Time Travel... the fly in the ointment

[Speakpigeon]

By asking about the past I am including the when.

But the past is not just a when. It is also a where. It is a specific arrangement of all the matter and energy and other "stuff" the universe is made up of.

In the past there was a piece of paper. In the history of that piece of paper it was burnt up.

To go back to the past is to go back to a time and place where that paper had not burnt yet.

Therefore if it is possible to go back to the past that piece of paper is out there somewhere just waiting for something to return to it.

Does anybody actually believe that piece of paper is out there somewhere, and when, just sitting there so that it is possible to return to it?

If they believe that, we know when the piece of paper was, but where is it hiding so that it is possible to return to it?

I can certainly conceive of the past as still in existence (and the future too, already there so to speak). If the piece of paper has existed in certain places at certain times before being burnt then in some sense the piece of paper is still in those places at those times.

However, it should be made clear that the sense in which the past still exists is not necessarily the same as the sense in which things exist in the fast, the present or the future. The piece of paper is as you say a certain arrangement of matter at various places and times. These arrangements therefore exist within time and space. We shouldn't mix up this idea with the idea that the whole past still exists (or the future already exists).

Finally, it's not because the past still exists in some sense that we should be able to go back to it. Maybe we can't. We certainly can't go back to a moment in the past we've never been to. Maybe it's conceivable, perhaps even possible, possibly actual that space-time folds unto itself and in this case we would have a cycle between two areas in space-time but whatever would happen in there would only be whatever has always happened. It's the eternal return or nothing. So we can't really expect to be able to organise to have some kind of experiement to travel back in time. Rather, it happened or it didn't. If it did then some area(s) of the universe are forever recycled between two points in time, and it it didn't then it will never do.

Of course, we can imagine sundries other possibilities...
EB

 

[braces_for_impact]

It's arguable that "now" even exists. It's basically the infinitely thin edge between the past and the future.

...and no two people can agree on that infinitely thin "now" because of the spacetime separating them. Any one person will see others (and everything else) existing in the past - though the very recent past.

 

[bilby]

Only because we move in time with constant velocity. In a four dimensional universe, change in three dimensions is an illusion; nothing changes; we just experience a different point of view as our T coordinate changes - in exactly the same way that our point of view changes with variations in the X, Y and Z coordinates.

To use an analogy in three dimensions, a series of two dimensional slices through a cone, each slice further from the tip than the last, produces the illusion of an expanding two-dimensional circle. But the cone stays the same; only the perspective changes. The circle is a slice of a higher dimensional reality, and change only occurs as an artefact of thinking about the situation in terms only of the two dimensions we are considering at the time.

Ach, that's exactly the kind of angle I've tried to work out since ages and ages but you see it doesn't really work too well. The problem is that you still need to explain the changing of the T coordinate. We can imagine a still universe with four coordinates as you say, one being T, and sure our experience is dependent on the T coordinate. However, in your model, there is no longer any reason for us to be conscious of any one particular point in space-time and there is no longer any reason not to be conscious of all space-time points (not all together, all separately). You would also have to explain the apparent relation of cause and effect. Since everything is still, why would there be a relation between the universe at T and the universe at T + 1?
EB

That is not a requirement of the model; but it is equally not ruled out by it.

To use an analogy in three dimensions, one can see that subsequent slices through a girder will result in the shape of each slice being very similar to the ones on either side; but with a sudden breakdown of that pattern when the end of the girder is reached.

The Big Bang is a discontinuity. Causality need not be universal; it may only be characteristic of the local area of a few tens of billions of years.

 

[bilby]

Equally, it is impossible to tell whether or not the past exists in a four dimensional spacetime (or even whether the fourth dimension itself is 'real') by reference to the three dimensional space we can observe.

According to the empirical method the past exists because we have very many first-hand witness accounts about it. We have nearly all been there, we have observed the past, we have memorised how it looks like, we have recorded data about it in our information systems. We have accumulated more than enough evidence to claim that the past does exist. It's not really scientific because we can't reproduce any observation of the past. Well, you need to be there (then) to observe the past. And so the problem is we don't know how to go back to it to dupplicate observations. If space-time is still as you suggested then we can't go back to a point in time when we weren't there in the first place so no time-travel. Just the impression of travelling in the direction of the future. But maybe space-time is folded unto itself in such a way that here and there (now and then) there is a merging of big chunks of space-time from the future and from the past. That's probably against some law about entropy but maybe the law is wrong.
EB

All the evidence, memories etc. of the past exist in the present. We do not refer to the past at all; we have present memories that may or may not relate to the past.

If the present moment is all that exists, then those records and memories do not relate to anything real. If they do relate to something real, then in principle we could observe that reality (and given that the speed of light is an absolute limit, we can do so by standing back).

 

[untermensche]

Time is the freedom that allows 3 dimensional objects to move and change. It allows travel through space.

You would need some other kind of dimension to travel through time.

A fanciful dimension that doesn't exist.

Interesting idea.

Still, space could be the dimension that allows you to travel through time.

And one can argue that we are already travelling through time, in the direction of the future, without any apparent need for another dimension for achieving that.
EB

Traveling "through" time is just a metaphor.

If you are being pushed along by a raging river and have little control, are you traveling "through" the river, or within the river?

We travel within time, it pushes us along, not through it.

 

[bilby]

Interesting idea.

Still, space could be the dimension that allows you to travel through time.

And one can argue that we are already travelling through time, in the direction of the future, without any apparent need for another dimension for achieving that.
EB

Traveling "through" time is just a metaphor.

If you are being pushed along by a raging river and have little control, are you traveling "through" the river, or within the river?

We travel within time, it pushes us along, not through it.

True enough. And at any point your experience of the river is only of the bit you are currently in. But only a total idiot would suggest that the rest of the river, from headwaters to estuary, did not exist.

 

[DBT]

Maybe, but the nature of time has still not been fully explained.

The past is just recorded memory and future does not exist. The only experience we have of 'time' is an ever changing present moment, which our clocks measure as the rate of change (local).

Perhaps you are able explain the nature of time?


''Efforts to understand time below the Planck scale have led to an exceedingly strange juncture in physics. The problem, in brief, is that time may not exist at the most fundamental level of physical reality. If so, then what is time? And why is it so obviously and tyrannically omnipresent in our own experience? “The meaning of time has become terribly problematic in contemporary physics,” says Simon Saunders, a philosopher of physics at the University of Oxford. “The situation is so uncomfortable that by far the best thing to do is declare oneself an agnostic.”

While it is true that it is a bitch trying to get a handle on the nature of time, I find it just as difficult to try to get a handle on any of the other dimensions. Try painting a mental image of an X-axis all by itself without respect to the other dimensions. Now try explaining the "nature" of that dimension.

Models like spacetime or any other model are not necessarily “reality”, whatever the hell that is. Models are mathematical constructs that allow us to predict events and to “understand” the connections between events. Modeling time as a fourth dimension has proven to be better at allowing us to do this than any other model so far. Does that mean it is “reality”? Not necessarily, but it is the best at predicting events in “reality” we have yet found

I mostly agree, but as far as I can see there is no credible model that is able to demonstrate the ability to return to past states of the world, to travel back in time.

Brian Greene proposes fiddling with ''wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time'' - but without adequately explaining how going back to your own past is achieved.

''Greene, an expert on string theory—which views matter in a minimum of 10 dimensions and tries to bridge the gap between particle physics and nature's fundamental forces, questioned this scenario.

“Many people who study the subject doubt that that approach has any chance of working,” Greene said in an interview . “But the basic idea if you’re very, very optimistic is that if you fiddle with the wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time.”

 

[untermensche]

Traveling "through" time is just a metaphor.

If you are being pushed along by a raging river and have little control, are you traveling "through" the river, or within the river?

We travel within time, it pushes us along, not through it.

True enough. And at any point your experience of the river is only of the bit you are currently in. But only a total idiot would suggest that the rest of the river, from headwaters to estuary, did not exist.

Time pushes us forward like a raging river, but that doesn't mean it is a river.

Gravity pulls us to the earth like a magnet, that doesn't gravity is a magnet.

 

[bilby]

True enough. And at any point your experience of the river is only of the bit you are currently in. But only a total idiot would suggest that the rest of the river, from headwaters to estuary, did not exist.

Time pushes us forward like a raging river, but that doesn't mean it is a river.

Gravity pulls us to the earth like a magnet, that doesn't gravity is a magnet.

You post analogies like you understand what you are going on about, but apparently that doesn't mean you understand what you are going on about.

There is no more reason to declare the past nonexistent than there is to declare the headwaters of a river nonexistent.

Or if there is, you either don't know it or are determined to keep it a secret from me.

 

[skepticalbip]

While it is true that it is a bitch trying to get a handle on the nature of time, I find it just as difficult to try to get a handle on any of the other dimensions. Try painting a mental image of an X-axis all by itself without respect to the other dimensions. Now try explaining the "nature" of that dimension.

Models like spacetime or any other model are not necessarily “reality”, whatever the hell that is. Models are mathematical constructs that allow us to predict events and to “understand” the connections between events. Modeling time as a fourth dimension has proven to be better at allowing us to do this than any other model so far. Does that mean it is “reality”? Not necessarily, but it is the best at predicting events in “reality” we have yet found

I mostly agree, but as far as I can see there is no credible model that is able to demonstrate the ability to return to past states of the world, to travel back in time.

Models don't "demonstrate the ability" to exist now, proceed into the future, or return to the past. They are only mathematical constructs and, as such, we can only determine how well they reflect "reality" by testing them. Models work just as well in the "reverse time" direction as they do in the "forward time" direction. It is just a matter of changing a sign. The bitch is in figuring out how to test them to see if they really are modeling "reality". We have extensively tested Einstein's models and found that they are damned good at predicting future events and at postdicting past events. However, the models don't "demonstrate the ability" for us to exist in the "now" or travel to either the future or the past.

Brian Greene proposes fiddling with ''wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time'' - but without adequately explaining how going back to your own past is achieved.

''Greene, an expert on string theory—which views matter in a minimum of 10 dimensions and tries to bridge the gap between particle physics and nature's fundamental forces, questioned this scenario.

“Many people who study the subject doubt that that approach has any chance of working,” Greene said in an interview . “But the basic idea if you’re very, very optimistic is that if you fiddle with the wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time.”

You have to take Brian Greene with a large dose of salt, just as with Michio Kaku when they are on programming aimed at the general public. Their public persona tends to present the fringes of speculative interpretation on science because it makes for more interesting TV than a lecture on generally accepted understandings in science.

 

[untermensche]

There is no more reason to declare the past nonexistent than there is to declare the headwaters of a river nonexistent.

?

I can provide you with evidence of the existence of the headwaters.

There is no reason to believe in anything that has no evidence to support it's existence.

Put up or shut up.

What actual evidence do you have?

 

[bilby]

There is no more reason to declare the past nonexistent than there is to declare the headwaters of a river nonexistent.

?

I can provide you with evidence of the existence of the headwaters.

There is no reason to believe in anything that has no evidence to support it's existence.

Put up or shut up.

What actual evidence do you have?

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

 

[skepticalbip]

?

I can provide you with evidence of the existence of the headwaters.

There is no reason to believe in anything that has no evidence to support it's existence.

Put up or shut up.

What actual evidence do you have?

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

Plus we can see it right now. I see the Sun (present tense) a little over eight minitues ago (past tense).

Well, to be fair, I haven't seen it today because it is so fucking overcast so maybe it doesn't exist today.

 

[untermensche]

?

I can provide you with evidence of the existence of the headwaters.

There is no reason to believe in anything that has no evidence to support it's existence.

Put up or shut up.

What actual evidence do you have?

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

The past includes more than what you have observed.

But evidence is something you can present, not claim.

What evidence do you have?

- - - Updated - - -

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

Plus we can see it right now. I see the Sun (present tense) a little over eight minitues ago (past tense).

Well, to be fair, I haven't seen it today because it is so fucking overcast so maybe it doesn't exist today.

The light that you see is the light that hits your eye when it hits your eye. You are not seeing the past you are seeing the light in the present. That is the only light you have ever seen, present light.

 

[skepticalbip]

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

Plus we can see it right now. I see the Sun (present tense) a little over eight minitues ago (past tense).

Well, to be fair, I haven't seen it today because it is so fucking overcast so maybe it doesn't exist today.

The light that you see is the light that hits your eye when it hits your eye. You are not seeing the past you are seeing the light in the present. That is the only light you have ever seen, present light.

And only a couple pages back you claimed to accept that Einstein's spacetime was the correct interpretation. Now you reject it to give your Newtonian analysis of where the sun is rather than Einstein's analysis of where/when the sun is.

 

[Juma]

Assume, to make us able to even properly imagine this, that space-time consited of only two space coordinates (x,y) and time. Its the space-time pf flatland. Now let space origo be at the center of the sun then the earth will be a very thin pasta spiral spiralling in the direction of time. Let us view time as up and we will see the circling earth as a spiral around the vertical rod that is the sun. Now think of a flatland being on the flatland earth. (Assume it is a circle) then its movements on the earth will result in a complicated swirl around this spiral that is the earth. But how about the flatlanders "now"? That is at each position of the flatlanders life curve. But why then do we only experience a short now? Because we perceive locally: at each time we only perceive what is at that position in space time.

But how/why do we move in time?

 

[untermensche]

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

Plus we can see it right now. I see the Sun (present tense) a little over eight minitues ago (past tense).

Well, to be fair, I haven't seen it today because it is so fucking overcast so maybe it doesn't exist today.

The light that you see is the light that hits your eye when it hits your eye. You are not seeing the past you are seeing the light in the present. That is the only light you have ever seen, present light.

And only a couple pages back you claimed to accept that Einstein's spacetime was the correct interpretation. Now you reject it to give your Newtonian analysis of where the sun is rather than Einstein's analysis of where/when the sun is.

We are not seeing the past when we look at the sun. We are looking at the light as it exists in the present, not as it looked when it left the sun. It has traveled through gravity fields and the earth's atmosphere. It is not the same light as it was when it left the sun.

 

[skepticalbip]

Observational.

I know the past existed; I was there. I have no reason to think it was destroyed.

Plus we can see it right now. I see the Sun (present tense) a little over eight minitues ago (past tense).

Well, to be fair, I haven't seen it today because it is so fucking overcast so maybe it doesn't exist today.

The light that you see is the light that hits your eye when it hits your eye. You are not seeing the past you are seeing the light in the present. That is the only light you have ever seen, present light.

And only a couple pages back you claimed to accept that Einstein's spacetime was the correct interpretation. Now you reject it to give your Newtonian analysis of where the sun is rather than Einstein's analysis of where/when the sun is.

We are not seeing the past when we look at the sun. We are looking at the light as it exists in the present, not as it looked when it left the sun. It has traveled through gravity fields and the earth's atmosphere. It is not the same light as it was when it left the sun.

Since you are simply repeating the same thing, I will just give the same response.

Now you reject it to give your Newtonian analysis of where the sun is rather than Einstein's analysis of where/when the sun is.

ETA:
I could ask a question that I asked earlier but you didn't answer. You have heard that Newton didn't get it quite right haven't you?

 

[DBT]

However, the models don't "demonstrate the ability" for us to exist in the "now" or travel to either the future or the past.

That was my point. Nor do the models describe how the past exists in order to be able to 'rewind' time, or return to past states of the world, other than vague mentions, such as worm holes, etc.

You have to take Brian Greene with a large dose of salt, just as with Michio Kaku when they are on programming aimed at the general public. Their public persona tends to present the fringes of speculative interpretation on science because it makes for more interesting TV than a lecture on generally accepted understandings in science.

True, but that doesn't mean the scientific principles they are referring to are not valid, even if some of it is not testable such as string theory. If you have access to a plausible description for a return to past states of matter/energy, I'd interested in seeing it.

 

[untermensche]

Now you reject it to give your Newtonian analysis of where the sun is rather than Einstein's analysis of where/when the sun is.

Nonsense.

You simply have muddled thinking.

You think you are looking at the past when you are looking at light hitting your eye in the present.

And you think the light that leaves the sun is the same light that hits your eye, but it has been altered by it's journey.

This has nothing to do with a dichotomy between Newtonian understanding and current understandings.