Theory of relativity

[Anupam Sinha]

I would like to know what exactly is the theory of relativity. I tried a bit of googling and found some interesting sites. But they were full of maths formulas. Can someone explain the theory of relativity in smple words. All I know about this is when you travel close to the speed of light then the time you have actually taken is more/less than the time at say some other place. How can this happen. Can this theory be used to go back in time.

[Jignesh Malavia]

Originally posted by Anupam Sinha:

Can someone explain the theory of relativity in smple words
If we measure something, then the measurements are relative to from where we are measuring and what is used to measure. Simplest example: the speed of a moving train can be measured using a simple watch and observing two points on the tracks. But if the train was moving at speeds close to that of light, then the observer will not be able to measure those speeds acurately just by observing with his eyes because his 'view' is dependent on the light travelled from that train to his eyes. He will start and stop the timer when he 'sees' the train passing those points on the tracks.
Can this theory be used to go back in time.
There's an old limerick about it
There was a girl named Ms. Bright
Who could travel faster than light
She set out one day
The Einsteinian way
And returned the previous night!

[SJ Adnams]

google for "entangled particles"

[Michael Morris]

Originally posted by Anupam Sinha:
I would like to know what exactly is the theory of relativity. I tried a bit of googling and found some interesting sites. But they were full of maths formulas. Can someone explain the theory of relativity in smple words. All I know about this is when you travel close to the speed of light then the time you have actually taken is more/less than the time at say some other place. How can this happen. Can this theory be used to go back in time.

Here is an easy to understand article on Einstein's Special Theory of Relativity.
My understanding (take this with a grain of salt, I am no quantum physicist) is that time regression (going back to the past) is not possible because of the fact that no object can travel faster than the speed of light. Mass, time and space are all affected by velocity. At the speed of light, mass is infinite (hence the reason that you cannot travel faster than the speed of light), space or physical distance is zero, and time ceases to elapse. Time progression (future travel) can be accomplished, at least the appearance of time progression to outside observers, if the time traveler can muster speeds close to the velocity of light because time elapses more slowly as velocity increases relative to outside observers. Here is a link showing a Flythru of a ship accelerating to very near the velocity of light and showing the difference between the ship's clock and outside observer's clock.

[Greg Harris]

i read a paper about an experiement that was conducted a couple years ago. the scientists accelerated a beam of light through a tube of cesium gas and the light actually shined on the wall across the room "before it traveled through the tube..."
this, of course, is what michael is talking about with time progression.
this link goes to an article on CNN.com... not the scientific paper i read, but it give the idea.
[ July 02, 2003: Message edited by: Greg Harris ]

[Jim Yingst]

I would like to know what exactly is the theory of relativity.
One other thing to beware of - there are actually two theories of relativity. The first is the Special Theory of Relativity, and it is mostly about what happens when you travel really fast. It also contains the E = mc^2 formula. There's also the General Theory of Relativity, which deals with how gravity warps space and other fun stuff. That one's harder to understand, and irrelevant to Michael's "time and space" problem, so I'd skip it for now.

[Anupam Sinha]

Can this be an implication of the theory of relativity : You at the age of 27 decide to go on a space ship that travels quite close to the speed of light and when you have actualy travelled some distance you decide to come back. According to your watch the time elapsed is 1 day and by the earth's watch it is 1 year. Secondly if this is true it means that you will now actually be younger by a year - a day now.
So that means that when you should have been 28 years you are only 27 years and 1 day.
[ July 02, 2003: Message edited by: Anupam Sinha ]

[Jim Yingst]

Can this be an implication of the theory of relativity : You at the age of 27 decide to go on a space ship that travels quite close to the speed of light and when you have actualy travelled some distance you decide to come back. According to your watch the time elapsed is 1 day and by the earth's watch it is 1 year.
Yes.
Secondly if this is true it means that you will now actually be younger by a year - a day now.
Well, you're still older than you were before the trip. It's just you're less old than the poeple on Earth were expecting. You can't actually use relativity to get younger - you can only delay growing old.
So that means that when you should have been 28 years you are only 27 years and 1 day.
Right.

[David Hibbs]

Originally posted by Jim Yingst:
[b]One other thing to beware of - there are actually two theories of relativity. The first is the Special Theory of Relativity, and it is mostly about what happens when you travel really fast. It also contains the E = mc^2 formula. There's also the General Theory of Relativity, which deals with how gravity warps space and other fun stuff. That one's harder to understand, and irrelevant to Michael's "time and space" problem, so I'd skip it for now.

Actually, the latter theory (as above) is quite straightforward. To put it simply, "It all depends on your point of view." To put it not so simply, things all depend on what you're comparing them to.
Consider a car driving 90 MPH. (It's Texas; that's not unusual. :roll: ) If you stand still and watch the car go by, it's appears to be moving 90 MPH. On the other hand, if you're in your own car going 70 MPH (get in the right lane, slowpoke! ) , the same car appears to be moving 20 MPH.
Hence the problem with figuring out how fast the universe is expanding--Earth, the solar system, indeed the whole Milky Way Galaxy is moving. Everything else is moving, too, but all we can compare against is ourselves. But we don't know if we're standing still or moving 70 MPH, so all we know is the "relative" velocity.
So, simply, the basic (or general) theory of relativity can be summed up as "What's it related to?"
The special theory is much more complicated and involves complex mathematical formulae, and is what the problem relates to.
[ July 02, 2003: Message edited by: David Hibbs ]

[Michael Morris]

Originally posted by David Hibbs:

Actually, the latter theory (as above) is quite straightforward. To put it simply, "It all depends on your point of view." To put it not so simply, things all depend on what you're comparing them to.
Consider a car driving 90 MPH. (It's Texas; that's not unusual. :roll: ) If you stand still and watch the car go by, it's appears to be moving 90 MPH. On the other hand, if you're in your own car going 70 MPH (get in the right lane, slowpoke! ) , the same car appears to be moving 20 MPH.
Hence the problem with figuring out how fast the universe is expanding--Earth, the solar system, indeed the whole Milky Way Galaxy is moving. Everything else is moving, too, but all we can compare against is ourselves. But we don't know if we're standing still or moving 70 MPH, so all we know is the "relative" velocity.
So, simply, the basic (or general) theory of relativity can be summed up as "What's it related to?"
The special theory is much more complicated and involves complex mathematical formulae, and is what the problem relates to.
[ July 02, 2003: Message edited by: David Hibbs ]

Actually, I would have to agree with Jim. What you are describing is the Special Theory of Relativity which Einstein released in 1905. The General Theory deals with the complex subject of how space-time is affected by gravity. Specifically that matter warps space-time and that the universe (or space-time continuum) is curved. That theory was released in 1916.

[Jim Yingst]

The special theory is much more complicated and involves complex mathematical formulae, and is what the problem relates to.
Oh, GR has some nasty fomulas associated with it too - most of which are too complex to offer in any "intro to relativity" article. One of the simpler results though is for Gravitational time dilation. A decent light-mathematical overview of various parts of relativity can be found here. A sample of the math that's really involved in GR is here. Go on, click on some of those links on that last page - I dare ya.
[ July 03, 2003: Message edited by: Jim Yingst ]

[Mark Herschberg]

(Shouldn't this really be Meaningles Drivel and not Programming Diversions?)
It's been about 7 years since I was a physics major, but here goes.
Plenty of people gave you info on the basic concept, so I'll adress time travel. Is time travel possible? Yes, no, and maybe. When you look at the math, it does allow faster then light travel, and the equations suggest that if you do go faster then light, you will go back in time. Unfrotunately, moving past the light barrier (whether speeding up past it, or slowing down past it), doesn't seem practically feasible. Speeding up past it would require infinite energy.
There are experimental cases of quantum tunneling and similar work which push the limits of our theories. You can interpret some of these experiements as allowing time travel. Keep in mind, this is all happening at subatomic levels.
I recommend Faster Than Light: Superluminal Loopholes in Physics as a good, non-technical book on this issue.
--Mark

[Michael Morris]

Originally posted by Mark Herschberg:
(Shouldn't this really be Meaningles Drivel and not Programming Diversions?)
It's been about 7 years since I was a physics major, but here goes.
Plenty of people gave you info on the basic concept, so I'll adress time travel. Is time travel possible? Yes, no, and maybe. When you look at the math, it does allow faster then light travel, and the equations suggest that if you do go faster then light, you will go back in time. Unfrotunately, moving past the light barrier (whether speeding up past it, or slowing down past it), doesn't seem practically feasible. Speeding up past it would require infinite energy.
There are experimental cases of quantum tunneling and similar work which push the limits of our theories. You can interpret some of these experiements as allowing time travel. Keep in mind, this is all happening at subatomic levels.
I recommend Faster Than Light: Superluminal Loopholes in Physics as a good, non-technical book on this issue.
--Mark

I was wondering when you were going to weigh in on this Mark. Meaningless Drivel? Meaningless maybe but certainly not Drivel.

[Timothy Chen Allen]

Originally posted by Anupam Sinha:
I would like to know what exactly is the theory of relativity. I tried a bit of googling and found some interesting sites. But they were full of maths formulas. Can someone explain the theory of relativity in smple words. All I know about this is when you travel close to the speed of light then the time you have actually taken is more/less than the time at say some other place. How can this happen. Can this theory be used to go back in time.

This made me think about a puzzler I thought of when I heard that travelling close to the speed of light would make time on board the space ship move at a different rate than on the world you are coming from (or going to):
Let's say you are going from Earth to Relojus. Relojus is a planet one light year away from earth. It has a single moon which circles Relojus exactly one time in a year. The moon is on a plane such that the moon is always visible from Earth. Basically, if Relojus were a big clock, the moon would be its "year" hand.
You have really good eyes so you can see the moon all the time, and you're not much of a sleeper, so you watch that moon constantly. Your eyes are so good that you can see the people living on the moon. You have conveniently put up a clock on the wall next to the window, and the shipboard clock has a single hand that rotates the face once in a year. So you can see the "clock" of Relojus's moon, and the ship board clock at the same time.
You take off from earth, and the moon of Relojus is at the 12 o'clock position. You have synchronized the shipboard clock with the moon of Relojus. You watch both those clocks constantly during your journey. Your ship is travelling at 1/2 the speed of light (.5C), so for the people on Relojus, you will arrive when the moon has gone around the clock's "face" two times and is at 12 o'clock. But on board, your clock will register less time (what will the clock read on board the space ship?)
What will this all look like from within the space ship? Will the Relojus "clock" appear to run faster than the shipboard clock? Will the on-board clock seem to go at a regular rate? Will your looking at the two clocks have some effect on how fast they run? If you had really, really good eyes and could see people living on the moon of Relojus, would those people seem to be living faster than normal, like a sped-up movie?
I haven't a clue about any of these questions, but Anupam Sinha's question about relativity made me remember this and wonder. Thanks in advance.

[R K Singh]

There is very good book "A Brief History of Time � from Big Bang to Black Holes " By Dr. Hawkins
Its language is very simple and its has explained some complex physical theory like relativity in a very interesting and simple language.

[Joel McNary]

I would think that you would see the moon revolve three times around Relojus. When you are taking off from Earth, you are seeing the moon as it was 1 year in the past (in absolute, universal time terms). So on your travel, it will take two revolutions of the moon to get there, and you will see the missing revolution that represents the light-travel-time-differential thingie (technical terms here) between Relojus and Earth. Of course, this is based on my limited understanding of the theory and I could be off base here....

[Timothy Chen Allen]

Originally posted by Joel McNary:
I would think that you would see the moon revolve three times around Relojus. When you are taking off from Earth, you are seeing the moon as it was 1 year in the past (in absolute, universal time terms). So on your travel, it will take two revolutions of the moon to get there, and you will see the missing revolution that represents the light-travel-time-differential thingie (technical terms here) between Relojus and Earth. Of course, this is based on my limited understanding of the theory and I could be off base here....

Joel, you just hit on something I had not really thought about-- of course, the light from the moon of Relojus is a year old when the ship takes off. And the light received by the ship gets progressively "younger" as the ship approaches Relojus, until it is almost real time when the ship reaches the planet. Wow, what a weird thought experiment. Good catch.

[Jim Yingst]

I think this really is MD more than PD. PD may have slowed down, but I don't think we want it to be MD2. We've had a lot of non-programming puzzles lately - and this thread wasn't even a puzzle. Accordingly, I'm moving it to MD.

[Jim Yingst]

Regarding the most recent questions, there are two effects going on here. There's an apparent speed-up effect as you approach Relojus and observe it - you move from viewing old light to viewing new light, and as your info becomse progressively more up-to date, it looks like the poeple of Relojus are moving faster. However time dilation is another effect independent of this. Let's pretend that you had the magic ability to view Relojus in "real time", not limited by the speed of light. Or more realistically, pretend that you made detailed recordings of your observations, and afterwards you run them through some nifty programs that correct for the time it took the light to get you (which varied as you got closer) and now you're looking at a reconstructed view of what happened in "real time". In this case - you still observe the time dilation effect. I will now respond to Tim's questions assuming we've already corrected for the delay in light reaching us.
What will this all look like from within the space ship? Will the Relojus "clock" appear to run faster than the shipboard clock?

Yes
Will the on-board clock seem to go at a regular rate?
Yes.
Will your looking at the two clocks have some effect on how fast they run?
No. (You're probably thinking of quantum mechanics here; that's another discussion.)
If you had really, really good eyes and could see people living on the moon of Relojus, would those people seem to be living faster than normal, like a sped-up movie?
Yes. Specifically, as your ship starts to accelerate, you would see Relojus start to speed up. While you maintain a constant high speed, Relojus will look like it's in constarnt fast-forward mode. And when you decelerate later, you see Relojus gradually slow down to normal speed.

[John Smith]

Can this theory be used to go back in time.
From what I remember, the special theory of relativity denies the possibility of moving back in time, while moving forward in time is an everyday experience that was proven scientifically (two atomic clocks synchronized on Earth, and then one clock is put on board of the plance; upon return, the atomic clock on the plane is behind).

[John Smith]

JY: Go on, click on some of those links on that last page - I dare ya.

So, does this mean that the Universe is a friendly place?

[Mark Herschberg]

Originally posted by Eugene Kononov:
Can this theory be used to go back in time.
From what I remember, the special theory of relativity denies the possibility of moving back in time, while moving forward in time is an everyday experience that was proven scientifically (two atomic clocks synchronized on Earth, and then one clock is put on board of the plance; upon return, the atomic clock on the plane is behind).

This is incorrect. The equations of special relativity certainly allow time travel. It's just that achieveing it, as I noted earlier, is, under the laws of special relativity, practically speaking impossible, for slower-than-light objects such as ourselves.
Tachyons are faster the light particles. They can be interpreted to be moving backwards in time. SR very much allows them; we just haven't found them yet.
--Mark

[John Smith]

Tachyons are faster the light particles. They can be interpreted to be moving backwards in time.
So, if I send a tachyon to the past and program it to kill my father, what would happen to my grandchildren?

[Mark Herschberg]

Originally posted by Eugene Kononov:
So, if I send a tachyon to the past and program it to kill my father, what would happen to my grandchildren?

That's a good question; no one knows.
It may be that even if tachyons exist, we can't interact with them.
It may that even if things can go back in time, they can't be used to signal. We know that things do travel faster then light, it just doesn't transmit information.
Even if you can kill him, there are all sorts of questions about how it may effect your reality and/or alternative universes.
Physicists, philosophers, theologians, science fiction writers, and people from all walks of life have debated this.
--Mark

[Michael Morris]

Originally posted by Mark Herschberg:

That's a good question; no one knows.
It may be that even if tachyons exist, we can't interact with them.
It may that even if things can go back in time, they can't be used to signal. We know that things do travel faster then light, it just doesn't transmit information.
Even if you can kill him, there are all sorts of questions about how it may effect your reality and/or alternative universes.
Physicists, philosophers, theologians, science fiction writers, and people from all walks of life have debated this.
--Mark

Even without tachyons, is it not theorectically possible to warp space-time in the negative direction due to the Casimir effect to produce a wormhole? If so, time travel into the past would be possible.

[Thomas Paul]

Originally posted by Eugene Kononov:
So, if I send a tachyon to the past and program it to kill my father, what would happen to my grandchildren?

Tachyons are pacifists.

[Jim Yingst]

Tachyons are pacifists.
Except for a few fringe groups which for years have been demanding official recognition as particles coequal with luxons and tardons. They've been increasingly frustrated at the denial of their right to interact with other particles, and have grown impatient with the inertia of the tardon community in particular. Some of these tachyons just might be willing to assist Eugene's attempted patricide as a means of attracting attention to their causality.

[Michael Morris]

Originally posted by Jim Yingst:
Tachyons are pacifists.
Except for a few fringe groups which for years have been demanding official recognition as particles coequal with luxons and tardons. They've been increasingly frustrated at the denial of their right to interact with other particles, and have grown impatient with the inertia of the tardon community in particular. Some of these tachyons just might be willing to assist Eugene's attempted patricide as a means of attracting attention to their causality.

Good point Jim as the overachieving tachyons consider themselves much superior to the other subatomics listed, especially the tardons which the tachyons call the tards in a diminutive tone.

[Nirvan Sagar]

Originally posted by Eugene Kononov:

So, if I send a tachyon to the past and program it to kill my father, what would happen to my grandchildren?

Is there evidence of people being visited by "people from the future".?

[Bert Bates]

Old Albert himself wrote a pretty good little book on some of these topics. (A great example of complex topics made accessible.) I think he called it 'Relativity' or some such random title.

[Richard Hawkes]

Originally posted by Nirvan Sagar:
Is there evidence of people being visited by "people from the future".?
These dramatised documentaries are quite convincing though some of the evidence is debatable.