What is Time?

[RedDog]

Thanks project,

That is an interesting story of light and it's relational effects from relative positions. As I suspected, once our combined recessional speed exceeds the speed of light, we can no longer see the recessional object.

I'm curious, visually, what would now be in that spot??

Time, then, is also relative to the observer.
Say we have three observers.
The first one is in our galaxy, The second one is in the recessional galaxy, and the third is at a fixed mid-point between the two.
We and number two are moving at the same relativistic speed, but we can no longer see each other as we exceed the speed of light.

What would the third person see us doing if he had a magic viewport that showed him life on both our worlds? Would we appear to be moving very slowly?

What if we had the same magic viewport, at both Two and Three's location?
What would we see timewise?

[Project]

Thanks project,

That is an interesting story of light and it's relational effects from relative positions. As I suspected, once our combined recessional speed exceeds the speed of light, we can no longer see the recessional object.

I'm curious, visually, what would now be in that spot??

Well, vision depends on photons hitting a sensor, be it an eye or a telescope, so we would (assuming it is in a totally black spot in space) see nothing at all if the photons never reached us.

Time, then, is also relative to the observer.
Say we have three observers.
The first one is in our galaxy, The second one is in the recessional galaxy, and the third is at a fixed mid-point between the two.

We and number two are moving at the same relativistic speed, but we can no longer see each other as we exceed the speed of light.

We would have to be moving at different relative speeds, since the galaxy would have to be rapidly accelerating away from us...

What would the third person see us doing if he had a magic viewport that showed him life on both our worlds? Would we appear to be moving very slowly?

What if we had the same magic viewport, at both Two and Three's location?
What would we see timewise?

Thought Experiment time:

Mrs. Einstein is standing in a field. Next to her is a light clock. That is, two mirrors that are reflecting a beam of light back and forth, and the journey from one mirror to the other and back again counts as one tick of the clock. Also, Mrs. Einstein is wearing a watch that is synchronized with her light clock. Standing on a railroad car is Mr. Einstein. He also has a light clock, and his clock is synchronized with Mrs. Einstein's and his own wristwatch. The railroad car is now moving to the left with a velocity v.

Question: What happens?

Answer: From Mr. Einstein's perspective, the beam of light keeps going up and down between the mirrors, but from Mrs. Einstein's perspective, the light now has to travel a diagonal path from one mirror to the other. Since Mrs. Einstein still measures the speed of light as c, she is now going to observe Mr. Einstein's light clock as ticking slower than hers since the light now has a longer distance to travel. However, since Mr. Einstein still experiences his watch as being synchronized with his clock, Mrs. Einstein will see his watch slow down along with his clock!

Conclusion: If someone moves in a straight line with velocity v with respect to you, then you will observe time passing more slowly for them.

Optionally view the math here.

The twins paradox:

In physics, the twin paradox is a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he or she has aged less than an identical twin who stayed on Earth. This result appears puzzling, since the situation seems symmetrical, as the latter twin can be considered to have done the travelling with respect to the former. Hence it is called a "paradox". In fact, there is no contradiction and the apparent paradox is explained within the framework of relativity theory, that only one twin has undergone acceleration and deceleration, thus differentiating the two cases. The effect has been verified experimentally using precise measurements of clocks flown in airplanes.

http://en.wikipedia.org/wiki/Twin_paradox

[Divinorumus]

Well, vision depends on photons hitting a sensor, be it an eye or a telescope, so we would (assuming it is in a totally black spot in space) see nothing at all if the photons never reached us.

If you could exceed the speed of the light approaching you and outrun the incoming photons, you would still be traveling along a path where previously passed photons are still traveling along, right? You would be catching up to yesterdays protons. As you sped away, your slowing down the "now" and catching up to yesterday. Assuming of course that were physically possible. Put aside the "visual" aspect of this. Even if the red shift drifts into the (seemingly) invisible, the wave and particles are still there. The eyes are inefficient and limited detectors.

[earthist]

Wow, Project, kudos for your explanations.

I've got a question for you, too, if you don't mind. I've asked a couple of those "ask a scientist" websites, and even e-mailed to a couple of physicists, but never received a reply. I hope you'll be more patient.....

My main question arises from the simple premise that when we look out into space, no matter how many light-years back in time we see, we can never see the Earth as it was in the past. Obviously, the light generated from that time and location has already passed us. The same would remain true for the entirety of Earth's progenitor stars, galaxies, etc. As long as we say that the expansion of the universe is slower than light speed, we can never see into our own past. In fact, the light from everything within a few (perhaps many?) light years of our progenitors has also already passed us. So this brought up the question in my mind, "If we can see a galaxy whose light comes from maybe 12 billion light-years away from where we are today, how far away must it have been from our then location at the time 12 billion years ago when it emitted the light we see now?"

FWIW, what I'm really trying to understand is how can the astrophysicists say we are 'seeing' nearly back to the Big Bang? Since we cannot see into our own past, it seems to me there must be a 'cone of invisibility' of sorts behind us in time. There must be a fair amount of stuff we can't see with a telescope. Am I making sense?

If you're still not worn out, here's an easier one:

"If the universe was expanding more rapidly 12 billion years ago than it is today, then isn't it only logical that "the farther something is from us, the faster it appears to be moving away from us?" It would appear to be moving faster because we are looking back to a time when everything was moving faster. Do our red-shift calculations account for this?

Thanks!

[joequinn]

Saint Augustine, The Confessions, Book XI:

'Quid est ergo tempus? Si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio'.

What then is time? If no one asks me, I know, but if I want to explain it to someone who asks, I do not know.

[Project]

I should note I am doing my best with a totally complex and crazy subject, I may be factually or theoretically wrong I will try to back stuff up at least with links etc.

If you could exceed the speed of the light approaching you and outrun the incoming photons, you would still be traveling along a path where previously passed photons are still traveling along, right? You would be catching up to yesterdays protons. As you sped away, your slowing down the "now" and catching up to yesterday. Assuming of course that were physically possible. Put aside the "visual" aspect of this. Even if the red shift drifts into the (seemingly) invisible, the wave and particles are still there. The eyes are inefficient and limited detectors.

It does not have to be an eye, any detector needs a particle to hit it. If the particle cannot speed back as fast as it is speeding forward, it will never get to us, so will not be visible. But, if it was previously visible, you could see it until a certain point, what that point is I am not sure, and this is really just conjecture

"If we can see a galaxy whose light comes from maybe 12 billion light-years away from where we are today, how far away must it have been from our then location at the time 12 billion years ago when it emitted the light we see now?"

It could have been very close, relatively. The idea of the big bang is we all came from a singularity, a single point, and grew from there.

Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years

So, the galaxy would have been about 1.7b yrs old, things were much closer together. The math to figure out where it must have been is over my head, but essentially, you have to figure in acceleration, placement, etc. Some of the links above to speed of light do have this material, but it is very serious math

FWIW, what I'm really trying to understand is how can the astrophysicists say we are 'seeing' nearly back to the Big Bang? Since we cannot see into our own past, it seems to me there must be a 'cone of invisibility' of sorts behind us in time. There must be a fair amount of stuff we can't see with a telescope. Am I making sense?

Horizons

Main article: Cosmological horizon
An important feature of the Big Bang spacetime is the presence of horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. If the expansion of the universe continues to accelerate, there is a future horizon as well

see also http://en.wikipedia.org/wiki/Frame-dragging

"If the universe was expanding more rapidly 12 billion years ago than it is today, then isn't it only logical that "the farther something is from us, the faster it appears to be moving away from us?" It would appear to be moving faster because we are looking back to a time when everything was moving faster. Do our red-shift calculations account for this?

Current theories say it is actually accelerating faster as time goes on, and this in fact is what the red-shift is showing us. This explains it well http://en.wikipedia.org/wiki/Redshift

Most of the info here is from this article: http://en.wikipedia.org/wiki/Big_Bang

You can learn everything at wikipedia

[Project]

The effect has been verified experimentally using precise measurements of clocks flown in airplanes.

Re: the Twins Paradox above... this is the really amazing, almost spooky thing to me, this is not theory that affects planets only and so on, in this experiment clocks actually went out of sync because of this effect. Time itself was different for them.

Hafele and Keating Experiment

"During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40+/-23 nanoseconds during the eastward trip and should have gained 275+/-21 nanoseconds during the westward trip ... Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59+/-10 nanoseconds during the eastward trip and gained 273+/-7 nanosecond during the westward trip, where the errors are the corresponding standard deviations. These results provide an unambiguous empirical resolution of the famous clock "paradox" with macroscopic clocks."
J.C. Hafele and R. E. Keating, Science 177, 166 (1972)

http://hyperphysics.phy-astr.gsu.edu...airtim.html#c1

[Divinorumus]

Current theories say it is actually accelerating faster as time goes on, and this in fact is what the red-shift is showing us. This explains it well http://en.wikipedia.org/wiki/Redshift

That would seem to make sense. After the so-call big band (which is actually an inaccuracy), the circumference of the universe began to expand (or rather it began being sucked into the absence of any space that surrounds it). Kind of like the opposite of blowing up a balloon from the inside out, the surrounding absence of anything including space around it sucked it into a round expanding balloon. As the circumference of our space expands, the greater the outer surface of our universe there is for the absences of space that surrounds it to pull upon to expand it, and at an increasing pace as it gets larger.

[RedDog]

Well, vision depends on photons hitting a sensor, be it an eye or a telescope, so we would (assuming it is in a totally black spot in space) see nothing at all if the photons never reached us.

I know this gets complicated very fast, so I want to keep it simple. I can google these standard answers also, but they don't help us think outside the box we now find ourselves in.

So, a totally black spot in space?? LIke a hole?

What do we usually call that?

[earthist]

I should note I am doing my best with a totally complex and crazy subject, I may be factually or theoretically wrong I will try to back stuff up at least with links etc.

You did great! Many thanks for the time and effort!

It could have been very close, relatively. The idea of the big bang is we all came from a singularity, a single point, and grew from there.

So, the galaxy would have been about 1.7b yrs old, things were much closer together. The math to figure out where it must have been is over my head, but essentially, you have to figure in acceleration, placement, etc. Some of the links above to speed of light do have this material, but it is very serious math

Yeah, I can't do the math either, that's why I'm asking.

Main article: Cosmological horizon
An important feature of the Big Bang spacetime is the presence of horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not had time to reach us. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the FLRW model that describes our universe. Our understanding of the universe back to very early times suggests that there was a past horizon, though in practice our view is limited by the opacity of the universe at early times. If the expansion of the universe continues to accelerate, there is a future horizon as well

Cosmological horizon: a term I hadn't heard so could not have looked up. Thanks! Still, the past horizon it describes deals with light that hasn't reached us yet, while my question deals with light that has already passed us.

I'm not trying to argue with the 'experts' per se, but it does bother me that none of them ever seem to mention or care about it (or to reply when I ask). Seems to me it would matter with all the ponderous calculations they run, and the authoritative pronouncements they make based on them. Oh, well, I am just an egg (Robt. Heinlein quote).

see also http://en.wikipedia.org/wiki/Frame-dragging

Hey, neat stuff. I never heard of frame dragging before, so I couldn't have looked that up, either. Thanks again, I learned something today. I love learning stuff!

Current theories say it is actually accelerating faster as time goes on, and this in fact is what the red-shift is showing us. This explains it well http://en.wikipedia.org/wiki/Redshift

Sure, but we'd get the same result if, as I said, the universe is slowing down. In that case, everything we see that's farther away (ie: older) would appear to be going faster due to age. Again, I'm not trying to argue, I just don't get why it has to be expanding faster when this idea makes so much sense to me.

Most of the info here is from this article: http://en.wikipedia.org/wiki/Big_Bang

You can learn everything at wikipedia

Yeah, but it's like using a dictionary: you can't look up the spelling unless you know what spelling to search for. You've given me some good info, Project. Thanks again! This stuff fascinates me. I'm too old and too poor to get a physics PhD at this point, so I'm stuck asking others. I have a son in 3rd year electrical engineering with a physics minor at Pitt. I'm hoping eventually to grow my own answer machine -- assuming I live long enough.

Edited to add:
Your answer re: cosmological horizons has finally led me to the answer I sought, namely that the light from our past is now seen as light from our nearby present. ie: we cannot see Alpha Centauri, for instance, as it was a billion years ago because we can see it as it was a mere 4.5 years ago.

Duh! I can be really dense at times. Thanks again, Project, you've 'shown me the light.'

[Project]

I know this gets complicated very fast, so I want to keep it simple. I can google these standard answers also, but they don't help us think outside the box we now find ourselves in.

So, a totally black spot in space?? LIke a hole?

What do we usually call that?

I think you want me to say a black hole, but that is a very different thing than "nothing". My answer would be that there is empty space there seemingly, you could not see whatever it is. A black hole is the furthest thing from nothingness, it is the most dense thing we know of in physics, it has more matter than anything else in a given area.

In my understanding of it, when you are looking at a point in 3d space, and you cannot see a receding galaxy, it would mean that the particles coming from that galaxy are too far away and traveling away from us too fast for us to detect.

[Project]

Your answer re: cosmological horizons has finally led me to the answer I sought, namely that the light from our past is now seen as light from our nearby present. ie: we cannot see Alpha Centauri, for instance, as it was a billion years ago because we can see it as it was a mere 4.5 years ago.

Well put, and this would seem to make sense to me. It is funny how "seeing back in time" really just means seeing light that took a long time to get to us. It really points out the fact that time and space are intertwined.

[Divinorumus]

It really points out the fact that time and space are intertwined.

Everything is intertwined. Everything.