Comments on: Beetlejuiced

By: Ben

Ben — Fri, 11 Jun 2010 23:53:17 +0000

Steve: Does this mean that an observer in a car moving 70 mph decides that the GPS signal he just received was transmitted 5 nanoseconds earlier than the observer who just got his toes run over? It seems like this error would be significant; do GPS receivers actually do this relativistic correction?

By: David

David — Fri, 11 Jun 2010 04:37:48 +0000

Bad Astronomy – one of the best science blogs let alone for this. Phil Plait is a great writer, I’m glad you linked his post on this.

By: Steve Landsburg

Steve Landsburg — Thu, 10 Jun 2010 16:57:35 +0000

Roger: If my arithmetic is right, the rotation of the earth causes people in China and the U.S. to disagree about the timing of events on the sun by up to .0007 seconds. We disagree about the timing of events on earth, of course, by far far less than that. Only very rarely do we contemplate events farther away than the sun. So it’s not surprising that we don’t bother to make corrections for these discrepancies. I’d say that ignoring time discrepancies of far less than .0007 seconds is a very good example of what is meant by roundoff.

By: Roger Schlafly

Roger Schlafly — Thu, 10 Jun 2010 15:40:05 +0000

Steve, China and the USA do not have different notions of time that they change every 12 hours based on their orientation towards Betelgeuse. It is not just a matter of roundoff.

By: Steve Landsburg

Steve Landsburg — Thu, 10 Jun 2010 06:21:49 +0000

Stchoty: I fully agree with your characterizations of the questions you and I were answering. If we continued to discuss this, we might or might not continue to disagree about which question is more natural. But I think we at least fully understand each others’ positions.

By: Stchoty

Stchoty — Thu, 10 Jun 2010 04:21:11 +0000

Steven, thanks for your last comment. I think I now better understand your argument.

“I agree that if one of the observers has accelerated then his watch will continue to show the affects of that acceleration.
But we’re not talking (or at least I wasn’t talking) about what’s on their watches; I was talking about how they currently calculate the time of the explosion.”

I think we have some fundamental disagreement about the meaning of relativity, which is both exciting and strange. To me your last two statements contradict each other. There is no other time for an observer than the time on his watch, so “time of the explosion” should mean to any observer “time his watch displayed at the event simultaneous to the explosion.” Questions about time in relativity should IMHO always be answered in terms of proper time, because this is the time that is actually measured by some observer.

You, if I understand you correctly, are using a different notion of “time” in your last statement, meaning something like “time coordinate in the observers current inertial frame.” To me there is no meaning to such a time coordinate other than as “proper time
of some unaccelerated observer”, which is why I concluded that the driver has to be in uniform motion for approximately 600 years, for your statement to make sense.

By: Neil

Neil — Thu, 10 Jun 2010 02:33:55 +0000

@dave: I’ve got no problem with that because I hold the view that there is no free will, because the conditions that lead the decision makers to their decision are pre-determined. But, according to The Big Questions, Steve does think there is free will, so he might have a problem with it.

By: dave

dave — Thu, 10 Jun 2010 01:52:21 +0000

@neil what if there was no free will? and therefore no ‘decision’.

By: Steve Landsburg

Steve Landsburg — Thu, 10 Jun 2010 00:19:21 +0000

Roger Schlafly:

Your same argument would say that people in the USA and China would disagree about the age of the supernova, and that their disagreement would depend on the time of day.

Absolutely.

No one measures time that way.

How do you know? People in the USA and people in China will disagree about the age of the supernova, but they’ll both round off their estimates to 600 years. So when they both say “600 years”, that doesn’t mean they’re in agreement.

By: Roger Schlafly

Roger Schlafly — Thu, 10 Jun 2010 00:05:21 +0000

Steve, this problem has nothing to do with relativity. You are just formulating a question in an ambiguous way, and then giving an answer that seems surprising only because your interpretation of the question is contrary to common sense.

The essence of your point is that if a car is driving 70 mph and you extrapolate that car backwards 600 years, then the car might be in Jupiter’s orbit somewhere. Then you are pointing out that a car in Jupiter’s orbit might see the supernova 30 minutes later than Earth, because that is how long it takes light to get from Earth to Jupiter. That’s all you’re saying.

But your interpretation of the problem is contrary to common sense because cars do not drive from Jupiter to Earth. That 70 mph is completely dwarfed by the 1000 mph velocity from the rotation of the Earth, and the much larger velocity from the revolution about the Sun. Your same argument would say that people in the USA and China would disagree about the age of the supernova, and that their disagreement would depend on the time of day. No one measures time that way.