[Ignorant Enthusiast (OP)]

I was reading a book the other day (Why E = MC squared) and there was some talk about time dilation, namely that if you could build a spaceship that could travel at 99.99999999% the speed of light you could travel to the Andromeda galaxy in 50 years, while 3000000 years or so would pass on earth. And this made me wonder, if as you made the trip you could watch earth as you speed away from it, would everything on Earth appear to be moving in super fast forward? After all if your time is running 60,000 slower than earth's time wouldn't everything on Earth appear to be moving 60,000 times faster to you?

If that is the case, then consider the following, while you are making your trip to Andromeda scientist run an experiment on Earth that makes a car travel from 1 location to another at 10% the speed of light. If you on your spaceship observed this car, would it appear to you to be travelling at 6000 times the speed of light due to your own local time dilation?

If this is the case (that to us it appears to be travelling much faster than the speed of light) then could the same phenomenon explain things such as instant information exchange between pairs of quantum entangled particles? If the information that one particle passes to the other is travelling in a slower frame of reference than ourselves, but close the the speed of light, then wouldn't our time dilation make it appear to travel faster than the speed of light to us?

Or am I completely misunderstanding time dilation?

[Bill S]

As another "ignorant enthusiast" I would have to think about that for some time to have a chance of thinking of an answer, but the first thing that comes to mind is that observing this phenomenon would give the observer clear evidence that he/she was in motion.

[Ignorant Enthusiast (OP)]

True, but with a sufficiently powerful telescope it would be an observable phenomena, if not the effect of seeing something travel faster than the speed of light at the very least we would observe the earth complete 3000000 orbits of the sun in our 50 year trip, which again would confirm that we were the ones in motion.

Indeed time dilation should be an observable thing anyway, the ISS travels fast enough so that time dilation happens, albeit only a very small amount, but with suitably accurate atomic clocks (one on the ISS and one remaining on earth) then eventually the crew of the ISS should notice that their clock is running slower than the one on earth, telling them that they are travelling at a faster relative speed than the clock on earth.

[alancalverd]

Suppose you were travelling away from earth at "very close" to c. Say it takes you a year and a second to travel one light year. Then when you had reached 1 light year distant and looked back, you would see events that occured one second after you left, and after 2 light years' travel, events that occured a second later. So you wouldn't see time compressed, but expanded as you travel. When you reach your destination, you will see earth events in their normal time sequence but delayed by 50 years. But the moment before you land, you will see events that took place 50 years and 50 seconds ago.

Time dilatation is indeed observable and a relativistic correction is applied to the clocks used for GPS. 

[jeffreyH]

Say you are at 99.9% light speed and travelling away from earth. Assume for a moment that 'time dilation' (I hate that term) did not exist. The photons following you from events on earth would appear to move slower from your perspective and when measured would record a slower speed for light. However because all observers record the same value for the speed of light you must be 'slowed down' in order for this to be true. What actually happens is all particle reactions occur more slowly because of the energy trade-off with your momentum. So all the particles that make up you and your spaceship environment are reacting at a slower rate than those of people on earth. This does not only happen at velocities but also under the influence of gravitation. This is because gravity is a momentum potential.

[Ignorant Enthusiast (OP)]

Fair enough alancalverd, so what happens if you make the return journey back to Earth? Surely you would have to see events on Earth unfold in fast forward at some point? So if our scientists tested their superfast car on earth at that point, would we observe something that appears to be travelling faster than light?

[jeffreyH]

In travelling close to light speed your perceptions would slow so you wouldn't see things speed up. Your interaction with the universe would be slowed.

[Ignorant Enthusiast (OP)]

But surely jeffreyH, at some point things would have to speed up. If you have made the round trip to Andromeda in 100 years and 6,000,000 years have passed on Earth at some point, even if it's as you slow from light speed as you approach Earth things would have to seem to speed up wouldn't they? Using the orbit of the Earth around the sun as an example, if you spent all 100 years counting how many times Earth orbits the sun wouldn't you have to end up with 6,000,000 by the time you returned to Earth, after all that is what the people (assuming there are any left in six million years) on Earth have experienced, or does it not work like that? If the people on Earth have witnessed 6,000,000 orbits of the sun does our traveler at close to light speed who was watching the whole time not have to come up with the same figure?

[imatfaal]

In travelling close to light speed your perceptions would slow so you wouldn't see things speed up. Your interaction with the universe would be slowed.

Nope.  You are always at rest within your own frame - thus your own wrist watch, brain, bodily functions etc all proceed at a normal pace.  Whether due to gravitational potential or relative velocity - you never notice the change to yourself, only to those in other frames.  You can calculate the change that they would perceive in you - but never observe it.

Travelling toward an object at relativistic speed would blue shift (increase the frequency, decrease the time between peaks and shorten the wavelength) any em radiation  that you encountered.

[alancalverd]

Fair enough alancalverd, so what happens if you make the return journey back to Earth?

Use the same analysis. We leave Andromeda at time t=0 when we have just observed an event that took place on earth 50 years previously. After one year and a day we are at a point where we observe events that took place 49 years previously. That's the "blue shift".   

Now how do we know how fast the car was travelling? Our experimenter on earth will have told us the length of the test track, and will transmit a signal every second. To make it simple, let's say the car travels 100m in 100s. So we receive 100 time pulses, after which we see that the car has travelled 100m. The effect of the blue shift has indeed been to increase the frequency with which we perceive the time signals c ompared with our clock, but whilst the car may appear to have been travelling faster than 100m/s by our clock, it hasn't done so by his clock - and that is the one we use to measure speed.

[Bill S]

Nice one, I E!  This is the sort of problem I have wrestled with for years, but I have not considered this one.  I shall probably print out this thread to give the aging brain cells a chance.

[jeffreyH]

In travelling close to light speed your perceptions would slow so you wouldn't see things speed up. Your interaction with the universe would be slowed.

Nope.  You are always at rest within your own frame - thus your own wrist watch, brain, bodily functions etc all proceed at a normal pace.  Whether due to gravitational potential or relative velocity - you never notice the change to yourself, only to those in other frames.  You can calculate the change that they would perceive in you - but never observe it.

Travelling toward an object at relativistic speed would blue shift (increase the frequency, decrease the time between peaks and shorten the wavelength) any em radiation  that you encountered.

I do understand all that already. The poster was missing the point that light speed is constant. You would simply be passing through all the photons that had not caught up with you yet when on the return journey. You cannot break through the speed of light so no speeding up would occur from the traveler's frame of reference.

[jeffreyH]

But surely jeffreyH, at some point things would have to speed up. If you have made the round trip to Andromeda in 100 years and 6,000,000 years have passed on Earth at some point, even if it's as you slow from light speed as you approach Earth things would have to seem to speed up wouldn't they? Using the orbit of the Earth around the sun as an example, if you spent all 100 years counting how many times Earth orbits the sun wouldn't you have to end up with 6,000,000 by the time you returned to Earth, after all that is what the people (assuming there are any left in six million years) on Earth have experienced, or does it not work like that? If the people on Earth have witnessed 6,000,000 orbits of the sun does our traveler at close to light speed who was watching the whole time not have to come up with the same figure?

You can't outrun photons, that is simply a thought experiment. On your journey back you would just experience photons at the same rate you did when travelling away. If you COULD break light speed then theoretically, yes, you could see a speeding up but what sort of time dilation would this involve. Negative? However you can still only travel in a positive direction of time and never arrive back at earth before the first photon that set of with you.

[Pmb]

After all if your time is running 60,000 slower than earth's time wouldn't everything on Earth appear to be moving 60,000 times faster to you?

You would measure clocks on Earth to be running slower and observers on Earth would measure your clock to be running slower.

What most people miss when they're considering this problem what happens to the clocks on board the ship when it turns around. The Earth observers will determine the clocks to run slow depening on the speed of the ship. However the earth clocks will run at a rate which depends not only on the speed of the ship but also according to the position of the earth in the ships gravitational field. If the earth is high in the "apparent" gravitational field that exists in accelerating frames of reference then gravitational time dilation must be taken into account. When all these affects are considered then it all works out as described in the grandfather paradox.

[alancalverd]

However the earth clocks will run at a rate which depends not only on the speed of the ship but also according to the position of the earth in the ships gravitational field.

Are you sure of that statement? Suppose there are two ships travelling at different speeds with respect to earth. Which one determines the rate of the earth clock?

The essence of relativity is that no clock (or indeed anything else) knows that it is moving if it is not accelerating. You can only detect relative motion.

Those who claim that understood relativity, automatly acknowledge that they understood nonsense

Yet, remarkably, we dulwitted selfdelusionists manage to build particle accelerators, nuclear reactors, PET scanners, electron microscopes, Cerenkov detectors and GPS systems, to name but a few things, that actually work exactly as predicted by relativistic physics. Magic, just like the transit of Venus, computed by guesswork and observed by sheer luck. Long may the luck continue. 

[Pmb]

Are you sure of that statement?

Yes. And to make sure I was on track I talked to a GR expert or two and then found it described as such in a modern physics text. This isn't my first rodeo you know.

Note: I'm looking for that text right now. After I scan that portion into a PDF file would you like a copy of it?

[alancalverd]

If I read you correctly, you are saying that the frequency of an atomic clock depends on the speed with which things are approaching it. Thus two atomic clocks that are stationary with respect to one another, and initially in sync with each other, would lose sync if another body was to move between them.

The paradox here is that an observer moving from A to B would see a red shift of A and an equal blue shift of B with respect to his own clock, from which he would calculate that A and B remain in sync, yet the very fact of his presence and motion would destroy that synchronism. 

[imatfaal]

niebieskieucho's hijack regarding alternative theories which replace einstein's relativity has been moved to new theories.  please don't invade mainstream threads with speculative ideas - post those in new theories.  thanks

imatfaal

[jeffreyH]

If I read you correctly, you are saying that the frequency of an atomic clock depends on the speed with which things are approaching it. Thus two atomic clocks that are stationary with respect to one another, and initially in sync with each other, would lose sync if another body was to move between them.

The paradox here is that an observer moving from A to B would see a red shift of A and an equal blue shift of B with respect to his own clock, from which he would calculate that A and B remain in sync, yet the very fact of his presence and motion would destroy that synchronism.

You could argue that the relationships are basically pythagorean. But pythagoras on steroids.

http://www.science20.com/hammock_physicist/whats_wrong_relativity

[alancalverd]

But in my example A, B and the observer are all in a straight line. What PmB implied was that B would see A's clock blue-shifted simply because someone was travelling form A to B, which seems at the very least improbable.