Is it Possible to Travel Faster than the Speed of Light?

  • #101
I'll sum it up in one sentence for you:
You are wrong in your assumptions that measurements are valid outside the frame they were made and that velocities may be linearly added together. You have been given the correct measurements and litterature where you can read about the theory but have ignored both.
 
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  • #102
James S Saint said:
The physical impossibility of instant acceleration is not relevant. I could make the scenario where they backed off further and the ships merely took a little extra time to get up to speed. The question would turn out the same. It merely complicates the issue. How fast does a photon accelerate to get up to light speed when it leaves an atom? Certainly not instantly.

The acceleration wasn't the point. My post is still valid regardless of how they get up to speed.

Also, a photon ALWAYS travels locally at c.
 
  • #103
James S Saint said:
Not so. Both brothers (and all observers always) see themselves as "the rest frame". That is what relativity is about. There is no actual rest frame for them to assume that they are the one traveling.
What do you mean 'not so'? You just repeated back what I said.


The only measurements that make in this scenario is the 2Ls distance and the 1.333 secs duration.
According to what frame? Answer this or this thread is done.
They both read the same time change. Neither knows who moved except that they moved with respect to each other.
No one is talking about 'absolute' movement. All movement is relative.

Perhaps, from their perspective, one traveled at .9c and the other at .6c. Or perhaps one at .75 and the other at .75. They cannot know. They have nothing with which to measure such.

ALL they know is that there was 2Ls of distance and 1.333 secs later, there is none.
Again, according to what frame?
 
  • #104
Doc Al said:
What do you mean 'not so'? You just repeated back what I said.
Oh oh.. I'm sorry. I read your post as "Neither".. my mistake.

Doc Al said:
According to what frame? Answer this or this thread is done.
The INITIAL frame, which is also the FINAL frame. It is in that frame that both measurements, both initial and final time and distance, get measured by both parties.
 
  • #105
James S Saint said:
The INITIAL frame, which is also the FINAL frame. It is in that frame that both measurements, both initial and final time and distance, get measured by both parties.
OK, so what's the big issue? In the ground frame, the ships close at 1.5c. So?

Note that that says nothing about the relative speed of the ships when they are moving. If you wanted that speed, you can calculate it to be 0.96c.
 
  • #106
But now there is where things seem to get a little confusing. The distance of 2 Ls between us got reduced to 0 in only 1.333 secs. That means that he traveled a 2Ls distance toward me in only 1.333 secs. That is 1.5 times the speed of light.
No, your twin only traveled 1Ls, because you meet at the sign. So you calculate that both of you traveled at 0.75c.

How did you double the distance traveled ?

This is how it looks from the sign's frame. This diagram is accurately to scale. You can read off distances and times and work out the time on your clock T2 = t2 - x2 and divide by distance to get 0.96 !
 

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  • #107
Doc Al said:
OK, so what's the big issue? In the ground frame, the ships close at 1.5c. So?

Note that that says nothing about the relative speed of the ships when they are moving. If you wanted that speed, you can calculate it to be 0.96c.
What it says, because there are no "ground observers" (no absolute frame), the OBSERVED speed by the only observers present is 1.5c. Thus in the scenario pointed out, the observers, the travelers, would "see", by their measurements, that one of them traveled at 1.5c.

THAT is relevant.

The theory is that no such observation, measurement, could ever be made. "No observer will measure anything traveling faster than light."

But there is an even greater consequence.
 
  • #108
Mentz114 said:
No,
your twin only traveled 1Ls, because you meet at the sign. So you calculate that both of you traveled at 0.75.

How did you double the distance traveled ?
I am measuring the distance between the two travelers. Later in the thread, I removed the ground from the story all together leaving the only observers seeing only each other and thus measuring only that in 1.333 secs, a distance of 2Ls vanished. Thus they would measure 1.5c
 
  • #109
James S Saint said:
The INITIAL frame, which is also the FINAL frame. It is in that frame that both measurements, both initial and final time and distance, get measured by both parties.

You have taken a distance measurement and time measurement (2ls and 1.333s) in one frame (the ground frame, which you're now trying to forget) and assuming those distances and times are the same in other frames (the traveler's frame and his brother's). This is not true.

there are no "ground observers" (no absolute frame)
Having no ground observers is completely different to having no absolute frame. The ground is a perfectly valid reference frame, it's not absolute.
 
  • #110
James S Saint said:
What it says, because there are no "ground observers" (no absolute frame), the OBSERVED speed by the only observers present is 1.5c. Thus in the scenario pointed out, the observers, the travelers, would "see", by their measurements, that one of them traveled at 1.5c.

Nope. As has been explained, that's not the case.
 
  • #111
James S Saint said:
What it says, because there are no "ground observers" (no absolute frame), the OBSERVED speed by the only observers present is 1.5c.
Again that's the closing speed only in the ground frame!

Thus in the scenario pointed out, the observers, the travelers, would "see", by their measurements, that one of them traveled at 1.5c.
Nope! If they used measurement made while they were moving, they would measure their relative speed as 0.96c. If they just use the measurements made in the ground frame, their closing speed would be 1.5c. In no frame is anything moving greater than c.

The theory is that no such observation, measurement, could ever be made. "No observer will measure anything traveling faster than light."
Given the statements you've made in this thread, I don't think you're in a position to tell us what relativity says.

But there is an even greater consequence.
Beware: Garbage in = garbage out.
 
  • #112
I am measuring the distance between the two travelers. Later in the thread, I removed the ground from the story all together leaving the only observers seeing only each other and thus measuring only that in 1.333 secs, a distance of 2Ls vanished. Thus they would measure 1.5c

See the diagram I added. I think you are still miscalculating it by mixing frames.
 
  • #113
Doc Al said:
Again only in the ground frame!
They are BOTH in the ground frame when they take their measurements.


Doc Al said:
If they used measurement made while they were moving, they would measure their relative speed as 0.96c.
But they didn't
Doc Al said:
If they just use the measurements made in the ground frame, their closing speed would be 1.5c. In no frame is anything moving greater than c.
Those two statements are contradictory. In the ground frame where they made their measurements, they measured 1.5c (not supposed to be possible). But then you said, "In no frame is anything moving greater than c.".

Make up your mind.
 
  • #114
Well, this has been fun. I've got to go for now but, I'm definitely going to check back to see if:

A. My question from post 76 ever gets answered.
B. The thread gets locked.
C. James gets banned.
D. B and C.
 
  • #115
James S Saint said:
But they didn't

They did. You just can't/won't realize it.

James S Saint said:
Those two statements are contradictory. In the ground frame where they made their measurements, they measured 1.5c (not supposed to be possible). But then you said, "In no frame is anything moving greater than c.".

Make up your mind.

Closing speed =/= velocity
 
  • #116
Mentz114 said:
See the diagram I added. I think you are still miscalculating it by mixing frames.
I think you missed my last point. There are only two measurements (now). There is a distance of 2Ls between two people. 1.333 secs later, there is no distance between them. To both of those people, the distance between them vanished in only 1.333 secs yet they both know tht light could have only traveled that distance in 2 secs.

From their perspective, one of them traveled at 1.5c.
 
  • #117
James S Saint said:
They are BOTH in the ground frame when they take their measurements.
OK, then you admit that measurements are being made in the ground frame. Good!

Those two statements are contradictory. In the ground frame where they made their measurements, they measured 1.5c (not supposed to be possible). But then you said, "In no frame is anything moving greater than c.".

Make up your mind.
In the ground frame, the closing speed is 1.5c. That's obviously not the speed of the ships! That was given as 0.75c. Or have you forgotten already?

Please tell us what frame measures the speed of a ship to be 1.5c? Answer: No frame does!
 
  • #118
James S Saint said:
I am measuring the distance between the two travelers. Later in the thread, I removed the ground from the story all together leaving the only observers seeing only each other and thus measuring only that in 1.333 secs, a distance of 2Ls vanished. Thus they would measure 1.5c

If you understand logic then you must understand the limitations of abstractions.
Your assumption of 1.5 c is an abstraction based on a concept of distance /time that has no physical meaning. In any frame. No entity travels from your intial point to the other initial point i.e. 2 ls
It is no different that the faster than c sweep of a laser dot which also has no physical meaning because no single entity actually traverses the distance from the first point to the last.
 
  • #119
I'm just wondering if you're trolling everyone at this point. You're saying yourself that all your measurements are made from the ground's frame--that is, when everything is standing still before the experiment. You've been told again and again that from the ground's frame--from those measurements you took--you can conclude that the closing speed was 1.5c. You seem to think that means something, but it doesn't. All you've done is closed your eyes through part of the experiment, looked at the results from the ground's point of view, and come to the incorrect conclusion that one of you moved faster than light. This isn't what relativity talks about at all.
 
  • #120
Doc Al said:
In the ground frame, the closing speed is 1.5c. That's obviously not the speed of the ships!
Not obvious to whom?? The only observers measure 1.5c.

Doc Al said:
That was given as 0.75c. Or have you forgotten already?
That was the reality. But from that reality, we get the only observers seeing a 1.5c speed. There is no observer who stands on any ground and watches the event so as to tell the travelers how fast they were going.

Doc Al said:
Please tell us what frame measures the speed of a ship to be 1.5c? Answer: No frame does!
You JUST said yourself that the 1.5c is measured from the ground frame!? "In the ground frame, the closing speed is 1.5c." == 1.5c as seen by the ONLY observers.
 
  • #121
I think you missed my last point. There are only two measurements (now). There is a distance of 2Ls between two people. 1.333 secs later, there is no distance between them. To both of those people, the distance between them vanished in only 1.333 secs yet they both know tht light could have only traveled that distance in 2 secs.

From their perspective, one of them traveled at 1.5c.

The diagram is never wrong. Here it is from your frame. You calculate your twin's speed to be ~10.1/10.5 = 0.96, which is close enough considering the read-off error.
 

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  • #122
Closing speed is NOT A REAL SPEED OF ANY OBJECT.
 
  • #123
James S Saint said:
Not obvious to whom?? The only observers measure 1.5c.
No. The ground observers measure the speeds of the ships as 0.75c.


That was the reality. But from that reality, we get the only observers seeing a 1.5c speed. There is no observer who stands on any ground and watches the event so as to tell the travelers how fast they were going.
Huh? You started this thread by saying that the ships both moved at 0.75c with respect to the ground!


You JUST said yourself that the 1.5c is measured from the ground frame!? "In the ground frame, the closing speed is 1.5c." == 1.5c as seen by the ONLY observers.
Yes, it's a closing speed. It is not the speed of the ships with respect to any frame. It just measures how quickly they close the distance according to ground observers.

Nothing is moving at that speed!
 
  • #124
Austin0 said:
If you understand logic then you must understand the limitations of abstractions.
Your assumption of 1.5 c is an abstraction based on a concept of distance /time that has no physical meaning. In any frame. No entity travels from your intial point to the other initial point i.e. 2 ls
It is no different that the faster than c sweep of a laser dot which also has no physical meaning because no single entity actually traverses the distance from the first point to the last.
The theory concerns the ability to observe anything travleing faster than light. It is irrelevant as to whether anything actually was with respect to some absolute frame, because it is accepted that no absolute frame exists.

The ONLY frames that have any observer are the travelers. But the travelers make no measurement as they travel, they merely measure before and after and conclude a speed of 1.5c. There is no one else there to tell them how it occurred, thus THEY must perceive a 1.5c occurrence.
 
  • #125
James S Saint said:
Not obvious to whom?? The only observers measure 1.5c.


That was the reality. But from that reality, we get the only observers seeing a 1.5c speed. There is no observer who stands on any ground and watches the event so as to tell the travelers how fast they were going.


You JUST said yourself that the 1.5c is measured from the ground frame!? "In the ground frame, the closing speed is 1.5c." == 1.5c as seen by the ONLY observers.

But when you revert to their frames, their relative speeds are less than c. If there was no obserevr on the ground then no-one observes their closing speed to be 1.5 c.

What you're saying would be valid (well not valid, but in some way correct) for Gallilean relativity, but it just doesn't work that way for special relativity.
 
  • #126
Doc Al said:
Yes, it's a closing speed. It is not the speed of the ships with respect to any frame. It just measures how quickly they close the distance according to ground observers.
How fast a distance closes IS the speed.
 
  • #127
James S Saint said:
the travelers make no measurement as they travel


Which is exactly why you're getting a result according to the ground frame, where the closing distance is allowed to be 1.5c. If you come to the conclusion that someone moved faster than c from that then it's your own fault for using measurements from one frame in a different frame.
 
  • #128
James S Saint said:
The theory concerns the ability to observe anything travleing faster than light.
Right. And nothing does.

For some reason, you seem desperate to conclude that somehow that proscription has been violated by your ships. Not so.

I can only suspect some hidden 'philosophical' agenda.
 
  • #129
James S Saint said:
How fast a distance closes IS the speed.

Only when one object is stationary, but when you choose a frame when one of the objects is stationary their closing speed is less than c.
 
  • #130
If I understand James S Saint's 'logic' correctly he thinks that if an ultra-relativistic rocket reaches a star system 10 million light years away in one year on the rocket's clock the rocket's speed must have been 10 times the speed of light. If so, he clearly does not properly understand time dilation and length contraction.
 
  • #132
Thing to note here is that closing speed is invariant in gallielean relativity in inertial frames, but not in special relatvity, that's one of the many differences.
 
  • #133
jcsd said:
But when you revert to their frames, their relative speeds are less than c.
Only IF there was someone watching them travel. There wasn't. They have no way to know which of them traveled or how fast except to surmise that one had to be going faster than light to get to the other in 1.333secs.

jcsd said:
If there was no obserevr on the ground then no-one observes their closing speed to be 1.5 c.
THEY measured, from the "ground" (initial frame) a distance of 2Ls and a time (0). After the event, THEY measured a distance of 0 and a time of 1.333secs. Thus THEY both measured a distance of 2Ls reduced to 0 in 1.333 secs.
 
  • #134
jcsd said:
Only when one object is stationary, but when you choose a frame when one of the objects is stationary their closing speed is less than c.
From the frame of either observer, the distance of 2Ls vanished in 1.333secs = 1.5c

There is no "stationary".
 
  • #135
James S Saint said:
From the frame of either observer, the distance of 2Ls vanished in 1.333secs = 1.5c
From the ground frame only, the closing speed is 1.5c.

You are switching frames all over the place.
 
  • #136
Passionflower said:
If I understand James S Saint's 'logic' correctly he thinks that if an ultra-relativistic rocket reaches a star system 10 million light years away in one year on the rocket's clock the rocket's speed must have been 10 times the speed of light. If so, he clearly does not properly understand time dilation and length contraction.
The time dilation involved at merely .75c is minuscule to the 1.5c measurement realized. Neither travelers clock would slow by 25%.
 
  • #137
jcsd said:
Thing to note here is that closing speed is invariant in gallielean relativity in inertial frames, but not in special relatvity, that's one of the many differences.
"Closing speed" is the ONLY speed there is unless you accept an absolute frame.
 
  • #138
James S Saint said:
Only IF there was someone watching them travel. There wasn't. They have no way to know which of them traveled or how fast except to surmise that one had to be going faster than light to get to the other in 1.333secs.


THEY measured, from the "ground" (initial frame) a distance of 2Ls and a time (0). After the event, THEY measured a distance of 0 and a time of 1.333secs. Thus THEY both measured a distance of 2Ls reduced to 0 in 1.333 secs.

Stick to measurements tkaen in the same frame. The distance and time measured byu a gorund based observer is not the distance and time measured by our two traveling observers.

Relatvity is specifically constructed so that an object traveling at c travels at c in all inertial frames. One of the side effects of this is that objects traveling at less than c in some inertial frame always appear to be traveling at less than c in all other inertial frames.
 
  • #139
James S Saint said:
THEY measured, from the "ground" (initial frame) a distance of 2Ls and a time (0). After the event, THEY measured a distance of 0 and a time of 1.333secs. Thus THEY both measured a distance of 2Ls reduced to 0 in 1.333 secs.

They have to BOTH have been stationary in the ground frame in order to make that initial measurement. In order to reach 0 distance in 1.333 seconds, they both have to have traveled towards each other. This means they BOTH ACCELERATED. Acceleration is absolute in special relativity. The only way for them to meet up in 1.333s is to both accelerate- which they can objectively measure.
 
  • #140
James S Saint said:
"Closing speed" is the ONLY speed there is.

No when we talk about speed we usually mean dx/dt in some (let's assume inertial) cooridnate system and that is the speed we're talking about when we say it can't exceed the speed of light. If we use another definition of speed we may find what we have previously said not to be true about that definition which is what you're doing.
 
  • #141
Doc Al said:
From the ground frame only, the closing speed is 1.5c.

You are switching frames all over the place.
I am not switching frames AT ALL. THERE IS NO OTHER FRAME involved. What other frame are you thinking of??
 
  • #142
James S Saint said:
From the frame of either observer, the distance of 2Ls vanished in 1.333secs = 1.5c

There is no "stationary".

There is stationary when you're talking in the context of a single frame.
 
  • #143
James S Saint said:
The time dilation involved at merely .75c is minuscule to the 1.5c measurement realized. Neither travelers clock would slow by 25%.
The 'time dilation' factor at 0.75c is about 1.5 (not minuscule), but it has nothing to do with the 1.5c closing speed. At .96c, the relative speed of the moving ships, the time dilation factor is even greater: about 3.6.
 
  • #144
James S Saint said:
The theory concerns the ability to observe anything travleing faster than light. It is irrelevant as to whether anything actually was with respect to some absolute frame, because it is accepted that no absolute frame exists.

The ONLY frames that have any observer are the travelers. But the travelers make no measurement as they travel, they merely measure before and after and conclude a speed of 1.5c. There is no one else there to tell them how it occurred, thus THEY must perceive a 1.5c occurrence.

If the only measurements they make are before and after then the only measurements they actually have are there own times for their own travel from beginning to end. They have no observations of any kind regarding the other system while in motion. Therefore they can only derive their own velocity of .75c from actual observation.
You do understand that the observations from ground observers, if any , of 1.5 c closing speed has no physical meaning I hope?
 
  • #145
James S Saint said:
I am not switching frames AT ALL. THERE IS NO OTHER FRAME involved. What other frame are you thinking of??
You seem to want to state that the speed of one ship in the moving frame of the other is somehow 1.5c. Not so.

If you stay in the ground frame, then there's no problem. The ships move at 0.75c and they close at 1.5c. Done!
 
  • #146
jcsd said:
Stick to measurements tkaen in the same frame. The distance and time measured byu a gorund based observer is not the distance and time measured by our two traveling observers.

Relatvity is specifically constructed so that an object traveling at c travels at c in all inertial frames. One of the side effects of this is that objects traveling at less than c in some inertial frame always appear to be traveling at less than c in all other inertial frames.
You seemed to have missed a point somewhere. The only measurements made are in the "initial/ground" frame.

The measurement is made of 2Ls at t(0), travel is made. When they meet, they stop and look at their clocks. Both clocks read that they traveled for 1.333secs.

All measurements are from the initial frame.
 
  • #147
From the frame of either observer, the distance of 2Ls vanished in 1.333secs = 1.5c
Wrong. Look at the diagram. The time elapsed on the worldline clock is simple to calculate.
 
  • #148
James S Saint said:
I am not switching frames AT ALL. THERE IS NO OTHER FRAME involved. What other frame are you thinking of??

There are 3 frames involved here, the ground frame and the frame of each observer. Don't let the symmetry of the situation fool you these are 3 very distinct frames.
 
  • #149
Doc Al said:
You seem to want to state that the speed of one ship in the moving frame of the other is somehow 1.5c. Not so.

If you stay in the ground frame, then there's no problem. The ships move at 0.75c and they close at 1.5c. Done!
There is no measuring going on as they move. They take ALL measurements together at the beginning and end while they were not moving with respect to each other. Thus there is no frame changing involved in any measuring. But from the PERSPECTIVE of each, the other seems to have come to them at 1.5c.
 
  • #150
James S Saint said:
You seemed to have missed a point somewhere. The only measurements made are in the "initial/ground" frame.

The measurement is made of 2Ls at t(0), travel is made. When they meet, they stop and look at their clocks. Both clocks read that they traveled for 1.333secs.

All measurements are from the initial frame.

Okay, but in the ground frame, neither of the travellers is traveling at greater than c. We started by specifying that each was traveling at 0.75 C in opposite directions in that frame.

We cannot apply measurements made in one frame to another, so in order to work out what was going on in the other frames we'd either need to take new measurements in those frames or alternatively use the Lorentz transformation to convert our previous measurements into the other frames. And lo and behold when we do this we find that neither traveller obserevrs the other traveling at greater than c.
 

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