Thread: Passage of time,the of speed light, and Reletivity

Originally Posted by Homo bibiens

Agree with Tom above, but just wish to emphasize - if a spaceship is travelling close to the speed of light (relative to us), we would observe that time has slowed down for the people on the spaceship. But for them, everything seems perfectly normal on the spaceship, relative to them, it is us moving (in the opposite direction) moving close to the speed of light, and they see time slowing down for us.

Tom references the twin paradox, and the resolution to that one has to do with the fact that one of the twins accelerates, and the other doesn't.

What happens when the twin decelerates? How do you tell whether it is acceleration or deceleration?

Originally Posted by Joe McCarron

How does speed affect the passage of time?

Time is space, Minkowski 1908. We call it the space-time continuum. Speed bends space making it appear slower. Space is curved in the presence of mass. Mass increases with speed and increases the curvature of space accordingly. When time bends the increased curvature makes it appear slower. If you go from Miami to Los Angelos through Texas it is faster. If you go through Canada it is slower. The straighter path is faster than the curved path.The faster you go, the more curved your path. This is an inverted simplification, but it fits nicely.

It is possible to show that applied force resulting in acceleration and near light speed may, by increasing mass, actually cause space to straighten and thus make time appear to slow. It is not a commonly held view. However, it provides a possible foundation for the theory of wormholes.

Originally Posted by Wayne Bruinekool

What happens when the twin decelerates? How do you tell whether it is acceleration or deceleration?

One needs use energy to accelerate. Deceleration is just form of acceleration in the opposite direction of motion.

f(x) = 10t - 2t^2
is decelerating when t = 2.

Originally Posted by tom

One needs use energy to accelerate. Deceleration is just form of acceleration in the opposite direction of motion.

f(x) = 10t - 2t^2
is decelerating when t = 2.

Agree completely, except that you might want to write f(t) instead of f(x) Just want to add, what constitutes deceleration is reference-frame dependent. If I am standing "still" inside a moving train, and then start to run towards the back of the train, from the perspective of people on the train, I have "accelerated" (I wasn't moving before, now I am moving towards the back of the train), but from the perspective of people standing on the side of the tracks watching the train go by, I have "decelerated", since I was moving at the same speed of the train before, now I am moving slightly more slowly, since my speed (relative to the train) is now subtracted from the trains speed.

Originally Posted by tom

One needs use energy to accelerate. Deceleration is just form of acceleration in the opposite direction of motion.

f(x) = 10t - 2t^2
is decelerating when t = 2.

Algebra; I haven’t seen one of those in forty years.
Are you sure?
(f(t),t); (8,1), dt = 8; (12,2), dt = 4; (12,3), dt = 0; (18,4), dt = 6; (0,5), dt = -18; If we define dt > 0 as acceleration then I get deceleration beginning at t = 4 for t > 0.

What do you mean by use energy? If you add X energy to twin A to accelerate to light speed and then add X energy to decelerate back to twin B speed, then A will be younger then B, but with 2X energy more then B.

If A has more energy at light speed and less energy at B speed, then wouldn’t we have to add X and then subtract X amount of energy? Wouldn’t we be adding energy to accelerate and subtracting it to decelerate? Then why wouldn't it balance out and return A to the same age as B as well as the same energy level?

Are you sayng that the time dialation is more dependant on direction than acceleration or speed?

Originally Posted by Joe McCarron

On almost every Discovery networks, History Channel, Or PBS show I see about outer space, they almost always go over relativity and light speed. They say that time is relative to the observer, and thy use the same stupid analogy on every show.

"Spaceman Bob goes the speed of light for one year, comes back, and earth has aged
50 years"

.......then they stop explaining why and say "Because time is relative, see?" Then they move on. Thats not a very good answer if you ask me. Leaving me on my couch, swearing at my TV. It's an example of relativity, sure, but one that opens the door for other questions.

I also saw a segment on a show(I think it was The Universe w/ Stephen Hawking on Sci channel) and they were talking to a guy that works at a detection station for particles that enter the earths atmosphere but get (destroyed/evaporate) within a thousandth of a second.(I don't think it was wimps) So, if they get destroyed so fast they would all be in the upper atmosphere and undetectable on the surface. But this guy shows that his detector is getting hits from these particles, and he says that it's because they're traveling at the speed of light, so time goes slower for them, so they decay slower......something along those lines. I like this example alot better.

I understand the concept of relativity.(not the math) I understand that as you approach the speed of light time slows down.(I think) But what I don't understand is what speed has to do with the space-time continuum. I don't know why time slows down for spaceman bob. I know that space-time can be warped by gravity/mass, but by speed as well? Can someone shed some light on this?

How does speed affect the passage of time?
Why does time seem to slow down?

Yes
Yes
It bends space
One way to measure time is by the length of the radius of the curvature of space. As speed approaches light speed, the radius of curvature becomes shorter and the curvature becomes tighter. As you approach light speed, mass approaches infinity, length approaches zero and the radius approaches zero. When the length equals the radius equals zero, time no longer passes.
Stick with Bob.
The detector guy is pulling your leg. A lot of the things that they say on TV is good for ratings only, but does not accurately reflect physics.

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Originally Posted by Homo bibiens

Tom references the twin paradox, and the resolution to that one has to do with the fact that one of the twins accelerates, and the other doesn't.

I've always wanted to see the math for the acceleration part of that.

Originally Posted by Wayne Bruinekool

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I've always wanted to see the math for the acceleration part of that.

The way to do that rigorously is not all that easy to follow, it requires you to go in and out of rindler space (see here), and if you're assuming instantaneous accelerations (as is usually done in the twin paradox) it gets even a bit more difficult to do right.

However, perhaps instead of the acceleration picture, might i suggest a different, but strictly speaking more correct picture.

You can think of each twin taking a different path through spacetime. Each path connects the same two events (the first event is when they are both at the same place at the start, the second event is when they are both at the same place when he returns).
Let's give a certain class of paths a name. Inertial paths will be called geodesics. Inertial means that you are not feeling any fictitious forces. For example, if you're sitting in a car that's simply moving, you don't feel any force. However whenever that car is accelerating (fast enough) you will feel a force "pushing" you backwards into your seat. Since we cannot distinguish between different inertial paths (is your car moving or is the earth underneath?), we could in a sense say that all geodesics are "equal".
Now we still need a measure of time. Let's call the time that a clock will read along a path the "proper time" of that path.
It can be proven that a geodesic is the unique path between any two events that maximizes proper time. In other words, along all the possible paths between any two events, there will be one unique geodesic. And the time a clock will read along that path will be more than along any other path.
Now back to our two twins. The twin that stays at home is inertial, and thus on a geodesic, in other words his clock will have the most time elapsed. The twin that goes away is on a geodesic all the way to his return point. However the geodesic would continue going straigth on, but he "jumps" on another geodesic coming back. It is this "jump" that makes his clock the one that "really" has slower time, since the path he took between the two events was not a geodesic.

It appears that everyone is in agreement that the speed of light is an atomic constant. Kind of like the flat earthers...just sayin. It wasn't long ago that the scientific community was convinced that the universe was infinite and that time was constant. What implications would a slowing speed of light have on modern science?

I'm not trying to be antagonistic, and forgive me if it appears that i am. I am just fascinated with this "little fish bowl" that we are all in together.

History of the Light-Speed Debate - eNews for March 23, 2010

The speed of light is just a mathmatical term in some regards. Speed is distance over time. The idea is that the max ratio ( for massless particles ) is c. This falls out of the formulae presented by einstein. Basically it says that time and distance stay proportionate to each other. If there is time dilation ... there must also be proportional spacial contraction. c is that ratio. Note :special relativity does not forbid the existence of particles that travel faster than light at all times.

Originally Posted by Blue Collar Know Nothing

It appears that everyone is in agreement that the speed of light is an atomic constant. Kind of like the flat earthers...just sayin. It wasn't long ago that the scientific community was convinced that the universe was infinite and that time was constant. What implications would a slowing speed of light have on modern science?

I'm not trying to be antagonistic, and forgive me if it appears that i am. I am just fascinated with this "little fish bowl" that we are all in together.

History of the Light-Speed Debate - eNews for March 23, 2010