Physics

Gravity

Question:

I have read a great deal about the cause of gravity, including Einstein’s theory of curved space/time and attraction of large objects. I have the impression that scientists really do not know what is the cause. Is there some truth in that impression?

There is indeed some truth to that impression. But I think you could also say it applies to all scientific explanations. Eventually if you keep asking “what is the cause,” you get to the answer “that’s just the way the universe is”. Some people would try to address this with something called the “anthropic principle”: “the universe is the way it is because otherwise we wouldn’t be here to observe it”. Not everyone finds this idea convincing.

As working astrophysicist, I tend to leave questions like yours to people who study the philosophy of science. I am content to accept that there is gravity, for which Einstein’s theory of general relativity is the best current description we have,and to try and figure out its observable consequences. It’s not that these deeper questions are not important, but a typical scientist’s training doesn’t equip us to address them.

Increasing the temperature

Question:

Beaker A with 250ml water and Beaker B with 250ml vegetable oil are heated from 10 Celsius to 20 Celsius. which beaker requires more heat to achieve the stated result? explain why?

Beak A:

250 mL H2O X 0.9997g/mL(density) X 4.184 J/g C (specific heat) x 10 oC (change in temperature)= c.a. 10.5 kJ

Beak B:

250 mL of veg. oil X 0.918 g/mL(density) X 2.000 J/ g oC(specific heat X 10 oC (change in temperature)= c.a. 4.59 KJ

Here is the concept: The higher the specific heat, the higher the energy requirement to increase the temperature of the material by one degree Celsius. The determining factor in this question is the specific heat and not the density. If you see the calculation above, it takes more heat to get water to move from 10 degree Celsius to 20 degree Celsius.

Absolute Zero

Question:

Can it get more colder than absolute zero? Is this where the temperature stops?

No, it can’t get colder than absolute zero. The temperature of a substance is defined by the motion energy of particles that make up that substance. The coldest temperature on the temperature scale, -273 C or absolute zero, is defined as the point where the motions of particles become effectively zero. Indeed, the qualities of matter at such low temperatures can be quite unusual (e.g., superconductivity, bose-einstein condensates) and are still being actively studied today in condensed matter physics laboratories.

- Dr. James Di Francesco

Question:

If particles in the LHC are circulation in opposite directions near the speed of light, is their relative speed almost twice the speed of light when they collide?

That would be true following the classical physics of our everyday world.  Over the last 100 years, however, we have learned that Nature acts in a much more peculiar way when objects gain speeds close to the speed of light.  Following Einstein’s Special Theory of Relativity, space and time themselves distort so no object can appear to be moving faster than the speed of light.  So, from the perspective of either particle, the other actually appears to approach it at a speed very close to the speed of light, not twice that.  (Since the particles have finite mass, they cannot actually reach the speed of light itself, just get very close.)

You may have recently heard news about European observations of subatomic particles where the speed of light was exceeded.  If true, this measurement could lead to serious reconsiderations of Einstein’s theories which until now had been proven to high degree of precision in many experiments.  Although these recent measurements have been examined carefully for flaws by the scientists involved, none have been found.  It still remains possible, however, that their measurements were off by an unusual statistical fluke.  Repeat, independent experiments are planned to revisit their findings.

- Dr. James Di Francesco

Nuclear Waste

Question

Is it possible to use containers made of Cadmium to save the nuclear waste and avoid the risk of Nuclear radiation?

The concerns are for the more penetrating type of radiation.  If the type of the radiation is x-ray or gamma,  then materials that are dense and have high atomic number are best suited for the job.  The higher the energy of the radiation, the thicker the material has to be in order to shield the radiation.  Rock, soil, and concrete can do the same job, but larger amounts of it will be required.  I suppose cadmium could be used as a barrier to such radiation, but the cost and health risks most likely would out way its benefits.  If the radiation is neutron based than hydrogen-based materials are necessary to absorb the radiation.

Laser Beam

Question:

What happens to air as a laser beam travels through it?

This depends on the wavelength of the laser beam.

As this picture shows: http://fuse.pha.jhu.edu/~wpb/spectroscopy/figures/trans.gif, there are very narrow windows in which laser light can pass (i.e. the atmosphere is transparent and has no effect on the laser light). By contrast, where the atmosphere is “opaque”, all the light is absorbed by the atmosphere. Of course, air is composed of several different gases, each of which respond differently to different wavelengths of laser light. So, depending on the wavelength, the laser will either interact with the gas or it won’t.

For example, one can consider a typical microwave oven and how it works to heat food. Microwaves, as found in a microwave oven (122mm wavelength), interact and are absorbed by water molecules causing them to vibrate more vigorously, resulting in heat, but leave the other food molecules unchanged. By the same token, a laser beam of the appropriate wavelength can interact with one of the gases in air causing the air to heat up (absorption), refract or reflect the light (change the direction in which the light is moving), or even change the wavelength of the light (change of colour of the laser beam).

Weight at the Equator

Question:

What would be the difference in my weight (force) between standing at the equator vs standing at one of the poles? I suspect I would weigh less at the equator due to the centrifugal force. Say I weigh 80 kg.

At first glance, yes you would weigh a very small amount less at the equator, but one day of over-eating on the airplane to the pole, or of strenuous sweating exercise, will easily mask the predicted weight difference. This answer considers the effect of rotation at the equator, that uses “approximate values” for factors, and ignores the non-spherical nature of the earth and other effects. This simple approach compares

• The acceleration (a) of a body on the equator (a=dv/dt = omega ^ 2 * r) (here the carat implies “power of,” so it is omega squared)
• To the acceleration due to gravity at the equator, g
• Where
• v is the tangential speed of the body due to the Earth’s rotation
• omega is the “rotational speed” (degrees or radians per second)
• r is the radius of the Earth at the equator

3.30E-06  Fraction decrease in weight

2.6E-04 - kg for an 80 kg person

0.26 - grams

Based on that result, one day of over-eating on the airplane to the pole, or of strenuous sweating exercise, will easily mask the predicted weight difference.

Electromagnetic Spectrum

Question:

Every textbook/science magazine I have seen, always show the same looking diagram of the electromagnetic spectrum. We are all familiar with it: radio waves on the left (low frequency) side, and gamma on the right (high frequency side). What I want to know is what is the origin (0 cycles/sec.) of this spectrum called. Is it something like DC current (flat line)? Also, how high is the highest frequency that can be produced? Will it also have the appearance a flat line at the extreme frequency?

As stated in the question, most information sources show the same looking diagram of the electromagnetic spectrum (http://en.wikipedia.org/wiki/Electromagnetic_spectrum). The labels of the origin and end of the frequency axis are not identified as well as for the corresponding wavelength and energy axes.

DC can generate an electromagnetic field but for generating a wave in steady state, some oscillation is required which is not the case for DC.

Also note that from the energy scale of an electromagnetic wave, the energy is proportional to the frequency with a lower bound at minimum frequency (e.g., at 3 Hz with 12.4 feV) and an upper bound at the highest frequency (Gamma rays next to antimatter) around 2.4×1023 Hz (1 GeV).

“What I want to know is what is the origin (0 cycles/sec.) of this spectrum called.”
For the two extremes of the frequency axis I suggest to associate them to “static” and “known upper frequency limit”. “Static” to say it is not oscillating. The “known upper frequency limit” (2.4×1023 Hz) seems to be associated with Gamma rays and antimatter based on current information from the Wikipedia reference.

“Will it also have the appearance a flat line at the extreme frequency?”
If it is a wave, it exhibits an oscillation, not a flat line. So at extreme frequency it should not display a flat line. The real bounds, at lowest and highest frequencies, seem to be expressing the facts that as the wavelength approaches infinity, the energy approaches zero, and when the wavelength approaches zero, the energy approaches infinity.

On a measuring instrument display, such as an oscilloscope, one can observe sinusoidal signals in the radio and audio frequency range for examples. For DC, it could be a straight line but if a straight line is observed at higher frequencies it is mainly because the high frequency signal in question is either too small in amplitude or too high in frequency for that measuring instrument (beyond its sensitivity and maximum operating frequency). This is the case for the upper limit of the electromagnetic spectrum, which is difficult to observe this way.

Multi-Spectrum Light

Question:

If multi-spectrum light is reflected off of a mirror, does it reflect the same multi-spectrum?

Let’s rephrase that question in plain language: How do the colours within the light reflected off a mirror compare with the colours within the light that hit the mirror.

The simplest answer is yes, everyday mirrors are made to reflect as much of every visible colour as possible; they are made to reflect the same spectrum incident on them. The colours of the rainbow, from red through violet, form a continuous band of colours that comprise the “visible spectrum”. When we look at something in a mirror we want the image to resemble the object in every way, including its colour. For that to be possible, the mirror has to be constructed from a material with the proper reflection spectrum. When light hits a material it can be reflected, transmitted, and absorbed. Usually, all three processes occur in different proportions, depending on the material. Metals are great at reflecting light, but metals don’t all reflect all colours equally: Silver and aluminum are the most commonly used for making mirrors because they reflect most of the light incident on them and because they reflect all colours (nearly) equally. Alternatively, gold and copper, for example, are excellent at reflecting red and yellow, but not as good at reflecting blue and violet. That’s why they look orange-red or yellowish. That’s also why we don’t use gold or copper for mirrors. Because silver and aluminum are so expensive, we’ve been making mirrors for the last few hundred years by evaporating them in a thin layer on the back of polished glass.

Electron subshell

Question:

In an element, how do electrons fill a subshell? Is there some sort of attraction force that does it?

An electron has four quantum numbers when it is looking for its seat assignment, so to speak, in the element. “No two electrons in the same atom can have the same four quantum numbers”, we call this the Pauli exclusion principle. The first quantum number is the principal quantum number which indicates the distance of the electron from the nucleus of the atom. The larger the number, the farther the electron is away from the nucleus. The second quantum number is called the angular momentum quantum number. It basically communicates the shape of the orbital. Liken it to the path that one travels on a given day, If you have travel only to work and home the shape would be a straight line. If you traveled from home to school and then work and finally back to home it may look like a circle, a triangle, etc. The angular momentum quantum number is the sublevel. Each sublevel has more options of shape when the principle quantum number increases. The third quantum number is called magnetic quantum number or orbital-orientation quantum number. It tells you how the angular momentum quantum number is oriented in 3 dimensional space. Is the same laying on the x-y axis or x-z axis, etc. The last quantum number is spin number, this number communicates the direction of the spin.

In summary, when the electron finds its seat, it is balanced generally by two forces a) the attraction to the charge of the nucleus and b) the repulsion by other electrons.

Is time part of the universe?

Question:

Is time part of the universe or does the universe exist in time? Which incorporates the other? What position do scientists take on this matter?

Until the beginning of the 20th century, scientists considered time to be separate from motion within the universe, thought to proceed at the same rate, no matter your reference frame. In that way, it was the universe which existed in time, with time being an independent quantity. However, since Einstein came up with his theory of Relativity, we now consider time and space to be intertwined into a quantity we call space-time.

Einstein predicted that the passage of time would appear to change depending on your frame of reference, that is, it is not an absolute quantity. For example, time would pass more slowly for someone travelling at great speeds than for someone at rest. Similarly, a strong gravitational field will also cause time to slow down for someone who is closer to it. It is this knowledge that allows GPS to work, since time is passing more quickly for satellites in orbit around the Earth than for those of us down on the ground, and this time difference must be accounted for to pinpoint exact locations on the Earth's surface.

In short, time and space are not separate entities, but two sides of the same coin. Time and space are both part of the universe.

Hole through the earth

Question:

If a hole was dug all the way through the earth coming out on the opposite side of the planet, and someone jumped in it, what would happen? (Of course the sides of the hole are sealed and we don't have to worry about pressure and temperature, just curious what gravity would do.)

Just like above the Earth's surface, the Earth's gravitational force would accelerate you toward the center of the Earth. Because gravity is a "conservative" force, if you start on one side of the Earth and step into the hole from rest, you would accelerate down towards the center and reach your maximum velocity at the center of the Earth. Then you would continue down the hole, decelerating until you reached the exact same radius you started at on the other side of the hole. If you don't quickly grab onto the side, you would again start accelerating down to the center of the Earth and subsequently reach your first starting point. This could continue forever (as long as we assume there is no friction) so you would oscillate back and forth through the hole. (Small caveat: I've assumed the Earth is a perfect sphere so that the gravitational force only depends on your distance from the center).

Also, the magnitude of the gravitational acceleration which is about g=9.8m/s^2 on Earth's surface changes as you go deeper into the Earth. It turns out that g=GM/r^2 where G is known as the gravitational constant and M is the mass that is enclosed in the sphere BELOW you. So all the mass in the spherical shell above you doesn't affect your gravitational acceleration. Although the enclosed mass decreases as you descend into the earth, your "r" also gets smaller and so it turns out that in the mantle of the Earth (the outer rocky part that extends from the surface to a depth of about 2800km) g stays roughly constant at about 10m/s^2. This means its fairly straightforward to calculate what speed you would have when you reach the bottom of the mantle (which is the top of the iron core): v=2gy where y is the distance you've travelled. This works out to about 7.5km/s! (which is pretty fast). In the core of the Earth, gravity decreases in magnitude roughly linearly so you will still speed up as you descend the core, but the rate of speed up will be less than in the mantle. You will have your fastest speed at the very center.

Quantum entanglement theory

Question:

I was wondering how exactly quantum entanglement theory has been proven. I've read about Bell's experiments, but it just seems that it is right because it can't be proven wrong. Nothing seems to discard Einstein's idea that the particles' states were determined before they we split. Since you can't know what characteristics your particle has before you've looked at it, how can you be sure that it's your action that defined it and its entangled partner?

Your question seems to be focused on the validity of entanglement. Strictly speaking we don't have a "quantum entanglement theory" but rather just "quantum theory" and, strictly speaking, no scientific theory is every proven but rather refuted or supported. The tests of the Einstein-Podolsky-Rosen notion you described is commonly called "local realism", which is  tested by performing appropriate measurements and checking whether the data violate a form of Bell's inequality.  Many experiments to date violate a form of Bell's inequality known as the Clauser-Horne-Shimony-Holt inequality, which makes reasonable but uncomfortable assumptions about random sampling to account for detector inefficiency. We are hoping in the near future that a tighter form known as a Clauser-Horne inequality, which does not rely on this uncomfortable assumption (uncomfortable because a malicious local-realistic universe could have a detector-dependent sampling feature), will be testable. The barrier has been reaching a minimal detection threshold, but we are at that point now so stay tuned: we are close to refuting or not refuting the Einstein-motivated local realistic theory.

Motion in a vacuum

Question:

In a vacuum, if the applied force and the frictional force on an object is the same will the object keep moving for ever?

This is an important question, the answer to which is a key concept distinguishing how the Greeks thought about motion and how we now think about motion! In short, the answer is yes. “An object in motion will remain in motion as long as all forces acting on it are balanced”. This statement is known as Newton’s First Law of Motion! As posed in the question, the “frictional force” and the “applied force” are the same, and one assumes that they act in opposite directions, and thus the forces are balanced. If the object is at rest, it will remain at rest until one of these two forces (probably the applied force) changes. If the object was already moving, it will continue to move at precisely the same speed and direction, until, again, one of the two forces changes. Note that Newton’s First Law of Motion does not require the object to be in a vacuum. Indeed, the qualifier of “In a vacuum” in the question is unnecessary in concept, but is somewhat important to explaining why this law of motion seems so unintuitive to us. For example, once a baseball pitcher releases a fastball at 100 miles per hour there are no “applied [horizontal] forces” acting on the ball, yet we know it slows down. This is because of air resistance. So we are used to seeing objects slow down (or alternatively…not “keep moving forever”) for no apparent reason. If the ball game were to be played on the moon (i.e. in near vacuum), the horizontal speed of the ball would remain 100 miles per hour as it crosses the batter’s box. However, most of the frictional forces we experience every day are contact forces between objects and surfaces and thus would not diminish in a vacuum. For example, a hockey puck sliding on the ice slows down because of small amounts of friction with the ice. The same slow-down would happen on the moon.

In conclusion: yes, if the applied force and the frictional force on a moving object are equal in magnitude and operate in opposite directions, the object’s motion will remain unchanged forever. The corollary is true as well: If an object’s motion is changing, there must be some unbalanced force acting on it. Often, as in the case of frictional forces, it may not be immediately apparent what that force is.

Water evaporation

Question:

When a drop of water is dropped on to a hot iron plate, why does it take a spherical shape before it evaporates?

The droplet shape is not really related to the temperature of the plate, but rather to what type of surface is the water dropped on. Essentially, how much does the “iron plate” like water is the question we are asking. In a normal kitchen application, an iron plate would have a fair amount of grease even though it appears to be clean. So to be more precise, we are really dealing with a “greasy iron plate”. Grease and water don’t like each other, so water tries to minimize contact with the “greasy iron plate” by beading into a droplet-like shape.

Subcooled liquid

Question:

I left two full 500 ml bottles of water in my truck overnight and the temperature went down to -21°C. One bottle was frozen solid while the other appeared to remain liquid. When I picked up the unfrozen bottle and held it in my hand, like a cascade from top to bottom it froze rapidly. What caused this reaction? Why did it not freeze overnight like the other bottle?