The OFFICIAL IntenseMuscle Physics Q&A Thread

[Sammich]

This is an excellent question/observation and I will answer it more fully after I get back from my GRE later today. For now I will just say that the effect IS present during the space travel that we do, but it is so incredibly small that humans cannot perceive it. However we have extremely precise atomic clocks that CAN detect it. I will go more into this later today.

For the second question, I’ll take the latter half first: “How does gravity affect time?” It turns out that gravity makes time go MORE SLOWLY. Let’s say we have two astronauts in orbit around a very large star. The astronauts synchronize their clocks so that they read the same time (12:00 PM) and are running at the same speed. Then one of the astronauts travels right next to the star, so that he can feel its gravity very strongly. He stays there for a while, then travels back to where the second astronaut is. When they compare their clocks, let’s say the clock of the astronaut who was stationary reads 2:00 PM. The clock of the astronaut who was near the star will read a time that is BEFORE 2:00 PM. This is because the presence of gravity SLOWS DOWN TIME!

I have a wierd question for you about this...

Since gravity affects time, How exactly does NASA keep track of time? I mean in regard to things like sending rovers to Mars, even sending a shuttle to the moon or having men in the spacestation. As a shuttle leaves earth for instance, the gravity lessens, thereby speeding up time. It travels on its way to Jupiter. The farther the shuttle gets away from earth, the less gravity there is in space, therefore speeding up time even more. As it arrives at its Jupiter destination, the gravity increases, thereby causing time to slow down again. If an astronaut on the shuttle steps outside of the craft onto the surface of Jupiter to take a stroll, would the watch on his arm read the same time as the watch of somebody on earth? Or is the variation in gravity during the journey not enough to alter time? I have no idea why I need to know this... :-)


Great thread by the way! I just found it...

[olinerules87]

good luck on the GRE...took mine a few months ago. it isn't that bad

[Jeff Grant]

This is an excellent question/observation and I will answer it more fully after I get back from my GRE later today. For now I will just say that the effect IS present during the space travel that we do, but it is so incredibly small that humans cannot perceive it. However we have extremely precise atomic clocks that CAN detect it. I will go more into this later today.

Awesome bud... can't wait to hear what you have to say on this.

[Sammich]

good luck on the GRE...took mine a few months ago. it isn't that bad

This was the Physics GRE, not the general one. I took the general last December.

Awesome bud... can't wait to hear what you have to say on this.

Nap now, general relativity explanation later.

[Sammich]

Ok, here we go. The best way humans have to keep track of time so far is by atomic clocks. We essentially take a few atoms of an element, typically cesium, and supercool it to just above absolute zero. At this temperature, the effects of quantum mechanics become visible. It turns out the the atom can only have certain discrete energy levels that are a multiple of some number E. So the energy of the atom can by 1E, 2E, 3E, and so on. By continuously exposing the cesium atom to a constant (but very small) amount of energy, we can make the atom oscillate between the lowest and the next-to-lowest energy levels. The atom does this at a very predictable and steady rate, which we can calculate from atomic theory and quantum mechanics. In fact it oscillates back and forth so precisely that we have now defined the second to be exactly the time it takes the cesium-133 atom to go back and forth 9,192,631,770 times. (Cesium-133 means that the atom has 55 protons and 78 neutrons. The 133 is the number of protons plus the number of neutrons.) The National Institute of Standards and Technology are the official timekeepers for the United States. We measure time with atomic clocks SO exactly that sometimes we have to add leap-seconds on to the end of years to compensate for the Earth's gradual slowing down of its rotation.

So that's the best way we have to measure time. Now in the example you gave, we are comparing the RATE of time passage between, say a person on Earth and some people in the space shuttle. Earlier I said that being in a strong gravitational field slows down your passage of time relative to someone who is not near a gravitational field. According to this, people in the space shuttle experience time passing as more slowly than people on the surface of the Earth, since the people on Earth are closer to the mass. This is absolutely true, and we have measured it with atomic clocks. The catch is that the effect is so extremely small that humans cannot perceive it. Below I have written a short PDF document that shows you how small the effect is. There is math in it, so if you're not good with math you can skip to the end for the results. (PROTIP: The effect is REAAAAAALLY small.)

Although the effect is extremely small, it is enough that GPS satellites have to take the gravitational time dilation into account when they make their measurements. If they did not, the locations would be off by miles.

For the second question, I’ll take the latter half first: “How does gravity affect time?” It turns out that gravity makes time go MORE SLOWLY. Let’s say we have two astronauts in orbit around a very large star. The astronauts synchronize their clocks so that they read the same time (12:00 PM) and are running at the same speed. Then one of the astronauts travels right next to the star, so that he can feel its gravity very strongly. He stays there for a while, then travels back to where the second astronaut is. When they compare their clocks, let’s say the clock of the astronaut who was stationary reads 2:00 PM. The clock of the astronaut who was near the star will read a time that is BEFORE 2:00 PM. This is because the presence of gravity SLOWS DOWN TIME!

I have a wierd question for you about this...

Since gravity affects time, How exactly does NASA keep track of time? I mean in regard to things like sending rovers to Mars, even sending a shuttle to the moon or having men in the spacestation. As a shuttle leaves earth for instance, the gravity lessens, thereby speeding up time. It travels on its way to Jupiter. The farther the shuttle gets away from earth, the less gravity there is in space, therefore speeding up time even more. As it arrives at its Jupiter destination, the gravity increases, thereby causing time to slow down again. If an astronaut on the shuttle steps outside of the craft onto the surface of Jupiter to take a stroll, would the watch on his arm read the same time as the watch of somebody on earth? Or is the variation in gravity during the journey not enough to alter time? I have no idea why I need to know this... :-)


Great thread by the way! I just found it...

[DCBliever]

Listen I'm finishing my degree in electrical egineering at the moment and trying to find Vc(t)=(Vo-Vf)e^t/Tau + If where Tau=RC. I ran the stupid circuit through pspice with no success, so Mr. Physics genius tell me how to solve this first order RC circuit. R=2K and C=1n with a voltage step of 0V to 5V and 0<t<7tau. and find Ic(t), where I= C(dv/dt), Vc(t)=1/c integral of idt + Vo with repsect from some time 0 to some time t? I know circuits sucks. By the way I currently desing power transformers, voltage regulators, and reclosers for power companies and I get paid like I work at McDonalds.

Robb

[Sammich]

I am a little confused by the notation. Is the stuff I put on the below PDF correct? Is there any chance you could scan the problem (from the text or the homework or whatever) so I wouldn't get confused on the notation?

I suck at circuits but it look like just solving a differential equation which I can definitely do if I can clear up my confusion on the notation.

[DCBliever]

Right on Brother, however it's a basic RC circuit which I'll transfer from my pspice program to microsoft word. The only problem is you have to look at the circuit from t<0 before the voltage source is turned on t=0, and after it is turned on t>0.

[Jeff Grant]

Thanks for putting that pdf file together man. This stuff is fascinating! Kind of boggles the mind... Of course, you explaining this only leads to more questions I have about time ;-) Gotta work though... I'll ask later. Thanks again.

[Sammich]

Right on Brother, however it's a basic RC circuit which I'll transfer from my pspice program to microsoft word. The only problem is you have to look at the circuit from t<0 before the voltage source is turned on t=0, and after it is turned on t>0.

As soon as you can get me that Word file I will see what I can do with it. I never was good at circuits, through, but I'll do my best.

Thanks for putting that pdf file together man. This stuff is fascinating! Kind of boggles the mind... Of course, you explaining this only leads to more questions I have about time ;-) Gotta work though... I'll ask later. Thanks again.

Keep asking, man, that's why I started the thread. I love answering these questions and teaching people.

[TommyKav]

dinobird - im having a bad brain day, you know one of those days you cant get the head functioning properly. i think its called quantumn entanglement. where 2 particles behave identically over a distance instantly. say you move and electron in one the same electron in the other moves the same way instantly. theres like 3 people on the planet that really understand this phenomenon. anyway, what is your opinion on using this quirk of nature for FTL communication?

question 2.
this one involves SETI. in your opinion is SETI wasting theyre time. if you think about it the odds of finding intelligent life that within the time frame of us having radio astronomy sent out a strong enough signal not to dissapate by the time it reaches us and coming from the direction we happen to be looking in are astronimical.

[Sammich]

dinobird - im having a bad brain day, you know one of those days you cant get the head functioning properly. i think its called quantumn entanglement. where 2 particles behave identically over a distance instantly. say you move and electron in one the same electron in the other moves the same way instantly. theres like 3 people on the planet that really understand this phenomenon. anyway, what is your opinion on using this quirk of nature for FTL communication?

question 2.
this one involves SETI. in your opinion is SETI wasting theyre time. if you think about it the odds of finding intelligent life that within the time frame of us having radio astronomy sent out a strong enough signal not to dissapate by the time it reaches us and coming from the direction we happen to be looking in are astronimical.

1) Quantum entanglement CANNOT be used to transmit information at all, and thus does not violate relativity. It is, however, extremely cool and interesting. It doesn't make electrons "move." Here's essentially how it works. Say a particle decays and creates two electrons. If the original particle had zero spin angular momentum, then the electrons have to have opposite spins, one up and one down, in order to conserve angular momentum. But before any measurement is made on either of the two electrons, they don't HAVE a definite spin, since the process of measuring is what gives the spin a definite value. What they have is a certain probability of measured the spin to be up or down. But since we know the spins must be opposite, if we measure the spin of one particle then we automatically know the spin of the other particle.

Now let's move the electrons REALLY far away from each other WITHOUT making any observations on them. Thus the state of them having an indeterminate spin is preserved. If they are, say, 1000 miles away and we make a measurement of one particle, then it appears that the other particle IMMEDIATELY gets this information that it should have the opposite spin. Any measurement now made on the second particle will give the proper result of the opposite spin of the first particle. It seems like we have made information travel faster than the speed of light by doing this. However we cannot use this method to transmit any information about anything. All the person with the second electron knows is that he made a measurement on the electron and found its spin. He now knows the spin of the other electron, but absolutely nothing else. Since you cannot control what the outcome of the measurement of spin will be, you can't use it to communicate information.

If you COULD control the measurement, then you could tell a person ahead of time by assigning meaning to what the spin is. However the measuring of the spin is completely random (having a probability based on the wavefunction of the system), so you can't transmit information that way.

2) Do I think SETI will ever find an extraterrestrial civilization? Nope. Are they wasting their time? Not at all. You can't possibly predict what outcomes will come of this. I mean for one thing, it demonstrated the incredible power of distributed processing. And it could also be used to find interesting astronomical phenomena. Plus it's not really using any resources that wouldn't be going to waste anyway. The radio telescopes are collecting the data whether it's analyzed or not, and the computers only use the processing power that would be otherwise wasted when they're idle. So it's kind of a "why not?" situation, in my opinion.

[TommyKav]

there is an experiment slated by nasa to test using quantumn entanglement to transmit information from the ground to the space station. i cant remember when it is actually schedualed for but i think it was on discovery.com that i read it. ill try to find it later. i believe if i remember correctly that theoretically the particle can be manipulated in some fashion and have the other particle behave identically. sorry im really having trouble remembering where i read that but im trying to find it right now. if i can find it ill post it as soon as i do.

[Jay Cook]

:sammich:

[Sammich]

:sammich:

The integral of z-squared dz
from one to the cube root of three
times the cosine
of three pi over nine
equals log of the cube root of e.