# The Naked Scientists Forum

### Author Topic: How will time change with velocity?  (Read 1655 times)

#### thedoc

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##### How will time change with velocity?
« on: 08/08/2013 04:30:01 »
matthew deeds  asked the Naked Scientists:

I have been thinking about how as you approach the speed of light time slows down. I would agree  that the way you perceive events around you would change and possibly look like  they are going back in time but I am not convinced that you will actually go back in time. I know that this is a  trivial thing and I am probably thinking to simplistically about this problem but here it goes. (I have not gone to
the trouble of working out all of the math but it appears logical to me)

Thought Experiment on the Conservation of Time

We are going to undertake a thought experiment to see if we can travel backward or forward in time through the use of faster than light travel. We will need a few things for this experiment. First a craft that is capable of faster than light travel. Next we will need three clocks synchronized to the same time. Next we will need a display panel that is capable of instantaneously displaying a read out of the clocks over vast distances. Finally we will need two observers. To set the stage we will place two
of the clocks ten light years apart. We will call these two clocks clock A and B. Next we will place our vehicle half way between the two clocks. Now we will put the third clock in the craft and call this clock C. Observer one will then check the instantaneous display and verify that all of the clocks are synchronized. Then observer two will enter the craft with the clock. Observer one is seeing that all
three clocks are reading the same time. To observer two however clock C reads the present time and cocks A and B are reading five years in the past. This is because it takes the light emitted / reflected from the clocks five years to reach observer
two. Now we can set things in motion. Our craft is going to travel towards clock A and away from clock B at an average speed of two times the speed of light. As observer two accelerates away from clock B it will appear to slow down because it takes the light longer to reach the craft due to both the increasing distance and the acceleration. When the craft reaches the speed of light the clock will appear to stop because observer is now traveling at the same speed as the light and there for no new data is reaching them. As the craft breaks the speed of light the clock will appear to run in reverse because observer two is traveling faster than the light and they are now able to observe things that they have previously observed due to re receiving the old data had already passed the starting point of the journey. So far it
appears that we may be traveling back in time. Now let us see what is happening to clock A from the point of view of observer two. As the observer accelerates toward clock A it would appear to speed up and when the speed of light is reached the clock would appear to be moving forward at twice the normal rate. This is because the
observer is closing the distance with the light and twice as much of the inputs
would reach them. As the craft increases in speed past the speed of light the clock would continue to appear to move forward in time at an even more accelerated rate.
When our craft reached clock A what would observer two see? The trip would
have taken 2.5 years so the clock on board the craft would read 2.5 years more
than at the beginning of the experiment. Clock A would now display the same time as clock C. It would appear to observer two that clock A moved forward a total of 7.5 years. Now clock B would appear to be 10 years behind clock A and C and seemingly ran back 2.5 years. To observer one looking at their screen all three clocks still agrees and read the same time. This is because of the instantaneous display that observer one is using. Observer two has to rely on their perspective and therefore registers a perceived change in the speed of time passing as recorded on clock A and B. This perceived change would just be a function of the speed that observer two is traveling at, the distance traveled and the speed that the light is traveling at to reached observer two. Observer one is receiving an instant readout
and therefore receiving the times on each of the clocks as it is displayed just
as if they were standing right next to all three clocks. If observer two gets back in the craft and travels back to the starting point the same things will happen in reverse
and when the trip is over another 2.5 years later. Clock C will read five years more than its starting point. Clocks A and B will again read five years behind clock C. The only change in time / perceived time during the entire trip for either observer one or two will be the five years of time that it took the craft to travel from the starting point to clock A and back.

From This thought experiment we can conclude that any perceived change in the speed of time (forward or backward) by traveling the speed of light or faster will be offset by the return trip. It is my opinion that it will take more than simply traveling the speed of light to travel back in time.
What do you think?
« Last Edit: 08/08/2013 04:30:01 by _system »

#### lightarrow

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##### Re: How will time change with velocity?
« Reply #1 on: 08/08/2013 12:00:54 »
I have been thinking about how as you approach the speed of light time slows down.
It doesn't. Sorry for the telegraphic answer but we have already discussed this infinite times in this forum and you can find with a search as many threads as you want about it.

#### Bill S

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##### Re: How will time change with velocity?
« Reply #2 on: 09/08/2013 00:50:53 »
Matthew, I am responding to this, not because I am a mathematician or a scientist – far from it, nor because I think that lightarrow has not given you the best advice, but because I think I recognise someone else who is trying to work through SR in much the same way I have been doing for quite a long time.

I think you may have overlooked a few points:

1. You are trying to apply relativity to a situation involving FTL travel, which is (in this sort of scenario) forbidden by relativity, so your results are bound to be suspect.

2.  Time dilation has been experimentally established as correct, and correctly described by relativity, so you would need to take it into account in your calculations.

3.  The capability to display, instantaneously, a readout of clocks over vast distances is wishful thinking.  It’s tempting to give in to the “what if” factor, but that is really best avoided in discussions with scientists.  :)

4.  Whatever the motion of the observer, light must be observed as travelling at “c”. Have you considered what effect this would have on your thought experiment?

5.  Have you considered what the outcome of your thought experiment would be if instead of travelling along a line joining A and B, your craft travelled the same distances in a direction perpendicular to that line?

#### lightarrow

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##### Re: How will time change with velocity?
« Reply #3 on: 09/08/2013 10:08:21 »
2.  Time dilation has been experimentally established as correct, and correctly described by relativity, so you would need to take it into account in your calculations.
Certainly, but this has *nothing* to do with the statement:

"as you approach the speed of light time slows down".

Do you want a prove? Here it is: My time flows as always, exactly in the same way, but in this moment I am travelling at almost light speed, with respect to some extremely energetic cosmic ray, and at the same time I'm obviously stationary, with respect to another frame of reference co-moving with me.

Time flows exactly in the same way independently of speed.

Time dilation is a consequence of the fact 2 different bodies makes different paths in spacetime between two events. But time for every of the 2 bodies flows exactly as before.

A metaphor.
Let's say you with your car A and you friend with his car B go from Rome to Paris, starting simultaneously at 00:00:00 (you have synchronized your perfectly identical clocks) from Rome. You travel *at the same exact constant speed*.

At the end you compare your clocks: your signs 15:00:00, the one of your friend signs 13:00:00.

Has time flowed differently for you and your friends?
Or is it, maybe, that you have taken two different paths to go to Paris?

--
lightarrow

#### Pmb

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##### Re: How will time change with velocity?
« Reply #4 on: 09/08/2013 14:17:12 »
Quote from: thedoc

I have been thinking about how as you approach the speed of light time slows down.
Please note that time is relative. This means that someone traveling with a moving clock will not observe it to be moving slowly but at a normal rate. Its only when two clocks in relative motion are compared does one appear to be running slow relative to the other.

Quote from: thedoc

I would agree  that the way you perceive events around you would change and possibly look like.
This has nothing to do with perception but what can be measured.

Quote from: thedoc

. they are going back in time ..
That is incorrect. Motion slower than the speed of light does not mean that anything is going back in time.

Quote from: thedoc

but I am not convinced that you will actually go back in time.
That’s good because special relativity makes no such assertions.

Quote from: thedoc

To observer two however clock C reads the present time and cocks A and B are reading five years in the past.
That is incorrect. Observer two must determine how far the light has traveled and then correct for it in order to determine when the events he’s “seeing” happened. If he “sees” the other clock reading five years in the past then all he knows is that the light left the clock five years ago.

Quote from: thedoc

Our craft is going to travel towards clock A and away from clock B at an average speed of two times the speed of light.
This requires the observer to move in an altered spacetime since no observer in an unaltered spacetime can move at or faster than the speed of light. If you’re making such an assumption then you’re assuming that physics as we know it is wrong and as such anything goes. You’ll then have to  And for this reason I’ll stop here.

#### The Naked Scientists Forum

##### Re: How will time change with velocity?
« Reply #4 on: 09/08/2013 14:17:12 »