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### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Thu Mar 16, 2017 11:32 pm UTC**

by **eSOANEM**

idk, the usual arguments seem to require some sort of horizon (be it an event horizon or an unruh one) but then I've never actually seen a proper QFT in curved space derivation of it

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 17, 2017 1:11 am UTC**

by **cyanyoshi**

Sizik wrote:It's a little ironic that the rubber sheet model of gravity relies on gravity to work.

In principle, you could also use iron balls being attracted to a giant magnet of some sort. That way, people can mistakenly think that gravity can be explained by electromagnetism!

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 17, 2017 1:26 am UTC**

by **doogly**

eSOANEM wrote:idk, the usual arguments seem to require some sort of horizon (be it an event horizon or an unruh one) but then I've never actually seen a proper QFT in curved space derivation of it

yeah, those are what you need

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 17, 2017 3:06 am UTC**

by **Yakk**

Quizatzhaderac wrote:Do objects without an event horizon emit hawking radiation?

Looking at

the math on Wikipedia: it appears that all space with changing surface gravity should emit it, but that objects much larger than their schwarzchild radius should emit much less radiation than the black hole they could make.

From what little I understand, when you move you get Hawking radiation from the event horizon induced by movement...

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 17, 2017 7:39 pm UTC**

by **Quizatzhaderac**

eSOANEM wrote:idk, the usual arguments seem to require some sort of horizon (be it an event horizon or an unruh one) but then I've never actually seen a proper QFT in curved space derivation of it

That's the thing, the equations are for blackbody-matter holding it's coordinate position at a spatial location approaching (but not at) event horizon.

As for "proper QFT in curved space derivation of it": the explanation of the

Unruh effect doesn't depend on curved space. As I understand it, there are necessarily two apparent horizons for a constantly accelerating particle, but those aren't directly related to unruh radiation. The "heat" comes from the blue-shifting of the vacuum.

wikipedia, Unruh_effect page wrote:The temperature of the vacuum, seen by an isolated observer accelerating at the Earth's gravitational acceleration of g = 9.81 m s−2, is only 4×10^−20 K

If an observer accelerating at g, experiences that temperature, then (by the equivalence principle) someone staying still on the surface of the earth should also experience it.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 1:22 pm UTC**

by **djangochained**

Yakk wrote:Quizatzhaderac wrote:Do objects without an event horizon emit hawking radiation?

Looking at

the math on Wikipedia: it appears that all space with changing surface gravity should emit it, but that objects much larger than their schwarzchild radius should emit much less radiation than the black hole they could make.

From what little I understand, when you move you get Hawking radiation from the event horizon induced by movement...

Yea I remember learning this aswell

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 3:32 pm UTC**

by **makeitstop**

Can anyone help me with a hypothetical time dilation question? Just for fun

So let's say in the case of the traveling twins they had an open line of communication between the rocket and earth. What if the twin in the rocket was playing "Glide" by Stone Temple Pilots which is exactly 5 minutes long? At what speed would the song play twice as fast as normal to the twin on earth? Five times as fast? I feel like there should be a handy chart for this issue that we will surely face some day in the future.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 4:00 pm UTC**

by **Zohar**

There are plenty of time dilation calculators online if you just google them, which you can use to get close to the correct result (didn't see one that goes by "enter time dilation factor > get velocity"). You can also just look at the formula for time dilation and solve for a ratio of 1/2 or 1/5 for the dilation factor. It's pretty simple algebra.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 6:09 pm UTC**

by **Eebster the Great**

The factor is called λ (lambda) and is equal to (1-^{v²}/_{c²})^{-½}, where v is the speed and c is the speed of light in a vacuum. So for instance, if the moving twin has a speed of v = ^{4}/_{5} c = 0.8 c, then λ = ^{5}/_{3} ≈ 1.67. In that case, the song which seems 5 minutes long to the twin on the rocket will seem λ · (5 minutes) = ^{25}/_{3} minutes ≈ 8.33 minutes (8 minutes 20 seconds) long to the twin on Earth. On the other hand, if the relative speed is only v = ^{3}/_{5} c = 0.6 c, then λ = ^{5}/_{4} = 1.25, and the song will only appear λ · (5 minutes) = ^{25}/_{4} minutes = 6.25 minutes (6 minutes 15 seconds) long to the twin on Earth.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 8:24 pm UTC**

by **gmalivuk**

You sure you don't mean gamma instead of lambda?

And remember there's also a Doppler shift depending on whether the traveling twin is moving toward or away from the stationary one.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Mar 28, 2017 8:34 pm UTC**

by **doogly**

Whether it's gamma or lambda is relative to your orientation in the plane.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Mar 29, 2017 12:02 am UTC**

by **Eebster the Great**

doogly wrote:Whether it's gamma or lambda is relative to your orientation in the plane.

This. In my reference frame I wrote a γ, but it Terrell-rotated to λ.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Mar 29, 2017 4:18 am UTC**

by **Hypnosifl**

gmalivuk wrote:And remember there's also a Doppler shift depending on whether the traveling twin is moving toward or away from the stationary one.

Yes, as gmalivuk says, due to the doppler effect, the rate at which a transmitter moving relative to you will seem sped up (if it is moving towards you) or slowed down (if it is moving away from you) is

not equal to the time dilation factor of the transmitter in your frame. To answer makeitstop's question, we have to use the relativistic doppler effect formula given

here, in which the frequency of a signal you get from a transmitter is shifted by a factor of sqrt(1 + v/c)/sqrt(1 - v/c) relative to the frequency that would be measured by someone traveling along with the transmitter, where v is the velocity of the transmitter in your frame (positive if it's moving towards you, negative if moving away) and c is the speed of light. So if you want the frequency you measure to be twice the frequency seen by someone moving along with the transmitter, take the equation sqrt(1 + v/c)/sqrt(1 - v/c) = 2 and solve for v, which gives v = (3/5)c. So, a signal from a transmitter moving at 0.6c towards you would appear sped up by a factor of 2.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Mar 29, 2017 7:13 am UTC**

by **Eebster the Great**

The important difference is that the observer on Earth knows there will be a Doppler shift and can correct for it. But there is no "correcting for" time dilation, because it is correct.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Mar 29, 2017 1:47 pm UTC**

by **gmalivuk**

Yeah, but that wasn't the question.

Without Doppler, there's no way to get the song to play twice as fast.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Thu Mar 30, 2017 6:35 am UTC**

by **Hypnosifl**

Eebster the Great wrote:The important difference is that the observer on Earth knows there will be a Doppler shift and can correct for it. But there is no "correcting for" time dilation, because it is correct.

In what sense is the Doppler shifted rate less correct than the time dilated rate? After all, there is no

frame-independent truth about the instantaneous rate one distant clock B is ticking relative to another clock A. It's true that in the type of coordinate system we call an "inertial frame", the time dilation equation gives the correct rate of B's ticking relative to coordinate time in the inertial frame where A is at rest. But you could also, if you liked, construct a non-inertial coordinate system where A is at rest, and where the Doppler shift equation would give the correct rate of B's ticking relative to coordinate time. If you're going to draw the line between "objective" or "physical" facts and facts which are more a matter of perspective, I think it makes the most intuitive sense to label as "objective" only those facts which are completely independent of the choice of coordinate system, like the proper time along a particular world between two events that lie along that worldline.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 31, 2017 5:15 pm UTC**

by **Quizatzhaderac**

I think Eebster meant that the Doppler shift isn't a relativistic effect.

A stereo traveling toward you at half the speed of sound would seem to play twice as fast and at a higher pitch. We could easily correct for pitch. With a little patience, we could correct for playback rate.

If we tried to correct for time dilation with these effects, we'd make mistakes because the brothers genuinely experience different amount of proper time.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Fri Mar 31, 2017 5:26 pm UTC**

by **gmalivuk**

gmalivuk wrote:Yeah, but that wasn't the question.

Without Doppler, there's no way to get the song to play twice as fast.

Either you account for the Doppler shift, or you say there's no answer.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 12:57 am UTC**

by **Eebster the Great**

Quizatzhaderac wrote:I think Eebster meant that the Doppler shift isn't a relativistic effect.

Yes, this.

gmalivuk wrote:Either you account for the Doppler shift, or you say there's no answer.

Or you make the song take twice as long instead of half as long. After all, time is dilated, not contracted. In that case, the answer is:

v =

^{√3}/

_{2} c ≈ 0.866

c ≈ 260,000

^{km}/

_{s}.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 3:29 am UTC**

by **gmalivuk**

Eebster the Great wrote:Or you make the song take twice as long instead of half as long.

Well sure, there are

lots of ways you could answer different questions than the one that was asked.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 3:10 pm UTC**

by **morriswalters**

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 6:39 pm UTC**

by **Eebster the Great**

gmalivuk wrote:Eebster the Great wrote:Or you make the song take twice as long instead of half as long.

Well sure, there are

lots of ways you could answer different questions than the one that was asked.

makeitstop wrote:Can anyone help me with a hypothetical time dilation question?

I am merely suggesting he might not have known that the song will play slower for the twin on Earth due to time dilation, not faster.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 8:37 pm UTC**

by **speising**

Assume instead of a song, the ship sends a ping at regular 1 second intervals. They'll be received, if i'm right, at longer intervals than that, eg. 2 second for v=0.866c.

The thing is, the one on the ship will receive earth's pings at 2s intervals, also! As long as there's a relative movement, both will see the other as slowed down.

But what happens when the ship stops, and then reverses course? We know that the situation isn't symmetrical anymore, as both will agree that time on earth passed faster when the ship arrives back home.

Do all the missing pings of earth pile up in the instant (disregarding de/acceleration times) of course reversal?

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 9:34 pm UTC**

by **morriswalters**

I hate April 1.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sat Apr 01, 2017 10:04 pm UTC**

by **gmalivuk**

speising wrote:Assume instead of a song, the ship sends a ping at regular 1 second intervals. They'll be received, if i'm right, at longer intervals than that, eg. 2 second for v=0.866c.

The thing is, the one on the ship will receive earth's pings at 2s intervals, also! As long as there's a relative movement, both will see the other as slowed down.

But what happens when the ship stops, and then reverses course? We know that the situation isn't symmetrical anymore, as both will agree that time on earth passed faster when the ship arrives back home.

Do all the missing pings of earth pile up in the instant (disregarding de/acceleration times) of course reversal?

No, they come in at a greater speed during the whole trip back.

That's also a bit of a Doppler thing, though. If the shipboard twin calculates the time on Earth, according to the ship's reference frame, there will be a sudden jump when the direction reverses.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Sun Apr 02, 2017 12:22 am UTC**

by **Hypnosifl**

Quizatzhaderac wrote:I think Eebster meant that the Doppler shift isn't a relativistic effect.

There is a non-relativistic Doppler effect, but the relativistic version obeys a different equation, so I think a Doppler shift obeying that equation would count as a "relativistic effect".

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Mon Apr 03, 2017 8:05 pm UTC**

by **Quizatzhaderac**

The relativistic Doppler effect and it's equation are just the straightforward combination of the concepts/equations of the Doppler effect and time dilation.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Apr 04, 2017 12:14 am UTC**

by **jewish_scientist**

I just wanted to let everyone know that

this unit converter exists. To everyone who just thought 'So what, there are a million unit converters across the online', tell me how many of them convert

Aardwolfs to nautical miles/ hour? When Wolfram|Alpha

failed me, some random

silversmith on the internet was there

to save the day.

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Tue Apr 04, 2017 1:57 am UTC**

by **Copper Bezel**

That's fascinating! It does seem like a malformed question, though; I would have thought one would be more likely to need a figure for aardwolves ~~/ hr~~ per nautical miles ~~/ hr~~ / kg!

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Apr 05, 2017 1:36 am UTC**

by **Hypnosifl**

Quizatzhaderac wrote:The relativistic Doppler effect and it's equation are just the straightforward combination of the concepts/equations of the Doppler effect and time dilation.

If you derive it in an inertial frame, sure. But all the terms of Doppler equation can be understood in terms of local measurements which don't depend on any particular choice of coordinate system (even the "v", which can be understood in terms of the change in delay for two successive light signals you send out to bounce back and return to you), which means you could derive the same equation using some non-inertial frame, including one which used a definition of simultaneity such that the two clocks remained synchronized as they moved apart (no time dilation).

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Apr 05, 2017 12:59 pm UTC**

by **jewish_scientist**

Using quantum physics (specifically the phenomena of quantum entanglement), it is possible to send information faster than the speed of light. This is a direct contradiction to Einstein's Theory of Special Relativity. Einstien, Podolsky and Rosen were the first ones to point this out, so this problem is called the EPR Paradox. Their solution to this paradox was that there was stuff we do not know, which they named local hidden variables (LHV).

Although Bohr gave a response that this was not a paradox at all, more research is currently being done on Bell's response, which is called

Bell's Theorem (a.k.a. Bell's Inequalities). I am not even going to pretend that I understand any of this in any way. The EPR Paradox and Bell's Theorem is so complex and has such important imprecation regarding the nature of The Universe, that the Stanford Encyclopedia of

Philosophy has a article dedicated to each of these topics.*

*

https://plato.stanford.edu/entries/qt-epr/ and

https://plato.stanford.edu/entries/bell-theorem/

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Apr 05, 2017 1:19 pm UTC**

by **doogly**

jewish_scientist wrote:Using quantum physics (specifically the phenomena of quantum entanglement), it is possible to send information faster than the speed of light. This is a direct contradiction to Einstein's Theory of Special Relativity. Einstien, Podolsky and Rosen were the first ones to point this out, so this problem is called the EPR Paradox. Their solution to this paradox was that there was stuff we do not know, which they named local hidden variables (LHV).

This is basically the opposite. Quantum mechanics can most certainly not send information faster than the speed of light. EPR did not understand what it meant to send information when they proposed their initial work. There are correlations between spacelike separated observables, which require quantum mechanics to explain. Local Hidden Variables

cannot explain the observed violations of Bell's Inequality (Bell's Inequality says that classical physics must give < a certain amount of correlation, and because it is exceeded, we require quantum mechanics.) But quantum mechanics is entirely local. All observables are defined locally, and all commutations are zero if the two observables are not casually connected (in a light cone.)

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Apr 05, 2017 2:01 pm UTC**

by **gmalivuk**

jewish_scientist wrote:Using quantum physics (specifically the phenomena of quantum entanglement), it is possible to send information faster than the speed of light. This is a direct contradiction to Einstein's Theory of Special Relativity. Einstien, Podolsky and Rosen were the first ones to point this out, so this problem is called the EPR Paradox. Their solution to this paradox was that there was stuff we do not know, which they named local hidden variables (LHV).

Although Bohr gave a response that this was not a paradox at all, more research is currently being done on Bell's response, which is called

Bell's Theorem (a.k.a. Bell's Inequalities). I am not even going to pretend that I understand any of this in any way. The EPR Paradox and Bell's Theorem is so complex and has such important imprecation regarding the nature of The Universe, that the Stanford Encyclopedia of

Philosophy has a article dedicated to each of these topics.*

*

https://plato.stanford.edu/entries/qt-epr/ and

https://plato.stanford.edu/entries/bell-theorem/

Did you have, like, an actual question here, or just a summary of what some other people said?

### Re: RELATIVITY QUESTIONS! (and other common queries)

Posted: **Wed Apr 05, 2017 2:03 pm UTC**

by **doogly**

gmalivuk wrote:Did you have, like, an actual question here, or just a summary of what some other people said?

I'm taking the question to be an implicit, "Am I correctly stating this situation," and the answer is no.

### Qustion about ATP

Posted: **Sun Apr 16, 2017 5:07 am UTC**

by **Tevin Chung from South Korea**

While I learned about ATP and process of photosynthesis at school, (by the way, I'm highschool kid) I found that energy efficiency of ATP is almost equal to that of carnot heat engine's.

I wonder that if we if we applied this ATP to energy engines like batteries, wouldn't it be possible to create a very efficient energy engine?

and I also wonder that if so, how could we apply ATP to batteries?

Any answer would be welcomed.

### Re: Miscellaneous Science Questions

Posted: **Fri May 12, 2017 1:37 pm UTC**

by **jewish_scientist**

This is so funny. When I first joined this forum in high school, this same question was one of the first things I asked. I think I even posted it on this thread.

They problems with using ATP for energy is basically the same ones Randall addresses

here.

### Re: Miscellaneous Science Questions

Posted: **Thu Sep 14, 2017 3:05 pm UTC**

by **commodorejohn**

Okay, explain this to me, all you lay physicists (or, um, even any professional physicists, you're cool too):

- Make a nice steaming mug of hot cocoa. Use a reasonably sturdy mug.
- Put a spoon in the glass and begin steadily tapping at the bottom. The sound of the tapping will rise in pitch.
- Do it less steadily. The pitch will not cease to rise. There will be no noticeable correlation between the rate or regularity of the tapping and the rate of change in pitch.
- Walk away and leave the entire thing alone for a few minutes.
- Come back and start tapping again. IT WILL CONTINUE AT THE SAME PITCH IT LEFT OFF AT.

Why is this? If the pitch rises due to, say, energy imparted to the glass or the beverage, it should decay over time, which would mean that a slower rate of tapping would result in a smaller rate of change in pitch, and walking away for a few minutes should cause the pitch to drop noticeably. If it's some kind of resonance in the chamber, it should simply emphasize frequencies close to the resonant frequency of the glass and de-emphasize others. The heck's going on here?

### Re: Miscellaneous Science Questions

Posted: **Thu Sep 14, 2017 5:13 pm UTC**

by **Zohar**

I don't think I've ever encountered this phenomenon. It might have something to do with temperature changes, or just your brain misinterpreting things.

### Re: Miscellaneous Science Questions

Posted: **Thu Sep 14, 2017 5:17 pm UTC**

by **commodorejohn**

I'm pretty sure it's not just a weird perceptual glitch, because A. I've repeatedly tested this and got the same results, and B. I've verified it with other people. Tried it with coffee (cream & sugar) and got the same result. Though I admit I've never recorded the audio and analyzed the pitch thusly.

### Re: Miscellaneous Science Questions

Posted: **Thu Sep 14, 2017 5:47 pm UTC**

by **Flumble**

commodorejohn wrote:Though I admit I've never recorded the audio and analyzed the pitch thusly.

Yes, do science to it.

And see if you can measure the temperature too (preferably of the cup and the liquid, firstly to see if there's any change of temperature, secondly to see if the change corresponds to a change in pitch).