Ask any question you want about Physics

originally posted by: Arbitrageur
I'm not questioning the expansion of the universe, just the accuracy of this tape measure claim. I think part of the reason that space is able to expand is that gravity between galaxies is relatively weak, however, the intermolecular forces in a tape measure are not so weak since EM forces in general (which bind the tape measure atoms) are over a trillion trillion trillion times stronger than gravity.

I think it would depend on how the string is bound to both galaxies. I free floating string should not expand noticeably.

Some quick online search brings up a similar thought experiment (in "Measuring the Universe in Space and Time" by Simon Lilly, maybe the original source?) which uses an unrolling ball of string instead and doesn't require plastic stretching.

a reply to: Arbitrageur

The point isn't so much about the tape measure it's hypothetical the point of thr tape measure is to show you the measurements changed and distances were stretched. But the original measurement in say light years remain the same. As long as our scale of measument is expanding with the expansion of space. Galaxies stay relative to each other.

originally posted by: moebius
Some quick online search brings up a similar thought experiment (in "Measuring the Universe in Space and Time" by Simon Lilly, maybe the original source?) which uses an unrolling ball of string instead and doesn't require plastic stretching.

I found that as a chapter in the book "An den Grenzen des Wissens" and Google Books let me read 6 pages of it (143-148) before saying I reached the limit, and there was nothing about string in those 6 pages, so I don't know exactly what he said. However, if I understand your paraphrased description, that can't be the source because the "Ask an Astronomer" page I cited says specifically that what Simon Lilly says is NOT what would happen (Am I correct in my interpretation of your comments that Simon Lilly is describing what is referred to as "The old picture" here?):

curious.astro.cornell.edu...

What is the distance between two galaxies? In the old picture, this is an easy question to answer theoretically (though not necessarily in practice!). Just get yourself a giant tape measure and clip it to a faraway galaxy, then come back to our galaxy and hold on tight. As the galaxy moves away, it will pull on the tape measure, and you will easily be able to read off the distance as the tape measure unwinds... one billion light-years, one and half billion light-years, two billion light-years, etc.

In our new picture of the universe, however, with the raisins and the dough, the tape measure will not unwind at all as the universe expands, because the galaxies are not actually moving with respect to each other! Instead, it will read one billion light-years the whole time. You could be perfectly justified in saying that the distance between the galaxies has not changed as time goes on.

I think this author is actually contradicting himself and not by comparing this "old picture" with the "new picture", but above he says in this "new picture" that the "You could be perfectly justified in saying that the distance between the galaxies has not changed as time goes on", which doesn't mesh with his explanation that cosmological redshift is a result of "stretched light" because why would the light have cosmological redshift if "the distance between the galaxies has not changed as time goes on"?

The space between the galaxies does increase, but one problem with this tape measure is, the tape measure is not "space".

In my view the cosmological redshift is evidence of the expansion of space, but I think this simply doesn't apply to a tape measure because a tape measure is not space. If you say the tape measure expanded along with the expansion of space between the galaxies, the inference to me is that space is expanding in all directions so the tape measure would too if it was expanding along with space, so if the space doubled in direction as he suggests, this would be an 8-fold increase in volume (2x2x2) and the density of the tape measure would then be 1/8 of its original density. I don't think this would happen for numerous reasons.

If instead the claim is changed to saying the tape measure was simply stretched via tensile stretching between galaxies, this would be like saying if you take a 1 meter measuring tape you could stretch it to 2 meters. Sure that can be done if the tape measure is made out of something stretchy, but would you really say that "the length of the tape measure has not changed" if you stretched it?

If someone can clear this up for me that would be great, but until that happens I think the tape measure claim is flawed, and we should just stick to using light for measuring cosmological redshift and the inferred stretching of space, because light works, the tape measure either doesn't work or again someone needs to show the math of exactly what happens to the tape measure in a way that doesn't have the flaws described above or other flaws.

originally posted by: dragonridr
The point isn't so much about the tape measure it's hypothetical

If you or the "ask and astronomer" source had used a qualifier like "imaginary tape measure" then I could try to imagine it didn't have any atoms or chemical bonds, then maybe it wouldn't create such problems. But without such a qualifier it sounds like a real tape measure is being described and I don't think a real tape measure would do what's described.

I really dont know if this Physics's related but I'll ask anyways ,and forgive me for my English.
Does "Time" really exist ?
I mean time its a factor humans created to measure aging / days , or in general because we need the ability to set meetings or whatever... you got my point.

Time , as we know him dont really exist because there is no such thing as time , their is only aging / desolution /transformation .
I mean I never saw even 1 prove that time exist , and you basicly cant give me one.
because time is not a natural object / measuring tool , it something "we" created.
I can only tell that life is cyclical (Day/ nights ) , and everything grows and eventually dies or gets transformation into something else.

time is our tool to measure transformations.
so in the end of the day , time travel or all that nonsense , cant really work ? does it ?
Because you cant travel in invented measuring tool you in you self created .

Hope u understood my poor English, I really tried my best here.

originally posted by: FederWBush
I really dont know if this Physics's related but I'll ask anyways ,and forgive me for my English.
Does "Time" really exist ?
I mean time its a factor humans created to measure aging / days , or in general because we need the ability to set meetings or whatever... you got my point.

Time , as we know him dont really exist because there is no such thing as time , their is only aging / desolution /transformation .
I mean I never saw even 1 prove that time exist , and you basicly cant give me one.
because time is not a natural object / measuring tool , it something "we" created.
I can only tell that life is cyclical (Day/ nights ) , and everything grows and eventually dies or gets transformation into something else.

time is our tool to measure transformations.
so in the end of the day , time travel or all that nonsense , cant really work ? does it ?
Because you cant travel in invented measuring tool you in you self created .

Hope u understood my poor English, I really tried my best here.

Simple answer yes it does. Time is just a sequence of events. Doesn't matter if it's a ticking clock or rotation of a planet. Humans developed a way to measure time but it existed long before we did and will be around long after.

if you are driving a car at the speed of light and turn your headlights on....

what happens ?

originally posted by: syrinx high priest
if you are driving a car at the speed of light and turn your headlights on....

what happens ?

Bugs would hit your windshield and shatter it. That aside you wouldn't notice the difference from your perspective the light would still be ahead of your car.

originally posted by: syrinx high priest
if you are driving a car at the speed of light and turn your headlights on....

what happens ?

The lights come on, as normal, because the faster you get to the speed of light, the slower you are perceived going. Or something like that.

I'm no phycisist. I'm just bored.

that's one of the questions we used to ponder years ago, looking up at the stars.....

that and where did all the beer go ? lol

originally posted by: syrinx high priest
where did all the beer go ? lol

Sorry. I drank it.

Oh, I found a spare.

CATCH!

originally posted by: dragonridr
a reply to: joelr

The best way to look at it is simply the galaxies don't move much thr space around them expands.

what do you say if I tell you that in local group Andromeda is on a collision course with Milky Way? Why space is not expanding between those two when it should based on your theory (or how you understand it)?

could it be that vector sum expansion is faster than light speed but in reality it is only a sum, a collective indication and nothing still can be faster than light? And in fact locally, as joelr has said, local groups tend to group together ignoring faster than light rate of expansion in between?
I mean, vector sum would include left, right, up and down and every possible combination in between. The sum will give you faster than light.

originally posted by: syrinx high priest
if you are driving a car at the speed of light and turn your headlights on....

what happens ?

First, thank you for asking a question as the topic of this thread suggests. Some posters apparently think the topic of this thread is something else, so it's nice to see a genuine question.

You probably heard about the space shuttle burning up in the Earth's atmosphere when it had a heat shield problem, right? That was only going a little less than orbital speeds which are far less than the speed of light, so your car can't go very fast in Earth's atmosphere without burning up like the space shuttle did.

So what happens if you strap some rockets on your car bigger than Bob Lazar did, and take it to space?

You still can't go the speed of light because that would require an infinite amount of energy and the energy in the observable universe isn't infinite.

You could theoretically travel close to the speed of light like the particles at the LHC do, but it would be better to send a radiation-hardened robot to do that because you'd be dead very quickly from the radiation.

Super-Fast Space Travel Would Kill You in Minutes

Unfortunately, as spaceship velocities approach the speed of light, interstellar hydrogen H, although only present at a density of approximately 1.8 atoms/cm3, turns into intense radiation that would quickly kill passengers and destroy electronic instrumentation. In addition, the energy loss of ionizing radiation passing through the ship's hull represents an increasing heat load that necessitates large expenditures of energy to cool the ship.

In other words, travel close to the speed of light and you'll be bombarded with so much radiation that you kick the bucket. The knock-on effect is that even if it's possible to create a craft capable of traveling close the speed of light, it wouldn't be able to transport people.

Instead, there's a natural speed limit imposed by safe levels of radiation due to hydrogen, which means humans couldn't travel faster than half the speed of light unless they were willing to die almost immediately. Dammit.

So, keep your speeds to less than half the speed of light unless you want to die. That would be worse than getting a speeding ticket.

originally posted by: FederWBush
Time , as we know him dont really exist because there is no such thing as time , their is only aging / desolution /transformation .
I mean I never saw even 1 prove that time exist , and you basicly cant give me one.
because time is not a natural object / measuring tool , it something "we" created.

You raise two different issues.
1. Does time exist?
2. How do you measure time?

For your first point, I can't say whether time exists in a black hole singularity which is beyond the scope of the ability of our theories to make accurate predictions, but outside of black holes, time exists. The Earth was rotating long before humans existed, and the Earth can't rotate without time. So humans didn't invent time, it existed before we did, so the answer to #1 is that yes time exists.

Your second point you answered yourself correctly, which is that humans devised a method for measuring time. Probably one of the first methods was to notice the sun rising every morning and call each one of those sunrises a new day in caveman-speak, so that was a human invention to measure time.

But the Earth was rotating before we showed up to measure the rotation. As far as we can tell it's been rotating for billions of years but the rotation has probably slowed down a bit because of transfer of angular momentum from the Earth to the moon. So the human invention of a "Day" is not a perfect clock as the "Days" are getting longer over "deep time" meaning time long compared to human lifetimes. On average a day is something like 1.3 milliseconds longer at our death than at our birth because the Earth's rotation slows down that much over 75 years.

So a day isn't such an accurate measurement system for scientific purposes, that's why we invented atomic clocks, they are very accurate.

originally posted by: Arbitrageur

originally posted by: dragonridr
a reply to: joelr

The best way to look at it is simply the galaxies don't move much thr space around them expands. Say we could take a tape measure and and spool it from one galaxy to another. You would expect our tape measure to read 1 million light years then two etc as it continued to spool out. But what would happen is we would see no change in distance . You could be perfectly justified in saying that the distance between the galaxies has not changed as time goes on.

This astronomer says the same thing but I'd like to see the math on exactly what happens to the tape measure, because its mass won't increase, right? So if the tape measure is longer with no increase in mass what happens to its width, thickness, and density, and how do you calculate that?

The regions that both ends of the tape lie are way too far to make accurate measurements. The far end will appear to be accelerating away faster than light but from the other side it's still and we are moving faster than light.
But visual observations would be millions of light years off(if it was that long). Among other issues.

Space is expanding but not atoms so objects don't get bigger. Did someone say something about that?

Anyway you can measure distant objects that have more than light speed separation because of expansion or extreme curvature with General Relativity using tensors but Special Relativity measurements probably will not work.

originally posted by: FederWBush
I really dont know if this Physics's related but I'll ask anyways ,and forgive me for my English.
Does "Time" really exist ?
I mean time its a factor humans created to measure aging / days , or in general because we need the ability to set meetings or whatever... you got my point.

Time , as we know him dont really exist because there is no such thing as time , their is only aging / desolution /transformation .
I mean I never saw even 1 prove that time exist , and you basicly cant give me one.
because time is not a natural object / measuring tool , it something "we" created.
I can only tell that life is cyclical (Day/ nights ) , and everything grows and eventually dies or gets transformation into something else.

Time exists, it's one of the vectors of "position" in defining a location. There is a common mis-understanding about time that comes from some Physicists saying that the FLOW of time may be an illusion.
Flow of time may be psychological but time as an aspect of reality is 100% real. Not only can it be measured but we can predict exact changes in it's rate of change depending on it's speed and gravitational influence.

Virtual particles have a type of time limit as to how long they can exist without violating laws of physics. The more energy they have the quicker they must annillate.

But time is still very mysterious in many ways. Many unanswered questions.

originally posted by: Arbitrageur

originally posted by: syrinx high priest
if you are driving a car at the speed of light and turn your headlights on....

what happens ?

First, thank you for asking a question as the topic of this thread suggests. Some posters apparently think the topic of this thread is something else, so it's nice to see a genuine question.

You probably heard about the space shuttle burning up in the Earth's atmosphere when it had a heat shield problem, right? That was only going a little less than orbital speeds which are far less than the speed of light, so your car can't go very fast in Earth's atmosphere without burning up like the space shuttle did.

So what happens if you strap some rockets on your car bigger than Bob Lazar did, and take it to space?

You still can't go the speed of light because that would require an infinite amount of energy and the energy in the observable universe isn't infinite.

You could theoretically travel close to the speed of light like the particles at the LHC do, but it would be better to send a radiation-hardened robot to do that because you'd be dead very quickly from the radiation.

Super-Fast Space Travel Would Kill You in Minutes

Unfortunately, as spaceship velocities approach the speed of light, interstellar hydrogen H, although only present at a density of approximately 1.8 atoms/cm3, turns into intense radiation that would quickly kill passengers and destroy electronic instrumentation. In addition, the energy loss of ionizing radiation passing through the ship's hull represents an increasing heat load that necessitates large expenditures of energy to cool the ship.

In other words, travel close to the speed of light and you'll be bombarded with so much radiation that you kick the bucket. The knock-on effect is that even if it's possible to create a craft capable of traveling close the speed of light, it wouldn't be able to transport people.

Instead, there's a natural speed limit imposed by safe levels of radiation due to hydrogen, which means humans couldn't travel faster than half the speed of light unless they were willing to die almost immediately. Dammit.

So, keep your speeds to less than half the speed of light unless you want to die. That would be worse than getting a speeding ticket.

[

thanks for the reply ! This was waaaaay more just simple hypothetical, I understand the limitations of taking something the size of a car to light speed.

its more a question of if light is sort of a particle and sort of a wave would it benefit from the momentum of the source of the light already being at light speed ?

could you see it in front of you ?

originally posted by: greenreflections
what do you say if I tell you that in local group Andromeda is on a collision course with Milky Why? Why space is not expanding between those two when it should based on your theory (or how you understand it)?

The Hubble constant relates to a straight line approximation of the "best fit" line for distance versus recession velocity data which has some variation in it at all distances. When those distances are close the variation from the "best fit" line can have negative values, like Andromeda does. This is perfectly consistent with theory, it doesn't change the "best fit".

For your "straw-man" model, (that is saying Andromeda should be moving away), you would have to assume that the only factor affecting galactic motion is the distance between galaxies, and every single galaxy has to be exactly on the line, and that is NOT what mainstream theory suggests, and it never did. This is Hubble's own plot showing the velocity versus distance data doesn't form a perfectly straight line, and it shows negative recession values for nearby galaxies so he was well aware of that when he drew his straight line through the data:

Hubble's Law


One thing that has changed however is that at great distances (off the scale of that plot) the relationship is no longer a straight line. Hubble didn't know that.

originally posted by: syrinx high priest
thanks for the reply ! This was waaaaay more just simple hypothetical, I understand the limitations of taking something the size of a car to light speed.

its more a question of if light is sort of a particle and sort of a wave would it benefit from the momentum of the source of the light already being at light speed ?

could you see it in front of you ?

You get all kind of nonsensical results if you assume that mass can travel at the speed of light. Theory says mass can't travel at the speed of light so that's the only way I see to avoid a discussion of nonsense.

Some people try to ask "well a photon travels at the speed of light, so what does the photon observe"? The answer to that is that the photon does not have a valid frame of reference from which to make any observations according to existing theories. If you try to assume it does you get nonsense, for example, there would be no passage of time at all for the photon, so you couldn't observe anything moving at all, not the car, and not the light from the headlights. You need time to observe motion.

a reply to: Arbitrageur

The Hubble constant is a straight line approximation of the "best fit" line for distance versus recession velocity data which has some variation in it at all distances. When those distances are close the variation from the "best fit" line can have negative values, like Andromeda does. This is perfectly consistent with theory, it doesn't change the "best fit".

Thanks.

so locally effect of gravity is stronger than the rate of space-time volume increase locally? So why currently space-time expansion thought getting close or faster to the speed of light?

Space-time is expanding everywhere at every point of space volume but despite increasing rate of acceleration Andromeda is still on a collision course.

originally posted by: greenreflections
so locally effect of gravity is stronger than the rate of space-time volume increase locally?

Yes, this is also why a single galaxy isn't expanding due to the expansion of space, gravity is stronger locally.

So why currently space-time expansion thought getting close or faster to the speed of light?

Because gravity follows the inverse-square law which means it's 100 times weaker at 10x the distance, so at greater distances gravity doesn't have as much effect and the expansion becomes more significant.

space-time is expanding everywhere at every point of space according to data but despite increasing rate of acceleration Andromeda is still on a collision course.

Not everywhere, mostly in empty space in the voids between galaxies and galaxy clusters and superclusters. As mentioned earlier, atoms aren't getting bigger, molecules aren't getting bigger, and galaxies aren't getting bigger from expansion, though obviously when the Milky Way and Andromeda collide we'll end up with one larger galaxy instead of two smaller ones.

a reply to: Arbitrageur

Yes, this is also why a single galaxy isn't expanding due to the expansion of space, gravity is stronger locally.

If that's so, then expansion should slow down when known mass is taken into consideration with its mutual gravity bond.)))

a reply to: greenreflections
That's what scientists expected to find in 1998 when they tried to measure how much the expansion was slowing down. Instead they found the expansion was accelerating and discovered "Dark Energy", so now there's another factor in the equations of the model that describes these things, called the Lambda-CDM model.