly man is destroying the environment, the fact that he does have a negative impact is obvious to most. Between pollution and consuming natural resources at exponential rates, we will be in trouble at some point. The pictures of air quality at the olympics in China suggest it may be sooner then later.

If it is a matter of when and not if, what should we be doing now to correct the problem? While technology is moving quickly, I do not see it staying up with with the population explosion. The population of the world goes up by over 200,000 per day. That is like adding the population of my entire state (Oklahoma) plus another 1 million people to the world every month. How long can that go on? The waste alone from the 90,000 chickens a minute we consume is a problem by itself.

http://www.peterrussell.com/Odds/WorldClock.php

### what is the universe expanding into?

would it be possible to explain hubbles law a bit simpler and detailed and without the equations

http://en.wikipedia.org/wiki/Hubble's_law

"Hubble's law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance."

It is often explained using a balloon with dots on it representing the galaxies. As the balloon expands, the distance between the dots increases no matter where you are on the surface of the balloon. The further away you are from a dot, the faster it is moving away from you. This expansion is from space expanding and not from the galaxies moving. Local gravitational forces keep galaxies and galaxy clusters from expanding.

### Light speed

One could alternately treat light as a wave, and simply accept the fact that the wave slows down as it encounters a material with permeability and permittivity that is different from vacuum.

It does not seem that is going to be the case around here.

### are protons manipulable?

A magnetic field strong enough to protect you would probably cause all sorts of problems. Digital media and instruments for one plus anything attracted by magnets would be stuck to the wall.

### Light speed

Whilst the photon is absorbed it does not exist.

With the first law of thermodynamics in mind, what is the photon converted into and out of when being absorbed?

### Light speed

Read my post above. You're confusing average and instantaneous.

I did read your post above. The average speed and instantaneous speed being different is what may be confusing to some. Then you have the observed speed from different frames to complicate it more.

### Light speed

I am not disputing what is being said here but the speed of light, relative to different observers, is not observed as constant. Warped space or water, the time to travel a given distance is variable. Hence, the speed of light being constant may confuse many.

### Light speed

How so?

You drive in your car for 100 km, and it takes you 2 hours. But you stopped along the way, for a total of 1 hour. What was your average speed? How fast was your car traveling while it was in motion?

Does stopped or moving sound like a constant speed? If the photon can only maintain its speed the distance between molecules, then would it be harmonious with "Distance traveled divided by the time of travel"? It would be relative to a particular distance and your point of view.

### Light speed

you got me with the new avatar i took a couple swings at that bug on my screen before i realized it wasn't.

It does make you want to slap at it.

As far as light speed being constant. I can see why many would be confused. The explanations given and the physics definition of speed are not harmonious.

### Light speed

Photons ALWAYS move at c.

It appears that light moves slower through materials due to the absorption and re-emission of photons.

Most probably consider this thread over but could someone please explain how absorption and re-emission of photons is not slowing them down? I visualize a drop of water falling into a sponge and eventually making it out the other side, effectively being slowed down. I also do not see how refraction of something moving 186,000 miles per second can make it look like it is standing still. You can't shine a light into two opposing mirrors and expect it to continue when the source is turned off. It would seem the time between two points determines the speed.

### What's in a Black Hole - Can't We Find Out?

...If you beamed back the progress of your journey into a black hole, your friend would have to tune to progressivly longer wavelengths (lower frequencies) as you approached the event horizon. This is the effect of gravitational redshift'' (see General Relativity predictions section). Eventually, the photons would be stretched to infinitely long wavelengths...

Very interesting. Is this saying that to the naked eye, something would disappear as it falls into a BH since the photon frequency would be outside the visible light range?

### What's in a Black Hole - Can't We Find Out?

Actually, it would just appear to freeze... to be forever static right there at the edge of the event horizon... from the perspective of an outside observer, that is...

Like a big magnet collecting matter? Could you elaborate on that? Or a link.

### could distant galaxies be moving faster than lightspeed relative to each other?

In general, discussions about galaxies receding from each other is a discussion of general relativity, where the galaxies are not in the same local spacetime. Then, recession speeds can exceed c. Hubble's law is linear, so recessional speeds are proportional to distance. Two galaxies receding from a point at .75c, are twice as far away from each other, and should recede from each other at 1.5c.

Good point. Right words for the wrong situation. I knew better if I would have thought about it. I see why they let you stay around.

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Wow!

So there really is an end to the visible universe!

This raises questions in my mind as to weather there may be galaxies that are blue shifted beyond our visibility! suggesting different regions of the universe expand and contract!

Hubble's law says that galaxies are moving away from us at a speed proportional to their distance but this does not always apply to nearby galaxies effected by local gravity. I see no reason why the expansion would not apply to spacetime outside of our visible universe.

"There are about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. Most of these galaxies are in our own local group, and are all in orbit about each other. Most are dwarf galaxies among them include the Andromeda Galaxy,"

### could distant galaxies be moving faster than lightspeed relative to each other?

...If a distant galaxy's red shift is observed to give a speed of 3/4 the speed of light and another galaxy in the opposite direction is also observed to be moving away from us at 3/4 the speed of light, does this mean that galaxy A is traveling faster than light speed relative to galaxy B ?

Welcome!

The observer in between sees them seperate a 1.5c but to each other the galaxies are seperating at .96c.

A few calculators to help with the math.

http://www.cthreepo.com/cp_html/math1.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html

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so, will space ever stop expanding? if so, how long until then? and will life still be a possibility with expanded space?

At this point the rate of expansion is actually increasing.

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