I've been reading A Brief History of Time lately, and I have just a couple of questions(preferably someone who read the book and understood it well/knows a lot about the subjects will answer):

1) During one of the first chapters he explained that the observation of what time an event occurs is based on the velocity and location(?) of the viewer. I did not understand this at all, so hopefully someone could try to explain to me more clearly that this is true.

2) I'm not so sure I really understand what the uncertainty principle states. Could someone briefly explain it for me.

The uncertaintly principle is just what it sounds like. That physicists are uncertain about anything, and that speeds, distance, anything with physics cannot be measured perfectly, and are just a very, very close approximation.
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No - uncertainty principle shows that at any one point in time only one property can be measured, either the position or velocity. Nothing to do with approximation.
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No - uncertainty principle shows that at any one point in time only one property can be measured, either the position or velocity. Nothing to do with approximation.

ok...I just did this yesterday in physics class so maybe I misunderstood. Something along those lines
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For question 1, an easy explanation is an elevator. Imagine one viewer on the outside and one on the inside with a box on the ground. The elevator is traveling upwards from the ground at 10 m/s. To the person who is still at the ground floor, the box, the elevator and you are all travel at 10 m/s. To the person inside the elevator, the box doesn't have any velocity relative to the person- they are both moving in the same direction at the same velocity, so it appears as if the box isn't moving at all, but to the viewer on ground level it is. The velocity of the object can change depending on the perspective that the viewer takes on the object. That is the gist of it. Also, Dr Pain gave the correct simple answer to your second question.
Last edited by curtis15 at Jan 15, 2009,
Havn't read the book at all, but I assume the first part is based on the fact that light is not instant (300 000 000 m/s). Ie, it has to reach you before you can see what happened. For instance, a star (not ours) may be many light years away, so it could have burnt up already, but we still see the star there because it has to reach us first.
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For question 1, an easy explanation is an elevator. Imagine one viewer on the outside and one on the inside with a box on the ground. The elevator is traveling upwards from the ground at 10 m/s. To the person who is still at the ground floor, the box, the elevator and you are all travel at 10 m/s. To the person inside the elevator, the box doesn't have any velocity relative to the person. The velocity of the object can change depending on the perspective that the viewer takes on the object. That is the gist of it. Also, Dr Pain gave the correct simple answer to your second question.
Ok, but how does that apply to time?

And lol, 10 m/s is a bit fast for an elevator.
Last edited by guitarhero_764 at Jan 15, 2009,
In a nut shell.

1) Think of relativity. If you are driving down the street at 30 mph, someone standing on the sidewalk sees you moving 30 mph. But someone else driving next to you at 25 mph in the same direction will only see you moving at 5 mph. Your speed is relative to who is watching you.

2) Its a quantum thing. You can only know either the velocity, or the position of a particle. Not both at the same time.
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1) Think of relativity. If you are driving down the street at 30 mph, someone standing on the sidewalk sees you moving 30 mph. But someone else driving next to you at 25 mph in the same direction will only see you moving at 5 mph. Your speed is relative to who is watching you.

Again, I don't see how that applies to time.
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Ok, but how does that apply to time?

And lol, 10 m/s is a bit fast for an elevator.

As you approach the speed of light, time slows for you.
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Ok, but how does that apply to time?

And lol, 10 m/s is a bit fast for an elevator.

My mistake, i misread your question a bit. Cowfish has the right explanation for what you meant. Light isn't instantaneous, so if one person gets hit by lightning, that person will say it happened at "x" time. but another person watching the person get hit will have to wait for the light to get transmitted from the struck person to the viewers eyes, which will mean the viewer will see the event at a different time than the person who got hit.
http://www.garlikov.com/teaching/time

we just started einstein's theory of relativity the other day and to be honest I don't really understand it. The main principle to remember is that any observation made in any frame of reference can be correct. But I really can't help much, try reading the article

edit:
Quote by curtis15
My mistake, i misread your question a bit. Cowfish has the right explanation for what you meant. Light isn't instantaneous, so if one person gets hit by lightning, that person will say it happened at "x" time. but another person watching the person get hit will have to wait for the light to get transmitted from the struck person to the viewers eyes, which will mean the viewer will see the event at a different time than the person who got hit.

like I said I am no expert, but I actually don't think that this is right. It has to do with einstein's theory of relativity, not the amount of time it takes light to travel from one point to another. Although your explanation is technically correct, the amount of time it would take for light to travel that distance is so small its basically inadmissible.

OP: As someone else mentioned, time 'dilates' or slows as you go faster, although for the difference to be measurable you have to be going extremely fast (ie close to the speed of light) this paragraph may explain the problem a little bit better

Bondi later describes the case of what is, or was, often called "the twin paradox" whereby one of two twins leaves earth in a rocket ship that constantly accelerates at the rate of 1 g (gravity) for 10 years, then reverses its acceleration direction, but at the same rate, for the next 20 years, so that it essentially comes to a halt relative to earth's velocity 20 years after it has left the earth, and then at the end of 30 years is hurtling back toward the earth at the great velocity it was leaving earth 10 years after it left -- i.e., at the point it reversed its acceleration. At that point, it again reverses the direction of its acceleration so that it can slow down to a soft landing by the time it reaches earth. Hence the twin in the rocket ship will be 40 years older. Bondi says that the time which has passed on earth, however, will have been 48,000 years by earth clocks/calendars. Hence, the twin left on earth would have been 47,960 years older than the twin in the rocket, had he still been alive.
Last edited by priest.fan. at Jan 15, 2009,
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Ok, but how does that apply to time?

Time is commonly regarded as an invariant quantity. It is assumed to carry on at a constant rate. This is not actually the case. Space and time are intrinsically linked, and therefore, time is dependent upon the speed at which you are travelling, and your mass. As you go faster, time slows down more in YOUR frame of reference. If you are in a rocket travelling at a significant proportion of the speed of light, you would experience only a fraction of the time that a person at rest, or moving at a slower speed would experience. Imagine you take off in this rocket at the age of thirty. You come back to earth after travelling at a significant proportion of lightspeed, at the age of what was to you, 50. In your frame of reference, 50 years have passed. On Earth, however, much more time will have passed.

So as you get faster, time gets slower. As you get closer to lightspeed, the change in time gets closer to zero. Hypothetically AT lightspeed, it stops. You would simply not experience time.

This effect also applies to gravity fields - the stronger the gravity field in, the slower local time flows, but that wasn't your question so I won't go into it.
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We didn't learn uncertainty principal in physics yet but we did it in chemistry when we were covering quantum mechanics and it states that you can never know both the position and velocity simultaneously, only one or the other i'm guessing it'd be the same for physics.
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The uncertainty principle says nothing about velocity. It says you can't know the momentum of a particle, and it's position. Momentum is a function of velocity, but also a function of mass, which changes at high speeds.
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My mistake, i misread your question a bit. Cowfish has the right explanation for what you meant. Light isn't instantaneous, so if one person gets hit by lightning, that person will say it happened at "x" time. but another person watching the person get hit will have to wait for the light to get transmitted from the struck person to the viewers eyes, which will mean the viewer will see the event at a different time than the person who got hit.
Yeah I know, but all you would have to do is take that into effect and then you could calculate the exact time. It isn't like light controls time, right?
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Yeah I know, but all you would have to do is take that into effect and then you could calculate the exact time. It isn't like light controls time, right?

We do take it into effect.The speed of light IS an invariant quantity. It does NOT change depending on your frame of reference. Time, however, does.
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It isn't like light controls time, right?

Well, not technically.
The speed of an object affects the frame of reference and therefore you see one time while the object experiences another.
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Think of it like this: An object travelling to a star, exactly 10 light years away, will reach there in 10 years when (theoretically) travelling at c (c=speed of light, but I'm not going to keep writing it out).

However, to anyone INSIDE the rocket, the chronometers would not show any change in time at all. For them, it's as if the rocket moved instantaneously from the point it achieved c to the point it decelerated.
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Yeah I know, but all you would have to do is take that into effect and then you could calculate the exact time. It isn't like light controls time, right?

no, listen to Lord Bishek, he has the correct explanation.
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Time is commonly regarded as an invariant quantity. It is assumed to carry on at a constant rate. This is not actually the case. Space and time are intrinsically linked, and therefore, time is dependent upon the speed at which you are travelling, and your mass. As you go faster, time slows down more in YOUR frame of reference. If you are in a rocket travelling at a significant proportion of the speed of light, you would experience only a fraction of the time that a person at rest, or moving at a slower speed would experience. Imagine you take off in this rocket at the age of thirty. You come back to earth after travelling at a significant proportion of lightspeed, at the age of what was to you, 50. In your frame of reference, 50 years have passed. On Earth, however, much more time will have passed.

So as you get faster, time gets slower. As you get closer to lightspeed, the change in time gets closer to zero. Hypothetically AT lightspeed, it stops. You would simply not experience time.
Ah, I get what you're saying. Thanks for that explantion.

This effect also applies to gravity fields - the stronger the gravity field in, the slower local time flows, but that wasn't your question so I won't go into it.

Actually, I would appreciate it if you would briefly explain this as well I know how it works somewhat, but I'm not entirely sure about it(still very new to physics).
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Think of it like this: An object travelling to a star, exactly 10 light years away, will reach there in 10 years when (theoretically) travelling at c (c=speed of light, but I'm not going to keep writing it out).

However, to anyone INSIDE the rocket, the chronometers would not show any change in time at all. For them, it's as if the rocket moved instantaneously from the point it achieved c to the point it decelerated.

But they would feel incredibly heavy and bloated.
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The uncertainty principle says nothing about velocity. It says you can't know the momentum of a particle, and it's position. Momentum is a function of velocity, but also a function of mass, which changes at high speeds.

since it's a function of velocity, isn't it entirely possible that you have some variable velocity (acceleration) meaning a variable momentum? I don't know, I'm not the best at this, I'm still studying basic mechanics

although in that case, with a variable velocity you'd have to consider impulse, I don't know, this part makes some sense, but at the same time, I'm not quite following it all.
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But they would feel incredibly heavy and bloated.

Just like the bastard child of Rosie O'Donnell and Michael Moore

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Ah, I get what you're saying. Thanks for that explantion.

Actually, I would appreciate it if you would briefly explain this as well I know how it works somewhat, but I'm not entirely sure about it(still very new to physics).

Um, I don't understand it well meself, to be honest, I'm an engineer, not a physicist. But I'm sure the wiki articles will be helpful
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The uncertainty principle says nothing about velocity. It says you can't know the momentum of a particle, and it's position. Momentum is a function of velocity, but also a function of mass, which changes at high speeds.

since it's a function of velocity, isn't it entirely possible that you have some variable velocity (acceleration) meaning a variable momentum? I don't know, I'm not the best at this, I'm still studying basic mechanics

although in that case, with a variable velocity you'd have to consider impulse, I don't know, this part makes some sense, but at the same time, I'm not quite following it all.

Yeah, you've essentially got it, but remember, uncertainty only deals with Quantum physics. In the real world, we just don't see uncertainty. But when you get down to quantum levels, and are discussing, say, electrons, then those particles usually travel at an appreciable fraction of the speed of light. Hence, their mass will change as well - considerably.

Now, I'm guessing you know the momentum equation in classical mechanics, p = mv.

But in relativistic mechanics, momentum is a bit more complex -

p = m0 x Gamma x v

V, again, is Velocity in this case.
But our m has become m0 - this is called the rest mass, and is the mass of an object completely at rest.

Now for Gamma - it's called the lorentz factor, and is given by this equation:

Gamma = 1 / Square root ( 1- (v^2/c^2))
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Think of it like this: An object travelling to a star, exactly 10 light years away, will reach there in 10 years when (theoretically) travelling at c (c=speed of light, but I'm not going to keep writing it out).

However, to anyone INSIDE the rocket, the chronometers would not show any change in time at all. For them, it's as if the rocket moved instantaneously from the point it achieved c to the point it decelerated.

The thing I don't get is, if time is not an "invariant quantity", then velocity is not an invariant quantity either, then the speed of light is not an invariant quantity either...
If you are travelling near the speed of light, then the "speed of light" is no longer the regular speed of light in your frame of reference because you preceive time differently, meaning you perceive velocity different, meaning the speed of light changes with your velocity too, meaning everything, except space, is variable....

How come it is not that way?
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The thing I don't get is, if time is not an "invariant quantity", then velocity is not an invariant quantity either, then the speed of light is not an invariant quantity either...
If you are travelling near the speed of light, then the "speed of light" is no longer the regular speed of light in your frame of reference because you preceive time differently, meaning you perceive velocity different, meaning the speed of light changes with your velocity too, meaning everything, except space, is variable....

How come it is not that way?

Other way round - SPACE is variable, or more accurately, SPACE TIME is. Now local velocities are affected by gravitational fields, because they usually occur in objects that have mass. Bizarrely, photons (and other force carriers, such as the hypothetical graviton, and the gluon) do not have any rest mass. I don't understand the mechanics of it myself, but this lack of rest mass is what enables the speed of light to be invariant.

Actually, speed of light is a misleading term. We usually name velocities after the stuff that travels - hence the airspeed of a plane is the velocity of the plane with respect to the fluid. This is misleading in terms of the speed of light which actually has nothing to do with light.

Speed of light should really be called speed-at-which-things-with-no-rest-mass-travel-at - since ALL of these travel at the same speed. Remember my spiel about stuff getting heavier the faster it got? The speed of light is the hypothetical limit for this speed - at the speed of light, any massive object will have infinite mass (hint: look at the lorentz factor. Assume lightspeed is 1. If v=1, then the Lorentz factor becomes 0).

Now clearly, the lighter your mass, the less energy required to accelerate you to a given speed. (E=mc^2)
So the lightest you can possibly get is...not having any mass at all. But light isn't the only thing to fall into this category - all EM radiation falls into it as well. Radiowaves, Infra-red radiation. In fact, photons, which are the particles of the EM spectrum are force carriers for the electromagnetic force. Two of the other four fundamental forces, the Strong Nuclear force and gravity both have "massless" particles (although the gluon is purely hypothetical at this stage, but we pretty much know gravity, at least works at c) - so these two particles also travel at lightspeed.
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Other way round - SPACE is variable, or more accurately, SPACE TIME is. Now local velocities are affected by gravitational fields, because they usually occur in objects that have mass. Bizarrely, photons (and other force carriers, such as the hypothetical graviton, and the gluon) do not have any rest mass. I don't understand the mechanics of it myself, but this lack of rest mass is what enables the speed of light to be invariant.

Actually, speed of light is a misleading term. We usually name velocities after the stuff that travels - hence the airspeed of a plane is the velocity of the plane with respect to the fluid. This is misleading in terms of the speed of light which actually has nothing to do with light.

Speed of light should really be called speed-at-which-things-with-no-rest-mass-travel-at - since ALL of these travel at the same speed. Remember my spiel about stuff getting heavier the faster it got? The speed of light is the hypothetical limit for this speed - at the speed of light, any massive object will have infinite mass (hint: look at the lorentz factor. Assume lightspeed is 1. If v=1, then the Lorentz factor becomes 0).

Now clearly, the lighter your mass, the less energy required to accelerate you to a given speed. (E=mc^2)
So the lightest you can possibly get is...not having any mass at all. But light isn't the only thing to fall into this category - all EM radiation falls into it as well. Radiowaves, Infra-red radiation. In fact, photons, which are the particles of the EM spectrum are force carriers for the electromagnetic force. Two of the other four fundamental forces, the Strong Nuclear force and gravity both have "massless" particles (although the gluon is purely hypothetical at this stage, but we pretty much know gravity, at least works at c) - so these two particles also travel at lightspeed.

What I meant with "variable" is that the speed of light or speed of massless things wouldn't be x (can't remember the value) everytime.
If you were to travel at the speed of light, and you looked backwards over your shoulder at a planet, the "light" would not have speed, or you would see everything standing still, since light can't reach you, etc...
If you were to travel near the speed of light, "light" would travel slower in your frame of reference or you would see things happening "slower" or something like that (unless there is a visible massless particle you can verify).

Hmm, the whole "gluon" thing and lorentz stuff goes over my head since I don't know anything about physics....

What I don't get is how "time" changes, is it time the only thing that changes or does light make it seem like it changes..
Also I don't quite get how the space-time is formed, is it only a frame that includes both of them, or are they stuck together for another reason? Light or mass?