Physics

Haha this is the entire premise of the tv show "Andromeda". has anyone seen that?

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Originally posted by Gogita
Actually gentlemen, lets not forget gravity. Gravity is said to play a major role in the perception and passage of space/time. If an observer was in high orbit around the earth for say 10yrs. He will be a few milliseconds younger than an observer residing on Earth for the same period. Very accurate atomic clocks can pick up the time dilations even when only a few metres off the ground. The difference of course is only measurable in hundredths of a nanosecond. This can be explained since the Earths gravity weakens the farther you are from the surface.

Having said this, it is possible then to time travel many hundreds or thousands of years into the future by approaching a very dense object, it is said that a nuetron star would probably shave off a few hours if you remained in close orbit for several days. Of course the gravimetric forces would be incredible. On the other hand a black hole would be the ultimate vehicle for time travel. Lets say for arguments sake, a pilot could survive approaching a black hole, it is said that even a few seconds exposure could apparently throw the hapless traveller many thousands or millions of years in the future.

Of course "throw" isn't an accurate description, it is more like allowing the passage of time to almost freeze for the test subject, and the closer one approaches the event horizon the slower time passes, until it almost seems that time stops, by which time it is possible for the person to experience eternity itself, if he isn't crushed by the indescribable gravimetric forces.

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Originally posted by elimc
This may be a simple problem but it has been bugging me. If you make a car that can travel at .9999999999999 the speed of light (which 1986 Ford Tauruses can do) and you turn on your headlights, will you be able to see the headlights in front of you?

If the answer is yes:
Then wouldn't this violate the Law of General Relativity?

If the answer is no:
This would mean that the speed of light is relative. In that case, how can we observe the speed of light without it breaking the Law of General Relativity since earth is moving at half a million miles per hour.

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*Creative Statement*

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I would say that you would see the light reflected in the cosmic dust ;)

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Originally posted by russ_watters


Actually, gravity slows time - so the affect of being in orbit would increas the rate of the passage of time due to less gravity. However, speed is the overriding factor, making the net time rate decrease.




You probably understand it, but its not clicking. I highly recommend you read A Brief History of Time - Hawking explains this stuff very well (though it can still take a couple of reads to make it click). I have mine here....

Correction: The Michaelson/Moreley experiment was carried out in Cleveland (in 1887).

The idea of distance being a variable is easy though (maybe too easy). If you play ping pong on a train, how far is the ball moving when it crosses the net? That depends on your frame of reference. For the players, it might move 8 feet. For an observer on the gound, it might move 80 feet.

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nice points, and the reference frame is always important to remember.;) I was just wondering though if you had anymore information on this experiment that was conducted. Sonds really interesting. I will look on google a bit, but I was wondering if there was a book or anything that would talk more about the results obtained or any noteworthy conclusions made. Thanks

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I was just wondering though if you had anymore information on this experiment that was conducted. Sonds really interesting. I will look on google a bit, but I was wondering if there was a book or anything that would talk more about the results obtained or any noteworthy conclusions made. Thanks

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Well, the results and conclusions are the simple part - quite simply the Michaelson-Morely experiment is the foundation on which Einstein built Relativity. A quote from A Brief History of Time:

"In 1887 Albert Michelson (who later became the first American to receive the Nobel Prize for physics) and Edward Morley carried out a very careful experminet at the Case School of Applied Science in Cleveland. They compared the speed of light in the direction of the earth's motion with that at right angles to the earth's motion. To their great surprise they found they were exactly the same."

In essence, this means that unlike the ping-pong ball on the train, the speed of light does NOT depend on your inertial frame of reference. The explanation of why is relativity.

A google search for "michelson morley experiment" returns tons of good results. A few exerpts from the first(excellent article):

http://galileoandeinstein.physics.vi...michelson.html

"Detecting the aether wind was the next challenge Michelson set himself after his triumph in measuring the speed of light so accurately. Naturally, something that allows solid bodies to pass through it freely is a little hard to get a grip on. But Michelson realized that, just as the speed of sound is relative to the air, so the speed of light must be relative to the aether. This must mean, if you could measure the speed of light accurately enough, you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind, and the difference of the two measurements should be twice the windspeed. Unfortunately, it wasn't that easy. All the recent accurate measurements had used light travelling to a distant mirror and coming back, so if there was an aether wind along the direction between the mirrors, it would have opposite effects on the two parts of the measurement, leaving a very small overall effect. There was no technically feasible way to do a one-way determination of the speed of light.

At this point, Michelson had a very clever idea for detecting the aether wind...[swimmer in a river analogy omitted for brievity]...

Michelson's great idea was to construct an exactly similar race for pulses of light, with the aether wind playing the part of the river. The scheme of the experiment is as follows: a pulse of light is directed at an angle of 45 degrees at a half-silvered, half transparent mirror, so that half the pulse goes on through the glass, half is reflected. These two half-pulses are the two swimmers. They both go on to distant mirrors which reflect them back to the half-silvered mirror. At this point, they are again half reflected and half transmitted, but a telescope is placed behind the half-silvered mirror as shown in the figure so that half of each half-pulse will arrive in this telescope. Now, if there is an aether wind blowing, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since one would have gone more upstream and back, one more across stream in general. To maximize the effect, the whole apparatus, including the distant mirrors, was placed on a large turntable so it could be swung around.

Michelson calculated that an aether windspeed of only one or two miles a second would have observable effects in this experiment, so if the aether windspeed was comparable to the earth's speed in orbit around the sun, it would be easy to see. In fact, nothing was observed. The light intensity did not vary at all. Some time later, the experiment was redesigned so that an aether wind caused by the earth's daily rotation could be detected. Again, nothing was seen. Finally, Michelson wondered if the aether was somehow getting stuck to the earth, like the air in a below-decks cabin on a ship, so he redid the experiment on top of a high mountain in California. Again, no aether wind was observed. It was difficult to believe that the aether in the immediate vicinity of the earth was stuck to it and moving with it, because light rays from stars would deflect as they went from the moving faraway aether to the local stuck aether.

The only possible conclusion from this series of very difficult experiments was that the whole concept of an all-pervading aether was wrong from the start. Michelson was very reluctant to think along these lines."

um....I dunno!?!?

I don't think we will ever truly know

great thread will read through it all when got time

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I believe that they have an extremely accurate clock in the International Space Station. Strangely enough, it is slower than a clock here on earth. The clocks were both set on earth at exactly the same time.

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anouther thing that may have been mentioned already i havnt read the hole thred yet is that gravity also has an effect on space time-so the greater a gravatational pull the faster time will travel.ie on the surface of earth time will travel faster than 100 miles up where3 the grational pull is weaker.thius is part of the theory of realativity i belive

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Originally posted by fozz
great thread will read through it all when got time
anouther thing that may have been mentioned already i havnt read the hole thred yet is that gravity also has an effect on space time-so the greater a gravatational pull the faster time will travel.ie on the surface of earth time will travel faster than 100 miles up where3 the grational pull is weaker.thius is part of the theory of realativity i belive

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I believe that's backwards :) gravity "slows" time not speeds it up.

Gravity slows time, but so does speed - so the net effect of orbiting the earth is a slowdown in time rate. Being further away from earth speeds it up, but moving fast slows it down more.

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I believe that's backwards gravity "slows" time not speeds it up.

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thats what i was thinking but that space station thing got he thinking backwards for some reason-should have thought a little bit before posting:(

Hey I got a question for you guys: If you have a perfectly smooth cube of some material (basically, a cubic "object;" the material doesn't matter to me), and you have another cube of something, and you place them next to one another in a vacuum, will they stick together? Will they effectively become a single object? (i.e. without a "weak spot" in the middle?) Assume perfection here -- it may not be humanly possible, but theoretically, what would happen? If you could point me towards a website with answers to these kinds of questions, that would be great, too. :)

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If you have a perfectly smooth cube of some material (basically, a cubic "object;" the material doesn't matter to me), and you have another cube of something, and you place them next to one another in a vacuum, will they stick together? Will they effectively become a single object? (i.e. without a "weak spot" in the middle?) Assume perfection here -- it may not be humanly possible, but theoretically, what would happen?

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In the absence of other forces, the gravitational attraction of the two objects would weakly (depending on their mass and density) hold them together. They would not become one object without something to bond them together - chemical bonds, welds, glue, duct tape, etc.

How about metal? I suspect two metal objects would become one...

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How about metal? I suspect two metal objects would become one...

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Why? the change in your pocket doesn't end up as a solid mass of metal, does it? Metal objects need to be chemically bonded together to become one object (a weld is chemical bonds).

I'm basically wondering what causes a single object to be a single object, as opposed to a bunch of dust. Two separated liquids of the same material will combine without any chemical bonds, right? So what makes solids different? All I can think of are microscopic imperfections on the surface, a "buffer" of air between objects, and maybe an oxidized surface layer. (My education on this stuff doesn't go beyond high school, so my understanding may be a bit weak ;)) Take water for example. If you take two ice cubes and put them together, they don't bond. But if you melt them down (just a layer of liquid on the surface may suffice) and refreeze them, they've become a single object. There aren't any chemical reactions going on there, right? What was preventing them from bonding when solid?

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Originally posted by russ_watters
Gravity slows time, but so does speed - so the net effect of orbiting the earth is a slowdown in time rate. Being further away from earth speeds it up, but moving fast slows it down more.

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It depends on what reference frame you're talking about. If you're talking versus the "proper" reference frame, then velocity, gravity etc do cause a dilation of various parameters including time and length. However, if you're discussing this versus your own reference frame, then there is no such dilation.

Let me give you an example. Say you're on Earth which in this case we call one reference frame (that is we'll discuss everything relative to the earth). Say a spaceship is moving towards the earth at some speed close to that of the speed of light. To the earthbound observer, the spaceship will appear to be shorter in length (length dilation) then to someone on the spaceship at rest relative to the spaceship (that is they're in the "proper" reference frame). This factor is gamma in the Lorentz transformation equations. Time is inversely affected by gamma.

However, to anyone in the same inertial reference frame, objects within that reference frame won't experience any relative dilations (time, length, velocity etc.). The only time you have any dilation is when you're comparing two reference frames which, relative to each other (or the observer's reference frame if they're in a third frame) are moving at speeds close to that of c (the speed of light), or there's one of a few key differences of a large magnitude between them.

What I'm (Einstein, Lorentz etc. rather ;)) basically saying is there is no such thing as absolute time, length, velocity, etc. You can only compare such attritbutes within your own reference frame without taking into account dilation effects. One must be careful so as not to assume there is an absolute standard of time.