Question related to speed of light.

[Galaxiom]

if you'll look at the post directly above mine, you'll note he asked if a denser medium actually changes the speed of light.

I believe Mike's question was rhetorical.

 

[Mike375]

If I calculate the kinetic energy of a rifle bullet then is it correct that my answer will be 99.9999999 etc.% of the true figure as the mass of the bullet will have increased?

Likewise, if we had a ballistic pendulum then it would swing back by a greater amount than the transfer of momentum would illustrate?

 

[Galaxiom]

Mass, length, time and hence velocity and kinetic energy are all affected by Relativity.

The only property that measures the same from all frames of reference is momentum, the product of mass and velocity. Work backwards from that and all the answers can be derived.

Remember it is the relative motion of the object and the measurement frame of reference that matters.

 

[Galaxiom]

BTW Earlier it was pointed out that time stops for the speed of light space traveller. However this does not give them the impression that they travelled at infinite speed because the distance to the destination also becomes zero as observed by the traveller.

 

[Mike375]

Back to my original question as to time and mass if our traveller slows down.

Let's take the Apollo astronauts. Stage 3 reignites and takes them to escape velocity of about 26,000 mph and the stage is shut down and earhs gravity slows them to about 3000 mph when they are about 30,000 miles from the moon at which point the moon's gravity has more influence than Earth's.

Does time speed up for them during this "slow down" and does mass decrease?

Now to the next part. When the Lunar Module docks with the command and service module the service module fires up and takes them to about 5,000 mph to moon escape velocity and then they gradually accelerate to about earth's escape velocity of 26,000 mph and re entry. Does this cancel out of "even up" what has happened with time/mass on the trip to the moon?

As a side note, I was able to secure about a half an hour session with Eugene Cernan (Apollo 17) when he came to Australia....I have been a space freak since I was a young kid. The one thing that stands out from that meeting with him was his total inability to convey/explain what it was like to sit on top of the Saturn V as ignition sequence started and of course hitting the starter button on the Luna Module to leave the moon.

It is no wonder so many of those blokes finished up going a bit "funny" as not much else you could do......A Formula 1 is not quite like sitting 36 stories up while propellant is being consumed at 15 tons per second under you.

 

[The_Doc_Man]

Again, there is no slowdown. There is only a change of relative velocity. "Slow down" (as you are trying to use it in this discussion) implies that there might be an absolute frame of reference and thus there would be a strange effect of suddenly going at a "true velocity of 0" - but not only do we not think that such a thing CAN occur, but even if it DID, we don't think we would know it happened (since we still see things relatively). Your questions all seem to ignore the fact that everything you measure is relative to your own frame of reference, or to some other frame of reference that you can observe conveniently.

All you can ever do is ACCELERATE in a direction to cancel existing velocity with respect to your frame of reference, which is to say that if you have layered frames of reference, an outside observer in the outermost frame would see you and that space capsule synchronize speeds. I.e. the capsule would join your frame of reference. Thereafter, if there WERE any time dilation effects, you and the capsule would have the same level of that dilation.

 

[Mike375]

Your questions all seem to ignore the fact that everything you measure is relative to your own frame of reference, or to some other frame of reference that you can observe conveniently.

I don't that way at all. For example if I leave the earth and accelerate up towards the speed of light and return to earth several years have passed on earth but me 1 year or whatever has passed.

If we take two cars that leave each other at 50 mph then 1 hour later they are 100 miles apart. However, if one car remains stationery and the other car departs at 100 mph then again the cars a 100 miles apart after 1 hour but it will be the car that accelerated to 100 mph that experiences the time change and increase in mass. At least I think that is how it works.

I think I might go and play with Access for a while

 

[The_Doc_Man]

If we take two cars that leave each other at 50 mph then 1 hour later they are 100 miles apart. However, if one car remains stationery and the other car departs at 100 mph then again the cars a 100 miles apart after 1 hour but it will be the car that accelerated to 100 mph that experiences the time change and increase in mass. At least I think that is how it works

Nope, not how it works. Let's call that car M and car S. If car M moves and car S stands still (according to your setup),

- Car M moves at 100 MPH with respect to (w.r.t.) Car S, and thus experiences the mass and time effects based on an observer in car S.

- Car S moves at 100 MPH w.r.t. Car M, and thus appears to have the mass and time effects based on an observer in car M.

If you have an outside observer O (onlooker) who is standing still, this person is in the same frame of reference as car S, so isn't actually an outside observer.

If you have a traffic observer H (helicopter traffic report) whose pilot happens to be exactly following car M, this person is in the same frame of reference as car M, so isn't actually an outside observer.

If you have a passenger in a third car (Car T) moving at 50 MPH w.r.t. to car M, this person sees both Car M and Car S moving at 50 MPH away from him and sees corresponding changes in time and mass. (Miniscule for 50 MPH, but still there.)

Ain't relativity grand?

 

[Mike375]

If you have a passenger in a third car (Car T) moving at 50 MPH w.r.t. to car M, this person sees both Car M and Car S moving at 50 MPH away from him and sees corresponding changes in time and mass. (Miniscule for 50 MPH, but still there.)

Ain't relativity grand?

But it is Car M (its driver) that sees the greatest change in mass and time?

 

[Galaxiom]

But it is Car M (its driver) that sees the greatest change in mass and time?

No. A Doc said, it is all about relative motion.

None of the drivers see any change in the mass of their own car.

 

[Mike375]

No. A Doc said, it is all about relative motion.

None of the drivers see any change in the mass of their own car.

I don't get it.

If our intrepid traveller jumps on a light beam them from his perspective he reaches a planet at Alpha Centauri in zero time and there was zero distance. However, for observers on earth and the planet at Alpha Centauri he has travelled about 4 light years and he has travelled for about 4 years.

When he arrives at the planet they send a message back to earth. Our traveller spends 24 hours on the planet and then jumps on the light beam to head back to earth.

When he arrives back at earth 24 hours of his time will have passed and the message sent from the planet at Alpha Centauri will have arrived 24 hours before he arrived.

For the people on earth he will have been away for 8 years plus the 24 hours he spent at Alpha Centauri.

When he arrives back at earth they send a message to Alpha Centauri which arrives 8 years plus 24 hours after they sent their message to earth on his arrival at Alpha Centauri

Is this correct?

 

[The_Doc_Man]

When he arrives back at earth 24 hours of his time will have passed and the message sent from the planet at Alpha Centauri will have arrived 24 hours before he arrived.

Assuming instantaneous acceleration to speed c, yes. And your problem is?

I think that where you run into SERIOUS problems with this thought experiment is that by moving at an impossible speed you get an impossible result.

Shall I trigger yet another blown mind? If you could somehow convert to antimatter, you could travel as a tachyon, which can move faster than light ... but never slower. In fact, in theory a tachyon takes LESS anti-energy to move at speeds much faster than light. The closer to c it gets, the more massive (in anti-matter terms) the tachyon gets. Because the Lorenz-Fitzgerald equations work for tachyons, too. They just have a whole bunch of imaginary numbers floating around.

 

[Galaxiom]

Assuming instantaneous acceleration to speed c, yes. And your problem is?

I think that where you run into SERIOUS problems with this thought experiment is that by moving at an impossible speed you get an impossible result.

I don't see anything in the thought experiment inconsistent with Relativity.

Mike is actually struggling with the concept that it is the difference between the travellers' frame and the observers' frame that is the whole story with Relativity.

 

[Frothingslosh]

I don't see anything in the thought experiment inconsistent with Relativity.

Mike is actually struggling with the concept that it is the difference between the travellers' frame and the observers' frame that is the whole story with Relativity.

This.

The sticking point seems to be that he's welded to the idea of an absolute frame of reference, and seems to have some difficulty with the idea that there is literally no such thing as 'slowing down' or 'de-acceleration' when discussing physics, at least in regards to Relativity.

From what I've seen before, that's definitely one of the most difficult points for most people when they start wrestling with Relativity.

Asked in 1919 whether it was true that only three people in the world understood the theory of general relativity, [Arthur Stanley Eddington] allegedly replied: 'Who's the third?

 

[Mike375]

Let's try a real world one instead.

Astronaut leaves earth to dock and board the ISS.

He goes from zero to low earth orbit at about 17,500 mph in about 10 minutes. During this phase what happens with his time as compared to back on earth. My understanding has been his clock will be slightly behind earth time.

He stays on the ISS for 6 months. What happens to time in this "steady" velocity situation?

He then re enters and so goes from 17,500 mph to zero. What happens to time during this phase?

Lastly, it is simple to calculate the kinetic energy and momentum of a rifle bullet. However, is the "true" KE and momentum 1.0000000 etc.% greater than my calculation gives. If the bullet strikes a ballistic pendulum does the pendulum swing back a distance that is 1.0000000 etc.% greater than the calculation would show?

 

[The_Doc_Man]

While theoretically present in all cases, most speeds that we can reach do not trigger effects that are measurable on a gross object. Therefore, any pendulum resilient enough to still exist after being hit by your rifle bullet will change momentum based on the simple formula of mass x velocity giving a number of foot-pounds or kg-meters or whatever system you are using, and the relativistic effects of something so slow as a rifle bullet will be very nearly too small to measure.

Here is the case that maybe will anchor some of this discussion. There is no absolute frame of reference, but there is an absolute limit for one thing - speed. Nothing in the physical universe travels faster than light.

One can argue that a particular photon could represent a frame of reference. The noticeable aging and mass effects predicted by the Lorenz-Fitzgerald (and related) equations only occur when you try to join the frame of reference of a photon that happens to be going in your desired direction. As you approach that frame of reference, your time dilation and mass accretion become significant. When you accelerate in the opposite direction from that photon (notice I did not say "decelerate"), your time flow becomes closer to other frames of reference that might even have people in them. They would see effects based on just how close you came to joining that photon's frame of reference. But YOU wouldn't see any effects at all until you rejoined a frame of reference where comparison became possible. (See? You can't get away from relativity because such comparisons are how you RELATE to something.)

Outside of the case of trying to join a photon, no other speeds make a significant difference, though long-term effects HAVE been measured for objects that reached orbital speeds, stayed that way, and then returned to frames of reference slower than the orbital situation. The more significant issue was that the returned objects were ALSO farther away from the frame of reference for photons.

I believe one of the objects in question was a digital clock or watch with a mechanism that counted vibrations of a quartz-based oscillator. There were predictable effects that included time dilation and mass inflation. Mass affects quartz oscillator vibration rates AND increased speed ALSO slows down apparent vibration rate, both of which were computable for the long-term conditions of being in orbit for a longer period of time. So we know that the effects are real.

If you can't wrap your head about the relativity issues any other way, here is your anchor point. Your speed with respect to cars, planes, boats, or trains is immaterial. The ONLY thing that noticeably affects is you is how close or how far you are from the frame of reference of photons that happen to be going in your direction. If you accept that the term "deceleration" only makes sense as "accelerating in the opposite direction from where I was originally pointed" then you can try to accelerate to the speed of light and can then decelerate from it. (But notice that "decelerate" only makes sense in RELATIVE terms, even as ACCELERATE only makes sense as relative to something else.)

 

[Galaxiom]

Let's try a real world one instead.

Astronaut leaves earth to dock and board the ISS.

He goes from zero to low earth orbit at about 17,500 mph in about 10 minutes. During this phase what happens with his time as compared to back on earth. My understanding has been his clock will be slightly behind earth time.

He stays on the ISS for 6 months. What happens to time in this "steady" velocity situation?

He then re enters and so goes from 17,500 mph to zero. What happens to time during this phase?

It isn't about acceleration. Only the speed counts in the time dilation. It will just be the dilation integrated over the trip according to the instantaneous speeds.

Lastly, it is simple to calculate the kinetic energy and momentum of a rifle bullet. However, is the "true" KE and momentum 1.0000000 etc.% greater than my calculation gives. If the bullet strikes a ballistic pendulum does the pendulum swing back a distance that is 1.0000000 etc.% greater than the calculation would show?

There is no "true" kinetic energy. The measurement depends on the frame of reference from where it is measured because both the mass and velocity depend on the measurement frame.

The relativistic effects seen by an observer who is at the same frame as the fulcrum of the pendulum would be: The length of the bullet is reduced, the clock inside the bullet is ticking slowly and the bullet is measured to be approaching at a lower velocity than the speedo on the bullet shows.

However, the bullet's momentum is not affected by relativistic effects from all frames of reference. The reduction in measured speed (as measured from the fulcrum) will be compensated by an increase in the mass.

The pendulum will be moved by the amount of momentum transferred, which will be the same from all frames. But both the pendulum speed and mass will be different for different observer frames.

If you think about it, the momentum cannot be affected by Relativity, otherwise the resulting paths after a collision between two bodies would be different depending on the observation frame of reference, which would clearly not make any sense since there is only one path the object can take.

 

[Mike375]

The pendulum will be moved by the amount of momentum transferred, which will be the same from all frames. But both the pendulum speed and mass will be different for different observer frames.

If you think about it, the momentum cannot be affected by Relativity, otherwise the resulting paths after a collision between two bodies would be different depending on the observation frame of reference, which would clearly not make any sense since there is only one path the object can take.

So are you saying the mass of the pendulum is increased like the bullet and so the distance the pendulum swings back is the same as the simple calculation would show.

If apply a force to give a body 1g then a simple V=U+AT will have that body at the speed of light in just under a year. I have read that when the increase of mass is factored in (but force stays the same) it will be at 77% speed of light and I think it was something like 90% after 12 years. Is this correct?