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

[Borg]

I wasn't referring to HIM shining the light. I was referring to the fact that there was 2Ls between us and 1.333 secs later, there was no distance between us. Light couldn't do that.

Please explain what you think you see if he does shine a light when he crosses that point. Do you think that you would see his light traveling at 1.75 times the speed of light?

 

[Doc Al]

Do we agree that there is no absolute frame?

Right!

The ground merely serves as an initial means to measure the distance between us both before and after. It is NOT an absolute frame, right? It merely let's us have the same reference when we start and end.

Nothing absolute about the ground frame, that's for sure!

The fact is that we were 2Ls apart and merely 1.333 secs later, we were together by BOTH clocks. We can ignore the ground. Forget the ground.

What do you mean by 'forget the ground'? You gave distances and times that only make sense in the frame of the ground! Specifying a distance without mentioning the frame you are using is meaningless, right? (Unless you secretly do believe in some absolute frame!)

Speed is measured by distance divided by time => 2Ls/1.333secs = 1.5c

Again, that's the closing speed according to the ground frame. You can't 'forget the ground'.

 

[Bussani]

Forget the ground.

Speed is measured by distance divided by time => 2Ls/1.333secs = 1.5c

You can't say, "Forget the ground," and then use distances according to the ground.

 

[DrGreg]

Speed is measured by distance divided by time

Speed relative to a frame is measured by distance in the same frame divided by time in the same frame.

You are mixing frames.

 

[James S Saint]

Like Doc Al's pointed out, relativity doesn't forbid the distance between the two ships from receding at a faster than light rate according to a ground observer. That's nothing special.

Yes, but I'm not talking about what a ground observer sees. I see the distance of 2Ls get reduced to 0 in only 1.333 secs. That is what matters. Perhaps I cannot actually see the ground at all or even know it exists. As stated just prior, the ground only serves as an equal frame for us to begin and end. Both of us would end up seeing that 2Ls of distance vanished in only 1.333 secs.

Side question, if I may ask: who would reach the sign first according to all 3 observers ("you" in the vehicle, your brother in the oncoming vehicle, and a person standing beside the sign)? I assume if the person by the sign sees them reach it simultaneously, the other two won't agree with him.

All observers would see us meet at the sign. The ground person would see each of us traveling .75c toward the sign. We would each see ourselves approaching the sign at .75c. But regardless of what we might observe of each other, our end measurements would be that we reached each other in only 1.333 secs causing a 2Ls distance to be traversed.

 

[espen180]

I wasn't referring to HIM shining the light. I was referring to the fact that there was 2Ls between us and 1.333 secs later, there was no distance between us. Light couldn't do that.


Guys...

Do we agree that there is no absolute frame?

The ground merely serves as an initial means to measure the distance between us both before and after. It is NOT an absolute frame, right? It merely let's us have the same reference when we start and end.

The fact is that we were 2Ls apart and merely 1.333 secs later, we were together by BOTH clocks. We can ignore the ground. Forget the ground.

Speed is measured by distance divided by time => 2Ls/1.333secs = 1.5c

You are - whether you ralize it or not - working in the ground frame, in which the relative speed of the ships is only constricted to be smaller than 2c, and in which your result is correct. You cannot, however, extrapolate the measurement of the ground frame to the ship frame without using the Lorentz transformation equations, which will tell you that according to an observer on either of the ships, the other ship is approaching at .96c.

 

[HallsofIvy]

You cannot just assert "I can see and measure that he is approaching me at 1.5c". HOW would you "see and measure" that?

I would see and measure his speed as
\frac{.75c+ .75c}{1+ \frac{(.75c)(.75c)}{c^2}}= \frac{1.5c}{1+ 0.5625}= \frac{1.5}{1.5625}c which is 96% the speed of light.

And btw, that math reflects me traveling to the sign and him traveling from the sign in the same direction. That is not my scenario.

No, it doesn't. If you and he were traveling in the same direction, at the same .7t c, then it would be
\frac{.75c- .75c}{1+ \frac{(.75c)(.75c)}{c^2}}= 0, of course.

 

[James S Saint]

Again, that's the closing speed according to the ground frame. You can't 'forget the ground'.

No it is according to US.

We measure 2Ls of distance. In merely 1.333 secs, we measure NO distance between us. How is that not 1.5c?

 

[jcsd]

Yes, but I'm not talking about what a ground observer sees. I see the distance of 2Ls get reduced to 0 in only 1.333 secs. That is what matters. Perhaps I cannot actually see the ground at all or even know it exists. As stated just prior, the ground only serves as an equal frame for us to begin and end. Both of us would end up seeing that 2Ls of distance vanished in only 1.333 secs.


All observers would see us meet at the sign. The ground person would see each of us traveling .75c toward the sign. We would each see ourselves approaching the sign at .75c. But regardless of what we might observe of each other, our end measurements would be that we reached each other in only 1.333 secs causing a 2Ls distance to be traversed.

But think what happens in the ground observers frame when one of you shines a light at the other, the light, traveling at the speed of light, reaches the other before the pair of you meet.

Is the light therefore traveling at greater than 1.5 times the speed of light? That would be totally illogical as we started by assuming it travels at the speed of light.

 

[HallsofIvy]

Wow, 57 posts in less than 3 hours! That may be a record. James S Saint, you are doing all of your computations as if Newtonian physics applied. Naturally, you are going to get a contradiction to relativity.

 

[James S Saint]

To put it simply (I hope)

We measure 2Ls of distance.

1.333 secs later, we measure 0 distance.

How is that possible?

 

[Doc Al]

No it is according to US.

Who is "US"? You? Your brother? The ground? All three frames will measure different distances and times.

You say "US", but you are actually using ground frame measurements.

We measure 2Ls of distance. In merely 1.333 secs, we measure NO distance between us. How is that not 1.5c?

It is 1.5c, but only in the ground frame. You used distances and times in the ground frame, so the closing speed is in the ground frame.

 

[Austin0]

Like Doc Al's pointed out, relativity doesn't forbid the distance between the two ships from receding at a faster than light rate according to a ground observer. That's nothing special.

Side question, if I may ask: who would reach the sign first according to all 3 observers ("you" in the vehicle, your brother in the oncoming vehicle, and a person standing beside the sign)? I assume if the person by the sign sees them reach it simultaneously, the other two won't agree with him.

They would all agree on the simultaneity of the intersection. If they don't steer carefully they could headon and then it would be obvious they met simultaneously. Seriously though, all observers agree on local events at a single location. SImultaneity becomes relevant when events are separated spatially

 

[Doc Al]

To put it simply (I hope)

We measure 2Ls of distance.

1.333 secs later, we measure 0 distance.

How is that possible?

Where's the problem?

You are confusing a closing speed greater than c with something actually moving with a speed greater than c in some reference frame. But that's not the case.

 

[James S Saint]

Wow, 57 posts in less than 3 hours! That may be a record. James S Saint, you are doing all of your computations as if Newtonian physics applied. Naturally, you are going to get a contradiction to relativity.

Actually I'm tickled to find a forum where people are so active.

Now if I can just get them to follow logic... grin. :)

 

[jcsd]

To put it simply (I hope)

We measure 2Ls of distance.

1.333 secs later, we measure 0 distance.

How is that possible?

Becaus we're measuring the distance and time in the ground observers frame, where both of you are moving. Simples.

Imagine a 100m race. If someone runs 100m in 10 secs (assuming constant speed), but the finsihing linen approaches them at the same speed, do we say they ran 100 m in 5 secs? No we don't bceause we're talking about the frame in which both the runner and the finsihing line are moving,therefore the runner ran 50m in 5 secs.

 

[Borg]

Is the light therefore traveling at greater than 1.5 times the speed of light? That would be totally illogical as we started by assuming it travels at the speed of light.

Also, what about someone standing at the one second mark and in the signpost's reference frame. If your brother shines a light back at that observer after he passes, will that observer see light traveling at 1/4 the speed of light? No.

 

[Doc Al]

Now if I can just get them to follow logic... grin. :)

You abuse the term 'logic'. What you really are saying: "If I can just get them to agree with me." But many of your statements are incorrect!

It's not a problem with 'logic', but with your understanding of physics.

 

[James S Saint]

Where's the problem?

You are confusing a closing speed greater than c with something actually moving with a speed greater than c in some reference frame. But that's not the case.

The "problem" is that speed is ONLY measured by distance and time, agreed?

We (my brother and I - the only people involved) measured 2Ls of distance between us. It doesn't matter where we were. It doesn't matter if there was a ground. We could have simply met in space and slowly backed away to a distance of 2Ls. No distortions involved.

But 1.333 secs later, we are back where we started.

The "problem" is that we are measuring speed by time and distance which tells us that one of us traveled at 1.5c. We can't really know which one of us. But SOMETHING happened faster than light.

 

[James S Saint]

You abuse the term 'logic'. What you really are saying: "If I can just get them to agree with me." But many of your statements are incorrect!

It's not a problem with 'logic', but with your understanding of physics.

Trust me for a moment. You do NOT want to lecture me about what logic is or isn't. I am letting that go as it is a different thread topic.

 

[Doc Al]

You do NOT want to lecture me about what logic is or isn't.

 

[Borg]

I'm still waiting on your explaiination of how fast light travels when it is coming from your brother's ship. I would love to hear how light travels faster than light.

 

[Doc Al]

The "problem" is that speed is ONLY measured by distance and time, agreed?

Of course. But what you don't seem to be getting is that distance and time are with respect to some reference frame.

We (my brother and I - the only people involved) measured 2Ls of distance between us. It doesn't matter where we were. It doesn't matter if there was a ground. We could have simply met in space and slowly backed away to a distance of 2Ls. No distortions involved.

Sorry, still wrong. A distance of 2 Ls implies some reference frame. In this case, the ground frame.

But 1.333 secs later, we are back where we started.

Again, only with respect to that ground frame.

The "problem" is that we are measuring speed by time and distance which tells us that one of us traveled at 1.5c. We can't really know which one of us. But SOMETHING happened faster than light.

Nonsense. All it tells you is that with respect to the ground frame, the distances closed at a rate of 1.5c.

 

[James S Saint]

I'm still waiting on your explaiination of how fast light travels when it is coming from your brother's ship. I would love to hear how light travels faster than light.

I am trying desperately to keep this as simple as possible. The issue of how fast light might be going here or there is not really the issue.

Now i have even removed the ground from the scenario to simplify further. Someone please just answer this;

The "problem" is that we are measuring speed by time and distance which tells us that one of us traveled at 1.5c. We can't really know which one of us. But SOMETHING happened faster than light.

 

[Borek]

Assuming that just because one of them moves at 0.75c means the other can get faster than 0.25c seems wrong to me. Following this line of thinking, what happens to the light emitted by our Sun? If it goes in the direction of the Earth with c, everything emitted in the opposite direction has to stop?

As for the light question, a photon doesn't have an intertial reference frame, you we cannot ask what the world looks like for a photon.

I was referring to this statement by DaleSpam:

Here is the problem. You are starting with a flawed premise. In this arrangement the fastest your brother can possibly be closing with the sign is <0.25c.

 

[Borg]

I am trying desperately to keep this as simple as possible.

Sorry, it's not as simple as you envision. Please answer the question. How fast do you (in your ship) think that the light from your brother's flashlight travels?

 

[James S Saint]

Nonsense. All it tells you is that with respect to the ground frame, the distances closed at a rate of 1.5c.

We didn't even know there was a ground. We saw, measured 2Ls. Yet 1.333 secs later, we BOTH saw 0 distance.

From our perspective, we would not be able to know who "traveled", but only that one of us obviously did. And that one got to the other at 1.5c.

 

[Doc Al]

I am trying desperately to keep this as simple as possible. The issue of how fast light might be going here or there is not really the issue.

The issue with the light beam was just used to point out the logical consequence of your statements and how they contradict what we know about how light works. There's logic coming to bite you in the butt again!

Now i have even removed the ground from the scenario to simplify further. Someone please just answer this;

It's been answered several times.

 

[espen180]

I am trying desperately to keep this as simple as possible. The issue of how fast light might be going here or there is not really the issue.

Now i have even removed the ground from the scenario to simplify further. Someone please just answer this;

You cannot remove the ground frame, because that's where you are making for measurements, whether you realize it or not.

In your ship's rest frame the time from start to meetup is approx 0.9 seconds, and your brother is approx 0.2 light seconds away from the signpost when the race starts. You are approx 0.66 light seconds away from the same signpost.

 

[Doc Al]

We didn't even know there was a ground. We saw, measured 2Ls. Yet 1.333 secs later, we BOTH saw 0 distance.

Measured 2 Ls according to what frame? 1.333 s according to whom?

You are stuck in your pre-relativistic notions that distance and time are independent of the frame doing the measurements. Physics has moved beyond that!

From our perspective, we would not be able to know who "traveled", but only that one of us obviously did. And that one got to the other at 1.5c.

Again, only as measured with respect to the ground frame.

You don't seem to be interested in learning about relativity, so this thread is going nowhere fast.

 

[Bussani]

Yes, but I'm not talking about what a ground observer sees. I see the distance of 2Ls get reduced to 0 in only 1.333 secs. That is what matters. Perhaps I cannot actually see the ground at all or even know it exists. As stated just prior, the ground only serves as an equal frame for us to begin and end. Both of us would end up seeing that 2Ls of distance vanished in only 1.333 secs.

That's where people start telling you you're wrong. What you're not getting is that you're using distances measured in one frame of reference and then telling us to ignore that frame of reference. It doesn't matter if there's ground or not. The closing speed is only 1.5c according to the measurements made in that frame. That's allowed and isn't anything special--there's nothing in relativity that forbids that. If you're saying, "Ignore what you see in the ship and just go from what you know from before the experiment," then you're working from the "ground frame", whether you like it or not.

They would all agree on the simultaneity of the intersection. If they don't steer carefully they could headon and then it would be obvious they met simultaneously. Seriously though, all observers agree on local events at a single location. SImultaneity becomes relevant when events are separated spatially

Hm, I guess that makes sense. Thanks.

 

[James S Saint]

Let's see what we CAN agree to. Assuming the floating in space modification to the original OP;

We didn't even know there was a ground. We saw, measured 2Ls. Yet 1.333 secs later, we BOTH saw 0 distance.

From our perspective, we would not be able to know who "traveled", but only that one of us obviously did. And that one got to the other at 1.5c.

Does anyone disagree with that post?

 

[espen180]

Let's see what we CAN agree to. Assuming the floating in space modification to the original OP;

Does anyone disagree with that post?

Yes. See Post #79, where I have calculated the actual measurements you would make.

 

[Doc Al]

Does anyone disagree with that post?

Of course we disagree with it! Where have you been?

 

[James S Saint]

Yes. See Post #79, where I have calculated the actual measurements you would make.

So everyone agrees that we both measured 2Ls and within 1.333 secs, we both measured 0?

A) Now the whole point to relativity is that there is no actual absolute frame, thus neither brother can claim to be the one traveling. All we see is 2Ls of distance vanish in 1.333 secs.

B) We have no means to use the equation involved, because we do not know where ground would be. We do not know who was traveling. ALL we know is that within 1.333 secs, what was 2Ls disappeared.

C) From ALL we know, one of us traveled at 1.5c.

Now which of A,B, and C is disagreeable?

 

[Borg]

Sorry, it's not as simple as you envision. Please answer the question. How fast do you (in your ship) think that the light from your brother's flashlight travels?

Still waiting...

 

[James S Saint]

Of course we disagree with it! Where have you been?

Ok, what part of THAT post do you disagree with?

 

[James S Saint]

Still waiting...

One traincrash at a time, please. I'll get to it if I can get anywhere with what I am pursuing first.

Do you disagree with that post in question and if so with what part?

 

[Borg]

One traincrash at a time, please. I'll get to it if I can get anywhere with what I am pursuing first.

Do you disagree with that post in question and if so with what part?

Yes, I completely disagree with your post(s). My question was made to get you to realize the illogic of your assumptions. If you answer the question, you might see that there are problems with your assumptions.

 

[espen180]

So everyone agrees that we both measured 2Ls and within 1.333 secs, we both measured 0?

A) Now the whole point to relativity is that there is no actual absolute frame, thus neither brother can claim to be the one traveling. All we see is 2Ls of distance vanish in 1.333 secs.

B) We have no means to use the equation involved, because we do not know where ground would be. We do not know who was traveling. ALL we know is that within 1.333 secs, what was 2Ls disappeared.

C) From ALL we know, one of us traveled at 1.5c.

Now which of A,B, and C is disagreeable?

All of them are wrong. Assuming instant acceleration, the actual situation, in either of the ships' rest frames, is 0.86Ls being traversed in 0.9 secs.

 

[James S Saint]

Yes, I completely disagree with your post(s). My question was made to get you to realize the illogic of your assumptions. If you answer the question, you might see that there are problems with your assumptions.

i understand that was your intention and I will respect it, but not yet. I need to find out why people cannot see what I see so very clearly concerning "no absolute frame".

I have reduced the scenario to a simple case where there is no ground reference with which to use that equation because no one knows who is traveling.

The only things known are that there was 2Ls of distance and 1.333 secs later, there was none.

 

[James S Saint]

All of them are wrong. Assuming instant acceleration, the actual situation, in either of the ships' rest frames, is 0.86Ls being traversed in 0.9 secs.

So you disagree with (A), the very notion of relativity. I think therefore your input is a bit pointless.

And sorry, I had misread your reply just before.

 

[espen180]

So you disagree with (A), the very notion of relativity. I think therefore your input is a bit pointless.

And sorry, I had misread your reply just before.

The second part of A is wrong. The distances you report are not those observed by the ship observers, and the time is wrong according to ship observers. Of course I agree with relativity!

 

[James S Saint]

The second part of A is wrong. The distances you report are not those observed by the ship observers, and the time is wrong according to ship observers. Of course I agree with relativity!

EXACTLY what is in error with (A)?

The statements were;

1) Now the whole point to relativity is that there is no actual absolute frame, thus neither brother can claim to be the one traveling.

2) All we see is 2Ls of distance vanish in 1.333 secs.

The second sentence is the very setup to the story. (1) The brothers measured the 2Ls. (2) There was travel between them such that they both saw the distance vanish in 1.333 secs.

How can that be wrong?

 

[Doc Al]

1) Now the whole point to relativity is that there is no actual absolute frame, thus neither brother can claim to be the one traveling.

Either brother can (and does) view himself as being at rest and the other brother moving towards him.

2) All we see is 2Ls of distance vanish in 1.333 secs.

The second sentence is the very setup to the story. (1) The brothers measured the 2Ls. (2) There was travel between them such that they both saw the distance vanish in 1.333 secs.

Those measurements are only true in the ground frame. As soon as the brothers move, they are no longer at rest in the ground frame. They will use their own measurements of distance and time to measure how fast they move with respect to each other. (If you understand relativity, you can calculate what they will measure.)

 

[espen180]

EXACTLY what is in error with (A)?

The statements were;

1) Now the whole point to relativity is that there is no actual absolute frame, thus neither brother can claim to be the one traveling.

2) All we see is 2Ls of distance vanish in 1.333 secs.

The second sentence is the very setup to the story. (1) The brothers measured the 2Ls. (2) There was travel between them such that they both saw the distance vanish in 1.333 secs.

How can that be wrong?

Those starting conditions are physically impossible if reported in either of the ships' rest frames.

If reported in the ground frame, the correspond measurements the observers in the ships with make are the ones I gave above and in post #79.

 

[Borg]

i understand that was your intention and I will respect it, but not yet. I need to find out why people cannot see what I see so very clearly concerning "no absolute frame".

So you realize that the intention is to get you to see your illogical assumptions but, you're going to continue making statements that are based on those assumptions anyway? You aren't going to get anywhere with this so why not answer the question from post 76 while you're waiting for everyone else to see the light?

 

[James S Saint]

Either brother can (and does) view himself as being at rest and the other brother moving towards him.

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.

They will use their own measurements of distance and time to measure how fast they move with respect to each other. (If you understand relativity, you can calculate what they will measure.)

The only measurements that are made in this scenario is the 2Ls distance and the 1.333 secs duration. They both read the same time change. Neither knows who moved except that they moved with respect to each other.

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.

 

[James S Saint]

So you realize that the intention is to get you to see your illogical assumptions but, you're going to continue making statements that are based on those assumptions anyway? You aren't going to get anywhere with this so why not answer the question from post 76 while you're waiting for everyone else to see the light?

Note that you really aren't answering my questions either. I am trying to narrow exactly where I am actually wrong to one particular incorrect statement (at least). You are not helping with that. I understand that you might have a good point, but like I said. one train wreck at a time, please.

 

[James S Saint]

Those starting conditions are physically impossible if reported in either of the ships' rest frames.

If reported in the ground frame, the correspond measurements the observers in the ships with make are the ones I gave above and in post #79.

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. So is the photon not traveling at the speed of light?