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

[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.