Is Time Travel (going backwards) Possible?

Morrus · Nov 5, 2012

GMforPowergamers said:
it is the first i have heard about columbus not seting out to prove that the eqrth is not flat

Are you being serious? You guys are actually being taught that?

No, Columbus did not set out to prove the Earth was not flat. As Russell said, "no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat" and other historians say "there was scarcely a Christian scholar of the Middle Ages who did not acknowledge [Earth's] sphericity and even know its approximate circumference".

Then again, this thread has revealed far more bizarre viewpoints than that to me.

jonesy · Nov 5, 2012

Some of Colombus' opponents were of the opinion that according to their calculations the trip to Asia would be far too long for his crew to survive it without starving to death. I think that might have been where the whole 'fall off the face of the Earth' thing started from. The interesting thing is that they weren't wrong. If the Americas hadn't been there the trip really would have been too long.

Umbran · Nov 5, 2012

GMforPowergamers said:
so p,ease tell me some moreabout this, and where can I find out more?

As Jonesy said, it is referred to as the "Myth of the Flat Earth". Wikipedia has an article on it, which may be a good place to start:

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

Wikipedia said:
The issue in the 1490s was not the shape of the Earth, but its size, and the position of the east coast of Asia, as Irving in fact points out. Historical estimates from Ptolemy onwards placed the coast of Asia about 180° east of the Canary Islands. Columbus adopted an earlier (and rejected) distance of 225°, added 28° (based on Marco Polo's travels), and then placed Japan another 30° further east. Starting from Cape St. Vincent in Portugal, Columbus made Eurasia stretch 283° to the east, leaving the Atlantic as only 77° wide. Since he planned to leave from the Canaries (9° further west), his trip to Japan would only have to cover 68° of longitude.

Columbus mistakenly used a much shorter length for a degree (he substituted the shorter 1480 m Italian "mile" for the longer 2177 m Arabic "mile"), making his degree (and the circumference of the Earth) about 75% of what it really was. The combined effect of these mistakes was that Columbus estimated the distance to Japan to be only about 5,000 km (or only to the eastern edge of the Caribbean) while the true figure is about 20,000 km. The Spanish scholars may not have known the exact distance to the east coast of Asia, but they believed that it was significantly further than Columbus' projection; and this was the basis of the criticism in Spain and Portugal, whether academic or amongst mariners, of the proposed voyage.

The disputed point was not the shape of the Earth, nor the idea that going west would eventually lead to Japan and China, but the ability of European ships to sail that far across open seas. The small ships of the day (Columbus' three ships varied between 20.5 and 23.5 m – or 67 to 77 feet – in length and carried about 90 men) simply could not carry enough food and water to reach Japan. The ships barely reached the eastern Caribbean islands. Already the crews were mutinous, not because of some fear of "sailing off the edge", but because they were running out of food and water with no chance of any new supplies within sailing distance.

GMforPowergamers · Nov 5, 2012

Morrus said:
Are you being serious? You guys are actually being taught that?

No, Columbus did not set out to prove the Earth was not flat. As Russell said, "no educated person in the history of Western Civilization from the third century B.C. onward believed that the earth was flat" and other historians say "there was scarcely a Christian scholar of the Middle Ages who did not acknowledge [Earth's] sphericity and even know its approximate circumference".

Then again, this thread has revealed far more bizarre viewpoints than that to me.

I hate to tell you the lies we teach... columbus discovered america well trying to prove that the earth was round and he could get to china.

We are also tought (in public hs in 1996 anyway) that the civil war was fought becuse aberham lincon wanted to free the slaves, in collage I learned of the social economics that may also play into it

In 2002 a friend was told by a chemistry teacher in hs that there are only 3 stages of mattter, and another friends older brother went to the school (he is a chemical engener) that she was just plain wrong.

In my older sister's HS she was told aids was an african deases that started in monkies, 4 years later my teacher laughed at the idea that it was from a monky, but it was from africa ???

Even today with a 1st grade nephew we teach basic but wrong things.then correct them 4-5 years later but still misslead until collage gives you all the information.

Dannyalcatraz · Nov 5, 2012

Part of the reason for some of the jaw-dropping oddities in American education is that schools in the USA are controlled at the state level- IOW, almost no nationwide standards exist. Each state gets to choose how and what its students learn about math, history, science, etc. It is as if there were not objective standards!

in addition, the process of choosing books has become politicized. I live in Texas, and there are so many students in our system, that other states often use our approved books list, and you may have seen articles about the shenanigans that goes on here.

One of the ones approved by Texas a few years ago talked about how we won the Korean War by dropping the Atomic Bomb.

But Texas isn't alone- I'm an army brat, and I got to go to public schools in many states. I once had a school book that talked about how we may someday land a man on the Moon (this was in the mid-1970s).

Thank God I'm a "reader" whose grandparents & Mom were ALL educators (from K to grad school), whose Dad is a MD- no way my education was going to be compromised by the idiocy of others.

Umbran · Nov 5, 2012

Morrus said:
Are you being serious? You guys are actually being taught that?

Well, I don't think many of your users are currently in grade school, Morrus. They were teaching that in American schools in the 1970s. I don't know about now.[/QUOTE]

GMforPowergamers said:
In my older sister's HS she was told aids was an african deases that started in monkies, 4 years later my teacher laughed at the idea that it was from a monky, but it was from africa ???

Current theory is that HIV had its origins in non-human primates in west-central Africa.

http://en.wikipedia.org/wiki/HIV/AIDS#Origins

GMforPowergamers · Nov 5, 2012

Umbran said:
Well, I don't think many of your users are currently in grade school, Morrus. They were teaching that in American schools in the 1970s. I don't know about now.

Current theory is that HIV had its origins in non-human primates in west-central Africa.

http://en.wikipedia.org/wiki/HIV/AIDS#Origins[/QUOTE]

Now going back to 92-93 my sister was tought that, and 96-97 I was tought that was prajadice propaganda. NEW ENGLAND middle school and grade schools are horrable with it, and High schools were only a little better, I graduated in 1998 from high school and still to this day find things like this columbus thing.

freyar · Nov 5, 2012

Wowsers, get busy for a couple of days and watch the thread explode! I'm a bit tired and in the middle of some work, but I want to address a couple of points quickly.

tomBitonti said:
If I may: A photon starts a billion light years away. In the frame of an observer on the earth, is the photon already red shifted, or does the red shift occur as the photon travels to the earth? If the red shift travels while the photon travels to the earth, what is different about the photon's experience as it travels to cause it to red shift, compared, say, to a photon in a perfect resonant cavity bouncing back and forth for a billion years? Why does that photon never red shift, but the one that travels from a billion light years away does?

TomB

Umbran said:
Um, neither? Doppler shifting is best modeled by thinking of the wave nature of light, not the particle nature. Then, it the shifting Hubble observed is a result of relative motion between the source and observer.

Expansion of space only comes into it as a cause of that relative motion.

In the cavity example, the source and observer are not moving relative to one another - they are both pretty much at rest with respect to the resonant cavity (I assume), so no redshift is observed.

tomBitonti said:
Isn't the billion mile away emitter technically at rest (more-or-less) relative to us? I thought the passage through the billion light years was what caused the red shift. (This all is confusing because of the seeming difference between everyday velocity and apparent velocity due to space expanding.) Then, the red shift is caused by a curvature along the way. That curvature must not be uniform, otherwise, the light in the resonant cavity should experience a similar red shift. Is the effect imperceptible because it adds non-linearly (in a manner to reduce the effect) in a region already curved due to gravity?

Note: This is all at the edge of my understanding. I'm prepared for any one of these statements to be utterly wrong. What I'm interested in as much as the correct answer (to current levels of understanding) is a correct approach to obtaining the answer.

TomB

Nellisir said:
I think this falls over the edge of my understanding, but here goes. Keep in mind that this is the first time I've ever actually tried to make sense of redshifting and wavelengths, so it's very possible I've gotten something totally wrong or backwards.

Redshift isn't a property of the photon itself. The photon doesn't age or turn red as it travels. Redshifting is a property of one object travelling away from another object, and the wave (not particle) of light between the two. If the two objects are static in relation to each other, there is no redshift, regardless of the time or distance.

In our universe, however, on a intergalactic scale, everything is moving away from everything, in all directions. We are all dots on a balloon, and the balloon is inflating. (If this gets you going about curvature, then it's a flat piece of balloon being pulled from the perimeter). Because the expansion is speeding up, older things are moving faster, and are more redshifted.

Since time = distance, and time = speed, therefore further = older = faster = more redshift.

Light is photons, which are particles, but it is also a wave. The wavelength is the distance over which the wave's shape repeats. Imagine that there's a line between two objects, and that line has a repeating wave pattern. Let's say that there are....30 waves between Object A and Object B. The length between each wave equals X. If the objects are static, X is unchanged. If the objects begin to move together,the waves get tighter and X becomes shorter (X-, aka blueshift). If the objects move apart, X becomes longer (X+, aka redshift). The distance between waves is increasing, but the quantity of waves remains the same. There's still 30 waves, but they have to stretch to accommodate the increasing distance. That's redshift. The wavelength (distance between the same point on two adjoining waves) of light increases as objects move apart.

tomBitonti said:
That part I think I get. But, I think this answers my question by building it into the presises of your answer. Why only on an intergalactic scale? Is that because the effect is too small to be measured on a smaller scale (similar to the precession of Mercury, which is barely measurable, compared to the presession of Earth, which I gather is not. Or does the effect not happen at all within a galaxy? Does the wave stay unshifted until it reaches a very flat part of intergalactic space and then start to shift? (With a suitable smoothing in transition?)

If we model this as a set of rigid discs on an elastic surface, say, lots of CDs on a trampoline with sequins sprinkled here and there in-between, then we tighten the trampoline, then the distance between each of the rigid CDs and sequens grows, but the trampoline stretches, sliding, beneath the wide CDs.

But, if that happens, will there be an extra effect applied to light, similar to gravitational lensing, but not accounted for entirely by gravity, as we measure light passing galaxies from distant sources?

TomB

Umbran said:
No, merely having the wave (or photon) travel a long distance won't cause a red shift. The Doppler Effect comes from motion of the source or receiver.

However, I find that at 12:30 AM, I'm not so good at explaining. I will take a stab at it after I've gotten some sleep, if that's okay.

That's a lot of words about the Doppler effect and cosmological redshift! Going through that, I'm finding some good explanations and a little bit of confusion, so I'll just try my best at explaining rather than pick through all the previous posts (I will try to reference them at appropriate points if I can keep them all in mind).

OK, as Umbran says, the usual Doppler effect is not something that "happens" to the photon but is due to the relative motion of the photon's source and receiver. At low relative speeds, you can think that the receiver moves past the peaks of the light wave (photon) more quickly/slowly if the receiver is moving toward/away from the source. At these low speeds, the relative change in the light's wavelength = v/c, the relative speed over the speed of light (assuming head-on motion). This picture isn't quite right at relativistic speeds (a significant fraction of the speed of light), but it is a reasonable picture if you also throw in some words about "relativistic time dilation" (the equation changes a lot, though). A proper explanation would traditionally come after a couple of years of university physics, but I could try to give you something better in a few days (when I'm less busy) if someone wanted to start a different thread. I'm happy to do physics Q&A when I'm not swamped at work. Anyway, Nellisir also, I think, gave something similar to this explanation but may have been mixing in some of what I'm about to say next.

All that said, the normal Doppler effect is not quite what happens in the cosmological expansion of the universe, at least not in Einstein's general relativity, although you can think about it that way when you talk about light moving from galaxy to galaxy. However, it's more useful (and generalizes better mathematically) to think that the galaxies are really not moving with respect to some grid you've laid down in space (at least not due to the expansion of the universe). Instead, it's the space inside the grid lines that's getting bigger. In this way of thinking, the photon/light wave itself really does get stretched out as space grows between successive peaks of the wave. In my work, I find this to be the clearest way of thinking about it, especially in the very early universe, which is just a big plasma without galaxies (or much other structure), so it's very hard to think about objects moving apart unless you go down to the single particle level. On the other hand, it's very easy to think about the overall plasma being at rest and just diluting as the universe expands --- and the photons redshift as it does so.

freyar · Nov 5, 2012

The other thing I wanted to talk about (hope I'm coherent now, 'cause I'm way past bedtime) is KarinsDad's discussion with Morrus and others. I don't want to get into the whole thing about open-mindedness other than to make one point: I am a physicist by profession and know a lot of other physicists. With very few exceptions, we try to be open-minded in the sense of weighing every new idea. We might make quick judgements sometimes, we might disagree with each other's opinions, and sometimes we (either individually or as a community consensus) are wrong, but we have well-thought out reasons for those decisions nearly all the time. And, frankly, those reasons have to do with the data at hand.

But I do want to say a bit more about dark matter and alternatives, since that came up and it's an area of my research. Dark matter was first discovered about 80 years ago, like KarinsDad mentioned, but most scientists didn't pay much attention at the time because there wasn't enough data to demand it --- the observations could have been fluky in some way. So the first big "demand" for dark matter was the discovery that galaxies rotate faster than they ought to if (1) gravity works the way Newton and Einstein said and (2) there's just normal matter. But it works if you add a invisible, nearly pressureless form of matter that interacts via gravity with about 5 times as much total mass as normal matter. That's the how we define "dark matter." You can also describe galaxy rotation well if you change gravity. This is the idea of MOdified Newtonian Dynamics (MOND) and similar theories (I'll lump them together). However, you can also look at galaxies moving in clusters of galaxies. They also move faster than they "should" based on normal gravity and just normal matter. If you add 5 times as much dark matter, you get a prediction consistent with observation. If you use MOND, you also have to add some kind of invisible, nearly pressureless matter usually identified by MOND enthusiasts as neutrinos (from the Standard Model of particle physics). So MOND doesn't work perfectly on its own. Next, you can look at the cosmic microwave background light (CMB), which is the oldest light that's possible to see (any older, and the universe was opaque). It looks very uniform, but there are tiny variations in its temperature over the sky. Based on dark matter, people made detailed predictions for the patterns of the CMB years in advance of their measurement. These predictions match the measurements very well. For MOND to make these predictions, you need a relativistic version, and there are 2 problems: (1) it's not clear that the relativistic version of MOND is a self-consistent theory of physics as it looks pretty ugly and (2) there's controversy -- meaning conflicting calculations -- about whether it can predict (postdict now, maybe) the pattern of the CMB.

So, dark matter makes good predictions, MOND, not so much. Nonetheless, MOND is an intriguing option, so there are physicists still working on it, getting grants, etc. As long as there is a way it might fit the data, people will talk and think about it. But I hope I've explained why dark matter is the prevalent idea. It just works with the data, and it has the added bonus of being something that makes sense in terms of particle physics (I haven't gotten into that). I also hope you can see that it's not like what KarinsDad has said about Neptune's winds; it's not that we think we know what's going on and make a prediction in the absence of data (though that is supposed to be one part of the scientific method). There really is a lot of data consistent with and predicted by the dark matter hypothesis.

KarinsDad · Nov 5, 2012

Umbran said:
Here's the thing - that's not true. Yes, in grade school, they may have taught you that the learned people of Columbus' day thought the Earth was flat. But that was incorrect. And, these days, we know it is incorrect.

What you've just proven is that sometimes laymen didn't/don't know a whole lot about what scientists think. Which, as Morrus points out, is kind of ironic.

Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.

What you just proved is that many people do not read what is actually written, but put their own spin on it.