Hawking didn't address the biggest problem, which, as everyone knows, is grammar:
"The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr Dan Streetmentioner's Time Traveller's Handbook of 1001 Tense Formations. It will tell you for instance how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. Most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up: and in fact in later editions of the book all the pages beyond this point have been left blank to save on printing costs."
Naming time the "fourth dimension" is a misnomer, it's not a dimension in the usual sense of the. Hence the spacetime of relativity, formulated in Minkowski space, is different than standard 4-ED Euclidean space (see http://en.wikipedia.org/wiki/Minkowski_space).
As Einstein says, "Make things as simple as possible, but not any simpler." The time as fourth dimension thing, I think, is an example of a wrong analogy pushed to make it easy for people to understand what in effect is quite hard to grasp.
I always understood a dimension as "a variable that you can change to avoid occupying the spot." In 3d space, two objects cannot be in the same spot so either their x,y, or z coordinate must be different. If I add time as a fourth dimension then two objects can be at the same x,y and z, but not at the same time. You are right that time is a special dimension, but there are many things it has in common with the 3 spacial dimensions as well, my example being one.
You're right, in an intuitive sense. In math, especially dealing with weird stuff like extra dimensions, intuition may break down, though. For example, in the 4D Minkowski space you can have a unit sphere with a radius of -1! One has to understand what is meant when we say that "time is a special dimension". otherwise teh analogies we draw may fail. The Wikipedia article I linked explains these in a simple way.
I've always heard dimensions being referred to as "timelike" or "spacelike", similar in vein (though not in concept) to the difference between "flat" and "rolled up" dimensions.
While you're right that it's not a dimension in the sense that most people are used to, I think that it appropriately conveys the fact that time and space are intertwined. When you start traveling very quickly your sense of distance and time change in a way that is exactly proportional to each other. The fact that space and time need to be treated as a single entity was in fact one of the key conceptual advances that Einstein made.
Of course it doesn't :-) However, I wonder how many people reading this article in the Guardian know about different metric spaces. Hawking's presumed audience here is "laymen".
I've never really understood why we shouldn't be able to travel backwards in time.
Most sci-fi movies and shows seem to represent time as a line. I think of our history as a line, but every step along that history had an infinite number of possibilities it could have gone. To travel back in time would be to move back along the timeline that our reality exists - but once at that point, simply by being there, you would never be able to move forward into the timeline you came from. You're now on a new timeline, and anything you do has no effect on the timeline you came from.
Also, I don't know about these forward "time travel" methods described in the article. By those definitions we are all time traveling right now - we are moving forward in time. Its just the idea that some people move at different speeds so that when you slow catch up to the speed of everyone else, you're now "in the future". Really you were always there, you were just moving at a different speed.
> I've never really understood why we shouldn't be able to travel backwards in time.
Because time is not a place you can go to. Time is a measure how how fast the various reactions, and forces, and processes that make up your atoms, moves.
Those reactions can move faster, or slower. But there is no place you can go to to travel backward in time. Time is NOT a river or a line. Time is just a measure of how fast your reactions processed relative to the reactions of other things.
The reason time is sometimes talked about as a line is that it works mathematically. And the mathematics works if you reverse it too. But that doesn't mean the math has a physical reality.
Yeah I understand what you're saying from a practical approach. I'm looking at it more from a sci-fi (emphasis on FI) angle I guess. I don't really consider what you're talking about as "time travel" any more than how I traveled a minute or so into the future sitting here typing this.
When I said "I've never really understood why we shouldn't be able to travel backwards in time." I was referring to the statement from the article: "Ultimately, scientists may find that only travel into the future is possible, as the laws of nature may make travel to the past impossible so the relationship between cause and effect is maintained." particularly the part about cause and effect.
Traveling backwards in time is functionally equivalent to exceeding the speed of light; in fact, an object is traveling faster than the speed of light in one inertial frame of reference if and only if there is a second inertial frame of reference, with relative velocity to the first less than the speed of light, in which it is traveling backwards in time.
Technical matters of relativity aside, consider that you can't actually change how fast you move through time. You can't make time move faster or slower, at least within your own frame of reference. Time doesn't just move in a fixed direction, it moves at a fixed rate. Note that this is pretty close to how special relativity actually works, modulo semantics of "rate" when dealing with time.
There are also thermodynamic considerations. One direction of time does seem to be privileged in terms of entropy. This doesn't necessarily mean that time travel is impossible, as far as I'm aware, but it does mean that "forward" in time isn't completely arbitrary, and reversing that may be thermodynamically unfavored.
We always knew if you travel really fast, time slows down for you, and people on Earth have aged faster.
We always knew time moves at a different rate depending on gravity, too. Its there in SH's Brief History.
Using BH's and WH's has been talked about for years, too.
You're right, that classical special theory of R, but I don't think that's what people have in mind when they think about time travel. If they're so inclined (like me), they think of it as a way to visit a person from the past, e.g. Cleopatra in her imperial boat. Or, going to the year 2500.
As far as I understand, the crucial physical and philosophical problem is: Why is time's direction one way? Why can't we reverse it. Some physicists explain this by saying time moves in the direction of increasing entropy. But WHY does it have to be so? There's no inherent physical law, AFAIK, that says that this must be the case.
In your example the direction is still the same for both people, it's just the difference of inertial frames.
That's actually also true. The only thing that physicists have identified so far as being linked to time's directionality is entropy. Pretty much every other aspect of physics works with time's arrow pointed in either direction.
So from a physical point of view, the only thing keeping the egg you dropped from putting itself back together is the slope of the entropy curve associated with that event.
Apart from the quantum foam wormholes, it's just describing almost century-old (wow!) results of special and general relativity. Nothing especially exciting if you know the tiniest amount of physics.
But this was printed in a national Sunday paper here in the uk. Which I think is great - my dad was talking about it for ages.
EDIT: huh? my point being that the audience in question, for the most part, have little active interest in physics - thus this was a great way to expose people to some new ideas :)
Meh, I don't find it to be in unacceptably bad taste. The comment was made more out of sympathy at the horrors of paralysis than out of cruelty or a desire to offend. It's "gallows humour", not "a sick joke".
Let's save that discussion for when you've spent a good part of your life in a wheelchair without a means of expressing yourself directly, other than through some electronic gizmo.
You're making a joke at the expense of a guy that has had just about every card in the deck stacked against him and that made in spite of that more of his life and did more for us all than you will probably ever do.
If you don't see why it is in bad taste then I'm really sorry for you, if you think that that was 'out of sympathy' then I must have a big problem understanding English, and I hope I'll never be on the receiving end of your sympathy.
If I visit the future, say 25 years hence, I'd like to be able to look back and see what i did in that time. I'd also expect to see grown up kids, maybe grand-kids.
If i visit the future the way Hawkins suggests, by slowing down my present... I would land up in a future where there's been no me for that period. All there is a memory of 5 years in my mind (while I was traveling in space). There's a missing 20 years. I can't then (hypothetically) jump back in time and see the last 20 years, since i was not "there" during that time.
In quantum foam, miniscule tunnels between distinct times appear. Is it possible to put something in such a tunnel, and push to widen it? What forces would be exerted to counter that? Or do these "tunnels" obey totally foreign laws of physical interaction?
Hawking is great. Not because these types of articles are particularly detailed or illuminating, but because this is the kind of stuff that lures young people into becoming rocket scientists. Go Steven!
A few years back there were articles that you could travel back in time, but only to the point when the time travel machine was invented. Now you can't travel back at all.
If we upload our minds into computers, we will be able to code up and inhabit a simulated universe where time travel and teleportation are possible. I wonder if we will want that, though.
Randy D. Allen, a biologist at Oklahoma State University, argued that a smart-enough species could develop a quantum computer and eventually transfer their consciousnesses into it.
"No more inefficient metabolism requiring huge energy input, no chemically derived bodies to wear out, no reproduction, no death, no taxes. Just supermassively parallel collective consciousness with unlimited capabilities," he wrote. "Perhaps, through super symmetry or entanglement, they can "see" or "feel" the entire universe. Maybe, they've gained the ability to manipulate elementary particles and can control its evolution and its fate. They would have become, by any human definition, Gods."
Another way to go would be to link our brains to some computer that can simulate experience. Tired of what the physical world has to offer, people of the future may prefer to live in such virtual worlds, ... as long as the experience seems real, who cares.
"The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr Dan Streetmentioner's Time Traveller's Handbook of 1001 Tense Formations. It will tell you for instance how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. Most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up: and in fact in later editions of the book all the pages beyond this point have been left blank to save on printing costs."